Stress Measurement / Strain Gauging

Stress in engineering components can be calculated both analytically and using numerical techniques such as finite element analysis or can be measured directly. Measurement of the stress levels can be carried out using brittle lacquer, photo elastic, DIC and strain gauging techniques.

Origen offers both dynamic and residual stress measurement services.

What Is Stress Measurement?

Stress in engineering components can be calculated both analytically and using numerical techniques such as finite element analysis or can be measured directly. Measurement of the stress levels can be carried out using brittle lacquer, photo elastic and strain gauging techniques.

Strain gauging techniques, as employed by Origen, make use of electrical resistance strain gauges bonded to the surface of the component in question. These thin foil gauges, essentially a length of wire formed into a grid, extend with the component as it strains to accommodate load which causes slight change in resistance of the length of wire. For a known gauge type, the change in resistance can therefore be related to strain and hence stress.

Origen has used these techniques to good effect in a number of challenging applications and measurements have been successfully undertaken on amongst others - high pressure valves, waste containers, earthmoving equipment and even on a rotating underwater drilling system operating 120m below the sea surface.

Why Stress Measurement?

Although stresses in a component can be calculated using analytical and numerical techniques in components with complex geometries, such analyses are often complicated, time consuming and expensive. Furthermore the applied loads, on which these analyses are critically dependent, are often unknown or at best have been simply estimated. Direct measurement of the strains allows the stress state to be ascertained and the loads to be back calculated.

In combination with finite element analysis this direct measurement of the stress allows the model of the system to be 'calibrated' and supplies the information regarding the loads. In turn, the finite element analysis allows the results from a finite number of measurement positions to be extended to the entire component.

Furthermore stresses due to both the applied loads as well as residual stresses can be measured. Residual stresses are very difficult to analyse as they are dependent on the exact manufacturing process employed - heat treatment, machining processes, mechanical deformation and chemical processing all introduce residual stresses. These stresses can, however, be measured using a number of techniques, one of the most definitive of which is the centre hole drilling technique - a strain gauge based, semi destructive technique in which a small hole is drilled at the focus of a special format strain gauge. Residual stresses of this nature often lead to failure as they contribute to stress corrosion cracking and fatigue ('Residual Stress, the Nemesis of Structural Integrity', SA Mechanical Engineer, October 2000)

How Is Stress Measured?

Origen measures stress using strain gauging techniques in which stresses are calculated from measured strains. Foil strain gauges (or gauge rosettes) are affixed to the surface of the component and hard wired to amplification, transmission and data capture equipment. This allows the deflection of the structure/component to be measured and recorded in real time. Depending on the requirement, other system parameters are also monitored to allow a meaningful cause and effect time trace to be developed (eg stress goes up when vehicle decelerates).

Residual stresses are measured in a similar manner. After affixing the strain gauge rosette to the surface of the component a small hole is formed at the focus of the gauge using a suitable technique (air abrasion or high speed end mill). Strains relieved during the formation of the hole are then recorded as a function of hole depth.