Abstract
At temperatures ranging from -50° to 80°C polymethyl methacrylate (PMMA) plates of 5mm thickness were impacted with PMMA flyer plates of 1mm thickness, which were accelerated by the foil explosion method. Spall cracks of various growth stages produced in specimens were examined in detail by the aid of an optical microscope.
It was found that in general the spall in PMMA plates was not an extension of a single crack but consisted of many small cracks of disc shape. (Each of them shall be called as a unit crack.) The diameter of the unit cracks in a specimen, which extended perpendicularly to the direction of tensile stress, varied from 30 to 700μ. Most of the unit cracks had circularly symmetrical patterns with discernible origins at their centers. This point of origin was surrounded by a region of mirror-like smoothness and a number of fine cracks spreaded radially from the periphery of the mirror region. The distance of crack separation at the mirror region was comparable to the wave length of visible light.
When the amplitude of stress waves applied to specimen plates was not large enough, such mirror region was not distinguishable and the whole pattern of the unit crack became similar to that of the well-known“parabola-mark”in the vicinity of its focus. When the amplitude was sufficiently high, the separation of fracture surfaces became very distinct all over the area of well-grown unit cracks. It was characteristic of this case that another region of mirror-like smoothness appeared near the outer circumference of the crack disc. As an intermediate case between the above two extremes, very fine cracks with a cloud-like appearance sometimes appeared radially outside a rough area of an irregular shape with a shell-like appearance adjacent to the inner mirror region.
Neither specimen temperatures nor annealing conditions affected the pattern of spall cracks. Their sizes were also independent of specimen temperatures in the range of 23°-9°C. The present results suggest that the initiation of spalling in PMMA is substantially dominated by the location and properties of internal nuclei such as flaws and that both the configuration of the nuclei and the time required for them to form origins of unit cracks determine the final crack growth.
