# Bone: An Outstanding Composite Material
## 5 Mechanical Behavior  
The ordered lamellar structure confers bone tissue an anisotropic behavior with direction-dependent material properties. Cortical bone strength, yield stress, and Young's modulus have higher values in compression than in tension along the direction aligned with the diaphyseal axis rather than transversely. In the latter, it is weaker in tension than in compression. Although the microstructural mechanism that causes the differences in yield and post-yield behavior is still not completely understood, it is generally accepted that when the cortical bone is loaded past the yield point, it accumulates permanent damage within the bone tissue. Such phenomena may also occur even when the cortical bone is unloaded near the yield point. After the yield point is exceeded, the reloading modulus presents a lower value. This is associated with the cortical bone degradation characteristics. The tissue microstructure deterioration appears as \"microdamage\" accumulation as a result of static or cyclic loading. In such a case, cortical bone has greater resistance in compression than in tension. Damage initiation and growth should be analyzed using fracture mechanics concepts. The objective is to determine fracture toughness (the mechanical property that describes the bone resistance to crack initiation and propagation) under different loading modes, i.e., opening (mode I), shear sliding (mode II), and shear tearing (mode III). These loading modes, and a combination of them, occur frequently during normal daily activities, such as walking or running, for example. Aiming to perform bone fracture characterization, several different tests have been used. The compact tension [118], single-edge notched beam [119] and double cantilever beam [120] tests have been employed for fracture characterization of bone under mode I loading. Under mode II loading, [PERSON] et al. [121] proposed the compact shear test, and [PERSON] et al. [122] used the end notched flexure test. The mixed-mode I + II loading fracture characterization of bone has been studied considering the asymmetric four-point bending test using single-edge notched specimens [123], the single-leg bending test [124], and the mixed-mode bending test [125]. The initial methods utilized to study the fracture risks of bone under different conditions were based on linear elastic fracture mechanics concepts [118, 126]. However, the bone complex microstructure and composition gives rise to the development of a non-negligible fracture process zone, which is the region close to the crack tip, where several phenomena such as microcracking, crack deviation, and fiber bridging occur. In this context, non-linear fracture mechanics-based methods, such as cohesive zone modeling, are more appropriate [120, 127]. Another important characteristic of cortical bone is its viscoelasticity, although only moderately. This means that the stress developed within the bone, and consequently its stiffness and strength, are dependent on the rate at which bone is strained. A six order of magnitude increase in strain rate will only change the Young's modulus by a factor of two and strength by a factor of three. The positive effect of this property is accentuated during impact loading (e.g., high energy impact), where strength can increase by a factor of three [21, 128, 129, 130, 88]. In cancellous bone, the mechanical properties of the constituents and trabecular architecture are the main factors determining its mechanical performance [131]. In trabecular bone, the strength is highest in compression rather than in tension and lowest in shear, which is likewise in cortical bone. Oppositely to the fairly linear behavior verified at low stress values in cortical bone stress-strain curve, in trabecular bone, such a performance is absent, and there is no clear linear region. The nonlinearity is even present at low stress levels. An interesting property of the trabecular bone is that it can absorb substantial energy on mechanical failure.