Papers by Keyword: Surface Flaws

Abstract: This paper presents the crack growth mechanism from a 3-D surface flaw on gabbro specimens using strain measurement and acoustic emission (AE) technique. Based on the results of strain and AE measurement, microcracks initiated inside the rock and extend to the surface of the specimen. With the observation from the measurements, four types of crack patterns initiate wing crack, anti-wing crack (opposite direction of wing crack), petal crack and compressive crack. The strain values of anti-wing cracks are larger 1 to 2 times than that of wing crack. The AE energy release from anti-wing crack is higher 2.5 times than that of wing crack, while the energy release form wing crack is the least but the compressive crack is the highest. Thus, the appearance of initiation and propagation of the anti-wing crack and compressive crack are very actively than that of the wing crack. The strain and AE measurement is not only to provide a clear concept on the mechanisms of crack growth form a 3-D surface flaw but also to provide useful knowledge on the AE property of the crack patterns.

Abstract: A number of instability problems in rock engineering projects are caused by crack propagation. However, crack growth mechanisms from 3-dimentional flaw are not fully understood, in particular for 3-D flaw case with varied dipping angle. This study focuses on 3-D surface flaw using real rock specimens containing a flaw with varied inclination angle α from axial loading and dipping angle γ from specimen surface under uniaxial compression. Acoustic emission technique was used for tracing the initiation and growth of micro-cracks inside of specimen. It was found that crack growth process is affected by the dipping angle γ of the 3-D flaw. When dipping angle γ ≠ 90º, the thickness of rock above the flaw plane is thinner than that of below the flaw plane. As a result, compressive crack and wing crack initiated easily from the thinner flaw tips. And, the normalized stress for crack initiation σi /σc, AE events and the AE energy for crack growth decreases with the dipping angle γ. However, for γ = 90º, the thickness of rock above and below of the flaw tips is the same, it was observed that anti-wing crack (crack growth direction opposite to wing crack) initiated first at a certain place away from the flaw tips, then wing crack and compressive crack emerged at the late stage. For this case, the stress σi /σc, AE events and the AE energy for crack initiation and propagation are at a high value. Thus, for rock mass contains flaws geometry with small dipping angle, some problems of crack propagation may be induced easily during excavation.

Abstract: JSME Code on Fitness-for Service for Nuclear Power Generation Facilities defines allowable subsurface flaws for Class 1 ferritic vessels in the Acceptance Standard. When detected subsurface flaws are located near vessel surface, the subsurface flaws are transformed to be surface flaws for assessment of acceptability in JSME Code. This paper describes inconsistency of the subsurface flaws transforming to surface flaws and introduces an improvement method for the subsurface flaws.

Abstract: Biaxial testing was used to get insight into the incidence of the surface flaw properties (size, shape) on the strength of float glass specimens. Grinding grooves, Vickers' indentations and scratches as defects were introduced at the surface of annealed float glass specimens. The machined and fractured surfaces were observed using optical and confocal microscopes. The influence of the flaw characteristics on the strength of glass was evaluated and analyzed in the light of the fracture mechanics.

Abstract: Grinding may create flaws that control strength and limit the performance of finished ceramics. Machining cracks sometimes have been difficult or impossible to find especially in toughened ceramics with interlocking grain microstructures that create rough fracture surfaces. Our fractographic examinations show that machining damage leaves telltale markings on fracture surfaces that may be easily detected using common fractographic techniques. A comprehensive study with over 400 ground rods and rectangular bars was conducted on several commercial silicon nitrides to study the effects of various machining conditions. Similarities and differences in behavior were observed. A paradoxical finding was that tougher silicon nitrides developed deeper grinding cracks. Machining crack size and shape strongly depended on the grinding wheel grit size.