Determination of fracture mechanics parameters by digital image correlation method of surface crack for 7075 Al alloy under nonproportional cyclic loading (Growth life prediction method considering shear stress on the crack surface)

Abstract

A method for predicting the propagation life of a crack initiated on the surface of a 7075 aluminum alloy under nonproportional loading was proposed using the modified Paris law. The nonproportional loading conditions in previous studies are limited to derive an experimental formula that can estimate the effective resultant stress intensity factor range used in the modified Paris law. Therefore, crack growth tests were first conducted under new nonproportional loading conditions and the effective resultant stress intensity factor range was determined using the continuous crack opening displacement measured by digital image correlation method. Then, the contribution of the shear stress acting on the crack plane to the effective resultant stress intensity factor range was clarified by referring to well-known Findley's model, which takes into account the interlocking effect due to the shear stress acting on the crack plane. In addition, a new experimental formula that can determine the effective resultant stress range from nonproportional loading conditions without measuring the crack opening displacement was proposed. This formula enables us to predict the crack growth life until the surface crack penetrates the plate with an error of approximately ±30% based on a modified Paris-type equation through numerical integration.

Keywords

• digital image correlation
• fatigue
• stress intensity factor
• crack closure
• crack propagation