Measuring and testing – Testing by impact or shock
Reexamination Certificate
2000-06-16
2002-05-21
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Testing by impact or shock
C073S012040, C073S012090
Reexamination Certificate
active
06389876
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to materials testing and, in particular, to material testing machines, test piece attachments used with the material testing machines and methods for conducting material tests with the material testing machines.
2. Description of the Related Art
There have been proposed and utilized various material testing methods for obtaining values of parameters characterizing different materials. Many material tests are conducted by applying a load to a test piece. In some material testing methods, a test piece is subjected to a static or quasi-static load, and the resultant strain of the test piece is measured to obtain a stress-strain relationship for the material. In some other material testing methods, an increasing load is applied to a test piece until it fractures, so as to determine the stress acting on the test piece at the fracture. There are still other material testing methods for various purposes. In any case, it is highly important to obtain exact values of the loads actually applied to the test pieces.
Most material testing machines include a load measuring means for determining the actual load acting on the test piece. There are proposed many types of load measuring means, among which a suitable one meeting the requirements and purposes of a particular material testing machine is selected and used in that material testing machine. The requirements depend in part on the material testing methods to be conducted with the machine. Material testing methods may be categorized, in terms of the load to be applied to the test piece, into tensile test, compression test, torsion test, shearing test, bending test and others. Material testing methods may be also categorized, in terms of the strain rate of the test piece to be produced, into high-strain-rate test, moderate-strain-rate test and low-strain rate test. Those material tests which are conducted at high strain rates may be also called impact tests.
For purposes of ensuring safety of buildings and other structures against collapse, protecting passengers of automobiles safe at collision, or achieving appropriate numerical simulations of working and/or forming processes of metal parts at actual deformation rates, it becomes more and more important to determine characteristics of materials when they are subjected to deformation occurring at different strain rates in a wide strain-rate range covering from relatively low strain rates to relatively high strain rates. Accordingly, there have been strong needs for material testing machines and material testing methods, in which material tests may be conducted at different strain rates in a wide strain-rate range, and in which, in particular, those of tensile tests which necessitate relatively large deformation of the test piece and require relatively high strain rate higher than 10
3
/sec. may be conducted with only a low level of noise found in the measured load waveform. So far, any tensile tests conducted at strain rates higher than 10
3
/sec. have been typically subjected to a relatively high level of noise in the measured load waveform.
With the difficulties in obtaining precision measurements of material tests conducted at relatively high strain rates, measurements of material tests conducted at relatively low strain rates have been commonly used as approximations of the actually required measurements, which have been, however, often only insufficient approximations. In contrast, by utilizing the present invention, one can obtain, with ease, precision measurements of material tests conducted at different strain rates in a wide strain-rate range including relatively high strain rates corresponding to the deformation rates frequently found in an actual environment. The availability of such measurements is highly useful for many applications. For example, it may be useful in development of durable materials for structural components and automobile's parts and components. It may be also useful in improvement of accuracy in various numerical simulations for determining the behavior of a designed structure or determining the mechanisms of forming and/or working processes which produces deformation of materials occurring at different strain rates in a wide strain-rate range.
In order to cause a test piece to produce a strain at a high strain rate, an impact load is applied to the test piece. A material testing machine including load applying means for applying an impact load to a test piece may typically also include load measuring means for measuring an impact load actually applied to the test piece.
There are proposed several load measuring methods for measuring an impact load actually applied to a test piece, among which Hopkinson bar method is commonly known and accepted. The original Hopkinson bar method has been modified in various ways into a range of variations of the Hopkinson bar method, some of which are used for conducting compression test, others are used for conducting tensile test, shearing test or other material tests.
Briefly, Hopkinson bar method uses one or two elongated bars (often called the Hopkinson bar) made of a strong and resilient material such as steel. A test piece is installed to one end surface of the single bar, or between the end surfaces of the two bars facing each other. When an impact load is applied to the test piece, a stress wave is produced at the end of the bar and propagates along the longitudinal axis of the bar toward the other end. The propagating stress wave produces a corresponding dynamic strain of the bar. Strain gages are affixed on the side surface of the bar, near the end of the bar to which the test piece is installed, in order to sense any dynamic strain of the bar. The sensed dynamic strain is used to determine the dynamic stress of the bar, which in turn is used to determine the dynamic load applied to the bar. The dynamic load applied to the bar corresponds to the dynamic load actually applied to the test piece.
From the dynamic load to the test piece thus determined, the dynamic stress of the test piece can be determined. Another means is used to determine the dynamic strain of the test piece. Then, the dynamic stress and the dynamic strain of the test piece are analyzed to determine characteristics of the material of the test piece.
The stress wave propagating from the first end (to which the test piece is installed) to the second end of the bar will be reflected by the second end to return back to the first end. The reflection would provide severe noise and disturbance to the measured load waveform if the measurement is not completed before the reflection reaches the strain gages. Thus, if one wishes to apply an impact load (or a load pulse) of relatively long duration to the test piece, the Hopkinson bar has to be long enough to provide a sufficiently long turnaround time of the stress wave propagating in the bar. Otherwise, the load measurement will be practically impossible due to the reflection of the stress wave. Indeed, for allowing use of a load pulse of significantly long duration, the bar may possibly have to be as long as ten meters or more. This leads to one of drawbacks of Hopkinson bar method that a material testing machine adopting Hopkinson bar method tends to occupy an extraordinary space. Further, for such a long bar, it is practically difficult to make precision calibration of the outputs of the strain gages with reference to the magnitude of the dynamic load (impact load) actually acting on the bar.
More recently, as an attempt to overcome the drawbacks of Hopkinson bar method described above, there has been developed another method for measuring dynamic load actually acting on a test piece, which uses, in place of a Hopkinson bar, a block of steel having a small projection. Examples of devices and methods using such a steel block are taught by Yoshitake CHUMAN, Kazuhiko KOTOH, Koichi KAIZU and Shinji TANIMURA in an article “Improvement of an Apparatus for Measuring Impulsive Force Generated at a Contact Part in
Takada Susumu
Tanimura Shinji
Yin Rongsheng
Allen Andre
Fuller Benjamin R.
Kabushiki Kaisha Saginomiya Seisakusho
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