Ulf Stigh

Influence of Layer Thickness on Cohesive Properties of an Epoxy-Based Adhesive : An Experimental Study

Cohesive laws are determined for different layer thicknesses of an engineering adhesive. The shape of the cohesive law depends on the adhesive layer thickness. Of the two parameters of the cohesive law—the fracture energy and the strength—the fracture energy is more sensitive to thickness variation than the strength. The fracture energy in peel mode (Mode I) increases monotonically as the thickness is increased from 0.1 to about 1.0 mm. At an adhesive thickness of 1.5 mm, the fracture energy is slightly lower than for a 1.0 mm adhesive thickness, indicating a maximum between 1.0 and 1.5 mm. In shear mode (Mode II), the thickness dependence is not as strong, but an increasing trend in fracture energy with increasing adhesive thickness is evident. A slight decrease in strength with increasing adhesive thickness is found in both loading modes.

On the determination of the constitutive properties of thin interphase layers — An exact inverse solution

On the determination of the constitutive properties of thin interphase layers : an exact inverse solution

Damage and crack growth analysis of the double cantilever beam specimen

The DB(T) specimen has recently been analysed, [i] and [2], considering nonlinear mechanical properties of the adhesive. In [i] the adhesive is modelled as an elastic ideal plastic material and in [2] strain softening is considered. This interest in nonlinear properties originates in part from the increasing use of tough adhesives, i.e., rubber-toughened epoxies; also strain softening has been observed experimentally in wood adhesives [3]. Strain softening as a useful model to analyse adhesive fracture has recently been proposed in [4] and for the more general problem of interface fracture in [5] and [6].

A Nonlinear Kinematic Hardening Model for Elastoplastic Deformations in Grey Cast Iron

A kinematic hardening model including an associated flow rule is proposed for elastoplastic deformations in graphitic grey cast iron. Quantitatively good results are obtained when comparing with previously performed biaxial experiments. Use of a nonassociated flow rule is found to result in an undesirable weakening behavior that can be explained as a deficiency with the combination of kinetic hardening and the present choice of yield potential. The model proposed is also extended to include multilinear kinematic hardening. With this model qualitatively good agreement with experimental cyclic results from the literature is obtained. A three-dimensional FE-analysis of a cylinder head for a heavy duty Diesel engine is performed as an application. To predict initiation of thermal fatigue cracks, it is essential to use an elastoplastic material model.

MEASUREMENT OF COHESIVE LAWS AND RELATED PROBLEMS

Cohesive modelling provides a simple method to intr oduce a process region in models of fracture. It is computa tionally attractive since it blends into the structure of fi nite element programmes for stress analysis. The development of computational methods and applications of cohesive modelling has accelerated during recent years. Methods to mea sure cohesive laws have also been developed. One class o f such methods is based on the path-independence of the J-integral. By choosing a path encircling the cohesive zone, J can be shown to be given by the area under the traction-se paration relation for the cohesive zone. Using an alternativ e path, J can in some cases be directly related to the applied lo ad and deformation with relatively modest or no assumptions on the material behaviour. Thus, the cohesive law can be measured. Methods to measure cohesive laws for different spec imen geometries are presented. The methods are used to measure the cohesive law in peel, shear and mixed-mode for an a dhesive layer. A new method to measure cohesive laws in shear is presented. The method is shown to give accurate dat a with a much smaller test specimen than earlier methods.

Influence of damage on ultrasonic velocity and strength—analysis and experiments

Abstract Measurements of ultrasonic velocity in creep damaged material are reviewed. The influence of damage on ultrasonic velocity is analysed and the relation between them is derived. It is shown that measurements of ultrasonic velocity can be used to predict the remaining life. Measurements of ultrasonic velocity in a damaged beam are presented along with measurements of the ultimate strength. A relation is found to exist, between reductions in ultrasonic velocity and ultimate strength, which might be used to assess the load carrying capacity of a structural element.

Continuum Damage Mechanics and the Life-Fraction rule

This paper gives a short review of two different methods for life prediction at high temperature; namely continuum damage mechanics (CDM) and the linear life-fraction rule (LFR). It is well known t ...

Explicit FE-Formulation of Interphase Elements for Adhesive Joints

The potential of adhesive bonding to improve the crashworthiness of cars is attracting the automotive industry. Large-scale simulations are time consuming when using the very small finite elements needed to model adhesive joints using conventional techniques. In the present work, a 2D-interphase element formulation is developed and implemented in an explicit FE-code. A simplified joint serves as a test example to compare the interphase element with a straightforward continuum approach. A comparison shows the time-saving potential of the present formulation as compared to the conventional approach. Moreover, the interphase element formulation shows fast convergence and computer efficiency.

A finite element study of threading dislocations

Abstract A finite element procedure is developed to analyse the force on a threading dislocation. The finite element method facilitates the study of dislocations in finite 3-dimensional bodies and the study of effects of anisotropic material behaviour. The FE-results show a strong sensitivity to the modelling of the core. This is in sharp contrast to the core insensitivity of the classical problems of interacting dislocations. Reasonably good agreement with classical results is achieved for the circular edge loop and for the circular shear loop. However, the classical results are derived using an artificial cut-off parameter and the present procedure introduces a simple core model.

High cycle fatigue crack growth in Mode I of adhesive layers: modelling, simulation and experiments

The capability to predict high cycle fatigue properties of adhesive joints is important for cost-efficient and rapid product development in the modern automotive industry. Here, the adaptability of adhesives facilitates green technology through the widening of options of choosing and joining optimal materials. In the present paper a continuum damage mechanics model is developed based on the adhesive layer theory. In this theory, through-thickness averaged variables for the adhesive layer are used to characterise the deformation, damage and local loading on the adhesive layer. In FE-simulations, cohesive elements can thereby be used to model the adhesive layer. This simplifies simulations of large scale complex built-up structures. The model is adapted to experimental results for two very different adhesive systems; one relatively stiff rubber based adhesive and one soft polyurethane based adhesive. The model is able to reproduce the experimental results with good accuracy except for the early stage of crack propagation when the loads are relatively large. The model also predicts a threshold value for fatigue crack growth below which no crack growth occurs. The properties of the model are also compared with the properties of Paris’ law. The relations between the parameters of the continuum damage mechanics law and the parameters of Paris’ law are used to adapt the new law. It also shows that the properties of a joined structure influence the Paris’ law properties of the adhesive layer. Thus, the Paris’ law properties of an adhesive layer are not expected to be transferable to joints with adherends having different mechanical properties.