No abstract

No abstract

This paper reports the findings of fatigue crack growth measurements on a 2014 aluminium alloy that had been anodised using the recently developed boric acidsulphuric acid process. The results demonstrate that fatigue crack growth rates in this alloy can be markedly enhanced by the presence of a thin anodic oxide film. This enhancement can be explained in terms of reduced plasticity induced closure assisted by surface cracking of the brittle oxide film ahead of the propagating fatigue crack.

Stiffness under uniaxial tension of injection-moulded polypropylene was found to increase much more rapidly with the addition of glass fibres than with the same amounts, by weight, of glass spheres and to decrease in both cases with strain. The observed increases in stiffness due to the fibres correlated closely with the results obtained from a simple model of a two-phase material, when allowances were made for the apparently low degree of alignment of the fibres which decreased with increasing fibre content. The stiffening effect of the fibres was virtually constant over the strain range considered. The lower increases in stiffness due to the glass spheres also correlated with the results obtained from the simple model but only for strains of less than 0.5 per cent: at higher strains the inclusion of glass spheres reduced the stiffness of the polypropylene.

No abstract

In recent years there has been an increasing interest in investigating the openings of cracks in materials under load.In particular this interest has concentrated on the value of the critical displacement at the crack tips at fracture (variously designated as 6c, COS, COD, 2Vc) since these displacements, being essentially determined by the plastic deformations in the crack tip region, should provide a fracture criterion to characterize plastic fracture behaviour.In metals a direct measurement of the critical crack opening is seldom practicable and is, therefore, usually determined by an extrapolation of crack openings measured remote from the crack tip.In optically transparent materials such as glassy thermoplastics, on the other hand, it is possible to measure not only the crack opening very close to the crack tip but also the plastic zone (or craze zone) ahead of the crack tip using optical interference microscopy [1][2][3][4]. Essential requirements for the application of this method are that the crack opening and craze zone thickness should both be comparable in order of magnitude to the wavelength of l~ght and that, additionally, for the craze zone, the refractive index of the crazed material should differ significantly from that of the bulk material and the boundary between the two regions should be sharp.In this work we would like to report results of measurements of the critical displacements of the elastic-plastic boundary directly at the crack tip (here designated as 2v c and marked by an arrow in Fig. 4) and their variation with temperature in some glassy thermoplastics.These measurements were performed on miniature compact tension specimens in an experimental arrangement which permitted the simultaneous recording of the interference fringe pattern and the load acting on the specimen [4]. The materials were PMMA of two different molecular weights (M w ~ 2,200,000 and M w = 120,000) and PVC. Examples of the interference fringe patterns just before the onset of slow crack propagation at room temperature are shown in Fig. i. From the different numbers of fringes in the craze zones it is directly apparent that there are differences in the critical crack tip opening displacements.Based on a quantitative analysis of such fringe patterns (details of which have been given in earlier work [3,411 Fig. 2 shows the critical displacements at the crack tip 2v c in the three materials for temperatures between 0 deg C and 80 deg C. Three quite different patterns of behaviour are apparent: the 2v c values for PVC fall sharply with increasing temperature, those for the lower molecular weight PMMA remain practically constant whilst those for the h~gher molecular weight PMMA show a significant increase.*now at the Open University, Milton Keynes MK7 6ka, UK. Int Journ of Fracture 14 (1978) RI90In contrast to this, the corresponding results for the fracture toughness at the onset of slow crack propagation, Kio , decrease, albeit at different rates, with temperature for all three materials as is shown in Fig. 3. Since, in ge...

No abstract

No abstract