Optical 2D Displacement and Strain Sensor for Creep Testing of Material Samples in Transparent Fluids

The designer of mechanical products needs to know the behaviour of the applied materials under different environmental conditions. A standard experiment used to determine these behavioural parameters is the creep test. Certain groups of materials such as polymers exhibit large sensitivities of these parameters with respect to environmental conditions such as temperature and exposure to chemicals. In these cases creep tests need to be performed under conditions that match as closely as possible the service conditions, i.e. conditions under which the material will be used. The physical measurands observed during theses tests are displacement and consequently strain. In general, it is difficult to achieve the long-term stability and accuracy required by creep tests as these experiments usually last for more than thousand hours. The present paper reports on the design and implementation of an optical sensor for 2D displacement and strain measurements in transparent fluids. The proposed sensor meets the accuracy requirements imposed by the creep test procedure. Further, the sensor is insensitive to homogeneous changes in the optical path and thus is well suited for long-term displacement measurements in transparent fluids.