Poly(ethylene terephthalate) (PET) filaments were degraded with aqueous solutions of n-propylamine and methylamine. The heated materials were examined by chemical microscopy and physical and mechanical methods. After partial degradation by aqueous solutions of amines, the surfaces of the filaments cracked. Direct observations of the cracking pattern formation and the changes in the physical properties of the filaments led to an explanation for the cracking pattern formation. The cracked patterns showed the swelled and deformed structure of the filaments. This explanation of the possible mechanisms for crack formation goes further than those discussed in previous research.