Showing papers in "Journal of Strain Analysis for Engineering Design in 1997"

The interpretation of the strain-dependent Poisson's ratio in auxetic polyethylene:

Abstract: The strain-dependent behaviour characteristic of auxetic (i.e. having a negative Poisson's ratio) polymers has been modelled using a simple geometric model which consists of rectangular nodules intecronnected by fibrils. Careful consideration of the correct form of the model to use depending on the experimental method employed to test samples of auxetic ultra high molecular weight polyethylene (UHMWPE) has resulted in very good agreement between the experimental and theoretical Poisson's ratios and total engineering strain ratios when the deformation is predominantly due to hinging of the fibrils.Auxetic UHMWPE has been processed to yield a very wide range of Poisson's ratios depending on its microstructural parameters (i.e. nodule shape and size, fibril length and the angle between the fibril and nodule). These can be predicted using the model, allowing the possibility of tailoring Poisson's ratio of the material.

A finite element simulation of the cold working process for fastener holes

Abstract: Two-dimensional axisymmetric finite element simulations have been conducted for the cold working of a fastener hole in an aluminium plate. The simulation models the actual cold working process where an oversize mandrel is pulled through the fastener hole. The results of the simulation are compared with a simplified finite element model where the cold working process is reduced to applying a uniform radial expansion to the hole edge. It is shown that substantial differences exist between the finite element simulations; specifically, the simulation of the actual process shows tensile residual radial stresses on the surface of the plate after cold working whereas the simplified simulation shows only compressive ones. Further comparisons are made for the axial deformation of the plate by using the results of an experimental measurement of the surface profile around a cold worked hole. There is good agreement between the finite element and experimental results. The results of this work show that accurate simulations of cold working are necessary if predicted residual stresses are to be used to assess fatigue life.

On the singular stress field near the edge of bonded joints

Abstract: A stress singularity of the form Hr λ-1 develops at the corner where the interface between two bonded elastic solids intersects a traction-free edge. Here H is the intensity of the singularity, λ-1 is the order of the stress singularity and r is the radial distance from the interface corner. The intensity H of the singularity is evaluated for a butt joint consisting of a thin elastic layer sandwiched between two substrates and subjected to a uniform remote tension. The magnitude of H is determined using a combination of the finite element method (FEM) and a path-independent integral. The obtained values of H depend on the material elastic properties and on the magnitude of the applied loading. The results for H are compared with those obtained via the commonly used extrapolation method. The extent of the elastic singularity is determined as a function of the material elastic properties, and the implications of the results in developing an H-based free-edge failure initiation criterion are discussed.

Cyclic creep and creep-fatigue interaction in sandwich beams

Abstract: The main emphasis of this paper is to identify long-term effects such as creep and creep-fatigue interaction for a typical sandwich structure incorporating FRP (fibre-reinforced plastic) faces and a PVC (polyvinyl chloride) foam core. For the creep modelling a Burger model and a power law model are investigated and compared with experimental results. Fatigue results pertaining to a cyclic trapezoidal waveform are presented in the context of the creep-fatigue interaction. A cyclic creep model is developed in order to investigate the effect of cyclic creep deformation on a response containing elements of fatigue and creep. A simple linear damage rule separating the creep and fatigue components for the chosen cyclic waveform is used in order to quantify the relative influence of both components on the lifetime of the structure. Finally conclusions are drawn regarding the aptness of the chosen creep and creep-fatigue interaction models.

Creep Continuum Damage Constitutive Equations for the Base, Weld and Heat-Affected Zone Materials of a Service-Aged {1 over 2}cr{1 over 2}mo{1 over 4}v:2{1 over 4}cr1mo Multipass Weld at 640 °c

Abstract: Creep continuum damage constitutive equations have been determined for the base, weld and heat-affected zone materials of a service-aged 1/2Cr1/2Mo1/4V:21/4Cr1Mo multipass weld at 640°C. The base and weld material properties were determined from uniaxial creep and notched bar creep rupture test results. The properties of the heat-affected zone material were determined from impression creep tests, waisted cross-weld specimen tests and notched cross-weld specimen tests. At the test temperature of 640 °C, surface oxidation occurs. For the uniaxial creep and notched bar creep tests, this oxidation was considered not too significant. However, for the low-load impression creep tests, the oxidation depth was comparable to the indentation depth.

Residual stresses in alumino-thermic welded rails:

Abstract: Neutron strain scanning has been used to map the residual stress field that is generated in a railway rail by a standard gap alumino-thermic weld made using routinely specified procedures. The longitudinal and vertical residual stress fields in the sections well away from the weld are characteristic of many unwelded rails, being generally tensile in the head and foot with balancing compression in the web. In the vicinity of the weld the residual stress patterns are very different. At the top and bottom surfaces of the rail the longitudinal residual stress field is strongly compressive, which is generally beneficial in that it would tend to inhibit the initiation and propagation of fatigue cracks from surface defects. Just at the surface the vertical residual stress attenuates to zero but internally, in the web region, both longitudinal and vertical components of the residual stress field are strongly tensile, which increases the susceptibility of that region to crack initiation and propagation fro...

Static lateral compression of aluminium tubes: Strain gauge measurements and discussion of theoretical models

Abstract: This paper illustrates experimental results obtained by means of strain gauges applied on circular tubes during static lateral crushing. The aim of the work is to obtain an experimental ins...

The linear elastic stress analysis of curved pipes under generalized loads using a reduced integration finite ring element

Abstract: This paper consists of a linear elastic stress analysis of curved pipes having all the possible boundary conditions in structural engineering and submitted to a generalized in-plane or out-...

In vitro measured strains in the loaded femur: Quantification of experimental error:

Abstract: The application of strain gauges to bone surfaces has been extensively employed as a method of determining, strain fields in response to implanted devices in orthopaedics. The aim of this study was to determine some of the experimental errors associated with the use of strain gauges in in vitro experimental investigations of the loaded femur. An experimental protocol was devised to obtain strain data at 20 strain gauged locations on the proximal femur. These data were interpolated using a parametric model. The parametric model was then used to estimate the errors associated with mispositioning of the gauges and deviations in their direction of application to the bone. This sensitivity analysis was also supported by a finite element analysis for the purposes of comparison and cross-validation. The results indicated that the nature of the loading normally employed in the literature can contribute to making the readings for some of the gauges (anterior and posterior) unreliable and redundant, even fo...

Numerical analysis and experimental verification of the radial growth of a turbocharger centrifugal compressor impeller

Abstract: Steady state stress analysis was carried out on a typical automotive turbocharger centrifugal compressor impeller using the finite element method and the concept of cyclic symmetry. Since the rotational speed of the impeller is very high and the size of the impeller is quite small, experimental verification of the stresses is extremely difficult. A novel method of measuring the radial growth of the impeller tip has been presented and a suitable instrumentation was designed and developed. This paper describes the numerical and experimental work carried out on a centrifugal compressor impeller.

Stress singularities at the free surface of an axisymmetric two-material creep test specimen:

Abstract: The nature of the stress singularity that occurs at the surface of an axisymmetric, two-material creep test specimen has been investigated. Steady state finite element analyses were obtained for this purpose, using Norton's power law creep equations having the same stress exponent for each material. The strength of the singularity was found to be strongly dependent upon the relative creep strengths of the two materials, but was surprisingly insensitive to the relative dimensions of the two materials. The implications of the results for practical situations are discussed.

Stress intensity factors for semi-elliptical internal surface cracks in autofrettaged thick-walled cylinders:

Abstract: Stress intensity factors for internal semi-elliptical surface cracks in autofrettaged cylinders with and without internal pressures applied are presented. The three-dimensional finite element based displacement method with the crack tip square-root singularity of stresses and strains simulated is used to evaluate the stress intensity factors along the crack front. Both allowing and disallowing the overlapping of crack faces are considered in this investigation, the latter being simulated by considering crack surface contact through a kind of interface element introduced into the cracked area. The residual stress distribution assumed to act on the crack face is obtained according to Tresca's yield criterion with the material assumed to be elastic-perfectly plastic. Three different overstrains of autofrettage are chosen. The results show that the stress intensity factor is generally underestimated if the crack contact that has actually occurred is ignored, which may lead to a danger in the assessment of either fracture strength or fatigue life. Implications of the stress intensity factor results are also briefly discussed, particularly for the prediction of fatigue lives, and it is shown that the full autofrettage treatment might be the most beneficial for increasing the fatigue life of cracks initiated from the inner core.

A new static failure criterion for ductile materials

Abstract: A new static failure criterion is proposed with a function of the stress state as well as the material properties (the ratio of ty /Sy), which is based on the experimentally determined ratio of Ty to Sy and includes the maximum shear stress theory and the von Mises-Hencky theory as special cases. The new criterion has a great potential to fit the experimental data and appears to fit better the very limited experimental data found in the open literature.

Stresses in a flat plate due to a loose pin pressing against a cracked hole

Abstract: The elastic contact stress field caused by a radially loaded pin in a nearly conforming hole is studied. The elastic limit of the configuration is found, together with crack tip stress intensity factors for radial cracks emanating from the bolt hole, the latter using the distributed dislocation approach. A closed-form solution is therefore generated to enable an elastic design philosophy to be followed for both perfect and flawed configurations, providing that there are no nearby boundaries.

Influence of fibre on the cutting stress state in machining idealized glass fibre composite

Abstract: The machining of an idealized glass fibre reinforced plastic (GFRP) was examined using photo-elasticity, dynamometry and optical microscopy. Cutting stresses at the glass roving and roving-matrix interface were evaluated using experimental and numerical methods. Experimentally observed isochromatics and measured forces in the orthogonal cutting of GFRP were shown to be affected by the reinforcement and its orientation. Machining stresses and machined surface damage were found to be highest when machining materials with roving oriented 45° towards the cutting edge.

Prediction of elastic—plastic displacements of tubular joints under combined loading using an energy-based approach

Abstract: A series of 24 geometrically and materially non-linear finite element analyses of a simply supported YT tubular joint, with axial loads on the T- and Y-brace ends, was carried out to collap...

The measurement of prior plastic deformation in metallic alloys using the neutron diffraction technique

Abstract: The neutron diffraction technique has been used to determine the extent of prior plastic deformation in an aluminium alloy weld and preloaded cracked steel beams. The change in peak width has been shown to be a function of the prior plastic deformation, and is found to be insensitive to the direction of prior straining, material and the orientation of the diffraction vector. Prior plastic deformation has also been shown to influence the reference peak angle. The distributions of prior plastic strain in the steel cracked beam subjected to an overload were found to be in good agreement with results from finite element analyses.

Strain gauge measurement of wheel-rail interaction forces

Abstract: A theoretical basis for quasi static determination of wheel—rail interaction forces using strain measures in the foot of the rail is given Vlasov's theory for thin-walled beams is used in combination with continuous translational and rotational elastic supports based on smoothing out the stiffness of the rail sleepers The smoothing out of the rotational elastic support has traditionally not been done The use of this model is validated by the decay lengths of the problem and through finite element analysis The finite element analysis is performed using discrete sleeper stiffness and Vlasov beam elements The sensitivity of the measuring technique to parameter variations is illustrated and an example shows the simplicity of the proposed direct measuring technique

Stress distribution in gudgeon pins

Abstract: Frozen stress photoelastic tests were carried out on four shapes of gudgeon pins, loaded in a piston of realistic shape by a realistic connecting rod. The results showed that the greatest stresses occurred in the gaps between the piston and connecting rod at the horizontal position in the bore. Measured compressive hoop stresses and calculated shear stresses there were up to 2 1/2 times the mean shear stress. Thick ring calculations slightly overpredicted the compressive stresses and the tensile hoop stresses at the vertical position, i.e. they are 'safe’. The greatest axial stresses were smaller than the above. Measured pin deflections between the piston bosses were less than the results of simple beam calculations, which ignored the restraints exerted by the piston. Ovalization of the cross-section was largest in the middle.

Orientation and formability of orthotropic sheet metals

Abstract: This paper examines the formability of automotive sheet metals: CR steels and 6000 series aluminium-magnesium alloys. Necking strains are used to determine the forming limits; i.e. a diffuse instability condition is reached under in-plane biaxial stressing. The theory admits material anisotropy, work-hardening and sheet orientation under any ratio of applied principal stresses. It has been programmed to accept orientations between the principal stress axes and the sheets' rolling direction in 15° increments between 0° and 90°. The ratio between the principal stresses may vary between 0 and ± 1. The input data required are the width-thickness strain ratios (r values) in directions 0°, 45° and 90° to the roll and the Hollomon hardening exponent (n value). The output is presented in four diagrams: the critical subtangent-stress ratio and plots between three combinations of the limiting principal engineering strains: (a) two in-plane strains, (b) major in-plane strain versus thickness strain and (c) m...

Stress intensity factors for semi-elliptical surface cracks in semicircularly notched tension plates:

Abstract: Stress intensity factors for semi-elliptical surface cracks located at the centre of a semicircular edge notch in a finite thickness plate subjected to a remote tensile load are presented in a tabulated format. A wide range of geometry ratios are considered. They are all combinations of the following ratios: the ratio of crack surface half-length to plate half-thickness, c/t = 0.2, 0.4, 0.6, 0.8 and 0.95; the ratio of crack depth to surface half-length, a/c = 0.2, 0.4, 0.6, 0.8 and 1; and the ratio of notch radius to plate half-thickness, r/t = 0.5, 1, 2 and 3. Both the quarter-point displacement and J.-integral methods based on three-dimensional finite element analyses were employed for the calculation of stress intensity factors. The calculation accuracy was studied by analysing the J.-integral path independence and comparing stress intensity factor results with other solutions available in the literature.

Benchmarking of a destructive technique to determine residual stresses in thick-walled axisymmetric components

Abstract: A proposed destructive technique to determine residual stresses in thick-walled axisymmetric pressure vessels employs incremental strain gauge data in a finite element algorithm to construct the initial residual stress distribution. The data acquisition component of the proposed technique has similarities with Sachs' bore out method, enabling independently obtained data to be employed by the finite element algorithm. A comparison between the two methods based on the same strain data enables the accuracy of the finite element algorithm to be assessed. Results to date suggest that the new algorithm determines residual stress distributions to an accuracy comparable with Sachs' bore out method. An advantage that the proposed technique has over Sachs' bore out method is that it can be applied to axisymmetric thick-walled pressure vessels of geometries more complex than plain cylinders.

A practical guide to the measurement of the elastic stress intensity factor in engineering materials by the method of caustics

Abstract: The optical method of caustics is an experimental technique which, particularly when used in reflection, has recently found increasing applicability to the fracture mechanics of engineering materials. Having been widely applied to stress intensity factor measurement, it has found a niche in areas where an analytical prediction has been impossible or in need of experimental verification, such as in dynamic fracture. This paper provides an accessible introduction to the method, covering its historical background and theoretical basis, together with a comprehensive guide to, and review of, its practical application to materials such as aluminium and steel.

Effects of the state of stress on the tensile and creep properties of γ-TiAl

Abstract: Tensile stress-strain tests at 20, 450 and 750 ° C and creep experiments at 750 ° C have been performed on plain and circumferentially notched bars of the candidate gas turbine blade material γ-titanium aluminide (γ-TiAl). Two notch shapes were examined to produce different stress states in the notch root region. In all cases notch strengthening was observed. Finite element calculations have also been carried out to determine the stress distributions developed in the notched bars during the tensile and creep tests. The tensile properties were represented by a Ramberg-Osgood relation with a range of strain-hardening exponents, m, and the creep behaviour by a Norton-type creep law with different stress indices, n. It has been found that a skeletal point exists in the notch cross-section where the stress state is independent of the values chosen for m and n. It is shown that the experimental results can be explained in terms of the state of stress developed at the skeletal point of each notch shape.

A procedure for equation fitting of computer-generated design data

Abstract: A procedure is developed to enable the data obtained from an exhaustive computer exercise, using finite elements, to be presented in the form of best fitted equations for use in Design Standards. Although the example chosen is directed towards the analysis of a horizontal hot oil storage vessel, the method developed can be used to obtain a best fitted equation for any set of generated or measured data. The resulting parametric equations can, thereafter, be used manually or, alternatively, programmed for a personal computer.

Variation of the residual stress state in a welded joint during plastic deformation in a 5.0%Cr and 0.5%Mo steel

Abstract: The aim of the present work is to verify the variation of the initial residual compressive stress induced by shot-peening, during plastic deformation under uniaxial tension, in a steel with 5.0%Mo and 0.5%Cr, employing test pieces taken from a welded joint where, due to the welding process, there are regions with different mechanical properties (yield strength and tensile strength). The regions of the joint, the base metal, the heat-affected zone and the weld bead exhibited different behaviour. In the heat-affected zone the residual compressive stresses in the longitudinal direction changed to tensile stresses with 0.5-1 per cent plastic deformation. At 3 per cent the stresses attain a maximum tensile value, decreasing in an oscillating manner up to 6 per cent plastic deformation. Beyond this point the test pieces reached a constant tensile value until the fracture. The transverse residual stress values were also modified during the tensile test but no full stress relaxation was observed. Indeed, the initial compressive stress values changed under small applied plastic deformation and after 4 per cent plastic deformation they changed weakly until the test piece fracture. The weld bead was weakly deformed and no effective stress relaxation was observed. In both the longitudinal and the transverse directions, the residual compressive stresses are brought sharply from an initial value of -470MPa to a minimum of -250MPa for only 0.5 per cent plastic deformation. No further changes were observed until the fracture of the test piece.

Localized metal forming simulation by r-s-adapted arbitrary Lagrangian-Eulerian finite element method

Abstract: In this paper, an adaptive arbitrary Lagrangian—Eulerian (ALE) large deformation finite element method (FEM) is developed for solving metal forming problems with strain localization. The ALE mesh movement is coupled with r-adaptation of automatic node relocation to minimize mesh distortion during the process of deformation. A strain localization phenomenon is incorporated through constitutive relations for porous ductile materials. Prediction of localized deformation is achieved through a multilevel mesh superimposition method, called s-adaptation. A few metal forming problems are simulated to test the effectiveness of this model.

Machining of simulated defects in pipeline steels: A neutron diffraction study of local stress effects:

Abstract: The most commonly used in-line pipeline inspection tools utilize the magnetic flux leakage (MFL) technique, which is sensitive to the stress state of the pipe wall. Calibration of these MFL tools is often carried out using unstressed pipeline sections containing simulated defects, usually produced by mechanical drilling. There is also strong interest in creating simulated defects in stressed pipe walls, thus simulating corrosion in the field. In this study, neutron diffraction is used to map the local stresses surrounding mechanically and electrochemically drilled holes in a steel plate sample. Holes were drilled in unstressed samples, as well as samples held at a constant 80 MPa uniaxial stress. Mechanical drilling into a stressed sample was found to induce considerable residual stresses around the hole. Conversely, electrochemical drilling did not create a residual stress field in the vicinity of the hole.

Deformation behaviour of the cross-section of a heat-sealed area in plastic film used for liquid package bags

Abstract: Displacements on a cross-section of a heat-sealed area in laminated plastic film used for liquid package bags are measured by a digital image correlation. All the specimens used for obtaining the images necessary to measure the displacements are cut into the required shape from heat-sealed package bags. The images are recorded by an 8mm video camera via a microscope, and digitized by an image-processing device. Twenty-six images are used for obtaining the displacements between the no-load condition and failure of the specimens. As a result, a characteristic displacement is found in the heat-sealed area.

Computer aided photoelastic stress analysis of a variable-pitch lead screw

Abstract: Three-dimensional stress-freezing photoelasticity is applied to analyse the stress in a variable-pitch lead screw at contact with two conical meshing elements A PC-based image-processing system and the relevant digital techniques are used to collect the photoelastic data, and the cubic technique is used to evaluate the effective stress at the contact region The model-making and computer aided data collection procedures are described Contour plots of the effective stress at the contact regions are presented It was found that the highest effective stress values on the two contact thread surfaces are approximately the same, although the exerting forces are quite different