Repeatability of digital image correlation for measurement of surface strains in composite long bones

Digital image correlation (DIC) can measure full-field surface strains during mechanical testing of hard and soft tissues. When compared to traditional methods, such as strain gauges, DIC offers larger validation data (∼50,000 points) for, e.g., finite element models. Our main aim was to evaluate the repeatability of surface strain measurements with DIC during compressive testing of composite femurs mimicking human bones. We also studied the similarity of the composite femur samples using CT. Composite femurs were chosen as test material to minimize the uncertainties associated with the use of cadaveric tissues and to understand the variability of the DIC measurement itself. Six medium-sized fourth generation composite human proximal femora (Sawbones) were CT imaged and mechanically tested in stance configuration. The force-displacement curves were recorded and the 3D surface strains were measured with DIC on the anterior surface of the femurs. Five femurs fractured at the neck-trochanter junction and one at the site below the minor trochanter. CT image of this bone showed an air cavity at the initial fracture site. All femurs fractured through a sudden brittle crack. The fracture force for the composite bones was 5751±650N (mean±SD). The maximum von Mises strain during the fractures was 2.4±0.8%. Noise in one experiment was 5-30µε. When applied loads were equalized the variation in strains between the bones was 20-25%, and when the maximum strains were equalized, variation in the other regions was 5-10%. DIC showed that the ability of nominally identical composite bones to bear high strains and loads before fracturing may vary between the samples.