Strength-limiting Flaw Research Articles

Mechanical evaluation of WC-Co materials with varying microstructures

Tungsten carbide‑cobalt materials are useful in a variety of extreme applications due to a desirable blend of properties, yet the technology has not significantly changed since their initial development in the 1920s. The mechanical properties of this class of materials is highly dependent on two variables, the size of the tungsten carbide grains, and the amount of binder phase present in the final body. In this study, the amount of binder phase is isolated across three commercial materials from the same manufacturer with three different grain sizes to investigate the effect on mechanical properties. The mechanical properties investigated are indentation hardness, flexure and tensile strength, as well as fracture toughness. In general, an increase in hardness and tensile strength with decreasing grain size was observed, while the fracture toughness showed the opposite trend with toughness increasing with increasing grain size. The flexure strength results did not show a correlation to grain size. Fractographic analysis identified the dominant strength-limiting flaw for each sample, which largely were in the form of porosity. Other flaws types, such as inclusions from the milling process, clusters of large grains, and machining cracks from the surface finishing process, were also identified. Weibull analysis was performed and deemed appropriate for analysis of these materials, but strength-size scaling was not conducted due to the variability in the strength-limiting feature between the different specimen geometries.

Investigating the flexural strength of recycled cast glass

Currently, tons of high quality commercial glass are down-cycled or landfilled due to contaminants that prevent close-loop recycling. Yet, this glass is potentially a valuable resource for casting robust and aesthetically unique building components. Exploring the potential of this idea, different types of non-recyclable silicate glasses are kiln-cast into 30times 30times 240 mm beams, at relatively low temperatures (820–1120,^{circ }hbox {C}). The defects occurring in the glass specimens due to cullet contamination and the high viscosity of the glass melt, are documented and correlated to the casting parameters. Then, the kiln-cast specimens and industrially manufactured reference beams are tested in four-point bending, obtaining a flexural strength range of 9–72 MPa. The results are analysed according to the role of the chemical composition, level of contamination and followed casting parameters, in determining the flexural strength, the Young’s modulus and the prevailing strength-limiting flaw. Chemical compositions of favourable performance are highlighted, so as critical flaws responsible for a dramatic decrease in strength, up to 75%. The defects situated in the glass bulk, however, are tolerated by the glass network and have minor impact on flexural strength and Young’s modulus. The prerequisites for good quality recycled cast glass building components are identified and an outline for future research is provided.

Uniaxial tensile strength and fracture analysis of a hot-pressed boron carbide

Tensile specimens with three different gage dimensions were used to determine the tensile strength of a hot-pressed boron carbide. The strength-limiting flaw population and information on the fracture behavior under an applied tensile stress were also determined. The material follows traditional strength size-scaling for advanced ceramics, and flexure strength data for the same boron carbide was an excellent fit with the uniaxial tensile strength data. The strength-limiting flaw in all specimens was a carbon-based inclusion or agglomerate. The size of the fracture mirror around the origin and the fracture toughness value estimated from the measured strength-limiting flaw size are in excellent agreement with previously reported data. This information shows that standard four-point flexure strength tests can be used to confidently predict the uniaxial tensile strength of this hot-pressed boron carbide.

Thermal shock behavior of pressureless liquid phase sintered SiC ceramics

The thermal shock behavior of pressureless liquid phase sintered (PLPS) SiC ceramics with Al2O3 and Y2O3 as sintering additives was investigated by using water quenching method with the temperature difference from 200°C to 1400°C. It was found the residual flexural strength of PLPS SiC dropped quickly to about 100MPa when the temperature difference reached 600°C due to the cracks induced by thermal stress. The residual flexural strength of the PLPS SiC increased when the temperature difference reached 1300°C and 1400°C due to the formation of SiO2 oxide layers, which acted as thermal barriers to reduce the thermal stress on the surface of the SiC ceramics. However, comparing the microstructure of specimens quenched at ΔT=1300°C and 1400°C, the slight decrease of residual flexural strength at ΔT=1400°C was due to the formation of large amount of through pores in the oxide layer, which was speculated to be the strength-limiting flaw above 1300°C.

C-Sphere Strength-Size Scaling in a Bearing-Grade Silicon Nitride

A specimen geometry was used to determine the failure strength distributions of a commercially-available bearinggrade silicon nitride (Si₃N₄) with ball diameters of 12.7 and 25.4 mm. Strengths for both diameters were determined using the combination of failure load, C-sphere geometry, and finite element analysis and fitted using two-parameter Weibull distributions. Effective areas of both diameters were estimated as a function of Weibull modulus and used to explore whether the strength distributions predictably scaled between each size. They did not. That statistical observation suggested that the same flaw type did not limit the strength of both ball diameters indicating a lack of material homogeneity between the two sizes. Optical fractography confirmed that. It showed there were two distinct strength-limiting flaw types common to both ball diameters, that one flaw type was always associated with lower strength specimens, and that a significantly higher fraction of the 25.4-mm-diameter Csphere specimens failed from it. Predictable strength-size-scaling would therefore not result as a consequence of this because these flaw types were not homogenously distributed and sampled in both C-sphere geometries.

Probabilistic strength of {1 1 1} n-type silicon

The two-parameter Weibull strength distribution of {1 1 1} n-type silicon prismatic bars was determined in four-point bending and analyzed as a function of specimen size (width), loading rate, two different crystallographic orientations, and specimen orientation (polished or etched surface in tension). 100% fractography was performed to classify strength-limiting flaw types and to censor the strength data. All flaw types were extrinsic. Machining or cutting damage in the form of chipped edges (an edge-type flaw) was the dominant strength-limiting flaw when the polished surface was subjected to tensile stress, while a flat-bottomed etch pit (a surface-type flaw) was the dominant strength-limiting flaw when the etched surface was subjected to tensile stress. The censored Weibull strength distribution was independent of specimen width, loading rate (indicative of slow crack growth insusceptibility), and the two crystallographic orientations; however, it was dependent on specimen orientation. Pooling of the strength data was employed to tighten the confidence intervals about the censored parameters. The results from this study indicate that different extrinsic strength-limiting flaws and strength distributions will be operative depending on the manner in which a silicon component is stressed.

Prediction of the Strength of Ceramic Tubular Components: Part II—Experimental Verification

Abstract The strength distribution of reaction-bonded silicon carbide tubes that failed by internal pressurization was predicted from strength distributions obtained from simple laboratory test specimens at room temperature. The strength distributions of flexure bars, C-rings tested in tension, C-rings tested in compression, diametrally compressed O-rings, and internally pressurized short tubes were compared with the strength distribution of internally pressurized long tubes. The methodology involved application of Weibull statistics using elasticity theory to define the stress distributions in the simple specimens. The flexural specimens did not yield acceptable results, since they were ground before testing, thereby altering their flaw population in comparison with the processing-induced flaw populations of the tubular specimens. However, the short tube internal pressure test, the C-ring tested in tension, and the diametrally compressed O-ring test configurations yielded accurate strength predictions of full-scale tubular components, since these specimens more accurately represent the strength-limiting flaw population in the long tubes.

Rupture of inhomogeneous spherical clusters by simple flows

We extend models for the rupture of particle clusters developed by a number of authors to the case of the rupture of inhomogeneous spherical clusters of low porosity having a single strength-limiting flaw. With a view toward the design of mixing equipment, we examine the tendency of these clusters to break in 4 prototypical flow geometries: simple shear, pure elongation, uniaxial extension and biaxial extension