The plastic in rigid thermoplastic parts consists of long chain molecules entangled together like spaghetti noodles in a bowl of spaghetti. The strength of the plastic is directly proportional to the extent to which the polymer chains are entangled with each other. When a plastic part is placed under stress, the part is held together because of the chain entanglements. Every time the part is placed under stress, the polymer chains disentangle slightly. After thousands of stress events, the polymer chains become disentangled enough on the surface of the part that a crack develops. Repeated cyclical stress events causes the crack to propagate deeper into the part until the crack works its way completely through the part and the part completely fractures. This failure mode is generally characterized by the appearance of hundreds of parallel striations on the fracture surface when viewing the surface using a microscope as shown below:
