Split-ring Test Research Articles

Machine Learning Augmentation of the Failure Assessment Diagram Methodology for Enhanced Tubular Structures Integrity Evaluation

Failure assessment diagrams (FADs) are essential engineering tools for evaluating the structural integrity of components. However, their widespread application can be limited by complexity and computational expense. This study presents a novel machine learning-based approach to streamline FAD analysis, offering accuracy and efficiency while overcoming these limitations. The approach integrates numerical contour integral-based FADs with artificial neural networks (ANNs). To ensure reliable material modeling for the Finite Element Analysis (FEA) used to generate J-integral based FADs that train the ANNs, careful experimental and numerical procedures were employed. This involved uniaxial tensile tests, an iterative method for obtaining precise true stress–strain curves, and a Ramberg–Osgood material model for accurate material behavior representation. The ANNs themselves not only analyze large datasets to generate precise FAD envelopes but also predict limit loads and the Φ parameter, incorporating the effect of residual stress on the FAD methodology. To verify and test the proposed method, hypothetical fitness-for-service assessment cases were conducted, incorporating experimental residual stress measurements from split-ring tests on P110 and L80 pipes. These assessments were compared to both traditional FAD methods and computationally intensive FEA-based FADs. Results demonstrate a closer agreement with FEA-based calculations than traditional methods provided in engineering standards. Ultimately, this work provides a rather innovative and adaptable approach for structural integrity evaluations and critical engineering assessments through the proposal of an ANN enhanced FAD approach, simplifying these calculations while maintaining high fidelity.

MECHANICAL CHARACTERISATION OF FIBER-REINFORCED GLASS POLYMER COMPOSITE PIPES

This paper presents the details of experimental testing for the suitability of glass fibre-reinforced polymer (GFRP) pipes for gas transmission lines. Hydrostatic bursting pressure and split ring tests were performed on 150 mm (6 in.) and 250 mm (10 in.) diameter pipes made by both the pultruded and filament wound processes. The filament wound pipes were tested to evaluate the butt joint failure strengths. The classical lamination theory (CLT) was used to predict the elastic behaviour of these pipes. It was found that filament wound pipes offer superior resistance to internal pressures compared to pultruded pipes. Furthermore, the joints were found to be the most limiting component in terms of pipeline burst pressure capacity. The observed elastic behaviour by the employed hydrostatic pressure test method showed strong agreement with the CLT predictions which confirms its applicability for testing GFRP pipes for bursting pressure. Failure predictions between the experimental data and theory had minor discrepancies.

Experimental and finite element studies of springback using split-ring test for Inconel 625 alloy

ABSTRACT In many manufacturing and forming industries, springback effect in case of sheet metals is a major cause of concern. Thus, an effort has been made in present work for analysing the spring back effect with the help of split-ring test in deep drawn cups. The uniaxial tensile test has been performed and flow stress has been found to be decreased while ductility of material increased with rise in temperature. Subsequently, the deep drawing experiments have been conducted and overall thickness deviation was found to be increased with increase in temperature and decrease in blank holding pressure. The springback in deep drawn cups are measured in terms of average opening gaps in cut ring portion. Furthermore, Sellar’s-based constitutive model with different anisotropic yield criteria namely; Hill’48 and Barlat’89 have been developed for Inconel 625 alloy. The combination of Sellar’s model with Barlat’89 yield criterion is found to be more accurate for deformation behaviour prediction. Finally, these models have been implemented in finite element (FE) analysis of split-ring test using user defined material (UMAT) script. The mean relative error between numerically and experimentally computed results was found to be less than 8.11% which is well within the 10% of acceptance level.

Comparison of Microstructure and Residual Stress Affecting Stress Corrosion Cracking Susceptibility of Austenitic Stainless Steel Tubes

Three 304L austenitic stainless steel feedwater heater (FWH) tubes used in pressurized water reactors (PWRs) were investigated in this study. By comparing metallurgical differences, such as microstructural features, micro-hardness, and circumferential residual stress of the failed and non-failed FWH tubes by stress corrosion cracking (SCC), factors which could affect SCC susceptibility were assessed. Considering the impurity controlled and hydrogenated water chemistry, it is likely the mechanism of sudden SCC failures under PWR operation would be associated with material aspects rather than water chemistry. In tubes which experienced a large number of SCC failures, significant tensile circumferential residual stresses were measured. Such residual stress could be generated by work hardening during straightening of the rolling process, which directly applies stress on the surface of the tubes. The non-failed tube showed very low residual stress, by using straightening or stretching process. The rolling process induces a large amount of dislocations and an increasing tendency in micro-hardness values toward the outer region. Although the hardness values satisfied the material limitation requirement, the presence of large residual stress is thought to increase the SCC susceptibility of FWH tubes. Thus, this study indicates that straightening by stretching process has benefits in terms of low residual stress, assuring the long-term integrity of FWH tubes against SCC failure. Meanwhile, when the rolling process is used, additional heat treatment could be a practical method to reduce residual stress in FHW tubes. Key words: austenitic stainless steel, work hardening, micro-hardness, residual stress, split-ring test

Robust multi objective optimization of anisotropic yield function coefficients

The coefficients of yield functions have been conventionally determined based on limited number of experiments such as yield stresses and/or R-values along different material orientations. In the present study, a multi objective genetic algorithm (MultiGA) based approach was implemented to obtain coefficients of anisotropic yield functions by simultaneous error minimization for yield stresses and R-values. Three frequently employed yield functions, Hill 1948, Barlat 1989 and Barlat Yld2000-2d, in the sheet metal forming simulations were considered. The performance of the determined coefficients for each yield function was judged by comparing the predicted yield stress and R-values with experimental values. Fundamental questions regarding the effect of experimental data on determining the coefficients were analyzed using the proposed approach. It is generally perceived that increase in experimental data enhances the accuracy of the determination of coefficients. Some counter intuitive results obtained on this regard are discussed. Finally, finite element (FE) simulations were performed to predict earing profile in deep drawn cups and springback profile for split-ring test, which were validated with experimental results. From the comparative study with conventional method and experiment, the yield function coefficients optimized by the MultiGA performed well for predicting the deformation behaviors of various anisotropic sheet metals.

Experimental Investigation of Springback in Forming of High Strength Steel Using Split-Ring Test

Light weight and high strength steel has been used as a substitute for conventional steel in sheet metal forming operations. High strength steel is less formable than conventional steel. A thorough understanding of the factors limiting the formability of high strength steel is important from engineering as well as economic viewpoint. In this work three different materials, galvanic coated steel, EN42J steel and stainless steel (Grade 304) are taken for analysis. The influence of blank holding force on the formation of cups is investigated. The springback behavior of high strength steel sheet is investigated by conducting split ring tests.

Split-Ring Springback Simulations with the Non-associated Flow Rule and Evolutionary Elastic-Plasticity Models

Finite element simulations and experiments for the split-ring test were conducted to investigate the effect of anisotropic constitutive models on the predictive capability of sheet springback. As an alternative to the commonly employed associated flow rule, a non-associated flow rule for Hill1948 yield function was implemented in the simulations. Moreover, the evolution of anisotropy with plastic deformation was efficiently modeled by identifying equivalent plastic strain-dependent anisotropic coefficients. Comparative study with different yield surfaces and elasticity models showed that the split-ring springback could be best predicted when the anisotropy in both the R value and yield stress, their evolution and variable apparent elastic modulus were taken into account in the simulations. Detailed analyses based on deformation paths superimposed on the anisotropic yield functions predicted by different constitutive models were provided to understand the complex springback response in the split-ring test.

Multistage Deep Drawing with Ironing of Al-killed AISI 1040 Graded Medium Carbon Steel: a Parametric Study

Analysis of multistage deep drawing with ironing was made while manufacturing of cups from 12 mm thick, 60 mm diameter circular blanks of Al-killed AISI 1040 graded medium carbon steel. Quality requirements of the steel in heat treated condition were examined in terms of tensile properties, formability characteristics, and formability limit diagram. Drawability and ironability parameters in multistage cup drawing processes were evaluated in terms of draw ratio; draw reduction; ironing ratio; ironing reduction and thus its press formability in an actual state of practice was shown. Further, were analysed the influences of process parameters such as: semi die angle; interfacial friction coefficient; die-punch clearance on draw and ironing punch forces in each draw stage. The spring back tendency of cups in terms of their trends and degrees with respect to cup heights and draw stages were experimentally determined by conducting split ring test. Thus, a data bank was created as potential references for process engineers to improve the manufacturing process and draw tools as well, particularly for the selected steel.

Thermo-mechanical finite element analysis of the AA5086 alloy under warm forming conditions

Warm forming processes have been successfully applied at laboratory level to overcome some important drawbacks of the Al−Mg alloys, such as poor formability and large springback. However, the numerical simulation of these processes requires the adoption of coupled thermo-mechanical finite element analysis, using temperature-dependent material models. The numerical description of the thermo-mechanical behaviour can require a large set of experimental tests. These experimental tests should be performed under conditions identical to the ones observed in the forming process. In this study, the warm deep drawing of a cylindrical cup is analysed, including the split-ring test to assess the temperature effect on the springback. Based on the analysis of the forming process conditions, the thermo-mechanical behaviour of the AA5086 aluminium alloy is described by a rate-independent thermo-elasto-plastic material model. The hardening law adopted is temperature-dependent while the yield function is temperature-independent. Nevertheless, the yield criterion parameters are selected based on the temperature of the heated tools. In fact, the model assumes that the temperature of the tools is uniform and constant, adopting a variable interfacial heat transfer coefficient. The accuracy of the proposed finite element model is assessed by comparing numerical and experimental results. The predicted punch force, thickness distribution and earing profile are in very good agreement with the experimental measurements, when the anisotropic behaviour of the blank is accurately described. However, this does not guarantee a correct springback prediction, which is strongly influenced by the elastic properties, namely the Young's modulus.

Numerical study of springback using the split-ring test: influence of the clearance between the die and the punch

The split-ring test consists in cutting a ring from the wall of a drawn cylindrical cup, which is split to measure the springback. This springback measure can also be used to estimate the circumferential residual stresses in the ring. It is known that the distribution of the residual stresses in the cup depends on the specific combination of forming parameters selected to perform the deep drawing operation, which include the depth of the cup and the clearance between the die and the punch. Moreover, the forming parameters also seem to affect the distribution of the axial and circumferential through-thickness stress profiles along the cup height, since different ring opening trends have been observed, when rings are cut from the cup wall at different heights. The focus of this numerical study is the analysis of the impact of an ironing stage on the residual stress distributions in the cylindrical cup and, consequently, in the split-ring test. The analysis is performed considering a 6016-T4 aluminium alloy, for which experimental results are available. The results show that the ironing of the vertical wall changes the characteristic distribution of the axial and circumferential residual stresses in all locations of the cup wall, even for relatively small ironing strains. This affects the trend observed for the ring opening value, when rings are cut at different heights.

Remapping algorithms: application to trimming operations in sheet metal forming

Most of sheet metal forming processes comprise intermediate trimming operations to remove superfluous material. These operations are required for subsequent forming operations. On the other hand, the springback is strongly influenced by the trimming operations that change the part stiffness and the stress field. From the numerical point of view, this involves the geometrical trimming of the finite element mesh and subsequent remapping of the state variables. This study presents a remapping method based on Dual Kriging interpolation, specifically developed for hexahedral finite elements, which has been implemented in DD3TRIM in-house code. Its performance is compared with the one of the Incremental Volumetric Remapping method, using the split-ring test to highlight their advantages and limitations. The numerical simulation of the forming processes is performed with DD3IMP finite element solver.

Benchmark 3 - Springback of an Al-Mg alloy in warm forming conditions

Accurate prediction of springback is a long-standing challenge in the field of warm forming of aluminium sheets. The objective of this benchmark is to predict the effect of temperature on the springback process through the use of the split-ring test [1] with an Al-Mg alloy. This test consists in determining the residual stress state by measuring the opening of a ring cut from the sidewall of a formed cylindrical cup. Cylindrical cups are drawn with a heated die and blank-holder at temperatures of 20, 150 and 240°C. The force-displacement response during the forming process, the thickness and the earing profiles of the cup as well as the ring opening and the temperature of the blank are used to evaluate numerical predictions submitted by the benchmark participants. Problem description, material properties, and simulation reports with experimental data are summarized.

Residual stress in the cylindrical drawing cup of SUS304 stainless steel evaluated by split-ring test

The residual stresses in the wall of a SUS304 stainless steel cylindrical drawing cup were evaluated by split-ring tests, and the influences of stamping die parameters on the residual stress were investigated. A new theoretical model of a split-ring test was developed to evaluate the residual stress in a ring, which was verified to be reasonable and reliable by numerical simulations with ABAQUS code and by nanoindentation tests. Seven groups of split-ring tests were completed, and the residual stresses were calculated according to the theoretical model. The split-ring test results showed that the circumferential residual stress in the wall of the SUS304 stainless steel cylindrical drawing cup was very large and did not change with the different die corner radius. The circumferential residual stress first increased with the increase of drawing punch–die clearance, then was almost unchanged when the clearance increased greater than blank thickness 1 mm. Thus, a smaller clearance was suggested to be chosen to reduce the residual stress in the wall of the SUS304 stainless steel drawing cup. Seven groups, three samples in each group, and a total of 21 qualified cylindrical drawing cups in Fig. 2 were made from a circular SUS304 stainless steel sheet about 72 mm in diameter and 0.94 mm in thickness by deep drawing with different processing parameters. After the rings were cut from the cup walls, each ring was split along the radial direction. Two ends of each split ring opened a distance to release the circumferential residual stress in the closed ring. After measuring the opening distance, the residual stresses in the drawing cup walls were calculated by Eq. (18) in the newly developed split-ring test theoretical model of this paper. Then reasonable technology measures were found to reduce the residual stress by analyzing the influence of the mold parameters on residual stresses.

A novel method to evaluate Young׳s modulus of ceramics at high temperature up to 2100 °C

Although it is important to understand Young׳s modulus and creep behavior of ceramics at ultra-high temperature from 1500°C to 2300°C, measuring these properties accurately at high temperature above 1600°C is not very easy due to the difficulty of the deformation measurement or invalid testing instruments at such high temperature. This work presented a solution to this problem via a combination of the split ring test and relative method (viz. relative split ring method). The real deformation of the split ring at ultra-high temperature could be easily determined from the difference of cross-beam displacements between the split ring and a rigid disk under the same loads. Experiments of graphite and C/SiC/ZrB2 composite at ultra-high temperature demonstrated the validity and convenience of the relative split ring method, which would provide a simple and efficient approach to evaluate Young׳s modulus of ceramic materials at ultra-high temperature.

Experimental and numerical studies on the warm deep drawing of an Al–Mg alloy

The warm deep drawing of circular AA5754-O aluminium alloy blanks was investigated both experimentally using specially designed equipment and numerically using a fully coupled thermo-mechanical finite element model. Cylindrical cups were prepared with a heated die and blank-holder. The split-ring test was used to measure the effects of the temperature on the springback process from room temperature to 200°C. Temperatures above 150°C were found to greatly affect the force/displacement response during the forming process and the ironing phase, the earing profile and the springback effect. Tensile and shear tests were also performed to study the temperature-dependent mechanical responses.To simulate this process, a temperature-dependent anisotropic model for the material was implemented in the commercial code ABAQUS/Standard, based on the UMAT interface for user-material models. The parameters of a phenomenological Hockett–Sherby hardening model and a power law strain rate dependency were identified using data obtained in uniaxial tensile and shear tests at various temperatures and strain rates, in order to account for both the temperature and the viscous effects in the coupled thermomechanical constitutive law. Von Mises isotropic criterion and Hill׳48 anisotropic yield criterion were also adopted to describe the material mechanical behaviour. The influence of the contact with friction conditions in the forming process (i.e. punch force evolution, thickness distribution along the cup wall and earing profiles) was also analysed. The numerical results obtained with the calibrated parameters generally showed a good match with the experimental temperatures. The results highlighted the importance of the correct choice of the yield criteria. The Hill48 criterion showed difficulties in the correct prediction of the springback process for this aluminium alloy.

Investigation of residual stress development in spiral welded pipe

The residual stresses in spiral welded pipe (SWP) are generated due to localized plastic deformation in the weld-metal and the heat affected zone (HAZ), which cause detrimental effects on the reliability of the pipes. Therefore, it is necessary to investigate these stresses to ensure safe service and operation of pipes used in oil and gas industry. The most commonly used techniques for the evaluation of residual stresses in industries are the split ring test and the hole drilling method. In the experimental part of this work, the semi-destructive hole drilling experimental technique has been adopted for analyzing the stresses in the pipe. The distribution of residual stresses is calculated through different mathematical procedures, such as uniform method, power series method and integral method. The second part of this work consists of finite element (FE) modeling of the split ring test. It is done by sequentially coupling the thermal and structural analysis of welding to assess the adequacy of the split ring method for the estimation of weld residual stresses. Furthermore, the numerically predicted residual distributions during welding are compared with experimentally calculated values using hole drilling method and it is found in good agreement.

Springback Prediction Using the Split-Ring Test Based on a Combined Anisotropic Hardening Model

Material model which could describe the Bauschinger effect is essential to accurately predict springback since strain path reverse is quite common in sheet metal forming. In this paper, a combined isotropic-kinematic hardening law which can capture the Bauschinger effect, transient behavior and permanent softening was used to model the hardening behavior. Also, the non-quadratic anisotropic yield function, Yld2000-2d, was chosen to describe the anisotropy. An inverse identificat- ion method was carried out to calibrate the material parameters by using uni-axial tension and Bauschinger simple shear tests. Experiments of cylindrical cup drawing and following split-ring tests were performed. Similar processes were carried out by numerical simulation, and springback predicted by present model was compared with experiments and that of vonMises model. The result shows that the current model significantly improves the accuracy of springback prediction.

Mechanical Behaviour and Springback Study of an Aluminium Alloy in Warm Forming Conditions

This study deals with the mechanical behaviour and material modelling of an AA5754-O alloy at elevated temperature. Experimental shear tests were performed from room temperature up to 200°C, and the material behaviour has been identified with both shear and tensile tests, as a function of temperature. To analyse the influence of temperature during forming over springback, a split-ring test is used. Experimental results are obtained and compared to numerical simulations performed with the finite element in-house code DD3IMP. The numerical process of ring splitting is performed with the in-house code DD3TRIM. The main observed data are force-displacement curves of the punch during forming, cup thickness at the end of forming, and ring gap after splitting. It is shown that all these parameters are strongly dependent on the forming temperature. A correlation is obtained between experimental data and numerical simulation for the evolution of punch force and opening after springback as a function of temperature. The distribution of the tangential stress in the cup wall is the main factor influencing the springback mechanism in warm forming condition.

Numerical study of springback using the split-ring test for an AA5754 aluminum alloy

The split-ring test provides a simple benchmark for correlating springback obtained by finite element analysis (FEA) with experimental measurements. This test consists in cutting a ring specimen from a full drawn cup and then to split the ring longitudinally along a radial plane. The difference between the ring diameters, before and after splitting, gives a direct measure of the springback phenomenon, and indirectly, of the amount of residual stresses in the drawn cup. In this paper, the numerical simulation of the deep drawing process and splitting of an aluminum alloy AA5754-O ring is performed using both the finite element code ABAQUS and the in-house code DD3IMP. The trimming of the ring is carried out with a devoted program, DD3TRIM, which allows the geometrical and material state remapping treatment of solid hexahedral element meshes. The punch-force evolution during deep drawing and thickness distribution obtained numerically are compared with experimental ones provided in [1] (Laurent et al., 2009). The influence of finite element formulation, mesh size and yield criteria on the numerical results for the ring opening is evaluated. The stress distributions in the cup, at the end of the drawing stage are analyzed and some explanations concerning their influence on springback mechanisms are given.

Influence of the temperature on residual stresses and springback effect in an aluminium alloy

This study deals with the experimental and numerical investigation of springback in an aluminium alloy at different temperatures. An experimental split-ring test is performed on a AA5754-O alloy using a laboratory drawing device. The influence of temperature during forming over springback is measured from room temperature to 200 °C. The temperature is the same for all tools and is maintained constant during all the forming process. The experimental results are compared to numerical simulations performed with the finite element code Abaqus. Material parameters are identified using uniaxial tensile tests at different temperatures and several strain rates in order to take into account both temperature and viscous effects in a coupled thermomechanical constitutive law. The stress and strain states in the cup at the end of the drawing and after springback are analyzed as a function of temperature and a detailed study of stress distributions in the thickness of the blank is proposed. It is shown that the effect of temperature tends to decrease the stress gradient in the cup wall that is directly linked to the decrease of the springback opening of the ring. The distribution of the hoop stress in the cup wall is the main factor influencing the springback mechanism in warm forming condition.