The effect of mechanical loading on the size and shape of bone in pre-, peri-, and postpubertal girls: a study in tennis players

Exercise during growth results in biologically important increases in bone mineral content (BMC). The aim of this study was to determine whether the effects of loading were site specific and depended on the maturational stage of the region. BMC and humeral dimensions were determined using DXA and magnetic resonance imaging (MRI) of the loaded and nonloaded arms in 47 competitive female tennis players aged 8-17 years. Periosteal (external) cross-sectional area (CSA), cortical area, medullary area, and the polar second moments of area (I_p, mm⁴) were calculated at the mid and distal sites in the loaded and nonloaded arms. BMC and I _p of the humerus were 11-14% greater in the loaded arm than in the nonloaded arm in prepubertal players and did not increase further in peri- or postpubertal players despite longer duration of loading (both, _p < 0.01). The higher BMC was the result of a 7-11% greater cortical area in the prepubertal players due to greater periosteal than medullary expansion at the midhumerus and a greater periosteal expansion alone at the distal humerus. Loading late in puberty resulted in medullary contraction. Growth and the effects of loading are region and surface specific, with periosteal apposition before puberty accounting for the increase in the bone's resistance to torsion and endocortical contraction contributing late in puberty conferring little increase in resistance to torsion. Increasing the bone's rt.osistance to torsion is achieved hy modifying bone shape and mass, not necessarily bone density.

Using geometric shape variations to create an inverse model for a sheet metal process

The output of the sheet metal forming process is subject to much variation. This paper develops a method to measure shape variation in channel forming and relate this back to the corresponding process parameter levels of the manufacturing set-up to create an inverse model. The shape variation in the channels is measured using a modified form of the point distribution model (also known as the active shape model). This means that channels can be represented by a weighting vector of minimal linear dimension that contains all the shape variation information from the average formed channel.

The inverse models were created using classifiers that related the weighting vectors to the process parameter levels for the blank holder force (BHF), die radii (DR) and tool gap (TG) of the parameters. Several classifiers were tested: linear, quadratic Gaussian and artificial neural networks. The quadratic Gaussian classifiers were the most accurate and the most consistent type of classifier over all the parameters.

A shape error metric for sheet metal forming and its application to springback

There are many variations within sheet metal forming, some of which are manifest in the final geometry of the formed component. It is important that this geometric variation be quantified and measured for use in a process or quality control system. The contribution of this paper is to propose a novel way of measuring the geometric difference between the desired shape and an actual formed "U" channel. The metric is based upon measuring errors in terms of the significant manufacturing variations. The metric accords with the manually measured errors of the channel set. The shape error metric is then extended to develop a simple empirical, whole-component, springback error measure. The springback error measure combines into one value all the angle springback and side wall curl geometric errors for a single channel. Two trends were observed: combined springback decreases when the blank holder force is increased; and the combined springback marginally decreases when the die radii is increased.

Spherical indentation hardness of shape memory alloys

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.

Theoretical investigation of wear-resistance mechanism of superelastic shape memory alloy NiTi

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.

Geometric shape errors in forging: developing a metric and an inverse model

The complexity of the forging process ensures that there is inherent variability in the geometric shape of a forged part. While knowledge of shape error, comparing the desired versus the measured shape, is significant in measuring part quality the question of more interest is what can this error suggest about the forging process set-up? The first contribution of this paper is to develop a shape error metric which identifies geometric shape differences that occur from a desired forged part. This metric is based on the point distribution deformable model developed in pattern recognition research. The second contribution of this paper is to propose an inverse model that identifies changes in process set-up parameter values by analysing the proposed shape error metric. The metric and inverse models are developed using two sets of simulated hot-forged parts created using two different die pairs (simple and 'M'-shaped die pairs). A neural network is used to classify the shape data into three arbitrarily chosen levels for each parameter and it is accurate to at least 77 per cent in the worst case for the simple die pair data and has an average accuracy of approximately 80 per cent when classifying the more complex 'M'-shaped die pair data.

Effect of transformation volume contraction on the toughness of superelastic shape memory alloys

Shape memory alloys (SMAs) exhibit two very important properties: shape memory phenomenon and superelastic deformation due to intrinsic thermoelastic martensitic transformation. To fully exploit the potential of SMAs in developing functional structures or smart structures in mechanical and biomechanical engineering, it is important to understand and quantify the failure mechanisms of SMAs. This paper presents a theoretical study of the effect of phase-transformation-induced volume contraction on the fracture properties of superelastic SMAs. A simple model is employed to account for the forward and reverse phase transformation with pure volume change, which is then applied to numerically study the transformation field near the tip of a tensile crack. The results reveal that during steady-state crack propagation, the transformation zone extends ahead of the crack tip due to forward transformation while partial reverse transformation occurs in the wake. Furthermore, as a result of the volume contraction associated with the austenite-to-martensite transformation, the induced stress-intensity factor is positive. This is in stark contrast with the negative stress-intensity factor achieved in zirconia ceramics, which undergoes volume expansion during phase transformation. The reverse transformation has been found to have a negligible effect on the induced stress-intensity factor. An important implication of the present results is that the phase transformation with volume contraction in SMAs tends to reduce their fracture resistance and increase the brittleness.

Spatial information in histograms for shape representation

Histograms have been used for Shape Representation and Retrieval. In this paper, the traditional technique has been modified to capture additional information. We compare the performance of the proposed method with the traditional method by performing experiments on a database of shapes. The results show that the proposed enhancement to the histogram based method improves the effectiveness significantly.

Coherence based histograms for shape retrieval

Histograms have been used for Shape Representation and Retrieval. The drawback of the histograms method is that histograms can be same for dissimilar shapes, which renders the method less effective for retrieval of shapes. In this paper, we describe the concept of coherence. We show how coherence can be used with distance and angular histograms. We perform experiments to test the effectiveness of the proposed method. It is found that coherence improves accuracy of retrieval significantly.

Angular histograms for shape retrieval

Distance histograms have been used for Shape Representation and Retrieval [1][2]. In this paper, we have proposed the use of angular histograms for shape representation. We have implemented a system for conducting experiments and evaluating the effectiveness of the proposed method. The proposed method is compared with the distance histograms method. It is found that the
proposed method is effective.

Discriminating shape descriptors based on connectivity

We propose a method for enhancing the accuracy of shape descriptors. The concept of connectivity to obtain discriminating shape descriptors, is introduced. We show how connectivity is applied to two popular shape descriptors. Experiments are performed to test the effect of using connectivity with generic Fourier descriptors and distance histograms. Item S8 within the MPEG-7 still images content set is used for performing experiments. This dataset consists of 3621 still images. The experimental results show that connectivity enhances the performance of the methods significantly.