<p>Mechanical fatigue data is generated for many purposes and characterization metrics are needed to interpret the data. Fatigue characterization is examined here using an epoxy-fiberglass laminate as a model polymer matrix composite. Detecting, measuring, and assessing fatigue aging and damage prior to failure is especially difficult for polymer matrix composites. This study introduces a new metric to monitor changes due to mechanical aging and quantify low levels of fatigue damage. Encouraging results are shown for K_D , the dynamic work hardening modulus. These moduli were determined for samples with prior thermal aging and controlled fatigue exposure. A relationship between K_D and work hardening was detected. The metric links macroscopic measurements to changing mesostructural dynamic properties as the composite ages under cyclic fatigue. Dynamic mesostructural processes and their aging effects are often overlooked to the detriment of understanding fatigue behaviors. The dynamic extensional storage modulus at zero K_D could become a useful indicator of resistance to dynamic work hardening for a composite material. Further investigations using the dynamic work hardening modulus metric are also anticipated to clarify the transition between ductile and brittle behavior during cyclic fatigue of polymer matrix composites.</p>