Through-thickness Residual Stress Profiles Research Articles

Residual stress prediction of arc welded austenitic pipes with artificial neural network ensemble using experimental data

The prediction of weld-induced residual stress assists the structural integrity assessment of welded structures. In this study, residual stress measurements of girth welded austenitic stainless-steel pipes were used to develop two Artificial Neural Network (ANN) ensemble models to predict through-thickness residual stress profiles in Weld Centre Line (WCL). One model was developed for axial and the other for hoop residual stress prediction. The inputs of the models were the pipe radius to thickness ratio, the thickness, the heat input (weld arc electrical energy per unit run length [kJ/mm]) and the normalised through-thickness position. The hyperparameters were tuned, and the models were trained with various initial weight vectors, creating an ensemble of ANNs. The models’ performance was assessed by a test set and by sensitivity studies which revealed the models’ output trends.

Influence of WC Particle Size on the Mechanical Properties and Residual Stress of HVOF Thermally Sprayed WC-10Co-4Cr Coatings.

Cermet coatings deposited using high-velocity oxy-fuel (HVOF) are widely used due to their excellent wear and corrosion resistance. The new agglomeration–rapid sintering method is an excellent candidate for the preparation of WC–Co–Cr feedstock powders. In this study, four different WC–10Co–4Cr feedstock powders containing WC particles of different sizes were prepared by the new agglomeration–rapid sintering method and deposited on steel substrates using the HVOF technique. The microstructures and mechanical properties of the coatings were investigated using scanning electron microscopy, X-ray diffraction, nanoindentation, and Vickers indentation. The through-thickness residual stress profiles of the coatings and substrate materials were determined using neutron diffraction. We found that the microstructures and mechanical properties of the coatings were strongly dependent on the WC particle size. Decarburization and anisotropic mechanical behaviors were exhibited in the coatings, especially in the nanostructured coating. The coatings containing nano- and medium-sized WC particles were dense and uniform, with a high Young’s modulus and hardness and the highest fracture toughness among the four coatings. As the WC particle size increased, the compressive stress in the coating increased considerably. Knowledge of these relationships enables the optimization of feedstock powder design to achieve superior mechanical performance of coatings in the future.

Effect of Al2TiO5 Content and Sintering Temperature on the Microstructure and Residual Stress of Al2O3-Al2TiO5 Ceramic Composites.

A series of Al2O3–Al2TiO5 ceramic composites with different Al2TiO5 contents (10 and 40 vol.%) fabricated at different sintering temperatures (1450 and 1550 °C) was studied in the present work. The microstructure, crystallite structure, and through-thickness residual stress of these composites were investigated by scanning electron microscopy, X-ray diffraction, time-of-flight neutron diffraction, and Rietveld analysis. Lattice parameter variations and individual peak shifts were analyzed to calculate the mean phase stresses in the Al2O3 matrix and Al2TiO5 particulates as well as the peak-specific residual stresses for different hkl reflections of each phase. The results showed that the microstructure of the composites was affected by the Al2TiO5 content and sintering temperature. Moreover, as the Al2TiO5 grain size increased, microcracking occurred, resulting in decreased flexure strength. The sintering temperatures at 1450 and 1550 °C ensured the complete formation of Al2TiO5 during the reaction sintering and the subsequent cooling of Al2O3–Al2TiO5 composites. Some decomposition of AT occurred at the sintering temperature of 1550 °C. The mean phase residual stresses in Al2TiO5 particulates are tensile, and those in the Al2O3 matrix are compressive, with virtually flat through-thickness residual stress profiles in bulk samples. Owing to the thermal expansion anisotropy in the individual phase, the sign and magnitude of peak-specific residual stress values highly depend on individual hkl reflection. Both mean phase and peak-specific residual stresses were found to be dependent on the Al2TiO5 content and sintering temperature of Al2O3–Al2TiO5 composites, since the different developed microstructures can produce stress-relief microcracks. The present work is beneficial for developing Al2O3–Al2TiO5 composites with controlled microstructure and residual stress, which are crucial for achieving the desired thermal and mechanical properties.

Residual stresses in narrow-groove girth welds and applications for fitness-for-service assessment

Narrow gap welds have been increasingly used in pressure vessels and pipe components due to their unique advantages. A recent comprehensive investigation into residual stress distributions in narrow gap is presented in this paper, covering component wall thickness from 1” (25.4 mm) to 10” (254 mm), component radius to wall thickness ratio from 2 to 100, and linear welding heat input from low (300 J/mm) to high (18,000 J/mm). By means of a residual stress decomposition technique, two key parameters that govern through-thickness residual stress distributions in terms of their membrane and bending content have been identified. One is component radius to wall thickness ratio (r/t) and the other is a characteristic heat input density (Qˆ) having a unit of J/mm3. With these two parameters, a unified functional form for representing through-thickness residual stress profile in narrow gap welds is proposed for supporting defect assessment. Its adequacy for estimating residual stress distribution trends is further confirmed by comparing with thermomechanical finite element residual stress analyses for a number of selected narrow gap weld cases.

The role of the fiber and the bond in the hygroexpansion and curl of thin freely dried paper sheets

A computationally efficient method to study the in-plane and out-of-plane dimensional instability of thin paper sheets under the influence of moisture changes is presented. The method explicitly resolves the bonded and the free segments of fibers in the sheet, capturing the effect of anisotropic hygroexpansion at the fiber level. The method is verified against a volumetric model. The importance of longitudinal fiber hygroexpansion is demonstrated in spite of the absolute value of longitudinal hygroexpansion being an order of magnitude lower than the transverse hygroexpansion component. Finally, the method is used to demonstrate the formation of macroscopic sheet curl due to a moisture gradient in structurally uniform sheets in the absence of viscoelastic or plastic constitutive behavior and through-thickness residual stress profiles.

Modeling the Evolution of Residual Stresses in Thermally Sprayed YSZ Coating on Stainless Steel Substrate

This study is based on Eulerian method to model residual stresses for yttrium-stabilized zirconia coating applied to stainless steel substrate. A commercially available finite element software ABAQUS/Explicit is used to conduct this study. Single and multiple-particle impact analyses were carried out, and the residual stress data have been reported. The analysis is performed for two different values of thermal contact resistance and the through-thickness residual stress profiles obtained within the coating for single particle are tensile, while the substrate has a mixture of tensile and compressive residual stresses. For multiple impact model, the residual stress data have been presented for substrate with and without cooling. The residual stresses within the coating without substrate cooling are mostly tensile while the substrate is compressive. The residual stresses within the coating with substrate cooling are mostly tensile with compressive stresses on the top of the coating, while the substrate consists of compressive stresses. The obtained residual stresses are compared with experimental and analytical data.

Numerical and Experimental Studies on Subscale Behaviors of Ultrasonic Surface Peening

Displacement-controlled and force-controlled surface-striking conditions were applied for finite element method simulations of the single path ultrasonic nanocrystal surface modification (UNSM) surface peening process. The single path UNSM treatment was done on a 4 mm-thick 6061 T6 Al sheet, and the surface topology and through-thickness residual stresses were measured and compared to the simulation results. Both displacement- and force-controlled simulations predicted maximum compressive residual stresses near the surface and subsurface for the stresses along the strike progression direction and transverse direction, respectively. The experimentally measured through-thickness residual stress profile also showed the maximum compressive residual stress near the subsurface. Both simulation conditions showed a topological pile-up on the surface in front of the leading strike. However, only the force-controlled simulation showed the pile-up breakdown at a critical number of strikes followed by the redistribution of subsurface strain fields, and lead to topological undulations along the edge and valley of the UNSM path, which were confirmed to be consistent with the surface topology measured from the experiment. It was concluded that the actual UNSM strike is close to the force-controlled surface-striking condition.

Effects of Laser Shock Peening over Minimally Detectable Partial Through-Thickness Surface Cracks

Abstract Laser shock peening (LSP) has evolved as a viable alternative to other surface treatments (shot peening, burnishing, etc.) that induce beneficial residual stress in structural components. While fatigue life improvements have been recognized by the aerospace industry, in-service components may have surface flaws with sizes below current inspection limits. A concern is that the application of LSP over an existing crack may cause unintended detrimental consequences. To address this concern, two different LSP processes were applied over a 0.25 in. (6.35 mm) partial through-thickness surface fatigue crack in 7075-T651 aluminum. The resulting residual stresses were measured using combinations of included X-ray diffraction with layer removal, incremental center-hole drilling, and neutron diffraction. Both LSP processes resulted in a through-thickness residual stress profile in which compressive stresses at either surface were balanced by tensile stresses in a band centered in the mid-thickness of the specimen. Peened and unpeened (baseline) specimens were then tested under constant-amplitude cyclic loading to assess the fatigue response. The average fatigue life of the baseline specimens was about 49,000 cycles. The peened specimens (both treatments) survived to runout, exhibiting no crack growth when examined with an optical microscope even when the crack tip was either very close to or inside the region of tensile residual stress.

The effect of plasticity on residual stress generation and redistribution in offshore pipelines

The effect of plasticity induced by bending-reverse bending-straightening on residual stress generation and redistribution in pipes is examined through an analytical model, Finite Element Analysis and experimental measurements. The analytical model is based on elastic-perfectly plastic material properties, and correlates with Finite Element Analysis results, based on both elastic-perfectly plastic and Ramberg-Osgood strain-hardening material models. Comparison with experimental measurements at the pipe's crown position, furthest from the bending neutral axis, shows a similar through-thickness trend between the three methods. The proposed analytical model has enabled simplified, closed-form equations for predicting the through-thickness residual stress profile in pipes after one cycle of the bending-reverse bending-straightening process. In addition, parametric analyses using the extended analytical model with due consideration for initial residual stresses has produced practical recommendations for estimating the maximum residual stress magnitude at the pipe's crown position and around the pipe's circumference for a range of pipe sizes (outer diameter between 8in and 16in).

Through-Thickness Residual Stress Profiles in Austenitic Stainless Steel Welds: A Combined Experimental and Prediction Study

Economic and safe management of nuclear plant components relies on accurate prediction of welding-induced residual stresses. In this study, the distribution of residual stress through the thickness of austenitic stainless steel welds has been measured using neutron diffraction and the contour method. The measured data are used to validate residual stress profiles predicted by an artificial neural network approach (ANN) as a function of welding heat input and geometry. Maximum tensile stresses with magnitude close to the yield strength of the material were observed near the weld cap in both axial and hoop direction of the welds. Significant scatter of more than 200 MPa was found within the residual stress measurements at the weld center line and are associated with the geometry and welding conditions of individual weld passes. The ANN prediction is developed in an attempt to effectively quantify this phenomenon of ‘innate scatter’ and to learn the non-linear patterns in the weld residual stress profiles. Furthermore, the efficacy of the ANN method for defining through-thickness residual stress profiles in welds for application in structural integrity assessments is evaluated.

Experimental and numerical investigation of through-thickness residual stress of laser-bent Ti samples

One of the important parameters that can influence on performance of a part and its life is residual stress. All manufacturing processes can create residual stress in different ranges. Laser bending is one of the thermoforming processes that has been used in different industries. The aim of this study is investigation of through-thickness residual stress profile of Grade 2 titanium samples that were subjected to laser bending process. Slitting method was used to measure the residual stress profile experimentally. Both series expansion and pulse-regularization methods that are common computational techniques in the slitting method were employed to calculate the residual stress. Besides, finite element simulation of the laser bending process was done to study the through-depth residual stress distribution numerically. The results of two computational techniques were compared to each other and with the finite element method results. Furthermore, the effect of laser bending process on surface roughness and microhardness of the samples at the heat-affected zone was investigated. The results showed that laser bending process can induce considerable residual stress in the heat-affected zone of the part. Comparing the experimental and finite element method results showed good agreement between them. In comparison with the control sample, both surface roughness and microhardness of the samples increased after the laser bending process.

Residual Stress State of X65 Pipeline Girth Welds Before and After Local and Furnace Post Weld Heat Treatment

This research investigated the effects of global (in other words, furnace-based) and local post weld heat treatment (PWHT) on residual stress (RS) relaxation in API 5L X65 pipe girth welds. All pipe spools were fabricated using identical pipeline production procedures for manufacturing multipass narrow gap welds. Nondestructive neutron diffraction (ND) strain scanning was carried out on girth welded pipe spools and strain-free comb samples for the determination of the lattice spacing. All residual stress measurements were carried out at the KOWARI strain scanning instrument at the Australian Nuclear Science and Technology Organization (ANSTO). Residual stresses were measured on two pipe spools in as-welded condition and two pipe spools after local and furnace PWHT. Measurements were conducted through the thickness in the weld material and adjacent parent metal starting from the weld toes. Besides, three line-scans along pipe length were made 3 mm below outer surface, at pipe wall midthickness, and 3 mm above the inner surface. PWHT was carried out for stress relief; one pipe was conventionally heat treated entirely in an enclosed furnace, and the other was locally heated by a flexible ceramic heating pad. Residual stresses measured after PWHT were at exactly the same locations as those in as-welded condition. Residual stress states of the pipe spools in as-welded condition and after PWHT were compared, and the results were presented in full stress maps. Additionally, through-thickness residual stress profiles and the results of one line scan (3 mm below outer surface) were compared with the respective residual stress profiles advised in British Standard BS 7910 “Guide to methods for assessing the acceptability of flaws in metallic structures” and the UK nuclear industry's R6 procedure. The residual stress profiles in as-welded condition were similar. With the given parameters, local PWHT has effectively reduced residual stresses in the pipe spool to such a level that it prompted the thought that local PWHT can be considered a substitute for global PWHT.

A framework for estimating residual stress profile in seam-welded pipe and vessel components part I: Weld region

A recent comprehensive investigation into residual stress distributions in pipe and vessel longitudinal seam welds is presented in this paper, covering component wall thickness from 1/4″ (6.35 mm) to 10″ (254 mm), component radius to wall thickness ratio from 2 to 20, and linear welding heat input from low (50 J/mm) to high (6000 J/mm). Through the use of a residual stress decomposition technique, two key parameters that govern through-thickness residual stress distributions in terms of their membrane and bending content have been identified. One is component radius to wall thickness ratio (r/t) and the other is a characteristic heat input density (Q^) having a unit of J/mm3. With these two parameters, a unified functional form for estimating through-thickness residual stress profile in seam welded components is proposed in this paper (Part I) for supporting fitness for service assessment for crack-like flaws in weld region. A curved beam bending theory based model is introduced in Part II as a means of analytically describing through-thickness residual stress profile as a function of circumferential position away from the weld region until residual stresses become zero. The effectiveness of this proposed framework for achieving residual stress profile estimation within weld region (Part I) for longitudinal seam welds in pressure vessel and piping components has been confirmed by finite element residual stress analysis results on a large number of component configurations and different welding conditions.

A framework for estimating residual stress profile in seam welded pipe and vessel components Part II: Outside of weld region

Part I of this study has identified two key parameters (r/t ratio and characteristic heat input Qˆ) that dominate important through-thickness residual stress distribution characteristics, with which a residual stress profile estimation scheme has been developed for weld region, i.e., at weld centerline and weld toe. In Part II, we present an analytical model for achieving residual stress profile estimation for through-thickness sections away from the weld region in seam welded pipe and vessel components. A curved beam bending theory based model is analytically constructed through an assembly of two parts: One is weld fusion zone region and the other is the rest of the component section along circumferential direction. The final assembly of the two parts leads to a closed form solution to both axial (longitudinal) and hoop (transverse) residual stress components as a function of circumferential angular position away from weld toe. The effectiveness of the full-field residual stress estimation scheme is demonstrated by comparing with finite element modeling results over a broad range of weld geometries and welding conditions. The present development should provide a consistent and effective means for estimating through-thickness residual stress profile as a continuous function of pipe geometry and welding heat input.

An IIW residual stress profile estimation scheme for girth welds in pressure vessel and piping components

Over the years, one major objective of IIW Joint Working Group (JWG) on residual stresses and distortion prediction (RSDP) has been the development of consistent residual stress profiles for supporting fitness-for-service assessment of welded components. As a result of the JWG activities and further in-depth investigations, this paper presents a major development on a novel full-field residual stress estimation scheme that can be used for describing through-thickness residual stress profiles not only within a weld region, but also at any location away from a weld in pressure vessel and piping components. Within weld region, a systematic parametric finite element analysis is used to establish key parameters that govern through-thickness residual stress distribution characteristics by introducing membrane, bending, and self-equilibrating based decomposition technique. Away from weld region, a shell theory based method is developed for describing through-thickness residual stress distribution at any distance away from weld toe. This paper presents detailed results for single “V” joint preparation with pipe wall thickness varying from 6.25 to 250 mm and wall thickness to pipe radius ratio (r/t) varying from 2 to 100. The residual stress estimation method can be applied for double V and narrow groove joint preparations for girth welded components since all three joint preparations have been shown to follow a similar functional form to that seen in single V joint preparation.

Neutron diffraction residual stress analysis of Al2O3/Y-TZP ceramic composites

Residual stress measurements were conducted by time-of-flight neutron diffraction and Rietveld analysis method in Al2O3/Y-TZP ceramic composites fabricated by different green processing techniques (a novel tape casting and conventional slip casting) and with different Y-TZP content (5 and 40vol.% Y-TZP). The results show that the residual stresses in Y-TZP particulates are tensile and the ones in Al2O3 matrix are compressive, with almost flat through-thickness residual stress profiles in all bulk samples. As Y-TZP content increased, tension in Y-TZP phase was decreased but compression in Al2O3 matrix was increased (in absolute value). The values of residual stresses for both phases were mainly dependent on the Y-TZP content in the studied Al2O3/Y-TZP composites, irrespective of sample orientation and fabrication processes (a novel tape casting and conventional slip casting).

A full-field residual stress estimation scheme for fitness-for-service assessment of pipe girth welds: Part II – A shell theory based implementation

With the two key controlling parameters identified and their effectiveness demonstrated in Part I of this study series for constructing a continuous residual stress profile at weld region, a classical shell theory based model is proposed in this paper (Part II) for describing through-thickness residual stress distributions of both axial and hoop components at any axial location beyond weld region. The shell theory based model is analytically constructed through an assembly of two parts: One represents weld region and the other represents the remaining component section away from weld. The final assembly of the two parts leads to a closed form solution to both axial and hoop residual stress components as a function of axial distance from weld toe position. The effectiveness of the full-field residual stress estimation scheme is demonstrated by comparing with a series of finite element modeling results over a broad range of pipe weld geometries and welding conditions. The present development should provide a consistent and effective means for estimating through-thickness residual stress profile as a continuous function of pipe geometry, welding heat input, as well as material characteristics.

A full-field residual stress estimation scheme for fitness-for-service assessment of pipe girth welds: Part I – Identification of key parameters

In this two part series of papers, a recent comprehensive investigation of residual stress distributions in pipe and vessel girth welds is presented, covering component wall thickness from 6.35 mm (1/4″) to 254 mm (10″), radius to wall thickness ratio from 2 to 100, weld preparation from single “V” to double “V” to narrow-gap welds, and welding linear heating input from low (300 J/mm) to high (7,500 J/mm). Through the use of a residual stress decomposition technique, two key parameters that control through-thickness residual stress distributions in terms of their membrane and bending content have been identified by examining a large number of parametric residual stress analysis cases. One is component radius to wall thickness ratio (r/t) and the other is a characteristic heat input (Qˆ) having a unit of J/mm3. With these two parameters, a unified functional form for representing through-thickness residual stress profiles, along with its solution scheme, is proposed and demonstrated in this paper (Part I) for applications in weld region. A shell theory based analytical procedure is then introduced in Part II for estimating through-thickness residual stress profile at any axial distance away from weld regime. The effectiveness of the analytical estimation procedure is then confirmed by finite element residual stress solutions on a large number of component configurations and welding conditions.

Residual Stress in Coatings Produced by Cold Spray

Cold spray technology is used to produce metal coatings with a variety of functions, including surface corrosion protection, improvement of wear resistance, etc. Cold sprayed materials exhibit a wide range of behaviours resulting in large variation of spraying efficiency, coating properties, quality and performance in service. Residual stress, being a result and attribute of the deposition process, can be studied to test whether the coating is in tension/compression stress state and also to provide information about the thermo-mechanical history of the material during the deposition process. Residual stress distributions in a variety of coating materials have been studied by neutron diffraction. Through-thickness residual stress profiles show that the stress magnitude varies significantly and depends mainly on the mechanical properties of the coating material.

Measurement of Residual Stresses within a PWR Swaged Heater Tube

Frequent failures of the pressuriser heater tubes used in Pressurised Water Reactors (PWRs) have been found. Axial cracks initiating from the tube outer diameter have been detected in some tubes as well as the resulting electrical problems. Replacement of the heater tubes requires an undesirably prolonged plant shutdown. In order to better understand these failures a series of residual stress measurements were carried out to obtain the near surface and through-thickness residual stress profiles in a stainless steel pressuriser heater tube. Three different residual stress measurement techniques were employed namely, Deep-Hole Drilling (DHD), Incremental Centre Hole Drilling (ICHD) and Sachs’ Boring (SB) to measure the through thickness residual stress distribution in the heater tubes. Results showed that the hoop stresses measured using all three techniques were predominantly tensile at all locations, while the axial stresses were found to be tensile at the surface and both tensile and compressive as they reduce to small magnitudes within the tube. The magnitude of the in-plane shear stresses was small at all measurement depths at all locations. The various measurement methods were found to complement each other well. All the measurements revealed a characteristic profile for the through-thickness residual stress distribution.