Fracture strength and crack propagation behaviour of MgO sheet specimens under tension and an analysis of X-ray diffraction profile under bending

Small-size smooth specimens of the high purity MgO and the similar fine-ceramics sheet materials are fractured under tension at room temperature. Here, the specimen's design and size and the way of loading tensile force, used for the previous investigation on the fracture of SIALON sheet specimen, are employed and the variation and scattering in fracture strength due to the materials i.e., the high purity MgO and the other five kinds of similar fine-ceramics materials, are studied and compared. A center-notched and a single edge-notched medium-size tensile sheet specimens having various tip-radius are fractured under tension at temperatures of 20℃ and 1000℃. Notched fracture strength is found to be controlled by not only the notch geometries but also the stress field around the notch tip-radius due to the thermal expansion. Fracture strength, of course, shows the strong dependence on the notch tip-radius (ρ) down to ρ≒15μm and it shows very little dependence at ρ<15μm, and we estimate the fracture toughness value of the high purity MgO sheet specimen based on the fracture strength of the specimens having ρ≒15μm. The crack propagation behaviour of the MgO sheet specimen during a cycle of heating up to 1000℃ and cooling down to 20℃ under free hanging in the furnace is examined. The crack proceeded straightly in the first stage and curved upside or/and downside and then it stopped. The detail mechanism of this phenomena is very complicated and is not known at present. The average crack propagation velocity of the single edge-notched specimen is measured at 300℃ and 20℃. The former gives about 53m/s and the latter does about 1830m/s and this says that the average crack velocity is faster the stronger the fracture strength. Analysis of the X-ray diffraction pattern (Cu target) is made for the MgO sheet specimen at various elastic strain under bending. The integrated intensity ratio and the relative half-width ratio of the (111) peak profile show the more correspondence than the other planes' profiles with respect to the elastic tensile strain and the former decreases with increasing the strain and the latter increases with increasing the strain.