Ring-core Method Research Articles

Uncertainty quantification of residual stress evaluation by the FIB–DIC ring-core method due to elastic anisotropy effects

• Compact matrix formulation is presented for residual stress calculation from residual strains using Voigt's 6-vector notation. • By performing the calculation for 8000 randomly selected crystal orientations, Uncertainty Quantification is obtained of residual stress evaluation for cubic materials with different anisotropy factors. Elastic anisotropy can have a significant effect on the reliability and precision of residual stress evaluation, due to the uncertainty in the elastic constants multiplied by the measured strains. For the focused ion beam – digital image correlation (FIB–DIC) ring-core method taken as an example, a Mathematica package was developed to evaluate the complete in-plane residual stress state from the measured strain relief values using known material orientation and anisotropic elastic properties for materials displaying cubic symmetry. However, in many practical situations the underlying material orientation is unknown, and nominal isotropic continuum elastic constants are used. This leads to a systematic error in the stress calculation. The present analysis focuses on the statistical evaluation of the uncertainty in stress evaluation due to the unknown material orientation as a function of its degree of anisotropy. We demonstrate an experimental application of this procedure to a real case of micron scale residual stress analysis in a nickel-base superalloy.

Verification of the Geometric Parameters of the Ring-Core Method

The Ring-Core method is a semi-destructive method for residual stress determination inside materials. The evaluation of residual stresses using Ring-Core method requires complex analysis of the geometric parameters. This work deals with the uniformly distributed residual stress components through the material thickness.

Program Tools for Residual Stress Evaluation by Ring-Core Method

Residual stress determination requirements of production and research users grow rapidly. Commercially available programs enable relatively quick residual stress evaluation with certain level of accuracy and with limited user access to used compliance matrices and calculations. However precise analyses require sensitive approach to compliance matrices determined for individual case e.g. for specific specimen dimensions. Therefore program tools for complex residual stress evaluation by Ring-Core method were developed using finite element analysis and Visual Basic scripts.

Residual micro-stress distributions in heat-pressed ceramic on zirconia and porcelain-fused to metal systems: Analysis by FIB–DIC ring-core method and correlation with fracture toughness

ObjectivesThe production of fixed partial dentures (FPDs) induces complex residual stress profiles, due to both the thermal expansion coefficient mismatch between the veneering ceramic and the framework and to the thermal gradients occurring during the final cooling. Detailed knowledge of residual stress distributions in the veneering ceramics is important to understand the interface phenomena with the framework and the consequences of the different firing systems. The first objective of this study was to analyse the residual stress distribution in heat-pressed ceramic on zirconia core with micrometer spatial resolution, with also a focus on the stress at the interface versus porcelain-fused-to-metal samples. The second purpose was to correlate the residual stress with the fracture toughness. MethodsThe micron-scale focused ion beam (FIB) ring-core method was used to map the residual stress over the cross-sections of the veneering ceramics. The methodology is based on FIB micro-milling of annular trenches, combined with high-resolution in situ scanning electron microscope (SEM) imaging, a full field strain analysis by digital image correlation (DIC) and numerical models for residual stress calculation. Fracture toughness was evaluated by using high load Vickers indentation and hardness/modulus were measured by nanoindentation testing also across the interfaces. ResultsBoth prosthetic systems showed a compressive stress at the ceramic surface on a micron-scale. The stress profile for porcelain fused to metal (PFM) showed a transition to tensile stress at the half of the layer, whilst the stress in proximity of the interface was more compressive in both the cases. Residual stress on a micron scale are higher in magnitude than the corresponding macro-scale values reported in the literature, due to the stress relaxation given, at larger scales, by micro-voids and cracks. The stress field was directly correlated with the indentation fracture toughness, which was higher in those areas where the compressive stress is greater. Stress analysis in correspondence of interfacial porosity for the zirconia sample also showed that micro-defects could induce local modifications of the residual stress field, which may even locally generate a tensile stress state. SignificanceThe interfacial stress in dental systems was analysed on a micron scale and can give further insights into the process/property/performance correlation for this class of materials. In particular, interfacial and/or local modifications of the residual stress are expected to have a significant influence on crack nucleation mechanism in correspondence of micro-defects. A direct correlation between residual stress distribution and fracture toughness was proposed. It is noteworthy that the method can be used to study real crowns and bridges. In fact, complex geometries can be easily analysed by this procedure.

基于云纹干涉技术的环芯和切槽残余应力测量方法的实验研究

In this study, we combined moire interferometry with ring-core and slitting to measure the residual stress in the interference assembled specimens. For ring-core method coupled with moire interferometry, the moire patterns of the tested region were obtained through observing the specimen in different magnifications. And a rectification method was proposed for slitting method coupled with moire interferometry, using which more accurate measurement results can be obtained. Moreover, residual stress of each specimen was measured using a strain rosette, whose result was taken as a reference value. Through comparisons, we found that residual stresses in the specimens measured by moire interferometry-based methods agreed well with their reference values, which validated the feasibilities of these two approaches. Besides, when coupled with moire interferometry, ring-core method was found of higher accuracy compared with slitting method.

Analysis of Differential Method Used for the Evaluation of Uniform Residual Stresses by the Ring-Core Method

Despite the significant advantages of the Ring-Core method over the hole-drilling method, the Ring-Core method is still less common. Recent developments at our department showed the full potential of the method when using the adequate calibration coefficients. In this paper the differential method, a quick and easy method for uniform residual stress evaluation is described and the influence of its relaxation coefficients on the specimen’s dimensions is obtained.

Finite Element Analysis of Calibration Coefficients for Residual Stress Measurements by the Ring Core Procedure

AbstractFinite element analysis (FEA) and experiments have been conducted to analyze the calibration coefficient for measuring residual stress in the ring core method. In this publication, firstly, the calibration coefficients were determined using FEA. Then, factors which affect the calibration coefficients were investigated, such as the tensile load applied to the components, the geometric sizes of the annular grove and the location of the strain gage. The results of FEA were consistent with the experimental results. The most appropriate geometric sizes of the annular groove and location of the strain gage were obtained. This study also defines the distortion energy parameter S to calculate and modify the calibration coefficients. Hence, it is reasonable to apply FEA to calibration coefficients in the ring core method.

Comparison of Different Simulation Approaches in Ring-Core Method

Ring-Core, a semi-destructive method enables measurement of the residual stresses inside the material by the sensor attached at the top of the specimen. Such stress reconstruction requires proper determination of the calculation coefficients. Thus the simulation model for finite elements analyzes is created by means of two different commercial software. Subsequently both simulation results are compared experimentally by measurement on the specimen loaded by known uniaxial state of stress.

Four-Point Bending Tests to Study the So-Called Plasticity Effect on the Residual Stress Results Determined by the Hole-Drilling and Ring-Core Methods

The hole-drilling method (HDM) and ring core method (RCM) are limited to low residual stresses under 60% of the yield stress. This issue will experimentally analyze the method of adaptive calculation function, presented by the author on the ICRS9. With a four-point-bending test machine, a defined stress can be triggered between the middle bending. In this defined loading area, the strains in two load cases with the HDM and the RCM configuration were measured. With these measured strains the residual stress calculation will be analysed.

Integral Method Coefficients and Regularization Procedure for the Ring-Core Residual Stress Measurement Technique

The ring-core method allows the determination of residual stresses at high depth from the surface. The numerical calculation integral method, commonly used for the hole-drilling, can also be applied to the ring-core. The integral method coefficients were obtained for several depth steps after axial-symmetric FE simulations with harmonic load. These coefficients were then validated with a 3D finite element model. Finally, an application was reported, showing the performance of the Tikhonov regularization on experimental data.

A Measuring and Evaluation Program for the Application of the Ring-Core or Hole-Drilling Method

There is a strong need for reliable residual stress measurements. On the one hand, residual stresses can be beneficial, when they are adapted to external loads. On the other hand, they can be detrimental, when they are unknown. Thus, their occurrence can lead to an uneconomical oversizing of components or in their failure, as well. Apart from diffraction methods, mechanical methods are well recognized in order to determine unknown residual stress states. Depending on the applied method, specific boundary conditions have to be taken into account. In the case of mechanical methods like the hole-drilling and the ring-core method, the characteristics of the geometry of the component should be in accordance with an ideal and thick plate. The reason behind is the need for a calibration data set to transform strains into stresses. The calibration is usually carried out numerically. For the sake of simplicity, the geometry of the component is an ideal thick plate and the hole is introduced in its center. However, in most cases, this is not identical with the geometry of the component under investigation. Hence, an application tool was designed that enables the parametric design of a Finite Element Model, the determination of calibration coefficients, the evaluation of the experiment and the visualization of the results for geometries of practical importance. So far, the application tool can represent plates variable in their geometries and in positioning of the point of measurement. The option for other geometries are also possible e.g. a turbine blade.

Residual Stress Prediction Using Neural Network Approach

Procedures for residual stress evaluation by using Ring-Core method have not been standardized. Most common techniques are derived from the ones used for similar semi-destructive hole-drilling method. The differences between research approaches can cause significant errors. New approach using neural network is proposed as a more adaptive way for residual stress evaluation than the commonly used methods.

Estimation of Residual Stress Field Uniformity when Using the Ring-Core Method

The evaluation of residual stresses using Ring-Core method requires complex analysis of the acquired strain values. The development is based on ASTM Standard E837-08 for hole-drilling method, but the specific approach for Ring-Core is needed. The input data sets obtained from strain gage rosette are categorized by proposed uniformity test. The influence of geometric parameters of the specimen on this test and subsequently on the both uniform and nonuniform calculations is considered.

Effect of the Kinematic Hardening in the Simulations of the Straightening of Long Rolled Profiles

Residual stresses and lack of straightness appear during the cooling of sheet piles where the initial temperature field is not homogeneous. To meet the standards, the long hot rolled pieces are straightened using a series of rollers placed alternately above and below the pieces with shifts which create a succession of bendings. The process is modeled to study the impact of the industrial parameters (the duration of the cooling and the rollers positions), to improve the final geometry and to reduce the residual stresses. Tests are carried out on this structural steel to observe the material behavior, then material laws are chosen and the parameters of these laws are defined using an inverse method. Two sets of material data are obtained: for the first one, the hardening is supposed to be isotropic, and for the second one, additional tests are performed to describe isotropic and kinematic hardenings. The cooling followed by the straightening is then simulated by the finite element method with these two sets of data. The comparison of the rollers forces, the deformation and the residual stresses show the impact of the kinematic hardening on such a process where the material undergoes a succession of tensions and compressions. The real forces applied by the rollers, the real curvature of the interlocks at the end of the straightening process and the distribution of the residual longitudinal stresses measured on the web using the ring core method are used to validate the numerical model.

Experimental Validation of the Calibration Function of the Hole Drilling Method and Ring Core Method for Residual Stress Measurement

Abstract Residual stresses in mechanical components can result in both detrimental but also beneficial effects on the strength and lifetime of the components. The most detailed knowledge of the residual stress state is of advantage or a pre-requisite for the assessment of the component performance. The mechanical methods for residual stress measurement are divided into the groups of non-destructive and destructive methods. Two commonly used mechanical methods for determination of residual stresses are the hole drilling method (HDM) and the ring core method (RCM) which can be regarded as semi-destructive. This investigation will experimentally determine the calibration functions for residual stress measurements of the HDM and RCM. These functions are used for RS analyses using the differential method. With a four point bending testing machine a defined stress can be triggered between the middle bearings. The strain in this area can be measured with strain gauges, so the stress is well-known. In this defined loading area, the strains in two load cases with HDM configuration and RCM configuration were measured using strain gauge rosettes. With these measured strains and the actually known stresses, the specific calibration functions can be calculated and were presented together with the numerical functions.

A micro-scale strain rosette for residual stress measurement by SEM Moiré method

In this paper, a new method combining focused ion beam (FIB) and scanning electron microscope (SEM) Moire technique for the measurement of residual stress at micro scale is proposed. The FIB is employed to introduce stress relief like the macro ring-core method and fabricate gratings with a frequency of 5000 lines/mm on the measured area of the sample surface. Three groups of gratings in different radial directions are manufactured in order to form a micro-scale strain rosette. After milling ring-core by FIB, the deformation incurred by relief of the stress will be recorded with the strain rosette. The displacement/strain field can be measured using SEM scanning Moire with random phase-shifting algorithm. In this study, the Nickel alloy GH4169 sample (which was processed by laser shock peening) is selected as a study object to determine its residual stress. The results showed that the components of the in-plane principal stresses were −359 MPa and −207 MPa, respectively, which show good agreement with the results obtained from the available literature.

Residual stress of veneered porcelain by ion beam ring-core method

Residual stress of veneered porcelain by ion beam ring-core method

Analysis of the Geometric Shape of the Cutter in Ring-core Measurement

Estimation of the magnitude, sense and direction of the residual stress components in mechanical parts is a key factor for its lifespan and safety prediction. The principles of the semi-destructive Ring-Core method for residual stress determination are already well known. Commercially available measuring systems now enables to connect numerical and experimental evaluation techniques. Just by this appropriate connection new analyzes of evaluation accuracy by studying necessary influencing factors are formed. This paper deals about the influence of specific mill cutter geometry.

Determination of the Necessary Geometric Parameters of the Specimen in Ring-Core Method

There are several measuring techniques for determining residual stress which can be divided according to the created damage in to the construction in non-destructive, semi-destructive and destructive ones. One of the most common is semi-destructive hole-drilling method. This paper deals about Ring-Core method which is based on the similar principles. Today, there is no standard for the Ring-Core method, thus it is important to consider various influential factors. One of them are the dimensions of specimen. Calibration coefficients are determined by finite element (FE) analysis using the commercial software Solidworks. These coefficients are used for residual stress evaluation by incremental method used in Ring-Core method. The influence of different specimen dimensions on the accuracy of the evaluated residual stresses is considered.

New Method of Residual Stress Evaluation and its Advantages in Comparison with More Common Hole-drilling Method

Residual stress evaluation in constructions and mechanical parts is a necessary task in every modern industrial factory. Right evaluated changes in lifetime and loading conditions could save amount of money and increase the human safety. The Department of Applied Mechanics and Mechatronics has a long tradition in developing and practical usage of semi-destructive methods for determining residual stresses. Especially by using hole-drilling method and recently also Ring-Core method. Main characteristics and comparison of both methods with emphasis on Ring-Core analysis are discussed in this article.