Models for Time-Variation of Mechanical Strength for Materials and Machine Parts

Abstract

The paper presents experimental results which demonstrate that both elasticity and fracture strength decrease during useful life. Experiments allow the determination of the qualitative characteristics of phenomena such as lowering elasticity and strength caused by intrinsic interference between the strength of metals and their usage stress. Based on experiments, the paper models the prediction with time of fracture probability and safety coefficients in two frequent practical situations: 1. Deterministic strength according to mean value and 2. Probabilistic strength with normal (Gaussian) distribution. In both cases, it is considered an ergodic distribution of stress, a variable between the extreme values adopted in the design phase.