BS ISO 18608-2015 pdf download – Fine ceramics (advanced ceramics, advanced technical ceramics) — Mechanical properties of ceramic composites at ambient temperature — Evaluation of the resistance to crack propagation by notch sensitivity testing.
4? Significance? and? use The fracture toughness is a material property which characterizes the initiation of fracture from a sharp crack (usually obtained by fatigue cracking under plane strain conditions). The fracture toughness of materials at the onset of crack extension from a pre-existing fatigue crack is characterized by the value of one of the following parameters: a) K Ιc, a critical value of K Ι (the stress intensity factor of the elastic stress field in the vicinity of the crack front), at the point of instability of the crack extension; b) G Ιc , a critical value of G Ι (the strain-energy release rate with crack extension per unit area of newly created crack surface) at the point of instability of the crack extension; c) J Ιc , a critical value of J Ι (a line or surface integral used to characterize the local stress-strain field around the crack front) at the onset of stable crack extension. The J integral plays an important role in nonlinear fracture mechanics. It applies to nonlinear elastic bodies, whereas linear elastic fracture mechanics (K Ιc and G Ιc ) consider linear elastic bodies. Several problems arise in determining and even in defining K Ιc , G Ιc and J Ιc in fibre reinforced ceramic matrix composites, as a result of the following features: a) CMC are generally highly heterogeneous, consisting of different constituents (fibres and matrix), and containing pores and cracks; b) in some CMC a damage zone of multiple matrix cracks forms ahead of a notch prior to ultimate failure; c) the associated deformations are nonlinear.
The load versus load line displacement curve from a fracture test on a notched specimen involves a nonlinear domain induced by diffuse micro-cracking within the matrix at the notch tip. The damage zone is in the millimetre to centimetre scale (from one to several tow diameters). At maximum load, a macroscopic crack is created from the random failure of fibres within those tows located in the damage zone. Crack extension in CMC, hence, does not result from the mechanism of extension of a single macroscopic crack as observed in monolithic materials. Because of the presence of the damage zone and of heterogeneous microstructure, the stress distribution in the damage zone differs from the one induced ahead of the crack tip in linear elastic bodies. The K Ι parameter does not describe the stress field in the region ahead of the crack tip. A critical value K Ιc cannot be defined. The main difficulty in the determination of the strain energy release rate G Ι , as well as the J integral, results from the presence of the micro-cracked zone at the notch tip (which is not small compared with the specimen dimensions) and the jagged surface of the macroscopic crack. As a consequence an increase in crack length can neither be easily defined nor measured.