Some aspects of cohesive models and modelling with special application to strength of adhesive layers (original) (raw)
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Influence of temperature and strain rate on cohesive properties of a structural epoxy adhesive
International Journal of Fracture, 2009
Effects of temperature and strain rate on the cohesive relation for an engineering epoxy adhesive are studied experimentally. Two parameters of the cohesive laws are given special attention: the fracture energy and the peak stress. Temperature experiments are performed in peel mode using the double cantilever beam specimen. The temperature varies from-40°C to +80ºC. The temperature experiments show monotonically decreasing peak stress with increasing temperature from about 50 MPa at-40ºC to about 10 MPa at +80ºC. The fracture energy is shown to be relatively insensitive to the variation in temperature. Strain rate experiments are performed in peel mode using the double cantilever beam specimen and in shear mode, using the end notch flexure specimen. The strain rates vary; for peel loading from about 10-4 s-1 to 10 s-1 and for shear loading from 10-3 s-1 to 1 s-1. In the peel mode, the fracture energy increases slightly with increasing strain rate; in shear mode, the fracture energy decreases. The peak stresses in the peel and shear mode both increase with increasing strain rate. In peel mode, only minor effects of plasticity are expected while in shear mode, the adhesive experiences large dissipation through plasticity. Rate dependent plasticity, may explain the differences in influence of strain rate on fracture energy between the peel mode and the shear mode.
2009
To be able to predict the strength of adhesive joints accurately, correct material data of adhesives are essential. Hence, it is critical to develop reliable testing methods to obtain the constitutive behaviour of adhesive layers. In use, adhesives are constrained to thin layers. Thus, an adhesive constrained into a layer is expected to behave differently compared to the adhesive as a bulk material. Under loading, the size of the Failure Process Zone (FPZ) in the adhesive layer is often much larger than the thickness of the layer. Thus, the small scale FPZ condition is not fulfilled and the traditional Linear Elastic Fracture Mechanics (LEFM) can not be applied. At the same time, experiments show that test specimens are prone to produce unstable crack propagation and combined adhesive/cohesive fracture patterns appear frequently, especially when mixed mode loading (peel and shear) is involved. Cohesive law should be taken as the basic fracture property for adhesives characterization...
The evolution of damage at the tip of cracks in adhesive bonds deforming in shear was monitored in real time using a high-magnification video camera. Brittle and a ductile epoxy resins were evaluated, with the bond thickness t being an experimental variable. An extensive zone of plastic deformation developed ahead of the crack tip prior to fracture. In the case of the brittle adhesive, for relatively thick bonds tensile microcracks formed within that zone. Increased loading caused the microcracks to grow from the interlayer to the interface, which led to a complete bond separation after interface cracks emanating from adjacent microcracks linked. In contrast, for the ductile adhesive the crack always grew from the tip. Strain gradients tended to develop there when the bond thickness was large. The adhesive shear strain was determined from fine lines scratched on the specimen edge. For both adhesives, the average crack tip shear strain at crack propagation rapidly decreased with increasing t. This effect was attributed to the changing sensitivity of the bond to the presence of flaws; thicker bonds can accommodate larger microcracks or microvoids which cause greater stress concentration. For a given bond thickness, the critical crack tip shear strain agreed well with the ultimate shear strain of the unflawed adhesive ?s previously determined using the napkin ring shear test 1-12]. This suggests that the ultimate shear strain is a key material property controlling crack growth. The critical distortional strain energy/unit area of the unflawed adhesive W~ was determined from the area under the stress-strain curve in the napkin ring test. Good agreement between W, and the adhesive mode II fracture energy was found for all joints tested except for relatively thick bonds. For the particular case of an elastic-perfectly plastic adhesive, the agreement above implies Gnc = W s =-tzy)~f.
Effect of in-plane deformation on the cohesive failure of heterogeneous adhesives
Journal of the Mechanics and Physics of Solids, 2013
The effect of in-plane deformations on the failure response of heterogeneous adhesives with a second phase of spherical elastic particles is investigated numerically using a 3D cohesive framework. The methodology includes a new interface-enriched generalized finite element scheme for the solution of structural problems with weak discontinuities, allowing for the efficient and accurate prediction of the stress and displacement fields in the adhesive based on finite element meshes that do not conform to the heterogeneities. A rate-dependent isotropic failure model is adopted to capture the failure in the matrix, while the stiff inclusions are assumed to be linearly elastic. Cohesive failure envelopes resulting from the micro-to-macro analysis are extracted for a wide variety of failure mode conditions. A study of 1611the impact of in-plane tensile and shear strains on the macroscopic failure response under tensile (mode I) loading is also presented.
The Use of Bilinear Cohesive Zone Model to Study the Fracture of Mode I in Adhesive Joints
Adhesively bonded joints can be numerically simulated using the Cohesive Zone Model (CZM) concepts. CZM are widely used for the strength prediction of adhesive joints. The critical strain energy release rate and critical interface strength are the parameters which must be known when cohesive elements in ABAQUS software are used. The formulation of the cohesive finite elements is based on the CZM approach with the bilinear traction-separation law. In this work, the parameters of two industrial adhesives Huntsman Araldite 2015 and resin LY3505/XB3405 for bonding of epoxy composites are identified. Double Cantilever Beam (DCB) test data was used for the identification. Finally, cohesive parameters are identified comparing numerically simulated load-displacement curves with experimental data retrieved from literature. Parametric study is performed to evaluate the variation of input parameters like initial stiffness, element size, peak stress and energy release rate 'G'. From the numerical evaluation, it was noted that CZM simulation relies largely on element size and peak cohesive strength.
International Journal of Fracture, 2000
The deformation and fracture in shear of a structural adhesive undergoing large-scale yielding is studied as a function of bond thickness, h, temperature, T , and strain rate using the Napkin Ring specimen. The lack of edges in this test, and the fact that the strain rate can be locally controlled, allow for a meaningful evaluation of the mechanical response throughout the deformation process. In accord with Airing's molecular activation model, the yield stress linearly decreases with T while logarithmically increasing with the strain rate. The ultimate shear strain, γ F , is little sensitive to rate while decreasing with h and increasing with T. Some complementary fracture tests are carried out using the ENF bond specimen in order to explore the relation between the mechanical properties of the nominally unflawed adhesive and the mode II fracture energy, G IIC. For sufficiently thin bonds, G IIC /h correlates well with the ultimate energy density (i.e., the area under the stress-strain curve in the Napkin Ring test), given, to a first approximation, by τ Y γ F , where τ Y is the yield stress in shear. Accordingly, the fracture energy of the bond would be greatly affected by temperature, tending to a small value at the absolute as well as the glass transition temperatures while attaining a maximum in between these two extremes. Because the yield stress does not vary much with h, the variation of G IIC with the bond thickness reflects that of γ F. A large-deformation fracture analysis, based on a cohesive zone like model, is developed to account for the observed variations of γ F with h. The analysis assumes that a crack preexist in the bond, either at its center or at the interface. The results suggest that the observed increase of γ F with decreasing h is due mainly to two geometric effects. The first is due to the interaction of the bonding surfaces with the stress field generated by the crack and the second has to do with the probability of finding large flaws in the bond to trigger the fracture.
Influence of the Cohesive Law Parameters on the Strength Prediction of Adhesively-Bonded Joints
Materials Science Forum, 2012
Adhesive joints are largely employed nowadays as a fast and effective joining process. The respective techniques for strength prediction have also improved over the years. Cohesive Zone Models (CZM's) coupled to Finite Element Method (FEM) analyses surpass the limitations of stress and fracture criteria and allow modelling damage. CZM's require the energy release rates in tension (G n ) and shear (G s ) and respective fracture energies in tension (G n c ) and shear (G s c ). Additionally, the cohesive strengths (t n 0 for tension and t s 0 for shear) must also be defined. In this work, the influence of the CZM parameters of a triangular CZM used to model a thin adhesive layer is studied, to estimate their effect on the predictions. Some conclusions were drawn for the accuracy of the simulation results by variations of each one of these parameters.
Experimental estimation of the mechanical and fracture properties of a new epoxy adhesive
Applied Adhesion Science
The reduction of structural weight and the enhancement of vehicle safety are currently two of the most important research subjects for the automotive industry. The demand for lighter and safer structures has led the designers to increasingly employ alternative joining methods, replacing the more commonly used spot welding. Adhesive bonding is one of these methods and its use has expanded significantly, driven by the development of improved high performance adhesives and bonding techniques. While previous adhesives were relatively strong but brittle, the adhesives currently used for structural bonding by the automotive industry are designed with the aim of providing the joint with high ductility and high mechanical strength . These materials are commonly referred
Mode I fracture of adhesive joints using tailored cohesive zone models
International journal of …, 2009
Cohesive zone models are explored in order to study cleavage fracture in adhesive bonded joints. A mode I cohesive model is defined which correlates the tensile traction and the displacement jump (crack faces opening) along the fracture process zone. In order to determine the traction-separation relation, the main fracture parameters, namely the cohesive strength and the fracture energy, as well as its shape, must be specified. However, owing to the difficulties associated to the direct measurement of the fracture parameters, very often they are obtained by comparing a measured fracture property with numerical predictions based on an idealized traction separation relation. Likewise in this paper the cohesive strength of an adhesive layer sandwiched between elastic substrates is adjusted to achieve a match between simulations and experiments. To this aim, the fracture energy and the load-displacement curve are adopted as input in the simulations. In order to assess whether or not the shape of the cohesive model may have an influence on the results, three representative cohesive zone models have been investigated, i.e. exponential, bilinear and trapezoidal. A good agreement between experiments and simulations has been generally observed. However, a slight difference in predicting the loads for damage onset has been found using the different cohesive models.
Composite Structures, 2018
The increasing use of adhesive joints in dynamic applications require reliable measurements of the rate-dependent stress-displacement behaviour. The direct measurement of the stress-displacement curve is necessary when using cohesive models in discretised solutions of boundary value problems in solid mechanics. This paper aims to investigate the rate-dependent tensile failure of adhesive joints by using a new experimental methodology-it relies upon the combination of the stress wave propagation theory and digital image correlation methods on high speed footage to quantify the tensile stress and the dissipated energy respectively. For this purpose, the Split Hopkinson Bar methodology was employed-the experimental configuration was optimised using numerical modelling. To prove the sensitivity of our framework, two different adhesives are characterised at different loading rates: the adhesive failure strength was found to increase considerably with the strain rate, while the plastic deformation of these adhesives was reduced. The film adhesive showed superior performance over the particle toughened one. In the final part, a rate-dependent cohesive zone model is proposed, one which captures the measured behaviour and which has the potential to be used in industrial applications.