The Development of Accelerated Test Methods for Environmental Stress Cracking

A more global question was how to determine that design or process changes applied to new products would not cause ESC. How could we prove that the mechanism really was ESC? In our investigation of the stress-cracking mechanism, we recognized several things; ESC cannot occur without strain and the mechanism could not be duplicated in vitro. Obviously, if ESC requires a residual strain, then it should depend on processes that increased or reduced residual strain. It was also known that ESC processes required induction periods and critical strains; however, one cannot accelerate ESC in a thermoplastic by elevating temperature. These polymers are viscoelastic, which means that they flow or "creep" under load. As temperature increases, the creep rate increases and stresses are relieved. Therefore, we strained a number of samples over mandrels and implanted them in the subcutis of rabbits. We found that the time to failure (induction period) varied as a function of the magnitude of applied strain and the polymer's thermal history. Extrusion conditions and poststrain annealing (a thermal treatment to reduce stress) were found to have significant effects. Indeed, these properties fit all of the hallmarks of a stresscracking phenomenon.74 The animal results matched exactly with the findings on explanted and returned cracked leads.

We settled on a set of standard conditions and developed an accelerated, in vivo test that could be used to evaluate new processes and new materials. We learned how to optimally stress-relieve the devices by annealing to prevent ESC failure and incorporated those processes in evolving next-generation devices.

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