Test method for determining imminent failure in metals

Abstract

A method for determining a “safe-operation” point for a metal structural element subjected to repeated loading, the same or different, generating variable levels of strain and residual stress in the worked element; and, predicting the imminent failure of the structural element. The surface of the metal element is worked to provide a residual strain, for example, by shot-peening. Measurements of electrical conductivity are compared at various chosen frequencies corresponding to different depths in the “near-surface” of the element. Similar measurements are made in the near-surface of a “standard” and a first difference is computed between the conductivity of the shot-peened surface and the “standard” surface. This first difference provides a basis for comparison of the effects of residual stress after successive loadings of the shot-peened metal element. The effect of residual stress on the peened portion after it is subjected to a number of successive loadings is measured and compared to the reference. When the effect of residual stress is measured as conductivity or resistivity, it is found that there is a progressive increase in conductivity (and decrease in resistivity) as the metal element begins to succumb to fatigue. It is found that when the resistivity decreases to a value in the range from about 50% to 60% of the measured first difference in resistivities, the metal element has reached its 50% “safe-operation” point beyond which it rapidly,progresses to fatigue failure.