Bone-Inspired Microarchitectured Materials with Enhanced Fatigue Life

Abstract

Text 2 Microarchitectured materials achieve superior mechanical properties through geometry 17 rather than composition 1-4 . Although lightweight, high-porosity microarchitectured materials can 18 have high stiffness and strength, stress concentrations within the microstructure can cause flaw 19 intolerance under cyclic loading 5,6 , limiting fatigue life. However, it is not known how 20 microarchitecture contributes to fatigue life. Naturally occurring materials can display 21 exceptional mechanical performance and are useful models for the design of microarchitectured 22 materials 7,8 . Cancellous bone is a naturally occurring microarchitectured material that often 23 survives decades of habitual cyclic loading without failure. Here we show that resistance to fatigue 24 failure in cancellous bone is sensitive to the proportion of material oriented transverse to applied 25 loads -a 30% increase in density caused by thickening transversely oriented struts increases 26 fatigue life by 10-100 times. This finding is surprising in that transversely oriented struts have 27 negligible effects on axial stiffness, strength and energy absorption. The effects of transversely 28 oriented material on fatigue life are also present in synthetic lattice microstructures. In both 29 cancellous bone and synthetic microarchitectures, the fatigue life can be predicted using the 30 applied cyclic stress after adjustment for apparent stiffness and the proportion of the 31 microstructure oriented transverse to applied loading. In the design of microarchitectured 32 materials, stiffness, strength and energy absorption is often enhanced by aligning the 33 microstructure in a preferred direction. Our findings show that introduction of such anisotropy, 34 by reducing the amount of material oriented transverse to loading, comes at the cost of reduced 35 fatigue life. Fatigue failure of durable devices and components generates substantial economic 36 costs associated with repair and replacement. As advancements in additive manufacturing expand 37 the use of microarchitectured materials to reusable devices including aerospace applications, it is 38 increasingly necessary to balance the need for fatigue life with those of strength and density. 39 Text 3Fatigue failure is caused by the accumulation of microscopic damage following repeated loading 40 and generates substantial economic costs associated with repair and replacement of durable devices. 41Microarchitectured materials are particularly susceptible to fatigue failure because the complex 42 geometry can result in local stresses an order of magnitude greater than stresses applied to the bulk 43 material 5,6,9 . The presence of large stress concentrations can promote damage initiation and 44 accumulation under cyclic loading and thereby reduce the fatigue life. Naturally occurring 45 microarchitectured materials also experience cyclic loading and provide a model for strategies to resist 46 fatigue failure. 47

The plate-to-rod transition in trabecular bone loss is elusive

“…However, all three of these parameters had a significant and strong correlation with bone volume fraction, and this study therefore does not constitute evidence for geometrical changes in the trabecular compartment driving mechanical properties beyond the loss of material. Fatigue failure of trabecular bone may further be related to elements oriented transversely to the main loading direction, which have little effect on stiffness and strength [49].…”

“…On the other hand, this intentional feature of EF may be useful to detect situations where a pure change in shape (without differences bone volume fraction or local thickness) is responsible for altered mechanical properties. Torres et al [49] found that the size, shape and orientation of small off-axis members can be critical for the mechanical performance of porous materials. The maximal ellipsoids' EF, orientation and volume could provide this additional information.…”

Frugal manufacturing in smart factories for widespread sustainable development

“…Furthermore, AM is leading to novel ways of enhancing product performance such as printing of a bone-inspired architecture for substantially improving fatigue life of foam-like products. [58].…”

The plate-to-rod transition in trabecular bone loss is elusive

“…However, all three of these parameters had a significant and strong correlation with bone volume fraction, and this study therefore does not constitute evidence for geometrical changes in the trabecular compartment driving mechanical properties beyond the loss of material. Fatigue failure of trabecular bone may further be related to elements oriented transversely to the main loading direction, which have little effect on stiffness and strength [45].…”