AbstractAcoustic emission (AE) during irreversible deformation in short glass fiber reinforced poly(vinyl chloride) (PVC) composites was studied using a piezoelectric crystal transducer. Compared to the well‐coupled composites, many more AE events were observed during tensile deformation in the poorly‐coupled composites, presumably due to failure at the fiber‐matrix interface. No fiber fracture was detected in the tensile tests for either well‐coupled or poorly‐coupled composites. Irreversibility of acoustic emission was observed in repeated tensile loading experiments. Unlike PVC, the short fiber composites fractured during stress relaxation at 1 percent elongation. Studies of acoustic emission behavior during stress relaxation indicated that interfacial debonding is a time‐dependent process. Relaxation fracture time was strongly increased by chemical coupling at th