Showing papers in "Strain in 2011"

Effects of Fibre Geometry and Volume Fraction on the Flexural Behaviour of Steel‐Fibre Reinforced Concrete

Abstract: This work aims in studying the mechanical behaviour of concrete, reinforced with steel fibres of different geometry and volume fraction. Experiments include compression tests and fourpoint bending tests. Slump and air content tests were performed on fresh concrete. The flexural toughness, flexural strength and residual strength factors of the beam specimens were evaluated in accordance with ASTM C1609/C1609M-05 standard. Improvement in the mechanical properties, in particular the toughness, was observed with the increase of the volume fraction of steel-fibres in the concrete. The fibre geometry was found to be a key factor affecting the mechanical performance of the material.

Acoustic and Electromagnetic Emissions as Precursor Phenomena in Failure Processes

Abstract: In this work we measured the electromagnetic field, given by the moving charges, during laboratory fracture experiments on specimens made of different heterogeneous materials. We investigated the mechanical behaviour of concrete and rocks samples loaded up to their failure by the analysis of Acoustic Emission (AE) and Electromagnetic Emission (EME). All specimens were tested in compression at a constant displacement rate and monitored by piezoelectric (PZT) transducers for AE data acquisition. Simultaneous investigation of magnetic activity was performed by a measuring device calibrated according to metrological requirements. In all the considered cases, the presence of AE events has been always observed during the damage process, whereas it is very interesting to note that the magnetic signals were generally observed only in correspondence of the final collapse or sharp stress drops.

Measurements and Calculations of the Width of the Fracture Process Zones on the Surface of Notched Concrete Beams

Abstract: The paper presents the results of experimental and theoretical investigations of the width of the fracture process zone (FPZ) on the surface of notched concrete beams during quasi-static three-point bending. To measure two-dimensional deformations on the surface of beams, a Digital Image Correlation (DIC) technique was used. Laboratory tests were performed with different notched concrete beams. The experiments were simulated with two different isotropic continuum crack models under two-dimensional conditions: an elasto-plastic and a damage one with non-local softening.

Delamination Effect on Bending Behaviour in Carbon–Epoxy Composites

Abstract: This paper presents the results of experimental and numerical studies on the bending behaviour of carbon fibre-reinforced epoxy composite containing delamination located at different positions along the laminate thickness. Experimental tests were conducted in three-point bending using specimens with and without delamination to evaluate their bending behaviour. Numerical simulations were also performed in order to evaluate the maximum load as a function of the defect position and its size. The numerical model includes two-dimensional solid elements of the ABAQUS software and a cohesive mixed mode damage model to simulate delamination propagation. The numerical and experimental results concerning the maximum load were found to be concordant. It was concluded that delaminations affect the bending behaviour of laminates mainly due to alterations in shear stress profiles.

The b‐Value Analysis for the Stability Investigation of the Ancient Athena Temple in Syracuse

Abstract: Some of the most significant architectural works are monumental masonry constructions. Among these, the Cathedral of Syracuse can be viewed as a fundamental element in the cultural heritage of Europe. For the preservation of these monuments, it is necessary to assess their durability by taking into account cumulative damage and cracking conditions in the structures. The paper describes the methods used by the authors to determine the conditions of the materials and the crack patterns in the stonework structures of the Cathedral. In particular, the acoustic emission (AE) technique is used to evaluate the time dependence of damage and the onset of critical conditions in a pillar, which is part of the vertical load-bearing structures. The authors show that the damage evolution in the stonework structures, experimentally investigated in situ by the AE technique, can be described by a power law characterised by a non-integer exponent, βt. In this way, the time dependence of damage is evaluated by working out the βt exponent and making a prediction of the stability conditions of the structure. Furthermore, the achievement of the critical condition is characterised through another synthetic parameter, the b-value of the Gutenberg-Richter law. The b-value systematically changes during the different stages of the failure process and tends to 1.0 as the structure reaches the final collapse. In the present study, this behaviour is documented by several AE tests carried out on specimens of different dimensions extracted from the pillar. In addition, these results are compared with the AE data obtained from the in situ-monitored pillar.

An Understanding of the Width Effect in FRP–Concrete Debonding

Abstract: The scaling of the ultimate load in fibre-reinforced polymer (FRP)–concrete debonding with the relative width of the FRP is experimentally investigated in this paper. Shear debonding tests are performed to evaluate the cohesive stress transfer between the adherents during the interface crack growth which produces debonding. Concrete specimens with two different widths and different widths of FRP are used in the experimental programme. The nominal stress at debonding increases with the FRP-to-concrete width ratio. For a given width of FRP composite sheet, lower debonding stress is obtained from concrete specimens with a larger width. The strain distribution on the FRP and concrete free surface at different stages of debonding was determined using a full-field optical technique known as digital image correlation. The contribution of the two factors, the boundary effect and the restraint from the surrounding concrete, was studied from the measured strain distribution. The strain distributions across the FRP composite sheet and the concrete within the cohesive stress transfer zone associated with the interface crack are shown to be very inhomogeneous. A region of constant width associated with high shear strains is found at the edge of the FRP sheet during the entire debonding process. The increase in the ultimate nominal stress at debonding is shown to be due to the decrease in the proportion of the total width of the FRP occupied by the edge region. It is shown that the boundary region within the FRP is of a fixed width. The width of concrete close to the edge of the FRP involved in stress transfer, however, increases with the width of FRP. It is established that when the FRP-to-concrete width ratio is smaller than 0.5, the level of restraint from concrete increases with the FRP width.

Acoustic emissions at high and low frequencies during compression tests of brittle materials

Abstract: The damage process in a concrete specimen subjected to uniaxial compression test is investigated by detection of the propagating elastic waves because of micro- and macrocrack growth. Besides the high-frequency acoustic emissions (AEs), the presence of low-frequency elastic emissions (ELEs), from 1 to 10 kHz, is detected just before the specimen failure. A spectral analysis of the ELEs is performed by measuring with a calibrated transducer the local acceleration of the specimen surface. Quantitative information about the macrocrack effects in terms of released energy is thus obtained. Furthermore, the evolution of damage is followed through the analysis of the amplitude distribution of AE and ELE signals, distributed according to the Gutenberg–Richter (GR) statistics.

Experimental Analysis of Shear Zone Patterns in Cohesionless for Earth Pressure Problems Using Particle Image Velocimetry

Abstract: The evolution of shear zones in cohesionless sand for earth pressure problem of a retaining wall is experimentally investigated using a non-invasive method called particle image velocimetry. It is an optical technique for measuring surface displacements from successive digital images. Small scale laboratory tests are performed for active and passive cases of a rigid retaining wall subjected to horizontal translation, rotation about its toe and rotation about its top. Attention is focused on the effect of initial sand density on distribution of volumetric and deviatoric strain. The results for initially dense sand are qualitatively compared with corresponding ones obtained with X-rays at Cambridge University.

Compositional and Microchemical Evidence of Piezonuclear Fission Reactions in Rock Specimens Subjected to Compression Tests

Abstract: Energy-dispersive X-ray spectroscopy (EDS) is performed on different samples of external or fracture surfaces belonging to specimens used in piezonuclear tests [Strain 45, 2009, 332; Strain (in press); Phys. Lett. A. 373, 2009, 4158]. For each sample, different measurements of the same crystalline phases (phengite or biotite) are performed to obtain averaged information of the chemical composition and to detect possible piezonuclear transmutations from iron to lighter elements. The samples were carefully chosen to investigate and compare the same minerals both before and after the crushing failure. Phengite and biotite, which are quite common in the Luserna stone (20 and 2%, respectively), are considered owing to the high iron concentration in their chemical compositions. The results of EDS analyses show that, on the fracture surface samples, a considerable reduction in the iron content (∼25%) is counterbalanced by an increase in Al, Si, and Mg concentrations

Practical Applicability and Limitations of the Elastic Modulus Degradation Technique for Damage Measurements in Ductile Metals

Abstract: The use of damage models require a number of material parameters the identification of which should be made, when possible, based on direct measures of the damage that develops in the material Among the possible experimental techniques to measure the occurrence of ductile damage in metals, the measure of the damage through the degradation of the material Young's modulus has been indicated as one of the more effective technique and used by several authors In this paper, this technique is critically reviewed highlighting the number of issues that may affect the measure of damage In particular, the attention is focused in the case where damage process initiates at the onset necking and develops in the post-necking regime where stress, strain and damage are no longer uniform in the gauged section Since geometry variations alter the reference base length for both stress and strain definition, a procedure based on the use of finite element simulation is proposed in order to account for this effect on the measure of the stiffness loss The procedure has been applied to two class of metals: a high strength steel and high purity copper

Saturated Lightweight Aggregate for Internal Curing in Low w/c Mixtures: Monitoring Water Movement Using X-ray Absorption

Abstract: The susceptibility for early-age shrinkage cracking in low w/c concrete has led to the development of new technologies that can reduce the risk of cracking Internal curing, one of these new technologies, uses saturated lightweight aggregate to supply ‘curing water’ to low w/c pastes as they hydrate While significant research has been performed to determine the effects of internal curing on shrinkage and stress development, relatively little detailed information exists on movement of water from the LWA to the paste The details of when water moves and how far it travels has implications on the volume of aggregate and the aggregate gradation that should be used in mixture proportioning with internal curing This study examines the timing of the release of water from saturated lightweight aggregate (LWA) and the distance that water can travel X-ray absorption is used to determine the time at which water is drawn out of the lightweight aggregate and the loss of water is traced over time Further, it examines the travel distance of the water into the paste Experimental details associated with the X-ray technique are provided to describe the importance of specimen alignment Further, in sealed specimens, the results indicate little water movement prior to set After set however, the distance of water movement was as much as 2 mm The importance of water movement is discussed in terms of mixture proportioning and its influence on shrinkage and stress development

The Influence of the Boundary Conditions on Low‐Velocity Impact Composite Damage

Abstract: The objective of this work was to study the influence of the boundary conditions on low-velocity impact behaviour of carbon-epoxy composite plates. Experimental work and numerical analysis were performed on [04,904]s laminates. The influence of different boundary conditions on the impacted plates was analysed considering rectangular and square plates. The X-radiography was used as a non-destructive technique to evaluate the internal damage caused by impact loading. A three-dimensional numerical analysis was also performed considering progressive damage modelling. The model includes three-dimensional solid elements and interface finite elements including a cohesive mixed-mode damage model, which allows simulating delamination between different oriented layers. It was verified that plate’s boundary conditions have influence on the delaminated area. Good agreement between experimental and numerical analysis for shape, orientation and size of the delamination was obtained.

Geomechanical and geochemical evidence of piezonuclear fission reactions in the Earth's crust

Abstract: Piezonuclear reactions, which occur in inert and non-radioactive elements, are induced by high pressure and, in particular, by brittle fracture phenomena in solids under compression. These low-energy reactions generally take place in nuclei with an atomic weight that is lower or equal to that of iron (Fe). The experimental evidence, obtained from repeatable measurements of neutron emissions [Strain 45, 2009, 332; Strain (in press); Phys. Lett. A. 373, 2009, 4158], can be also recognised considering the anomalous chemical balances of the major events that have affected the Earth's crust, oceans and atmosphere, over the last 4 billion years. These anomalies include (i) abrupt variations in the most abundant elements in correspondence with the formation of tectonic plates; (ii) the ‘Great Oxidation Event' (2.7-2.4 billion years ago), with a sharp increase in atmospheric oxygen and the subsequent origin of life; (iii) the current climate acceleration partially because of ‘carbon pollution'. Natural piezonuclear reactions are induced by fault sliding and plate subduction phenomena

Identification of Damage and Plasticity Parameters for Continuum Damage Mechanics Modelling of Carbon and Glass Fibre-Reinforced Composite Materials

Abstract: An experimental test series was carried out to determine input parameters for a well-known continuum damage mechanics elementary ply plasticity model. A full suite of data was obtained for a carbon fibre and an S2-glass fibre-reinforced composite material, both currently used in the aerospace industry. Models were implemented using the experimentally determined input parameters and predictions for in-plane behaviour found good agreement with experiments for both material systems. In addition, model predictions for cyclic loading accurately captured reload moduli and plastic strain magnitude.

AE monitoring and numerical simulation of a two-span model masonry arch bridge subjected to pier scour

Abstract: A scaled model of a two-span masonry arch bridge has been built to investigate the effect of the central pile settlement because of riverbank erosion. The bridge geometry and the structural details, included the masonry bricks and mortar joints, are realized in the scale 1 : 2. The model bridge has been equipped with different Non-Destructive Testing (NDT) instruments, including accelerometers, displacement transducers LVTD, strain gages, optic fibres strain sensors and Acoustic Emissions (AE) transducers. The model bridge has been subjected to incremental settlement of the pile, which was sustained on a mobile support. The exact mechanism and temporal evolution of the pier scour has been investigated numerically and experimentally by mean of a hydraulic model. In this article, a detailed description of AE data and damage localization is provided. During the first stage of the settlement, the AE counting has been recorded. Based on the interpretation of the AE rate, it is possible to monitor the criticality of the ongoing process. In addition, thanks to the AE equipment, it has been possible to localize the main damaged zones. The statistical properties AE time series have been analysed using an estimation of the b-value of the Gutenberg-Richter (GR) law permitting to determine the damage level reached in the model. The damage evolution has been also interpreted numerically with the aid of a finite element programme able to predict the nucleation and propagation of fracture. In this way, some criteria for the monitoring and interpretation of full-scale structure cracking are provided for the assessment in presence of riverbank erosion phenomena.

Cross‐Correlation and Differential Technique Combination to Determine Displacement Fields

Abstract: This study presents a method to measure the displacement fields on the surface of planar objects with sub-pixel resolution, by combining image correlation with a differential technique. First, a coarse approximation of the pixel level displacement is obtained by cross-correlation (CC). Two consecutive images, taken before and after the application of a given deformation, are recursively split in sub-images, and the CC coefficient is used as the similarity measure. Secondly, a fine approximation is performed to assess the sub-pixel displacements by means of an optical flow method based on a differential technique. To validate the effectiveness and robustness of the proposed method, several numerical tests were carried out on computer-generated images. Moreover, real images from a static test were also processed for estimating the displacement resolution. The results were compared with those obtained by a commercial digital image correlation code. Both methods showed similar and reliable results according to the proposed tests.

Study on Failure Characteristic of Rock-Like Materials With an Open-Hole Under Uniaxial Compression

Abstract: The failure strength model of brittle materials with a pre-existing open-hole defect is proposed in this paper. A modified Sammis–Ashby model is deduced, in which it can be used to calculate the peak strength of brittle materials. It shows the law between peak strength σp and independent variable μ, which is the ratio of open-hole radius (a) to half-width of the specimen (t). Moreover, numerical and experimental investigations on failure process of rock-like materials with an open-hole imperfection were carried out. In the experiments, 3D-digital image correlation method, an optical technique which utilises the full-field and non-contact measurement, was employed. A progressive elastic damage method realistic failure process analysis (RFPA) was used in the numerical investigation to inspect and verify the modified model and simulate the failure process. The investigation finds that there are good correlations between the experimental, numerical and theoretical results. Moreover, because of the influences of boundary conditions, shear failure type was obtained both experimentally and numerically.

An analytical model based on strain localisation for the study of size-scale and slenderness effects in uniaxial compression tests

Abstract: In this paper, an analytical model based on the concept of strain localisation is proposed for the analysis and prediction of the response of quasi-brittle materials in uniaxial compression tests, such as mortar, plain concrete with different compression strengths, as well as fibre-reinforced concrete. The proposed approach, referred to as Overlapping Crack Model, relies only on a pair of material constitutive laws, in close analogy with the Cohesive Crack Model: a stress- strain relationship describing the pre-peak behaviour of the material and a stress-interpenetration relationship for the description of the post-peak response. In the paper it will be shown how the stress-interpenetration relationship can be deduced from experimental data and how it depends on the compression strength and on the crushing energy of the tested materials. A wide comparison between the stress-displacement curves predicted by the proposed model and those experimentally found in the literature will show the effectiveness of the present approach to capture both stable softening or sharp snap-back post-peak branches by varying the slenderness or the size-scale of the tested samples.

Condition Monitoring of Slow-Speed Shafts and Bearings with Acoustic Emission

Abstract: For various rotating machinery applications, condition monitoring using high-frequency Acoustic emission (AE) technology has remained a subject of intense study since the late 1960s. This paper demonstrates the results of an investigation for detecting and monitoring natural crack initiation and propagation in slow-speed rotating machinery with the AE technology. In addition to highlighting previous work published by the authors [1–3] on condition monitoring of slow-speed rotating machinery, this paper presents experimental results on accelerated bearing fatigue under starved lubricating conditions. For these investigations, purpose-built test rigs were employed for generating natural degradation on both a bearing and a shaft. It is concluded that crack initiation and propagation on slow-speed bearings and shafts can be detected with the AE technology.

Strain Localisation and Damage Measurement by Full 3D Digital Image Correlation: Application to 15‐5PH Stainless Steel

Abstract: 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.

Effect of Styrene–Butadiene Rubber Latex on Mechanical Properties of Cementitious Materials Highlighted by Means of Nanoindentation

Abstract: In this paper, micro-mechanical properties of styrene–butadiene rubber (SBR) latex-modified cement pastes identified by means of the nanoindentation (NI) technique are related to macro-mechanical properties of SBR latex-modified mortars obtained from standard test methods, considering an SBR latex/cement ratio varying from 0% to 20%. For this purpose, the average value of the hardness and the so-called indentation modulus of the different material phases of the cement paste, i.e. calcium–silicate–hydrate (CSH), portlandite, anhydrous cement, etc., obtained from NI are compared with the compressive and flexural strengths, on the one hand, and the dynamic elastic modulus of SBR latex-modified mortars, on the other hand. This comparison revealed a linear correlation between the dynamic elastic modulus and the indentation modulus and between the compressive strength, flexural strength and hardness. Thus, the obtained results clearly indicate the finer-scale origin of the macroscopic elastic and strength properties, linking the mechanical properties at the so-called mortar scale to the cement-paste scale.

Real‐Time Visualisation of the Portevin–Le Chatelier Effect With Mechanoluminescent‐Sensing Film

Abstract: The use of the mechanoluminescence (ML) technique to directly observe the Portevin–Le Chatelier (PLC) effect is described. A uniaxial tensile test of an aluminium alloy plate coated with SrAl2O4 : Eu2+ (SAOE) ML-sensing film was performed, and a high-speed camera recorded ML images simultaneously. Strong ML light in response to the PLC bands was emitted from the coated SAOE ML-sensing film and could be seen clearly with the naked eye. The position, the slope angle of the band front and propagation velocity of the PLC bands were measured directly from the ML images. Momentary interplay between two face-to-face propagating PLC bands was observed and captured in the ML images. The results show that real-time visualisation of the PLC effect was achieved using this ML technique.

Determination of Elastic Properties of Resected Human Breast Tissue Samples Using Optical Coherence Tomographic Elastography

Abstract: The present study is concerned with the application of optical coherence tomographic (OCT) elastography technique for quantitative assessment of the elastic properties of resected human breast tissue samples subjected to axial compressive loading in vitro. Three classes of breast tissue samples, namely normal, benign (fibroadenoma) and malignant (invasive ductal carcinoma), were considered. A speckle tracking technique based on two-dimensional cross correlation was employed to track the speckle motion between original (pre-compressed) and the displaced (post-compressed) OCT images of the tissue samples for the measurement of displacement and strain maps. The overall data reduction approach for quantitative assessment of elastic properties was validated against the results of gelatin phantoms containing activated charcoal particles as scattering centres. Results are presented in the form of OCT images and displacement and axial strain maps for normal, benign and malignant breast tissue samples. Based on the stress–strain relationship obtained for these three classes, the values of stiffness coefficients were reported in terms of modulus of elasticity. Results of the study reveal significant differences between the two-dimensional displacement vector maps of normal and cancerous breast tissue samples. The stiffness of benign tissue samples is found to be about two times higher than that of normal tissue samples, whereas for malignant samples, it is about four times higher, thereby signifying appreciable differences in the stiffness of cancerous and normal tissue samples.

Application of Stereo‐Digital Image Correlation to Full‐Field 3‐D Deformation Measurement of Intervertebral Disc

Abstract: This paper presents the application of a Stereo-Digital Image Correlation (Stereo-DIC) based approach for full-field deformation measurements on a porcine intervertebral disc (IVD) under in-vitro loading. Full-field capabilities of Stereo-DIC provide useful information on the IVD structure-function relationship needed for designing novel disc replacement devices on the basis of biomimetic concepts. In this work, the use of a multi-view Stereo-DIC system allows full-field measurement over more than 180° of the IVD surface. This is achieved by sequentially moving a single camera through seven fixed positions in order to cover the desired angle of vision. Ad hoc data processing and merging procedures are calibrated for a cylindrical sample. Strain maps are determined for a rubber cylinder subjected to rigid-body motions and then to uniform compression. These preliminary procedures serve to evaluate strain-mapping errors as well as to assess repeatability and accuracy of camera re-positioning. Once the entire procedure is verified and calibrated, a fresh porcine functional spine unit is loaded under anterior, posterior and lateral compression. From displacement maps obtained with the multi-view set-up, it is possible to gather information on both IVD bulging and surface strains. Results are consistent with the IVD global behaviour under standard testing protocols reported in the recent literature. Furthermore, detailed information on local variations in structural response and stiffness properties occurring when load is applied in different regions of the IVD are obtained. The present approach allows to correlate structural response with the entire 3-D deformation and strain field and not just with a single displacement/strain component and thus it could be effectively used for explaining in-homogeneity observed in human discs degenerative patterns.

Effect of Loading Rate on Fracture Energy of High-Strength Concrete

Abstract: This research deals with the sensitivity of several types of performance-designed high-strength concrete to the loading rate. Variations in the composition of the concrete produce the desired performance, for instance having null shrinkage or being able to be pumped at elevated heights without segregation, but they also produce variations in the fracture properties that are reported in this paper. We performed tests at five loading rates spanning six orders of magnitude in the displacement rate, from 1.74 × 10−5 mm s−1 to 17.4 mm s−1. Load-displacement curves show that their peak is higher as the displacement rate increases, whereas the corresponding displacement is almost constant. Fracture energy also increases, but only for loading rates higher than 0.01 mm s−1. We use a formula based on a cohesive law with a viscous term [Anales de Mecanica de la Fractura 25 (2008) 793–797] to study the results. The correlation of the formula to the experimental results is good and it allows us to obtain the theoretical value for the fracture energy under strictly static conditions. In addition, both the fracture energy and the characteristic length of the concretes used in the study diminish as the compressive strength of their aggregates increases.

Buckling of carbon/glass composite tubes under uniform external hydrostatic pressure

Abstract: This paper describes an experimental and an analytical investigation into the collapse of 44 circular cylindrical composite tubes under external hydrostatic pressure. The results for 22 of these tubes were from a previous investigation and the results for a further 22 models are reported for the first time in this paper. The investigations concentrated on fibre-reinforced plastic tube specimens made from a mixture of three carbon and two E-glass fibre layers. The lay-up was 0°/ 90°/0°/90°/0; the carbon fibres were laid lengthwise (0°) and the E-glass fibres circumferentially (90°). The theoretical investigations were carried out using a simple solution for isotropic materials, namely a well-known formula by ‘von Mises’. The previous investigation also used a numerical solution based on ANSYS, but this was found to be rather disappointing. The experimental investigations showed that the composite specimens behaved similarly to isotropic materials previously tested, in that the short vessels collapsed through axisymmetric deformation while the longer tubes collapsed through non-symmetric bifurcation buckling. Furthermore, it was discovered that the specimens failed at changes of the composite lay-up due to the manufacturing process of these specimens. These changes seem to be the weak points of the specimens. For the theoretical investigations, two different types of material properties were used to analyse the composite. These were calculated properties derived from the properties of the single layers given by the manufacturer and also the experimentally obtained properties. Two different approaches were chosen for the investigation of the theoretical buckling pressures, of the previously analysed models, namely a program called ‘MisesNP’, based on a well-known formula by von Mises for single-layer isotropic materials, and two finite element analyses using the famous computer package called ‘ANSYS’. These latter analyses simulated the composite with a single-layer orthotrophic element (Shell93) and also with a multi-layer element (Shell99). The results from Shell93 and Shell99 agreed with each other but, in general, their predictions were higher than the analytical solution by von Mises. The von Mises solution agreed better than the finite element solutions for the longer vessels, which collapsed by elastic instability, particularly when the experimentally obtained material properties were used. Thus, it was concluded that the results obtained from the finite element analyses predicted ‘questionable’ buckling pressures. The report provides design charts by all approaches and material types, which allow the possibility of obtaining a ‘plastic knockdown factor’ for these vessels. The theoretical buckling pressures obtained using the computer programs MisesNP or ANSYS can then be divided by the plastic knockdown factor obtained from the design charts, to give the predicted buckling pressures. It is not known whether or not this method can be used for the design of very large vessels.

Virtual Fields Method on Planar Tension Tests for Hyperelastic Materials Characterisation

Abstract: New methods are emerging in the material characterisation field with the aim of exploiting innovative full-field strain measurement techniques. Besides experimental issues, also numerical procedures for inverse problems should adapt to a new philosophy: the large amount of data referred to local strains should be used in an appropriate way to obtain as much benefits as possible. In this context, an experimental and numerical procedure for the characterisation of hyperelastic materials is proposed. Planar tension tests have been performed on flat rubber specimens of different geometries. Strain maps obtained by means of a 2D Digital Image Correlation system are used to implement the virtual fields method, to estimate material dependent parameters of two of the most known hyperelastic constitutive laws: Ogden and 2nd order Mooney-Rivlin models. Numerical results and comparisons with experimental data are shown, analysing also aspects concerning implementation of the numerical procedures and computational efficiency of the algorithms.

Experimental Investigation of Load Sharing in Multiple Gear Tooth Contact Using the Stress‐Optical Method of Caustics

Abstract: In this paper, a comprehensive methodology for calculating load sharing in multiple tooth contact is presented based on the experimental stress-optical method of caustics. The technique is applied to a set of poly-methyl-methacrylate gears at various meshing positions covering a complete meshing cycle, including single and multiple gear tooth contact. The load sharing factor (LSF) is calculated using well-established mathematical formulae from the photographs of the transmitted caustics and the obtained results are compared with the pertinent International Organisation for Standardisation and American Gear Manufacturers Association standards with which good agreement is verified. The proposed method is a reliable alternative for measuring load distribution in gear teeth compared with photoelasticity and other experimental techniques. ct Stress-optical constant

Asymmetrical Bending Model for NiTi Shape Memory Wires: Numerical Simulations and Experimental Analysis

Abstract: A detailed numerical and experimental study of a NiTi Shape Memory Alloy (SMA) wire subjected to bending is presented in order to provide a complete characterisation of the material under this load case. The beam model presented is based on the classic Euler-Bernoulli theory and uses De la Flor one-dimensional constitutive equations modified to take into account different material responses to tension and compression as well as the different elastic properties of austenite and martensite. All the necessary experimental techniques were designed to determine the bending behaviour of NiTi SMA wire over the entire range of transformation temperatures. These experimental bending results were then compared with the numerical results. The numerical model proposed agrees quantitatively and qualitatively with the experimental bending results obtained for NiTi wire, representing an effective tool for the analysis of one-dimensional structural devices. A comparison of the numerical results that assume symmetrical behaviour between tension and compression with the results that assume asymmetrical behaviour has shown that the tension/compression asymmetry is more pronounced in the martensitic range and has little influence on the response of the SMA wire subjected to bending at temperatures above Af. These results are analyzed and discussed.