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Abstract: Metallic foams are a new class of functional materials. They have found their applications as sandwich cores in lightweight structures and as implant materials in bioengineering. To characterize the mechanical properties of these materials becomes an interesting and relevant research topic. In the mean time, indentation method has been well accepted as a simple and effective way to measure the mechanical properties of solid materials. We believe that it is possible to study the averaged mechanical properties of a metallic foam from a spherical indentation test. In this paper, theoretical investigation to understand the spherical indentation responses of metallic foams is presented. Based on the dimensional analysis, several scaling relationships in the indentation of metallic foams with a spherical indenter are obtained. Numerical results from the finite element simulations are used to examine the dependence of the indentation response on the basic material parameters, such as the porosity, the work hardening exponent and the shape factor, which characterizes the plastic deformation of metallic foams due to hydrostatic loading. Our numerical results show that the maximum indentation force has a linear relationship with the indentation depth for different shape factor values. It is therefore proposed to calibrate the shape factor value from the slope of the maximum indentation force versus the indentation depth from a spherical indentation test, instead of a complicated hydrostatic loading test. We also find that the spherical indentation hardness varies about 11% within the examined indentation depths. The range of the ratio of the hardness to the yield strength of metallic foams is from 2.17 to 2.95, which is different from that of solid materials. Our study provides the basis for applying a simple spherical indentation test to investigate the mechanical properties of metallic foams. [Copyright &y& Elsevier]

Abstract: Recent experimental research indicates that superelastic shape memory alloy nickel–titanium (NiTi) is superior to stainless steel against wear and could be applied in tribological engineering. It is believed that the super wear resistance of shape memory alloys is mainly due to the recovery of the superelastic deformation. Our recent wear study indicates that wear rate is very sensitive to the maximum contact pressure. In the present investigation, which involves applying Hertz contact theory and the finite element method, the wear behaviour of shape memory alloys is examined against that of stainless steels through analyzing the maximum contact pressure and the plastic deformation. Our investigation indicates that the contribution of superelasticity to the high wear resistance of NiTi is directly linked to the low transformation stress and the large recoverable transformation strain. Furthermore, the low Young''s modulus of this alloy also plays an important role to reduce the maximum contact pressure and therefore reduce the wear rate. Additionally, the high plastic yield strength of transformed martensite NiTi enhances its wear resistance further. [Copyright &y& Elsevier]

Abstract: Using dimensional analysis and the finite element method, the spherical indentation hardness of shape memory alloys (SMAs) is investigated. The scaling relationship between the hardness and the mechanical properties of a SMA, such as the forward transformation stress, the maximum transformation strain magnitude, has been derived. Numerical results demonstrated that the hardness increases with the indentation depth but there is no three-fold relationship between the hardness and the forward transformation stress. Increasing the maximum transformation strain magnitude would reduce the hardness of the material. These research results enhance our understanding of the hardness from the spherical indentation of SMAs. [Copyright &y& Elsevier]

Stimulated by recent experimental results on superelastic NiTi shape memory alloy, a theoretical study is carried out to quantify the effect of plasticity on stress-induced martensite transformation, using a constitutive model that combines phase transformation and plasticity. A constraint equation is introduced to quantify the phenomenon of the stabilisation of plasticity on stress-induced martensite. The stabilised martensite volume fraction is determined by the equivalent plastic strain. The transformation constitutive model is adopted from a generalised plastic model with Drucker–Prager type phase transformation functions, which are pressure sensitive, while the plasticity is described by the von Mises isotropic hardening model. The martensite volume fraction is chosen as the internal variable to represent the transformation state and it is determined by the consistency transformation condition. An approach to calibrate model parameters from uniaxial tensile tests is explored, as well as the issue of elastic mismatch between austenite and martensite is discussed. Based on the proposed constitutive model, the influence of hydrostatic stress on transformation is examined. As an example of application, this new constitutive model is employed to numerically study the transformation field and the plastic deformation field near a crack tip. [Copyright &y& Elsevier]

The impacts of preheating conditions and carbon dilution on the microstructural and mechanical properties of laser cladded rails using single and double cladding layers have been investigated for a hypereutectoid steel grades typically used under heavy haul conditions. The microstructures in the HAZ showed that formation of martensite, which has a detrimental effect on behaviour in wheel-rail contact, was successfully inhibited by increasing the length of the preheated region using a preheating temperature of 350 °C. Dilution of carbon from the hypereutectoid substrate was observed and its effect on the microstructures of the 410L ferritic stainless-steel deposits was investigated. The formation of ferrite in the 410L cladding layers was attributed to the very low carbon content, and no carbide formation was observed on boundaries of the ferritic grains. The thickness of dilution band was determined to be approximately equal to the thickness of the first cladding layer. Texture measurement obtained by EBSD showed a random trend owing to the formation of martensite in diluted bands. Strong solidification fibre texture was developed for double deposition, particularly in the second deposit. Mechanical characterization of the 410L deposits undertaken in terms of Vickers microhardness, shear and tensile yield strengths, and ultimate tensile and shear strengths were correlated with the observed microstructural morphologies. [ABSTRACT FROM AUTHOR]

Abstract: The fracture toughness and mode I fatigue crack growth (FCG) tests for open cell stainless steel foam with 45 pores per inch (ppi) have been carried out. In this study, the R-curve of crack propagating from a pre-crack was measured for a compact tension specimen by fracture toughness test. The fracture mechanics response was simulated by using an inverted spherical foam modeling approach. The results attained for crack extension rates were described by ΔK, using the Paris-power law approach. The compact tension porous stainless steel specimens have shown full plastic collapse along the ligaments. The microstructure of the foam had a significant influence on the fatigue crack propagation rate. It was found that stainless steel foam has higher Paris exponent than solid stainless steel and the high Paris exponent was explained by crack bridging. The simulation results showed that initiation toughness values strongly depend on failing struts, resulting in cracks that are significantly curved and kinked along their weakest path in stainless steel foam. The results from this study help in predicting and improving mechanical properties of metallic foams with open cell structure. [Copyright &y& Elsevier]

Abstract: There is an urgent need to understand the failure behavior of titanium foams because of their promising application as load-bearing implant materials in biomedical applications. Following our recent study on fracture toughness of titanium foams , this paper investigates the mode I fatigue crack propagation in 60% porous open pore titanium foams both with and without solid coated surface. Fatigue crack propagation tests were performed on compact tension specimens at load ratios of R =0.1 and R =0.5 and the fracture surfaces were examined using scanning electron microscopy. The crack growth rate, da/dN, versus the stress intensity factor range, ΔK, curves were measured and compared using two different techniques; image processing and compliance methods. The crack extension rates were well described by ΔK, using the Paris-power law approach. Coated and non-coated titanium foams with 60% porosity had a significantly higher Paris exponent than solid titanium, which can be explained by crack closure and crack bridging. It was also shown that the fatigue crack grows along the centerline, following the weakest path throughout the foam. The results obtained from this work provide important information for evaluating the structural integrity of porous titanium components in the future biomedical applications. [Copyright &y& Elsevier]

Abstract: The fracture behavior of titanium open foam is characterized and the R-curves of crack propagation from pre-cracks are measured. The crack growth has been optically observed, the measured initiation toughness, J IC, has been analyzed and the effect of material morphology on the J IC is discussed. The fracture toughness was found to be dependent on the expanding crack bridging zone at the back of the crack tip. The compact tension specimens also have some plastic collapse along the ligaments and it has shown that the titanium foam with a higher relative density is tougher. The non-uniform stressing within the plastic zone at the crack tip and the plastic collapse of cell topology behind the tip was found to be the primary cause of the R-curve behavior in low relative density titanium foams. [ABSTRACT FROM AUTHOR]