Michal K. Budzik

Adhesive fracture of heterogeneous interfaces

We have studied the effect of interface heterogeneity on fracture, at both local and global scales. The single cantilever beam adhesion test was used to investigate interfacial fracture between polycarbonate plates and an elastic/fragile epoxy adhesive. Two surface treatments were applied to a (given) polycarbonate plate giving zones of strong and weak adhesion parallel to the crack direction. Calculated fracture energies differed from those expected from a simple rule-of-mixtures. A perturbation method, proposed by Rice, was used and results compared with crack fronts observed in situ. The technique was applied successfully but the difference in values of stress intensity factor between the zones was found substantially different from the experimental value. In an attempt to explain discrepancies, specimens with discontinuous crack fronts (adhesive and/or plates severed along the strong/weak adhesion frontier) were tested. Good agreement was found with the rule-of-mixtures predictions raising questions about the role of crack front continuity in load transfer.

Antagonist adhesion effects due to variable substrate surface

The effects of variability of intrinsic adhesion within a joint have been studied using a single cantilever beam (SCB) test. Fracture energy was found not to be a simple function of relative areas of 2 surface pre-treatments: a ‘weak’ zone decreased strength more than expected from simple, additive considerations. By severing the adhesive along the strong–weak transition, fracture energy increased. The prior antagonist effect appears to be directly linked to supplementary stresses generated near the crossover by continuity of the adhesive layer, existent despite differences in adhesion.

Impact of interface heterogeneity on joint fracture

The effects of heterogeneities (weak zones in particular) in adhesive joints and their importance on overall fracture properties are relatively unknown, but doubtlessly they may be crucial in many applications. Using a model heterogeneous adhesive bond, represented by a given mixture of regions of strong and weak adhesion, we have studied the influence of interface variability on overall fracture energy (global energy release rate). Adopting the original Griffith-Irwin arguments, we have employed a simple, fracture scaling law. By varying relative proportions of the weak and strong interfaces, a nonlinear evolution of fracture energy was observed. This was contrary to expectations, cf. rules of mixtures. Inspired by rheological models (Maxwell, Voigt/Kelvin, etc.), an appropriate model was found empirically.

Intersection of Interface Crack Front with Free Edge

Delamination in layered materials is analyzed in a fracture mechanical framework. The work deals with quasi-static propagation of crack fronts along planar interfaces between different isotropic, elastic layers. Special focus is here on local effects at the sides of the layers which are assumed to be stress free. The interface crack front meets the free sides of the specimen at an angle which depends on the elastic mismatch in the system. Finite element calculations allowing the shape of the crack front to be arbitrary are carried out for double cantilever beam type specimens. An iterative procedure is formulated which adjusts the shape of the crack front so that an interface fracture criterion is satisfied locally along the front. Apart from the overall shape of the crack front, the angle of intersection with the free sides is in particular determined numerically by this procedure. Comparisons with analytical formulations and experimental results are performed.

Water sorption and blistering of GFRP laminates with varying structures

Abstract The microstructures, water absorption as well as blistering was studied for bi- and tri- axial glass fibre vinyl ester and polyester -matrix laminates coated with gel coat layer and uncoated. The effect of manufacturing technique on water sorption characteristics was considered. It was found that water sorption characteristics of GFRP depend on manufacturing methods. The least water intake was found for specimens having perfect microstructure due to manufacturing by infusion process. Water sorption saturation value was reached after ca. 50 days of accelerated test at 70°C and was found to correspond to 250 days conditioning at 20°C. Blistering evolution was illustrated and SEM images shown corresponding to degradation of the gel coat layer and the laminate during the exposure in water.

ON DEGRADATION OF GLASS/POLYESTER LAMINATE IMMERSED IN WATER

ON DEGRADATION OF GLASS/POLYESTER LAMINATE IMMERSED IN WATER Mechanical behaviour was compared for glass/polyester laminates manufactured in the boatbuilding plant using three methods: hand lay-up, vacuum bagging, infusion. Specimens were tested in dry condition and following accelerated water immersion test (70°C- corresponding to the exposure of 30 years at 19°C). In three point bending test 40-50% reduction in laminate strength was observed due to water immersion. The highest degradation was in samples manufactured using hand lay-up method, the differences in strength between both vacuum methods were insignificant. Interlaminar shear strength was reduced by 25% for infusion method which is recommended as the most efficient. Matrix plasticization and debondings as well as surface microcracks were responsible for reduction in strength for water conditioned specimens. However, no microstructural difference in type or intensity of internal damage was observed for the three sample types.

Inverse End Loaded Split test configuration for stable mode II crack propagation in bonded joint: macroscopic analysis—effective crack length approach

Mode II crack propagation along a bonded joint is investigated using newly proposed Inverse-End Loaded Split experimental configurations. This test configuration allows stable crack propagation all along the crack propagation path. The specimen compliance and strain energy release rate for the new experimental arrangement are derived. An experimental data reduction procedure, based on the effective crack length approach, is also proposed. Two series of experiments are performed to assess the stable nature of the crack propagation and data reduction scheme associated to this new experimental arrangement. In addition to stable crack growth, the experiment may prove his worthiness in the study of crack onset under mode II loading.

Bonded bimaterial ring under compressive load

A bonded ring test is proposed to study crack onset under pure and mixed mode conditions. In the existing centre-cracked Brazilian disc configuration, introduction of a central crack may be arbitrary and can easily lead to initial asymmetric crack configuration. Moreover, the crack front geometry is, in general, unknown which may result in serious consequences on crack onset and fracture parameters. In the present work, a bonded ring loaded in compression was investigated numerically and experimentally. Two bimaterial singular points were introduced between the half discs and the adhesive providing privileged crack onset location – controllable during specimen fabrication. Three types of specimens were analysed using digital image correlation and finite element techniques: homogenous poly(methyl methacrylate), bonded poly(methyl methacrylate) and bonded poly(methyl methacrylate) to stainless steel rings. A satisfying agreement between experimental and numerical approaches was found enabling more complex analysis in future. Such test configuration can be of interest for crack onset studies and estimation of fracture parameters under mixed mode I/II loading conditions.

Evaluation of Defects in Adhesive Joint by Double Cantilever Beam Experiment

We analyzed effects of interface/adhesive defects during fracture mechanical Mode I Double Cantilever Beam (DCB) tests of an adhesive joint. Two aluminium slabs were bonded using structural epoxy adhesive. A DCB experiment under static loading was conducted to estimate the critical fracture energy. During the ‘steady-state’ fracture we noted oscillating, random fluctuation in the force vs. displacement curve, and thus in the fracture energy. This is associated to the local variation of properties within the bondline and the interfaces. A simple model is derived to quantify the probable density of flaws observed experimentally.

Experimental study of cohesive strain prior to crack initiation in constant force single cantilever beam test

Retarded deformation at the bond line edge of a thick, viscoelastic adhesive layer is observed using digital image correlation. Such information is required for proposing physical crack onset/propagation parameters which take into account viscoelastic and viscous mechanisms in the evaluation of dissipated energy. A complex stress/strain redistribution is observed in the early stage after specimen loading. The results illustrate that viscoelasticity dissipates a considerable amount of energy prior to crack nucleation and the lack of predictive models to account for that phenomenon.