Specimen geometric configuration for uniform shear distribution during testing

Abstract: The present invention deals with the geometric configuration of a (metallic, ceramic, composite or geo-material) specimen in order to achieve uniform distribution of shear stresses-strains in a predefined region during the application of countercurrent load pairs. In addition, this invention includes the development of a specimen heating technique adapted to the proposed geometric configuration and capable of attaining high temperatures during the testing procedure. The proposed geometry is characterised by the existence of a semicircular groove that runs along the (cubic or sheet shaped) body of the specimen (drawings 1,2). An extra thin heating element (wire covered by ceramic tape) is wound along the upper and lower grooves and covered by a boron nitride layer in order to increase the temperature in the test section. Representative shear tests prove the functionality and efficiency of the proposed configuration.

Description: The present invention refers to the geometric configuration of a specimen (metallic, ceramic, composite or geo-material) that, when properly subjected to the action of pairs of loads, ensures the development of uniformly distributed shear stress-strain in the test section interior, even in the case of large material deformation. In addition, this invention refers to the development of a technique adapted to the proposed specific design, in order to heat the specimen to temperatures that may reach 1900° C during the shear tests. The major scientific and technical fields related directly with the invention include Mechanics of Materials. Technology and Strength of Metals, Alloys, Ceramics, Composites and Geo-materials and Material Testing Procedures. Despite the fact that significant shear stresses may develop under realistic loading conditions in materials (with important effects on the design of structures), the efforts of acquiring proper design data (especially for composite materials) over the last decades have been encountered with severe difficulties concerning the determination of in-plane shear modulus and strength. The problem stems primarily from an inability to define a specimen geometry and loading configuration which will induce a state of uniform shear in the test section. Such a state is a necessary condition for the derivation of useful conclusions from the tests. Indeed, there exists presently no generally accepted experimental technique for the measurement of in-plane shear response of materials and particularly sheet shaped materials. While many different test methods are currently available (e.g. off-axis tension test, Iosipescu shear test, Arcan shear test, rail-shear test etc.), the results obtained from these tests have often proved inconsistent. In addition, the volume of the specimen undergoing uniform shear is very limited and, therefore, the measured strength may not be representative of the material. Depending on the particular testing procedure, the variations in the reported values can be attributed to the state of shear stress in the test sections of the specimens, which is neither pure nor in most cases uniform. A method for configuring a sample is known from SU-A-888 001. Referring to the two most common shear test methods for composites, i.e. the off-axis tension test and the Iosipescu shear test, previous studies have shown that consistent results can be obtained only on the basis of proper interpretation and after corrections that take into account the nonuniform test section stress distribution. In the case of the off-axis tension test, the end- constraint effect introduces an additional shear stress component at the clamped edge of the test coupon that must be included with the applied longitudinal stress in the transformation to the principal material coordinate system. When the Iosipescu shear specimen in considered, on the other hand, correction factors have been computed using finite element simulations for the nonuniform shear-stress distribution. The situation worsens in the case of large specimen deformation under the shear loading. Furthermore, the modern technological needs for high temperature testing add to the difficulties encountered, as the demand for uniform temperature distribution in the test section must be taken into account. The invention deals with the development of a specific geometric configuration of the specimen and a corresponding heating technique such that the above requirements are met. Specifically, following the examination of several geometries, the configuration shown in drawing 1 has been selected. The test section of interest, where the developing pure shear stresses are uniformly distributed, consists of a zone along the centerline of the specimen characterized by the semicircular groove on its surface. The shear stress is easily calculated from the applied boundary forces (without recourse to constitutive equations), assuming only specimen equilibrium (Drawing 1): τ xy = L 1 · σ a - L 2 · σ b AB . This specific geometrical shape has been selected among others (elliptical etc.) after detailed 3-D finite element simulations of the stress field on suitable grids for cubic (Drawing 2) and sheet shaped (Drawing 3) specimens. The same computational study revealed the optimal size and location of the regions on the specimens upper and lower boundaries where loading should be applied in order to achieve uniform shear conditions at the test section. Four pairs of paraller strip loads are applied in a way such that a uniform shear field is generated in the central region (Drawing 1: Y1 Y2 Y3 Y4). The distribution of the shear stress τ xy is depicted in Drawing 4, while Drawing 5 shows the distribution of τ xy , σ x , σ y from the specimen centre on the x, y plane. The sheet-shaped specimen, (usual case for composites) has dimensions typically of the order of 50x50x7 mm 3 . In the case of cubic specimen, the size may reach 20 cm. This is a significant feature for geo-materials because it allows taking into account the tectonic discontinuities of the material during mechanical testing. It is furthermore possible to study the behaviour of cubic concrete specimens with small dimensions of aggregates. Turning to the heating aspects, the proposed technique involves the use of an extra thin wire (resistance) that is carefully wound along the upper and lower grooves of the cubic or sheet shaped specimen. The heating element is a Cr-Ni wire 0.3 mm in diameter, which is covered by ceramic tape for better electrical insulation. The ceramic tape has been immersed in boron nitride to improve its thermal conductivity. The overall system (wire, ceramic tape) is finally covered by a boron nitride layer for better inslulation from the ambient and more efficient fixing during temperature variation and load application. The attained temperature uniformity depends essentially on the overall loading system design as this imposes the relevant boundary conditions. To avoid undesired overheating, cooling (by air or water circulation) should be applied on the loading system. The experiments done to verify the functionality and durability of the heating system and the insulators and to measure the uniformity of temperature, took place by letting the system to reach steady state at temperatures of the order of 400°C and 800 °C for several hours. No particular operational problems were observed with the required power, while the temperature variation in the test section was of the order of 1%. The specimen configuration as described above, with the loading and heating system, was used in a special device designed to apply the aforementioned pairs of loads and allowing observation of the specimen, with quite satisfactory results. More specifically, an adequately uniform distribution of shear stress-strain inside the test section was achieved, as shown by the comparison of the numerically calculated shear strain (under various stress states at twelve points near the centre of the specimen) with the measured strain using an optical system (drawing 6) and the recordings of a three-gage 45-deg rosette. All these independent methods resulted in good agreement of the data. To summarise, the new geometric configuration of the specimen (cubic or sheet shaped) generates a three dimensional region (with a width of at least 5 mm) characterised by uniformly distributed shear stresses, a feature that improves the current state-of-the-art. The latter exhibits uniformity of stresses that is restricted to a two-dimensional zone of the material.

Claims: A method of geometrically configuring a cubic or sheet shaped metallic, ceramic, composite or geo-material specimen, in order to achieve uniform shear stress-strain fields in its interior when subjected to shear loading, wherein two mutually opposite, symmetrical grooves extending along a centerline of the specimen body are formed and characterized in that the region of material left between them is only a few millimeters thick and in that the bottom of the grooves is semicircular, so that a uniform shear stress/strain field is attained in the central region (Y1, Y2, Y3, Y4) between said grooves of the specimen. The method of claim 1 further comprising the step of heating the specially configured specimen by winding a thin heating wire, covered by ceramic tape, along the grooves of the specimen, said wire is covered further by a boron nitride layer for better insulation, fixation and contact with the specimen body.

Inventors: MICHELIS PAUL, MICHELIS, PAUL
Applicants: INST MICHANIKIS YLIKON KAI GEO, INSTITUTO MICHANIKIS YLIKON KAI GEODOMON A.E.
Application year: 1995
Main classification: G01N3
IPC codes: G01N3