Alloys-by-design:A low-modulus titanium alloy for additively manufactured biomedical implants

The performance of many metal biomedical implants – such as fusion cages for spines – is inherently limited by the mismatch of mechanical properties between the metal and the biological bone tissue it promotes. Here, an alloy design approach is used to isolate titanium alloy compositions for biocompatibility which exhibit a modulus of elasticity lower than the Ti-6Al-4V grade commonly employed for this application. Due to the interest in alloys for personalised medicine, additive manufacturability is also considered: compositions with low cracking susceptibility and with propensity for non-planar growth are identified. An optimal alloy composition is selected for selective laser melting, and its processability and mechanical properties tested. Additive manufacturing is used to engineer an heterogeneous microstructure with outstanding combined strength and ductility. Our results confirm the suitability of novel titanium alloys for lowering the stiffness towards that needed whilst being additively manufacturable and strong

Alloys-by-design:A low-modulus titanium alloy for additively manufactured biomedical implants

The performance of many metal biomedical implants – such as fusion cages for spines – is inherently limited by the mismatch of mechanical properties between the metal and the biological bone tissue it promotes. Here, an alloy design approach is used to isolate titanium alloy compositions for biocompatibility which exhibit a modulus of elasticity lower than the Ti-6Al-4V grade commonly employed for this application. Due to the interest in alloys for personalised medicine, additive manufacturability is also considered: compositions with low cracking susceptibility and with propensity for non-planar growth are identified. An optimal alloy composition is selected for selective laser melting, and its processability and mechanical properties tested. Additive manufacturing is used to engineer an heterogeneous microstructure with outstanding combined strength and ductility. Our results confirm the suitability of novel titanium alloys for lowering the stiffness towards that needed whilst being additively manufacturable and strong

On the rate dependent behaviour of epoxy adhesive joints: experimental characterisation and modelling of mode I failure

The increasing use of adhesive joints in dynamic applications require reliable measurements of the rate-dependent stress-displacement behaviour. The direct measurement of the stress-displacement curve is necessary when using cohesive models in discretised solutions of boundary value problems in solid mechanics. This paper aims to investigate the rate-dependent tensile failure of adhesive joints by using a new experimental methodology – it relies upon the combination of the stress wave propagation theory and digital image correlation methods on high speed footage to quantify the tensile stress and the dissipated energy respectively. For this purpose, the Split Hopkinson Bar methodology was employed – the experimental configuration was optimised using numerical modelling. To prove the sensitivity of our framework, two different adhesives are characterised at different loading rates: the adhesive failure strength was found to increase considerably with the strain rate, while the plastic deformation of these adhesives was reduced. The film adhesive showed superior performance over the particle toughened one. In the final part, a rate-dependent cohesive zone model is proposed, one which captures the measured behaviour and which has the potential to be used in industrial applications

Estimating correspondences of deformable objects "in-the-wild"

This is the author accepted manuscript. The final version is available from IEEE via the DOI in this recordDuring the past few years we have witnessed the development of many methodologies for building and fitting Statistical Deformable Models (SDMs). The construction of accurate SDMs requires careful annotation of images with regards to a consistent set of landmarks. However, the manual annotation of a large amount of images is a tedious, laborious and expensive procedure. Furthermore, for several deformable objects, e.g. human body, it is difficult to define a consistent set of landmarks, and, thus, it becomes impossible to train humans in order to accurately annotate a collection of images. Nevertheless, for the majority of objects, it is possible to extract the shape by object segmentation or even by shape drawing. In this paper, we show for the first time, to the best of our knowledge, that it is possible to construct SDMs by putting object shapes in dense correspondence. Such SDMs can be built with much less effort for a large battery of objects. Additionally, we show that, by sampling the dense model, a part-based SDM can be learned with its parts being in correspondence. We employ our framework to develop SDMs of human arms and legs, which can be used for the segmentation of the outline of the human body, as well as to provide better and more consistent annotations for body joints.Engineering and Physical Sciences Research Council (EPSRC)TekesEuropean Community Horizon 202

On the dynamic response of adhesively bonded structures

Fracture mechanics experiments are used to investigate the rate-dependent failure of adhesively bonded structures under different deformation modes: I, II and I/II. First, the high-rate mechanical response of the adhesive interface is analysed with a newly developed method – which relies entirely upon digital image correlation. The method was purposely designed to avoid any dynamic effects which may be present. This novel method is verified against quasi-static standard methods showing good agreement. Finally, simulations of the experiments are used to validate a cohesive zone model of the adhesive. The ability of the model to predict cohesive failure under a wide range of strain rates and deformation modes is demonstrated

On the microtwinning mechanism in a single crystal superalloy

© 2017 Acta Materialia Inc. The contribution of a microtwinning mechanism to the creep deformation behaviour of single crystal superalloy MD2 is studied. Microtwinning is prevalent for uniaxial loading along 〈011〉 at 800°C for the stress range 625 to 675 MPa and 825°C for 625 MPa. Using quantitative stereology, the twin fraction and twin thickness are estimated; this allows the accumulated creep strain to be recovered, in turn supporting the role of the microtwinning mode in conferring deformation. Atom probe tomography confirms the segregation of Cr and Co at the twin/parent interface, consistent with the lowering of the stacking fault energy needed to support twin lengthening and thickening. A model for diffusion-controlled growth of twins is proposed and it is used to recover the measured creep strain rate. The work provides the basis for a thermo-mechanical constitutive model of deformation consistent with the microtwinning mechanism

Synthetic bone: design by additive manufacturing

A broad range of synthetic trabecular-like metallic lattices are 3D printed, to study the extra design freedom conferred by this new manufacturing process. The aim is to propose new conceptual types of implant structures for superior bio-mechanical matching and osseo-integration: synthetic bone. The target designs are 3D printed in Ti-6Al-4V alloy using a laser-bed process. Systematic evaluation is then carried out: (i) their accuracy is characterised at high spatial resolution using computed X-ray tomography, to assess manufacturing robustness with respect to the original geometrical design intent and (ii) the mechanical properties - stiffness and strength - are experimentally measured, evaluated, and compared. Finally, this new knowledge is synthesised in a conceptual framework to allow the construction of so-called implant design maps, to define the processing conditions of bone tailored substitutes, with focus on spine fusion devices. The design criteria emphasise the bone stiffness-matching, preferred range of pore structure for bone in-growth, manufacturability of the device and choice of inherent materials properties which are needed for durable implants. Examples of the use of such maps are given with focus on spine fusion devices, emphasising the stiffness-matching, osseo-integration properties and choice of inherent materials properties which are needed for durable implants. STATEMENT OF SIGNIFICANCE: We present a conceptual bio-engineering design methodology for new biomedical lattices produced by additive manufacturing, which addresses some of the critical points in currently existing porous implant materials. Amongst others: (i) feasibility and accuracy of manufacturing, (ii) design to the elastic properties of bone, and (iii) sensible pores sizes for osseointegration. This has inspired new and novel geometrical latticed designs which aim at improving the properties of intervertebral fusion devices. In their fundamental form, these structures are here fabricated and tested. When integrated into medical devices, these concepts could offer superior medical outcomes

Synthetic bone: design by additive manufacturing

A broad range of synthetic trabecular-like metallic lattices are 3D printed, to study the extra design freedom conferred by this new manufacturing process. The aim is to propose new conceptual types of implant structures for superior bio-mechanical matching and osseo-integration: synthetic bone. The target designs are 3D printed in Ti-6Al-4V alloy using a laser-bed process. Systematic evaluation is then carried out: (i) their accuracy is characterised at high spatial resolution using computed X-ray tomography, to assess manufacturing robustness with respect to the original geometrical design intent and (ii) the mechanical properties - stiffness and strength - are experimentally measured, evaluated, and compared. Finally, this new knowledge is synthesised in a conceptual framework to allow the construction of so-called implant design maps, to define the processing conditions of bone tailored substitutes, with focus on spine fusion devices. The design criteria emphasise the bone stiffness-matching, preferred range of pore structure for bone in-growth, manufacturability of the device and choice of inherent materials properties which are needed for durable implants. Examples of the use of such maps are given with focus on spine fusion devices, emphasising the stiffness-matching, osseo-integration properties and choice of inherent materials properties which are needed for durable implants. STATEMENT OF SIGNIFICANCE: We present a conceptual bio-engineering design methodology for new biomedical lattices produced by additive manufacturing, which addresses some of the critical points in currently existing porous implant materials. Amongst others: (i) feasibility and accuracy of manufacturing, (ii) design to the elastic properties of bone, and (iii) sensible pores sizes for osseointegration. This has inspired new and novel geometrical latticed designs which aim at improving the properties of intervertebral fusion devices. In their fundamental form, these structures are here fabricated and tested. When integrated into medical devices, these concepts could offer superior medical outcomes

Alloys-By-Design: a low-modulus titanium alloy for additively manufactured biomedical implants

The performance of many metal biomedical implants – such as fusion cages for spines – is inherently limited by the mismatch of mechanical properties between the metal and the biological bone tissue it promotes. Here, an alloy design approach is used to isolate titanium alloy compositions for biocompatibility which exhibit a modulus of elasticity lower than the Ti-6Al-4V grade commonly employed for this application. Due to the interest in alloys for personalised medicine, additive manufacturability is also considered: compositions with low cracking susceptibility and with propensity for non-planar growth are identified. An optimal alloy composition is selected for selective laser melting, and its processability and mechanical properties tested. Additive manufacturing is used to engineer an heterogeneous microstructure with outstanding combined strength and ductility. Our results confirm the suitability of novel titanium alloys for lowering the stiffness towards that needed whilst being additively manufacturable and strong