Stress Measurement Introduction

Why StressProbe?

What does StressProbe really give me and how does StressProbe differ from other technologies?

StressProbe makes measurements of an electromagnetic field close to the surface of a material. There is an induced input field and sensors measure particular components of the magnetic field. The magnetic field is affected by the stress level in the material so as the stress changes, the system responds.

StressProbe systems are capable of providing different levels of information. All StressProbe systems require calibration information for the material under investigation. This is often not as onerous as it seems because many 'families' of steel have similar responses. However it is not possible to use calibration data from mild steel to predict the behaviour of a high alloy steel. Calibration takes the form of measuring the material response to applied stress and is often achieved by making measurements on simple test pieces in the lab, these generally take the form of simple bending or uniaxial loads. StressProbe responds to both tensile and compressive loads.

In its simplest form, StressProbe can measure dynamic load cycles. These are typically required for fatigue analysis for example, or for measuring the effect of an event - this could be a lorry passing a over a bridge, the welding of a component or perhaps the effect of removing a support. The emphasis here is on dynamic loading because simple StressProbe systems are not able to make absolute measurements of stress but can accurately measure changes from a particular starting position. This is possible because the calibration curves are essentially linear responses. In practice this means you can measure the load cycle range without knowing the value of the mean.

The mean load is more difficult to determine using simple systems - and is often not required for basic fatigue analysis of structural steels in air. If you need to measure the mean, you require more information and need to consider what has happened to the steel in the past. Many steels exhibit a hysteresis effect due to previous loading and this can be pronounced when making magnetic measurements. In this situation we use a demagnetisation process to shake down the material into a known state. By measuring in that state a better estimate of the mean load in the material is obtained. Some StressProbe systems contain a demagnetisation facility which provides controlled harmonisation of the material. This type of measurement has been successfully used to measure the static load in structural clamp studs, thereby determining the distribution of loads in the system.

How different is it to using strain gauges?

StressProbe really responds to material strain and so can be considered analogous to a "non-contacting strain gauge" - but with some important differences.

You cannot apply strain gauges to a loaded structure and determine the load.

You cannot apply strain gauges through coatings without damaging the protective coating. StressProbe can measure the response of the steel through the coating - up to around 5mm depending on the probe.

Note however that interpretation of strain gauge data is well established, interpretation of data from StressProbe requires special operator training.

Remember that StressProbe requires a magnetic material to work. (But you can stick magnetic material onto a non-magnetic one and still get meaningful data).

Is StressProbe suitable for my application?

We hope so!. StressProbe is being used in a number of applications. TSC have experienced mechanical and structural engineers who understand your problems and can work with you to develop the best solution for your application. Will will be pleased to discuss your specific requirements.

Although TSC produce standard instrumentation, it is rarely possible to pick up an instrument and immediately make measurements. Correct choice of probe and the procedures for making the measurements are extremely important. Each application needs careful consideration and TSCs experience will prove invaluable in this regard.

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