The effect of crystallographic texture on stress-induced martensitic transformation in NiTi: A computational analysis.

NiTi׳s superelasticity is exploited in a number of biomedical devices, in particular self-expanding endovascular stents. These stents are often laser-cut from textured micro-tubing; texture is the distribution of crystallographic grain orientations in a polycrystalline material which has been experimentally shown to have a marked influence on mechanical properties. This study offers a computational examination into the effect of texture on the stress-induced martensite transformation (SIMT) in a micro-dogbone NiTi specimen subject to tensile loading. Finite Element Analysis (FEA) is employed to simulate the transformational behaviour of the specimen on a micro-scale level. To represent a realistic grain structure in the FEA model, grains present in a 200µm×290µm test site located at the centre edge of the specimen were identified using Scanning Electron Microscopy (SEM). Grains are assumed to have homogenous behaviour with properties varying according to their crystallographic orientation to the loading direction. Required material properties were extracted from uniaxial stress-strain curves of single crystals for each crystallographic orientation for input into the in-built UMAT/Nitinol. The orientation of each grain in the test site was identified using Electron Back-Scatter Diffraction (EBSD) techniques. In this way, a quantitative explanation is offered to the effect of crystallographic texture on SIMT. Finally, the evolution of grains in the specimen, during the transformation process, was experimentally investigated by means of an in-situ SEM tensile test.

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In the midst of a rich environment for medical device development and manufacturing, universities can play a critical role by developing relevant training programs to produce entrepreneurs who can be efficient and successful in creating early stage companies by understanding deeply the issues involved in creating a useful device, how to raise money, designing early clinical studies and locating manufacturing partners.

Differences in the crack resistance of interstitial, osteonal and trabecular bone tissue.

The purpose of this work was to investigate differences which may exist in the crack resistance of the microstructural bone tissues, i.e., osteonal, interstitial and trabecular bone. Indentations, using varying loads were used to initiate cracks of the same size scale as those which exist habitually in bone. The crack lengths and corresponding toughness values are presented for each of the tissues. Specimens were prepared using standard nanoindentation preparation techniques. Young's modulus and hardness were measured using a Berkovich tip, while cracks were produced using a cube-corner tip. Crack lengths were subsequently measured using scanning electron microscopy. Cracks produced at the same loads were significantly longer in trabecular bone than in interstitial and osteonal cortical bone. Similarly, within individual subjects, cracks produced in interstitial bone were longer than those produced in osteonal bone. These results provide significant experimental evidence that bone microstructural tissues exhibit differing resistance to crack growth and may help explain the incidence of more microcracks in interstitial than osteonal bone. The ability of the technique to distinguish differences between individual bone tissues is promising in an area where the focus has switched to the microscale, and in particular, to measures bone quality.

Calibration of a constitutive model for the post-yield behaviour of cortical bone.

In this work, the post-yield behaviour of cortical bone is investigated using finite element modelling, nanoindentation and atomic force microscopy. Based on recent investigations, it is proposed that, since pressure dependent deformation mechanisms may contribute to yielding in bone, constitutive models attempting to capture its post-yield behaviour should also incorporate pressure dependence. Nanoindentation testing is performed using a spheroconical indenter tip, and subsequent atomic force microscopy at the indented site shows that bone does not exhibit surface pile-up. By simulating the nanoindentation test, it is found that a Mises based constitutive law cannot simultaneously capture the deformations and load-displacement curve produced during nanoindentation. However, an extended Drucker-Prager model can capture the post-yield behaviour of bone accurately, since it accounts for pressure dependent yield. This suggests that frictional mechanisms are central to the post-yield behaviour of bone. In this work, the extended Drucker-Prager model is calibrated and validated using further simulations.

Measurement of the microstructural fracture toughness of cortical bone using indentation fracture.

The purpose of this work is to investigate the use of indentation fracture as a method of measuring toughness at the microscale in cortical bone. Indentation fracture employs sharp indenters to initiate cracks, whose length can be used to calculate the toughness of the material. Only a cube corner indenter tip is found to initiate cracks at a suitable size scale for microstructural measurement. Cracks from 7 to 56 microm in length are produced using loads from 0.05 to 3N. Preliminary data predicts rising toughness with increasing crack length (rising R-curve behaviour) at the microscale. This technique provides a new insight into fracture in cortical bone since it allows the investigator to observe mechanisms and measure toughness at a size scale at which in vivo damage is known to exist.

Micromechanical modelling of cortical bone.

Cortical bone is a heterogeneous material with a complex hierarchical microstructure. In this work, unit cell finite element models were developed to investigate the effect of microstructural morphology on the macroscopic properties of cortical bone. The effect of lacunar and vascular porosities, percentage of osteonal bone and orientation of the Haversian system on the macroscopic elastic moduli and Poisson's ratios was investigated. The results presented provide relationships for applying more locally accurate material properties to larger scale and whole bone models of varying porosity. Analysis of the effect of the orientation of the Haversian system showed that its effects should not be neglected in larger scale models. This study also provides insight into how microstructural features effect local distributions and cause a strain magnification effect. Limitations in applying the unit cell methodology approach to bone are also discussed.

Finite element comparison of performance related characteristics of balloon expandable stents.

Cardiovascular stents are commonly made from 316L stainless steel and are deployed within stenosed arterial lesions using balloon expansion. Deployment involves inflating the balloon and plastically deforming the stent until the required diameter is obtained. This plastic deformation induces static stresses in the stent, which will remain for the lifetime of the device. In order to determine these stresses, finite element models of the unit cells of geometrically different, commercially available balloon expandable stents have been created, and deployment and elastic recoil have been simulated. In this work the residual stresses associated with deployment and recoil are compared for the various stent geometries, with a view to establishing appropriate initial stress states for fatigue loading for the stents. The maximum, minimum, and mean stresses induced in the stent due to systolic/diastolic pressure are evaluated, as are performance measures such as radial and longitudinal recoil.