Strain

A nonlinear optimization procedure is established to determine the elastic modulus of slender, soft materials using beams with unknown initial curvature in the presence of large rotations. Specifically, the deflection of clamped-free beams under self-weight - measured at different orientations with respect to gravity - is used to determine the modulus of elasticity and the intrinsic curvature in the unloaded state. The approach is validated with experiments on a number of different materials - steel, polyetherimide, rubber and pig skin. Since the loading is limited to self-weight, the strain levels attained in these tests are small enough to assume a linear elastic material behavior. This nondestructive methodology is also applicable to engineered tissues and extremely delicate materials in order to obtain a quick estimate of the material's elastic modulus.

Computer-aided, personal computer (PC) based, optoelectronic holography (OEH) was used to obtain preliminary measurements of the sound-induced displacement of the tympanic membrane (TM) of cadaver cats and chinchillas. Real-time time-averaged holograms, processed at video rates, were used to characterise the frequency dependence of TM displacements as tone frequency was swept from 400 Hz to 20 kHz. Stroboscopic holography was used at selected frequencies to measure, in full-field-of-view, displacements of the TM surface with nanometer resolution. These measurements enable the determination and the characterisation of inward and outward displacements of the TM. The time-averaged holographic data suggest standing wave patterns on the cat's TM surface, which move from simple uni-modal or bi-modal patterns at low frequencies, through complicated multimodal patterns above 3 kHz, to highly ordered arrangements of displacement waves with tone frequencies above 15 kHz. The frequency boundaries of the different wave patterns are lower in chinchilla (simple patterns below 600 Hz, ordered patterns above 4 kHz) than cat. The stroboscopic holography measurements indicate wave-like motion patterns on the TM surface, where the number of wavelengths captured along sections of the TM increased with stimulus frequency with as many as 11 wavelengths visible on the chinchilla TM at 16 kHz. Counts of the visible number of wavelengths on TM sections with different sound stimulus frequency provided estimates of wave velocity along the TM surface that ranged from 5 m s(-1) at frequencies below 8 kHz and increased to 25 m s(-1) by 20 kHz.

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.

We demonstrate that polarization-sensitive optical coherence tomography (PS-OCT) is suitable to map the stress distribution within materials in a contactless and non-destructive way. In contrast to transmission photoelasticity measurements the samples do not have to be transparent but can be of scattering nature. Denoising and analysis of fringe patterns in single PS-OCT retardation images are demonstrated to deliver the basis for a quantitative whole-field evaluation of the internal stress state of samples under investigation.

The current development of digital image correlation, whose displacement uncertainty is well below the pixel value, enables one to better characterise the behaviour of materials and the response of structures to external loads. A general presentation of the extraction of displacement fields from the knowledge of pictures taken at different instants of an experiment is given. Different strategies can be followed to achieve a sub-pixel uncertainty. From these measurements, new identification procedures are devised making use of full-field measures. A priori or a posteriori routes can be followed. They are illustrated on the analysis of a Brazilian test.

  In this paper, physical parameters for the creep constitutive equations of the low alloy ferritic steel 1.25Cr0.5Mo have been determined using experimental data. This alloy is used mostly in power generation and petrochemical industries because of its high temperature creep resistance. Test samples have been obtained from a new super-heater pipe wall of a steam-generating boiler in Tabriz Petrochemical Plant according to the ASTM standards. By conducting creep rupture tests for 1.25Cr0.5Mo steel, creep behaviour and creep-rupture properties were examined for this material. Creep rupture tests have been carried out at four temperatures of 700, 725, 750 and 800 °C, under applied uni-axial stresses of 30, 35, 40 and 50 MPa. The experimental data have been used to obtain the constitutive parameters using numerical optimisation techniques. Also the temperature and stress dependency of the creep lifetime for this alloy has been investigated using Larson–Miller and Monkman–Grant parameters. The results show good agreement with other test data such as ASTM and API. Finally, these constitutive equations have been used to study the creep behaviour of the super-heater pipe. The results show that the super-heater tube has been over designed in terms of the creep lifetime and this is in accordance with the in-plant observations.

The design and development of Manganin wire sensors for use at pressures up to 200 000 lbf/in2 (1.38 GN/m2) is described and data presented for an alternative wire, Zeranin. A brief description of suitable bridge and digital read–out systems is given together with the design of a resistance strain gauge transducer suitable for control applications.

  The article focuses on the application of a recently developed damage operator-based lifetime calculation to a thermomechanically loaded exhaust downpipe. The damage operator approach enabling online continuous damage calculations for isothermal and non-isothermal loading with mean stress corrections is reviewed. The article also highlights an extension of the strain-life approach to take into account viscoplastic effects and creep. The transient results from thermal and structural analyses using finite element analyses have been applied to the exhaust downpipe in LMS Virtual.Lab and the damage predicted. Tested exhaust downpipes were then subjected to the same loading conditions as in the calculation, and load cycles were repeated up to the point of failure. Simulated and test results are comparable.

Standards, testing and certification are major topics of discussion in relation to the European single market in 1992. The British Society for Strain Measurement has taken initiatives since 1977, to try to raise the standards of validation of competence in testing and analysis particularly in the areas of stress and strain measurement and analysis. This paper sets out some of the current thinking of the BSSM, in an attempt to obtain wider support for and participation in, what it considered to be one of the vital approaches to improved product quality, performance, safety and reliability.

  The crack tip opening angle (CTOA) fracture criterion is one of the most promising fracture criterion used to characterise the stable tearing process in metallic materials. Traditional methods used for the experimental characterisation of the CTOA involve accurate identification of the crack tip at each tearing event. Recently alternative methods have been proposed that reduce the necessity of accurately defining the current crack and rely more on the shape of the crack flanks to define the CTOA. In addition, these methods define an ‘apparent crack tip’, which may be different from the actual surface crack tip and may provide insight into the amount of crack-front tunnelling that is occurring. In the current research, compact tension specimens fabricated from 6.35 mm thick 2024-T351 aluminium alloy plate were evaluated to investigate different CTOA measurement methods and their potential for estimating crack-front tunnelling. In addition to characterizing the CTOA, fatigue marker bands were employed to map the evolution of crack-front tunnelling. The experimental critical CTOA values obtained from the alternative methods were noticeably lower than that obtained from the traditional approach and showed noticeably more scatter. When compared to the experimentally obtained marker bands, the alternative methods indicated limited potential for predicting crack-front tunnelling.

Impression creep tests under multi-step load conditions were performed for a service-aged 1/2Cr1/2Mo1/4V steel at 565°C, corresponding to uniaxial stresses in the range of 100–150 MPa. Results of the tests have shown that the minimum creep strain rate data of the material, produced from the creep curves obtained under different load histories, are in good agreement. The results obtained indicate that the results from a multi-step load test of an impression creep specimen, which is particularly useful when the test material is very limited, can be used to obtain the secondary creep properties for a material from a very small test sample.

  In this article, the material and physical parameters for the creep constitutive equations of cold-drawn 304L stainless steel have been determined using experimental data. Austenitic stainless steel 304L is used mostly in power generation and petrochemical industries because of its high-temperature creep resistance even at above yield stresses. Test samples have been obtained from cold-drawn bars, and the material conforms to ASTM A276-05a specifications. The creep behaviour and properties have been examined for this material by conducting uni-axial creep tests. Constant temperature and constant load uni-axial creep tests have been carried out at three temperatures of 680,700 and 720 °C, subjected to constant loads which produce below and above yield initial stresses of 200, 250, 320, 340 and 360 MPa. The experimental data have been used to obtain the creep constitutive parameters using numerical optimisation techniques. In addition, the temperature and stress dependency of the creep properties for this alloy have been investigated using Larson–Miller and Monkman–Grant parameters.

  Forge welding is a process that can overcome the limitations of friction welding and diffusion bonding for joining dissimilar metals, while still permitting a large amount of plastic deformation. However, the available information about forge welding of dissimilar metals is rare as most studies have focused on joining similar metals. This paper reports an investigation on effects of process parameters in forge-welding bimetallic materials: AISI 316L stainless steel and 6063 aluminium alloy. Experiments were carried out under variations in process parameters, including forge-welding temperature, amount of deformation and forging speed. The results showed that the forge-welding temperature was the most significant process parameter and that this could highly influence the tensile strength of the joint. The quality joint was produced successfully. It could withstand the tensile strength of 111.3 MPa and this was comparable to the findings of other researchers. The diffusion zone of the joint was examined by the optical micrograph and elemental composition analysis. Intermetallic compounds were found in the diffusion zone, which was critical evidence for the validity of the experiment.

  The propagation of fatigue cracks under constant amplitude cyclic loading was studied in welded stiffened steel plates. The residual stresses in the stiffened plates were measured using the neutron diffraction strain-scanning technique. The neutron diffraction measurements indicated that, in general, the residual stresses were tensile near the welded stiffeners and compressive between the stiffeners and ahead of the starter notch tips. Fatigue testing indicated that the fatigue crack growth rates of the stiffened plates were, in general, lower than that of a corresponding unstiffened plate, especially near the notch tips, where compressive residual stresses existed. An analytical method, using Green's function, was developed to predict the fatigue crack growth rates. Reasonable accuracy was obtained.

  We have extended the Digital Image Correlation technique to the case in three dimensions. This new technique, allowing the full three-dimensional (3D) strain measurement in the bulk of a solid, needs volume images containing a 3D variation of the grey levels. Generally, volume images are obtained by X-ray computed tomography. In this paper, we present a procedure that is easier to implement and enables to generate volume image in transparent materials. The principle consists in the optical slicing of the specimen. To obtain a random distribution of grey levels within the volume image, we use the scattered light phenomenon induced by particles included in the specimen. The recording of 3D images by optical slicing is presented and the influence of different kinds of particles on the scattered light and on the accuracy of measurement is described. Through several tests involving rigid body displacements and a tensile test we show the performance of this technique and we evaluate the measurement error of displacement and strain components.

  Application of the three-dimensional digital image correlation technique (3D DIC) to the accurate measurement of full-field surface profile of a 730 mm-diameter carbon fibre composite satellite antenna is investigated in this article. The basic principles of the 3D DIC technique are described. The measured profile was compared with the one measured with a three-dimensional coordinate measuring machine. The results clearly indicate that the 3D DIC technique is suitable for full-field surface profile measurement of small satellite antenna, and further application of the 3D DIC technique to the measurement of thermal deformation of the antenna is expected.

  The objective of this paper is to propose a method for measuring damage in ductile materials, from its inception to rupture. In the first stage of damage, which occurs before localisation, the usual method for determining damage through the measurement of stiffness variation is used. A damageable elastic–plastic model of the modified Lemaitre/Chaboche type is identified from these tests. An original method is proposed for measuring damage following the initiation of strain localisation. This method is based on a full 3D image correlation analysis using four cameras. The principle of the method consists in identifying the damage through tensile experiments on thin, flat-notched specimens subjected to tensile loading. Speckles are applied on both faces of each specimen in order to follow the strain fields on the two faces at the same time. These two strain fields are digitised simultaneously by two synchronised sets of two digital cameras. This paper shows how this method enables one to identify strain localisation and deduce the evolution of damage directly. Here, the method is developed for 15-5 PH stainless steel.

The shearographic interferometry is employed as a nondestructive full field, optical testing and measuring method without contact. Fringes of constant strain (so called isotase, tasis (Greek)=strain) can he observed in real time on the surface of the investigated machine parts and structures of any material and are represented by the shearogram. Using shearography two states of “deformation are recorded by doubly exposing a Holotesl film in an ordinary camera or stored by an electronic image processing system. In the lens of the camera a shearing element inside or outside the focus is integrated or the lateral Michelson shearing interferometer is used. Rigid body motions of the object are not recorded. Local deformation irregularities caused by a defect under or on the surface of the specimen create, strain concentrations; the homogeneous surrounding is poorly superimposed by an interference pattern. The shearogram shows dark and bright fringes which are the functions of the displacement derivative. The holographic interferometry measures the out of plane deformations directly. Terms of the out of plane strain can be determined by the shearographic method as well as the in plane strain fringes which are described in the following.

  Aim of the work was the experimental strain analysis of a 420 sailboat mast during sailing: this type of data is of great interest in the structural design of sailboat masts and hulls as well as in the sail-making. The work involved the design, calibration and installation of strain gauge bridges at seven sections of an instrumented mast and at the shrouds: this was followed by the collection of strain data during simulated laboratory rigging tests and real sailing sessions with medium/strong wind. Compression loads at the mast step were recorded with a customised load cell calibrated on a force platform. On the basis of laboratory calibration constants, the strain measurements were converted into structural loads and averaged over steady states during rigging and sailing either close to the wind or beam reach: the longitudinal bending moment, the lateral bending moment and the axial loads were analysed and plotted along the 6.20 m mast length. Highest values of lateral bending (146 ± 14.5 Nm) were recorded at the mast head, whereas longitudinal bending showed highest values at the vang connection to the mast (229 ± 29 Nm). Tension loads acting on the shrouds were also measured with highest values of 2887 ± 167 N. The knowledge of loads acting along the mast will support the designers in improving the mast’s cross section profile, the material selection and the validation of numerical structural analysis. Moreover, experimental data about the loads acting on the standing rigging and on the mast-step during sailing will support the optimised design of the boat shell.

  The effects of shot-peening on fretting fatigue crack growth behaviour in titanium alloy, Ti-6A1-4V were investigated. Three shot-peening intensities: 4A, 7A and 10A were considered. The analysis involved the fracture mechanics and finite element sub-modelling technique to estimate crack propagation lives. These computations were supplemented with the experimentally measured total fretting fatigue lives of laboratory specimens to assess the crack initiation lives. Shot-peening has significant effect on the initiation/propagation phases of fretting fatigue cracks; however this effect depends upon the shot-peening intensity. The ratio of crack initiation and total life increased while the ratio of the crack propagation and total life decreased with an increase of shot-peening intensity. Effects of residual compressive stress from shot-peening on the crack growth behaviour were also investigated. The fretting fatigue crack propagation component of the total life with relaxation increased in comparison to its counterpart without relaxation in each shot-peened intensity case while the initiation component decreased. Improvement in the fretting fatigue life from the shot-peening and also with an increase in the shot-peening intensity appears to be not always due to increase in the crack initiation resistance from shot-peened induced residual compressive stress.

  This paper deals with the development of a methodology for the prediction of material failure in metallic aerospace alloys by evaluating changes in surface characteristics directly prior to unstable fatigue crack propagation. The study is based on in situ nondestructive characterisation of the depression zone ahead of the crack tip of fatigue-pre-cracked titanium alloy specimens subjected to static loading. A relationship between the surface characteristics of the deformation zone ahead of the crack and the stress intensity factor of the material was obtained. This relationship was common to a variety of microstructural conditions such as mill-annealed and β-annealed microstructures. Based on the analysis, prediction of the impending fracture in cracked samples of the material was enabled. The outcome of this study can be used for optimising the service life of structural components.

  In this study, the effects of electrode force on the static and fatigue strength of spot welded joints of 5083-O aluminium alloy sheets were investigated. The thickness of the sheet joints was 1.5 mm. Tensile-shear joints with one spot weld were considered and three different load levels for electrode force were selected as 2500 N, 3000 N and 3500 N while the welding time and electric current were fixed during resistance spot welding process. Also, micro-structures and micro-hardness of cross-sectional area of the test samples were investigated. The results show that increasing the electrode force from 2500 N to 3000 N has no major impact on the nugget size and fatigue strength of the specimens, but increasing the electrode force from 3000 N to 3500 N, despite reducing in the diameter of the nuggets, increases the fatigue life of the joints significantly. The results also indicate that increasing the electrode force increases the life associated with the crack initiation phase of total fatigue lifetime.

In recent years, the study of polymer/clay nanocomposites has attracted major research and commercial interests due to their superior mechanical and thermal properties to those of the neat polymers. Numerical modelling is an advantageous approach to understand the material behaviour. In this work a symmetric two dimensional finite element model is developed to simulate the fully exfoliated Polymer/Clay nanocomposite behaviour and evaluate its’ before stress stress–strain curve. The Nylon 66/Clay in this study has a nonlinear elastic behaviour. The influence of volume fraction and aspect ratio of clay platelet (defined as the ratio of the particle length to the particle thickness) on the tensile behaviour of exfoliated Nylon 66/clay nanocomposite is investigated with the aid of numerical simulations. Results show good agreement with experimental data from literature.

In this paper, a new method for producing high temperature gratings is put forward. With two deposited metal layers on the specimen, the oxidation resistant ability of this grating was improved, and its applications were enlarged, to cover almost all metals. The procedure for producing a grating and the controlling factors of grating quality are discussed in detail.