GB2348013A

GB2348013A - Bend test assembly

Info

Publication number: GB2348013A
Application number: GB0006056A
Authority: GB
Inventors: Julian Tubb; David Earnest Harrison
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1999-03-15
Filing date: 2000-03-14
Publication date: 2000-09-20
Anticipated expiration: 2020-03-14
Also published as: GB9905739D0; GB0006056D0; GB2348013B

Abstract

A bend test assembly capable of applying a flexural load to a test sample, the bend test assembly comprising at least three supporting strips 3,4,5 for supporting a test sample 6 and means for moving at least one of the supporting strips 3 so as to provide a flexural load on the test sample wherein at least two of the supporting strips 4,5 are configured so as to be capable of rotation about at least two axes of the supporting strip so as to maintain contact between the strip and the face of the test sample. The freedom of movement of the supporting strips thus allowing the sample material in contact with them to be non planar or to become distorted whilst under test and still maintain a continuous contact across the face of the sample.

Description

BEND TEST ASSEMBLY This invention relates to a bend test assembly.

Bend test assembly units are commonly employed to apply a flexural load to a standard rectangular sample of material in order to, for example, measure the strength of the material. They operate as follows: The material under test is supported on one face across its width on two fixed, parallel supporting strips. A third parallel supporting strip with one linear degree of freedom is made to bear on the opposed face of the sample in a position somewhere between the other two, generally midway. A means is then employed to force the third parallel supporting strip into the material under test and so apply the flexural load. An alternative method is to apply the force by keeping the central supporting strip fixed and moving the outer supporting strips.

The supporting strips are usually implemented as cylinders which can only rotate about their longitudinal axis and so act as rollers which allow the material to bend about the axis of the roller when stressed.

There is a problem with known bend test assemblies in that the supporting strips are mounted such that they have limited movement and so, if the material under test is not planar or becomes distorted under test, the parallel supporting strips then no longer make close and continuous contact across the face of the sample and inaccurate readings will be obtained.

According to the present invention there is provided a bend test assembly capable of applying a flexural load to a test sample having first and second opposed faces, the bend test assembly comprising at least first and second supporting strips for supporting the first opposed face and a third supporting strip for supporting the second opposed face at a position between the first and second supporting strips, and means for moving at least one of the supporting strips so as to provide a flexural load on the test sample characterised in that at least two of the supporting strips are configured so as to be capable of rotation about at least two axes of the supporting strip so as to maintain contact between the strip and the face of the test sample.

Giving at least two of the supporting strips a second degree of rotational freedom allows the sample material in contact with them to be non planar or to become distorted whilst under test and still maintain a continuous contact across the face of the sample.

The assembly of the present invention facilitates the measurement of the flexural behaviour of materials with low or high flexibility.

The bend test assembly preferably has only three supporting strips, first and second supporting strips for supporting the first opposed face and a third supporting strip for supporting the second opposed face at a position between the first and second supporting strips, and means for moving at least one of the supporting strips so as to provide a flexural load on the test sample characterised in that at least two of the supporting strips are configured so as to be capable of rotation about at least two axes of the supporting strip so as to maintain contact between the strip and the face of the test sample. This simple configuration allows the sample material in contact with them to be non planar or to become distorted whilst under test and still maintain a continuous contact across the face of the sample.

The second degree of rotational freedom can be achieved by any suitable means however, advantageously, the supporting strip can be mounted at its central load-bearing point thus allowing the supporting strip to rotate about its central load bearing point.

A further advantage can be achieved by mounting the supporting strip at its central load bearing point on a spherical surface such as a ball this arrangement allows the supporting strip a third degree of rotational freedom giving further flexibility.

Alternatively, one of the supporting strips is mounted between two spherical surfaces such that it can tilt about the axis of the two spherical surfaces, allowing a supporting strip an second degree of freedom. The spherical surfaces can be in the form of partial spheres or can be whole spheres such as balls. The supporting strip is advantageously held in place between the two spherical surfaces by a magnetic field which can be achieved by magnetising the spherical surfaces. This allows for the easy introduction and removal of the supporting strip A further advantage can be achieved by the use of a mechanism mounted on the base nearby the first and second supporting strips that can be engaged and subsequently disengaged. In the engaged position it holds the first and second supporting strips in a fixed position which allows for the easy mounting and positioning of the test sample. When disengaged the supporting strips are free to move.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying Drawings, of which: Figure 1 shows a part sectional front view of the three-point bend test assembly according to the invention with a sample of test material mounted upon it; Figure 2 is a detail view of the figure 1 assembly showing detail of supporting strip 3; Figure 3 is a detail view of the figure 1 assembly showing detail of supporting strip 5; and Figure 4 is a detail view of the figure 1 assembly showing detail of supporting strip 4.

Referring to figures 1 to 4, the bend test assembly comprises a base 2 with two supporting strips 4 and 5, a loading beam 1 and a further supporting strip 3 and its supporting structure 13 mounted on the beam 1. A rectangular sample of test material 6 is held between the two supporting strips 4 and 5 and the third supporting strip 3. The supporting strip 3 is held between two balls 7, which allow the supporting strip 3 to rotate about the axis of the balls 7. The supporting strip 3 is held in place due to the balls 7 being made of a magnetic material as shown in figure 2. Means (not shown) is provided to allow the supporting strip 3 and its supporting structure 13 to move along the axis of loading beam 1.

The supporting structure 13 is preferably machined from one piece of metal to increase the accuracy of the location of the load point generated by the contact of the sample 6 and supporting strip 3. Furthermore, grooves can be machined into supporting structure 13 to help focus the magnetic field experienced by the supporting strip 3. As shown in figure 3, the supporting strip 5 consists of a rectangular block of metal that is supported by a ball bearing 9 and can rotate freely about all axes. As shown in figure 4, the supporting strip 4 consists of a rectangular block of metal that is supported by a fixed cylindrical metal bar 11.

A locking plate 12 is fixed to the base 2 such that it can be raised and lowered by moving the crank arm 10. Crank arm 10 can be designed so as to minimise the backlash associated with the screw mechanism (not shown) which is used to raise and lower the locking plate 12. In its raised position locking plate 12 holds the two supporting strips 4,5 in a fixed attitude to allow easy and precise positioning of the test material 6. When the test material 6 has been suitably mounted the locking plate 12 is lowered out of the way using the crank arm 10 which then allows the supporting strips 4 and 5 the freedom to twist and rotate as needed. The locking plate 12 plays no part in the measurement process whilst it remains lowered. Supporting strips 4 and 5 do not have to be rectangular. The surfaces of the strips 4 and 5 which are in contact with the locking plate 12 during positioning of the sample 6 can be bevelled so as to interlock with reciprocal bevels in the locking plate 12. This ensures that strips 4 and 5 are initially'square'with the sample 6 and that the accuracy of location of the load points generated by contact between sample 6 and supporting strips 4 and 5 is maximised. Hence, the experimental errors associated with the uncertainties in the positions of the load points are minimised. This relative freedom of movement of the strips 3,4 and 5 allows all of them to maintain contact across the whole width of the test material 6 even if the test material 6 is not planar or distorts in some way due to the applied loading force. The forces acting on the test material 6 from each of the three supporting strips 3,4 and 5 are therefore spread over the whole width of the test material 6.

Load is applied to the test material 6 by forcing the supporting strip 3 against the test material using some known means (not shown) and the applied force measured, again by some known means (not shown). The deflection of the bar can be measured by known means (not shown) and can be related to the measured applied force. For example, a linear variable displacement transducer (LVDT) positioned beneath the centre of the sample 6 can measure the deflection of the sample 6 at that point.

Claims

CLAIMS 1. A bend test assembly capable of applying a flexural load to a test sample having first and second opposed faces, the bend test assembly comprising at least first and second supporting strips for supporting the first opposed face and a third supporting strip for supporting the second opposed face at a position between the first and second supporting strips, and means for moving at least one of the supporting strips so as to provide a flexural load on the test sample characterised in that at least two of the supporting strips are configured so as to be capable of rotation about at least two axes of the supporting strip so as to maintain contact between the strip and the face of the test sample.
2. A bend test assembly as claimed in claim 1 having only three supporting strips, first and second supporting strips for supporting the first opposed face and a third supporting strip for supporting the second opposed face at a position between the first and second supporting strips, and means for moving at least one of the supporting strips so as to provide a flexural load on the test sample characterised in that at least two of the supporting strips are configured so as to be capable of rotation about at least two axes of the supporting strip so as to maintain contact between the strip and the face of the test sample.
3. A bend test assembly as claimed in claim 1 or claim 2 wherein the at least two axes are not parallel.
4. A bend test assembly as claimed in any of the preceding claims wherein at least one of the supporting strips is mounted at its central load bearing point.
5. A bend test assembly as claimed in claim 4 wherein the at least one supporting strip is mounted on a spherical surface and so allows three rotational degrees of freedom.
6. A bend test assembly as claimed in claim 1 or claim 2 wherein at least one of the supporting strips is held between two spherical surfaces separated by a distance less than the width of the supporting strip.
7. A bend test assembly as claimed in claim 6 wherein the supporting strip is held in contact with the spherical surfaces by a magnetic field.
8. A bend test assembly as claimed in any one of the preceding claims further comprising means for engaging the first and second supporting strips in order to hold the first and second supporting strips in a fixed position the engaging means also being capable of disengaging to allow the first and second strips to rotate.
9. A bend test assembly substantially as described herein with reference to figures 1 to 4 of the accompanying drawings.