Measuring and testing – Testing by impact or shock
Reexamination Certificate
1999-09-16
2001-07-31
Noori, Max (Department: 2856)
Measuring and testing
Testing by impact or shock
C073S598000
Reexamination Certificate
active
06266994
ABSTRACT:
This invention relates to improvements in or relating to measuring properties of materials or structures and is more particularly but not exclusively concerned with measuring the mechanical properties of brittle construction materials or the mechanical properties of brittle geological structures. An exclusive performance of some improvements as described herein is that of measuring the mechanical properties under dynamic and quasi-static loading of a material specimen both as a global value and as value distribution over the specimen cross section.
Mechanical characteristics (given by stress strain curves, the influence on those curves of temperature and strain rate, fracture energy details of deformation mechanisms and so on) have been studied successfully in plastics, pure metals and alloys and the knowledge gained thereby has provided a scientific base towards optimum production and practical use of such materials. However, the situation with brittle materials or structures is much more complicated. From a practical standpoint, conventional test installations only allow the rupture limit of such materials to be measured with a corresponding large scatter or dispersion owing to the influence of surface micro-cracks induced in the material or structure under test. There is an ever increasing need to reduce the enormous losses due to earthquake and accident impact loading and thus there is a need to obtain reliable information about rupture generation and development in brittle materials of high importance such as concrete, reinforced concrete, ceramics, composites and rock materials: Here an important point is the measurement of the energy released as mechanical wave during the fracture development in the brittle construction materials or in the rock materials; when the masses and volumes of rocks are very large the energy release during fracture gives origin to seismic waves whose precise measurement remains an open problem.
It is an object of the present invention to provide apparatus (and method) for measuring properties of materials or structures more particularly during deformation and fracture which is improved in at least some respect.
According to one aspect of the present invention there is provided apparatus suitable for measuring mechanical properties of characteristics of a brittle material or structure, said apparatus comprising a Hopkinson bar or pressure bar system comprising an input bar bundle and an output bar bundle, each said bundle comprising a plurality of parallel bars each equipped with instrumentation for measuring values of longitudinal stress wave or crack propagation parameters in a brittle material specimen or structure under test located, in use, in between the input and output bar bundles.
Further according to this aspect of the present invention there is provided a method of measuring mechanical properties or characteristics of a brittle material or structure, said method comprising placing a Hopkinson bar or pressure bar system, comprising an input bar bundle and an output bar bundle, under load, and thus placing a brittle material specimen or structure located, in use, in between the input and output bar bundles, under load, and measuring values of shear wave or shear crack propagation parameters in the brittle material specimen or structure from instrumentation provided on a plurality of parallel bars forming each said bar bundle.
Usually, each bar o: the bundle gives the local mechanical properties of the part of specimen cross section facing of the bar; while summing the measurements of the bars of the bundle one obtains the global mechanical properties of the material specimen or structure.
Means may be provided for placing the bar bundles and thus the material specimen or structure under test in compression in a direction along the axis of the bar bundles. In one embodiment of the apparatus each bar of the bundles is equipped with instrumentation to measure locally the stress-strain relationship of the material specimen or structure under test in addition to measuring values of absorbed energy during rupturing of the material specimen or structure under test, as well as values of shear wave propagation with a known component of compression stress in a crack plane, the orientation of a rupture surface.
In a further embodiment of the apparatus, each bar of the bundles may be equipped with means for measuring the velocity of a shear crack propagation where the material specimen or structure under test is placed in “pure shear”. “Pure shear” means that there is no normal stress in the plane of shear. In this instance, the bar bundles are used in an unusual manner for a Hopkinson bar or pressure bar system in that they are used as receptors of waves emitted by propagating cracks and thus giving a measure of the energy released by crack propagation.
In a further embodiment of the apparatus, each bar bundle is arranged to measure the stress-strain relationship in the material specimen or structure under test in addition to measuring values of energy absorption during deformation of said material specimen or structure and the velocity of shear crack propagation therein in conditions of controlled “hydrostatic” pressure. The term “hydrostatic” as applied in this instance means that loading of the material specimen or structure under test is combined from equal stress-hydrostatic components plus excess load along one direction.
Although the above apparatus is suitable for measuring properties of brittle materials, it could also be used for investigating rupture processes in plastic materials at the stage of a neck development.
Additionally, means may be provided for placing the Hopkinson bar bundles and material structure under test in tension, each bar being provided with instrumentation to measure the stress strain relationship in a particular region of the material specimen or structure, values of absorbed energy during the rupture process and values of longitudinal wave propagation within the material or structure, as is already known from the Paper already presented by the inventors entitled “Study of the true tensile stress strain diagram of plain concrete with real size aggregate; need for and design of a large Hopkinson bar bundle” appearing in the Journal de Physique IV Colloque C8, supplement au Journal de Physique III, volume 4- September 1994, the entire content of which is hereby included into the present specification by reference. The aspect of the present invention as aforedescribed represents a further development over and above the arrangement shown in this paper.
The aforementioned paper only mentions placing the Hopkinson bar bundles in one dimensional tension and there is no mention of instrumentation for obtaining values of shear wave propagation or velocity measurement of shear crack propagation.
Embodiments of the present invention may provide instrumentation for measuring the following complex of deformation characteristics of brittle materials:
The stress-strain relationship and energy absorption in the material specimen or structure under test during the deformation process, including during the falling part of the load, in conditions of simple stress-state (e.g. tension or compression) and in conditions of complex stress-state (e.g. tension—tension, under hydrostatic pressure) and accurate measurements of propagation velocities of both longitudinal and shear waves in such conditions and therefore have a measure of energy release from a propagating fracture which is the basic phenomenon of an earthquake.
The Hopkinson bar system is widely used for the study of mechanical characteristics of materials in conditions of high velocity deformation (see for example the Paper by U.S. Linholm entitled “Some experiments with the split Hopkinson pressure bar, J. Mech.Phys Solids 1964 volume 12 317 to 335). The principal feature of such system is the use of two sufficiently long elastic bars (input and output bars) located on opposite sides of a sample for test and analysis of the deformation process through signals of incident, re
Albertini Carlo
Mogilevsky Mikhail
European Atomic Energy Community
Noori Max
Welsh & Katz Ltd.
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