Biaxial Strength Test Research Articles

Comparison of Methodologies for Determination of Fracture Strength of 8mol% Yttria-Stabilized Zirconia Electrolyte Materials

The widely used yttria-stabilized zirconia (YSZ) electrolyte is subjected to thermal and external stresses under operation, so that the enhancement of the mechanical properties is an important issue in planar solid oxide fuel cells. Fracture strengths of 8mol% YSZ electrolytes as 100×100mm squire plates, 23mm disks, and 17mm disks were evaluated using plate tensile, ball-on-ring, and pressure-on-ring testing methodologies, respectively. Finite element analysis (FEA) was validated and used to calculate the stress distribution and peak stress for the biaxial strength tests. A Weibull analysis was carried out on the test∕FEA-predicted peak stresses, and Weibull strength, modulus, and material scale parameters were found for each test methodology. The methodologies were compared and evaluated based on the results of the Weibull analysis; the pressure-on-ring test is preferred for brittle thin-film fracture strength testing.

The ball on three balls test for strength testing of brittle discs: Part II: analysis of possible errors in the strength determination

Biaxial strength testing of brittle materials using the ball on three balls test (B3B test) is a useful tool for direct strength testing of disc specimens offering some advantages compared with the testing of bended beams: there are less problems with the alignment of the specimens in the fixture, in many cases the production of the specimens is easier, the area of maximum tensile stress amplitudes is far from the edges of the specimen (were often machining defects exist) and the testing procedure can easier be miniaturised. In a preceding paper, a proposal for a suitable testing set up was made, the stress fields in the disc specimens were analysed and first testing results were reported. A simple solution for the maximum tensile stress amplitude in the discs was given for a range of parameters concerning the geometry of the specimen, the supporting situation during the test and the material properties. In this paper, possible sources for deviations of this stress from the reported idealised solution (which would result in measuring errors) are discussed. For example, the not accounted influence of friction forces between the disc and the support balls, the influence of a possible buckling of the disc and the influence of geometric inaccuracies are analysed. It is shown, that the thickness of the disc is the most sensitive parameter determining the maximum tensile stress. Therefore it has to be determined with high accuracy.

The ball on three balls test for strength testing of brittle discs: stress distribution in the disc

Biaxial strength testing of brittle materials is claimed to have some benefits compared to uniaxial testing, e.g. the much simpler specimen preparation, the avoiding of tensile loaded edges, the similarity of the stress state to those from typical loading (e.g. during a thermal shock loading) and the fact, that biaxial stress states are more revealing of defects than uniaxial stress states. The experience of the past showed, that biaxial strength testing has its own problems, to avoid these led to the development of several variants. One of these variants, the ball on three balls test, seems to be extremely simple: a disc is supported by three balls and then axially loaded from the opposite side via a fourth ball. In this system small deviations from the requested geometry, especially some out of flatness of the disc, are mentioned to be tolerable, but the threefold bending symmetry makes an exact analytical assessment of the stress state in the loaded disc extremely difficult. A numerical approach has yet not been performed. In this paper a FE analysis of the stress state in a ball on three balls tested disc is performed. The stress field scales with the maximum principle stress, which occurs in the centre of the tensile surface. For this stress an analytical approximation (which has been fitted to the numerical results) is given, which accounts for the influence of all relevant geometrical and material parameters. The investigated range of parameters considers the values typical for testing of brittle materials.

A parametric study of the biaxial strength test for brittle materials

A parametric study of the biaxial strength test for two ceramic materials (alumina and glass) is reported. An attempt is made to establish certain variables which influence the biaxial strength of brittle materials. In particular, the effect of disc thickness, test rig diameter, loading ball diameter, load spreader under ball and finally rig design on the measured biaxial strengths for alumina and glass were considered. The measured biaxial strengths were observed to be significantly affected by the type of loading, disc diameter and loading ball diameter.

Extrapolation of Fracture Mirror and Crack‐Branch Sizes to Large Dimensions in Biaxial Strength Tests of Glass

Annealed glass disks were fractured in biaxial tension under a variety of conditions; crack lengths prior to branching and the sizes of the mirror and mist regions were measured. The experimental results indicated the expected correlation between fracture stress and these fracture features, except for an extrapolation to a consistent, clearly defined y‐intercept. Neither environment, surface preparation, nor the rate of applied stress was a factor in the fracture stress versus branch, mirror, or mist size relationships.

Effects of Interaction Component of the Generalized Yeh-Stratton Criterion

The effect of the interaction component of the generalized Yeh-Stratton cri terion is investigated in this paper. The criterion must be calibrated with an experimental result because the interaction component of the anisotropic criterion is generally unknown. The conditions on the interaction components are studied for the off-axis and biaxial strength tests separately, and the behavior of the generalized Yeh-Stratton criterion is observed as a function of the interaction component.

Microstruciture and Properties of Intermetallic Matrix Composites Produced by Reaction Synthesis

AbstractIn this US Bureau of Mines study, a variety of TiAl based composites were produced in situ by reaction synthesis. Mixtures of elemental Ti, Al and B and Ti, Al, and Si powders were reactive hot-pressed to form TiAl reinforced with 10, 20, 25 or 60 vol. pct. TiB2 or Ti5Si3. Microstructural evaluation of the resultant composites confirmed that the reaction products were primarily TiAl and TiB2 or Ti5S3, with a small amount of Ti3Al. The hot-press temperature and pressure had a significant effect on the density of the composites. In general, higher temperatures and initiating the reaction under pressure promoted dense composites. Room temperature biaxial flexure strength tests indicated that the addition of the reinforcing phases can improve the strength of TiAl. Potentiodynamic experiments revealed that TiAl, TiAl+TiB2 and TiAl+Ti5Si3 composites display active-passive corrosion behavior in both acidic and alkaline solutions.

Ball‐on‐Ring Test Revisited

A literature survey indicates that the ball‐on‐ring test is the best biaxial strength testing method. There is, however, some controversy in the calculation of the strength. In this communication, the theroretical controversy of the ball‐on‐ring test is resolved by strain and deflection analysis. Consequently, the ball‐on‐ring test can be used as a reliable method for testing the strength of flat brittle specimens.

Design and Development of a Biaxial Strength Testing Device

Abstract The apparatus described herein is designed for the performance of biaxial stretching tests on flat cross-shaped specimens. It can be used with a conventional uniaxial tensile testing machine. The device converts the machine from one that exerts a uniaxial force to one that applies a system of forces in two perpendicular directions. A fixed value of the logarithmic strain ratio ρ = ϵ2/ϵ1 can be imposed on the test specimen along these two directions. The fixed ρ-value may be chosen between 0 and 1 according to the geometrical configuration of the device.

Reliability of Brittle Materials in Thermal Shock

The fracture‐of alumina disks subjected to thermal shock is predicted from Weibull parameters derived from isothermal three‐point‐bend tests. Both the shear‐insensitive Batdorf model and the principle of independent action model are used to compare results from three‐point‐benr1 and isothermal biaxial strength tests to results from thermal fracture tests. It is found that the shear‐insensitive Batdorf model gives good agreement between mechanical tests and fracture of the thermally shocked disks.

Appraisal of biaxial strength testing

Three biaxial strength tests (ring-on-ring, piston-on-3 ball, and ball-on-ring) were evaluated using finite element analysis. Although in all three tests some uncertainties exist regarding the calculation of fracture stresses from the analytical equation, if fracture occurs within the loading ring, the ring-on-ring loading is thought to give the most accurate measure of strength. In addition, it was found that specimen shape (square vs. circular) had no effect on the stress distribution within the supports and that the stress at the edge of the specimen was less than 10% of the maximum stress for an overhang greater than about 40%. Fracture strength measurements on soda-lime glass gave support to these finite element results.