Progress in Rubber,Plastics and Recycling Technology

Fracture mechanics analysis is of great value in adhesion studies since it defines adhesive failure parameters that are, in principle, independent of the test employed and thus characteristic of the adhesive bond itself⁽¹⁾. The chief drawback of fracture mechanics is that the fracture parameters obtained are purely empirical quantities and are not explicitly related to the surface or mechanical properties of the adhering phases.

Generalised fracture mechanics (GFM) seeks to overcome this problem by expressing the adhesive failure parameters in terms of the interfacial energy and the mechanical hysteresis properties of the bulk phases. This then permits the surface and bulk contributions to adhesive failure energy to be separated, allowing a more profound analysis of the fracture process and the way it is controlled by rate, temperature and other environmental factors. This will be illustrated by reference to specific studies on rubberlike and structural adhesives.

An attempt has been made to demonstrate how studies of morphology and crystal growth of polyisoprenes have furthered our understanding of crystallisation in general. This has included work on the whole range of polymer systems, homopolymers, copolymers, blends and crosslinked systems. It has been shown both that polyisoprenes can be considered as normal crystallisable thermoplastics and that polyisoprene systems may yet have information to yield applicable to relatively low crystallinity thermoplastics in general.

Stress corrosion in GRP is a result of acidic attack of the fibres acting in concert with applied stress on the fibres. Accordingly. the effects of fibre orientation are in line with the magnitude of resulting resolved stress acting in the fibre directions. with the effect of fibre composition reflecting the susceptibility of the fibres to acid degradation. As a general rule the matrix has an important role to play during stress corrosion with tough ductile matrices favouring a good resistance to stress corrosion crack propagation. Whereas however certain systems have been found to provide good stress corrosion resistance on the basis of laboratory tests. any estimate of component lifetime is the result inevitably of extrapolation to longer times than may be covered in most laboratories. In practice. the long lifetimes required mean that if stress corrosion ceases to become an issue then more conventional corrosion resistance needs to be evaluated. It is likely that corrosion and stress corrosion have different requirements from resins ensuring that all selections are a compromise. The great dependence of the stress corrosion cracking process on stress concentrations and the sharp nature of the resulting cracks means that unusually great care must be taken to avoid geometric stress concentrators in the design of GRP process vessels and pipe work. Stress induced damage. especially in any protective barrier layer must be avoided at all costs in order to eliminate unexpected. early nucleation of cracks. Finally. it would be desirable to have access to intermittent monitoring procedures (e.g. acoustic emission) that may detect stress corrosion cracking in vessels that show no external signs of damage.

This review considers the development of impact testing of polymeric materials over the last decade. Increased utilisation of instrumented falling weight impact (IFWIM) testing is considered, together with the interpretation of the detail of force-time curves so obtained. Auxiliary techniques to aid interpretation of impact test data are considered, namely the low energy falling weight technique, high speed photography, short pulse flash photography and novel grid techniques to monitor crack growth. Application of these techniques is illustrated by reference to examples of testing on a variety of plastics and composites. Recent developments in the theoretical analysis of the flexed plate method which seek to obtain intrinsic materials property data under impact situations.