Spherical indentation of superelastic shape memory alloys

Spherical indentation of superelastic shape memory alloys (SMAs) has been theoretically analyzed. Two characteristic points on the superelastic indentation curve have been discovered. The bifurcation force corresponding to the bifurcation point relies on the forward transformation stress and the return force corresponding to the return point relies on the reverse transformation stress.
Based on these theoretical relationships, an approach to determine the transformation stresses of superelastic SMAs has been proposed. To improve the accuracy of the measurement, a slope method to locate the two characteristic points from the slope curves is further suggested. Additionally, the spherical indentation hardness was also analyzed.

Analysis of spherical indentation of superelastic shape memory alloys

Dimensional analysis and the finite element method are applied in this paper to study spherical indentation of superelastic shape memory alloys. The scaling relationships derived from dimensional analysis bridge the indentation response and the mechanical properties of a superelastic shape memory alloy. Several key variables of a superelastic indentation curve are revealed and examined. We prove that the bifurcation force in a superelastic indentation curve only relies on the forward transformation stress and the elastic properties of the initial austenite; and the return force in a superelastic indentation curve only relies on the reverse transformation stress and the elastic properties of the initial austenite. Furthermore, the dimensionless functions to determine the bifurcation force and the return force are proved to be identical. These results not only enhance our understanding of spherical indentation of superelastic shape memory alloys, but also provide the theoretical basis for developing a practicable method to calibrate the mechanical properties of a superelastic material from the spherical indentation test.

Processing and mechanical properties of porous titanium-niobium shape memory alloy for biomedical applications

Ti-26 at.%Nb (hereafter Ti-26Nb) alloy foams were fabricated by space-holder sintering process. The porous structures of the foams were characterized by scanning electron microscopy (SEM). The mechanical properties of the Ti-26Nb foam samples were investigated using compressive test. Results indicate that mechanical properties of Ti-26Nb foam samples are influenced by foam porosity. The plateau stresses and elastic moduli of the foams under compression decrease with the increase of their porosities. The plateau stresses and elastic moduli are measured to be from 10~200 MPa and 0.4~5.0 GPa for the Ti-26Nb foam samples with porosities ranged from 80~50 %, respectively.

Effect of alloying additions on the SFE, Neél temperature and shape memory effect in Fe–Mn–Si-based alloys

A range of Fe–Mn–Si-based shape memory alloys has been investigated to examine the interplay of composition, stacking fault probability (SFP) and Neél temperature on the shape memory effect (SME). It has been found that the SFP (inversely proportional to stacking fault energy) showed little correlation to the SME for the range of alloy compositions examined. Further, the Neél temperature was not found to exhibit a significant effect on the SME. The addition of interstitial elements, however, was found to markedly decrease the SME.

Nanofretting behaviors of NiTi shape memory alloy

Nanofretting refers to cyclic movements of contact interfaces with the relative displacement amplitude at the nanometer scale, where the contact area and normal load are usually much smaller than those in fretting. Nanofretting widely exists in microelctromechanical systems (MEMS) and may become a key tribological concern besides microwear and adhesion. With a triboindenter, the nanofretting behaviors of a nickel titanium (NiTi) shape memory alloy are studied under various normal loads (1–10 mN) and tangential displacement amplitudes (2–500 nm) by using a spherical diamond tip. Similar to fretting, the nanofretting of NiTi/diamond pair can also be divided into different regimes upon various shapes of tangential force–displacement curves. The dependence of nanofretting regime on the normal load and the displacement amplitude can be summarized in a running condition nanofretting map. However, due to the surface and size effects, nanofretting operates at some different conditions, such as improved mechanical properties of materials at the nanometer scale, small apparent contact area and single-asperity contact behavior. Consequently, different from fretting, nanofretting was found to exhibit several unique behaviors: (i) the maximum tangential force in one cycle is almost unchanged during a nanofretting test, which is different from a fretting test where the maximum tangential force increases rapidly in the first dozens of cycles; (ii) the tangential stiffness in nanofretting is three orders magnitude smaller than that in fretting; (iii) the friction coefficient in nanofretting is much lower than that in fretting in slip regime; (iv) no obvious damage was observed after 50 cycles of nanofretting under a normal load of 10 mN.

TiNi shape memory alloy foams synthesized by spacer sintering and their properties

Titanium (Ti) and nickel (Ni) elemental powders were blened by ball milling and the ball milled powders were employed to fabricate NiNi shape memory alloy (SMA) foams by space sintering. Effect of ball milling time on phase constitutes of the sintered TiNi alloy foams was studied by X-ray diffraction (XRD) analysis.Scanning election microscopy (SEM) was used to characterize the porous structure, and compressive tests were carried out to evaluate the mechanical properties of the foams. Results indicate the porosities of the TiNi alloy foams can be controlled by using the spacer sincering method, and the porosities show a significant effect on the mechanical prperties and shape memory effect (SME).

Ontogeny of body size and shape of Antarctic and subantarctic fur seals

Pre- and post-weaning functional demands on body size and shape of mammals are often in conflict, especially in species where weaning involves a change of habitat. Compared with long lactations, brief lactations are expected to be associated with fast rates of development and attainment of adult traits. We describe allometry and growth for several morphological traits in two closely related fur seal species with large differences in lactation duration at a sympatric site. Longitudinal data were collected from Antarctic (Arctocephalus gazella (Peters, 1875); 120 d lactation) and subantarctic (Arctocephalus tropicalis (Gray, 1872); 300 d lactation) fur seals. Body mass was similar in neonates of both species, but A. gazella neonates were longer, less voluminous, and had larger foreflippers. The species were similar in rate of preweaning growth in body mass, but growth rates of linear variables were faster for A. gazella pups. Consequently, neonatal differences in body shape increased over lactation, and A. gazella pups approached adult body shape faster than did A. tropicalis pups. Our results indicate that preweaning growth is associated with significant changes in body shape, involving the acquisition of a longer, more slender body with larger foreflippers in A. gazella. These differences suggest that A. gazella pups are physically more mature at approximately 100 d of age (close to weaning age) than A. tropicalis pups of the same age

A composite descriptor for shape retrieval

In this paper, a composite descriptor for shape retrieval has been proposed. The proposed descriptor is obtained from Generic Fourier Descriptors (GFD) for the shape region and the shape contour. A composite descriptor derived from GFD of the shape region and the shape contour is used for indexing and retrieval of shapes. Difference between two images is computed as the Euclidean distance between their composite descriptors. Experiments are performed to test the effectiveness of the proposed descriptor for retrieval of 2d images. Sets of composite descriptors, obtained by assigning different weights to the region component and the contour component, are also evaluated. Item S8 within the MPEG-7 Still Images Content Set is used for performing experiments; this dataset consists of 3621 still images. Experimental results show that the proposed descriptor is effective.

A cross-sectional study of weight-and shape-related beliefs, behaviours and concerns of north Queensland adolescents

The purpose of this study was to examine differences in weight- and shape-related beliefs, behaviours and concerns across a high school population. Data were collected by questionnaire from 902 high school students from private schools in Townsville. Concerns about weight and shape were more apparent in students from the higher school years. More girls in higher than lower school years were dissatisfied with their bodies, concerned that many parts of their bodies were too fat and more of them attempted weight loss. More boys in higher than lower school years were satisfied with their bodies but they wanted to 'bulk up'. Societal expectations of weight and shape may adversely affect both males and females but in different ways. Thus, interventions aiming to promote the maintenance of a healthy body weight may need to account for the desire of the majority of young women to be thinner, while the majority of young men would like to be larger.