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Optimisation of a non-contact strain measurement technique based on subjective laser speckle patterns

Alexander Spaett M.Sc.

In materials testing the measurement of strain is inevitable. Therefore, there are several measurement techniques which can be roughly divided into two different groups. One of them being contacting extensometer, while the second group includes non-contact extensometer. Non-contact extensometer are making use of different optical methods, including strain estimation based on subjective laser speckle patterns. Speckles will always appear when an optically rough surface is illuminated by sufficiently coherent light. The emerging speckle pattern can then be utilised as a statistical marker for the measurement of the strain.

The overall objective is to optimise the signal processing for the measurement setup depicted in figure 1. Within this setup, two laser diodes are used as coherent light sources in order to generate two different spots, each spot having its own laser speckle pattern. With the help of a telecentric 1:1 optic, these patterns are then projected onto a linescan camera. Measurements, taken with the above mentioned measurement setup and a non-moving object are shown in figure 2. Currently it is possible to resolve displacements down to 100 nm which corresponds to 1/70 pixel.

Measurement setup Figure 1: Measurement setup for strain measurement via laser speckle correlation technique. easurement of a laser speckle pattern Figure 2: Measurement of a laser speckle pattern.

In the first phase of the project, different influencing factors are to be analysed. These include environmental as well as technical influences. The former includes fluctuations of the temperature distribution between the laser and the sample. Examples for the latter are intra-pixel-sensitivity variations or the wavelength of the laser which can change slightly during the taking of a measurement.

Secondly, the signal processing is to be improved based on the knowledge gained during the first phase. Also, if necessary, changes to the measurement setup will be carried out. Therefore, the strain shall be estimated as accurate and reliable as possible.