A novel hybrid damage monitoring approach to understand the correlation between size effect and failure behavior of twill CFRP laminates

Despite the presence of numerous studies about size effect in composite materials, controversy still exists regarding the relation between mechanical behavior and size of the laminates. Therefore, in this study, a comprehensive experimental approach using combined structural health monitoring techniques namely, acoustic emission, digital image correlation and infrared thermography is conducted to elucidate the physics behind size effect in twill woven carbon fiber reinforced polymeric composites. Laminates with different thicknesses are produced and tested under tensile and in-plane shear loading conditions. In depth analysis of the acoustic emission data shows that an increase in the thickness of laminate changes the fraction of microdamage related to fiber failures. Moreover, a noticeable stagnation period in acoustic emission activity prior to global failure is observed for thick samples which is negligible for thin specimens. Furthermore, analysis of damage accumulation rate via acoustic emission technique is cross validated with digital image correlation and thermography for tensile and shear test results. The full field monitoring results indicate the inverse relation between the thickness of the laminates and damage growth rate. The combined usage of damage monitoring techniques shows that thicker laminates experiences slower damage growth dynamics as compared to those of thin laminates, thereby, assisting to understand the size effect in laminated composites.