Local strain and damage measurements on a composite with digital image correlation and acoustic emission

Non-linear stress-strain behaviour under monotonic loading can be caused by continuous damage. In order to understand this non-linearity, simultaneous strain and damage measurements were taken on composite specimens using digital image correlation and acoustic emission as these techniques give valuable local information. The latter is essential in the case of inhomogeneous or anisotropic materials, such as continuous fibre composites. On the one hand, digital image correlation gives access to full field strain and on the other hand, acoustic emission recording can be used for damage monitoring and location if, at least two sensors are placed on the specimen under loading. In this work, these two techniques were combined to correlate strain measurements and damage location on a complex composite material during a monotonic tensile test. The composite is a continuous fibre-reinforced friction material used in car clutches. These measures were used to understand the non-linearity of the stress-strain curve of the as-received material as well as detect volume damage after thermal cycling. In depth study of the strain field and acoustic emission events location revealed a correlation between non-homogeneous damage kinetics throughout the specimen and the evolution of the strain distribution.