Högskolan i Skövde

High-quality simulation methods demand accurate material models. In simulations an adhesive can be represented by a cohesive layer. A cohesive layer model utilizes a cohesive law to represent the homogenized mechanical behaviour of a layer with a thickness. In the current paper we use three experimental methods to measure the cohesive law in shear using the ENF-specimen; one of the methods is novel and is also useful for evaluation of experiments with the ELS-specimen. Two sets of experiments are performed, one with elastic substrates and one with plastically deforming substrates. Each experiment is evaluated using all three methods. The evaluation shows that all methods provide reasonable data; the results are similar if the substrates are elastic. With smaller specimens, the substrates deform plastically and one of the methods is identified as the most accurate.

Pressure sensitive adhesives provide high toughness at low stress and stiffness. These properties are beneficial for bimaterial bonding. In the present study the tape is modelled with a cohesive layer characterized by a cohesive law. This is suitable for FE-analysis of bonded structures. The cohesive law is measured using a method based on the path independent property of the J-integral. Complementing an earlier study, we here focus on influences of loading rate on the properties of the pressure sensitive adhesive. Transparent PMMA substrates are used with the transparent tape in Double Cantilever Beam specimens. The transparency of both the tape and the substrates provides the possibility of in-situ studies of the fracture process. The results indicate that the fracture energy levels off at about 1 kN/m for small loading rates. Moreover, the cohesive law also appears to level off below an engineering strain rate of about 2 s-1. The cohesive law contains two peak stresses. The first is associated with the nucleation of cavities in the tape. This occurs at a stress level comparable to the critical stress associated with an unbonded growth rate of a spherical cavity in rubber. The second peak stress is associated to the breaking down of walls formed between the fully developed cavities. This process precedes the final fracture of the tape. It also appears as nucleation of cavities is influenced by the strain rate where slower rates give more time for cavities to nucleate. This results in larger cavity density at smaller loading rates. The results also indicate a similarity of the effects of loading rate and ageing of the macroscopic properties of the present pressure sensitive adhesive.

Confined layers may fracture in shear. This occurs, for example in adhesive joints and composite materials. A common mechanism for shear fracture is the formation of shear hackles associated with an expansion of the layer. This makes shear toughness and strength depend on the constraint of the expansion. By constraining the expansion using external loading in experiments, the expansion is reduced but not totally inhibited. The experiments are evaluated using the path independent properties of the J-integral. It is shown that the shear toughness increases for the more constrained case. Thus, from a strength analysis perspective, ignoring the expansion leads to a conservative estimate of the fracture properties. Extrapolation of the evaluated properties to totally inhibited expansions gives the traction separation relation and the fracture toughness for a layer in simple shear.

The stress-deformation relation i.e. cohesive law representing the fracture process in an almost incompressible adhesive tape is measured using the double cantilever beam specimen. As in many ductile materials, the fracture process of the tape involves nucleation, growth and coalesce of cavities. This process is studied carefully by exploiting the transparency of the used materials and the inherent stability of the specimen configuration. Utilising the path independence of the J -integral, the cohesive law is measured. The law is compared to the results of butt-joint tests. The law contains two stress peaks—the first is associated with nucleation of cavities at a stress level conforming to predictions of void nucleation in rubber elasticity. The second stress peak is associated with fracture of stretched walls between fully-grown cavities. After this second peak, a macroscopic crack is formed. The tape suffers at this stage an engineering strain of about 800%. A numerical analysis with the determined cohesive law recreates the global specimen behaviour.

Cracks normally propagate in the opening mode associated with a state of local symmetry at a crack tip. However, the micro- or macrostructure of a material or structure sometimes forces cracks to propagate in a shearing mode. Irrespective of the actual material studied, fracture in shear is frequently asso- ciated with the formation of a large number smaller sigmoidal-shaped cracks in the propagation direction of the major crack. Propagation of the major shear crack is accomplished by coalescing the sigmoidal-shaped cracks. Ex- periments show that the formation of sigmoidal cracks due to shear loading leads to a normal separation of the joined substrates. Theoretical studies show that constraining the local opening of the sigmoidal cracks increases the frac- ture resistance for the propagation of the major crack. In the present study, experiments with a ductile adhesive loaded in shear and where the normal sep- aration is constrained are presented. The experiments are evaluated using the path independent J-integral. The associated cohesive law shows that consid- erable normal compressive stress develops in the adhesive during macroscopic shear loading. It is also concluded that by ignoring the normal separation in the evaluation of the experiments, the strength of the adhesive is underesti- mated. Thus, the procedure developed in earlier studies is conservative from a strength analysis perspective. The present technique might be possible to extend to other materials to reveal their properties in shear fracture.

Structural tapes provide comparable toughness as structural adhesives at orders of magnitude lower stresses. This is potentially useful to minimize the effects of differences in thermal expansion in the joining of mixed materials. The strength properties are modelled using the cohesive zone model. Thus, a cohesive zone represents the tape, i.e. stresses in the tape are transmitted to the substrates through tractions determined by the separations of the surfaces of substrates. This simplification allows for structural analysis of large complex structures. The relation between the traction and the separation is measured experimentally using methods based on the path independence of the J-integral. Repeated experiments are performed at quasi-static loading. A mixed mode cohesive law is adapted to the experimental data. The law is implemented as a UMAT in Abaqus. Simulations show minor thermal distortions due to thermal loading and substantial structural strength in mechanical loading of a mixed material structure.

The use of MgO-paste as bone cement was tested on titanium cylinders implanted into rat tibia. The evaluation of bone healingwas made with the retention force (pull-out) test, light microscopy and ESEM/ EDX. Preimplantation of the MgO-paste into drillperforations of rat tibia increased the retention of the titanium implant 6-fold. The error was expressed as the 95% confidenceinterval of means (n=10 bones in each group). The observed difference between 3.46+/-0.71 N/mm2 for Ti-cylinders implantedwith MgO-paste and 0.56+/-0.26 N/mm2 for Ti-cylinders implanted directly into the bone, is statistically significant (p<0.01).The increase of retention force, caused by MgO is parallel to an increased thickness of the compact bone surrounding the implantand closer contact between bone and implant.Histological examination of the implant-related bone showed that the MgO-induced bone growth is mediated by the formation ofa bone-inducing matrix. The matrix contains organic substance, most likely proteins.

Adhesively bonded T-Peel specimens are loaded in a rotary impact device to investigate the behaviour of adhesive joints under high-strain rates. To gain a better understanding of that kind of tests and their results, the deformation of the samples as well as the movement of specimen support during the test is analyzed. A three-dimensional optical measurement system is used in combination with two synchronized high-speed cameras to obtain the deformations. The paper explains the experimental challenges and discusses the results of the analyses with respect to a planned usage of the experimental results in a finite element crash simulation. As main results of the investigations it can be summarized that the compliant clamping leads to an inadvertent out-of-plane-movement of the load introduction point. Based on finite element calculations it can be concluded that the measured out-of-plane-movement possesses a negligible influence on the obtained force signal, which is of primary interest in a T-Peel test.

Constitutive data are needed at extreme strains to increase the understanding of fracture processes. Ordinary tensile tests ends prematurely due to localization and large amounts of elastic energy stored in the specimens prior to fracture. A novel method is proposed to perform tensile tests using a double cantilever beam specimen. To verify the method a large specimen is developed and tested. Similar results are achieved with the present method as with more standardized methods giving confidence in the method. The specimen should be possible to minimise to provide data with small specimens.