The surface cracking of glassy polymers under a sliding spherical indenter

Abstract

Glassy polymers crack under a sliding hard spherical indenter in a way that is observed in other brittle materials. A series of curved cracks concave to the wake of the indenter are formed and these can penetrate to a depth of a few hundredths of a millimetre. With polystyrene the additional stress imposed by sliding reduces the critical load to fracture under normal loading conditions by 95% and the presence of an active environment, methanol, reduced the critical fracture load for sliding by 83%. Other glassy polymers which are generally considered tough, cracked in the presence of agents which could promote environmental stress cracking. The stress necessary to cause cracking and the crack size obeyed laws pertaining to fracture under elastic stress conditions. Although the flaws observed are referred to as cracks it is recognized that they could possibly be crazes. The introduction of surface damage in the form of cracks or crazes is important practically, since the inadvertent scratching or abrasion of the polymer surfaces may introduce flaws which under normally acceptable stress-environmental conditions could lead to failure. It is also noted that the flaws are detrimental to the polymer's optical properties.