Stress-free Specimens Research Articles

An Instrumented Sharp Indentation Method for Measuring Equibiaxial Residual Stress without Using Stress-Free Specimens

An Instrumented Sharp Indentation Method for Measuring Equibiaxial Residual Stress without Using Stress-Free Specimens

Taking residual stresses into account in low-cycle fatigue design using the adjustable localisation operator method

This paper assesses the ability of the Adjustable Localisation Operator (ALO) method to predict the influence of residual stresses on the fatigue behaviour of notched components. Different initial residual stress states are introduced into notched specimens, which are then tested under cyclic loadings in a fatigue range of 103 to 105 cycles. Experimental comparisons with initially stress-free specimens have shown how tensile residual stresses reduce fatigue life under repeated compressive loading while compressive residual stresses increase fatigue life under repeated tensile loading.Finite Element Analysis (FEA) and ALO method predictions of changes in residual stress are compared to experimental measurements beforehand to assess fatigue life predictions. A modified version of Morrow’s criterion is used to account for the mean stress effect. Results have shown that the ALO method can predict the influence of residual stresses on low-cycle fatigue with the same accuracy as finite element analysis and with a significant reduction in computation time.

Low velocity impact on pre-loaded composite plates: A novel standard-based experimental apparatus

Despite composite materials presenting a viable design solution for structural weight savings, their low resistance to impact damage portraits a potential drawback. This work presents a study on the compressive pre-loading effects on Low Velocity Impact, LVI, for composite laminated plates. Despite the absence of a standard testing procedure accounting for pre-load effects, a test rig was designed specifically for LVI on pre-loaded plates while preserving ASTM D7136M-15 guidelines for the stress-free specimens. Woven fabric composite laminated plates were manufactured using the Resin Transfer Moulding (RTM) process and subjected to impact on pre-loaded conditions. Three pre-load levels were prescribed, representing conditions ranging from low strain levels until post buckling regime. FE numerical models were implemented in ABAQUS FE code, comparing different Continuum Damage Mechanics (CDM) constitutive model formulations with experimental results. Analytical and numerical predictions showed good correlation with experimental results, allowing for further application of the test apparatus and methodology.

An indentation method for evaluation of residual stress: ESTIMATION of stress-free indentation curve using stress-independent indentation parameters

Abstract

Deformation Characterisations for the WEDM Contour Cut Surfaces

The contour method is a destructive technique to measure the residual stresses. The contour method involves cutting a specimen into two halves and provides a full cross-sectional map of the residual stress acting normal to the cut surface. The cutting process is the first and most important step of the method. Any error in this step can adversely affect all the subsequent steps of the method. Wire Electric Discharge Machining (WEDM) is the best choice cutting process for the contour method. Any deviation can cause inaccuracy and uncertainty in the contour stress results. Therefore, the most appropriate cutting conditions must be selected in order to minimise undesirable cutting effects and to obtain the best surface finish. This research covers the design of a test specimen to benchmark the quality of cutting for contour method measurement. The design allows sequential trial cuts on a nominally stress-free specimen. A high yield strength mild steel (EN3B) has been selected for this research on the basis that it is widely used in industrial applications. This research helps to identify the extent, nature and causes of undesirable effects of cutting process and aids to identify the important parameters that can be used to demonstrate the quality of the contour cut to help optimise the cutting process.

Assessment of residual stresses in steels and carbide composites by load and depth sensing indentation with spherical indenter

Last developments in indentation and computer simulation techniques for the evaluation of mechanical properties (hardness, fracture toughness) of materials are popular because the test conduction is simple. There is no need for precise and expensive specimens of specific geometry, standard tools (indenters) and equipment are used; a lot of measurements can be conducted on relatively small testpieces. Last studies have shown the possibility for the evaluation of the materials residual stresses using a combined indentation-simulation testing technique. The present study is an attempt to measure residual stresses arising in a carbide composite (conventional hard- metal and cermet) using this combined technique. Conventional steels are tested for comparison and validation of the testing technique. An improved simulation algorithm for the assessment of the residual stresses in materials (steels, carbide composites) by load and depth sensing indentation with spherical indenter of specified properties is proposed. The experimental validation of simulations is performed. Spherical indentation results are proved by scanning electron imaging and 3D optical microscopy. The results of the simulations are in a good agreement with experi- mental indentation data. The used method does not require imaging of the indentation impressions and testing of the stress-free specimen for the localized measurement of the residual stresses. It can be used as a reliable express tool for the assessment of the residual stresses with some approxima- tions regarding friction peculiarities of the Hertzian contact between the spherical indenter and specimen surfaces.

Swellings due to alkali-silica reaction and delayed ettringite formation: Characterisation of expansion isotropy and effect of moisture conditions

This paper investigates the effect of different conditions on the development of concrete expansions due to alkali-silica reaction (ASR), delayed ettringite formation (DEF) and their combination. The presence of products of the two reactions has been observed during structure diagnosis. The aim of this research is to study the two reactions in concretes with close mix designs but with various types of aggregate and moisture conditions. Measurements performed in the three directions of stress-free specimens showed that DEF expansions could be considered as isotropic for stress-free material. DEF expansions were largely influenced by the storage conditions (immersed in water or in sealed conditions). The volume of storage water modified the kinetics. Under sealed conditions, no expansions were measured for mortar containing non-reactive aggregate, while small positive strains were obtained for mortar containing reactive aggregate. In all cases, new water supply caused fast, large expansions. The different effects of alkali leaching and moisture conditions on DEF and ASR expansions are discussed.

Modeling Shrinkage Response to Tensile Stresses in Wood Drying: I. Shrinkage-Moisture Interaction in Stress-Free Specimens

This article reports on the wood shrinkage during drying in relationship with the temperature and moisture content. All tests were performed perpendicular to the grain on small clear wood specimens of green Western hemlock while drying at 40, 60, and 80°C to 17, 11, and 5% final moisture contents. Overall, wood dimensional changes and moisture loss phenomena were successfully analyzed and interpolated. The shrinkage strain followed a nonlinear pattern with the moisture loss being the driving force and exhibited good correlation with the square value of moisture content in tangential, and linear moisture values could be used to describe shrinkage in radial direction. Both shrinkage intersection points and end of capillary water values increased with temperature; the distinction between the two values could not be made at all times. A nonlinear function containing two regression coefficients (α and β) was found to be a good interpolation of the moisture loss experimental data. Further analyses revealed that β is independent of both target moisture content and temperature, whereas α appears to be influenced by both variables. The correlation between shrinkage and moisture loss rate is intended to be used as a stress prediction tool.

Influence of annealing temperature on the strength of dental glass ceramic materials

Dental glass ceramic components are always loaded by residual stresses because of the manufacturing process. For a reliable mechanical characterization, residual stress-free specimens are needed. Residual stresses can be removed by an annealing process. The hypothesis of this work was, that the annealing temperature must be individually chosen to reliably remove possible residual stresses without changing the characteristics of the glass ceramic material. Knoop-indented specimens made of the glass ceramic materials Duceram Opaker (Degudent, Hanau, Germany, formerly Ducera, Rosbach, FRG), Empress 1 (Ivoclar Vivadent, Schaan, Liechtenstein), In-Ceram Alumina (Vita Zahnfabrik, Bad Säckingen, Germany), Optec OPC (Jeneric Pentron, Wellingford, USA), and Vita Omega Opaker (Vita Zahnfabrik, Bad Säckingen, Germany) were annealed at various temperatures below and above the respective glass transition temperature. The Weibull strength parameters and the lengths of the indentations before and after annealing were determined. A strength increase caused by relaxation of residual stresses was found after annealing at elevated temperatures. A local maximum of increased strength was determined after annealing at a temperature of 100K below the glass transition temperature. After annealing at temperatures above the glass transition temperature, microplasticity was observed. It is recommended to anneal glass ceramic components that are intended to be mechanically characterized at a temperature 100K below the respective glass transition temperature before testing to determine 'true' mechanical values that are not influenced by possible residual stresses.

Residual stress measurement by Hertzian indentation

We have used Hertzian indentation to measure surface stresses on ceramics. Surface stresses in brittle materials have the effect of shifting the minimum load to produce Hertzian fracture. Quantitative use of the method requires knowledge only of the material’s Young’s modulus, Poisson’s ratio and its fracture toughness (or measurements from a stress-free specimen). We have used this method for surface stress measurements on Al 2O 3 and soda lime float glass. Specimens of both types were stressed by bending. Our results are consistent with those obtained by other methods. The Hertzian technique thus provides a promising simple method for measuring surface residual stress in brittle materials.

Determination of Surface Residual Stresses in Brittle Materials by Hertzian Indentation: Theory and Experiment

Hertzian indentation has been used to determine the surface residual stress levels in brittle materials. In this method, a hard sphere is pressed into the surface of the material: at a critical load a preexisting surface‐breaking crack in the neighborhood of the contact will propagate. There is a threshold load below which no such crack, of whatever size, can be propagated. The presence of a residual stress in the surface will lead to a shift in this threshold load. The effects of residual stresses on the minimum load to produce Hertzian fracture are predicted for alumina and glass, assuming that the variation of the residual stress over the length of the crack is small. Two methods of analysis (one approximate, one more general) are presented that enable the residual stress to be calculated from the shift in threshold load; the only further information required is a knowledge of the radius of the sphere, the elastic constants of the sphere and substrate, and also the fracture toughness of the substrate (or use of a stress‐free specimen as a reference). No measurement of any crack length is necessary. Experimental results are presented for the residual stress levels determined in glass strengthened by ion exchange. Indenting balls of a variety of materials with a range of elastic mismatch to the glass substrate were used, so as to evaluate the effects of elastic mismatch and interfacial frictional tractions on the results obtained. The results obtained by Hertzian indentation are consistent with residual stress levels determined by differential surface refractometry. We also present results on alumina specimens with induced surface stresses.

Analytical Study on Fatigue Strength Reduction Factor of Small-Diameter Socket-Welded Pipe Joints

Four-point bending and rotating bending fatigue tests were conducted on socket-welded joints made of carbon, stainless, and Cr-Mo steels for clarification of the effects of diameter, welding pass sequence and post-weld heat treatment (PWHT) on fatigue strength. The results were evaluated quantitatively. Fatigue strength of socket-welded joints was found to strongly depend on weld pass sequences in fillet welds, this being possibly due to large change in residual stress distribution at roots and toes. The effects of residual stress were thus examined quantitatively by comparison of fatigue strength of PWHT stress-free specimens with that of as-welded specimens. By the modified Goodman’s method, the lowest S-N curve corresponding to maximum tensile residual stress and the highest S-N curve corresponding to maximum compression residual stress were obtained for different steels and diameters. Conventional S-N data of socket-welded joints were situated between these two limiting curves. Based on the lowest curve, fatigue strength reduction factors of socket-welded joints were proposed.

Detection of strain during phase transformation of welds by the laser speckle method: Application of laser speckle method for strain measurement in welding (3rd Report)

This paper describes application of the laser speckle method for in situ dynamic strain measurements during welding. The principal objective pursued in the present investigation is to detect the strain during the phase transformation of welds by this method. The specimens used were 4 mm thick 9%Ni steel and SUS304 stainless steel plate. The former undergoes martensitic phase transformation at around T = 325 °C during cooling, whereas the latter does not. The specimen is heated by a gas tungsten arc (TIG arc) moving along its centreline. During the measurements, the laser beam is arranged to irradiate the specimen back surface at the required location at right angles to the plate surface. Then the change in waveform of the light intensity distribution of the speckle pattern formed by interference of the scattered light on a pair of linear image sensors symmetrically arranged about the irradiated spot is continuously recorded. Several types of specimen of different shape were used to compare the effect of restraint on the strain during phase transformation. The laser speckle method can be used for successful detection of the strain during phase transformation. Differences in the strain curves due to the direction, i.e. along or perpendicular to the heating line, are clearly detected. The amount of expansion during transformation varies considerably depending on the heat input, measurement direction and specimen size. In some cases where expansion in one direction is heavily restrained, expansion in the other direction greatly exceeds the value of a corresponding stress-free specimen. Such anisotropic strains due to phase transformation correspond well qualitatively with elastic-plastic FEM predictions and confirm the validity of the laser speckle method.

Alternate method for the analysis of residual strain in the arterial wall

If an artery is cut transversely into rings, and the rings are then cut radially, they spring open into sectors. This phenomenon implies the existence of residual stresses and strains in the arterial wall in the non-loaded state. In the present paper, we propose a new method to calculate the residual strain from the measured wall dimensions and a polar angle of a specimen in the stress-free state, assuming that the wall is homogeneous and incompressible, and that a radially cut, stress-free specimen forms a circular sector. For this analysis, edge angles were measured at the edges of the opened-up specimen. Residual strains were obtained for the descending thoracic aorta, the common carotid artery, and the femoral artery in the rabbit. The results obtained indicated that the magnitude of residual strain was largest in the femoral artery and smallest in the aorta among the three arteries. The opening angle did not depend upon the length of a ring specimen if the ratio of the length to the diameter was ≤ 3.

Influence of tensile stress on cavity growth in nickel under helium irradiation

The influence of tensile stress on cavity behavior in pure nickel under helium irradiation was investigated by in-situ observation using the transmission electron microscope (TEM) in which an ion gun is installed. Specimens were irradiated at 500 °C with 20 keV helium in the TEM. The dose rate was about 1014 He/cm2 s, and the angle between the helium beam and the normal direction of the specimens was about 60 °. The damage rate estimated by the E-DEP-1 code was about 0.6×10−3 dpa/s at its peak position. The main results are as follows: (1) cavity nucleation was accelerated by applying tensile stress, and cavity size in stressed specimens was several times larger than that in stress-free specimens; (2) cavity density in the stressed specimen increased more rapidly than in the stress-free specimen, and then decreased by cavity coalescences; (3) depth of cavity nucleation in the stress-free specimen was about 160 nm, while that in the stressed specimen was about 320 nm; that is, cavities nucleated in deeper regions in the stressed specimen than in the stress-free specimen. This result indicates that helium atoms and vacancies can migrate into the deeper region by applying tensile stress. (4) The experimental results obtained in this study can be explained qualitatively by the mechanism that mobile dislocations drag He-V complexes to the deeper region. This implies that there are similar phenomena in the case of compressive stress.

On the change in the austenite lattice parameter due to the martensitic transformation in an Fe-32Ni alloy

For the purpose of clarifying the mechanism of the often reported reduction in the austenite lattice parameter due to the martensitic transformation, the lattice parameters of an Fe-32Ni alloy prepared by different cooling rates from the austenitization temperature have been measured in the states before and after the martensitic transformation. From the measurements made, the stress levels have also been calculated for both states. In the slowly cooled stress-free specimen no significant change in the lattice parameter was found after the martensitic transformation, which is in agreement with the theoretical prediction, whereas in the quenched specimens, in which quenching stress is present, an appreciable reduction in the parameter was found. The latter is accounted for by stress relaxation due to the martensitic transformation.

The derivation of a simple void-swelling equation for cold-worked 316 austenitic steel

A simple theoretical equation to describe void swelling in 20% CW316 austenitic steel which embodies the important physical processes, including the effects of stress, is derived. The equation contains variables closely related to quantities appearing in the rate theory treatment of void swelling, but which are fitted to experimental data. The proposed equation, along with an appropriate incubation dose, has been calibrated using data for stress-free specimens for which electron microscope observations of the irradiation microstructure have been made. The equation gives an excellent fit to these data and to other stress-free specimen data when only the void densities are adjusted to the observed values. Comparisons with data obtained from electron microscope observations of the irradiation microstructure in similar material used as fuel pin cladding show that at temperatures well below the peak swelling temperature the simple scaling of the void densities provides excellent agreement between calculated and observed swellings. At and above the peak swelling temperature, for the data examined, we show that a consistent explanation of the observed swelling can be provided by invoking the presence of stress.

Cyclic oxidation and crack growth during high strain fatigue of low alloy steel

This paper attempts to assess the contribution of oxidation to crack growth during high strain fatigue of a Cr-Mo-V steel at 550°C from three separate oxidation studies. Stress-free data are clearly inapplicable to the highly strained crack tip and so it is first shown that internal stresses, as in bainitic material, promote enhanced oxidation. Weight gain experiments on oxide-free fracture surfaces are next described. It is shown that the stored energy of fatigue fracture likewise causes an increase in oxidation rate but that it is difficult to simulate the process at the tip of an advancing crack. Similarly, metallography of oxide in fatigue cracks does not reveal propagation. history. Finally, a dynamically worked surface, provided. by high-strain fatigue specimens deforming at 550°C, causes increasing oxidation with increasing total strain above a threshold value of oxide strain. It is shown that this is due to layering producing at least a tenfold increase compared with stress-free specimens. Therefore the cyclic surface growth data are used together with observed crack propagation rates in air and vacuum to estimate the oxidation rate in the highly-strained crack tip.

Investigation of the interdependence of stress corrosion of austenitic chromium-nickel steels and their susceptibility to intergranular corrosion

The susceptibility of chromium-nickel austenitic steels to intergranular corrosion was studied on stressed and stress-free specimens tested in accordance with GOST [Soviet standard] 6032-58 and in superheated water at high pressures. It was concluded that steels susceptible to intergranular corrosion should not be used in contact with boiler water and chloride solutions.