Determination of residual stress by use of phase shifting moiré interferometry and hole-drilling method

A phase-shifting moiré interferometry and hole-drilling combined system was developed to determine residual stresses. The relationship between the 2D displacement data of three points around the drilled hole and the residual stresses relieved by hole-drilling was established. The experimental setup consisted of a four-beam moiré interferometer and a computer-controlled hole-drilling system. Two phase shifters controlled by computer were fixed in two of the four optic paths to directly get the displacement data. With special residual stresses calculation software, the phase distributions of the u and v field obtained by moiré interferometry were quickly converted into values of residual stresses. To analyze the accuracy of this experimental system, an aluminum specimen with a blind hole in the center was real-time tensioned in this system. The displacement field obtained by phase shifting moiré interferometry was compared with the finite element method solution. Good agreement was found with respect to each other. As an application, the in-depth residual stresses of a shot-peened aluminum plate were measured by this method, and possible error sources were discussed.     

Monitoring of residual stresses in injection-molded plastics with holographic interferometry

Residual stresses are often trapped in injection-molded plastic parts due to the rapid cooling of the material in this manufacturing process. These stresses are a common source of failure in plastic components in automobiles, appliances and computers and are difficult to measure with conventional residual-stress experimental methods. Real-time holographic interferometry appears to be a viable technique to identify and monitor these stresses in plastic parts. In this investigation, holographic interferometry was used to monitor the relaxation of residual stresses in the plastic-molded actuator arm of a computer hard drive. In the first phase of this study, the relaxation of these residual stresses as a function of temperature was observed. In the second phase, the time to completely relax the residual stresses in the plastic part at an elevated temperature, the annealing temperature, was determined. In the third phase of this investigation, the rate of relaxation of these residual stresses as a function of time at various operating temperatures, was studied. Based on the results of this study, holographic interferometry appears to be a powerful research tool in the study of residual stresses in plastic parts. It also has the potential to be a practical tool for the inspection of manufactured plastic parts for the presence of residual stress.     

Combined implementing the hole-drilling method and reflection hologram interferometry for residual stresses determination in cylindrical shells and tubes

Main features inherent in simplified approach to residual stresses determination in cylindrical shells and tubes, external diameter of which is not less than 60 mm, by combing the hole-drilling method and reflection hologram interferometry are discussed in detail. Initial experimental information in a form of hole diameter increments in principal stress directions is derived from high-quality reflection holograms recorded near cylindrical objects of intermediate curvature value. Converting measured parameters into required stress values is based on the transition model that corresponds to plane stress conditions of pure membrane type. The technique developed is capable of determining residual stress component values within 5% accuracy in an absence of stress gradients over the probe hole diameter when a type of residual stress field corresponds to the transition model adopted. The accuracy analysis involved is based on matrix formulation of conventionally direct problem and an assumption on a pure membrane character of residual stress field under study for thin-walled shell. Required error estimations in a case of inspecting thick-walled cylindrical tube are obtained by combining the above-mentioned approach and an analogy of reconstructed fringe patterns with actual and artificial interferograms, which follow from drilling blind hole of the same geometrical parameters in thick-walled plates. Experimental verification of the developed approach is founded upon a determination of actual stresses in thin-walled cylindrical shell and obtaining residual stress distributions at the proximity of welded joint in thick-walled cylindrical tube.     

Accuracy and sensitivity of a hole drilling and digital speckle pattern interferometry combined technique to measure residual stresses

This paper reports on the accuracy and sensitivity of digital speckle pattern interferometry (DSPI) when it is combined with the hole drilling technique for measuring residual stresses. The in-plane displacement field generated by the introduction of a small hole is determined using an automated data analysis approach. This method is based on the calculation of the optical phase distribution through a phase-shifting method and the application of a robust iterative phase unwrapping algorithm. It is experimentally demonstrated that residual stresses can be measured with a relative uncertainty of 7.5%. It is also shown that the minimum value of residual stress that can be determined with the DSPI and hole drilling combined technique is about 10% of the yield stress of the material.     

Phase determination in holographic Moiré in presence of nonsinusoidal waveforms and random noise

This paper proposes a subspace parameter estimation method which besides allowing for accommodating multiple PZTs in an optical interferometer permits for extracting in real time values of phase shifts between data frames at each pixel point. The technique enables to freely choose values of phase shifts between 0 and π. A generalized phase measurement algorithm allows for computing multiple phase information present in the interferometer. The method facilitates the use of spherical beams, addresses errors arising from the miscalibration of the phase shifting devices, and is capable of handling nonsinusoidal waveforms in an effective manner. Numerical simulations demonstrate that phase distributions can be measured with high accuracy even in the presence of noise.     

基于Talbot-Moiré法测量透镜焦距的莫尔条纹的图像处理

利用Ronchi光栅的Talbot效应和Moiré条纹测量了长焦透镜焦距,并根据焦距与莫尔条纹斜率之间的定量关系可求得透镜焦距.为获得条纹斜率,用CCD采集莫尔条纹图像后,利用数学形态学滤波方法进行图像滤波平滑等预处理,有效地增强了图像的对比度.通过二值化、细化获得单像素宽连通的条纹,通过对条纹进行标记,为条纹斜率的直线拟合计算提供了可靠的数据,采用这些数据计算的焦距的误差为0.10%.     

Measurement of the thermal expansion coefficients of ferroelectric crystals by a moiré interferometer

A moiré interferometer is used to measure the thermal expansion of two ferroelectric crystals, LiNbO₃ and KTiOPO₄. The crystal samples are patterned with a chromium reflective grating and used as a diffractive component in a reflective grating interferometer. The thermal expansion of all the three axes of congruent LiNbO₃ and of x and y axes of the flux-grown KTiOPO₄ were measured from room temperature to 200 °C. For this temperature range the thermal expansion coefficient has been modeled by a second-order polynomial and its coefficients have been estimated by accurate analysis of the resulting moiré fringe pattern.     

On the visualization of heterogeneous plastic strains by Moiré interferometry

Moiré interferometry is a valuable tool for investigations of the mechanics of materials. It is characterized by high-sensitivity and full-field capability. In this paper, the applicability of moiré interferometry and microscopic magnification to the visualization of the heterogeneous nature of the plastic strains in a polycrystalline material is considered. Plastic deformation of a coarse-grained aluminum is considered in detail.     

Whole-field analysis of uniaxial tensile tests by Moiré interferometry

Moiré interferometry—a high sensitivity whole-field optical technique—was used to follow the evolution of the deformation process of an aluminium sample subjected to a uniaxial tensile test. This technique allowed us to measure the two in-plane displacement components undergone by the sample, to evaluate the strain, stress and rotation fields, and to appreciate the trends in the deformation pattern that characterized the different stages of the test. Through a subtraction process between fields obtained at two different load levels, we were able to identify the area where strains began to localize and to observe the appearance of the diffuse neck.     

A full-field method for measuring residual stresses in optical fiber

A method for measuring the axial residual stress profile in axisymmetric optical fibers is presented. The procedure is based on integrated photoelasticity and a fringe shifting technique is used to measure the optical retardation. The radial distribution of the axial residual stress is reconstructed using the inverse Abel transform. The paper describes the operating principle, the experimental setup and the results obtained on a multimode fiber are also reported. The influence of the measurement errors is finally discussed.     

A least-squares method to cancel rigid body displacements in a hole drilling and DSPI system for measuring residual stresses

This paper presents the evaluation of a method to cancel rigid body displacements that can be introduced when a hole drilling and digital speckle pattern interferometry (DSPI) combined system is used to measure residual stresses. The proposed method is based on a least-square calculation of three correction parameters determined from two evaluation lines located near the edge of the phase map where the displacement field generated by the drilling process is supposed to be negligible. The errors introduced by the method for different residual stress levels and rigid body displacements are analysed using a numerical simulation. An application of the method to experimental data is also presented.     

End-wall errors in temperature measurement of external convective heat transfer with Moiré deflectometry

Moiré deflectometry is a robust and simple optical method that allows obtaining the temperature field in flows with uniform pressure and two-dimensional and axisymmetric flows. Since in real configurations it is not possible to keep exactly the hypothesis of two-dimensional flow, it is necessary to asses this influence. Therefore this work studies a procedure to estimate the errors due to the end-wall and 3-D effects when Moiré deflectometry is used for measuring the temperature field of an external convective heat transfer flow with free edges.According to the value of the parameter Gr/Re², results show two tendencies. The error in temperature measurement is smaller than 1% for ratios of thermal boundary layer thickness to the test field width lower than 0.4, in turbulent forced convection flows for temperature differences of 40 K. Temperature effects are significative, multiplying the error by two for a temperature difference 50% greater. These results enable researchers to evaluate the errors of this measurement technique associated with the end-wall effect.     

Determination of residual stresses using hole drilling and digital speckle pattern interferometry with automated data analysis

A digital speckle pattern interferometry and hole drilling combined system is developed to determine the magnitude of the residual stress in a aluminum thin plate subjected to an uniform uniaxial tensile load. Performing automated fringe analysis, the optical data contained in the speckle interferograms are quickly converted into values of residual stress. The evaluation is carried out through the measurement of the in-plane displacement field generated by the introduction of the small hole. The displacement field is determined from the calculation of the optical phase distribution by means of a phase shifting method. The magnitude of the residual stress is finally evaluated through a least-squares calculation and compared with the stress value applied to the specimen.     

Experimental analysis of thermal displacement and strain distributions in a small outline J-leaded electronic package by using wedged-glass phase-shifting moiré interferometry

To evaluate the influence of a printed wiring board (PWB) with a high coefficients of thermal expansion (CTE) on the thermal deformation of a small outline J-leaded electronic package (SOJ), a newly developed phase-shifting method was applied to moiré interferometry. This phase-shifting moiré interferometry method uses a wedged glass plate as a phase shifter to obtain displacement fields with a sensitivity of 100 nm/line. This technique also enabled the quantitative determination of strain distributions in all observation areas. Thermal loading was applied from room temperature (25 °C) to an elevated temperature (100 °C), and then the thermal strains of SOJ with and without the PWB were compared. The results showed that the concentrations of the longitudinal strains ε_xx and ε_yy became increasingly prominent when mounted on the PWB, and the shear strains γ_xy were concentrated at the corners of the silicon chip. The values of these strains increased by about 50% when the SOJ was mounted on the PWB.     

Investigation on cracking behavior of Ni-based coating by laser-induction hybrid cladding

Based on the experiments of laser-induction hybrid cladding by powder feeding, the cracking behavior of Ni-based coating and solidification characteristic in molten pool were investigated. The results indicate that the hybrid cladding is effective to prevent from cracking in Ni-based coating. With the increase of induction energy density, the tensile stress and crack rate decrease obviously. When the induction energy density arrives at 36 J/mm², the free-cracks coating can be achieved. In laser-induction hybrid cladding, the martensite can be eliminated in the heat affected zone and the phase transformation stress is little. Moreover, the molten pool is solidified through two directions such as the coating surface and coating/substrate interface, i.e., firstly the top and bottom in molten pool are solidified, and then the middle in molten pool is solidified. Therefore, in hybrid cladding, the peak value of tensile stress is located in the middle of coating, which is different from that in laser cladding. This distribution status of residual stress is greatly helpful to restrict the cracks of Ni-based coating in laser-induction hybrid cladding.     

用于微观场小应变测量的云纹干涉载波条纹法

在云纹干涉法的基础上，研究了一种新的测量方法——云纹干涉载波条纹法，利用初始载波条纹根据物体变形前后的变化对应变进行分析。实验证明：在结合适当的数字图像处理技术情况下，应用频率为1200lp/mm的试件栅，测量试件（微观场）变形产生的应变时，精度基本控制在±10με的范围内。对该方法的研究表明：其灵敏度可在1με以下。方法可给出U场诸行、V场诸列的平均应变值，使研究微观场小应变及微观场临近区域微小应变的不同成为可能。这一技术为MEMS研究中对相关力学内容的分析提供了技术支持。     

Analysis by speckle interferometry of the dependency of yield stress on residual stress

We used electronic speckle pattern interferometry (ESPI) to measure in situ displacement fields nondestructively and with high resolution (10⁻² μm) by using the interferometry principle and the phase-shift technique. We measured the depth profile of the residual stress in steel pipe manufactured by thermomechanically controlled processing using a quantitative model, which explains the relationship between residual stress and displacement measured by ESPI in chemical etching. We analyzed the variation of yield stresses measured by the indentation technique and the residual stresses at various depths. The relationship between the residual stresses and the yield stresses was consistent with simulated results and can be used for indirect evaluation of the residual stresses from the yield stresses.