Showing papers in "Strain in 2005"

An Evaluation of Digital Image Correlation Criteria for Strain Mapping Applications

Abstract: The performance of four digital image correlation criteria widely used in strain mapping applications has been critically examined using three sets of digital images with various whole-field deformation characteristics. The deformed images in these image sets are digitally modified to simulate the less-than-ideal image acquisition conditions in an actual experiment, such as variable brightness, contrast, uneven local lighting and blurring. The relative robustness, computational cost and reliability of each criterion are assessed for precision strain mapping applications. Recommen- dations are given for selecting a proper image correlation criterion to efficiently extract reliable deformation data from a given set of digital images.

Development of a Vibrating Wire Strain Gauge for Measuring Small Strains in Concrete Beams

Abstract: A vibrating wire strain gauge capable of measuring strains in concrete elements to an accuracy of better than 0.5 μɛ is presented. This offers some advantages over conventional electrical resistance gauges, the quoted accuracy of which is typically 3 μɛ, and which are often considered unsuitable for concrete because of their inability to span cracks. While vibrating wire gauges are potentially more accurate, they are prone to significant errors because of temperature changes. In the purpose-built gauge described here, temperature correction is achieved using an unstrained reference gauge. The vibration data are analysed using a moving-window Fourier transformation in order to identify and remove the geometrically nonlinear portion of the response. The resulting system is accurate, economical and easy to use. The gauges have been used to study the behaviour of cracked concrete specimens. Typical results are presented and discussed.

Fracture-induced Physical Phenomena and Memory Effects in Rocks: A Review

Abstract: Fracture-induced physical phenomena allow a real-time monitoring of damage evolution in rocks induced by mechanical loading. Some of these phenomena, e.g. plastic deformation, changes in permeability and electric resistivity, provide an overall estimate of damage as a function of stress. Others, such as acoustic emission or electromagnetic emission, allow an exact location of cracks in space and time. Being nondestructive, monitoring techniques based on fracture-induced physical phenomena, are a basis for prediction of earthquakes and rockbursts in underground mines. They are also crucial for a fundamental understanding of rock fracture mechanisms and for developing constitutive models of rock behaviour. Memory effects take place in rocks subjected to cyclic loading with the peak stress (strain) value increasing from cycle to cycle. The effects are represented by certain characteristic changes in rock properties and parameters that are observed when the stress (strain) reaches its previous peak value. Potential applications of memory effects range from stress measurement techniques to earthquake prediction to damage surface scanning. Essential experimental and theoretical results on fracture-induced physical phenomena and memory effects in rocks are considered in the article. Possible applications as well as not-yet-clear points are discussed.

Analysis of the Residual Stress around a Cold‐expanded Fastener Hole in a Finite Plate

Abstract: In this paper, we present results of residual stresses around a cold-worked fastener hole in a thin, finite plate. An analytical model, with plane stress conditions, elastic nonlinear–plastic loading and unloading taken into account, is explicitly presented and has been developed from existing analytical models and methodologies. The solution also accounts for elastic deformation of the mandrel, and is applicable to different sizes of hole and mandrel. A parametric study of the strain-hardening exponent, plate size, Bauschinger parameter and expansion ratio was carried out in the context of the present solution. A sample study of a plate subjected to 4% hole-cold expansion was performed to compare this model with other analytical predictions, finite element simulation and X-ray diffraction experimental results. A brief review of related issues is given. This work is intended to present completely the closed-form equations able to generate residual stress profiles with ease around a cold-expanded hole in a thin, finite plate where boundary conditions are at issue.

Signal Processing for Remote Condition Monitoring of Railway Points

Abstract: The dependability of railway points (turnouts or switches) is a key part of any railway system; the Potters Bar crash (10 May 2002) in the UK, which led to seven fatalities is a key example of a failure of this subsystem. Present maintenance of points involves overly frequent inspection by maintenance staff. A remote condition monitoring approach would lead to more efficient inspection routines and directed anticipatory maintenance trips. To assist the creation of a suitable fault-detection algorithm, the authors analysed existing force and current data for the ‘as commissioned’ case of a turnout and for situations with different fault conditions. Signal processing of this data revealed several different methods that can be used to distinguish between fully functioning points, and different fault conditions of the points. Specifically, clustering of statistical parameters and their application to wavelet levels and coefficients, provided clear discrimination of most critical faults. This demonstrates a good first step towards a condition monitoring-based maintenance regime for points that is both safer for passengers and maintenance personnel and that has the potential to be more effective and economical.

Structural health monitoring of an annular component using a statistical approach

Abstract: For aerospace components there is undoubtedly a critical need to detect incipient damage in the structure, as any microscopic crack or defect can potentially lead to catastrophic failure and loss of human life. This paper investigates the scattering of an ultrasonic-guided wave into a hollow cylinder-like structure, under both damaged and undamaged conditions. Hollow cylinder structures are widely used not only in aerospace components but also in other engineering applications. The wave was sequentially transmitted and captured by means of a 'real-time data-acquisition system' combined with integrated disc-shaped piezoceramic transducers. The integration of the tested structure and the transducers formed a structural health monitoring system. Wave responses were recorded from both of the structural conditions for the purpose of damage identification using a novelty detection method called 'outlier analysis'. The principal component analysis method of reducing the dimensionality of the feature space is also presented in this paper, with its main aim being to visualise how the data sets behave as a function of the structural conditions.

Finite element analysis of footwear and ground interaction

Abstract: Military boots are designed to prevent the soft tissue and skeletal structure of the feet from damage under heavy usage. Good slip-resistant tread patterns of the outer-sole are vital to minimise the risk or severity of slipping under demanding conditions, most likely to result in accidents. However, boot design should also offer the customer flexibility, comfort, and shock absorption, be lightweight and be able to operate regardless of the ground surface texture and various weather conditions. The issue of footwear and ground interaction investigated in this study can be classified as a traditional stability problem. Solutions to these problems are often obtained using the theory of perfect plasticity. Therefore, elastic–perfectly plastic theory was adopted in this study and the Drucker-Prager (DP) material model was chosen to model the soil properties. Literature survey showed that little studies exist on the subject of interaction between foot and soft ground, in particular, using numerical modelling methods. However, there are numerous research works on some relevant domains, such as soil–tillage tool interaction, soil–wheel interaction and soil–structure interaction, etc. A three-dimensional finite-element (FE) analysis of a subsoiler cutting with pressurised air injection was performed by employing a DP harden material model without consideration of friction force by Araya and Gao [1]. Saliba [2] undertook elastic–viscoplastic FE modelling for tire/soil interaction and Mouazen and Nemenyi [3, 4] adopted a DP model for analysing soil–tillage tool interaction.

Strategies for Reducing Stress in Ultrasonic Cutting Systems

Abstract: Ultrasonic cutting is an established technology in the cutting of food products. As the high-cost components of an ultrasonic cutting system are the ultrasonic generator and transducer, some designs have concentrated on running several cutting blades from a single power source. This strategy is undoubtedly economic, but problems remain with reliability. Blade failure during cutting is a common problem in ultrasonic systems and therefore strategies are proposed to reduce stress levels at the failure locations. Many alternative blade profiles are investigated using finite element (FE) models, which are validated by experimental modal analysis (EMA) using a 3D laser Doppler vibrometer (LDV). Combining three strategies can reduce the stress level at the failure location on cutting blades. The first investigates the blade profile at the failure location with the aim of incorporating sufficient gain at reduced stress levels. The second concentrates on detuning the block horn and blades in order to move the longitudinal node away from the highest stressed section of the blade. Finally, a redesign of the block horn is studied in order to eliminate the effects of blade flexural vibrations in the longitudinal cutting mode.

Strain Measurement in a Shape Memory Alloy with Strain Gauges

Abstract: The recovery of shape due to temperature-induced martensite to austenite phase transformation in shape memory alloys (SMA), such as nickel titanium alloys, has been investigated and utilised in applications for many years. One of the problems in incorporating SMA in host materials, such as composites, is the poor bonding characteristics of the SMA. A closely related problem in experiments is the measurement of strains in SMA specimens; as thin wires have been used in many of the experimental investigations, strains have been deduced from length measurements. In the current work, electrical resistance strain gauges have been bonded on SMA strips. Tensile strains up to 8% during tensile loading as well as compressive strains during strain recovery due to subsequent heating have been measured. Strains determined by other methods, such as extensometer and length measurements, are also reported and compared. The major contribution of this work is the successful bonding of strain gauges on SMA and the measurement of large strains (up to 8%).

An Analysis of the Factors Influencing the Data Derived from a Plug Type Three-Dimensional Strain Rosette under Compression and Torsion

Abstract: The embedding of three-dimensional strain rosettes embedded into epoxy models provides an experimental technique for analysing complex structures; however, this technique has been known to produce data that were difficult to explain in terms of their physical significance. To gain a greater insight into the behaviour of a three-dimensional strain rosette used in this way, a three-dimensional strain rosette was embedded into each of two separate prismatic bars of square cross-section and subjected to fundamental tests of compression and torsion in standard commercial testing machines. In initial tests on a bar containing a three-dimensional strain rosette (Bar A) the data derived from the individual gauges sometimes departed from the theoretical values by more that 30 μe. After critical evaluation of the procedures used for making and testing Bar A, further tests were carried out on Bar B, which led to a reduction in the difference between theoretical and experimental data to 14 μe, acceptable for most practical purposes. The use of square plugs containing three-dimensional strain rosettes which are embedded into square cavities in the model, and the measurement of the actual direction cosines of the gauges on the square plug prior to embedment is a distinct advantage over the use of cylindrical plugs. In addition, the use of testing machines with a fixed base as opposed to a floating lower platen is recommended.

Thermal Properties Measurement of Micro-Electromechanical System Sensors by Digital Moire Method

Abstract: The thermal properties of a micro-electromechanical system sensor were analysed by a novel digital moire method. A double-layer micro-cantilever sensor (60 μm long, 10 μm width and 2 μm thick) was prepared by focused ion beam milling. A grating with frequency of 5000 lines mm−1 was etched on the cantilever. The sensor was placed into a scanning electron microscope system with a high temperature device. The observation and recording of the thermal deformation of the grating were realised in real-time as the temperature rose from room temperature to 300 °C at intervals of 50 °C. Digital moire was generated by interference of the deformed grating and a digital virtual grating. The thermal properties including strain distribution of the sensor and the linear expansion coefficient of polysilicon were accurately measured by the phase-shifted moire patterns.

Biologically Inspired Intelligent Robots using Artificial Muscles

Abstract: Humans throughout history have always sought to mimic the appearance, mobility, functionality, intelligent operation, and thinking process of biological creatures. This field of biologically inspired technology, having the moniker biomimetics, has evolved from making static copies of human and animals in the form of statues to the emergence of robots that operate with realistic appearance and behavior. Technology evolution led to such fields as artificial muscles, artificial intelligence, artificial vision and biomimetic capabilities in materials science, mechanics, electronics, computing science, information technology and many others. One of the newest fields is the artificial muscles, which is the moniker for electroactive polymers (EAP). Efforts are made worldwide to establish a strong infrastructure for this actuation materials ranging from analytical modeling and comprehensive understanding of their response mechanism to effective processing and characterization techniques. The field is still in its emerging state and robust materials are still not readily available however in recent years significant progress has been made. To promote faster advancement in the field, in 1999, the author posed a challenge to the research and engineering community to develop a robotic arm that would wrestle against human opponent and win. Currently, he is considering setting up the first competition in 2005. This paper covers the current state-of-the-art and challenges to making biomimetic robots using artificial muscles.

Instrumentation and Response Measurement of a Double‐Lane Bailey Bridge During Load Test

Abstract: During the past decades very significant advances in analytical techniques have led to the development of extremely powerful and versatile methods of structural analysis that are generally computer-based. Within the linear elastic range, these methods are supposed to make predictions with reasonable accuracy. These advanced methods of analysis (finite element, finite strip, grillage analysis, etc.) are used for both the design and strength evaluation of bridges. It has frequently been found that significant discrepancies exist between the predicted and observed responses, even when the loading is within the linear elastic range of the bridge. The discrepancies between the analytical and measured responses were subsequently found to be not due to inadequacies of the methods of analysis, but rather due to the presence of behavioural factors that could not be included in the mathematical modelling because of difficulties in their quantification. The theoretical evaluation of the bridge requires accurate information about material properties, support behaviour, contribution of non-structural members and other factors. Many a time, simplified assumptions are made to account for these parameters in analysis. It makes one believe that even highly rigorous methods of analysis cannot be relied upon unquestionably to predict the actual response of a bridge. There is no better way to understand the shortcomings of the mathematical models used for design (or) evaluation of bridges than to investigate the behaviour of bridges through load testing. Bridge testing is rapidly becoming a major tool in evaluation and may be seen as an acceptance proof test. Load testing provides several benefits from a safety point of view. Load testing helps to assess the behaviour of the bridge under specified loading condition and also to evaluate the load carrying capacity of particular structural configuration. This involves instrumenting the bridge at various critical locations and measurement of strain and deflection responses under the test loads. Selection of response locations, sensor types, installation techniques and data acquisition system are all very important and challenging in actual load testing of bridges. Load testing can have several benefits in evaluation, provided the tests are simple to perform and of reasonable cost. The results must also be able to be interpreted in a manner to realistically assess the behaviour and load carrying capacity of the bridge. Two types of bridge testing are available: (i) proof load testing, and (ii) diagnostic/behaviour tests. In the proof load test, a large load level is used, which assures that there is sufficient strength capacity for the bridge. Several problems exist with this type of test. The test is costly and a high level of risk is involved if the test programme is not properly planned. Hence, many bridge owners are reluctant to undertake such tests. The diagnostic/behaviour test is the most common type of bridge test in which some load, often much below the service load, is used and the response measured for the tested load level. The bridge response for higher classes of loading can be obtained by extrapolation. Behaviour tests are carried out either to study the mechanics of bridge behaviour or to verify other methods of analysis, the objective in the latter case being that after verification, the methods can be used with confidence in the design and evaluation of bridges. One double-lane Bailey bridge, 24.38 m (80 ft) span, was instrumented with 50 strain gauges and 14 dial gauges at various critical locations to obtain the structural responses under static loading. This paper describes the instrumentation details, loading arrangement, testing procedure and measurement of bridge response during the load test. The maximum load permitted on the bridge as per Indian Road Congress (IRC) guidelines was estimated from this investigation.

Embedded Strain Gauges: Effect of the Stress Normal to the Grid

Abstract: In general, a strain gauge embedded in a model is subjected to a stress normal to the grid, whereas a gauge on the external surface is free from such a stress. This paper concerns the effect of the stress normal to the grid on the output of the strain gauge; usually, the influence of such a stress has a negligible effect, however, in some cases a notable influence has been noted. Therefore, the output of the strain gauge is determined in function of the strains in the plane of the gauge, ɛl and ɛt, and of the stress, σn, normal to the grid. The analysis shows that the output of the strain gauge is influenced by the coupled effect of transverse sensitivity and pressure sensitivity of the strain gauge. In particular, the analysis shows that, for Constantan gauges compensated for steel, the influence of the transverse sensitivity is in general prevailing on that one due to the pressure sensitivity. The results reported in this paper explain the experimental data obtained by various researchers.

Complete Isochromatic Fringe-order Analysis in Digital Photoelasticity by Fourier Transform and Load Stepping

Abstract: In this paper, a new fully automated photoelastic technique based on both the Fourier transform and the envelope of the fringe centres, is presented. Using two images of the isochromatics acquired with different loads and a simple calculation routine, the method allows the user the complete determination of the isochromatic fringe retardation on any arbitrary line chosen over the model domain, without the need for supplementary external information or calibration. Various experiments have corroborated the accuracy of the method, estimated theoretically to about 0.07 fringe orders. As a result of its simplicity and sufficient accuracy, the method can be used in the industrial field also by a non-expert user.

StressProbe - a new non-contacting stress measurement technique

Abstract: A new non-contacting stress measurement technique, the StressProbe, based on the piezomagnetic effect has recently come into service. It utilises inducing and sensing coils that are arranged so that an almost linear response to stress is possible. Theoretical work already reported has shown that it can be used for measurement of residual stress, dead loads and cyclic stress. In general, a calibration is needed for the interpretation of StressProbe measurements and this may be obtained from a simple three-point bend or axial load tests. For some metals, the StressProbe response shows hysteresis during a static load–unload test. For these metals, it has been found that using a demagnetisation cycle between each measurement eliminates most of the hysteresis loop. For continuous cyclic loading, this behaviour does not occur.