Fatigue Crack Growth in Austenitic and Martensitic NiTi: Modeling and Experiments

AbstractFatigue crack growth of austenitic and martensitic NiTi shape memory alloys was analyzed, with the purpose of capturing the effects of distinct stress-induced transformation mechanics in the two crystal structures. Mode I crack growth experiments were carried out, and near-crack-tip displacements were captured by in-situ digital image correlation (DIC). A special fitting procedure, based on the William's solution, was used to estimate the effective stress intensity factor (SIF). The SIF was also computed by linear elastic fracture mechanics (LEFM) as well as by a special analytical model that takes into account the unique thermomechanical response of SMAs. A significant difference in the crack growth rate for the two alloys was observed, and it has been attributed to dissimilar dissipative phenomena and different crack-tip stress–strain fields, as also directly observed by DIC. Finally, it was shown that the predictions of the analytical method are in good agreement with effective results obtained by DIC, whereas a very large mismatch was observed with LEFM. Therefore, the proposed analytical model can be actually used to analyze fatigue crack propagation in both martensitic and austenitic NiTi

low to high cycle fatigue properties of a niti shape memory alloy

Abstract Low-to-high cycle fatigue behavior of a pseudoelastic NiTi SMA was analyzed. The evolution of both global and local strain were captured during fatigue tests. Local strains were measured in-situ by the digital image correlation (DIC) technique. Significant differences were observed at the two scales, due to the localized nature of stress-induced transformations. Strain-life fatigue curves obtained from global and local strain measurements were compared. It was demonstrated that local phenomena play a very important role on the fatigue properties of pseudoelastic SMAs

tribological properties of wear resistant coatings obtained by cold gas dynamic spray

Abstract The aim of this study was obtaining good deposits of stellite-6 by Cold Gas Dynamic Spray (CGDS), in terms of low porosity and good adhesion and cohesion. In fact, the high strength and melting point of the investigated alloy lead to a high value of the particle critical velocity in CGDS and, therefore, good quality results are difficult to achieve. The tribological properties of the coatings were analyzed by micro-hardness measurements and pin-on-disk wear tests. Results revealed that spraying parameters can be optimized to obtain almost pore-free coatings

multiaxial fatigue behavior of additive manufactured ti 6al 4v under in phase stresses

Abstract The development and application of additive manufacturing (AM) technologies is constantly increasing. However, in many applications, AM parts are subjected to multiaxial loads, arising from operating conditions and/or complex geometries. These make AM components serious candidates for crack initiation and propagation mechanisms. Therefore, a deep understanding of the multiaxial fatigue behavior of AM parts is essential in many applications where durability and reliability are core issues. In this study, multiaxial fatigue of Ti6Al4V thin-walled tubular specimens, made by Selective Laser Melting (SLM) process, was investigated by combined axial-torsional loads. Infrared thermography (IR) was also used to investigate the temperature evolution during fatigue tests. Results highlighted different damage mechanisms and failure modes in the low- and high-cycle fatigue regimes

Multiaxial fatigue behavior of additively manufactured Ti6Al4V alloy: Axial–torsional proportional loads

Additive manufacturing (AM) techniques are under constant development and selective laser melting (SLM) is among the most promising ones. However, widespread use of AM techniques in many industries is limited by the different/unusual mechanical properties of AM metallic parts, with respect to traditionally processed ones, especially when dealing with complex fatigue loading conditions. In fact, crack formation and propagation mechanisms are mainly affected by the development of internal defects, residual stresses, and microstructural changes. This is actually one of the major issues the materials engineering community is facing today. In many applications, AM components are subjected to multiaxial fatigue loads, arising from operating conditions and/or from complex geometries, that unavoidably generate crack initiation and propagation mechanisms. The aim of this study is to investigate the multiaxial fatigue behavior of additively manufactured Ti6Al4V samples, made by SLM. Fatigue tests, combining proportional axial and torsional loads, were performed on thin-walled tubular specimens. Full-field measurement techniques, such as the infrared thermography and digital image correlation, were also used to capture temperature and strain evolutions, at both local scales and global scales. Fatigue results highlighted damage mechanisms, and failure modes are strongly related to the applied stress level

Identification of Sclerostin as a Putative New Myokine Involved in the Muscle-to-Bone Crosstalk

Bone and muscle have been recognized as endocrine organs since they produce and secrete “hormone-like factors” that can mutually influence each other and other tissues, giving rise to a “bone–muscle crosstalk”. In our study, we made use of myogenic (C2C12 cells) and osteogenic (2T3 cells) cell lines to investigate the effects of muscle cell-produced factors on the maturation process of osteoblasts. We found that the myogenic medium has inhibitory effects on bone cell differentiation and we identified sclerostin as one of the myokines produced by muscle cells. Sclerostin is a secreted glycoprotein reportedly expressed by bone/cartilage cells and is considered a negative regulator of bone growth due to its role as an antagonist of the Wnt/β-catenin pathway. Given the inhibitory role of sclerostin in bone, we analyzed its expression by muscle cells and how it affects bone formation and homeostasis. Firstly, we characterized and quantified sclerostin synthesis by a myoblast cell line (C2C12) and by murine primary muscle cells by Western blotting, real-time PCR, immunofluorescence, and ELISA assay. Next, we investigated in vivo production of sclerostin in distinct muscle groups with different metabolic and mechanical loading characteristics. This analysis was done in mice of different ages (6 weeks, 5 and 18 months after birth) and revealed that sclerostin expression is dynamically modulated in a muscle-specific way during the lifespan. Finally, we transiently expressed sclerostin in the hind limb muscles of young mice (2 weeks of age) via in vivo electro-transfer of a plasmid containing the SOST gene in order to investigate the effects of muscle-specific overproduction of the protein. Our data disclosed an inhibitory role of the muscular sclerostin on the bones adjacent to the electroporated muscles. This observation suggests that sclerostin released by skeletal muscle might synergistically interact with osseous sclerostin and potentiate negative regulation of osteogenesis possibly by acting in a paracrine/local fashion. Our data point out a role for muscle as a new source of sclerostin.Bone and muscle have been recognized as endocrine organs since they produce and secrete “hormone-like factors” that can mutually influence each other and other tissues, giving rise to a “bone–muscle crosstalk”. In our study, we made use of myogenic (C2C12 cells) and osteogenic (2T3 cells) cell lines to investigate the effects of muscle cell-produced factors on the maturation process of osteoblasts. We found that the myogenic medium has inhibitory effects on bone cell differentiation and we identified sclerostin as one of the myokines produced by muscle cells. Sclerostin is a secreted glycoprotein reportedly expressed by bone/cartilage cells and is considered a negative regulator of bone growth due to its role as an antagonist of the Wnt/β-catenin pathway. Given the inhibitory role of sclerostin in bone, we analyzed its expression by muscle cells and how it affects bone formation and homeostasis. Firstly, we characterized and quantified sclerostin synthesis by a myoblast cell line (C2C12) and by murine primary muscle cells by Western blotting, real-time PCR, immunofluorescence, and ELISA assay. Next, we investigated in vivo production of sclerostin in distinct muscle groups with different metabolic and mechanical loading characteristics. This analysis was done in mice of different ages (6 weeks, 5 and 18 months after birth) and revealed that sclerostin expression is dynamically modulated in a muscle-specific way during the lifespan. Finally, we transiently expressed sclerostin in the hind limb muscles of young mice (2 weeks of age) via in vivo electro-transfer of a plasmid containing the SOST gene in order to investigate the effects of muscle-specific overproduction of the protein. Our data disclosed an inhibitory role of the muscular sclerostin on the bones adjacent to the electroporated muscles. This observation suggests that sclerostin released by skeletal muscle might synergistically interact with osseous sclerostin and potentiate negative regulation of osteogenesis possibly by acting in a paracrine/local fashion. Our data point out a role for muscle as a new source of sclerostin

Shape Memory Alloy—Polymer Composites: Static and Fatigue Pullout Strength under Thermo-Mechanical Loading

This work was carried out within the context of an R&D project on morphable polymer matrix composites (PMC), actuated by shape memory alloys (SMA), to be used for active aerodynamic systems in automotives. Critical issues for SMA–polymer integration are analyzed that are mostly related to the limited strength of metal–polymer interfaces. To this aim, materials with suitable thermo-mechanical properties were first selected to avoid premature activation of SMA elements during polymer setting as well as to avoid polymer damage during thermal activation of SMAs. Nonstandard samples were manufactured for both static and fatigue pullout tests under thermo-mechanical loading, which are made of SMA wires embedded in cylindrical resin blocks. Fully coupled thermo-mechanical simulations, including a special constitutive model for SMAs, were also carried out to analyze the stress and temperature distribution in the SMA–polymer samples as obtained from the application of both mechanical loads and thermal activation of the SMA wires. The results highlighted the severe effects of SMA thermal activation on adhesion strength due to the large recovery forces and to the temperature increase at the metal–polymer interface. Samples exhibit a nominal pullout stress of around 940 MPa under static mechanical load, and a marked reduction to 280 MPa was captured under simultaneous application of thermal and mechanical loads. Furthermore, fatigue run-out of 5000 cycles was achieved, under the combination of thermal activation and mechanical loads, at a nominal stress of around 200 MPa. These results represent the main design limitations of SMA/PMC systems in terms of maximum allowable stresses during both static and cyclic actuation