High Tensile Residual Stresses Research Articles

Critical Conditions of Cold Cracking in High Strength Steel Weld Based on the Local Stress Distribution and Hydrogen Accumulation

In this study, the occurrence of cold cracking in high strength steel welds were investigated in terms of residual stress and hydrogen diffusion behavior. The y-groove weld cracking test of TS780MPa grade steel plate was conducted with intentionally introducing hydrogen into the shielding gas during the gas-metal arc welding (GMAW). Since local stress is one of the most important factors for the cold cracking, residual stress distribution in the weld joint was measured by the neutron diffraction using TAKUMI in J-PARC. The root region, which is usually the crack initiation site in the y-groove cold cracking test, was under a multi-axial stress state and showed highest tensile residual stress in the transverse direction. It was considered that hydrogen diffusion and accumulation could be enhanced in the high stressed root region, resulting in cold cracking. Therefore, hydrogen diffusion behavior and stress distribution in the y-groove weld joint was investigated by a coupled thermo elastic plastic and hydrogen diffusion analyses. Hydrogen accumulation occurred in the root region where showed highest hydrostatic stress. The point where showed the hydrogen accumulation was well corresponded to the crack initiation site. It was indicated that local hydrogen concentration after welding was another important key factor for the cold cracking. From these investigations, it was essential to take the combination of local hydrogen concentration and residual stress distribution near the root region into account for the highly precise estimation of cold cracking.

Measurement and modelling of residual stress in wire-feed additively manufactured titanium

ABSTRACTResidual stresses were characterised in a wire-feed additively manufactured titanium alloy component. A numerical simulation based on the inherent strain method was used to model residual stresses arising from the manufacturing process. The contour method was used to experimentally determine the residual stress field. High tensile residual stresses were seen at and around the interface of the substrate and the deposited metal. Compressive residual stresses were present in the substrate and at the top of the deposit. The satisfactory correlation was achieved between the results from the numerical simulation and the contour method, except for the location of the root of the deposit. The effect of pre-heating the sample substrate on the residual stress distribution is also discussed.

Microstructural evolution and optical dispersion of sputtered ZnO thin films at low annealing temperature

In an attempt to understand microstructural evolution and its effect on optical properties, nanocrystalline ZnO thin films are deposited by pulsed DC magnetron sputtering and then heat treated at a low temperature for several hours. X-ray diffraction experiments confirm that texture and residual stress are sensitive to morphological changes from globular to plate-like grains. Nanocrystalline ZnO thin films are highly transparent to visible light but opaque to UV light below 350 nm. The transparency in the UV region is limited by its fundamental absorption below 400 nm. The spectral dependence of optical constants demonstrates normal dispersion and the films deposited at 70 W of sputtering power and then heat treated have higher values of refractive indices and significantly high extinction. The dispersion parameters evaluated under the purview of a single oscillator model propose vital linkages of optical properties with the microstructure of ZnO thin films. Dispersion parameters viz. plasma frequency can be attuned by controlling the microstructure of ZnO thin films via simply varying process parameters and post-deposition heat treatment at low temperature. This study confirms that high tensile residual stress and texture are favorable for the development of novel nanocrystalline ZnO thin films.

Residual Stress–Based Fatigue Design of Welded Structures

Abstract Fatigue design concepts for welded structures generally consider residual stresses as a factor affecting the mean stress influence. Residual stresses are mostly interpreted as mean stresses. In addition, high tensile residual stresses are conservatively assumed, resulting in maximum effective load stresses from fatigue loading in the order of the yield strength. The consequence of this is that additional static load stresses may have no influence on the resulting fatigue strength because the local effective mean stress (residual stresses and load mean stresses) is always high. The related evaluation concepts neither distinguish between different steel grades nor between different origins and amounts of possible residual stresses in welded joints. The real magnitude of existing residual stresses is also usually not considered, because in practice, usually no explicit knowledge of the residual stresses at critical sites of a construction is available because residual stress measurements are not state of the art in welding practice. For an explicit consideration of residual stresses in design concepts, the sign, the initial amount, and especially the amount of the residual stresses after a load induced relaxation must be considered. Therefore, the steel grade and the condition of the material are of great importance, as well as the local stress condition influenced by welding-induced notch geometry. The article shall give an overview about the state of the art of consideration of residual stresses in fatigue design concepts for welded structures and the background of their development. Finally, a new approach shall offer a possibility for an enhanced consideration of residual stresses in design concepts based on the explicit knowledge about the effective residual stresses that can actually be observed with different measurement concepts.

Preventing delayed cracks in SUS304 deep drawn cups using extreme blank holding forces aided by nanolubrication

Disappearance and reappearance of delayed cracks were observed in deep drawing process of SUS304 cylindrical cups under elevated blank holding forces (BHF) of 8~32 kN with nanolubricants containing 1~3 wt% SiO2 concentrations at limiting draw ratio. The widest crack-free BHF range determined experimentally, i.e., 29~31 kN was for 2 wt% SiO2. Delayed cracks were observed below and beyond this BHF range. The frictional force at the flange surfaces slightly increased when entering the crack-free BHF range, as evidenced by the rebound in sidewall thickness, relative intensity of α′-martensite, elongated height, and drawing force beyond 28 kN. The coefficients of friction for each BHF were obtained with finite element simulations. Since residual stresses developed along the outer surface of the cup due to the unbending of the material when the material left the draw die profile, the slight increase in frictional force in the radial direction along the flange surfaces reduced the circumferential tensile residual stress of the cup, particularly at 80% cup height. The elimination of delayed cracks was attributed to the lower tensile residual stress gradient, i.e., lower and higher tensile residual stresses at 80% and 100% cup heights, respectively. However, the excessive increase at 100% cup height was a result of the increase in longitudinal compressive stress at BHF beyond 31 kN due to the stress equilibrium, leading to the reappearance of cracks. The minimum BHF required to eliminate cracks changed minimally with increasing SiO2 concentration.

Effect of Cold Working on the Residual Stress around the Notch front of Stress Corrosion Cracking

Failures show that the austenitic stainless steel (SS) pipes with produced high weld residual stresses are more susceptible to stress corrosion cracking in the simulated high temperature water environments of pressurized water reactor. To understand the effect of cold working on the residual stress and strain of stress corrosion cracking in 304SS, the stress-strain field near the crack front of 304SS under different degree of cold working was simulated by ABAQUS. Results show that the higher tensile residual stress and the larger compressive residual stress simultaneously occurs in the lower cold working material, and cold working has greater effect on the residual stress than on the residual strain. The residual compressive stress and residual compressive strain both decrease rapidly in a small range around the notch fronts and tend to be consistent in a more larger distance. But cold working has little effect on the residual strain.

Raman Strain Sensing and Interfacial Stress Transfer of Hierarchical CNT-Coated Carbon Fibers

Hierarchical reinforcements of carbon fibers (CFs) coated with carbon nanotubes (CNTs) were produced via two different methodologies, namely chemical vapor deposition (CVD) and wet chemical treatment. These advanced smart structural materials are possible candidates for inherent strain sensing and improved interfacial properties when incorporated in the volume of structural fiber-reinforced polymer composites. The morphology and wetting properties of the hierarchical CFs were evaluated via SEM and contact angle measurements. Enhanced values in the strain-induced Raman sensing ability of the hierarchical reinforcement produced via CVD in comparison with the uncoated fibers were measured at single fiber level. The residual stress transfer profiles of both the hierarchical and the uncoated carbon fibers were also evaluated using the Raman spectroscopy in order to evaluate the phase interaction in the hierarchical structure. Compressive residual stresses in the order of 1.5 GPa were found to act on the CF surfaces, while tensile residual stresses were measured for both the CNT-coated systems. The effects of the preparation process of the CNT coatings on the residual stress profiles were also prominent, with the CVD-grown coatings resulting in higher tensile residual stresses, 1.76 GPa instead of 1.22 GPa for the coatings produced via wet chemical treatment. The change in the residual stress profiles established the interaction between the CFs and the CNTs via an anchoring mechanism which significantly altered the stress field at the vicinity of the interface.

Measurement of residual stresses in dissimilar friction stir-welded aluminium and copper plates using the contour method

ABSTRACTThe longitudinal residual stresses in the friction stir-welded plates of 5A06 aluminium and pure copper were determined using the contour method. The results revealed the presence of high tensile and compressive residual stresses on the aluminium and copper sides, respectively. The residual stresses were detected on the weld zone as well as the thermo-mechanically affected zone (TMAZ) of the aluminium plate. In contrast, the compressive residual stresses in the copper plate had a much narrower width along the weld line. Peak tensile stresses up to 240 MPa were found in the TMAZ of the aluminium plate.

Corrosion-mechanical resistance of arc-sprayed coatings made from cored powders

Result of investigations of resistance against corrosion and mechanical resistance of obtained by means of arc-spray metallization (with the use of cored wires) coatings are presented. The cored wires (CWs) enable us to regulate the chemical composition and, consequently, properties of the deposited coating in a wide range. With this, the characteristic feature is its high structural heterogeneity, which is caused by rapidness of the processes of melting of components of the CW in the arc; this promotes incompleteness of dissolving of change materials in the melt of the metallic shell, and thus, there forms of heterogeneous as to its chemical components melt. The determination of the first-type residual stresses in coatings was conducted according to the developed for bimetal rings technique. The tensile strength (cohesion) of ASC was determined with the use of an experimental set-up which consisted of two pipes. Electrochemical investigations were conducted in an electrochemical cell in potentiodynamic regime with the use of hard-ware-software complex which was designed for automation of investigations with the help of CBЛ-1Б-М voltamperometric system. The rate of corrosion was determined by means of extrapolation of linear segments of polarization curves to the potential of corrosion or on the basis of segments which corresponded to passive state. In order to develop experimental sets of CWs, there additionally were investigated some materials with different charge components (chromium, ferro-chromium, boron carbide, ferrochromium- boron, ferro-silicium, ferro-manganise, self-fluxing alloy) (Table 1). High hardness is characteristic of coatings made from CWs. Such a high hardness is due to 3 % of boron in the coating. However, the cohesive strength of such coating is low and does not exceed 100 MPa. This is caused by high tensile residual first-type stresses, which can lead to emergence of crack during machining. In order to reduce the level of residual stresses, it is necessary to preliminarily heat machine parts to 150–2000 °C. Electrochemical parameters and the character of polarization curves, despite some changes in chemical composition of coatings, do not essentially differ. With this, the potential of corrosion shifts towards the segment of negative values, and the corrosion current of such coatings are within one decimal order of their values. Open porosity, that is an important factor, which influences the corrosion behaviour of the material and its matrix is a characteristic feature of all the coatings. The corrosive medium, because of the presence of porosity, penetrates through such pores down to the matrix and creates conditions for proceeding of under-coating corrosion. In this case, products of corrosion accumulate at the coating – matrix interface, and they cause the separation of the coating from the basis (phenomena of ply-separation). The presence of chromium, ferro-chromium, ferro-silicon, and ferro-manganese in the charge for CW 90Cr17BMnSi leads to minimal chemical heterogeneity of the coating, and consequently to high corrosion resistance of the coating. The presence of ferro-chromium-boron, chromium, and self-fluxing alloying composition in the charge for CW 20Cr16B3Ni2SiAl ensures high content of chromium in the coating, low coefficient of microheterogeneity, and high resistance against corrosion.

Residual stress evaluation in selective-laser-melting additively manufactured titanium (Ti-6Al-4V) and inconel 718 using the contour method and numerical simulation

Residual stresses play an important role for the structural integrity of engineering components. In this study residual stresses were determined in titanium alloy (Ti-6Al-4V) and Inconel 718 samples produced using selective-laser-melting (SLM) additive manufacturing. The contour method and a numerical simulation approach (inherent-strain-based method) were used to determine the residual stress distributions. The inherent-strain-based method reduces the computational time compared to weakly-coupled thermo-mechanical simulations. Results showed the presence of high tensile residual stresses at and near the surface of both titanium and Inconel alloys samples, whereas compressive residual stresses were seen at the center region. A good agreement was seen between the results obtained from contour method and the numerical simulation, particularly 1 mm below the surface of the samples.

Effect of Microstructural Characteristics of Thick Steel Plates on Residual Stress Formation and Cracking during Flame Cutting

Thick wear-resistant steel plates are commonly used in demanding conditions, such as in the mining industry. In harsh environments, a high degree of both toughness and hardness is required, which extends the service life of the components but also makes the production of the plates difficult. Flame cutting is a generally applied cutting method in the heavy steel industry since it enables the cutting of thick steel plates at high production rates. However, flame cutting may cause cracks in the cut edge of the steel plates, leading to rejects for the steel industry and end-users. In addition, cutting generates a heat-affected zone at the cut edge, where volumetric and microstructural changes and hardness variations take place. A steep thermal gradient, generated during flame cutting, also produces high residual stresses on the cut edge. The goal of this study is to examine how microstructural features contribute to the residual stress formation and cracking probability of thick steel plates. Specific microstructural features can make the plates prone to cracking and tend to produce undesired stresses during the cutting process. The residual stress profiles of flame-cut specimens were measured using the X-ray diffraction method. In addition, the mechanical properties of steel plates were evaluated. The microstructures of the cut edge and the base material were characterized by electron microscopy. Results indicate that the shape of the prior austenite grains has an effect on both the cracking probability and residual stress formation. Longitudinally oriented prior austenite grain boundaries combined with a high residual tensile stress state provide potential sites for cracking.

The Influence of the Support Structure on Residual Stress and Distortion in SLM Inconel 718 Parts

The effect of support structure and of removal from the base plate on the residual stress state in selective laser melted IN718 parts was studied by means of synchrotron X-ray diffraction. The residual stresses in subsurface region of two elongated prisms in as-built condition and after removal from the base plate were determined. One sample was directly built on a base plate and another one on a support structure. Also, the distortion on the top surface due to stress release was measured by contact profilometry. High tensile residual stress values were found, with pronounced stress gradient along the hatching direction. In the sample on support, stress redistribution took place after removal from the base plate, as opposed to simple stress relaxation for the sample without support. The sample on support structure showed larger distortion compared to sample without support. We conclude that the use of a support decreases stress values but stress-relieving heat treatments are still needed.

R-curve behavior and flexural strength of zirconia-toughened alumina and partially stabilized zirconia composite laminates

A composite-laminate formed by thick layers (~ 320 µm) of zirconia-toughened alumina (ZTA) with thin (~ 50 µm) interlayers of zirconia partially stabilized (Y-PSZ) has been fabricated by tape casting and pressureless sintering. Fracture behavior and strength has been investigated and compared to a “monolithic” reference, e.g. a stack of zirconia-toughened alumina (ZTA) without interlayers. The fracture behavior has been analysed using stable crack growth in V-notched specimens loaded in 3-point bending. The ZTA+Y-PSZ composite laminate presented a rising crack resistance with maximum values between 6 and 14 MPa m1/2. In contrast, the “monolithic” ZTA laminate shows a plateau R-curve behavior at 2.7 MPa m1/2. Several toughening mechanisms were identified in the ZTA+Y-PSZ composite laminate, such as, crack arrest/slow down, micro cracking and bifurcation. These toughening mechanisms are most likely caused by high tensile residual stresses that were estimated theoretically.

Effect of hybrid wire EDM conditions on generation of residual stresses in machining of HCHCr D2 tool steel under ultrasonic vibration

This experimental examination exhibit a novel Hybrid Wire electrical discharge machining (H-WEDM) process of ultrasonic vibration combined with traditional Wire-EDM. The metal cutting for form tool and extrusion die with WEDM is liked to give better surface morphology. The effect of residual stresses has been dissected for H-WEDMed machined surface alongside the surface roughness and erosion rate to enhance surface integrity and longer service life of High Carbon High Chromium D2 tool steel. The process parameters chose for this investigation are type of vibration continuous/discontinues, amplitude of vibration, workpiece dimension, duty cycle, peek current and wire feed rate with objective to optimize the residual stresses and erosion rate. Portable X-ray Residual Stress Analyzer, a non-destructive X-ray analyzer is used to measure the residual stress efficiently by detecting the full Debye ring data from a single incident X-ray angle and Non goniometer stage influence on the measurements. An endeavor was made to compare the residual stresses for continuous/discontinues vibration and without vibration. The impacts of amplitude of vibration, peak current, duty cycle and wire feed rate variations on erosion rate was study using Taguchi method. From experimental study, it was observed that discontinuous vibration enhances the erosion rate and diminishes the resultant stresses. High Peak current and duty cycle altogether deteriorate the surface texture, which creates high tensile residual stresses because of debris and micro cracks. The optimum value of residual stresses 86.53 MPa and material erosion rate 6.45 mm/min was achieved using H-WEDM.

On the Simulation Scalability of Predicting Residual Stress and Distortion in Selective Laser Melting

Rapid heating and cooling thermal cycle of metals in selective laser melting (SLM) generates high tensile residual stress which leads to part distortion. However, how to fast and accurately predict residual stress and the resulted part distortion remains a critical issue. It is not practical to simulate every single laser scan to build up a functional part due to the exceedingly high computational cost. Therefore, scaling up the material deposition rate via increasing heat source dimension and layer thickness would dramatically reduce the computational cost. In this study, a multiscale scalable modeling approach has been developed to enable fast prediction of part distortion and residual stress. Case studies on residual stress and distortion of the L-shaped bar and the bridge structure were presented via the deposition scalability and validation with the experimental data. High residual stress gradient in the building direction was found from high tensile on the surface to high compressive in the core. The part distortion can be predicted with reasonable accuracy when the block thickness is scaled up by 50 times the layer thickness from 30 μm to 1500 μm. The influence of laser scanning strategy on residual stress distribution and distortion magnitude of the bridges has shown that orthogonal scanning pattern between two neighboring block layers is beneficial for reducing part distortion.

Modeling and simulation of weld residual stresses and ultrasonic impact treatment of welded joints

Most structures are fabricated using welded joints because of its low cost, structural strength and geometric flexibility. Welding is considered a highly complex metallurgical process that results in irregular geometries, material imperfections/flaws and tensile residual stresses. High tensile residual stresses and stress concentrations resulting from the weld process have a significant impact on fatigue life of structures, and thus a topic of great concern in product design. Ultrasonic impact treatment (UIT) is regarded as one of the most effective post welding treatment techniques to enhance the fatigue performance of welded structures. The UIT aims to introduce fatigue-beneficial compressive stresses by plastically deforming the weld toe and reduce stress concentrations by modifying local weld geometries. In this study, 3D modeling and simulation using finite element (FE) method has been performed to simulate welding process and numerical modeling of the UIT process to predict weld residual stress distribution of butt and T weld joints. The predicted numerical results under as-welded and UIT treatment conditions were compared to present weld residual stress improvements. Compared results shows that the UIT has potential applications on the fatigue design of welded structures, can lead to lighter structures and products, in which structures can be down-sized and optimized to reduce weights.

Characteristics of Residual Stresses Generated by Induction Heating on Steel Plates

This paper presents experimental and numerical investigations on the characteristics of residual stresses generated by induction heating (IH) on 12 mm thick steel plates. IH at 250 °C and 350 °C provided high tensile residual stresses in the heating field but high compressive stresses away from the heating field. The double heating case generated higher compressive residual stresses—around 200 MPa—than the single heating case because of the superposition of the compressive residual stresses. It will be expected to improve the fatigue performance of welded joints when IH is applied for repair work on existing steel structural members susceptible to fatigue damage. Numerical simulation models for predicting residual stresses by IH were proposed by adopting the body heat flux input and the surface heat flux input. They will be beneficial for identifying the optimum heating conditions, such as the target temperature and the heating field, for applying IH to the actual repair work of the steel structural members.

High cycle fatigue strength assessment methodology considering punching effects

Since a decrease of the fatigue strength may result from punching operations, this study proposes a methodology for designing punched parts against high cycle fatigue crack initiation. To reach this goal, high cycle fatigue tests are performed on different specimens configurations with either punched or polished edges. Due to punching effects, the fatigue strength of punched specimens is significantly decreased. Fracture surfaces observations reveal that crack initiation occurs always on a punch defect. Additional investigations are combined to characterize how the edges are altered by the punching operations. High tensile residual stress levels along the loading direction are quantified using X-Ray diffraction techniques. Furthermore, micro-hardness measurements and X-Ray diffraction results reveals a strong hardness gradient due to punching operation. For a better understanding of crack initiation mechanisms, the edge geometries have been scanned with 3D optical microscopy, allowing us to identify the most critical defect (and its real geometry) by comparing the edges before and after fatigue failure. Finally, FEA are performed on identified defects. A non-local high cycle multiaxial fatigue strength criterion has been used as post-processing of FEA to take into account the effect on the HCF strength of defects and the strong stress-strain gradients around them.

Cutting Zone Temperature Identification During Machining of Nickel Alloy Inconel 718

Abstract Quality of machined surface is affected by quality of cutting process. There are many parameters, which influence on the quality of the cutting process. The cutting temperature is one of most important parameters that influence the tool life and the quality of machined surfaces. Its identification and determination is key objective in specialized machining processes such as dry machining of hard-to-machine materials. It is well known that maximum temperature is obtained in the tool rake face at the vicinity of the cutting edge. A moderate level of cutting edge temperature and a low thermal shock reduce the tool wear phenomena, and a low temperature gradient in the machined sublayer reduces the risk of high tensile residual stresses. The thermocouple method was used to measure the temperature directly in the cutting zone. An original thermocouple was specially developed for measuring of temperature in the cutting zone, surface and subsurface layers of machined surface. This paper deals with identification of temperature and temperature gradient during dry peripheral milling of Inconel 718. The measurements were used to identification the temperature gradients and to reconstruct the thermal distribution in cutting zone with various cutting conditions.

Surface integrity of Inconel 718 by hybrid selective laser melting and milling

ABSTRACTWhile selective laser melting (SLM) offers design freedom of metal parts with much less material consumption, there exist several limitations, including high surface roughness, low-dimensional accuracy, and high tensile residual stresses. To make functional parts with high form accuracy and superior surface integrity, an as-SLM part needs finishing to remove the deposited surface material. The integration of machining and SLM creates a hybrid manufacturing route to overcome the inherited limitations of SLM. However, little study has been done to characterise surface integrity of an as-SLM part followed by machining (e.g. hybrid SLM-milling). In this paper, surface, integrity including surface roughness, microstructure, and microhardness, have been characterised for IN718 samples processed by the hybrid process. It has been found that microhardness varies with the scan direction and the use of coolant in the subsequent milling, and surface integrity can be significantly improved by the hybrid SLM-milling route.