Showing papers in "Journal of Strain Analysis for Engineering Design in 2014"

High-temperature fatigue strength of a copper-cobalt-beryllium alloy

Abstract: This article summarizes the results from uniaxial tension stress-controlled fatigue tests performed at 650°C on Cu-Be specimens. Two geometries are considered: hourglass-shaped specimens and plates...

High-temperature strain measurement using active imaging digital image correlation and infrared radiation heating:

Abstract: A technique for non-contact and full-field high-temperature strain measurement of a sample subjected to radiation heating using active imaging digital image correlation is described in this work. A high-performance quartz lamp heater system was designed to reproduce transient thermal environments experienced by hypersonic vehicles. The digital images of the test sample surface at various temperatures are captured using a novel active imaging optical system based on a combination of monochromatic light illumination and bandpass filter imaging. Subsequently, the captured images are processed by a robust reliability-guided displacement tracking algorithm with an automatic reference image updating scheme to extract full-field thermal deformation. With the improvements made in both the imaging system and correlation algorithm, the de-correlation problem of speckle patterns caused by the thermal radiation and surface oxidation of the heated test object are effectively addressed, enabling reliable deformation me...

Closed-form expressions of mode I apparent notch fracture toughness for key-hole notches

Abstract: Two closed-form expressions were developed in this research to estimate the mode I apparent notch fracture toughness of brittle materials weakened by key-hole notches based on the well-established brittle fracture criteria, namely, the point stress and the mean stress. The theoretical models were verified using the experimental results reported in open literature dealing with tensile fracture in rectangular isostatic graphite plates containing central key-hole notches of various notch tip radii. A very good agreement was found to exist between the predictions of both the models and the experimental results. The results showed that while the accuracy of each model was different for various notch tip radii, total accuracies of the two models were almost equal (both about 93%) demonstrating their effectiveness. Using such expressions, it is expected that one can simply estimate the load-carrying capacity of brittle and quasi-brittle components weakened by key-hole notches under mode I loading.

A constitutive model for inelastic behavior of casting materials under thermo-mechanical loading

Abstract: High-temperature components, for example turbochargers, are often subject to complex thermal and mechanical loading paths. Non-uniform temperature distribution and constraints by neighboring components result in complex timely varying stress and strain states during operation. The aim of this paper is to analyze inelastic behavior of a casting material Ni-resist D-5S in a wide stress, strain rate and temperature ranges. The material model including a constitutive equation for the inelastic strain rate tensor and a non-linear kinematic hardening rule is discussed. To calibrate the model, experimental databases from creep and low cycle fatigue tests are generated. They include creep curves for temperatures within the range 600–800 °C and stress levels from 10 to 150 MPa. The low cycle fatigue data collect a family of hysteresis loops for the strain rate of 10−3 1/s, the strain amplitude from 0.4% to 2% and temperature levels within the range 200–800 °C. For the verification of the model, simulations of the ...

A generalized cycle counting criterion for arbitrary multi-axial fatigue loading conditions

Abstract: In this article, a generalized cycle counting criterion applicable for both uniaxial and arbitrary multi-axial loading conditions is presented. This criterion consists of two parts: (1) an effective fatigue damage parameter definition in either stress or strain space and (2) a fatigue cycle definition corresponding to maximum damage possibly attained by the fatigue damage parameter within a given loading history. After discussing its mathematical and mechanics basis, we first provide a rigorous proof that the proposed generalized cycle counting criterion yields exactly the same results as the rainflow counting method under uniaxial variable amplitude loading conditions. Then, its applications in multi-axial fatigue are demonstrated by presenting a series of closed-form solutions of both the fatigue damage parameter and number of cycles for a general harmonic loading history of three independent traction stress components on a crack plane. The validity of the new generalized cycle counting criterion is dem...

Investigation of the effects of thermal gradients present in Gleeble high-temperature tensile tests on the strain state for free cutting steel

Abstract: A common feature of uniaxial high-temperature tension tests, and to a certain extent compression tests, performed using a Gleeble thermomechanical testing system that employs direct resistance heat...

Micro–macro wear–fatigue of modular hip implant taper-lock coupling:

Abstract: A multiscale methodology for fretting wear–fatigue analysis of a prosthetic hip implant taper-lock assembly is presented. On a macroscopic scale, the stem and cap surfaces are smooth. However, on a microscopic scale, surface roughness undulations are employed to accentuate the frictional contact for an enhanced mechanical locking effect. Macroscopic (global) modelling for fretting wear–fatigue prediction of two head/stem material combinations, namely, Co-28Cr-6Mo against direct metal laser sintering Ti-6Al-4V (Ti64) and forged Ti-6Al-4V, is investigated. The significance of fretting wear and fatigue in a hip joint for 10 years of service in a normal-weight male for moderately intense exercise is predicted for both material combinations. A micro-scale fretting wear–fatigue model of the surface undulation submodel is developed to predict wear and fatigue micro-cracking. A key aspect is the development of a multiscale wear–fatigue algorithm using adaptive meshing. Both material combination joints are shown t...

A critical assessment of automatic photoelastic methods for the analysis of edge residual stresses in glass

Abstract: The measurement of residual stresses is of great importance in the glass industry. The analysis of residual stresses in the glass is usually carried out by photoelastic methods since the glass is a photoelastic material. This article considers the determination of membrane residual stresses of glass plates by digital photoelasticity. In particular, it presents a critical assessment concerning the automated methods based on gray-field polariscope, spectral content analysis, phase shifting, RGB photoelasticity, “test fringes” methods and “tint plate” method. These methods can effectively automate manual methods currently specified in some standards.

Analysis of displacement fields from a high-speed impact using shape descriptors

Abstract: A composite bonnet liner subject to a high-velocity (70 m/s), low-energy (<300 J) impact by a 50-mm-diameter projectile has been investigated using computational simulation and by experiment High-

Digital image correlation study of mechanical response of nickel superalloy Hastelloy X under thermal and mechanical cycling: Uniaxial and biaxial stress states

Abstract: The cyclic plastic strain accumulation of a dynamic strain aging Ni-based superalloy, Hastelloy X, is investigated using digital image correlation under both uniaxial and biaxial thermomechanical l...

Integrating experiments and simulations to estimate uncertainty in lattice strain measurements

Abstract: The investigation of lattice strains has proven to be a viable method for probing the crystal level stress state in deforming polycrystalline samples. Building on the recent availability of high-ra...

Characterization of electrolytic tinplate materials via combined finite element and regression models

Abstract: In this article, a new method that combines finite element method with data mining techniques is proposed to obtain the mechanical properties of electrolytic tinplate. Using information provided by...

Assessment of creep constitutive properties from three-point bending creep test with miniaturized specimens:

Abstract: Three-point bending creep tests using miniaturized specimen are proposed to assess creep constitutive properties of materials. Based on Norton’s creep law, the analytical model for the three-point ...

Combining X-ray microtomography and three-dimensional digital volume correlation to track microstructure evolution during sintering of copper powder

Abstract: Optimising the manufacture, and ultimately the mechanical performance, of powder-processed components requires an understanding of how the state of the material evolves during processing and in particular during the final sintering stage. Synchrotron X-ray microtomography has been employed to follow in situ the evolution of particle microstructure during sintering of copper powder. In particular, three-dimensional digital volume correlation of a time-lapse computed tomography image sequence allows accurate quantification of the three-dimensional movements of the particles and thereby local strain for the first time. Strains are quantified both at a coarse scale across the whole powder assembly and at higher magnification for a smaller local region of interest. Unsurprisingly, the rate of shrinkage is observed to decrease with sintering time in accordance with changes in the overall density. Heterogeneities in straining within the body are observed at the several particle level, often associated with aniso...

An experimental study of the contact of a rounded rigid indenter with a soft material block

Abstract: Digital image correlation has been utilized for measuring the strains induced by the contact of a wedge-shaped indenter with a soft material block, which undergoes large deformation. The quality and extent of the experimental results allowed, for the first time, a detailed comparison with theoretical predictions using image decomposition and accounting for measurement uncertainties. Both in-plane strain and out-of-plane displacement calibrations of the digital image correlation system were conducted to establish the minimum measurement uncertainties (31.7 µe for in-plane strain and 0.65 µm for out-of-plane displacements) and to establish confidence in the measurements. It was concluded that the assumptions used in the theory were contravened when the depth of indentation exceeded 90% of the radius of the indenter tip so that the theory was no longer applicable.

In situ microtensile testing and X-ray microtomography-based finite element modelling of open-cell metal foam struts and sandwich panels:

Abstract: Microtensile testing was used to determine the mechanical properties of individual aluminium alloy foam struts Finite element modelling of as-tested struts was carried out using X-ray microtomography scans of the undeformed struts to define the geometry Strut deformation was described by continuum viscoplastic damage constitutive equations calibrated by microtensile test data of the aluminium alloy in its optimally aged condition The as-tested strut finite element model was used to develop a procedure that compensates for the effect of grip slippage inherent in the microtensile testing of metal foam struts, which results in a considerable reduction in observed elastic stiffness compared to the typical value of 70 GPa for aluminium alloys The calibrated constitutive equations were then implemented in finite element models of sandwich panels with the aluminium metal foam as its core material The finite element models were used in simulations of cases of low energy impact to represent tool drop conditio

Volume change in polyetheretherketone under compression loads over wide ranges of strain rate and temperature

Abstract: Although the understanding of volume change is of crucial importance to analyze and model the mechanical behavior of polymeric materials, it is highly understudied in the open literature. This article deals with the assessment of the volume change in polyetheretherketone when submitted to compressive loads. The volume change is measured by the end of mechanical tests before and after unloading. Therefore, the total (before unloading) and residual (after loading) volume changes are measured for strain rates ranging from 10−4 to 3000 s−1 and temperatures ranging from +133 to +433 K, that is, −140 °C to+160 °C. It is demonstrated that the total volume is almost constant in terms of the axial strain independent of strain rate and temperature. The residual volume slightly increases with increasing axial strain. This conclusion is obtained either by direct measurements or by a simplified model developed in this article. However, the volume change is softly affected by the specimen geometry.

Simulation and experiment on rubber components using rebound energy approach with stress softening

Abstract: In the current design of rubber anti-vibration components in industries, important design parameters, that is, load–deflection and fatigue requirement, are referenced only on the loading part of the loading–unloading histories. Hence, the performance of the rubber components may be substantially different and could lead to unexpected effects. There are different energy levels and stress values during loading–unloading at the same load value due to stress softening in rubber-like materials. A new model was introduced into a function of strain energy density accounted for this Mullins effect. A key engineering constant in this model, that is, rebound resilience in terms of ratio of rebound energy over the initial loading energy, was measured and incorporated into the function of strain energy density. Two typical rubber-to-metal components, one of them with contact, designed for engineering applications are selected for the verification with good agreements. The new approach matched the results on load–defl...

Controlling pretension of silicone membranes on micro air vehicle wings

Abstract: This study is concerned with a new method to apply consistent and known pretension to silicone rubber membranes intended for micro air vehicles. Pretension has a marked effect on the static and dynamic responses of membrane wings and controls the overall deflections, as such control and measurement of the membrane pretension are important. The technique developed employs thermal expansion to set the pretension and digital image correlation to assess the resulting spatial strain field. Two different frame geometries were fabricated to evaluate the technique. For open-cell frames, the pretension was not uniform, whereas it was for closed-cell frames. Results show developed full-field stress and strain fields as a function of membrane attachment temperature and frame geometry along with experimental iterations to prove repeatability. The membranes can be stretched to a specific pretension according to the temperature at which it adheres to frames.

Explicit equations for the half-plane sub-surface stress field under a flat rounded contact

Abstract: In this article, the interior half-plane stress field resulting from the contact between a half-plane and a flat rounded punch is obtained in an explicit form. The punch is first subjected to a normal load, N, and later to a tangential load Q = µN, so a global sliding condition is achieved. The equations presented here are obtained assuming that the contacting bodies exhibit isotropic elastic behaviour and have identical mechanical properties and that both bodies can be modelled as half-planes. In addition to the equations describing the interior stress field, the maximum value of the von Mises parameter and the location of this maximum value are obtained as a function of the b/a ratio, a and b being the semi-widths of the contact zone and flat section, respectively. Finally, the direct stress, σtxx(x, 0), due to the tangential load is calculated using the formulae developed here.

Ratcheting response of dented pipes under monotonic and cyclic axial loadings

Abstract: Offshore pipelines are usually subject to hostile environments. Temperature changes and repetitive start-up/shutdown in offshore pipelines generate cycles of axial compression and relaxation. Consequently, the pipes may become subject to loading magnitudes that could cause them to locally buckle and wrinkle, and if subject to repeated loading, they could undergo ratcheting. Pipelines could also be subject to damage in the form of dents. This form of damage would have a profound effect on a pipe’s ratcheting response when subjected to cyclic loading. In this article, the ratcheting response of locally dented pipes that were subject to axially applied cyclic loading has been experimentally investigated. Scaled pipes (tubes) were first laterally indented with a cylindrical indenter and then were subjected to a monotonic axial compressive load that initiated small amplitude wrinkles on them. The pipes were subsequently subjected to repeated axial strain. The experiments were conducted on low carbon steel pipe...

Asymptotic analysis of an adhered complete contact between elastically dissimilar materials

Abstract: A complete contact problem between elastically dissimilar materials is studied using an asymptotic analysis. A quarter plane wedge on a half-plane represents the contact edge geometry. Two eigenvalues are obtained for pairs of contacting materials, and their characteristics are classified on the Dundurs parallelogram. Generalized stress intensity factors, KI and KII, are derived to use a two-term stress equation of dimensionless form with developing a mode separation angle. It is found that the order of stress singularity increases as the wedge becomes more rigid than the half-plane. Slipping characteristics on the contact interface are investigated in detail, especially for the case of KI < 0 < KII that represents a typical adhesive complete contact condition. An example case is given using a finite element model to provide calibration of the stress intensities for a specific material, geometry and load combination.

Measurement of local relative displacements in large structures

Abstract: This article presents a novel measurement technique to measure local relative displacements between parts of large-scale structures. The measured deformations can be of significant importance for fracture analyses in many different types of structures in general and for adhesive connections in particular. The measurement of small local relative displacements in structures subjected to large global deformations is complex and hardly feasible with conventional measurement methods. Therefore, a small displacement measurement system has been devised. The small displacement measurement system is based on stereo photogrammetry and capable of measuring three-dimensional local displacements with a high degree of accuracy. In this article, the technique is used to measure local deformations in the vicinity of the adhesive trailing edge joint of a wind turbine rotor blade. The small displacement measurement system results correspond well with another independent measurement method.

Application of high-speed video extensometry for high-temperature tensile characterization of boron heat-treated steels

Abstract: The increased desire to use advanced, high-strength steels for lightweight automotive structural components requires better understanding of thermo-mechanical behavior and appropriate experimental data for developing constitutive models. Thermo-mechanical studies are particularly important for understanding and optimizing hot-stamping processes which produce both complex and high-strength components. The experimental setup presented herein is capable of characterizing the thermo-mechanical behavior of such steels with strain rates up to approximately 1 s–1 and temperatures as high as 850 °C. The main parts of the apparatus are a high-speed camera, a load frame, and a box furnace. For the determination of strain, a simple image-processing program was developed. The strain was determined in three sections that span the entire gauge length of the specimen. Thus, the onset of localization could be more accurately determined. Stress versus strain data for various strain rates and temperatures are presented.

Determination of the constants of material models at high strain rates and elevated temperatures using shot impact test

Abstract: Material models are widely used in finite element codes for analysis of material deformations particularly at high strain rates and elevated temperatures. The problems such as necking and bulging limit the conventional test techniques to measure the stress–strain curves only up to small strains. This is while in some deformation processes, the strain can be greater than 1. In this study, steel shots of 6 mm in diameter are impacted on specimens at high impact velocities and at elevated temperatures using shot impact test. Strains up to 1.6 and strain rates up to about 4 × 106 s−1 are achieved in this study. The geometry of the crater created by the shot impact on the specimen is used for determination of the constants of Johnson–Cook material model. A combined experimental, numerical and optimization approach is used for determination of the constants. The experimental and numerical crater geometries coincide when the constants of material are chosen correctly. The selection of the constants is performed ...

The effects of geometrical inaccuracies of the experimental set-up on small punch creep test results

Abstract: The small punch creep testing technique is able to provide creep properties from a very small amount of material. However, a universal and robust technique, to convert small punch creep testing results to corresponding uniaxial creep test data, has still not been established. In addition, the experimental output can be affected by several sources of uncertainty, such as friction between the components of the test rig and the specimen, and inaccuracies in the geometry of the experimental set-up and the testing procedures. This article reports the results of three-dimensional elastic/creep finite element analyses of small punch creep testing, taking into account geometrical inaccuracies in the initial punch position and the loading direction. The results of the calculations show that the initial position of the punch and the loading direction can considerably affect the variation in the specimen’s central deflection with time and the final time to failure. The minimum displacement rate was found to decrease...

Fracture mechanics approach to fretting fatigue behaviour of coated aluminium alloy components

Abstract: This article analyses the effect that different coatings have on the fretting fatigue behaviour of aluminium alloy mechanical components. To this end, a set of fretting fatigue tests of coated and uncoated specimens was carried out. The contact pads in the tests had a spherical surface, and a condition of partial slip was always maintained. The materials employed were aluminium alloy 7075-T651; coatings of molybdenum bisulphide, modified with tungsten carbide, MoS2–WC; and a commercial treatment known as Nituff®, which is a hard, anodized coating containing particles of polytetrafluoroethylene. The experimental results are discussed, and an analytical model based on fracture mechanics was applied to estimate the experimental fatigue lives obtained in the tests.

Digital image correlation at high temperatures for fatigue and phase transformation studies

Abstract: In this study, digital image correlation was used for two widely different cases to assess the potential and the limitations of the technique for applications at high temperatures. Specifically, digital image correlation was employed in high-temperature low-cycle fatigue experiments in a nickel-based superalloy and in phase transformation experiments conducted on bainitic steel in order to shed light on the microstructural processes. Depending on the type of experiments, the microstructure was characterized prior to the experiments (fatigue) or after the experiments (phase transformation). In the fatigue experiments, it was found that the features dominating damage evolution were the dendrites resulting from the solidification of the cast material. In the phase transformation experiments, variant selection is active when stresses are superimposed during the phase transformation process, which resulted in the evolution of transformation plasticity strains. Thereby, mainly the bainite variants oriented alon...

A comparison of the effect of riveting and cold expansion on the strain distribution and fatigue performance of fiber metal laminates

Abstract: Fatigue tests carried out on three configurations (unexpanded, cold expanded and riveted) of fiber metal laminate material clearly demonstrated the beneficial effect of riveting compared to cold ex...