Fracture behaviour and fracture toughness of ductile closed-cell metallic foams (original) (raw)

Local fracture behaviour and crack tip deformation in metal foams

2013

The renewed interest in metal foams, which has been induced by the transport industry in the last few years, leads to a need of detailed information about mechanical properties of this new material class. The aim of this study is to provide information on fracture toughness and fracture processes of ductile metal foams. Investigations were carried out on ALPORAS and ALULIGHT aluminium foams with different specimen sizes and different relative densities. Standard fracture mechanic tests were performed. The fracture toughness values are in the range of 0,1 to 1,5 MPa m for KQ and 0,1 to 1,0 kN/m for J 0,2, mainly depending on the relative density. In-situ tests in the scanning electron microscope and in the optical stereomicroscope were performed also to observe the crack growth processes and determine the local deformation in the cell walls in front of the crack tip. The crack follows apparently the weakest path through the foam structure and is accompanied by building of crack bridg...

Deformation and fracture of aluminium foams

Materials Science and Engineering: A, 2000

The tensile and compressive properties and the fracture resistance of two aluminium alloy foams have been measured. The yield strength, unloading modulus and toughness increase with relative density in such a manner that the closed cell foams of this study behave as open cell foams. These relationships can be described adequately by power law fits. Experimental results, when compared with theoretical models based on idealised foam structures, reveal unexpected discrepancies. We conclude that they are caused by morphological defects in the microstructures of the foams, the effects of which were not included in the models. Tests on samples with deep sharp notches show that the tensile and compressive strengths are notch-insensitive. Fracture toughness measurements show an R-curve behaviour. This is analysed in terms of the underlying microstructure-the major cause of the R-curve was observed to be the development of crack bridging ligaments behind the crack tip. The compact tension specimens employed were sufficiently small for the uncracked ligaments to suffer plastic yielding during the fracture tests. The crack bridging response was quantified in terms of the normal traction versus plastic displacement curve; the area under this curve for a deep double edge-notched specimen is approximately equal to the measured steady state toughness. The accuracy of an existing micromechanical model for the fracture toughness of brittle open cell foams is assessed, and a new toughness model for ductile foams is derived.

Toughness of aluminium alloy foams

Acta Materialia, 1999

ÐThe fracture behaviour of closed cell aluminium-based foams (trade-name``Alulight'') is characterized for the compositions Al±Mg1±Si0.6 and Al±Mg1±Si10 (wt%), and for a relative density in the range 0.1±0.4. The toughness testing procedures are critically analysed, and the origins of the observed Rcurve behaviour for metal foams are explored. A major contribution to the observed increasing crack growth resistance with crack advance is in the development of a crack bridging zone behind the crack tip. The crack bridging response is quanti®ed in terms of a crack traction vs extra displacement curve by performing independent tests on deep notch specimens. The area under the bridging traction vs extra displacement curve from the deep notch tests is approximately equal to the measured initiation toughness J IC , in support of the crack bridging concept. A line spring model is then used to interpret the fracture response. The eect of material composition and relative density upon the initiation toughness is measured, and the accuracy of an existing micromechanical model for the fracture toughness of a brittle foam is assessed. Finally, the reduction in tensile and compressive strengths due to the presence of an open hole is determined; it is found that the Alulight foams are notch-insensitive, with the net section strength equal to the unnotched strength.

The effect of cell wall microstructure on the deformation and fracture of aluminium-based foams

Acta Materialia, 2001

This study primarily concerns the role of cell wall microstructure in influencing the mechanical behaviour of metallic foams. Three closed-cell foams have been examined, having rather similar relative densities and cell structures but significant differences in cell wall microstructure. It is concluded that these differences can substantially affect the micro-mechanisms of deformation and failure under different types of loading and can also have an influence on the macroscopic mechanical response. Cell wall ductility and toughness are impaired by high volume fractions of coarse eutectic, fine oxide films and large brittle particles, all of which were present in one or more of the foams studied. This impairment can lead to extensive brittle fracture of cell walls, with little energy absorption, even under nominally compressive loading conditions. The influence of cell wall ductility tends to become more significant when the loading state is such that local tensile stresses are generated. 

Fracture behavior of low-density replicated aluminum alloy foams

Materials Science and Engineering: A, 2008

Tensile tests have been performed on replicated aluminum alloy foams of relative density between 4.5% and 8%. During the test the electrical resistance was measured with a four-point set-up and the displacements along the gage section were measured using a digital image correlation (DIC) technique. Right from the start of the tensile test, the strain as observed at the surface with DIC is not uniformly distributed over the sample, but concentrates in bands; this is attributed to density variations. The peak strain, i.e. the strain at the ultimate tensile strength (UTS), increases with decreasing density for densities below 5.5%. For densities above 5.5% fracture occurs in the band with the highest strain, yielding a roughly constant peak strain (near 1.5%). Resistivity data indicate an increased contribution of strut bending compared to stretching for densities below 5.5%, causing a decreased rate of damage accumulation and an increase in the tolerance to damage. Incremental strain maps show that the increased damage tolerance in the low-density samples allows initially formed deformation bands to harden resulting in multiple bands to be involved in the fracture process.

Fracture and microstructure of open cell aluminum foam

Journal of Materials Science, 2005

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Fracture toughness of open-cell Kelvin foam

International Journal of Solids and Structures, 2014

Brittle fracture behavior of a perfect open-cell Kelvin foam is considered. The foam is modeled as a spatial lattice consisting of brittle elastic struts rigidly connected to each other at the nodal points. The fracture toughness is determined from the analysis of a quasi-plane problem for a slice of the foam with an embedded finite length crack generated by broken struts. The crack plane is chosen on the basis of a previous study of crack nucleation phenomenon, and the crack length, which assures the self-similar K-field in the tip vicinity, is established by numerical experiments. For the considered densities range the crack includes several hundreds of broken struts and, consequently, the portion of the foam to be considered in the analysis has a very large number of nodal degrees of freedom. The computational cost is reduced significantly by using for the analysis the representative cell method based on the discrete Fourier transform. As a result, the initial problem for the foam slice is reduced to the problem for the repetitive cell which includes 12 struts. The dependence of the Mode I and Mode II fracture toughness of the considered bending dominated foam upon its relative density is determined and found to be different from known results for the stretch dominated cubic cell lattice. On the other hand, the results obtained for Mode I meet the experimental data and theoretical predictions for random foams. For the case of struts with hollow cross-section the analysis predicts linear dependence of the fracture toughness upon cross-section gyration radius.

Analysis and characterization of 20 ppi open cell aluminum foam under mechanical loading

Journal of Mechanical Engineering and Sciences

In this research the analysis and characterization of open cell aluminium foam with 20 pores per inch (ppi) of Alporas rout under mechanical loading is presented in order to provide a basic understanding with respect to pore size per unit length for the right selection in various engineering applications. For this purpose, three point bending test, tensile test, compression test, vickers hardness test and charpy impact test were performed to seek out the respective properties of each test. All samples and test procedure were performed as per ASTM standards. The scanning electron microscopy (SEM) will performed of the fractured surfaces of the specimens to investigate the failure mode. The SEM photograph shows that; some internal defects were found such as the tinny cracks some irregular shape holes in the cell wall which have been created during foaming process. The shredded cell wall is looked over which was ductile in nature and have occurred during flexural, tensile and charpy im...

Fatigue crack propagation in aluminium alloy foams

International Journal of Fatigue, 2001

The mode I fatigue crack propagation (FCP) response of the closed-cell aluminium alloy foams Alulight and Alporas have been measured for a relative density in the range 0.1 to 0.4. The validity of linear elastic fracture mechanics (LEFM) to characterise the fatigue crack propagation (FCP) response is demonstrated, and K-increasing and K-decreasing tests are used to determine the full shape of the FCP response. The classical sigmoidal variation of log da/dN with log ⌬K is evident, with a Paris-law exponent m=20 for Alulight and m=25 for Alporas. The effects of relative density, mean stress and a single peak overload on the FCP response are investigated. The study concludes by analysing the mechanism of fatigue crack growth; it is suggested that the fatigue crack growth rate is controlled by the progressive degradation of crack bridging by fatigue failure of the cell edges behind the crack tip.

Micro–macro investigation of deformation and failure in closed-cell aluminum foams

Macroscopic mechanical properties of metal foams originate from the deformation of forming cells which happens on a micro scale. In this sense, the geometry of cells as well as the thickness and property of cell walls play a role in determining overall mechanical properties. Using simulation methods combined with some experimental tests, the inter-relationship between the micro scale deformation and the macro scale properties is investigated in this paper. In the first part of the work, the effect of relative density on the mechanical properties of closed-cell aluminum foam is investigated by numerical methods, and the results are compared with analytical predictions and experimental data. In the second part, the effect of cell topology, including cell shape and cell size, on the material behavior is investigated for aluminum foams, regardless of relative density. It is shown that cell shape causes some changes in macroscopic material behavior, which can be explained by its effect on the pattern of deformation and local failure in the material. Different mechanisms of deformation at the cell level are considered in connection with closed-cell aluminum foams. And finally, in the third part, different patterns of failure are investigated on different scales. The deformation and failure at the cell level cause localization on the macro scale. The cell shape and the inhomogeneity of the foam structure are investigated as the primary factors affecting the deformation and failure modes, and the results give some deeper understanding about the effect of cell shape on mechanical behavior. Also, some experimental tests are carried out to validate the numerical results.