Cyclic Deformation Induced Residual Stress Evolution and 3D Short Fatigue Crack Growth Investigated by Advanced Synchrotron Tomography Techniques.

Diffraction and phase contrast tomography techniques were successfully applied to an austenitic-ferritic duplex stainless steel representing exemplarily a metallic material containing two phases with different crystal structures. The reconstructed volumes of both phases were discretized by finite elements. A crystal plasticity finite-element analysis was executed in order to simulate the development of the experimentally determined first and second order residual stresses, which built up due to the manufacturing process of the material. Cyclic deformation simulations showed the single-grain-resolved evolution of initial residual stresses in both phases and were found to be in good agreement with the experimental results. Solely in ferritic grains, residual stresses built up due to cyclic deformation, which promoted crack nucleation in this phase. Furthermore, phase contrast tomography was applied in order to analyze the mechanisms of fatigue crack nucleation and short fatigue crack propagation three-dimensionally and nondestructively. The results clearly showed the significance of microstructural barriers for short fatigue crack growth at the surface, as well as into the material. The investigation presented aims for a better understanding of the three-dimensional mechanisms governing short fatigue crack propagation and, in particular, the effect of residual stresses on these mechanisms. The final goal was to generate tailored microstructures for improved fatigue resistance and enhanced fatigue life.

Impact behaviors of human skull sandwich cellular bones: Theoretical models and simulation.

The impact behavior of human skull sandwich cellular bones with gradient geometric feature is investigated using theoretical and numerical methods. To predict the structural impact performance theoretically, the skull bone is considered as a multi-layer sandwich structure where the effect of the number of layers on its impact behavior is discussed. Three sections with different porosities and thicknesses obtained from the rebuilt 3D skull model are selected, and the numerical simulation is carried out to illustrate the reliability of the theoretical model. A close agreement between the numerical and theoretical results is observed. Moreover, the energy absorption capacity of the skull in the theoretical model is further demonstrated by experimental results of the human skull under impact loading from the literature. Numerical and experimental results show that the theoretical model can effectively predict the impact performance of the skull cellular bone. Therefore, this study can provide a reliable theoretical basis for the evaluation of the mechanical behavior of the human skull under dynamic loads.

Biomechanics of common fixation devices for first tarsometatarsal joint fusion-a comparative study with synthetic bones.

BACKGROUND: Hallux valgus disease is a common deformity of the forefoot. There are currently more than 100 surgical approaches for operative treatment. Because hypermobility of the first tarsometatarsal joint is considered to be causal for hallux valgus disease, fusion of the tarsometatarsal joint is an upcoming surgical procedure. Despite the development of new and increasingly stable fixation devices like different locking plates, malunion rates have been reported in 5 to 15% of cases. METHODS: Biomechanical comparison of three commonly used fixation devices (a dorsal locking plate, a plantar locking plate, and an intramedullary fixation device) was performed by weight-bearing simulation tests on synthetic bones. Initial compression force and stiffness during simulation of postoperative weight-bearing were analysed. RESULTS: Fixation of the first tarsometatarsal joint with the plantar plate combination demonstrated a higher stiffness compared to fixation with the intramedullary implant or the medial locking plate. The intramedullary device provided the highest initial compression force. Failure was detected in the following ranking: (1) the angle-stable intramedullary fixation device, (2) the medial located plate, and (3) the plantar locking plate. CONCLUSION: The intramedullary device demonstrated the highest initial compression force of the three tested implants. The plantar locking plate showed the best overall stability during weight-bearing simulation. Further clinical research is necessary to analyse if the intramedullary fixation device needs a longer period of non-weight-bearing to reach a better non-union rate compared to the plantar locking plate.

Impact response and energy absorption of human skull cellular bones.

A skull fracture, due to a composition of typical lightweight cellular structures, is the most common type of traumatic brain injury. This paper presents a systematic investigation on the failure mechanism and energy absorption of skull cellular bones under low- and medium-velocity impact loadings. Non-destructive three-dimensional micro-computed tomography (Micro-CT) is utilized to scan samples of human skull cellular bones, and relevant structural parameters are obtained to reconstruct a finite element (FE) model of these bones. Micro-structures, mechanical properties, and failure process analysis of human skull cellular bones under impact loadings are investigated. The effects of some typical parameters, such as impact velocity and angle, impactor shape and density, and various reconstructed sections on the impact behavior of human skull cellular bones are investigated. Their impact properties and energy absorption are summarized. The present work will be of great significance in understanding the mechanical mystery of human skull cellular bones under impact loading.

Intramedullary nailing in opening wedge high tibial osteotomy-in vitro test for validation of a method of fixation.

PURPOSE: Opening wedge high tibial osteotomy (HTO) as a treatment in unicompartimental osteoarthritis of the knee can significantly relieve pain and prevent or at least delay an early joint replacement. The fixation of the osteotomy has undergone development and refinements during the last years. The angle-stable plate fixator is currently one of the most commonly used plates in HTOs. The angular stable fixation between screws and the plate offers a high primary stability to retain the correction with early weight-bearing protocols. This surgical technique is performed as a standard of care and generally well tolerated by the patients. Nevertheless, some studies observed that many patients complained about discomfort related to the implant. METHODS: Therefore, the stability of two different intramedullary nails, a short implant used in humeral fractures and a long device used in tibial fractures for stabilization in valgus HTOs, was investigated as an alternative fixation technique. The plate fixator was defined as reference standard. Nine synthetic tibia models were standardly osteotomized and stabilized by one of the fixation devices. Axial compression was realized using a special testing machine and two protocols were performed: a multi-step fatigue test and a load-to-failure test. RESULTS: Overall motion, medial, and lateral displacements were documented. Fractures always occurred at the lateral cortex. Axial cyclic loading up to 800 N was tolerated by all implants without failure. The tibia nail provided highest fatigue strength under the load-to-failure conditions. CONCLUSIONS: The results suggest that intramedullary nailing might be used as an alternative concept in HTO.

Characterisation of a metallic foam-cement composite under selected loading conditions.

An open-cell metallic foam was employed as an analogue material for human trabecular bone to interface with polymethyl methacrylate (PMMA) bone cement to produce composite foam-cement interface specimens. The stress-displacement curves of the specimens were obtained experimentally under tension, shear, mixed tension and shear (mixed-mode), and step-wise compression loadings. In addition, under step-wise compression, an image-guided failure assessment (IGFA) was used to monitor the evolution of micro-damage of the interface. Microcomputed tomography (µCT) images were used to build a subject-specific model, which was then used to perform finite element (FE) analysis under tension, shear and compression. For tension-shear loading conditions, the strengths of the interface specimens were found to increase with the increase of the loading angle reaching the maximum under shear loading condition, and the results compare reasonably well with those from bone-cement interface. Under compression, however, the mechanical strength measured from the foam-cement interface is much lower than that from bone-cement interface. Furthermore, load transfer between the foam and the cement appears to be poor under both tension and compression, hence the use of the foam should be discouraged as a bone analogue material for cement fixation studies in joint replacements.

Fundamental mechanisms of fatigue and fracture.

A brief overview is given in this article on the main design philosophies and the resulting description concepts used for components which undergo monotonic and cyclic loading. Emphasis is put on a mechanistic approach avoiding a plain reproduction of empirical laws. After a short consideration of fracture as a result of monotonic loading using fracture mechanics basics, the phenomena taking place as a consequence of cyclic plasticity are introduced. The development of fatigue damage is treated by introducing the physical processes which (i) are responsible for microstructural changes, (ii) lead to crack initiation and (iii) determine crack propagation. From the current research topics within the area of metal fatigue, two aspects are dealt with in more detail because of their relevance to biomechanics. The first one is the growth behaviour of microstructural short cracks, which controls cyclic life of smooth parts at low stress amplitudes. The second issue addresses the question of the existence of a true fatigue limit and is of particular interest for components which must sustain a very high number of loading cycles (very high cycle fatigue).