Damage and fracture of biological and biomedical materials

In the last decade, the topic of damage and fracture of biological and biomedical materials not only became one of the central research areas in the healthcare engineering, but also drew attention of specialists in mechanics of materials and fracture. One of the motivations behind these developments is a continuing increase in the use of medical devices made of various materials that are exposed to challenging loading and environmental conditions. Many of them should have significant levels of durability to avoid recurring surgical interventions (typical examples being implants for hip and knee replacements or dental implants). A lack of understanding of their responses to specific conditions and interaction with biological environment can result in malfunctioning and failures or traumas to surrounding tissues. The typical application problems are additionally complicated by insufficient knowledge of mechanical behaviour of biomaterials at various length and time scales and under different loading conditions including their fracture and fatigue. These types of application presuppose the understanding of properties and performance of two classes of materials – natural (biomaterials) and engineering (biomedical materials), as well as their interaction at interfaces between, on the one hand, life tissues (or organs) and, on the other hand, implants and prostheses. Among engineering materials, used in such applications, are familiar metals and alloys, ceramics, polymers and composites. Their properties and performance seem to be well studied; still, biomedical applications are characterised by rather specific usability envelopes as well as, in most cases, additional constraints such as non-toxicity (biocompatibility) and/or resistance to harsh physiological environments. In some cases, a requirement, opposite to structural integrity, is needed, e.g. controlled degradation for scaffolds and stents..

Ultrasonically assisted drilling of aerospace CFRP/Ti stacks

Structural application involving aerospace stacks consisting of carbon-fiber-reinforced plastics (CFRP) and metals (such as aluminium and titanium) are characterized by their superior mechanical properties and relative ease of design. In many of such applications, drilling is required for hole making to facilitate fasteners for assembly. However, drilling with conventional methods pose several well-documented challenges including a requirement of an additional step for de-burring, increased tool wear, damage in the composite phase etc. Ultrasonically assisted drilling (UAD) is a hybrid machining technique, which has proven to enhance drilling quality in hard-to-machine materials. In this paper, UAD of stacks is implemented demonstrating significant improvement in hole quality produced in aerospace CFRP/Ti study with reduced drilling forces and energy spent

Ultrasonically assisted drilling of rocks

Conventional drilling of rocks can generate significant damage in the drilled material; a material layer is often split off a back surface of a sample during drilling, negatively affecting its strength. To improve finish quality, ultrasonically assisted drilling (UAD) was employed in two rocks - sandstone and marble. Damage areas in both materials were reduced in UAD when compared to conventional drilling. Reductions in a thrust force and a torque reduction were observed only for UAD in marble; ultrasonic assistance in sandstone drilling did not result in improvements in this regard

Characterising variability and regional correlations of microstructure and mechanical competence of human tibial trabecular bone: An in-vivo HR-pQCT study.

OBJECTIVE: Quantifying spatial distribution of trabecular bone mechanical competence and microstructure is important for early diagnosis of skeletal disorders and potential risk of fracture. The objective of this study was to determine a spatial distribution of trabecular mechanical and morphological properties in human distal tibia and examine the contribution of regional variability of trabecular microarchitecture to mechanical competence. METHODS: A total of 340 representative volume elements at five anatomic regions of trabecular bone - anterior, posterior, lateral, medial and centre - from ten white European-origin postmenopausal women were studied. Region-specific trabecular parameters such as trabecular volume fraction, trabecular thickness, trabecular number, trabecular surface area, trabecular separation, plate-like structure fraction and finite element analysis of trabecular stiffness were determined based on in-vivo high resolution peripheral quantitative computed tomographic (HR-pQCT) images of distal tibiae from ten postmenopausal women. Mean values were compared using analysis of variance. The correlations between morphological parameters and stiffness were calculated. RESULTS: Significant regional variation in trabecular microarchitecture of the human distal tibia was observed (0.001 ≤ p ≤ 0.05), with up to 106% differences between lowest (central and anterior) and highest (medial and posterior) regions. Higher proportion of plate-like trabecular morphology (63% and 53%) was found in medial and posterior regions in the distal tibia. Stiffness estimated from finite element models also differed significantly (0.001 ≤ p ≤ 0.05), with stiffness being 4.5 times higher in the highest (medial) than lowest (central) regions. The bone volume fraction was the strongest correlate of stiffness in all regions. CONCLUSION: A novel finding of this study is the fact that significant regional variation of stiffness derived from two-phased FEA model with individual trabecula representation correlated highly to regional morphology obtained from in-vivo HR-pQCT images at the distal tibia. The correlations between regional morphological parameters and mechanical competence of trabecular bone were consistent at all regions studied, with regional BV/TV showing the highest correlation. The method developed for regional analysis of trabecular mechanical competence may offer a better insight into the relationship between mechanical behaviour and microstructure of bone. The findings provide evidence needed to further justify a larger-cohort feasibility study for early detection of bone degenerative diseases: examining regional variations in mechanical competence and trabecular specifications may allow better understanding of fracture risks in addition to others contributing factors

Numerical analysis of thermo-mechanical behavior of indium micro-joint at cryogenic temperatures

This article was published in the journal, Computational Materials Science [© Elsevier]. The definitive version is available at: http://dx.doi.org/10.1016/j.commatsci.2010.12.026Microelectronic packaging plays an important role in cryogenic engineering; in particular, a solder joint as interconnection, which offers a mechanical, thermal and electrical support, undergoes much larger and harsher thermal changes during its service compared with conventional customer electronic products. The impact of thermo-mechanical properties of such solder joints under cryogenic service conditions becomes even more significant due to the continuing miniaturization of solder joints. Indium, a solder material with a low melting point and excellent cryogenic properties, has been favorable in various low temperature applications, in particular, to form solder joints for electronics interconnections. In order to understand the fundamental aspects of reliability of indium joints, this paper reports a constitutive model accounting for the effect of temperature change on thermo-mechanical behavior of indium joints. Especially, the model is used and subsequently implemented in a FE analysis to simulate a hybrid pixel detector system, in which indium micro-joints are manufactured to serve at cryogenic conditions. The response of indium joints to low-temperature cycling (300-76 K) was analyzed based on the proposed model, which not only offers a tool to understand the performance and experimental testing of solder joints under cryogenic temperatures, but can also be used for design optimization of the microelectronic package

Finite-element modelling of bending of CFRP laminates: Multiple delaminations

This article was published in the journal, Computational Materials Science [© Elsevier]. The definitive version is available at: http://www.sciencedirect.com/science/article/pii/S0927025611000826Carbon fibre-reinforced polymer (CFRP) composites are widely used in aerospace, automotive and construction structures thanks to their high specific strength and stiffness. They can also be used in various products in sports industry. Such products can be exposed to different in-service conditions such as large bending deformation and multiple impacts. In contrast to more traditional homogeneous structural materials like metals and alloys, composites demonstrate multiple modes of damage and fracture due to their heterogeneity and microstructure. Damage evolution affects both their in-service properties and performance that can deteriorate with time. These failure modes need adequate means of analysis and investigation, the major approaches being experimental characterisation and numerical simulations. This research deals with a deformation behaviour and damage in composite laminates due to quasi-static bending. Experimental tests are carried out to characterise the behaviour of a woven CFRP material under large-deflection bending. Two-dimensional finite element (FE) models are implemented in the commercial code Abaqus/Explicit. A series of simulations is performed to study the deformation behaviour and damage in CFRP for cases of high-deflection bending. Single and multiple layers of bilinear cohesive-zone elements are employed to model the onset and progression of inter-ply delamination process. Numerical simulations show that damage initiation and growth are sensitive to a mesh size of cohesive-zone elements. Top and bottom layers of a laminate experience mode-I failure whereas central layers exhibit a mode-II failure behaviour. The obtained results of simulations are in agreement with experimental data

Generation of higher harmonics in longitudinal vibration of beams with breathing cracks

This paper was accepted for publication in the journal Journal of Sound and Vibration and the definitive published version is available at http://dx.doi.org/10.1016/j.jsv.2016.06.025.Classical nonlinear vibration methods used for structural damage detection are often based on higher- and sub-harmonic generation. However, nonlinearities arising from sources other than damage – e.g. boundary conditions or a measurement chain – are a primary concern in these methods. This paper focuses on localisation of damage-related nonlinearities based on higher harmonic generation. Numerical and experimental investigations in longitudinal vibration of beams with breathing cracks are presented. Numerical modelling is performed using a two-dimensional finite element approach. Different crack depths, locations and boundary conditions are investigated. The results demonstrate that nonlinearities in cracked beams are particularly strong in the vicinity of damage, allowing not only for damage localisation but also for separation of crack induced nonlinearity from other sources of nonlinearities

Low cycle fatigue of a directionally solidified nickel-based superalloy: Testing, characterisation and modelling

Low cycle fatigue (LCF) of a low-carbon (LC) directionally-solidified (DS) nickel-base superalloy, CM247 LC DS, was investigated using both experimental and computational methods. Strain-controlled LCF tests were conducted at 850°C, with a loading direction either parallel or perpendicular to the solidification direction. Trapezoidal loading-waveforms with 2 s and 200 s dwell times imposed at the minimum and the maximum strains were adopted for the testing. A constant strain range of 2% was maintained throughout the fully-reversed loading conditions (strain ratio R = −1). The observed fatigue life was shorter when the loading direction was perpendicular to the solidification one, indicating an anisotropic material response. It was found that the stress amplitude remained almost constant until final fracture, suggesting limited cyclic hardening/softening. Also, stress relaxation was clearly observed during the dwell period. Scanning Electron Microscopy fractographic analyses showed evidence of similar failure modes in all the specimens. To understand deformation at grain level, crystal plasticity finite element modelling was carried out based on grain textures measured with EBSD. The model simulated the full history of cyclic stress-strain responses. It was particularly revealed that the misorientations between columnar grains resulted in heterogeneous deformation and localised stress concentrations, which became more severe when the loading direction was normal to a solidification direction, explaining the shorter fatigue life observed

Material model for modeling clay at high strain rates

This paper was accepted for publication in the journal International Journal of Impact Engineering and the definitive published version is available at http://dx.doi.org/10.1016/j.ijimpeng.2015.11.005Modeling clay is a soft malleable material made from oils and waxes. This material is fundamental for ballistic evaluation of body armors because it is used as backing material in ballistic tests. After a ballistic impact, a back-face indentation is measured to assess performance of the armor. Due to the important role of modeling clay in this particular application, its mechanical characterization and comprehension of penetration mechanics are essential for development of new personal protection systems. This paper presents a two-step computational methodology to calibrate parameters of a Cowper-Symonds material model for modeling clay at characteristic strain rates up to 1.8×104 s-1. In the first stage, a high-speed camera is used to record the penetration of a gas-gun launched cylindrical mass with a hemispherical cap into a block of clay. Image-processing software is used to capture the tail of the projectile as it penetrates into the clay. These data are then used to sample the penetration depth as function of time. In the second stage, an in-house developed model of penetration, based on both the spherical cavity expansion theory and the Tate penetration equation, is used to determine, by inverse analysis, the parameters of the Cowper-Symonds clay model. The proposed constitutive relationship for clay and the determined material parameters can be applied accurately to problems involving high strain rates