Layer Removal Technique Research Articles

Revision and extension of the standard laboratory technique for X-ray diffraction measurement of residual stress gradients

A new procedure is proposed to determine sub-surface residual stress gradients by laboratory X-ray diffraction measurements at different depths using a chemical layer-removal technique. The standard correction algorithm for stress relaxation due to layer removal is improved by including corrections for X-ray absorption, and by the addition of constraints imposed by the mechanical equilibrium conditions. Besides correcting the data,i.e.providing more reliable through-thickness residual stress trends, the proposed procedure also provides an elastically compatible and plausible estimate of the residual stress inside the component, well beyond the measured region. The application of the model is illustrated for a set of Al-alloy components shot-peened at different Almen intensities. Results are compared with those given by `blind hole drilling', which is an independent and partly destructive method.

Residual Strain Measurements in Thermal Spray Cermet Coatings via Neutron Diffraction

The impact and fatigue resistance of overlay coatings is significantly influenced by the residual strain (or stress) field induced during coating deposition, post-treatment, and in-service loading. Optimization of the residual strain field is therefore critical to the life and performance of components. Nondestructive measurement of these strain fields in relatively thin (300-400μm) thermal spray coatings, however, poses a challenge because conventional techniques, such as deep hole drilling, x-ray diffraction, synchrotron diffraction, and changes in beam curvature either make these techniques destructive and/or provides only a very near-surface strain measurement. This particularly complicates the strain analysis in cermet coatings, e.g., WC-Co deposited by the thermal spraying process, where the low penetration depth of x-ray and synchrotron-diffraction ray can only provide a through thickness measurement of stress or strain profile via the destructive layer removal technique. Recent investigations have therefore concentrated on the use of neutron diffraction technique for such analysis, and this paper reports some of the early findings of the comparison of through thickness strain measurements in relatively thin (400μm) as-sprayed and post-treated WC-12wt.%Co coatings via the neutron diffraction technique. Since neutrons are not charged, they do not interact with the electron cloud surrounding the atom (unlike x-ray); hence, diffraction results from the interaction with the atomic nucleus. Neutrons therefore have greater penetration depth in most engineering materials, and therefore provide a nondestructive through thickness strain measurement. Results of strain measurement are discussed with the structure property relationships and contact fatigue performance, and indicate that post-treatment of these coatings results in harmonization of the strain field within the coating, and at the coating substrate interface. This significantly influences the contact fatigue performance by improving both the cohesive and adhesive strength of these coatings.

Effect of Heat Treatment Conditions and Densities on Residual Stresses at Hybrid (FLN2-4405) P/M Steels

The characteristics of residual stresses occurring in PM steel based nickel (FLN2-4405) was investigated. Residual stresses were measured by electrochemical layer removal technique. The values and distributions of residual stresses occurring in PM steel processed under various densities (6.8, 7.05, 7.2 and 7.4 g/cm3) and heat treatment conditions (sintered at 2050 ºF, sintered at 2300 ºF, quenching-tempered, and sinter-hardened) were determined. In most of the experiments, tensile residual stresses were recorded on the surface of the samples. The residual stress distribution on the surface of the PM steels was found to be affected by the heat treatment conditions and density. The maximum values of residual stresses on the surface showed sinter hardened condition and a density of 7.4 g/cm3. The minimum level of recorded tensile residual stresses are150 MPa and its maximum level is 370 MPa.

Determination of residual stresses based on heat treatment conditions and densities on a hybrid (FLN2-4405) powder metallurgy steel using artificial neural network

This paper presents a new approach based on artificial neural networks (ANNs) to determine the residual stresses in PM steel based nickel (FLN2-4405). This study consists of two cases: (i) The experimental analysis: The measurements of residual stresses were carried out by electrochemical layer removal technique. The values and distributions of residual stresses occurring in PM steel processed under various densities (6.8, 7.05, 7.2 and 7.4 g/cm 3) and heat treatment conditions (sintered at 2050 °F, sintered at 2300 °F, quenching-tempered, and sinter-hardened) were determined. In most of the experiments, tensile residual stresses were recorded in surface of samples. The residual stress distribution on the surface of the PM steels is affected by the heat treatment conditions and density. Maximum values of residual stresses on the surface were observed sinter hardened condition and 7.4 g/cm 3 density. (ii) The mathematical modeling analysis: The use of ANN has been proposed to determine the residual stresses based on heat treatment conditions and densities in PM steel using results of experimental analysis. The back propagation learning algorithm with two different variants and logistic sigmoid transfer function were used in the network. In order to train the neural network, limited experimental measurements were used as training and test data. The best fitting training data set was obtained with four and five neurons in the hidden layer, which made it possible to predict residual stress with accuracy at least as good as that of the experimental error, over the whole experimental range. After training, it was found the R 2 values are 0.999244, 0.999025, 0.999664 and 0.999322 for sintered at 2050 °F, sintered at 2300 °F, quenching-tempered, and sinter-hardened, respectively. Similarly, these values for testing data are 0.998354, 0.99706, 0.999607 and 0.999205, respectively. As seen from the results of mathematical modeling, the calculated residual stresses are obviously within acceptable uncertainties.

Uncertainty of residual stresses measurement by layer removal

A model to evaluate the uncertainty in the measurement of the through-thickness residual stress distribution in plates by the layer removal technique is presented. Thin layers were chemically etched from a stripe on rectangular specimens cut from a low carbon cold-rolled steel sheet. Phase shifting laser interferometry was used to measure the ensuing curvature. Polynomials were least-squares adjusted to the curvatures as a function of the etched depth. The polynomials were inserted into an integro-differential equation relating the curvature to the residual stresses, which were assumed to be a function of depth only. A comparison with X-ray diffraction measurement of the surface residual stresses showed good agreement. The uncertainty was found to increase steeply at the surfaces and to depend mainly on the assumed value for the modulus of elasticity, on the curvature fit, and on the depth of etching.

Nickel diffusion in samarium sulfide

The diffusion of nickel atoms in samarium monosulfide (SmS) has been studied for the first time. Using the sequential layer removal technique, it was found that the coefficient of diffusion of the 63Ni radioactive isotope at a temperature of T = 1050°C amounts to D ∼ 1.8 × 10−10 cm2/s in SmS single crystals and to 5.3 × 10−9 and 1.2 × 10−10 cm2/s for the fast and slow diffusion components, respectively, in polycrystalline SmS. The process of nickel diffusion in thin polycrystalline SmS films was studied using an X-ray diffraction technique. The coefficient of diffusion at T = 400°C in thin-film samples according to these data is ∼10−13 cm2/s.

Mechanical Behaviour of Joints in FSW: Residual Stress, Inherent Strain and Heat Input Generated by Friction Stir Welding

For ordinary arc welding processes such as MIG, welding deformation, residual stress and inherent strain are important factors to control the reliability of weldments. Friction stir welding (FSW) is a new welding method, especially for light metals. The distortion of FSW is reportedly small but reasons can not be determined in detail. This paper deals with thermal cycles and inherent strains which create both residual stresses and welding deformations, in order to make clear effective factors that lower apparent strains on FSW than on MIG. Inherent strains were measured by using the layer removal technique. Thermal cycles and maximum temperature rise were measured by type K thermocouples. It is concluded from experimental data that the main cause lowering apparent strain in FSW is the compressive load that makes tensile plastic strain in weldments. The second cause is that the heat input is smaller than in ordinary arc welding processes.

Measurement of residual stress in engineering plastics using the hole-drilling technique

A comparative evaluation has been made of the effectiveness of the layer-removal and hole-drilling techniques for measurement of residual stress in polymeric materials. A quenched sheet of ABS containing equi-biaxial residual stresses was produced. Residual stresses in the polymer sheet determined from the layer-removal technique were found to be both equi-biaxial and repeatable. In contrast, residual stresses determined by hole-drilling were not equi-biaxial and did not balance through the thickness of the specimen. Although hole-drilling is a more flexible technique than layer-removal, the results obtained are not as reliable.

Modelling stress reduction techniques of cold compression and stretching in wrought aluminium alloy products

Heat treatable aluminium alloy aerospace products undergo a rapid quench from the solution heat treatment temperature into water/organic quenchant/spray quenching system during processing. As a result of this rapid quenching operation, residual stresses of yield strength magnitude can develop, leaving the material in an unsuitable condition for further machining operations and for service. Rectilinear, open-die forgings are generally cold compressed after quenching to relieve residual stresses. The effect of friction during cold compression between the forging and the steel compression platens, increasing/decreasing the amount of cold compression plastic deformation, natural ageing prior to cold compression, cold compression direction (LT v L v ST) and applying cold compression in ‘bites’ on residual stress magnitudes is not generally available. Residual stress development during quenching is modelled using the finite element technique and the effect on the final residual stress magnitude of varying these process parameters is evaluated. The effect of stretching to relieve residual stresses and the effect on residual stress magnitudes of sectioning samples of the material is also analysed using the finite element technique. The models of each of the stress relieving techniques are compared both by observing stress magnitudes and by using the FE model to predict the results from a layer removal technique.

Residual stresses and density gradient in injection molded starch/synthetic polymer blends

Residual stress distribution in injection molded starch/synthetic polymer blends was evaluated using the layer removal technique. The synthetic polymers in the blend were either polybutylene succinate (PBS) or polycaprolactone (PCL). The starch content ranged from 0 to 70% by weight in the PBS blend and was held constant at 70% in the PCL blend. The effects of various molding conditions, aging and starch content were investigated. The residual stress profiles were found to be parabolic in nature with surface compressive stresses and interior tensile stresses. Increasing the injection pressure and mold temperature decreased the tensile stresses but had no significant effect on the surface compressive stresses. Decreasing the packing pressure produced a significant decrease in the magnitude of residual stresses. Varying melt temperature and packing time did not significantly affect the residual stress distribution for the range of values investigated. The residual stresses relaxed with time, decreasing over a period of 57 days. The magnitude of residual stresses increased as the starch content in the PBS blends was varied from 0 to 70%. Density gradient measurements were made in a 70% starch/PBS blend. The density was found to be higher in the interior than at the surface with a steep gradient close to the surface. Varying the molding conditions had a complex effect on the average density and the density distribution.

Non-Destructive Method for Estimating Residual Stress Distribution in Component due to Shot Peening.

A non-destructive method is proposed for estimating residual stress distribution in an object part due to shot peening. The method involves measuring the eigen strain distribution in a small test piece made of the same material and peened under the same shot peening conditions as the object part using the layer removal technique. The residual stress distribution in the part is calculated from this eigen strain. An experiment was carried out to estimate axisymmetric residual stresses in a shot peened cylinder. The results measured directly from the cylinder show good agreement with the results predicted by the proposed method.

Residual stress in polymers—evaluation of measurement techniques

A comparative assessment has been made of techniques for evaluating residual stress in polymers. Emphasis was placed on the layer removal and hole-drilling methods. A more speculative approach, the chemical probe technique, using the sensitivity of the threshold stress for environment stress cracking to particular chemicals, has been developed to characterize near-surface stresses. Measurements were made for polycarbonate, filled and unfilled acrylo-nitrile butadiene styrene (ABS), and nylon. The materials had been processed by two distinct routes designed to generate different levels of residual stress. The layer removal technique gave the most consistent results for the stress distribution through the thickness but is time-consuming and limited to flat plates. The hole drilling method is potentially more flexible in determining the residual stress in more complex geometries because the measurement is over a small area only. However, although reasonable agreement with the layer removal technique was found for the same principal axis, the values of residual stress calculated for the other principal axis appeared conceptually unreasonable. The chemical probe technique has potential but uncertainty in the quantification of residual stress can arise if significant environment-enhanced relaxation occurs.

Non-Destructive Method for Estimating Residual Stress Distribution in Component due to Shot Peening.

A Non-destructive method is proposed for estimating residual stress distributions in a component due to shot peening. The method involves measuring the inherent strain distribution in a small plate, made of the same material and peened under the same shot peening conditions as the component, by layer removal technique. The residual stresses in the component are estimated from the measured inherent strain in the plate. An experiment was carried out to estimate axisymmetric residual stresses in a shot peened shaft. The results measured directly from the shaft show a good agreement with the results predicted by the proposed method.

Residual Stresses in Short-Fibre Reinforced Injection-Moulded Thermoplastic Parts

Having the prediction of the warpage and shrinkage behaviour of injection-moulded short-fibre reinforced thermoplastics in mind, a model is developed to calculate the residual stresses. In accordance with the structure and the behaviour of these materials, a layerwise orthotropic, viscoelastic constitutive equation is used. As a cause for the stress development, a process determining the history of cooling under pressure is considered. The volume expansion of the matrix is described using a time-dependent viscoelastic free volume concept. From this, the viscoelastic functions for thermal expansion are derived as well as a time-dependent temperature shift factor. The material law of the composite is developed with the help of micromechanical considerations assuming viscoelastic matrix and elastic fibre behaviour. The residual stresses are calculated for a specially designed testing device which possesses regions of different fibre orientation. Measured fibre orientation tensors are used as simulation input. In the following, the calculated stress profiles were compared with measured values, which were determined using the layer removal technique. The calculated and measured profiles are in good agreement and show a strong influence of the fibre orientation state. The highest occurring stresses are in the same order of magnitude as the long-term strength of these materials.

Residual Stresses in Plastic Pipes by Laser Speckle Technique

Abstract Laser speckle technique has been employed as a sensitive noncontact quantitative tool to measure the residual stress distribution developed in plastic pipes due to manufacturing processes. Pipes are classified into two main categories: thin-walled and thick-walled pipes. In thick-walled pipes, the laser technique has been combined with the layer removal method to describe accurately the stress distribution along the pipe thickness. A merit of the present investigation is the development of an empirical formula to assist the laser measurement in obtaining a more reliable stress distribution. Therefore, the need for the layer removal method has been eliminated, and the usual inaccuracy patterns resulting from excessive machining have been avoided. Since the results obtained are compared favorably with different techniques such as strain gages, ring slitting, and layer removal techniques, the proposed work possesses different advantages with respect to simplicity, practicality, and sensitivity.

An analytical model for predicting residual stresses in progressively deposited coatings Part 3: Further development and applications

Further development and applications of the analytical model, presented in Part 1 and Part 2, are presented in this paper. These involve extension of the analysis for bi-material systems to multi-layered cases and determination of the critical strain energy release rate from a knowledge of the point at which debonding occurs. Mention is also made of the possibility that, since relaxation of stresses parallel to the crack front cannot occur close to the crack tip, the associated strain energy may not contribute fully to the effective strain energy release rate. An outline is given of how the model can be used in conjunction with experimental data on specimen curvature to determine the deposition stress of the coating. Some suggestions are also made about how predicted residual stress distributions could be validated, using X-ray diffraction or layer removal techniques.

Modelled and measured residual stresses in plasma sprayed thermal barrier coatings

Thermal barrier coatings consisting of a NiCrAlY bond coating and a 1.4 mm thick partially stabilised zirconia top coating were air plasma sprayed onto grit blasted nickel-base substrates. Two samples were produced using different amounts of external cooling during spraying of the top coatings. The residual stress profiles in the samples were measured after each manufacturing process step with a layer removal technique. A finite element model including a thermal analysis and a stress-strain analysis of the deposition was developed to model the origin of the thermal stresses and to verify the measured residual stresses. The main components for the residual stresses in the sprayed coatings were identified as stresses developing during the rapid cooling of individual droplets (quenching stresses) and stresses formed during cooling from deposition temperature to room temperature. The quenching stresses were predicted to be low and tensile in the top coating, due to stress relaxation by formation of vertical microcracks. During cooling to room temperature, compressive stresses were superimposed on the top coating quenching stresses. The final residual stresses were predicted to be compressive in the top coating. This was confirmed by measurements (−15 MPa). In the bond coating, no stress relaxation by microcracking was observed and the residual stresses were found to be tensile (~100 MPa). In the substrate, compressive residual stresses reaching −200 MPa were found in a zone to a depth of 0.3, mm into the substrate. The stresses were found to have originated during the grit blasting of the substrates prior to bond coating deposition. A correlation between modelled inelastic strain and measured densities of vertical microcracks in the top coating was obtained. High values were found close to the bond coating, which were correlated to a low substrate temperature during spraying of the top coating material.

Distortion and Residual Stress in Quenched Aluminum Bars

Predictions of distortion and residual stress for rectangular bars, quenched on one surface, are compared with experimental measurements. The heat flow is unidirectional, and the dominant stress component acts along the length of the bars. Thermocouples measure the transient temperature distribution, and displacement transducers monitor the distortion of the bars into circular arcs. The thermal history inferred from an inverse heat transfer calculation drives the mechanical stress and deformation analysis. The numerical analyses demonstrate the sensitivity of the quench distortion to the thermal boundary conditions, the thermophysical properties and the constitutive model. Agreement between the predicted and measured distortion is only qualitative. The predicted residual stress field compares very well with the residual stress inferred from layer removal techniques.

CONFOCAL SCANNING LASER MICROSCOPY MEASUREMENTS OF THE GROWTH AND MORPHOLOGY OF MICROSTRUCTURALLY SHORT FATIGUE CRACKS IN A1 2024‐T351 ALLOY

Abstract— A new technique for studying the growth and morphology of microstructurally short fatigue cracks is described. The technique allows short crack growth, crack depth, and crack front configuration to be measured using Confocal Scanning Laser Microscopy (CSLM). Good agreement is shown between CSLM measurements of the initial stage of crack growth (along an inclined shear plane), crack depth and aspect ratio, together with the measurements obtained using a surface layer removal technique. It is also found that non‐propagating short fatigue cracks have approximately the same aspect ratios in different specimen orientations and that the aspect ratio is independent of stress amplitude.

An Assessment of Thermal Stresses in Injection Moulded ABS Copolymer

Abstract Thermal residual stresses in injection mouldings result from the non-uniform cooling of the melt in the mould. Normally, compressive stresses develop at the surface and tensile ones within the core of the mouldings. The magnitude of these residual stresses was examined for an acrylonitrile-butadienestyrene copolymer (ABS) under various injection moulding conditions. Although the occurrences of thermal stresses are inevitable, the warpage results only when the residual stresses are out of balance. The imbalance of the residual stresses are caused by the non-uniform temperature distribution through the thickness of the mouldings which may be caused by variation in cross section, part geometry and temperature difference between the two surfaces. The magnitudes of thermal residual stresses in injection moulded ABS flat plaques were determined using a layer removal technique. The development of warpage in a 3D component (an L-shaped bracket) was modelled using the residual stresses obtained for the flat plaques and the predictions were compared with the experimental warpage measurements.