Direct Measurement of Opening Mode Stress Intensity Factors Using Flexoelectric Strain Gradient Sensors

This paper presents a new method for direct measurement of Mode-I stress intensify factor of cracks using Ba_0.64Sr_0.36TiO₃ flexoelectric strain gradient sensors. Firstly, strain gradient field around the opening mode crack tip was analyzed, followed by the derivation of induced flexoelectric polarization in the strain gradient sensors attached in the vicinity of a crack tip. It was found that the constant stress term makes no contribution to the strain gradient, thus the flexoelectric output directly reflects the singular effect. This unique property eliminates the special requirement for sensor placement coordinates and improves the accuracy of the stress intensify factor predication. A specimen with Mode-I crack was then prepared with two strain gradient sensors (4.7 mm × 0.9 mm × 0.3 mm) attached close to the crack tip to verify the analytical model for measurements of stress intensity factor. The experimental results matched well with the empirical estimation confirming that flexoelectric strain gradient sensing can be a convenient and accurate avenue for measuring the stress intensify factor.