Residual-stress distributions produced by strain-gage surface preparation

Abrasion of a metallic surface to improve bonding during strain-gage installation is generally thought to produce negligible effect on the measurement of residual stresses by blind hole drilling. However, residual stresses induced by surface abrasion may affect residual-stress measurements in shallow subsurface layers of residual-stress fields produced by processes such as grinding and shot peening.The residual-stress and cold-work distributions produced by four methods of abrasive surface preparation and etching were studied by X-ray diffraction in fully annealed AISI 1018 steel. The surface residual stresses produced by abrasion ranged from tension to compression with magnitudes as high as 80 percent of the yield strength. Cold work was induced to depths of 20 to 60 m. Etching produced low magnitude surface stresses and negligible cold work.Paper was presented at the 1986 SEM Spring Conference on Experimental Mechanics held in New Orleans, LA on June 8–13.     

A Comparison of Residual-stress Measurements Using Blind-hole, Abrasive-jet and Trepan-ring Methods

This article describes the results of residual-stress measurements which were made on an artillery-projectile metal part to determine whether the state of the stress could be a factor in promoting a failure that had occurred during ballistic test firing. An additional objective of the work was to evaluate the suitability of several different methods for measuring residual stresses by the performance of these measurements on the same metal part.     

Residual stresses in reverse-bend test samples for stress-corrosion testing

Cracking of Inconel Alloy 600 (registered trademark) u-bend tubes used in pressure-water-reactor (PWR) steam generators has been a major concern in the nuclear-power industry over the past several years. The mechanism of cracking has been determined to be intergranular stress-corrosion cracking with residual stresses a major contributor. A simple specimen known as a reverse u-bend (RUB) has been used by a number of laboratories to simulate the high stresses and plastic strain extant in the most susceptible regions of the u-bend tubes. This paper presents the results of residual-stress measurements on four RUB samples, each from a different laboratory.The results indicate that the individual RUB fabrication procedures used by different laboratories tend to produce different residual-stress patterns in the highly strained regions over 700 tensile to nearly 700-MPa compressive on different samples. Stress gradients on the order of 140 MPa/mm were found on some samples. The residual-stress patterns were seen to qualitatively predict the stress-corrosion-cracking pattern experienced on similar samples.Electric Power Research Institute.     

Intermethod Comparison and Evaluation of Measured Near Surface Residual Stress in Milled Aluminum

Background

While near surface residual stress (NSRS) from milling is a driver for distortion in aluminum parts there are few studies that directly compare available techniques for NSRS measurement.

Objective

We report application and assessment of four different techniques for evaluating residual stress versus depth in milled aluminum parts.

Methods

The four techniques are: hole-drilling, slotting, cos(α) x-ray diffraction (XRD), and sin²(ψ) XRD, all including incremental material removal to produce a stress versus depth profile. The milled aluminum parts are cut from stress-relieved plate, AA7050-T7451, with a range of table and tool speeds used to mill a large flat surface in several samples. NSRS measurements are made at specified locations on each sample.

Results

Resulting data show that NSRS from three techniques are in general agreement: hole-drilling, slotting, and sin²(ψ) XRD. At shallow depths (<?0.03 mm), sin²(ψ) XRD data have the best repeatability (<?15 MPa), but at larger depths (>?0.04 mm) hole-drilling and slotting have the best repeatability (<?10 MPa). NSRS data from cos(α) XRD differ from data provided by other techniques and the data are less repeatable. NSRS data for different milling parameters show that the depth of NSRS increases with feed per tooth and is unaffected by cutting speed.

Conclusion

Hole-drilling, slotting, and sin²(ψ) XRD provided comparable results when assessing milling-induced near surface residual stress in aluminum. Combining a simple distortion test, comprising removal of a 1 mm thick wafer at the milled surface, with a companion stress analysis showed that NSRS data from hole-drilling are most consistent with milling-induced distortion.

     

Measuring Multiple Residual-Stress Components using the Contour Method and Multiple Cuts

The conventional contour method determines one component of residual stress over the cross section of a part. The part is cut into two, the contour (topographic shape) of the exposed surface is measured, and Bueckner’s superposition principle is analytically applied to calculate stresses. In this paper, the contour method is extended to the measurement of multiple residual-stress components by making multiple cuts with subsequent applications of superposition. The theory and limitations are described. The theory is experimentally tested on a 316L stainless steel disk with residual stresses induced by plastically indenting the central portion of the disk. The multiple-cut contour method results agree very well with independent measurements using neutron diffraction and with a computational, finite-element model of the indentation process.     

X-ray diffraction study of residual macrostresses in shot-peened and fatiqued 4130 steel

A study has been made of the effects of shot peening and fatigue cycling on the residual macrostresses determined by X-ray methods in an austenitized and tempered AISI 4130 steel (150–170 ksi). The results show that the effect of shot peening is to produce a residual compressive macrostress layer 0.014-in. deep. The residual-stress profile (stress vs. depth) exhibits a small negative stress gradient at and near the surface and a large positive stress gradient in the interior. Stress relaxation (due to fatique cycling) which occurred early in the fatigue history of the specimen was found greater at the surface than in the subsurface layers. Stress gradients of the stress profile increased with continued cycling and varied with depth. A correlation appears to exist between stress relaxation and stress gradients at the surface.     

Residual Stresses in a Shaft after Weld Repair and Subsequent Stress Relief

To evaluate the possibility of repairing and/or modifying shaft surfaces by welding, remachining, and low-temperature stress relieving without introducing high residual stresses, residual-stress measurements were made on an 8.0 in. diameter shaft of Type 347 stainless steel. The measurements were made on the shaft surface before welding, after welding and remachining, and after a 750°F stress relief. This report shows the residual-stress patterns in and near the weld area at the various stages of the investigation. Measurements were made using the hole-drilling method. The holes were 0.060 in. diameter and 0.06 in. deep. They were made using a unique, stress free abrasive jet machining technique.*     

Time-dependent motion of a two-dimensional floating elastic plate

Three methods are presented to determine the motion of a two-dimensional finite elastic plate floating on the water surface, which is released from rest and allowed to evolve freely. The first method is based on a generalized eigenfunction expansion and it is valid for all water depths. The second method is based on an integral equation derived from the Fourier transform, and it is valid for all water depths, although computations are made only for water of infinite depth. These two methods are both based on the frequency-domain solution—however no obvious connection exists between the two methods. The third method is valid only for shallow water, and it expresses the solution as the sum over decaying modes. We present a new derivation of the integral equation for a floating plate based on the Fourier transform of the equations of motion in the time domain. The solution obtained by each method is compared in the appropriate regime, and excellent agreement is found, thereby providing benchmark solutions. We also investigate the regime of validity of the infinite and shallow-depth solutions, and show that both give good results for a quite wide range of depths.     

A pilot study on the strength of 5Ni−Cr−Mo−V steel columns

These tests were part of an over-all study of the residual stresses in, and the column strength of, welded built-up and rolled heat-treated A514 steel columns. the present investigation on 5Ni?Cr?Mo?V steel extended the previous work on A514 steel columns. The test program consisted of one 10WF112 column tested in the “flat-end” condition, a stub-column test, seven standard tension-coupon tests, ten compression-coupon tests, and two sets of residual-stress measurements at two separate positions along the column. Results show that the stress-strain relationship of this high-toughness alloy steel can be well represented by the Ramberg-Osgood equation, and the column strength can be predicted by using the tangent-modulus concept.     

Measurement of residual-stress distribution by the incremental hole-drilling method

The hole-drilling method is widely used to measure residual stresses in mechanical components. Recent developments have shown that strains measured on the surface during an incremental drilling can be related to residual-stress distribution. Researchers throughout the world have proposed different calibration methods which lead to more or less accurate results.The present paper discusses different approaches used. A new calibration method is proposed. We also show how finite-element analysis can be used to determine the correlation coefficients. The variation of the strains measured on the surface for each increment is due to, first, the residual stresses in the layer and, second, the change of the hole geometry. Most authors do not consider the latter aspect. Our results show that this causes a significant error in the experimental data. The finite-element method has been used to compute the coefficients for all types of strain-gage rosettes when the hole diameter is predetermined.Another problem of the hole-drilling method is the selection of the drilling tool. Two systems have been studied: ultra-high-speed air turbine and conventional milling machine. The method has been applied on both shot-peened and water-quenched test specimens. The results are successfully compared with the bending-deflection and the X-ray method.     

Nondestructive residual-stress measurement on the inside surface of stainless-steel pipe weldments

The instrumentation, technique, and procedures are described for the nondestructive measurement of residual stresses on the inside surface of pipe as small as 10 in. in diam. The instrument is based upon a unique position-sensitive scintillation X-ray detector which provides for the most compact X-ray stress-measurement instrument available since the introduction of film cameras four decades ago. This instrument is capable of applying the single-exposure technique of X-ray stress measurements which results in unprecedented rapidity of stress measurement consistent with excellent precision and accuracy. The results of testing the precision and accuracy of the instrument on a zero-stress powder and four-point-bend specimen are given. Residual stresses in four austenitic stainless-steel girth-welded pipes are presented illustrating the effects of the different welding procedures. The results from the pipes confirm the beneficial residual-stress condition of heat-sink-welding procedures.     

Residual stresses in turned AISI 4340 steel

The residual-stress distribution in the surface region of workpieces of annealed AISI 4340 steel that is turned under unlubricated conditions is determined using a deflection etching technique. The absolute value of the residual stresses at the machined surface are low and increase with an increase in depth beneath the machined surface to a maximum. They then decrease with a further increase in depth eventually becoming vanishingly small. Peak residual stresses are tensile at cutting speeds of 0.5 and 1.0 ms⁻¹ and are compressive at a cutting speed of 1.5 ms⁻¹ for all feed rates and depths of cut. Peak residual stresses and depth of the stressed region increase with an increase in feed rate and depth of cut, but decrease with an increase in cutting speed. The results of this investigation can be interpreted in terms of the variation of tool forces with cutting conditions.     

Deformable spherical devices to measure stresses within field soils

The design, calibration and use of two deformable spherical stress transducers are described. They are suitable for detecting principal stresses in deforming media such as soil and have major advantages over many rigid transducers previously used in such situations. One of the transducers is a water-filled rubber ball (WFRB) sensitive to hydrostatic and deviator stresses, the other is a mastic ball which deforms plastically and is sensitive to only deviator stresses. When the two devices are used at similar depths under a surface load, e.g. a wheel, the combined measurements of internal pressure of the WFRB and axial deformation of the mastic ball can be used to derive values for first and third principal stresses (assuming second and third principal stresses are equal). Calibration of the transducers at different temperatures is described.

Field measurements made with the transducer under loaded wheels are compared with predicted values of first principal stresses using equations developed by Söhne. Close correspondence between predicted and measured values was observed, when the existing soil strength conditions were taken into account.

The transducers promise to be useful in the measurement of stresses in field soils.     

On residual-stress measurements in light truck wheels using the hole-drilling method

An investigation of the effect of drilling speed, milling-cutter wear, drilling mode, and applied drilling force on residual-stress measurements in a light truck wheel using a milling guide manufactured by Measurements Group, Inc. is described. The milling variables chosen were used to minimize the residual stresses induced by the introduction of a hole into the wheel material. An improved hole-drilling procedure was developed and found to be successful in the residual-stress measurements for a light truck wheel.     

Comparison Between Surface and Near-Surface Residual Stress Measurement Techniques Using a Standard Four-Point-Bend Specimen

Background

Determination of near-surface residual stresses is challenging for the available measurement techniques due to their limitations. These are often either beyond reach or associated with significant uncertainties.

Objective

This study describes a critical comparison between three methods of surface and near-surface residual stress measurements, including x-ray diffraction (XRD) and two incremental central hole-drilling techniques one based on strain-gauge rosette and the other based on electronic speckle pattern interferometry (ESPI).

Methods

These measurements were performed on standard four-point-bend beams of steel loaded to known nominal stresses, according to the ASTM standard. These were to evaluate the sensitivity of different techniques to the variation in the nominal stress, and their associated uncertainties.

Results

The XRD data showed very good correlations with the surface nominal stress, and with superb repeatability and small uncertainties. The results of the ESPI based hole-drilling technique were also in a good agreement with the XRD data and the expected nominal stress. However, those obtained by the strain gauge rosette based hole-drilling technique were not matching well with the data obtained by the other techniques nor with the nominal stress. This was found to be due to the generation of extensive compressive residual stress during surface preparation for strain gauge installation.

Conclusion

The ESPI method is proven to be the most suitable hole-drilling technique for measuring near-surface residual stresses within distances close to the surface that are beyond the penetration depth of x-ray and below the resolution of the strain gauge rosette based hole-drilling method.

     

Surface acoustic wave measurements of small fatigue cracks initiated from a surface cavity

A model for the low frequency scattering of a surface acoustic wave by a surface cylindrical cavity with two corner cracks is presented. It is applied to determine the depth of the small fatigue cracks initiated from a pit-type surface flaw. The general scattering formalism based on the elastodynamic reciprocity principle is employed. The effect of the cylindrical cavity on the surface wave reflection from cracks is considered using an approximate stress intensity factor for the corner cracks. In situ surface acoustic wave measurements have been performed during fatigue tests for an Al 2024-T3 sample. The surface wave signal was acquired continuously at different cyclic load levels. The model is verified by comparing calculated reflection signals and spectra with those from experiments. The depths of fully and partially open cracks are determined from the predicted and the measured surface wave reflections. The surface wave reflection is observed to be sensitive to crack closure.     

Residual stresses in thick electrodeposits of a nickel-cobalt alloy

Some electroplated metals contain residual stresses which can cause warpage or premature failure of parts plated or electrofomed with these materials. Noticeably absent from the literature are residual-stress data for finished parts. Typically for plated or electroformed parts, residual stresses are determined independently on thin strips and then piece parts are plated. This research describes a technique which can be used to measure stress on finished parts. The method involves drilling a hole in the part and measuring the resulting change of strain in the vicinity of the hole. Viability of this technique was demonstrated by measuring the stress in a nickel-cobalt deposit plated on an aluminum cylinder. Two separate runs, one 50 deg removed from the other, provided almost identical results; stress was 160 MN/m² (23,200 psi). Two other runs in a region where plating was somewhat thinner provided slightly lower results probably because all boundary-condition requirements were not met. The computed residual-stress values compared quite favorably with independent rigid-strip measurements of 131 MN/m² (19,000 psi) obtained for the solution before and after plating of the cylinder.     

Moiré interferometry determination of residual stresses in the presence of gradients

The full-field technique of high-sensitivity moiré interferometry in conjunction with a multiple-hole-drilling procedure is applied to residual-stress measurements in the presence of gradients. The method arrives at residual-stress estimates starting from in-plane displacement components. Successful applications of the method to problems simulating the nonuniform transverse residual stresses of welded joints are reported.Paper was presented at the 1990 SEM Spring Conference on Experimental Mechanics held in Albuquerque, NM on June 3–6.