A novel technique to generate sharp cracks in metallic/ceramic functionally graded materials by reverse 4-point bending

Accurate measurement of fracture toughness in brittle and ductile materials requires a sharp precrack at the machined notch. Cyclic tensile fatigue methods work well on ductile, but not brittle, materials. Uniform axial-compression methods developed for ceramic materials introduce microsturctural cracks leading to lower-than-expected fracture toughness. Pop-in methods usually develop longer precracks than desired for functionally graded material (FGM) beams and are difficult to control. Thus, precracking methods applicable to homogeneous metal and ceramic composite materials are not approapriate for FGMs. By using the present novel reverse four-point bending method, a controlled, sharp, short precrack can be developed at the machined notch in metallic/ceramic FGM beams, allowing accurate J-R measurements. This precracking method can be used in fatigue or monotonic loading. (CSA)