EFFECT OF RELAXATION ON FRACTURE AND CREEP OF POLYMERIC MATERIALS

Fracture of polymers is the result of the thermoactivated rupture of molecular chains. Thermoactivated rupture and rearrangement of intermolecular physical bonds leads to ductile yield and change of the initial shape of the body. The failure time, i.e., long-term strength tau is proportional to exp[U(sigma, T)/RT]. At brittle fracture, activation energy U0 is equal to the energy of thermal degradation of the polymer chain E(a); for soft polymers U0 = nu(1) (n is the number of elements in the segment, u(1) is the energy of the interaction between segments). If the polymer structure does not change, long-term strength decreases exponentially with an increase in applied stress due to a linear decrease in activation energy gammasigma. If the polymer structure changes under stress, fracture proceeds via two stages. In solid polymers the first stage is the destruction of crystals; the time of this stage is tau is similar to exp(E(a)/RT). The second stage is related to the segment motion and ductile yield followed by necking and microcracking; the time of this stage is described by factor [(U0' - E(a))/gamma' sigma] E(a)/2RT(m). In rubberlike polymers the sequence of these stages is reversed. The first stage is creep involving segmental motion and orientation of the chain for a time tau is similar to exp (u(1/RT). The second stage is the chemical degradation of the oriented molecular chains; the time of this process is described by the factor [(E(a) - u(1)/gamma'' sigma]u1/2RT(m)'. For rubbery polymers at high temperatures the long-term strength is not described by the Alexandrov-Zhurkov equation.