Crack tip stress fields revealed by infrared photoelasticity in silicon crystals

Stress fields around a crack tip in silicon crystals have been investigated by using infrared photoelasticity with the aim of clarifying the shielding effect due to crack tip dislocations on the steep increase of fracture toughness in the brittle-to-ductile transition (BDT). First, compact tension tests were carried out at room temperature to make in situ observation of elastic behavior of crack tip stress fields. The photoelastic images observed were in good agreement with those simulated for the usual elastic fields around the tip of a mode I crack. Next, to clarify the stress modification due to crack tip plasticity, three-point bending tests were also made by using notched specimens at high temperatures around 1000 K. After the high temperature test, in spite of the absence of the applied load, residual bright images were observed around the notch. Those images correspond to an internal stress due to dislocations multiplied around the notch, and they have an effect of shielding (accommodating) the stress concentration due to the applied load. The fracture toughness at room temperature was increased by the introduction of the residual stress.