Advances in Hole-Drilling Residual Stress Measurements

Residual stress measurements by hole-drilling have developed greatly in both sophistication and scope since the pioneering work of Mathar in the 1930s. Advances have been made in measurement technology to give measured data superior in both quality and quantity, and in analytical capability to give detailed residual stress results from those data. On the technology side, the use of multiple strain gauges, Moiré, Holographic Interferometry and Digital Image Correlation all provide prolific sources of high quality data. In addition, modern analytical techniques using inverse methods provide effective ways of extracting reliable residual stress results from the mass of available data. This paper describes recent advances in both the measurement and analytical areas, and indicates some promising directions for future developments.     

Residual stress measurement for injection molded components

Residual stress induced during manufacturing of injection molded components such as polymethyl methacrylate(PMMA) affects the mechanical and optical properties of these components. These residual stresses can be visualized and quantified by measuring their birefringence. In this paper, a low birefringence polariscope(LBP) is used to measure the whole-field residual stress distribution of these injection molded specimens. Detailed analytical and experimental study is conducted to quantify the residual stress measurement in these materials. A commercial birefringence measurement system was used to validate the results obtained to our measurement system. This study can help in material diagnosis for quality and manufacturing purpose and be useful for understanding of residual stress in imaging or other applications.     

MEASUREMENT OF NON-UNIFORM RESIDUAL STRESSES BY COMBINED MOIRE INTERFEROMETRY AND HOLE-DRILLING METHOD： THEORY, EXPERIMENTAL METHOD AND APPLICATIONS

Hole-drilling method is one of the most convenient methods for engineering residual stress measurement. Combined with moiré interferometry to obtain the relaxed whole-field displacement data, hole-drilling technique can be used to solve non-uniform residual stress problems, both in-depth and in-plane. In this paper, the theory of moiré interferometry and incremental hole-drilling (MIIHD) for non-uniform residual stress measurement is introduced. Three dimensional finite element model is constructed by ABAQUS to obtain the coefficients for the residual stress calculation. An experimental system including real-time measurement, automatic data processing and residual stresses calculation is established. Two applications for non-uniform in-depth residual stress of surface nanocrystalline material and non-uniform in-plane residual stress of friction stir welding are presented. Experimental results show that MIIHD is effective for both non-uniform in-depth and in-plane residual stress measurements. The project supported by the FRAMATOME ANP     

Repeatability of the Contour Method for Residual Stress Measurement

This paper describes the results of a residual stress measurement repeatability study using the contour method. The test specimen is an aluminum bar (cut from plate), with cross sectional dimensions of 50.8 mm?×?76.2 mm (2 in?×?3 in) with a length of 609.6 mm (24 in). There are two bars, one bar with high residual stresses and one bar with low residual stresses. The high residual stress configuration (±150 MPa) is in a quenched and over-aged condition (Al 7050-T74) and the low residual stress configuration (±20 MPa) is stress relieved by stretching (Al 7050-T7451). Five contour measurements were performed on each aluminum bar at the mid-length of successively smaller pieces. Typical contour method procedures are employed with careful clamping of the specimen, wire electric discharge machining (EDM) for the cut, laser surface profiling of the cut faces, surface profile fitting, and linear elastic stress analysis. The measurement results provide repeatability data for the contour method, and the difference in repeatability when measuring high or low magnitude stresses. The results show similar repeatability standard deviation for both samples, being less than 10 MPa over most of the cross section and somewhat larger, around 20 MPa, near the cross section edges. A comparison with published repeatability data for other residual stress measurement techniques (x-ray diffraction, incremental hole drilling, and slitting) shows that the contour method has a level of repeatability that is similar to, or better than, other techniques.     

Application of X-ray diffraction, micromagnetic and hole drilling methods for residual stress determination in a ball bearing steel ring

A basic understanding of distortion problems requires the analysis of a complete manufacturing process including an almost complete overview of residual stress states in the component during each production step. To reduce the measurement time in the future, three measurements methods (X-ray diffraction, micromagnetic and blind hole drilling methods) have been used to analyze residual stress states in machined AISI 52100 ball bearing rings. X-ray diffraction was used as a state-of-the-art method for machining induced residual stresses with pronounced gradients. The ring exhibited a complex residual stress state with high tensile residual stresses at the surface, a strong gradient in depth, and also showed some variation along the outer circumference due to a superimposition of machining induced residual stresses and effects from the clamping device process. Due to this surface state, micromagnetic signals depend on the analyzing frequency. A calibration of the signals was only possible with the X-ray diffraction data. The results of the three different measurement methods correlate reasonably well.     

A review on stress determination and control in metal-based additive manufacturing

Metal additive manufacturing (MAM) is an emerging and disruptive technology that builds three-dimensional (3D) components by adding layer-upon-layer of metallic materials. The complex cyclic thermal history and highly localized energy can produce large temperature gradients, which will, in turn, lead to compressive and tensile stress during the MAM process and eventually result in residual stress. Being an issue of great concern, residual stress, which can cause distortion, delamination, cracking, etc., is considered a key mechanical quantity that affects the manufacturing quality and service performance of MAM parts. In this review paper, the ongoing work in the field of residual stress determination and control for MAM is described with a particular emphasis on the experimental measurement/control methods and numerical models. We also provide insight on what still requires to be achieved and the research opportunities and challenges.     

X射线衍射法测量碳化硅单晶的残余应力

为了准确评估晶体的质量、提高器件的使用性能,本文围绕单晶碳化硅材料残余应力方面开展了相关研究工作.首先通过对原有的多重线性回归分析方法加以改进,推导出适用于求解六方晶系单晶碳化硅试样所处应力状态的相关理论.其次,采用该方法对沿着[1010]取向生长的6H-SiC单晶片进行了残余应力检测,同时选用{214}晶面族作为测量衍射面.最后,探究了来源于不同晶面组数的数据进行计算时对残余应力测量结果的影响.结果显示:采用多重线性回归分析方法可以实现单晶6H-SiC的面内残余应力的测定;当给定无应力晶面间距d;的精确值时,该应力结果的误差高于选用5组以上(hkl)晶面数计算得到的残余应力结果的误差;如果d;未知,则随着参与应力计算的晶面组数的增加,平面残余应力的误差结果会逐渐降低并趋于平稳.这表明实验测定的残余应力结果具有较高的精度.另外,为了保证实验测得的应力结果的可靠性,应该选用六组及以上衍射面数通过多元回归分析方法来求解单晶碳化硅试样所处的残余应力状态.     

X射线衍射法测量碳化硅单晶的残余应力

为了准确评估晶体的质量、提高器件的使用性能,本文围绕单晶碳化硅材料残余应力方面开展了相关研究工作.首先通过对原有的多重线性回归分析方法加以改进,推导出适用于求解六方晶系单晶碳化硅试样所处应力状态的相关理论.其次,采用该方法对沿着[1010]取向生长的6H-SiC单晶片进行了残余应力检测,同时选用{214}晶面族作为测量衍射面.最后,探究了来源于不同晶面组数的数据进行计算时对残余应力测量结果的影响.结果显示:采用多重线性回归分析方法可以实现单晶6H-SiC的面内残余应力的测定;当给定无应力晶面间距d;的精确值时,该应力结果的误差高于选用5组以上(hkl)晶面数计算得到的残余应力结果的误差;如果d;未知,则随着参与应力计算的晶面组数的增加,平面残余应力的误差结果会逐渐降低并趋于平稳.这表明实验测定的残余应力结果具有较高的精度.另外,为了保证实验测得的应力结果的可靠性,应该选用六组及以上衍射面数通过多元回归分析方法来求解单晶碳化硅试样所处的残余应力状态.     

Two-Dimensional Mapping of In-plane Residual Stress with Slitting

This paper describes the use of slitting to form a two-dimensional spatial map of one component of residual stress in the plane of a two-dimensional body. Slitting is a residual stress measurement technique that incrementally cuts a thin slit along a plane across a body, while measuring strain at a remote location as a function of slit depth. Data reduction, based on elastic deformation, provides the residual stress component normal to the plane as a function of position along the slit depth. While a single slitting measurement provides residual stress along a single plane, the new work postulates that multiple measurements on adjacent planes can form a two-dimensional spatial map of residual stress. The paper uses numerical simulations to develop knowledge of two fundamental problems regarding two-dimensional mapping with slitting. The first fundamental problem is to estimate the quality of a slitting measurement, relative to the proximity of a given measurement plane to a free surface, whether that surface is the edge of the original part or a free surface created by a prior measurement. The second fundamental problem is to quantify the effects of a prior slitting measurement on a subsequent measurement, which is affected by the physical separation of the measurement planes. The results of the numerical simulations lead to a recommended measurement design for mapping residual stress. Finally, the numerical work and recommended measurement strategy are validated with physical experiments using thin aluminum slices containing residual stress induced by quenching. The physical experiments show that two-dimensional residual stress mapping with slitting, under good experimental conditions (simple sample geometry and low modulus material), has precision on the order of 10 MPa. Additional validation measurements, performed with x-ray diffraction and ESPI hole drilling, are within 10 to 20 MPa of the results from slitting.     

A method for reconstruction of residual stress fields from measurements made in an incompatible region

A method is introduced by which the complete state of residual stress in an elastic body may be inferred from a limited set of experimental measurements. Two techniques for carrying out this reconstruction using finite element analysis are compared and it is shown that for exact reconstruction of the stress field via this method, the stress field must be measured over all eigenstrain-containing regions of the object. The effects of error and incompleteness in the measured part of the stress field on the subsequent analysis are investigated in a series of numerical experiments using synthetic measurement data based on the NeT TG1 round-robin weld specimen. It is hence shown that accurate residual stress field reconstruction is possible using measurement data of a quality achievable using current experimental techniques.     

Stress Monitoring of Post-processed MEMS Silicon Microbridge Structures Using Raman Spectroscopy

Inherent residual stresses during material deposition can have profound effects on the functionality and reliability of fabricated Micro-Electro-Mechanical Systems (MEMS) devices. Residual stress often causes device failure due to curling, buckling, or fracture. Typically, the material properties of thin films used in surface micromachining are not well controlled during deposition. The residual stress; for example, tends to vary significantly for different deposition methods. Currently, few nondestructive techniques are available to measure residual stress in MEMS devices prior to the final release etch. In this research, micro-Raman spectroscopy is used to measure the residual stresses in polysilicon MEMS microbridge devices. This measurement technique was selected since it is nondestructive, fast, and provides the potential for in-situ stress monitoring. Raman spectroscopy residual stress profiles on unreleased and released MEMS microbridge beams are compared to analytical and FEM models to assess the viability of micro-Raman spectroscopy as an in-situ stress measurement technique. Raman spectroscopy was used during post-processing phosphorus ion implants on unreleased MEMS devices to investigate and monitor residual stress levels at key points during the post-processing sequences. As observed through Raman stress profiles and verified using on-chip test structures, the post-processing implants and accompanying anneals resulted in residual stress relaxation of over 90%.     

喷丸镍基合金材料微区残余应力的切槽法测量研究

本文结合聚焦离子束-电子束(Focused ion beam-electron beam,简称FIB-EB)双束系统和真空镀膜工艺,进行微区散斑的制备工艺研究,并将所发展的微散斑制备工艺应用于喷丸镍基合金材料表面制斑,进而结合切槽法进行残余应力高温释放规律的测量研究。在FIB-EB双束系统下记录切槽前后制斑微区的图像,利用数字图像相关法计算切槽后的位移,结合InglisMuskhelishvili理论公式可计算得到残余应力。文中研究了不同温度及保温时间对残余应力释放的影响规律。结果表明,残余应力随保温时间的增长释放速度逐渐减小,最后残余应力趋于稳定值。同时,温度越高,残余应力释放越彻底,800℃下近乎完全释放。该工艺具有适用性好,效率高等优点,可望在材料微区变形测量中得到进一步应用。     

Chevron notched short bar testing of high toughness steels

Chevron notched short bar tests offer a simple, inexpensive method of fracture toughness measurement for both quality control and analytical purposes. They also provide data for thinner specimens than current techniques allow.This report describes an investigation into the applicability of this technique to medium strength, high toughness steel, which cannot easily be tested using plane strain fracture toughness methods. The results show that while the test procedure provides an accurate ranking of these steels in order of toughness, it systematically overestimates the values obtained from alternative test methods. In addition, current plasticity correction analyses are inadequate.     

Extension,inflation and torsion of a residually stressed circular cylindrical tube

In this paper, we provide a new example of the solution of a finite deformation boundary-value problem for a residually stressed elastic body. Specifically, we analyse the problem of the combined extension, inflation and torsion of a circular cylindrical tube subject to radial and circumferential residual stresses and governed by a residual-stress dependent nonlinear elastic constitutive law. The problem is first of all formulated for a general elastic strain-energy function, and compact expressions in the form of integrals are obtained for the pressure, axial load and torsional moment required to maintain the given deformation. For two specific simple prototype strain-energy functions that include residual stress, the integrals are evaluated to give explicit closed-form expressions for the pressure, axial load and torsional moment. The dependence of these quantities on a measure of the radial strain is illustrated graphically for different values of the parameters (in dimensionless form) involved, in particular the tube thickness, the amount of torsion and the strength of the residual stress. The results for the two strain-energy functions are compared and also compared with results when there is no residual stress.     

The accuracy of residual stress measurement by the hole-drilling method

This paper describes the verification of the accuracy of residual stress measurement by the hole-drilling method. The strain measurement is simulated by the use of the indirect fictitious-boundary integral method. As an example, a finite rectangular plate subjected to initial stress is treated, and a simulated measurement of the residual stress is made using the strain relieved during hole drilling. The accuracy of residual stress measurement is estimated by comparing the simulated measured residual stress with the actual residual stress, i.e., the given initial stress. The results are shown for various distances and angles of strain gages. Also, the influences of the eccentricity of the hole from the center of the strain gages and the effect of a boundary near the hole are examined.     

Superposition and Destructive Residual Stress Measurements

The paper describes destructive measurement of residual stress profiles across two perpendicular planes in a single coupon. Several established destructive measurement techniques can determine the residual stress locked in a coupon of interest, but in general, only a single measurement can be taken without releasing stress and affecting subsequent measurements. However, when the first measurement determines a stress profile across an entire plane, the released stress can be determined, through a supplemental stress analysis incorporating results from the first measurement, and then accounted for in a subsequent measurement. To demonstrate the capability, we describe measurements of residual stress profiles across two perpendicular planes in aluminum compact tension coupons. Residual stress was introduced into the coupons by laser shock peening, and measurements were performed using the slitting method. For each coupon, the measured stress profile on the first plane was used to compute stress released on the second plane. By adding this released stress to measured stress for the second plane, we obtain a stress profile for the second plane in the original configuration. Results of a numerical model that predicts residual stress due to laser shock peening are presented, and agreement between the model and experimental results gives confidence in the superposition method applied.     

Variational eigenstrain analysis of residual stresses in a welded plate

We present the formulation for finding the distribution of eigenstrains, i.e. the sources of residual stress, from a set of measurements of residual elastic strain (e.g. by diffraction), or residual stress, or stress redistribution, or distortion. The variational formulation employed seeks to achieve the best agreement between the model prediction and some measured parameters in the sense of a minimum of a functional given by a sum over the entire set of measurements. The advantage of this approach lies in its flexibility: different sets of measurements and information about different components of the stress-strain state can be incorporated. We demonstrate the power of the technique by analysing experimental data for welds in thin sheet of a nickel superalloy aerospace material. Very good agreement can be achieved between the prediction and the measurement results without the necessity of using iterative solution. In practice, complete characterisation of residual stress states is often very difficult, due to limitations of facility access, measurement time or specimen dimensions. Implications of the new technique for experimental analysis are all the more significant, since it allows the reconstruction of the entire stress state from incomplete sets of data.     

基于DIC方法的残余应力快速测量系统

结合数字图像相关(Digital Image Correlation,DIC)方法与钻孔法,开发了残余应力快速测量系统。该系统可分为两部分:适用于现场测量的便携式机械系统与针对残余应力测量而改进的基于DIC算法的程序。在四点弯曲加载平台上对工件进行载荷释放前后的残余应力测量试验,通过与应变片测量结果进行对比,该残余应力测量系统的精度达到了应变片测量的同等精度。同时,该测量系统解决了传统应变片测量系统对心误差大、操作繁琐、效率低和测量结果稳定性差等问题,具有较高的工程应用价值。     

钻孔法测量残余应力过程中钻孔附加应变

本文叙述了钻孔法测量残余应力过程中的附加应变.研究应力水平对附加应变的影响是在单向应力条件下进行的,结果表明,钻孔条件、材料状态以及残余应力达到一定值时,附加应变为零.     

Measurement of Torsionally Induced Shear Stresses in Shrink-Fit Assemblies

Shear stresses along the shaft/hub interface in shrink-fit components, generated by torsional loads, can drive premature failure through fretting mechanisms. It is difficult to numerically predict these shear stresses, and the associated circumferential slip along the shaft/hub interface, due to uncertainties in frictional behaviour and the presence of steep stress gradients which can cause meshing problems. This paper attempts to provide validation of a numerical modelling methodology, based on finite element analysis, so the procedure may be used with confidence in fitness-for-purpose cases. Very few experimental techniques offer the potential to make measurements of stress and residual stress interior to metallic components. Even fewer techniques provide the possibility of measuring shear stresses. This paper reports the results of neutron diffraction measurements of shear stress and residual shear stress in a bespoke test specimen containing a shrink-fit. One set of measurements was made with a torsional load ‘locked-in’. A second set of measurements was made to determine the residual shear stress when the torsional load had been applied and removed. Overall, measurement results were consistent with numerical models, but the necessity for a small test specimen to allow penetration of the neutron beam to the measurement locations meant the magnitude of shear stresses was at the limits of what could be measured experimentally.