Showing papers in "Journal of Strain Analysis for Engineering Design in 2010"

High-resolution electron backscatter diffraction: An emerging tool for studying local deformation:

Abstract: Electron backscatter diffraction (EBSD) is a widely available and relatively easy-to-use scanning-electron-microscopy-based diffraction technique. Recently, Wilkinson, Meaden, and Dingley presented two papers on a new cross-correlation-based analysis of EBSD patterns which allow variations in the elastic strain and lattice rotation tensors to be measured at a sensitivity of about 10×4 at high spatial resolution. This paper briefly describes the basis of the technique and how the resulting lattice curvatures can be used to estimate the geometrically necessary dislocation (GND) content in a sample. To illustrate the utility of the method for microscale deformation studies the following examples are described: first, nanoindentation near a grain boundary in α-Ti; second, transformation-induced GNDs in a dual-phase steel; third, thermally-induced and mechanically-induced deformation near carbides in a superalloy; fourth, GND accumulation during fatigue of a polycrystalline Ti–6Al–4V alloy.

Strategies and limits in multi-stage single-point incremental forming

Abstract: Multi-stage single-point incremental forming (SPIF) is a state-of-the-art manufac- turing process that allows small-quantity production of complex sheet metal parts with vertical walls. This paper is focused on the application of multi-stage SPIF with the objective of producing cylindrical cups with vertical walls. The strategy consists of forming a conical cup with a taper angle in the first stage, followed by three subsequent stages that progressively move the conical shape towards the desired cylindrical geometry. The investigation includes material characterization, determination of forming-limit curves and fracture forming-limit curves (FFLCs), numerical simulation, and experimentation, namely the evaluation of strain paths and fracture strains in actual multi-stage parts. Assessment of numerical simulation with experimentation shows good agreement between computed and measured strain and strain paths. The results also reveal that the sequence of multi-stage forming has a large effect on the location of strain points in the principal strain space. Strain paths are linear in the first stage and highly non-linear in the subsequent forming stages. The overall results show that the experimentally determined FFLCs can successfully be employed to establish the forming limits of multi-stage SPIF.

On the interpretation of results from small punch creep tests

Abstract: The small punch creep testing method is highly complex and involves interactions between a number of non-linear processes. The deformed shapes that are produced from such tests are related to the punch and specimen dimensions and to the elastic, plastic, and creep behaviour of the test material, under contact and large deformation conditions, at elevated temperature. Owing to its complex nature, it is difficult to interpret the small punch test creep data in relation to the corresponding uniaxial creep behaviour of the material. One of the aims of this paper is to identify the important characteristics of the creep deformation resulting from ‘localized’ deformations and from the ‘overall’ deformation of the specimen. Following this, the results of approximate analytical and detailed finite element analyses of small punch tests are investigated. It is shown that the regions of the uniaxial creep test curves dominated by primary, secondary, and tertiary creep are not those that are immediately apparent from the displacement versus time records produced during a small punch test. On the basis of the interpretation of the finite element results presented, a method based on a reference stress approach is proposed for interpreting the results of small punch test experimental data. Future work planned for the interpretation of small punch tests data is briefly addressed.

A procedure for evaluating high residual stresses using the blind hole drilling method, including the effect of plasticity

Abstract: When the blind hole drilling method is used to evaluate high residual stresses in a metallic component, plastic relaxed strain can be produced in the hole region because of the stress concentration...

Stress-separation techniques in photoelasticity: A review

Abstract: Photoelasticity has been used for decades in the experimental measurement of stresses and strains. As data-acquisition techniques only identify the differences between principal stresses and strains, stress-separation techniques exist to identify each of their values separately. A variety of techniques have been developed since the early days of photoelasticity, aiming to automate the measurement process, to exploit the full potential of photoelastic methods, and to facilitate its application. The purpose of this paper is to provide a general overview of the different stress-separation techniques, and their current state of development and usage. The paper presents the publications and the authors who have contributed to the development of new techniques.

Analysis of Stress and Deformation in Plastic Gears Used in Gerotor Pumps

Abstract: The mechanism of induction of stresses and deformations in plastic cycloidal gears used in gerotor pumps has been analysed using the finite element method and the ABAQUS program. It has been found that the gear system remains under the influence of the mechanical load resulting from the torque on the pump shaft and the hydraulic load resulting from the activity of pressure in the intertooth displacement chambers. It has also been discovered that the intertooth forces and stresses are formed only in the part of the gear that can be referred to as ‘active’. In the other part of the gear, forces and stresses do not occur, and the part can be referred to as ‘passive’. Another finding of the research is that gear deformations occur such that the teeth of the external gear are deformed and moved in the direction of the active part of the mesh, and the teeth of the internal gear in the direction of the passive part. Thus, radial and axial intertooth clearances are formed, which result in internal leakages in the...

Effects of fillers on the fracture behaviour of particulate polyester composites

Abstract: In the current work, the fracture toughness of sand-particle- and wood-flake reinforced polyester composites was studied under a linear elastic fracture mechanics approach. The effects of the particulate volume fraction (0–60 vol %) were studied. Scanning electron microscopy was used to observe the damage features on the composite surface. Results showed that sand-particle- and fine-wood-flake-reinforced polymer composites exhibited better results at 40 vol% than at other particulate volume fractions. Meanwhile, coarse-wood-flake-reinforced polymer composites showed higher properties at 30 vol % than at other particulate volume fractions. Observation of the composite surface after tests showed that sand particles have poor interfacial adhesion compared with wood flakes.

Experimental and crystal plasticity studies of deformation and crack nucleation in a titanium alloy

Abstract: This paper presents recent developments in crystal plasticity modelling to incorporate a non-local crack nucleation criterion and the comparison of experimental observations and model predictions for deformation and crack nucleation in a near-alpha titanium (Ti) alloy polycrystal. The model polycrystal was established so as to have grain morphology and crystallography nominally identical to that in the experiment, enabling direct interpretation and comparison of the results. The experimental measurements of strain fields were obtained using electronic speckle pattern interferometry on a large-grained polycrystalline sample.The crystal plasticity model predictions and experiments show strong agreement in a number of important features relating to crack nucleation: the location of crack nucleation, the direction changes in crack growth when the crack crosses two grain boundaries, and the important role of crystallographic orientation in nucleation site and in growth directions. There was also good agreement...

Identification of Poisson's ratios of standard and auxetic low-density polymeric foams from full-field measurements:

Abstract: This exploratory paper presents some preliminary results on the use of full-field deformation measurements on low density polymeric foams to identify the evolution of Poisson's ratio with compressive strain. Two types of foams were tested: a standard low density polyurethane foam and an auxetic foam manufactured from a similar precursor. 2D digital image correlation was used to measure the strain field at the specimens surfaces. Then, Poisson's ratios were identified using a dedicated inverse method called the Virtual Fields Method (VFM) and the results compared with the standard approaches. The results illustrate the advantages of the VFM compared to the standard procedure. It was also found that for the standard foam, very strong localization effects resulted in biased Poisson's ratio evaluation. It was shown that this could be corrected by taking into account these localization effects thanks to the full-field information.

Hysteresis-loop representation for strain energy calculation and fatigue assessment

Abstract: Improvements have been made to the cyclic strain energy density expression used in a fatigue life prediction method. The theory behind the prediction method is based on the understanding that the s...

Twinning in structural material with a hexagonal close-packed crystal structure

Abstract: The paper aims to provide a brief and comprehensible overview of the current understanding of twin nucleation and growth in structural metallic materials with a hexagonal close-packed crystal structure. It describes possible experimental methods to improve this understanding, which is required to implement twin nucleation and growth in crystal plasticity models using mechanistically meaningful criteria. These aspects are further discussed by presenting results from deformation experiments carried out on a zirconium alloy (ZIRLO (TM)) and Ti-6 wt% Al-4 wt% V (Ti-6Al-4V). It is shown that very significant texture changes after even small levels of plastic deformation can be observed in both materials, strongly suggesting the presence of extensive (10 (1) over bar2) twinning. The textures produced during high-temperature rolling and compression testing demonstrate that this twinning mode is still operative at elevated temperatures. A comparison between macroscopic texture changes during compression testing and fine-scale orientation mapping using electron backscatter diffraction revealed a significant discrepancy between the expected and observed twinned volumes in ZIRLO and Ti-6Al-4V. One explanation for this discrepancy is that, in both materials, twinning is progressing to encompass entire grains. However, the required accommodation of the transformation strain due to the shear produced by the twinned grain needs to be further investigated to confirm the possibility of such a mechanism.

Stress concentrations in keyways and optimization of keyway design

Abstract: Keys and keyways are one of the most common shaft–hub connections. Despite this fact very little numerical analysis has been reported. The design is often regulated by standards that are almost half a century old, and most results reported in the literature are based on experimental photoelastic analysis. The present paper shows how numerical finite element (FE) analysis can improve the prediction of stress concentration in the keyway. Using shape optimization and the simple super elliptical shape, it is shown that the fatigue life of a keyway can be greatly improved with up to a 50 per cent reduction in the maximum stress level. The design changes are simple and therefore practical to realize with only two active design parameters.

An experimental and numerical investigation of the static and dynamic constitutive behaviour of aluminium alloys

Abstract: A split Hopkinson pressure bar (SHPB) set-up was used to investigate the dynamic constitutive behaviour of commercial aluminium alloys both experimentally and numerically. The study was conducted i...

Inelastic response of structures due to large impact and blast loadings

Abstract: This article provides a broad overview of the contributions that relatively simple methods of analysis can make to the design of structural systems for the protection of the public and the security...

Plastic loads for 90° thick-walled elbows under combined pressure and bending:

Abstract: This paper extends plastic load solutions proposed by the authors for 90° thin-walled elbows, to thick-walled elbows, based on three-dimensional finite element analyses using the large geometry change option. For loading, internal pressure, in-plane, and out-of-plane bending, as well as combined pressure and bending, are considered. As no experimental data are available for thick-walled elbows, the proposed solutions are indirectly validated by comparing thin-walled solutions with exiting experimental data.

Application of a few necking criteria in predicting the forming limit of unwelded and tailor-welded blanks

Abstract: The limiting value of strain describing necking/failure during a sheet stamping operation is quantified, predicted by the forming limit curve (FLC). The main aim of the present work is to analyse the applicability of the few existing necking criteria, namely the effective strain rate based criterion (ESRC - RC1), major strain rate based criterion (MSRC - RC2), thickness strain rate based criterion (TSRC - RC3), and thickness gradient based criterion (TGNC - RC4), in predicting the forming limit of unwelded and tailor-welded blanks. In the case of unwelded blanks, dry (m 50.12) and low-friction (m 50.02) conditions are simulated and forming limit results are predicted. In the case of tailor-welded blanks, the forming limits of laser-welded steel blanks and friction-stir-welded blanks made of aluminium alloy are predicted using the chosen necking criteria and then compared with results in the literature. The influence of friction on the whole forming limit curve and the non-linearity of strain paths in the stretching side of the forming limit diagram (FLD) are also predicted and discussed. It is found that the FLCs predicted by ESRC, MSRC, TSRC, and TGNC are comparable with experimental FLCs. The predictions are accurate in the low-friction condition, indicating that the necking criteria are also valid under changing friction conditions. Moreover, the overall FLC does not change much with changing friction conditions. The strain paths are non-linear in the dry friction condition, but they become linear in the low-friction condition. In the case of laser- welded blanks, the original strain rate based criteria defined for unwelded blanks under- estimate limit strains on the stretching side of the FLD, where failure near the weld region is witnessed. Hence they are modified as RC1 >25, RC2 >32, and RC3 >32, which show a better prediction level compared with the original criteria. The original and modified limit strain criteria show better correlation with results in the literature and the TGNC in the case of friction-stir-welded blanks modelled using the Hollomon and Swift strain-hardening laws.

Impact, penetration, and perforation of a bonded carbon-fibre-reinforced plastic composite panel by a high-velocity steel sphere: An experimental study

Abstract: In this work, the response of a bonded carbon-fibre-reinforced plastic composite panel, which had been manufactured by bonding two laminates together, to impact, penetration, and perforation by a h...

A finite element application in the analysis and design of point-supported composite conoidal shell roofs: Suggesting selection guidelines:

Abstract: In the present paper, a finite element code is applied to study the bending behaviour of point-supported composite conoidal shells. These doubly curved surfaces may look similar to single curved conical shells, but the non-developable conoids present much stiffer surfaces. Laminated composites offer a high strength-to-weight ratio, and composite conoidal shells can cover large column-free areas. These shells on point supports have wide applications in car parks and theatres. Research reports are available regarding the static and dynamic behaviour of composite and isotropic conoidal shells, but with different combinations of simply supported, clamped, and free boundary conditions. Reports on corner-point-supported isotropic rhombic plates and doubly curved shells also exist, but data on the bending behaviour of conoidal shells supported at discrete points only are missing. Hence, in the present paper, three different point-supported boundary conditions are considered with four different laminations, the r...

Three-Dimensional Strain Analysis of Single-Lap Bolted Joints in Thick Composites Using Fibre-Optic Gauges and the Finite-Element Method

Abstract: Bolted joints involving composite plates used to be almost entirely dedicated to aerospace applications. As the need for energy conservation has increased, the field of composite bolted joints has found new applications in ground armoured vehicles. Thick panels able to withstand large in-plane and impact loads are critical. The present investigation evaluates the interior strain field, through the thickness, of a composite plate connected to an aluminium panel with a single-lap bolted joint. The area of interest is the bearing plane region close to the hole because of the presence of stress concentrations that heavily modify the stress field. Experimental data for the bolted joint were recorded by fibre-optic strain gauges that were embedded in the bearing plane of the composite plate. Numerical analyses were performed using ANSYS as a pre-processor and LS-DYNA as a solver. The overall goal was to evaluate the magnitude of contact strains around the hole and through the thickness of the composite. These v...

A Comprehensive Numerical Stress—Strain Analysis of Laser Beam Butt-Welded Titanium Compared with Austenitic Steel Joints

Abstract: Experimental tests on laser beam butt-welded joints made of commercially pure titanium and Ti—6Al—4V alloy showed a fatigue behaviour quite different from that usually observed in the case of AISI 304 joints connected with the same welding technique. In fact, fatigue failures of titanium joints were observed at the level of the base material but not in the notch region, as have been found for austenitic steel. In order to explain this experimental evidence, very detailed finite element models are developed in this paper. The models reproduce the real cord profiles of Ti—6Al—4V and AISI 304 steel welded joints detected by a high-precision coordinate measuring machine. Furthermore, mechanical properties given as input to the finite element models correspond to the real joint microstructure revealed by micro-hardness tests carried out in the different regions of the specimen. Finally, finite element models reproduce the distribution of porosity detected via microscopic inspection of the fractured surfaces. I...

Using Genetic Algorithms to Optimize the Material Behaviour Model in Finite Element Models of Processes with Cyclic Loads

Abstract: To ensure realistic results in modelling processes for analysing strains in material using finite element (FE) models, it is essential to have a model of the material that is as close to reality as possible, especially when materials are subject to cyclic loads, because the gap in behaviour between actual materials and simulated models widens as the number of cycles increases owing to the Bauschinger effect, ratchetting, and other effects. This paper sets out a fully automated method for determining the most appropriate material behaviour model (linear or non-linear) for use in numerical simulation programs and the optimum constitutive parameters that define that model, on the basis of experimental data and the combined use of genetic algorithms (GAs) and finite elements (FEs). As a practical example, the method is applied to determine the optimum material model for ZSTE 800 high-strength steel with a view to simulating its behaviour in a cyclic stress—compression process with controlled strain and a vari...

Size effect and forming-limit strain prediction for microscale sheet metal forming of stainless steel 304:

Abstract: Owing to the extensive applications of stainless steel 304 on a microscale, a series of microscale tensile and dome height tests were conducted to investigate its size effects on mechanical properties and formability. Based on the experimental results and observations and the Oh et al. fracture criterion, two new models were proposed in this paper for predicting the forming limit of stainless steel 304 foils in microscale sheet metal forming. For the mechanical properties study, foils of four thicknesses (150mm, 100mm, 50mm, and 20mm) heat treated at four temperatures (900uC, 950uC, 1000uC, and 1050uC) were used for the experiments. For the formability prediction, the first proposed model includes the effect of the strain path while the second proposed model considers the coupling effects of both the strain path and the size effects. The first model is superior to the Oh et al. criterion with respect to predicting the forming-limit strain of the foils, but it is not suitable for foils that are thinner than 100mm. However, the second proposed model can be used for stainless steel 304 foils irrespective of their thicknesses and thickness-to-average-grain-size ratios T/D. It can also be concluded that the size effects must be considered in microformability when the foil thickness is smaller than 100mm.

An assessment of the effect of cutting welded samples on residual stress measurements by chill modelling

Abstract: In making residual stress measurements in welds, a common problem is that the samples are often cut before measurement, which may alter the stresses. To estimate the effects of cutting, a simplified method of modelling residual stresses in welds was developed, known as chill modelling, which requires limited material data and no welding process data. The method can be performed with elastic finite element analysis. The method does not predict the value of the stresses, just the relative reduction that occurs after cutting. The method was validated against published synchrotron measurements of welded plates, which were cut and remeasured a number of times. For welded plates only, an empirical equation was developed that predicts the change in stress after cutting.

Characterization of low-impedance materials at elevated strain rates:

Abstract: Experimental characterization of materials at elevated strain rates of the order of 103 s×1 is typically performed by Hopkinson-bar-type facilities. The specific nature of Hopkinson bar tests is that a specimen is loaded by waves. As both the input and output bars are often made from high-impedance metallic materials, the ratio between transmitted and reflected wave components is very poor in the case of low-impedance specimen materials such as rubbers and foams. That poor wave transmission brings with it a limited quality of the signal-to-noise ratio. Changing the bar materials (e.g. to polycarbonate) is of little help because of the related viscous effects. A further deficiency of the wave-driven Hopkinson bar test for many rubber and foam applications is the comparably small amount of compression that is achievable. Maximum compressions of 95 per cent and above, often expected for damping and energy absorption applications, are out of the range of most Hopkinson bar set-ups. In order to overcome these ...

Modelling the hygrothermal stress in curved glulam beams

Abstract: Variations of relative humidity and temperature result in moisture and temperature gradients in wood, which induce stress perpendicular to grain and may even cause cracking of the wood. To address ...

Boundary element analysis of thermo-elastic problems with non-uniform heat sources

Abstract: A boundary element formulation for the analysis of two-dimensional and three-dimensional steady state thermo-elastic problems involving arbitrary non-uniform heat sources is presented. All domain integrals are expressed in terms of a heat source function instead of a temperature function. The proposed method alleviates the difficulty associated with finding the temperature distribution throughout the domain. A simple, yet robust, method referred to as the Cartesian transformation method (CTM) is developed that allows boundary-only evaluation of domain integrals. Unlike other transformation methods the CTM does not require the introduction of a domain point for the transformation. Domain heat sources defined either over the whole domain or over a specific part of the domain can be treated by the proposed method. Three examples including different forms of heat sources are analysed to show the validity and efficiency of the presented methods.

Predicting the forming limit diagram of AA 5182-O

Abstract: The prediction of a forming limit diagram (FLD) for aluminium alloy sheet using finite element analysis without implementing pre-defined geometrical imperfections or material imperfections is studi...

Concentration of Normal Stresses in Flat Plates and round Bars with Periodic Notches

Abstract: The stress concentrations produced by tensile loading of elastic solids with periodic notches are addressed. The criterion suggested by Neuber, which likens the periodic notch to a single notch of ...

Deformation characteristics and mechanisms of the cold rotary forging of a ring workpiece

Abstract: Cold rotary forging is an advanced but very complex incremental metal plastic-forming technology under coupled effects with multifactors. Previous research concentrates mainly on the study of the cold rotary forging process of a cylindrical workpiece and thus it is essential to explore the deformation characteristics and mechanisms of the cold rotary forging of a ring workpiece in order to use rotary forging presses with maximum efficiency. In the current paper, numerous simulations have been carried out based on a valid three-dimensional elastic-plastic dynamic explicit finite element model developed using ABAQUS software. Through these simulations, the interactive effects of three main processing parameters, the feed rate v of the lower die, rotational speed n, and inclination angle γ of the upper die on the cold rotary forging process of a ring workpiece, have been thoroughly explored. Furthermore, a comprehensive and decisive factor in cold rotary forging, namely the equivalent feed amount per revolut...

A method for the in situ measurement of evolving elliptical cross-sections in initially cylindrical Taylor impact specimens

Abstract: This paper presents a novel approach to the classic Taylor impact experiment using an ultra-high-speed camera and mirror arrangement to measure the elliptical cross-section of a specimen in situ as a function of time. This optical measurement technique is used to quantify the key aspects of material behaviour, such as the area strain perpendicular to the impact direction and the lengths of the semimajor and semiminor axes of the elliptical sample cross-section, which were caused by anisotropic plastic deformation, as functions of time and the axial position. The application of this technique gives access to previously unattainable data on the anisotropic plastic deformation of Taylor impact specimens and therefore has the potential to provide a more rigorous method of validation for anisotropic constitutive material models. To demonstrate the feasibility of the new method, experiments were carried out on cylindrical Taylor impact specimens machined from strongly textured high-purity zirconium plate. The surface geometry of a recovered specimen was measured using a coordinate measurement machine and compared with the optically measured surface geometry reconstructed from post-impact images. Excellent agreement between the two methods was found.