Localization of damage with speckle shearography and higher order spatial derivatives

• Shearography with stroboscopic illumination and temporal phase modulation is described.
• The phase maps of modal rotation fields of a damaged composite plate are measured.
• A decrease on the noise propagation is obtained by differentiating the phase maps.
• The differentiation of the phase maps improves the damage localization.
• Damage is better localized using fourth order spatial derivatives of mode shapes.

Two speckle shearography systems are described in this paper. The first is based on stroboscopic laser illumination and temporal phase modulation, whereas the second system relies on double pulse laser illumination and spatial phase modulation. These systems are applied to measure the phase maps of modal rotation fields of a damaged laminated composite plate. In order to decrease the propagation of noise, a new differentiation methodology is presented. It relies on the differentiation of the measured phase maps before they are post-processed. This leads to an improvement in the localization of damage. It was found that the fourth order spatial derivative of mode shapes also presents better damage localizations, in particular with the phase maps measured by the first shearography system.