Stiffness reduction, creep, and irreversible strains in fiber composites tested in repeated interlaminar shear

Interlaminar shear of a unidirectional carbon fiber/epoxy composite was studied experimentally by use of a tensile inclined double notch shear (IDNS-) test-setup. The non-linear constitutive behavior, the degradation and stiffness reduction, under monotonic and repeated loading with successively increasing peak strains were investigated. Shear strains (average values) up to 7–8% prior to failure were observed in the test region together with irreversible strains approaching 2.5% when unloaded. Hysteresis loops of notable width, increasing with higher peak strains, were observed. Several shear moduli measures were investigated. A notable decay in unloading secant moduli (∼30–40%) was present. Tangent (re-)loading and unloading moduli could be reliably measured only with a 2 h hold time prior to each load reversal. A quite early drop of >10% was observed in the elastic reloading tangent modulus, but not for the unloading tangent modulus. To the knowledge of the authors, no previous experiments have achieved global interlaminar shear strain levels of such magnitudes, and moreover, stress levels were higher than obtained with previously established methods. Several mechanisms were deemed to contribute to the overall strains; such were higher elastic-but also higher irreversible shear strains in the epoxy matrix and the fibers; damage within the epoxy; conceivably also in the fibers; appearance of microcracks in epoxy-rich regions; possibly separation of the interfaces, and also notable creep deformation at higher loads. None of these phenomena alone could account for the dominating part of the obtained strains. Matrix cracks (shear hackles) were found post mortem, but could not be confirmed during the test: a different damage mechanism is responsible for the early loss of elastic stiffness. Friction-like processes (sliding, viscosity) contributed substantially to the total strains.