Patent of the month - Tensile testing

Materials World magazine
29 Oct 2018

Patent Attorney, Gemma McGeough from Withers & Rogers, discusses a patent that could address cost and accuracy issues in tensile testing for mechanical strength.

In the simplest tensile test, a sample is marked with a series of points and their movement in response to an increasing tensile load is measured. This is a relatively basic but cost-effective method for determining various static mechanical properties of a material. An example of a mechanical property calculated from this type of test is the material’s Young’s Modulus – a measure of the material’s ability to withstand changes in length when placed under a tensile load.

In order to record the movement of points on the sample, a video camera can be used, from which the distance between points throughout testing is calculated. While effective, this method of measurement has limited accuracy. As a result, various sophisticated methods for more accurately measuring tensile testing data have evolved, but their complexity makes them more expensive to manufacture and operate.

UK-based company, Instron Ltd, identified that there was room for improvement in existing tensile test apparatuses and developed an alternative. In 2003, a European patent application was filed, which was granted around three years later. USA-based company, Illinois Tool Works Inc, identified the value in the application, so procured the application and resulting patent from Instron Ltd.

According to the published patent, the invention can produce more accurate results than the basic method described above, in a simpler and cheaper way than existing methods. The European patent, published as EP1424547B1, describes a device that uses a camera (12), or optical means to measure the deformation of the material being tested. While doing this, part of the apparatus provides a controlled environment.

Increasing the accuracy

As illustrated in Figure 1, the sample (11) to be tested is monitored with a camera, and between the camera and sample there is an open-ended tubular member (16), which is supplied with a flow of a gaseous medium.
Provision of the tubular member enables a controlled environment to be located between the camera and the sample. The axis of the tubular member is aligned with the optical axis of the optical arrangement, and the tubular member itself extends between the camera and the sample. Individual characteristics of the air within the tubular member can then be controlled, such as its density, humidity or temperature.

In an optional modification, the sample can be illuminated with light from a light source (22) that has passed through a polarising filter (25). There may also be a further polarising filter (27) at the camera. Without this modification, due to the sample surface often being specular, there would be relatively low contrast between the points marked on the sample and its remainder. So, using polarising filters has the advantage of increasing the contrast of the image received by the camera.

Optionally, a fan can be provided to control air flow through the tubular member, specifically to achieve homogenously mixed air between the sample and camera, at a pressure above atmospheric pressure. The open-ended configuration of the tubular member allows air to flow out of the tube and has the effect of sweeping away any ambient air from the front of the camera and the sample.

Using this apparatus with a charge coupled device (CCD) array with approximately 1,000 lines and a field view of 100mm, the accuracy of the device was such that points on the sample could be measured to within a precision of ±1µm.

This patent illustrates how clever inventions, capable of bringing real-world benefits, can be found in simple devices. In this case, by creating an accurate tensile testing measurement apparatus, the large price tag of existing accurate measurement devices can be avoided. The use of patent protection ensures that the patent owner has a period of 20 years from filing in which to capitalise on their investment by bringing an improved product to market.