Fracture Testing of a Layered Functionally Graded Material

This paper describes measurements of the K_I R-curve for a layered ceramicmetallic functionally graded material (FGM) composed of Ti and TiB phases. Single edge notch bend specimens were fabricated for crack propagation perpendicular to the graded layers from the brittle to the ductile side of the FGM. The precracking method and residual stresses affected the measured toughness. A new reverse bending method produced a sharp precrack without damaging the material. A representative sample indicates R-curve behavior rising from K_meas = 7 MPa∙m^1/2 in the initial crack-tip location (38% Ti and 62% TiB) to 31 MPa∙m^1/2 in pure Ti. Residual stresses acting normal to the crack-plane were measured in the parent plate using the crack compliance technique. The residual stress intensity factor in the SE(B) specimen determined from the weight function was -2.4 MPa∙m^1/2 at the initial precrack length. Thus, the actual material toughness at the start of the test was 34% less than the measured value.