Previous work by the authors has indicated that crack behaviour in PMMA (and thus probably in other materials as well) shows a secondary dependence on the degree of in-plane stress biaxiality, in addition to its established primary dependence on K~, the elastic stress intensity factor. Data published here shows the crack-length dependence of a parameter expressing the degree of stress biaxiality inherent to a number of standard specimen geometries. This should help to determine to what, if any, extent a given material's sensitivity to stress biaxiality is responsible for K~-independent variations in crack behaviour between specimens.

A number of fatigue crack propagation laws applied in the study of polymers is described. Consideration of the stress field distribution at the crack tip leads to the application of fracture mechanics. It is shown that a simplified relationship of the form da/dN = F~ ~, where ~b is a function of Klc, Km~x, Kmi, and KTH appears to be a convenient expression for cyclic crack growth. The effect of mean stress is more complicated than that in the field of metals, the compressive component of cyclic stress may delay the crack growth. Cyclic tests in tension performed on PMMA and PVC are dependent on AK and its mean value, K,~. The threshold value, KTH, is also influenced by Km but a more complicated behaviour due to strain rate effects may be observed. Other differences, such as the position of upper and lower transition points and growth rate changes with frequence, are noted. The effect of biaxial cyclic loading of PMMA and PVC plates is compared and some differences highlighted. The results available so far indicate little effect of the crack curving on its growth. However, it is shown that, while the increasing biaxiality can substantially retard the crack growth in PMMA, no such effect was recorded in PVC. Finally, it is shown that at very high stress levels (region III), the cyclic crack growth consists of two propagation modes, namely, a pure cyclic propagation, together with slow growth. At lower stress levels, slow growth disappears and the crack propagates in pure fatigue (region II). In region I, the propagation is very slow, without the usual correspondence between cycles and striations. The results recently obtained on glass reinforced plastics (GRP) are also presented and differences highlighted.

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Fracture toughness testing was carried out on an A1-4C~Cu alloy in the form of a fully heat treated 2½-in thick plate. Three types of test, the three point slow bend, instrumented Charpy impact, and double cantilever beam tests, enabled values of plane strain fracture toughness (Kit) to be studied over a range of temperature, testing speed and specimen size. Dependence of K lc upon temperature (-200°C to + 160°C) was found to be relatively small except at very low temperatures. The alloy was found not to be appreciably strain rate sensitive over the range of testing speeds (0.002 in/ rain to 2000 in/min) used. Higher toughness values, obtained from plane strain fractures of the smaller size specimens, were compared with low results from large specimens. The results became variable when curved fracture surfaces developed in the smallest DCB specimens. There was an apparent increase in toughness towards the centre of the plate and the effects of specimen size were most marked in the lower yield strength metal of this layer.The results indicate a genuine increase in toughness K~c with decreasing temperature, but also show that large specimens are required to approach the lowest and constant Kit value in circumstances where the yield strength decreases.The DCB test is also shown to be still valid for the measurement of K~ for this non-brittle alloy, where yield strength may be only about 40,000 lbf/in 2.Electron fractography showed a ductile intergranular fracture mode. The results are consistent with theories of solute precipitation in Al-alloys.

The fracture toughness of a variety of sharply notched tension, bending and rotating disc specimens of PMMA is examined using linear fracture mechanics. It is observed that rapid fracture with a brittle glassy appearance usually follows a period of slow crack growth, denoted by fan shaped markings of local ductility, though still brittle overall. In this near brittle regime the fracture toughness is sensitive to strain rate so that high values of effective surface energy are easily induced by rapid testing or notch bluntness. At impact rates the toughness increases again. For design purposes, in the absence of environmental effects, the onset of slow cracking and rapid (glassy) fracture, can be associated with fracture toughness K1, of about 800 lbf/in3/2 (90 kg/cm3/2) and 1800 lbf/in3/2 (180 kg/cm3/2) respectively. Detailed studies have not been made on other materials but a guide to the levels of notch toughness and notch brittle temperatures are given for several plastics.