Time-dependent damage accumulation under stress relaxation testing of bovine trabecular bone

Time-dependent loading of trabecular bone consists of both cyclic and sustained static loads. Numerous studies have investigated the effects of fatigue and creep loading on the apparent and damage behavior of bone; however, microdamage formation in trabecular bone under stress relaxation tests has not been characterized. Bovine trabecular bone specimens (1.5–3 years old) were loaded in uniaxial compression to 0.9% strain and held for six different time points: 0 s, 250 s, 2000 s, 5000 s, 10,800 s, and 36,000 s. Total microdamage increased with hold time and leveled off after 10,800 s. The qualitative patterns of microdamage progressed from surface damage for short hold times to microcracks extending through the trabecular thickness as hold time increased. Residual strain behavior corresponded well with microdamage formation over time. These results indicate that under static compressive deformation, microdamage accumulation leads to the development of residual strains with time. This mechanism has important implications on reloading of specimens where degradations in bone strength may contribute to age-related skeletal fragility and traumatic fractures.