Mechanical Testing

One of the key functions of the Test House at Keighley Laboratories is to physically test materials and components to establish their mechanical properties.

Impact Testing

Impact tests are used to measure the toughness/brittleness of a material and its capacity to resist mechanical shock. The two main types of testing are CHARPY and IZOD testing, which are undertaken to European and American standards. Izod testing is always carried out at room temperature, where as Charpy testing can be carried out at temperatures between –196°C and +200°C.

Hardness Testing

The hardness of a material is its resistance to indentation by a hardened steel/tungsten ball or diamond indentor. The following types of hardness testing are carried out at Keighley Laboratories; BRINELL, ROCKWELL, VICKERS and KNOOP.

Metallography

Metallography consists of preparing a piece of material by grinding, polishing and etching and subsequently examining it under magnification.

This process allows the metallurgist to determine certain characteristics regarding the material such as grain structure, case depth, quality of heat treatment and weld quality etc.

Macro examinations (up to x 10 magnification) and Micro examinations (up to x 1000 magnification) are undertaken on equipment including projection and stereo binocular microscopes which have photographic and video print facilities.

Tensile and Compression Load Testing

A test piece of known cross-section is put in tension or compressed until it breaks. These tests can be undertaken at ambient, sub-ambient and elevated temperatures, ranging from -100°C to +750°C. Stress strain and load deflection curves are produced for different materials for loads up to 1100kN.

Understanding Typical Stress Strain Curves

Determination of Proof Strength – Rp and Rt2

Rp – The proof strength (non-proportional extension) is determined from the force/extension diagram by drawing a line parallel to the straight portion of the curve and at a distance from this equivalent to the prescribed non-proportional percentage, for example 0.2%. The point at which this line intersects the curve gives the force corresponding to the desired proof strength (non-proportional extension). The latter is obtained by dividing this force by the original cross-sectional area of the test piece.

Rt - The proof strength (total extension) is determined from the force/extension diagram by drawing a line parallel to the ordinate axis (force axis) and at a distance from this equivalent to the prescribed total percentage extension. The point at which this line intersects the curve gives the force corresponding to the desired proof strength. The latter is obtained by dividing this force by the original cross-sectional area of the test piece.