This work is a part of a program aimed at understanding the response of polymer chains to external mechanical forces. The response may be nondestructive or destructive, and competition between the two types exists. Molecular dynamics was used to simulate systems of polymer chains on a computer. A constant strain was imposed, as in experimental stress relaxation tests. Free volume was varied, and its effect on relaxation rate and on bond breaking was studied. A crossover exists between the region in which the chain relaxation capability is sufficient and the region where the breaking of a small number of bonds leads to crack propagation. The results are significant for polymers, conventional polymer‐based composites, and liquid‐crystalline molecular composites.