Showing papers in "Strain in 2004"

Estimation of Electric Charge Output for Piezoelectric Energy Harvesting

Abstract: Piezoelectric materials (PZT) can be used as mechanisms to transfer mechanical energy, usually ambient vibration, into electrical energy that can be stored and used to power other devices. With the recent advances in wireless and micro-electro-mechanical-systems (MEMS) technology, sensors can be placed in exotic and remote locations. As these devices are wireless it becomes necessary that they have their own power supply. The power supply in most cases is the conventional battery; however, problems can occur when using batteries because of their finite life span. Because most sensors are being developed so that they can be placed in remote locations such as structural sensors on a bridge or global positioning service (GPS) tracking devices on animals in the wild, obtaining the sensor simply to replace the battery can become a very expensive task. Fur- thermore, in the case of sensors located on civil structures, it is often advantageous to embed them, making access impossible. Therefore, if a method of obtaining the untapped energy surrounding these sensors was implemented, significant life could be added to the power supply. One method is to use PZT materials to obtain ambient energy surrounding the test specimen. This captured energy could then be used to prolong the power supply or in the ideal case provide endless energy for the sensors lifespan. The goal of this study is to develop a model of the PZT power harvesting device. This model would simplify the design procedure necessary for determining the appropriate size and vibration levels necessary for sufficient energy to be produced and supplied to the electronic devices. An experimental verification of the model is also performed to ensure its accuracy.

Local Strain Measurement within Tendon

Abstract: Tendon is a dense connective tissue, responsible for transmitting the forces generated by muscles to the skeleton. It is composed of a hierarchical arrangement of crimped collagen fibres, interspersed with proteoglycan matrix and cells, known as tenocytes. During physiological loading, tendons are subjected to strains in the region of 5–6%, which result in the straightening and realignment of the collagen fibres, generating variable local strain fields within the tendon. This study demonstrates a technique for analysing local strains within viable tendon explants, during both loading and unloading of the tissue. Samples were strained in a custom-designed rig, allowing real-time visualisation of cell nuclei, used as local discrete markers, on a confocal microscope. Results indicated that local strains within the fascicle are smaller than the applied strains, never exceeding 1.2%, even at 8% gross applied strain. By contrast, the sliding of adjacent collagen units was recorded at each strain increment in this study, reaching a mean maximum of 3.9% of the applied displacement. Loading–unloading studies indicated that sliding behaviour is reversible up to strains of 5%, and provides the major extension mechanism within the rat-tail tendon. This technique can be extended to further analyse shearing behaviour within the matrix.

Progress in thermoelastic residual stress measurement

Abstract: The possibilities of using thermoelastic stress analysis to measure residual stresses are assessed, particularly in the context of the effect of plastic strain on the thermoelastic output. Components manufactured from aluminium and steel were deformed so that they experienced a plastic deformation and in some cases, a residual stress. Thermoelastic data from the components were compared with data from geometrically similar undeformed components. According to well-accepted theory, the thermoelastic data from both components should be practically identical. The results from the deformed components show departures from those of the undeformed components; the possible reasons for this are discussed in detail. A practical application of cold-expanded holes used in aircraft structures is described and recommendations are made for possible future work.

Experimental Assessment of Stress Patterns in Abdominal Aortic Aneurysms using the Photoelastic Method

Abstract: Abdominal aortic aneurysm (AAA) sac size is a major clinical problem. The maximum sac diameter is the major determining factor for treatment. There is a direct correlation between the wall stress and rupture sites for AAA with maximum wall stress not being dependent on the maximum diameter of the vessel but on the morphology of vessel. The reflective photoelastic method was developed to experimentally evaluate the stresses and strains in a model AAA. The epoxy resin material was used as the self-supporting structure and had similar mechanical properties to the aneurysmal aorta. For both static and dynamic fluid testing the high-stressed regions were located proximally and distally to the aneurysm. This correlates to reported in vivo rupture sites. There was a low-stress region at the location of greatest diameter at the centre of the aneurysm. The direction of the maximum principal stress was found to be in the circumferential direction. The maximum stress for a high blood pressure of 22.7 kPa (170 mmHg) was 35% greater than that for a normal blood pressure of 16.0 kPa (120 mmHg). The photoelastic method is a powerful and innovative method of analysing stresses in AAA models, producing results, which are visual and easy to interpret.

Measurement of the Relative Motion Between an Implant and Bone under Cyclic Loading

Abstract: The measurement of the relative motion between a prosthesis and the surrounding bone is important for the pre-clinical testing of prostheses and implants. A technique that allows measurements to be made over several million loading cycles was developed by Maher et al. (Clin. Biomech. 2001; 16: 307-314). However, the measurement of implant/bone motion is fraught with difficulties because: (i) testing over millions of cycles can take several days and errors because of diurnal temperature variations can occur, (ii) elastic deformations of the stem could have a dominant effect on 'inducible displacements', the rate of change of which is hypothesised to be a measure of loosening, and (iii) the use of this method for the analysis of cementless prostheses has yet to be demonstrated. This paper addresses these methodological issues with respect to testing hip prostheses, and concludes with a comparative evaluation of pre-clinical testing methods.

Experimental Device for Simulating Traumatic Brain Injury Resulting from Linear Accelerations

Abstract: Various methods are used to model and analyse traumatic brain injuries (TBI) in human beings. These include volunteer and cadaver experiments, anthropomorphic dummies, physical models, computational models and mathematical models. The pathophysiological response to mechanical impact of the human central nervous system and of in vivo neural tissue, is most realistically analysed using animal models, which provide the best surrogate for the human brain. During non-fatal impacts, a mechanical insult may trigger a cascade of physiological processes, many mediated by neurochemicals within the neural tissue which will, in turn, control the depth and extent of the brain injury. This paper reports the development of a novel experimental device which can apply different linear acceleration impacts directly to in vivo neural tissue in a manner which permits the experimental analysis of non-fatal TBI of varying severity.

A Simplified Model to Determine the Contribution of Strain Energy in the Failure Process of Thin Biological Membranes during Cutting

Abstract: Thin biological membranes such as skin are highly deformable, nonlinear in behaviour and fracture resistant. As a result of these properties, measuring the resistance to fracture of such materials is difficult. This paper investigates the resistance to fracture of a thin biological membrane, using the example of animal skin. Models of cutting using a fracture approach are examined and a review of the structure and mechanical properties of skin is given. A review of previous work in examining the fracture behaviour of skin is carried out and a strain energy-based failure model for skin is proposed. A method of measuring the fracture resistance of skin in opening mode (mode I) using this failure model is described. Values for the resistance to fracture of skin samples were calculated from experiments to be 2.32 ± 0.40 kj m-2. These results were found to be in good agreement with the literature. The model and experimental technique proposed here may be applied to establish the failure properties of membranes and, in particular, a range of soft tissues under a variety of cutting conditions.

Determination of Critical Tearing Energy of Tyre Rubber

Abstract: An experimental investigation was conducted to determine the crack growth characteristics and critical tearing energy of pure tyre rubber under mode-I and -III loading. Constrained tension and trousers specimens were used, respectively. In the trousers test it was observed that the crack does not propagate at a steady rate, but in a stick–slip way, i.e., it is arrested and re-initiated at fairly regular intervals. Thus, the force necessary to propagate the crack varies widely from a maximum value at crack initiation to a minimum value at crack arrest. In the constrained tension tests, no stable crack growth was observed and crack initiation coincides with catastrophic fracture. The critical tearing energies corresponding to crack initiation and arrest under mode-III and to unstable crack growth under mode-I loading were determined from the load–displacement records by an approximate analysis of the trousers and the constrained tension tests.

Measurement of Molecular Orientation in Retrieved Ultra-high-molecular-weight Polyethylene (UHMWPE) Hip Sockets using Fourier-transform Infrared Spectroscopy

Abstract: This paper describes the use of Fourier-transform infrared spectroscopy (FTIR) to measure the degree of orientation in retrieved ultra-high-molecular-weight polyethylene (UHMWPE) acetabular components of prosthetic hip joints quantitatively. Multidirectional shearing has been found to result in a wear rate that is several orders of magnitude higher than that for linear shear. This is because linear wear of UHMWPE is believed to induce orientation of the polymer lamellae. The FTIR technique described in this paper enables a direct comparison to be made between patient biomechanics and the molecular orientation of UHMWPE hip sockets. Patients were identified prior to revision surgery of the hip for loosening of components. Individual patient's hip-joint kinematics were quantified by the aspect ratio of movement loci developed from clinical gait analysis. It was found that patients with high aspect ratios and therefore more linear wear paths, exhibited measurable orientation of the UHMWPE polymer lamellae in their retrieved hip sockets. An aspect ratio of 5.36 resulted in 67% of the polymer lamellae being oriented in the a-axis direction. The technique was qualitatively validated by the use of transmission electron microscopy and it was found that an aspect ratio of 4.46 or greater resulted in the observed orientation of the polymer lamellae.

Sensor Optimisation using an Ant Colony Metaphor

Abstract: This paper describes an approach to impact detection and location using neural networks and an ant colony metaphor. Given the existence of an effective fault detection procedure, the problem arises as to how the sensors should be placed for optimal efficiency of the detector. In this paper, a neural network is used to locate and classify impacts on a composite panel and the ant colony metaphor is used to determine an optimal (or near optimal) sensor distribution. The results are compared with those from a previous study which used a genetic algorithm for the optimisation phase.

Homogenisation Metamodelling of Perforated Plates

Abstract: The use of finite element (FE)-based homogenisation has improved the study of composite material properties. However, it involves enormous computational effort when imple- mented in engineering design problems. Therefore an artificial neural network (ANN) surrogate model is proposed here to avoid this issue. In this study, a numerical homogenisation code was developed based on a commercial FE package. It is used to develop the ANN metamodel for an individual composite structure. The effectiveness of the metamodel was examined through an analytical optimisation procedure.

The simulation of impact loads on beam-type structures using a pseudo-dynamic procedure

Abstract: This study proposes an alternate method for the analysis of beams with solid cross-section or built as a framed structure and subjected to transverse impact loads from an external striker. The procedure used in the analysis is a combination of two essential tools using pseudo-dynamic techniques. The method reported here involves only one degree of freedom for the structure modelling and assumes an elastic contact between an external striker and the beam structure, which in reality does not happen. As only one degree of freedom is considered in the analysis, some important limitations are inherent to the method proposed here. Essentially, there is the difficulty of modelling the displacement field associated with the transient structure behaviour accurately, as a consequence of fast-rate impact loads. Another difficulty faced by the method refers to a local structure behaviour associated with contact loads. The present method can deal with large displacements in transversely loaded beams associated to a collapse mechanism having a simple geometry and defined with precision from a single parameter. This ensures reasonable accuracy in the evaluation of the strain energy absorbing capacity of transversely impacted beam structures using a single degree of freedom model in a pseudo-dynamic procedure.

Strain Measurement Techniques in Explosives

Abstract: Experiments to visualise the detailed microstructure of highly heterogeneous plastic-bonded explosives have utilised high-resolution moire interferometry to measure localised deformations under quasi-static loading. The information obtained has proved valuable in the development of finite element material models, and offers the potential to isolate and examine the influence of particular microstructural features (like binder layer thickness, or explosive crystal size) on deformation characteristics. New insight into failure processes and the implications of ageing effects have been obtained. The experimental methods and modelling have broad applicability to microstructural studies of a variety of composites.

Interlaminar Stress Determination in Carbon Fibre Epoxy Composites with the Embedded Strain Gauge Technique

Abstract: In order to determine interlaminar stresses, strain gauges have been embedded in carbon fibre/epoxy composites. Insulation problems occurring because of the electrical conductivity of the carbon fibres were successfully resolved with the help of special foils. Measurements on unidirectional and angle-ply laminates were in good agreement with calculations based on the classical laminate theory. Moreover, the laminate strength was not significantly affected by the embedded strain gauges. The technique may also be applied to residual stress determination in composite components.

40 Years of Strain: A Retrospective Review*

Abstract: The paper provides a picture of the changes and advances in the field of strain measurement and related topics, as represented by the contents of the journal Strain, from its inception in 1965 to the present day.

Validation of a New Approach to Condition Monitoring of Offshore Structures – Aspects of a Scale Model

Abstract: Offshore structures have a finite design life and many are approaching that limit. The expected life cannot be extended without some justification, such as the inclusion of a structural monitoring system. This project is aimed at using long-range propagation of acoustic signals through a structure to detect changes induced by damage, cracking or flooding of hollow members. It is based on the idea that, if a change occurs within the structure, this will induce changes in acoustic signals transmitted between two points on it. These changes will only occur after a certain time in the history of the signal, and this time corresponds to the minimum travel time from the transmission point to the reception point via the point of damage. A scale model in polycarbonate has been used to demonstrate that using acoustic waves to interrogate a multi-connected structure can successfully locate damage. This has been done with sensors all at the top (reflection mode) and with transmitters at the top and receivers at the bottom (tomographic mode). The method is effective when we use very small numbers of source and sensor positions in proportion to the number of structural members being monitored for damage. The reflection geometry is much more practical from the point of view of application to offshore structures because all transducers and wiring can be located above the splash zone.

A Theoretical Framework for Core Material Properties Identification in Cellular Solids using Novelty Detection

Abstract: This paper presents a novelty detection-based technique to identify core material properties of honeycombs and cellular structures. A numerical model (finite element) representing full scale and/or reduced size of the cellular solid is used to generate transmissibilities between topological homologous points at cells in different locations. In order to make the procedure robust against noise, these transmissibilities are artificially corrupted. This step is representative of a series of experimentally obtained measurements which automatically return information relating to the size and shape of data noise along with the mean measurement. The next stage, both in this paper and in the proposal for the experimental approach, is to generate several further sets of transmissibilities from the finite element model; the only difference being that the core material properties are altered from the original values. A novelty detection framework is then adopted to find a similarity measure between each of these ‘test’ transmissibilities and the original set thereby identifying the material properties. Although this work is concerned with identifying only one material property, the methodology extends to identifying several properties.