Abstract
Polycrystalline graphites are widely used in the metallurgical, nuclear and aerospace industries. Graphites are particulated composites manufactured with a mixture of coke with pitch, and changes in relative proportions of these materials cause modifications in their mechanical properties. Uniaxial tension tests must be avoided for mechanical characterization in this kind of brittle material, due to difficulties in making the relatively long specimens and premature damages caused during testing set-up. On other types of tests, e.g. bending tests, the specimens are submitted to combined stress states (normal and transverse shear stresses). The Iosipescu shear test, is performed in a beam with two 90° opposite notches machined at the mid-length of the specimens, by applying two forces couples, so that a pure and uniform shear stress state is generated at the cross section between the two notches. When a material is isotropic and brittle, a failure at 45° in relation to the beam long axis can take place, i.e., the tensile normal stress acts parallel to the lateral surface of the notches, controls the failure and the result of the shear test is numerically equivalent to the tensile strength. This work has evaluated a graphite of the type used in rocket nozzles by the Iosipescu test and the resulted stress, ~11 MPa, was found to be equal to the tensile strength. Thus, the tensile strength can be evaluated just by a single and simple experiment, thus avoiding complicated machining of specimen and testing set-up.
Highlights
Synthetic graphites are widely used in many engineering applications, such as those in the metallurgical, nuclear and aerospace industries, and as electrodes for ion-lithium batheries[1]
Tensile tests were performed on HLM graphite, with the applied load parallel to extrusion direction, following procedure described in the ASTM C 749 Standard by using the Instron universal testing machine[10]
The Iosipescu test procedure states that pure shear stress should occurs in the ac section (Figure 1) leading to a failure in between notch tips and shear strength can be calculated by Equation 2
Summary
Synthetic graphites are widely used in many engineering applications, such as those in the metallurgical, nuclear and aerospace industries, and as electrodes for ion-lithium batheries[1]. The most uniform and pure shear-stress state can be achieved in a material by applying torsional loading to a thin-walled tube specimen This kind of specimen is usually expensive to be produced and the testing procedure is not straightforward, because buckling failure along ±45° can take place. If the failure occurs perpendicular to this direction, which is caused by a principal normal tensile stress its value can be used to evaluate tensile strength This behavior is typical for brittle materials in which the deformations as well as the distortions are small, up to the failure load, and eventual geometric changes during the tests are negligible. Cracks and pores are common features that are found in graphite microstructure These defects are, undesirable structural parameters that significantly reduces the mechanical properties of the material, mainly the tensile strength. Weibull distribution statistics is currently a well established statistical tool to evaluate the significant variability of the mechanical test results which takes place in brittle materials and will be applied to the data collected from the Iosipescu shear test
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