Emanoil LINUL | Politehnica University of Timisoara (original) (raw)

ISI Journals by Emanoil LINUL

In this paper is presented the behaviour of PUR foams under mixed mode loading. Closed cell rigid... more In this paper is presented the behaviour of PUR foams under mixed mode loading. Closed cell rigid PUR foams having three different densities 100, 145, and 300 kg/m 3 were investigated. Experiments were performed using asymmetric semi-circular bend (ASCB) and Assymmetric Four-Point Bend (AFPB) specimens. The obtained crack initiation angles established for ASCB specimens were compared with four fracture criteria MTS, SED, G max and ESIF, and a good agreement was observed. When testing AFPB specimens, and calculating the normalized stress intensity factors it is important to obtain a correct crack propagation, that is to impose proper geometrical dimensions as not to affect the initial crack tip. The eXtended Finite Element Method (XFEM) is a complementary powerful numerical tool used to analyze crack initiation and propagation only if the numerical model is correctly calibrated and all the influencing parameters are properly understood.

Mixed mode four-point testing is performed on polyurethane foams. This paper presents only the re... more Mixed mode four-point testing is performed on polyurethane foams. This paper presents only the results on the stress intensity factors (SIFs) obtained experimentally for a density of 325 kg/m 3 , although tests were done for three foam densities. An asymmetric four-point bending setup was used for determining the critical SIFs in Mode I and Mode II, and discussions on the influence of the initial crack length on the SIF values are done. As initial crack length is increased the theoretical predictions give a better comparison to experimentally obtained results.

Micromechanical models used to predict mechanical and fracture properties of brittle metallic foa... more Micromechanical models used to predict mechanical and fracture properties of brittle metallic foams are validated experimentally for closed-cell aluminium foam (AlSi12Mg0.6) prepared by powder metallurgy route. Compression, tensile, tensile on notched specimens and fracture toughness tests were carried on, and the results are presented together with micromechanical models from literature. Moreover, the Digital Image Correlation technique was applied to identify the failure mechanisms of aluminium foams. Finally, the Theory of Critical Distances was employed to predict the fracture load of notched specimens. The novelty of the study is that the inherent stresses and critical distances were obtained by employing micromechanical analysis.

The failure-mode maps of composite sandwich beams can provide useful information about the influe... more The failure-mode maps of composite sandwich beams can provide useful information about the influence of different design parameters on the failure behaviour of such components. Failure mode of sandwich beams with different cores and different faces were investigated in the experimental program. Foams with 40 and 200 kg/m 3 densities were used as core material, while Glass-Fibre Reinforced Polymer (GFRP), polyester, epoxy and aluminium are the faces materials. Three-point bending tests were carried out for sandwich beams. In order to characterize these sandwich materials first were carried out a statistical analysis of the cellular structure for two different densities of the core material above mentioned. The sandwich core morphology and cells dimensions were studied before testing through scanning electron microscopy (SEM) and pore diameter versus frequency of pores histogram were plotted. After statistical analysis were performed static compression tests. These compression tests have had as objective the determining of the main mechanical properties such as Young's modulus and yield stress values. The results obtained from the static compression tests were used for the analytical determination of failure-mode maps of sandwich beams. Finally, the failure-mode maps were constructed for five considered sandwich types and validated by the experimental results. Each failure-mode map is characteristic for a family of sandwich beam designs.

This work investigates the mechanical properties of polyurethane rigid foams by means of Dynamic ... more This work investigates the mechanical properties of polyurethane rigid foams by means of Dynamic Mechanical Analysis (DMA) tests and Impulse Excitation Technique (IET). DMA tests were performed in single cantilever with a sweep in temperature (from-50 °C to 100 °C) and frequency (from 1 Hz to 100 Hz), not determining glass-transition in the test parameter interval. IET tests were used to determine the dynamic modulus of elasticity, showing good accordance with DMA results

The notch effect in Polyurethane materials of different densities is investigated. Tensile specim... more The notch effect in Polyurethane materials of different densities is investigated. Tensile specimens with lateral V and U symmetric notches and with holes of different diameters were tested. The material parameters inherent stress and critical distance are determined using a linear elastic finite element analysis in conjunction with experimental results of average maximum load. The paper proposed an original correlation between critical distance and the cell diameter of the cellular structure, respectively between inherent stress and the ultimate tensile strength of Polyurethane materials. Then for single edge notch specimens a relationship between critical distance and applied mixed mode is found and successfully applied to assess the brittle mixed mode of Polyurethane materials.

Polyurethane (PUR) foam materials are widely used as cores in sandwich composites, for packing an... more Polyurethane (PUR) foam materials are widely used as cores in sandwich composites, for packing and cushioning. They are made of interconnected networks of solid struts and cell walls incorporating voids with entrapped gas. The main characteristics of foams are lightweight, high porosity, high crushability, and good energy absorption capacity. Fracture toughness in mixed mode loading is of particular interest because foam cracking weakens the structure's capacity for carrying loads. Present paper assesses the shear elastic (shear modulus) and mechanical (shear strength) properties of polyurethane foams. Also, three different types of specimens were used to determine mode I and mode II fracture toughness. The shear modulus, shear strength and fracture toughness increases with increasing foam density. Also the effect of loading direction and loading speed is investigated. The authors propose a micromechanical model to estimate fracture toughness based on the tensile strength of the solid material and the topology of the cellular structure.

This paper deals with evaluating the elastic response of several micromechanical structures used ... more This paper deals with evaluating the elastic
response of several micromechanical structures used for simulating cellular materials under compression. For this
study polyurethane rigid foams were investigated, having
three relative densities: 0.085, 0.124 and 0.256. Their
microstructure was analysed using SEM images, determining
four types of cells that were consequently designed
using specialized CAD software: square cells with circular,
quadratic and/or hexagonal orifices and hexagonal
cells. An interdependent variation of the cells’ geometrical
parameters of the proposed structures was determined to
obtain geometrical variations at a required relative density.
Finite element analysis simulations were performed on
the designed microstructural models using a linear elastic
material model for the cell struts, resulting in the variation
of the elastic modulus of the structure with the variation in
cell geometry parameters. The final objective of this work
was to determine anisotropic bi-dimensional micromechanical
models for the studied cellular material that provides
accurate results in compression on both loading directions.
The anisotropic models for the proposed cell structures
were obtained by generating irregular geometries which
provided extra variables for the cell geometry parameters.
It was determined that some cell geometries are suitable
for simulating lower relative density materials while other cell geometries provide good accordance with experimental
data for higher relative density materials.

Many efforts have been made in recent years to determine the fracture toughness of different type... more Many efforts have been made in recent years to determine the fracture toughness of different types of foams in static and dynamic loading conditions. Taking into account that there is no standard method for the experimental determination of the fracture toughness of plastic foams different procedures and specimens were used. This paper presents the poly-urethane foam fracture toughness results obtained for different foam densities. Two types of specimens were used for determining fracture toughness in modes I, II and a mixed one, and also the size effect, loading speed and loading direction were investigated. The paper proposed correlations for density, cell orientation and mixed mode loading based on the experimental testing results.

International Journal of Solids and Structures, Nov 2, 2012

We investigate sandwich composite beams using a direct approach which models slender bodies as de... more We investigate sandwich composite beams using a direct approach which models slender bodies as deformable curves endowed with a certain microstructure. We derive general formulas for the effective stiffness coefficients of composite elastic beams made of several non-homogeneous materials. A special attention is given to sandwich beams with foam core, which are made of functionally graded or piecewise homogeneous materials. In the case of small deformations, the theoretical predictions are compared with experimental measurements for the three-point bending of sandwich beams, showing a very good agreement. For functionally graded sandwich columns we obtain the analytical solutions of bending, torsion and extension problems and compare them with numerical results computed by the finite element method.

Polymer Testing, Mar 9, 2013

The paper presents a correlation between dynamic and static fracture toughness of polyurethane ri... more The paper presents a correlation between dynamic and static fracture toughness of polyurethane rigid foams. Static three point bend tests and instrumented impact tests were performed using single edge notch specimens. The obtained results show that for all foam densities the dynamic fracture toughness is higher than the static toughness. Density appears to have the main influence on both static and dynamic fracture toughness. A quasi brittle fracture without plastic deformations and cushioning was observed for all foam densities.

Iranian Polymer Journal, Aug 17, 2011

T he fracture toughness was determined for cellular polymers by micromechanical modelling using f... more T he fracture toughness was determined for cellular polymers by micromechanical modelling using finite element analysis. In this study, mode I and mode II of fracture toughness were evaluated with a 2D-solid model using fracture analysis code FRANC2D/L. Simulation was performed for open cell polyurethane foams of different densities. Two cases were considered: constant cell length, l, and variable cell wall thickness; the former for constant cell wall thickness, t, and the latter for variable cell length. For estimation of fracture toughness the applied loads were progressively increased to the point reaching the fracture strength of the solid material (130 MPa) in an un-cracked strut in front of the crack. The estimated fracture toughness was independent on crack length, indicating that the obtained values could be considered as material property. The values of the fracture toughness of polyurethane foams are in the range of 10 -3 -10 -1 MPa.m 0.5 . Lower values were obtained for mode II fracture toughness. A strong dependency of the fracture toughness on the density of the cellular material was featured by present study. The obtained results for mode I fracture toughness were compared with Gibson-Ashby micromechanical model, by which a good agreement was obtained. While, mode II fracture toughness was compared with Choi and Sankar micromechanical models. The predicted fracture toughness was finally validated with some experimental data. Another advantage of this model was to obtain a fully described the stress field in the solid struts. The stress distribution in the first un-cracked strut showed a combined stress (bending and tension) for mode I loading, while for mode II loading a pure bending appeared.

Materiale Plastice, 2010

In the recent years the utilization of polymeric materials increased due to their good mechanical... more In the recent years the utilization of polymeric materials increased due to their good mechanical properties and easy manufacturing. Starting from automotive industry up to food industry polymeric materials are used for obtaining components by molding or by machining. Taking into account the wide range of applications, it is important to know the mechanical behaviour of these materials in different loading conditions. Accordingly, this paper presents an experimental study to determine mechanical properties on 11 commercial polymeric materials. Tensile and shear static tests and impact tests on notched and un-notched specimens were performed. The results are presented comparatively, and two new parameters were introduced for material selection purpose.

ISI Proceedings by Emanoil LINUL

Cellular plastics are light weight structures with many applications in civil, aeronautical, auto... more Cellular plastics are light weight structures with many applications in civil, aeronautical, automotive and mechanical engineering. Properties of cellular materials depend on the properties of the solid material, on the shape and dimensions of the cellular structure and on the relative density of the cellular material. Most of cellular plastic materials are crushing in compression and have a brittle behavior in tension. The effect of notches represents an important issue in such materials, taking into account that for packing applications for example, notches/holes should be introduced in the cellular material. This paper investigates the effect of notches in compression for three different densities 100, 145 and 300 kg/m 3 polyurethane (PUR) foams. Experimental investigations were performed on rectangular blocks of 100x100x25 mm with 16, 28 and 40 mm central holes. The mechanism of damage was monitored with an IR camera FLIR A40M. Purpose of the numerical simulations was to calibrate a material model, based on compression test for un-notched specimens using the CRUSHABLE FOAM models implemented in ABAQUS SIMULIA. Then the material models were used to simulate the experimental tests on notched blocks. Good agreement was obtained for the load-displacement curves obtained experimentally and from simulation. Also the plastic deformation patterns observed experimentally by IR thermograpghy were obtained numerically using the CRUSHABLE FOAM material model.

Cellular materials represent a new class of materials; main parameters that characterize the cell... more Cellular materials represent a new class of materials; main parameters that characterize the cellular structure are relative density, shape of the cell (open or closed), wall thickness and cell diameter. The purpose of this paper is to investigate the microstructure of foams materials and also to determine the flexural properties of this rigid PUR foams using Digital Image Correlation (DIC). The rigid PUR foams cells morphology and pore distribution for three densities (100, 145 and 300 kg/m 3) were studied before testing through scanning electron microscopy. Determination of flexural properties was carried out on rectangular beam samples using ARAMIS 2D system. This method provides a substantial increase in accuracy for measuring strain and is based on the calculation of surface deformation using a set of digital images from undeformed stage to different deformed stages. The specimens were subjected to static three points bending tests with loading rate of 2 mm/min, at room temperature and loading was applied in rise direction of the foam. Experimental results show that main mechanical properties such as flexural modulus and flexural strength values increases with increasing of density.

Journal of Physics: Conference Series , 2013

Effect of density, loading rate, material orientation and temperature on dynamic compression beha... more Effect of density, loading rate, material orientation and temperature on dynamic compression behavior of rigid polyurethane foams are investigated in this paper. These parameters have a very important role, taking into account that foams are used as packing materials or dampers which require high energy impact absorption. The experimental study was carried out on closed-cell rigid polyurethane (PUR) foam specimens of different densities (100, 160 respectively 300 kg/m 3 ), having a cubic shape. The specimens were subjected to uniaxial dynamic compression with loading rate in range of 1.37-3.25 m/s, using four different temperatures (20, 60, 90, 110°C) and two loading planes (direction (3) -rise direction and direction (2) -in plane). Experimental results show that Young's modulus, yield stress and plateau stress values increases with increasing density. One of the most significant effects of mechanical properties in dynamic compression of rigid PUR foams is the density, but also the loading speed, material orientation and temperature influences the behavior in compression D2FAM 2013

Key Engineering Materials, 2014

Polyurethane (PUR) foam materials represent a class of materials widely used for impact protectio... more Polyurethane (PUR) foam materials represent a class of materials widely used for impact protection and energy absorption. This paper presents a characterization of different rigid PUR foams under compressive impact loading by means of energy absorption and efficiency diagrams. Compressive properties were investigated on cubic specimens on the foams’ rise direction at room temperature with a loading rate of 3.09 m/s for three different closed-cell foams with densities of 100 kg/m3, 160 kg/m3 and 300 kg/m3 respectively. Experimental results show that the compression modulus, yield stress and plateau stress increase with density. Most of the energy is absorbed in the plateau region because of the cell deformation associated with this phenomenon, allowing greater absorption of impact energy at nearly constant load. Authors have found that both the energy absorption and efficiency diagrams are consistent and present similar results for studied foams.

Key Engineering Materials, 2014

This paper presents an experimental characterization of three different types of closed-cell alum... more This paper presents an experimental characterization of three different types of closed-cell aluminium alloy foams (AlMg1Si0.6, AlSi12Mg0.6 and AlMg0.6Si0.3) under static compressive loading. This study was carried out on half-cylindrical specimens with skin. The influence of foam density on compressive behaviour was investigated for densities ranging from 430 kg/m3 to 935 kg/m3. The compression tests were performed at room temperature (23°C) with a constant crosshead speed of 0.5 mm/min. Strain distribution, yield stress and compressive modulus values were recorded using Digital Image Correlation. Experimental results show that the mechanical properties (Young’s Modulus, yield stress and plateau stress) increase with density.

Key Engineering Materials, 2014

Closed cell polyurethane foams with densities of 100, 160 and 301 kg/m 3 were tested in compressi... more Closed cell polyurethane foams with densities of 100, 160 and 301 kg/m 3 were tested in compression at speeds of 1 and 5 mm/min. Digital image correlation (DIC) is used to determine the engineering characteristic curve, modulus of elasticity, Poisson's ratio and the deformation bands that appear during deformation and prior to the final failure of the specimens. By using this procedure both global and local phenomena are observed and analyzed. While each specimen is compressed the damage behaviour of the foams is directly observed in different stages, as being in the linear elastic domain, in the plateau region, and in the densification region. Several observations, characteristic for each foam density are discussed. As damage mechanisms are different, DIC allows the direct monitoring of the formation of the deformation bands and their propagation till the final failure of the foams, as long as calculations of the local strains are possible. Maps of the vertical displacements and local Mises strains are presented and comments on the characteristics of the deformation bands are done.

Key Engineering Materials, 2014

In this paper is presented the behaviour of PUR foams under mixed mode loading. Closed cell rigid... more In this paper is presented the behaviour of PUR foams under mixed mode loading. Closed cell rigid PUR foams having three different densities 100, 145, and 300 kg/m 3 were investigated. Experiments were performed using asymmetric semi-circular bend (ASCB) and Assymmetric Four-Point Bend (AFPB) specimens. The obtained crack initiation angles established for ASCB specimens were compared with four fracture criteria MTS, SED, G max and ESIF, and a good agreement was observed. When testing AFPB specimens, and calculating the normalized stress intensity factors it is important to obtain a correct crack propagation, that is to impose proper geometrical dimensions as not to affect the initial crack tip. The eXtended Finite Element Method (XFEM) is a complementary powerful numerical tool used to analyze crack initiation and propagation only if the numerical model is correctly calibrated and all the influencing parameters are properly understood.

Mixed mode four-point testing is performed on polyurethane foams. This paper presents only the re... more Mixed mode four-point testing is performed on polyurethane foams. This paper presents only the results on the stress intensity factors (SIFs) obtained experimentally for a density of 325 kg/m 3 , although tests were done for three foam densities. An asymmetric four-point bending setup was used for determining the critical SIFs in Mode I and Mode II, and discussions on the influence of the initial crack length on the SIF values are done. As initial crack length is increased the theoretical predictions give a better comparison to experimentally obtained results.

Micromechanical models used to predict mechanical and fracture properties of brittle metallic foa... more Micromechanical models used to predict mechanical and fracture properties of brittle metallic foams are validated experimentally for closed-cell aluminium foam (AlSi12Mg0.6) prepared by powder metallurgy route. Compression, tensile, tensile on notched specimens and fracture toughness tests were carried on, and the results are presented together with micromechanical models from literature. Moreover, the Digital Image Correlation technique was applied to identify the failure mechanisms of aluminium foams. Finally, the Theory of Critical Distances was employed to predict the fracture load of notched specimens. The novelty of the study is that the inherent stresses and critical distances were obtained by employing micromechanical analysis.

The failure-mode maps of composite sandwich beams can provide useful information about the influe... more The failure-mode maps of composite sandwich beams can provide useful information about the influence of different design parameters on the failure behaviour of such components. Failure mode of sandwich beams with different cores and different faces were investigated in the experimental program. Foams with 40 and 200 kg/m 3 densities were used as core material, while Glass-Fibre Reinforced Polymer (GFRP), polyester, epoxy and aluminium are the faces materials. Three-point bending tests were carried out for sandwich beams. In order to characterize these sandwich materials first were carried out a statistical analysis of the cellular structure for two different densities of the core material above mentioned. The sandwich core morphology and cells dimensions were studied before testing through scanning electron microscopy (SEM) and pore diameter versus frequency of pores histogram were plotted. After statistical analysis were performed static compression tests. These compression tests have had as objective the determining of the main mechanical properties such as Young's modulus and yield stress values. The results obtained from the static compression tests were used for the analytical determination of failure-mode maps of sandwich beams. Finally, the failure-mode maps were constructed for five considered sandwich types and validated by the experimental results. Each failure-mode map is characteristic for a family of sandwich beam designs.

This work investigates the mechanical properties of polyurethane rigid foams by means of Dynamic ... more This work investigates the mechanical properties of polyurethane rigid foams by means of Dynamic Mechanical Analysis (DMA) tests and Impulse Excitation Technique (IET). DMA tests were performed in single cantilever with a sweep in temperature (from-50 °C to 100 °C) and frequency (from 1 Hz to 100 Hz), not determining glass-transition in the test parameter interval. IET tests were used to determine the dynamic modulus of elasticity, showing good accordance with DMA results

The notch effect in Polyurethane materials of different densities is investigated. Tensile specim... more The notch effect in Polyurethane materials of different densities is investigated. Tensile specimens with lateral V and U symmetric notches and with holes of different diameters were tested. The material parameters inherent stress and critical distance are determined using a linear elastic finite element analysis in conjunction with experimental results of average maximum load. The paper proposed an original correlation between critical distance and the cell diameter of the cellular structure, respectively between inherent stress and the ultimate tensile strength of Polyurethane materials. Then for single edge notch specimens a relationship between critical distance and applied mixed mode is found and successfully applied to assess the brittle mixed mode of Polyurethane materials.

Polyurethane (PUR) foam materials are widely used as cores in sandwich composites, for packing an... more Polyurethane (PUR) foam materials are widely used as cores in sandwich composites, for packing and cushioning. They are made of interconnected networks of solid struts and cell walls incorporating voids with entrapped gas. The main characteristics of foams are lightweight, high porosity, high crushability, and good energy absorption capacity. Fracture toughness in mixed mode loading is of particular interest because foam cracking weakens the structure's capacity for carrying loads. Present paper assesses the shear elastic (shear modulus) and mechanical (shear strength) properties of polyurethane foams. Also, three different types of specimens were used to determine mode I and mode II fracture toughness. The shear modulus, shear strength and fracture toughness increases with increasing foam density. Also the effect of loading direction and loading speed is investigated. The authors propose a micromechanical model to estimate fracture toughness based on the tensile strength of the solid material and the topology of the cellular structure.

This paper deals with evaluating the elastic response of several micromechanical structures used ... more This paper deals with evaluating the elastic
response of several micromechanical structures used for simulating cellular materials under compression. For this
study polyurethane rigid foams were investigated, having
three relative densities: 0.085, 0.124 and 0.256. Their
microstructure was analysed using SEM images, determining
four types of cells that were consequently designed
using specialized CAD software: square cells with circular,
quadratic and/or hexagonal orifices and hexagonal
cells. An interdependent variation of the cells’ geometrical
parameters of the proposed structures was determined to
obtain geometrical variations at a required relative density.
Finite element analysis simulations were performed on
the designed microstructural models using a linear elastic
material model for the cell struts, resulting in the variation
of the elastic modulus of the structure with the variation in
cell geometry parameters. The final objective of this work
was to determine anisotropic bi-dimensional micromechanical
models for the studied cellular material that provides
accurate results in compression on both loading directions.
The anisotropic models for the proposed cell structures
were obtained by generating irregular geometries which
provided extra variables for the cell geometry parameters.
It was determined that some cell geometries are suitable
for simulating lower relative density materials while other cell geometries provide good accordance with experimental
data for higher relative density materials.

Many efforts have been made in recent years to determine the fracture toughness of different type... more Many efforts have been made in recent years to determine the fracture toughness of different types of foams in static and dynamic loading conditions. Taking into account that there is no standard method for the experimental determination of the fracture toughness of plastic foams different procedures and specimens were used. This paper presents the poly-urethane foam fracture toughness results obtained for different foam densities. Two types of specimens were used for determining fracture toughness in modes I, II and a mixed one, and also the size effect, loading speed and loading direction were investigated. The paper proposed correlations for density, cell orientation and mixed mode loading based on the experimental testing results.

International Journal of Solids and Structures, Nov 2, 2012

We investigate sandwich composite beams using a direct approach which models slender bodies as de... more We investigate sandwich composite beams using a direct approach which models slender bodies as deformable curves endowed with a certain microstructure. We derive general formulas for the effective stiffness coefficients of composite elastic beams made of several non-homogeneous materials. A special attention is given to sandwich beams with foam core, which are made of functionally graded or piecewise homogeneous materials. In the case of small deformations, the theoretical predictions are compared with experimental measurements for the three-point bending of sandwich beams, showing a very good agreement. For functionally graded sandwich columns we obtain the analytical solutions of bending, torsion and extension problems and compare them with numerical results computed by the finite element method.

Polymer Testing, Mar 9, 2013

The paper presents a correlation between dynamic and static fracture toughness of polyurethane ri... more The paper presents a correlation between dynamic and static fracture toughness of polyurethane rigid foams. Static three point bend tests and instrumented impact tests were performed using single edge notch specimens. The obtained results show that for all foam densities the dynamic fracture toughness is higher than the static toughness. Density appears to have the main influence on both static and dynamic fracture toughness. A quasi brittle fracture without plastic deformations and cushioning was observed for all foam densities.

Iranian Polymer Journal, Aug 17, 2011

T he fracture toughness was determined for cellular polymers by micromechanical modelling using f... more T he fracture toughness was determined for cellular polymers by micromechanical modelling using finite element analysis. In this study, mode I and mode II of fracture toughness were evaluated with a 2D-solid model using fracture analysis code FRANC2D/L. Simulation was performed for open cell polyurethane foams of different densities. Two cases were considered: constant cell length, l, and variable cell wall thickness; the former for constant cell wall thickness, t, and the latter for variable cell length. For estimation of fracture toughness the applied loads were progressively increased to the point reaching the fracture strength of the solid material (130 MPa) in an un-cracked strut in front of the crack. The estimated fracture toughness was independent on crack length, indicating that the obtained values could be considered as material property. The values of the fracture toughness of polyurethane foams are in the range of 10 -3 -10 -1 MPa.m 0.5 . Lower values were obtained for mode II fracture toughness. A strong dependency of the fracture toughness on the density of the cellular material was featured by present study. The obtained results for mode I fracture toughness were compared with Gibson-Ashby micromechanical model, by which a good agreement was obtained. While, mode II fracture toughness was compared with Choi and Sankar micromechanical models. The predicted fracture toughness was finally validated with some experimental data. Another advantage of this model was to obtain a fully described the stress field in the solid struts. The stress distribution in the first un-cracked strut showed a combined stress (bending and tension) for mode I loading, while for mode II loading a pure bending appeared.

Materiale Plastice, 2010

In the recent years the utilization of polymeric materials increased due to their good mechanical... more In the recent years the utilization of polymeric materials increased due to their good mechanical properties and easy manufacturing. Starting from automotive industry up to food industry polymeric materials are used for obtaining components by molding or by machining. Taking into account the wide range of applications, it is important to know the mechanical behaviour of these materials in different loading conditions. Accordingly, this paper presents an experimental study to determine mechanical properties on 11 commercial polymeric materials. Tensile and shear static tests and impact tests on notched and un-notched specimens were performed. The results are presented comparatively, and two new parameters were introduced for material selection purpose.

Cellular plastics are light weight structures with many applications in civil, aeronautical, auto... more Cellular plastics are light weight structures with many applications in civil, aeronautical, automotive and mechanical engineering. Properties of cellular materials depend on the properties of the solid material, on the shape and dimensions of the cellular structure and on the relative density of the cellular material. Most of cellular plastic materials are crushing in compression and have a brittle behavior in tension. The effect of notches represents an important issue in such materials, taking into account that for packing applications for example, notches/holes should be introduced in the cellular material. This paper investigates the effect of notches in compression for three different densities 100, 145 and 300 kg/m 3 polyurethane (PUR) foams. Experimental investigations were performed on rectangular blocks of 100x100x25 mm with 16, 28 and 40 mm central holes. The mechanism of damage was monitored with an IR camera FLIR A40M. Purpose of the numerical simulations was to calibrate a material model, based on compression test for un-notched specimens using the CRUSHABLE FOAM models implemented in ABAQUS SIMULIA. Then the material models were used to simulate the experimental tests on notched blocks. Good agreement was obtained for the load-displacement curves obtained experimentally and from simulation. Also the plastic deformation patterns observed experimentally by IR thermograpghy were obtained numerically using the CRUSHABLE FOAM material model.

Cellular materials represent a new class of materials; main parameters that characterize the cell... more Cellular materials represent a new class of materials; main parameters that characterize the cellular structure are relative density, shape of the cell (open or closed), wall thickness and cell diameter. The purpose of this paper is to investigate the microstructure of foams materials and also to determine the flexural properties of this rigid PUR foams using Digital Image Correlation (DIC). The rigid PUR foams cells morphology and pore distribution for three densities (100, 145 and 300 kg/m 3) were studied before testing through scanning electron microscopy. Determination of flexural properties was carried out on rectangular beam samples using ARAMIS 2D system. This method provides a substantial increase in accuracy for measuring strain and is based on the calculation of surface deformation using a set of digital images from undeformed stage to different deformed stages. The specimens were subjected to static three points bending tests with loading rate of 2 mm/min, at room temperature and loading was applied in rise direction of the foam. Experimental results show that main mechanical properties such as flexural modulus and flexural strength values increases with increasing of density.

Journal of Physics: Conference Series , 2013

Effect of density, loading rate, material orientation and temperature on dynamic compression beha... more Effect of density, loading rate, material orientation and temperature on dynamic compression behavior of rigid polyurethane foams are investigated in this paper. These parameters have a very important role, taking into account that foams are used as packing materials or dampers which require high energy impact absorption. The experimental study was carried out on closed-cell rigid polyurethane (PUR) foam specimens of different densities (100, 160 respectively 300 kg/m 3 ), having a cubic shape. The specimens were subjected to uniaxial dynamic compression with loading rate in range of 1.37-3.25 m/s, using four different temperatures (20, 60, 90, 110°C) and two loading planes (direction (3) -rise direction and direction (2) -in plane). Experimental results show that Young's modulus, yield stress and plateau stress values increases with increasing density. One of the most significant effects of mechanical properties in dynamic compression of rigid PUR foams is the density, but also the loading speed, material orientation and temperature influences the behavior in compression D2FAM 2013

Key Engineering Materials, 2014

Polyurethane (PUR) foam materials represent a class of materials widely used for impact protectio... more Polyurethane (PUR) foam materials represent a class of materials widely used for impact protection and energy absorption. This paper presents a characterization of different rigid PUR foams under compressive impact loading by means of energy absorption and efficiency diagrams. Compressive properties were investigated on cubic specimens on the foams’ rise direction at room temperature with a loading rate of 3.09 m/s for three different closed-cell foams with densities of 100 kg/m3, 160 kg/m3 and 300 kg/m3 respectively. Experimental results show that the compression modulus, yield stress and plateau stress increase with density. Most of the energy is absorbed in the plateau region because of the cell deformation associated with this phenomenon, allowing greater absorption of impact energy at nearly constant load. Authors have found that both the energy absorption and efficiency diagrams are consistent and present similar results for studied foams.

Key Engineering Materials, 2014

This paper presents an experimental characterization of three different types of closed-cell alum... more This paper presents an experimental characterization of three different types of closed-cell aluminium alloy foams (AlMg1Si0.6, AlSi12Mg0.6 and AlMg0.6Si0.3) under static compressive loading. This study was carried out on half-cylindrical specimens with skin. The influence of foam density on compressive behaviour was investigated for densities ranging from 430 kg/m3 to 935 kg/m3. The compression tests were performed at room temperature (23°C) with a constant crosshead speed of 0.5 mm/min. Strain distribution, yield stress and compressive modulus values were recorded using Digital Image Correlation. Experimental results show that the mechanical properties (Young’s Modulus, yield stress and plateau stress) increase with density.

Key Engineering Materials, 2014

Closed cell polyurethane foams with densities of 100, 160 and 301 kg/m 3 were tested in compressi... more Closed cell polyurethane foams with densities of 100, 160 and 301 kg/m 3 were tested in compression at speeds of 1 and 5 mm/min. Digital image correlation (DIC) is used to determine the engineering characteristic curve, modulus of elasticity, Poisson's ratio and the deformation bands that appear during deformation and prior to the final failure of the specimens. By using this procedure both global and local phenomena are observed and analyzed. While each specimen is compressed the damage behaviour of the foams is directly observed in different stages, as being in the linear elastic domain, in the plateau region, and in the densification region. Several observations, characteristic for each foam density are discussed. As damage mechanisms are different, DIC allows the direct monitoring of the formation of the deformation bands and their propagation till the final failure of the foams, as long as calculations of the local strains are possible. Maps of the vertical displacements and local Mises strains are presented and comments on the characteristics of the deformation bands are done.

Key Engineering Materials, 2014

Solid State Phenomena, May 14, 2014

This paper presents the size effect on fracture toughness of polyurethane foams (PUR 40), with no... more This paper presents the size effect on fracture toughness of polyurethane foams (PUR 40), with nominal density 40 kg/m 3 , closed-cell rigid foams widely used for sandwich cores. Determination of the fracture toughness was carried out by three-point bending tests (TPB), on notched specimens, at room temperature (20±2ºC). To determine the size effect in Mode I fracture toughness, specimens geometrically similar in two dimensions with length-to-width ratio 5:2 were selected. The specimens were subjected to a quasi-static loading with a speed of 2 mm/min, which was applied exactly on the notch direction. A strong size effect in the closed-cell PUR foam is experimentally demonstrated, by considering a smooth transition between strength of materials approach (with no size effect) and asymptotic case of linear elastic fracture mechanics.

Key Engineering Materials, 2010

At the stress-strain tests for the overhead electrical conductors stranded in alternate direction... more At the stress-strain tests for the overhead electrical conductors stranded in alternate directions (aluminum conductor steel reinforced -ACSR), the locking mode of the specimen ends on the tensile machine represents a difficult problem which if is not correctly solved may seriously influence the results obtained. The stress-strain tests are performed according to standards as the European Standard EN 50182:2001. The specimen must have a length imposed by the conductor diameter, but not less than 10m. During the stress-strain tests the specimen is loaded in steps at successive cycles of loadingunloading (30%, 50%, 70%, 85% of Rated Tensile Stress -RTS). Finally, in order to determine the breaking forces, the conductor is loaded up to total breaking. In these conditions, the test success is decisively influenced by the correct locking of the conductor in the grips. The locking of the conductor ends is usually performed by casting the conductor ends in the gripping devices, after the wires ends reflection. In this paper there is presented a new locking method of the conductor ends by separation of the steel wires from the aluminum ones and their separate winding on two drums with helical groove. The tests performed have confirmed that this method is cheaper, and not influencing the real behavior of the conductor.

Cellular solids are made up of an interconnected network of solid struts or plates which form the... more Cellular solids are made up of an interconnected network of solid struts or plates which form the edges and faces of cells. Cellular materials are widely used in engineering applications because they have certain properties that cannot be elicited from many homogeneous solids. Besides many other applications, polymeric cellular materials are used in the construction of wind turbine blade parts or entire sections. This research studies blade – section as a core (foam) inserted between the upper and lower surfaces (blade skin). For this purpose we will study the behaviour of PUR foam materials in various mechanical loading such as: compression tests (static and dynamic) and fracture toughness tests (static and dynamic) on notched specimens. We will study in detail the influence of density (in range of 40 – 200 kg/m3), influence of loading rate (from 1.67.10-4 m/s – static tests to 6 m/s – impact tests), influence of forming plane (in plane and out of plane loading direction), influence of temperature (from 20°C to 100°C) and size effect on the mechanical properties. Cellular materials have plastic plateau and densification in compression, while in tensile they are quasi – brittle. The most important mechanical properties which will be studied are Young’s modulus, yield stress, plateau stress, densification and fracture toughness. The experimental program was specifically designed according to the aim of the work. Also to the end of this paper is shown a microstructural analysis for both before (initial surface) and after (broken surface) compression and three – point bending tests. As a conclusion we can say that one of the most significant parameter on the mechanical properties for cellular materials is the density. Hence, the mechanical properties of foams can be controlled, making them attractive in structural application requiring particular strength or stiffness to weight ratios.

The scaling of compression strength with porosity for aluminium foams was investigated. The Al 99... more The scaling of compression strength with porosity for aluminium foams was investigated. The Al 99.96, AlMg1Si0.6 and AlSi11Mg0.6 foams of various porosity, sample size with and without surface skin were tested in compression. It was observed that the compression strength of aluminium foams scales near the percolation threshold with Tf ≈ 1.9-2.0 almost independently on the matrix alloy, sample size and presence of surface skin. The difference of the obtained values of Tf to the theoretical estimate of Tf = 2.64 ± 0.3 by Arbabi and Sahimi and to Ashby estimate of 1.5 was explained using an analogy with the Daoud and Coniglio approach to the scaling of the free energy of sol-gel transition. It leads to the finding that, there are two different universality classes for the critical exponent T f : when the stretching forces dominate T f = f = 2.1, respectively when bending forces prevail T f = .d = 2.64 seems to be valid. Another possibility is the validity of relation T f ≤ f which varies only according to the universality class of modulus of elasticity in foam.

This paper investigates the fracture properties and notch effect of PUR materials with four diffe... more This paper investigates the fracture properties and notch effect of PUR materials with four different densities. The asymmetric semi-circular bend specimen was adapted to perform mixed mode fracture toughness tests. This semi-circular specimen with radius R, which contains an edge crack of length a oriented normal to the specimen edge, loaded with a three point bending fixture, was proved to give wide range of mixed modes from pure mode I to pure mode II, only by changing the position of one support. Different types of notched specimens were considered for notch effect investigations and the Theory of Critical Distances was applied. It could be seen that the critical distances are influenced by the cellular structure of investigated materials.

Polyurethane foams crush in compression and have a brittle fracture in tension, so their failure ... more Polyurethane foams crush in compression and have a brittle fracture in tension, so their failure could be evaluated based on
Linear Elastic Fracture Mechanics. Fracture toughness in mixed mode loading is of particular interest because foam cracking
weakens the structure's capacity for carrying loads. Four fracture criteria (Maximum circumferential tensile stress, Minimum strain energy density, Maximum energy release rate, Equivalent stress intensity factor) were considered for evaluation of mixed mode fracture of three closed cell rigid polyurethane foams with densities: 100, 145 and 300 kg/m3. Mixed mode fracture tests were performed using asymmetric semi-circular specimen. The equivalent stress intensity factor criterion looks to give the better prediction of mixed mode fracture. Also the effect of cell orientation and the crack propagation angle were investigated.

Many efforts have been made recently to determine the fracture toughness of different types of fo... more Many efforts have been made recently to determine the fracture toughness of different types of foams in static and dynamic loading conditions. Taking into account that there is no
standard method for the experimental determination of the fracture toughness of plastic foams, different procedures and specimens were used. This paper presents the polyurethane foam fracture toughness results obtained experimentally for three foam densities. Asymmetric four-point bending specimens were used for determining fracture toughness in mode I and in a mixed one, and also the influence of the loading speed and geometry of the specimen were investigated.

In this paper are presented the crack initiation angles in polyurethane (PUR) foams under mixed m... more In this paper are presented the crack initiation angles in polyurethane (PUR) foams under mixed mode loading. Closed cell rigid PUR foams having three different densities 100, 145, and 300 kg/m 3 were investigated. Experiments were performed using Asymmetric Semi-Circular Bend and Single Edge Crack specimens. The obtained crack initiation values were compared with four fracture criteria Maximum Tensile Stress, Strain Energy Density, Maximum Energy Release Rate and Equivalent Stress Intensity Factor, and a good agreement was observed. This allow to conclude that the theoretical fracture criteria developed for solid material could be used with success to predict the crack propagation angles in cellular materials like PUR foams.

Polyurethane foams crush in compression and have a brittle fracture in tension, so their failure ... more Polyurethane foams crush in compression and have a brittle fracture in tension, so their failure could be evaluated based on Linear Elastic Fracture Mechanics. Fracture toughness in mixed mode loading is of particular interest because foam cracking weakens the structure's capacity for carrying loads. The mixed mode fracture of three closed cell rigid polyurethane foams with densities: 100, 145 and 300 kg/m 3 are experimentally investigated. Mixed mode fracture tests are performed using a single edge cracked specimen and a mixed mode loading device. The advantages of this specimen are the simple geometry and the ability to produce full range of mixed modes, from pure mode I to pure mode II, only by changing the loading direction. Fracture criteria based on the cellular topology and tensile strength of the solid material is assessed. It is found that the density of foams is the most important parameter influencing the fracture toughness. The crack propagation angles are also determined on the fractured specimens

This paper presents the effect of loading speed, material orientation and density on the properti... more This paper presents the effect of loading speed, material orientation and density on the properties of cellular materials, such as rigid polyurethane foams, subjected to compressive load. These parameters have a very important role because in many applications, foams are used as packing materials or dampers which require high energy impact absorption. The experimental tests were carried out on specimens in the form of cubes. The specimens were subjected to uniaxial compression with loading speed of 2 mm/min, except samples for determining the effect of loading speed where 1, 5, 10 and 20 mm/min loading speeds were used. One of the most significant effects of mechanical properties in compression of polyurethane foams is the density, but also the loading speed and material orientation influences the characteristics in compression.

This paper presents the effect of loading rate, (ELR), and direction of formation, (DF), of rigid... more This paper presents the effect of loading rate, (ELR), and direction of formation, (DF), of rigid polyurethane foams, (PUR 40 and PUR 140), on fracture toughness. Nominal densities of used foams in the experimental program were 140 kg/m 3 , (for ELS) and 40 kg/m 3 , (for DF), which is closed-cell rigid foams widely used for sandwich cores. Determination of fracture toughness for Mode I fracture of studied materials has made by three-point bending tests, (3PB), on specimens with notches, at room temperature (20 ± 2 ºC). All the specimens were cut from one and the same plate. The specimens were subjected to 3PB at a loading speed of 2 mm/min, except samples for determining the ELR where 2, 20, 200 and 400 mm/min loading speeds were used, and were taken into account the fact that the load must act exactly on the notch direction. All the specimens present brittle failure without plastic deformation.

This work presents the main numerical models used in the simulation of damage and failure of duct... more This work presents the main numerical models used in the simulation of damage and failure of ductile materials that are implemented in commercial software packages: equivalent strains criterion, shear criterion and the FLD criterion. Several numerical simulations were performed for impact loadings using one of the above mentioned degradation criteria as well as all criteria, in order to highlight the interaction between the models.

Key Engineering Materials, 2014

This paper presents an experimental characterization of three different types of closedcell alumi... more This paper presents an experimental characterization of three different types of closedcell aluminium alloy foams (AlMg1Si0.6, AlSi12Mg0.6 and AlMg0.6Si0.3) under static compressive loading. This study was carried out on half-cylindrical specimens with skin. The influence of foam density on compressive behaviour was investigated for densities ranging from 430 kg/m 3 to 935 kg/m 3 . The compression tests were performed at room temperature (23°C) with a constant crosshead speed of 0.5 mm/min. Strain distribution, yield stress and compressive modulus values were recorded using Digital Image Correlation. Experimental results show that the mechanical properties (Young's Modulus, yield stress and plateau stress) increase with density.

Theoretical and Applied Fracture Mechanics, 2016

Ecf19, Feb 22, 2013

Foams with densities of 35, 93, and 200 kg/m 3 were tested in compression at -60 C, 23 C, and 8... more Foams with densities of 35, 93, and 200 kg/m 3 were tested in compression at -60 C, 23 C, and 80 C, at speeds from 2 mm/min up to 6 m/s. One of the analyzed issues was the damage extension during testing and the formation of the bands of deformation in these foams and in sandwich composites with glass fiber sheets and a core made out of these foams. The damage was monitored using a Photron high speed camera at an acquisition rate of 17000 images per second by applying random speckles of paint on the surface of the samples as a digital image correlation (DIC) analysis has been done by using the ARAMIS system. Compression tests on the rise direction of the foam of 35 kg/m 3 show the appearance of crush bands inclined with approximately 45, but if compression is produced on an in plane direction the damage observed on the surface of the specimen is mostly uniformly distributed over the height. Compression on samples manufactured from sandwich composite material having a core with a density of 200 kg/m 3 give a maximum stress at yielding approximately equal to the values obtained on samples made only out of foam, but the modulus of elasticity is almost 50% greater. In the figure bellow are to be observed the damage mechanisms established at a testing speed of 2 mm/min in different stages (2 and 3) as being mostly localized at the interface between the sheet and the core. a) b) Damage determined with the ARAMIS system: a) displacement fields; b) Mises strains.

Key Engineering Materials, 2009

At the stress-strain tests for the overhead electrical conductors stranded in alternate direction... more At the stress-strain tests for the overhead electrical conductors stranded in alternate directions (aluminum conductor steel reinforced -ACSR), the locking mode of the specimen ends on the tensile machine represents a difficult problem which if is not correctly solved may seriously influence the results obtained. The stress-strain tests are performed according to standards as the European Standard EN 50182:2001. The specimen must have a length imposed by the conductor diameter, but not less than 10m. During the stress-strain tests the specimen is loaded in steps at successive cycles of loadingunloading (30%, 50%, 70%, 85% of Rated Tensile Stress -RTS). Finally, in order to determine the breaking forces, the conductor is loaded up to total breaking. In these conditions, the test success is decisively influenced by the correct locking of the conductor in the grips. The locking of the conductor ends is usually performed by casting the conductor ends in the gripping devices, after the wires ends reflection. In this paper there is presented a new locking method of the conductor ends by separation of the steel wires from the aluminum ones and their separate winding on two drums with helical groove. The tests performed have confirmed that this method is cheaper, and not influencing the real behavior of the conductor.