GB2313916A - Method and apparatus for testing strength of materials - Google Patents

Abstract

A method for testing the strength of materials in situ by measuring the torque necessary to deform the material and comparing to known values. A hole is drilled in the material to a specified depth and the hole is deformed by inserting a spline with a stellated circular cross section. A second spline of similar cross section but smaller radius is then inserted in the same hole ad the torque necessary to rotate the spline, further deforming the material, is measured. This torque is indicative of the strength of the material.

Description

Method and Apparatus For Testmg the Strength of Materials This invention provides a method and apparatus for testing the strength of materials, and more specifically but not exclusively to testing the strength of metal members in situ on structures.

The yield strength of metallic members in large fixed structures such as radio masts is of great importance to engineers in assessing the load-canying capacity of the structure. Unfortunately, records of materials used.

and their properties, are often incomplete. Tests frequently therefore have to be done on samples of material from the structure in the hope that these few samples will give data representative of all the members, thus allowing the strength of the structure to be assessed.

Surface hardness of materials, particularly metals, is normally tested by one of the Brinell, Rockwell or Vickers tests, all of which are very similar. The Brinell test involves the measurement of the diameter of an indentation made by a hard steel ball under a fixed steady load. The Vickers test is similar to the Brinell test, using a square pyramid instead of a ball, and measuring the indentation area rather than the diameter. The Rockwell test measures the depth of penetration of an indenter. In all cases, the measure of the indentation is inversely related to the surface hardness of the material which can therefore be calculated.

It can be shown that for some materials, surface hardness is approximately proportional to yield strength and to ultimate tensile strength. For other materials, including steel sections which have been produced by rolling, and substantially hot-dip galvanised, surface hardness testing is unreliable as a measure of yield strength or ultimate tensile strength.

It would be very useful to be able to administer a reliable strength test in situ, rather than removing material and transporting it to a laboratory for testing. Obtaining a sample of material would normally involve sawing pieces off the structure which will often have a serious effect on the strength of the member in question, both because of the lost section of member and the stresses involved in removing it.

A test is therefore required which can be carried out in situ without the need for complicated equipment which gives an accurate indication of the material yield strength.

The present invention provides a method for determining the yield strength of metals in situ.

Particular embodiments are hereinafter described with reference to the accompanying drawings in which: Figure 1 shows a piece of material being tested after a first step of the method according to a first embodiment of the present invention; Figure 2 shows a piece of material being tested after a second step of the method according to a second embodiment of the present invention; Figure 3 shows a piece of material being tested after a third step of the method according to a third embodiment of the present invention; and Figure 4 shows a piece of material being tested after a fourth step of the method according to a fourth embodiment of the present invention.

A first step in a method according to a first embodiment of the present invention is shown in Figure 1. An area of the surface of the material being tested is removed. thus exposing material of substantially the same properties as the majority of the mass of the member.

The second step in this method is shown in Figure 2. A cylindrical volume of material is removed with its axis substantially perpendicular to the surface of the surface created in the first step. The whole of the circular opening defined by the cylinder should be within the area exposed in the first step The third step in this method is shown in Figure 3. A deforming tool, substantially harder than the material being tested, is driven into the cylindrical cavity created by the second step. The deforming tool has rotational symmetry about an axis of the tool. the axis of the tool being substantially identical to the axis of the cylindrical cavity as it is driven in. The deforming tool is shaped to generate several equispaced ridges along the walls of the cylindrical cavity substantially parallel to the axis of the cylindrical cavity.

The fourth step in this method is shown in Figure 4. A testing tool, also substantially harder than the material being tested and also rotationally symmetrical about its axis, is inserted into the cavity deformedduring the third step with its axis substantially identical to that of the cavity. The tool also has equispaced ridges along its surface parallel to the axis and is shaped in such a way that it can be inserted into the cavity without being resisted by the cavity ridges, but such that it cannot be freely rotated within the cavity about its axis, due to engagement of the ridges on the cavity and the ridges along the surface of the tool.

The tool is inserted to a predetermined depth into the cavity, the ridges along the inside of the cavity generated by the deforming tool being substantially uniform at least as far as this depth.

A fifth step in the method according to this embodiment comprises rotating the testing tool about its axis. As the tool is rotated. a torque meter with a lazy pointer registers the torque necessary to rotate the tool. The torque required will rise to a maximum as the tool deforms the ridges on the cavity, As the rotation continues. the testing tool will become easier to rotate and the torque required will decrease. The maximum torque reached is recorded by the lazy pointer.

The final step of this method is to compare the maximum torque value reached with a calibration curve compiled from performing similar tests on similar materials of known yield strengths, and thus measure the yield strength of the material in question.

In furrier embodiments of the present invention the deforming tool or testing tools have different cross sectional shapes as shown in Figure 7b and 7c.

In yet further embodiments, the testing tool is the same tool as the deforming tool, the tool being withdrawn to an extent necessary to carry out the testing.

In yet further embodiments, the second step as described above is performed without the surface preparation described in the first step having been carried out.

The necessary reference surface is then achieved by counter sinking the rim of the hole to the precise depth. Subsequent steps then proceed as described.

This invention particularly has applications in the field of testing galvanised steel. for example an radio masts without significantly damaging the structure.

Claims (3)

CLAIMS:

1. A method for testing material strength comprising the steps of: generating a cavity in the material of predetermined shape; inserting a tool into said cavity, said tool being so dimensioned as to engage walls of said cavity on rotation within the cavity; applying torque to rotate said tool within said cavity: measuring the torque required; and comparing the torque with data from other materials of known strength to establish the strength of the material.

2. Apparatus for testing material strength comprising: first means arranged to generate a cavity in the material of predetermined shape; second means shaped to be insertable into said cavity and so dimensioned as to be engageable with the walls of said cavity on rotation within the cavity; and means for measuring torque applied to said second means.

3. Apparatus substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.