Opening Displacement Profiles Research Articles

Growth criteria for solvent crazes

Single methanol crazes are grown from sharp cracks in polymethylmethacrylate. Double exposure holographic interferometry is used to determine the sequential strain energy release rates G and opening displacement profile of the craze from the initiation of growth to its cessation. The craze stress profile is determined at various points in its growth from the opening displacement profiles using a Fourier transform method. The rapid increase in G observed just before the craze ceases to grow demonstrates that craze growth criteria based on the concept of a constant critical total strain energy release rate cannot be correct. Similarly, the large stress concentration which develops just behind the craze tip at growth cessation is incompatible with the assumption that the craze grows to a length that just eliminates a stress singularity at its tip (Dugdale model). This feature, however, would be expected if sufficient methanol cannot reach the fibrils just behind the tip of the craze to plasticize them fully.

Mechanical properties of methanol crazes in poly(methyl methacrylate)

AbstractMethanol crazes are grown from sharp cracks in poly(methyl methacrylate) (PMMA). The craze thickness profile is measured using a replica technique after the craze opening displacement profile of the growing craze has been measured with holographic interferometry. The craze strain profile is then computed from these data. The craze surface stress profile is determined by two methods: (1) from the uniaxial strain profile of regions adjacent to the craze as measured from the fringe spacing on the reconstructed hologram and (2) from the craze opening displacement profile using the Fourier transform method of Sneddon. From the surface stress and craze‐strain profiles a true stress‐strain curve for the craze fibrils has been constructed. The extrapolated fibril yield stress is in good agreement with the yield stress of bulk PMMA plasticized with methanol indicating that surface tension effects do not contribute importantly to craze fibril mechanical properties at room temperature. The craze strain increases from 0.4 near the craze tip to 1.4 near the craze base implying that methanol crazes in PMMA thicken by further straining of the existing craze fibrils and not by drawing new material into the craze from the craze surfaces. The primordial craze thickness, i.e., the original thickness of polymer which fibrillates to form the craze fibrils, is approximately 1 μm and is constant over most of the craze length. This thickness may be determined by diffusion of methanol normal to the craze surfaces in a process zone just behind the craze tip.

Micromechanics of solvent crazes in polystyrene

Double exposure holographic interferometry (DEHI) is used to determine the strain energy release rate, craze opening displacement profile, and craze stress profile ofn-heptane and methanol crazes growing from cracks in polystyrene.n-heptane crazes have strain energy release rates (SERRs) close to those of cracks and their stress profile is almost crack-like in that the tensile stress across the craze falls almost to zero. On the other hand, the SERRs of methanol crazes are only 30 to 55% the SERR of a crack depending on stress intensity factorK I of the precrack from which they are grown. The stress profile of the methanol craze shows it to be strongly load-bearing away from the craze tip, apparently as a result of the strain hardening of the craze fibrils. The stress concentration in front of the methanol craze tip is only 40% of that in front of then-heptane craze tip. The opening displacements of the methanol craze are almost as large as those of a crack very near its tip but are much less than those of a crack at large distances behind the tip. The Dugdale model of a strip yielding zone provides a poor representation of the craze opening displacements of the growing methanol craze. Dry (static) methanol crazes have larger opening displacements in response to an incremental tensile strain at moderate prestrains than at either low or high prestrains, suggesting that the craze fibrils undergo a yielding/strain-hardening process as the strain is increased similar to that observed in polycarbonate crazes by Kopp and Kambour. Dryn-heptane crazes do not show this response but rather open linearly with increasing prestrain. The opening displacement for long (dry)n-heptane crazes is almost crack-like whereas the largest opening of a dry methanol craze is only ∼ 20% of that of a crack. Dry methanol crazes break at aK IC that is ∼ 40% of theK IC of precracked but uncrazed specimens. The strongest (shortest) dryn-heptane crazes fail at only ∼ 7% ofK IC of uncrazed specimens and theK IC of the dryn-heptane crazes decreases markedly with increasing craze length.