FIELD OF INVENTION

The present invention relates to a shear extensometer for measuring the shear-strain a given specimen.

BACKGROUND OF THE INVENTION

Extensometers are commonly used for measuring very small extensions (axial strain) over a particular gauge length when tensile or compressive force is applied on a test specimen. These small extensions are not readily identifiable with the naked eye, or significant enough that someone can clearly visualize them. The method of measuring such minute variations using extensometers is used extensively in materials testing.

Consider the schematic of an axial displacement as illustrated in figure 1 a. Figure 1 a shows two points A and B on a surface. The distance separating them is the gauge length, L. Strain along the axis passing through A and B is termed axial strain. It will be given by the ratio of their relative displacement, -'Ί . This is the basis of operation of all axial extensometers used in material characterization to determine axial as well as transverse stress-strain properties. Such extensometers are required in materials testing to characterize material properties including modulus, yield stress, cyclic stress- strain curve, etc. During a tensile test, points A and B are aligned with the loading direction indicated by the bi-directional arrow in Fig. 1 a. Hence, the definition of axial strain. If the axis of extensometer mounting is rotated 90° with respect to the loading direction, the same extensometer can measure transverse strain that is essential, for example, to characterize Poisson Ratio. Axial and transverse strain can also be measured by pasting a strain gauge aligned along the targeted strain direction. Consider the schematic of a shear displacement as illustrated in figure 1 b. As opposed to axial strain that characterizes the extension or compression across a given gauge length, shear strain characterizes the sliding response of a material. An easy to comprehend example is the response of a cylinder when twisted around its axis. Another example is when a pure shear force is applied to a specimen of rectangular cross section. Neither of these responses can be correctly measured with conventional axial extensometers because they involve movement of reference points that are not coaxial. Consider the rectangle UVWX as shown in figure 1 b. If the line UX is displaced by ^ as shown in the figure, UVWX can be treated as having experienced a shear strain given by *L , where L is the gauge length represented by UV or WX. Shear strain is usually measured in materials characterization by pasting a strain gauge along the diagonal D1 or D2.

Conventional high temperature axial extensometers equipped with ceramic arms are used for measurements of axial displacement up to 1 200°C. These ceramic rods typically have a tapered edge that ensure line contact on flat specimen surfaces and point contact on cylindrical specimens. The contact points of the two rods with the specimen surface constitute points A and B mentioned earlier with reference to figure 1 a. Such an extensometer cannot be used for shear strain measurements because its contact points will always lie on a plane that is parallel, if not coincident, with the one that passes through the arm axes.

US 5,71 2,430 teaches a strain extensometer with knife-edge arm tips for measuring strain in a test specimen. The mounting tips of the '430 patent target displacement between the centres of their flat sharp tips, with both centres falling on the same axis of expected (axial) displacement. Such an extensometer cannot be used for shear strain measurements because its contact points will always lie on a single plane that is parallel, if not coincident, with the one that passes through the arm axes.

Thus, there remains a need to provide an extensometer that would serve the purpose of measuring shear displacement across a gauge length. SUMMARY OF THE INVENTION

The invention is based on the very nature of pure shear, which, over a gauge length, can be characterized by the relative movement of two diagonally positioned points, as shown in figure 1 b. The movement of these points is strictly along parallel axes, whose spacing remains unchanged. It would follow, that an extensometer suitably designed to make contact at points lying on parallel but offset axes, such as points U and W of figure 1 b would serve the purpose of measuring shear displacement across a gauge length determined by the lateral spacing of its mounting points. Accordingly, the shear extensometer for measuring shear responses of a specimen according to this invention comprises a first arm having a first mounting tip and a second arm, parallel to the first arm, and having a second mounting tip. The mounting tips are profiled to make point contact at diagonally opposite points on parallel, offset axes on the specimen. The arms of the extensometer according to this invention move strictly in the plane described by the arm axes. However, the arm tips make contact with diagonally opposed points on the surface of the specimen.

In an optional embodiment, a shear strain gauge is pasted on the specimen between the mounting tips of the shear extensometer of this invention. The shear strain gauge can serve as a reference to calibrate readouts if required and to account for possible errors in estimating effective gauge length of the extensometer, given that the actual mounting tip geometry may not conform to assumed point of geometry. A one-time calibration of the extensometer will suffice until the next scheduled extensometer calibration, since the effective gauge length is unlikely to change when moving from one specimen to another. BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings wherein the showings are for the purpose of illustrating a possible embodiment of the invention only, and not for the purpose of limiting the same,

Figure 1 a shows the schematic of axial displacement; Figure 1 b shows the schematic of shear displacement;

Figure 2 shows the ceramic rods of the shear extensometer according to the present invention;

Figure 3 shows the close-up view of the shear extensometer arm tips according to this invention; Figure 4 shows the shear extensometer mounted on a specimen loaded using the losipescu shear test fixture according to an exemplary embodiment of the invention.

Figure 5a shows an arrangement of an extensometer;

Figure 5b shows the side view of the arrangement of an extensometer;

Figures 6a and 6b show the different schematics of shear strain; Figure 7 shows the shear strain gauge versus shear extensometer output on two different materials.

DETAILED DESCRIPTION OF THE INVENTION

Figures 2 and 3 illustrate the core concept of the invention, namely, the profile of the extensometer rods (10). The extensometer ceramic rods (10) comprise a first arm (2) having a first mounting tip (6) and a second arm (4) having a second mounting tip (8). The axis (C-C) of the first arm (2) and the axis (C'-C) of the second arm (4) are parallel to each other. The first mounting tip (6) is located away from the axis (C-C) of the first arm (2) and the second mounting tip (8) is located away from the axis (C'-C) of the second arm (4).The arms (2, 4) of the shear extensometer move strictly in the plane described by their axes. The mounting tips (6, 8) are so profiled such that they make contact with diagonally opposed points on parallel, offset axes on a specimen surface, schematically similar to points V and X, or, U and W in figure 1 b. The profile of the mounting tips (6, 8) is the vital element of this invention because it ensures fidelity of shear displacement measurement by strictly measuring the movement of point U as shown in figure 1 b. The relative lateral offset of the two mounting tips (6, 8) constitutes the shear extensometer gauge length. The mounting tips (6, 8) are preferably round and sharp and they measure displacement along offset axes, with each tip sitting on different but parallel axis. The profile of the mounting tips (6, 8) is obtained preferably by a grinding process, but other methods of manufacturing will become readily apparent for a person skilled in the art. As in other (axial) extensometers, arm length affects both sensitivity and linearity of the extensometer. However, it is mounting tip lateral offset, rather than axial spacing, that determines the gauge length. Positioning a conventional axial extensometer along the diagonal D1 or D2 (of fig. 1 b) will not serve the purpose of accurate shear displacement measurement because of the unavoidable rotation of the mounting points (reflected in rotation of diagonals D1 , D2 under shear strain) that will distort the readouts.

The shear extensometer of the present invention is outwardly similar to an axial extensometer and, in fact, uses the same sensitive element. However, the shear extensometer of this invention is radically different from that of an axial extensometer in terms of the profile of its mounting tips (6, 8). An axial extensometer demands zero lateral offset between the two mounting points as a prerequisite for ensuring coaxiality. On the contrary, the lateral spacing of the mounting tips in the shear extensometer of this invention defining the gauge length serves as a prerequisite for shear displacement measurement. In all other respects, including the design of the sensitive element, wiring etc. the shear extensometer of this invention is similar to an axial extensometer. Therefore, by merely changing the arms, one can use the same transducer to perform both axial as well as shear displacement measurements.

The shear extensometer of this invention may be used to characterize shear response of materials including the determination of shear modulus, shear stress vs. strain curve, shear creep, etc. in both static as well as cyclic loading at any given test temperature. Figure 4 shows the shear extensometer according to this invention mounted on a specimen (S) loaded using the losipescu shear test fixture (20) with double V-notch coupons. Also, the same extensometer can be used to characterize shear response of cylindrical and tubular specimens in torsion, provided the diameter of the specimens is much larger than the gauge tip spacing. It may be noted that standard test practice with the losipescu fixture (20) calls for the use of shear strain gauges to monitor shear strain on the specimen (S). With the present invention, these measurements are considerably easier to perform.

The arrangement for a typical extensometer is shown in figures 5a and 5b. The arrangement includes a mounting plate (12) on which a linear bearing (13) of the type that prevents rotation is mounted. An extensometer holder (15) is connected to the linear bearing (13), said holder (15) adapted to grip the extensometer (17). The extensometer comprises ceramic rods (10), the profile of which have been explained in the preceding paragraphs. A heat reflector (21 ) is provided proximal to the extensometer (17) to shield the sensitive element of the extensometer (17) from being exposed to excessive heat when used in a high temperature application.

Figure 6a shows the "textbook" schematic of shear strain. As shown, the square element, when subject to shear strain, assumes the shape of a rhombus. In this event, the diagonal D1 will experience compressive strain, while D2 will experience tension. A strain gauge (22) is also pasted on the surface where the strain is to be measured. Strain gauges work on the principle of change in electrical resistance due to tension or compression. Thus, by pasting a strain gauge (22) as shown in the middle of the diagonal D2, the extension of this diagonal under shear strain can be estimated and assumed to serve as a measure of shear strain. Figure 6b shows the schematic of shear strain, but with the rhombus rotated to reflect the principle of operation of our invention. It can be readily shown that there is a unique trigonometric relationship between the measurement of ^ according to this invention and the extension along D2 or the response of a shear strain gauge as shown in figure 6a. Indeed, there is a linear calibration between shear strain gauge response and the response of our invention. As seen in figure 6b, the diagonals D1 , D2 do rotate, but the contact points do not rotate under shear.

In an optional embodiment, a shear strain gauge may be pasted on the specimen surface between the mounting tips of the shear extensometer to serve as a reference to calibrate the readouts, if required, and to account for possible errors in estimating effective gauge length of the extensometer, given that the actual mounting tip geometry may not conform to assumed point geometry. Strain is measured as the ratio of actual displacement to the gauge length over which it is measured. Shear strain in the schematic of figure 6b explained above is given by the ratio *L . This distance (L) cannot be estimated accurately for lack of accuracy in estimating actual lateral offset between rod tips. This problem is overcome by calibration of readout with the strain gauge. Calibration results estimates the value of L such that shear strain measurements match those seen by the strain gauge. As this distance is unlikely to change when moving from one specimen to another, a one time calibration will suffice until next scheduled extensometer calibration. Such a comparison carried out on any material can serve as measurements for any other material or testing condition because the correlation is geometric by nature and therefore independent of material response. This has been shown by comparing shear extensometer readout versus strain gauge readout obtained on identical AL-alloy and Nimonic alloy specimens tested on a losipescu fixture. Figure 7 shows the calibration of shear strain gauge versus shear extensometer output on Al-alloy and Nimonic alloy.

It follows that the shear extensometer can serve the purpose currently served by strain gauges that are by design non-reusable, require skill and effort to instrument and whose application becomes difficult if not impractical in hostile environment, such as elevated temperature. The shear extensometer arms, in contrast, can pass through access slots on furnaces, enviro-chambers, etc. There are no restrictions on usage with respect to material, provided the test coupon is in a position to withstand the bearing load from the mounting points of the extensometer. The foregoing description shows and describes preferred embodiments of the present invention. It should be appreciated that this embodiment is described for purpose of illustration only, and that numerous alterations and modifications may be practiced by those skilled in the art without departing from the spirit and scope of the invention. It is intended that all such modifications and alterations be included insofar as they come within the scope of the invention as claimed or the equivalents thereof.

The salient feature of this invention is the lateral offset of extensometer arm mounting (contact) points on the specimen surface. The same purpose can also be served by introducing a lateral offset in the mounting of the arms themselves through a suitable design change to the sensitive element. While this may somewhat change the appearance of the extensometer, the principle of operation of the device would remain unchanged. The drawings described herein are intended to merely describe its operation through a simple change to the arms of a conventional axial extensometer.