Fracture toughness

About: Fracture toughness is a research topic. Over the lifetime, 39642 publications have been published within this topic receiving 854338 citations.

Influence of hot pressing sintering temperature and time on microstructure and mechanical properties of TiB2 ceramics

Abstract: In this paper, a titanium diboride ceramic was produced by the hot pressing sintering method. The effects of hot pressing parameters on the TiB 2 ceramic microstructure and mechanical properties were studied. The bending strength and fracture toughness were measured by three point bending testing and single edge notched bending tests (SENB), respectively. The microstructure features of the TiB 2 sintered material were revealed by means of SEM and TEM. The results show that the TiB 2 grain size increases quickly with the increasing temperature and time during hot pressing sintering. The density and the TiB 2 grain size have a great influence on the mechanical properties. The bending strength decreases with increasing TiB 2 grain size, whilst the fracture toughness increases.

The Effect of Triaxial Stress Systems on the Strength of Rocks at Atmospheric Temperatures

Abstract: Summary The experimental work described in this paper was carried out in order to discover more about the laws governing the effect of the stress system on brittle fracture, with particular reference to rocks. Consideration of the implications of Griffith's theory of brittle fracture when an arbitrary, uniform plane stress system is applied (Murrell 1964a, b) enabled a number of fracture criteria to be postulated for triaxial stress conditions which required experimental investigation. An apparatus has been built which enables cylindrical rock specimens to be subjected to various combinations of principal stress (tension with compression, triaxial compression with the intermediate principal stress equal either to the major or the minor principal stress) with or without a controlled pore pressure, and at the same time enables the axial and diametral strains to be measured. Confining pressures of up to 4kb can be applied, with pore pressures up to 2kb. A new method of measuring the tensile strength of very brittle materials is described. Experiments were carried out on an isotropic sandstone. It is shown that with one principal stress tensile fracture occurs at a critical value of the tensile stress (=K′) within the range of hydrostatic stresses which were studied. When none of the principal stresses is tensile fracture occurs when the shear stress (τ) on the fracture surface reaches a value which depends on the normal stress (σ) according to the law where λ (4K′)½ and n 0.61. Where pore pressures are applied ‘effective stresses’ must be used in the equation (that is, s must be replaced by σ′=σ+p, where p is the pore pressure). The intermediate principal stress has a slight but measurable effect on the fracture criterion. After fracture the load supported by the rock falls to a value which can be supported by friction between the fracture surfaces. It is found that Amonton's law of friction is not obeyed, and instead the shear stress () due to friction and the normal stress () follow a law where μo 2 and n 0.9. Again ‘effective stresses’ must be used in the equation when pore pressures are applied. Pore water lowers slightly the parameters λ and μo by some form of chemical action. When the shear stress required to cause fracture becomes equal to or less than that required to overcome sliding friction, fracture is no longer possible and there is a transition to ductile behaviour. This occurs at a confining pressure of ∼ 17 000 Lb/in2 (1.17 kb) in the case of the sandstone used in the experiments. Interesting fractures occur sometimes, after the confining pressure is removed, in specimens which have been plastically deformed.

Effects of carbide size and Co content on the microstructure and mechanical properties of HVOF-sprayed WC–Co coatings

Abstract: Twelve commercially available WC–Co powders with different average WC grain sizes (0.2, 2, and 6 μm) and cobalt contents (8, 12, 17 and 25 wt.%) were sprayed on carbon steel substrates using High Velocity Oxy-Fuel (HVOF) spraying process. Hardness, Young's modulus, and fracture toughness of the coatings were measured. While the hardness and Young's modulus decreased with increasing cobalt content from 1600 to 1100 Hv and from 400 to 300 GPa respectively, the fracture toughness remained in the range from 4 to 6 MPam 1/2 . The coatings with 2 μm carbide showed lower hardness than those deposited from 0.2 and 6 μm carbide. These measured mechanical properties were discussed with the help of microstructures of the coatings investigated by scanning electron microscopy, X-ray diffraction and chemical analysis. Finally, the hardness of the binder phase in these coatings was estimated to range from 1000 to 1300 Hv by applying the mixture rule for composites to the experimental data, demonstrating that such hardening of the binder phase is a key factor affecting the mechanical properties of the coatings.