Measurement of the stress intensity factor and the effective stress intensity factor range of a mode I surface crack using the digital image correlation method

Abstract

Crack opening displacement around a mode I surface crack tip was measured by digital image correlation method to determine the stress intensity factor and the effective stress intensity factor range. First, the mode I surface crack was introduced into a specimen of A7075 aluminum alloy by a servo-hydraulic fatigue testing machine. Then, the specimen was annealed to remove a compressive residual stress around the crack and used to demonstrate the measurement of the stress intensity factor on the basis of crack opening displacement measured by digital image correlation method. Second, the effective stress intensity factor range was determined from the crack opening displacement during the pulsating tension test under three kinds of stress ratios and used to arrange the crack propagation rate. The difference between the measured stress intensity factors of the annealed specimen and the factors proposed by the numerical solution was less than 10%. Additionally, the crack opening displacement of the propagation process under pulsating tension was subjected to the influence of residual stress and became smaller than the crack opening displacements calculated by the theoretical solution. The crack opening stress calculated from the crack opening displacement was larger than the minimum stress in the case of R=0 and R=0.2. On the other hand, the crack opening stress was clearly smaller than the minimum stress in R=0.5. It was clear that the relationship between the crack propagation rate and the effective stress intensity factor range that determined by the crack opening displacement was barely dependent on the stress ratio.

Keywords

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