Non contact measurement an alternative to strain gauges

| Environmental Testing

Robot positioned 3D scanning laser vibrometer

University facility develops laser based non-contact method of measuring stresses and strains on lightweight surfaces as an alternative to foil strain gauges.

Knowledge of stresses and strains in a component and an understanding of their origins is vital to understanding the durability of a product. But obtaining a detailed map of component stresses and strains under dynamic loading is highly time consuming using traditional methods. Added to this, the placement of strain gauges on modern extremely lightweight materials can have a measureable effect on damping and stiffness causing uncertainty in the results.

Now, the University of Leicester facility Asdec is equipped to undertake detailed non-contact surface strain measurements to add to its capability in measurement and analysis of vibration. The Polytec 3D scanning laser vibrometer can undertake a detailed surface scan to measure component strains and to produce, by calculation, a full-field map of stresses and strains that is directly comparable to an engineer’s Finite Element (FE) results.

The 3D scanning laser is a non-contact method that does not add mass to the structure and therefore has no observer effect on the results. These high quality results are extremely quick to obtain as there is no need to invest time in the delicate art of affixing foil strain gauges to the component.

The system can be programmed to scan full surface strain of any object that can be fitted into the test chamber.  Additionally, Asdec can utilise robot-controlled placement of the system to fully track a curved surface.  This level of control helps maintain the orientation of the laser heads with a curved surface, allowing for – in extremely short timeframes compared to conventional methods – detailed measurements of strain on complex components, such as turbine blades.

The 3D laser measurement heads are mounted on a Kuka industrial robot which in turn mounts on a traversing platform to enable large objects to be measured and extensive data sets to be captured quickly.

“The capability of 3D full surface strain mapping on curved surfaces to obtain detailed maps of component stresses and strains under dynamic load conditions will be extremely interesting to engineers, particularly in the aerospace field where durability of components such as turbine blades is a critical concern, says Asdec General Manager Tim Stubbs. “Producing results with enough detail to directly compare the measured strains directly back to FE models will allow engineers to improve their durability design cycles.”

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