Interaction of welding residual stresses and warm pre-stressing on brittle fracture of a pipe containing an internal semi-elliptical crack

Residual stresses (RS) due to welding process, may change the load bearing capacity of cracked components. These stresses can also affect the benefit of warm pre-stressing (WPS) cycles which are used for improving structure behaviour. RS are obtained from a two-passes welding simulation of a pipe and verified by experiments. A semi-elliptical internal crack at the weld line is considered. Redistribution of RS field after introducing the crack shows a significant tensile RS are remained at the crack tip. Two common WPS cycles, load-cool-fracture (LCF) and load–unload-cool-fracture (LUCF), are applied using the model at room and low temperature subjected to axial loading. Using local approach to fracture shows that welding RS dramatically raise the fracture probability. LCF has more influence on reducing the fracture probability in comparison with LUCF. The interaction of welding RS and WPS cycles still improves the fracture properties, however, welding RS cause to decrease the benefit of WPS. Comparing RS distributions on crack-tip shows that applying WPS cause to release a significant amount of welding RS and therefore, WPS can be very useful for welded structures. The near crack-tip opening stresses at a same fracture load are further studied for all cases.     

Mechanism of effects of warm prestressing(WPS) on apparent toughness of notched steel specimens: Part I: Experimental

In this investigation, a series of experiments are carried out in blunt notched specimens to explore the various factors controlling the warm prestressing (WPS) effect on apparent toughness of a HSLA steel. A great number of specimens were tested using Cool-Fracture (CF) without WPS, Load-Cool-Fracture (LCF) and Load-Unload-Cool-Fracture (LUCF) cycles. More complex cycles have also been used to produce residual stress distributions and notch deformations different in quantities and signs. All fracture surfaces of the specimens were observed. Some specimens were unloaded after WPS and details of microscopic features in front of notch roots were investigated. Experimental results show that warm prestress cycles raising the residual compressive stress and opening the notch root improve notch toughness at low temperatures. Oppositely, WPS cycles raising the residual tensile stress and closing the notch root deteriorate notch toughness. One distinct effect of WPS involves deactivating inclusions and second phases particles. With increasing the preload of WPS, more and more particles being potential cleavage nuclei are decohered and blunted to cavities. This effect is proposed to be involved in improvement of notch toughness.     

INVESTIGATION OF THE ROLE OF RESIDUAL STRESSES IN THE WARM PRESTRESS (WPS) EFFECT. PART II—ANALYSIS

Abstract— Curry's model of the WPS effect has been applied to the results of a previous paper, and is extended to treat warm prestressing in blunt notched test-pieces. The effect of more complex prestress cycles is also predicted by an extrapolation of the model. The effects of the load-cool-fracture, LCF, cycle can be reasonably predicted for both sharply precracked and blunt notched specimens. For the sharply precracked specimens the effects of the load-unload-cool-fracture, LUCF, cycle at — 196°C are consistently overpredicted and this may be due to a decrease in the cleavage fracture stress at — 196°C of the material at the crack tip which has been subjected to repeated plastic straining by the combination of loading cycles. Modifications to the model are suggested which reduce the overproduction but a wide degree of scatter is observed in the experimental observations. Blunt notched specimens show a reasonable correlation between prediction and theory for the tensile LUCF cycle. Problems have been found in predicting the effect of various prestress cycles in different specimens due to the inherent variability in baseline fracture behaviour of the weld metal. It is concluded that the general trend of results is adequately explained by superposition models but that a greater understanding of local flow properties at a crack tip is required to achieve reasonable predictive success for weld metals such as A533BW.     

Effects of void damage induced by warm prestressing (WPS) on cleavage fracture of notched steel specimens

The effects of void damage induced by warm prestressing (WPS) on cleavage fracture of notched steel specimens were studied by experiments and FEM calculations. The results show that the local stress concentration around the voids promotes the cleavage initiation and decreases the notch toughness and cleavage fracture stress. The fibrous cracks ahead of notch tips caused by the ductile tearing in the WPS obviously raise the normal stress in front of their tips and decrease fracture load and notch toughness. When the beneficial effects of WPS on improving apparent fracture toughness for specimens or structures are used, the loads in WPS need to be limited so that no obvious void damage and ductile tearing are produced in front of defects.     

Fracture toughness and crack growth resistance of pressure vessel plate and weld metal steels

Compact tension specimens were used to measure the initiation fracture toughness and crack growth resistance of pressure vessel steel plates and submerged are weld metal. Plate test specimens were manufactured from four different casts of steel comprising: aluminium killed C-Mn-Mo-Cu and C-Mn steel and two silicon killed C-Mn steels. Weld metal test specimens were extracted from five weld joints of Unionmelt No. 2 weld metal. The welds were of double V butt geometry having either the C-Mn-Mo-Cu steel (three weld joints) or one particular silicon killed C-Mn steel (two weld joints) as parent plate. On the upper shelf, a multiple specimen test technique was used to obtain crack growth data which were analysed by simple linear regression to determine the crack growth resistance lines and to derive the initiation fracture toughness values for each test temperature. These regression lines were highly scattered with respect to temperature and it was very difficult to determine precisely the temperature dependence of the initiation fracture toughness and crack growth resistance. The data were re-analysed, using a multiple linear regression method, to obtain a relationship between the materials' crack growth resistance and toughness, and the principal independent variables (temperature, crack growth, weld joint code and strain ageing).     

A local approach to cleavage fracture in ferritic steels following warm pre-stressing

Quantification of the enhancement in cleavage fracture toughness of ferritic steels following warm pre‐stressing has received great interest in light of its significance in the integrity assessment of such structures as pressure vessels. A Beremin type probability distribution model, i.e., a local stress‐based approach to cleavage fracture, has been developed and used for estimating cleavage fracture following prior loading (or warm pre‐stressing, WPS) in two ferritic steels with different geometry configurations. Firstly, the Weibull parameters required to match the experimental scatter in lower shelf toughness of the candidate steels are identified. These parameters are then used in two‐ and three‐dimensional finite element simulations of prior loading on the upper shelf followed by unloading and cooling to lower shelf temperatures (WPS) to determine the probability of failure. Using both isotropic hardening and kinematic hardening material models, the effect of hardening response on the predictions obtained from the suggested approach has been examined. The predictions are consistent with experimental scatter in toughness following WPS and provide a means of determining the importance of the crack tip residual stresses. We demonstrate that for our steels the crack tip residual stress is the pivotal feature in improving the fracture toughness following WPS. Predictions are compared with the available experimental data. The paper finally discusses the results in the context of the non‐uniqueness of the Weibull parameters and investigates the sensitivity of predictions to the Weibull exponent, m, and the relevance of m to the stress triaxiality factor as suggested in the literature.     

Failure mode of laser welds in lap‐shear specimens of high strength low alloy (HSLA) steel sheets

In this paper, the failure mode of laser welds in lap‐shear specimens of non‐galvanized SAE J2340 300Y high strength low alloy steel sheets under quasi‐static loading conditions is examined based on experimental observations and finite element analyses. Laser welded lap‐shear specimens with reduced cross sections were made. Optical micrographs of the cross sections of the welds in the specimens before and after tests are examined to understand the microstructure and failure mode of the welds. Micro‐hardness tests were also conducted to provide an assessment of the mechanical properties in the base metal, heat‐affected and fusion zones. The micrographs indicate that the weld failure appears to be initiated from the base metal near the boundary of the base metal and the heat‐affected zone at a distance away from the pre‐existing crack tip, and the specimens fail due to the necking/shear of the lower left load carrying sheets. Finite element analyses based on non‐homogenous multi‐zone material models were conducted to model the ductile necking/shear failure and to obtain the J integral solutions for the pre‐existing cracks. The results of the finite element analyses are used to explain the ductile failure initiation sites and the necking/shear of the lower left load carrying sheets. The J integral solutions obtained from the finite element analyses based on the 3‐zone finite element model indicate that the J integral for the pre‐existing cracks at the failure loads are low compared to the fracture toughness and the specimens should fail in a plastic collapse or necking/shear mode. The effects of the sheet thickness on the failure mode were then investigated for laser welds with a fixed ratio of the weld width to the thickness. For the given non‐homogenous material model, the J integral solutions appear to be scaled by the sheet thickness. With consideration of the plastic collapse failure mode and fracture initiation failure mode, a critical thickness can be obtained for the transition of the plastic collapse or necking/shear failure mode to the fracture initiation failure mode. Finally, the failure load is expressed as a function of the sheet thickness according to the governing equations based on the two failure modes. The results demonstrate that the failure mode of welds of thin sheets depends on the sheet thickness, ductility of the base metal and fracture toughness of the heat‐affected zone. Therefore, failure criteria based on either the plastic collapse failure mode or the fracture initiation failure mode should be used cautiously for welds of thin sheets.     

Analysis of the unstable fracture behaviour of a high strength low alloy steel weldment

Local brittle zones (LBZ) cause the unstable fracture behaviour of weld metals. This threatens the safe service of welded structures and makes structural assessment procedures difficult. Therefore, the unstable fracture behaviour of an overmatched high strength low alloyed steel weldment was experimentally investigated. It showed that any interaction between two adjacent weld metal matrix and soft weld metal inclusions produces LBZ, causing local unstable fracture behaviour. The formation of a low hardness region is attributed to the multipass welding reheating process between Ac₁ and the self-tempering temperature. The presence of partly solid metallic inclusions with a high content of alloying elements and pro-eutectoid ferrite microstructure were found to be additional causes for the local unstable fracture behaviour of the weld metal. Local strength mis-match induced the yielding and strain hardening in the soft weld metal inclusions, contributing significantly to unstable fracture behaviour. Thus, a significantly different scatter of experimental results can be obtained. In the cases of specimens with through-the-thickness crack, not only is the scatter significantly lower, but the toughness itself.     

Fracture toughness behavior of weldments with mis-matched properties at elevated temperature

High temperature fracture toughness tests were performed on welded specimens of 1Cr-1Mo-14 V steel with different levels of mismatch between the base metal and the weld metal and the cracks lying along the fusion line. A wide range of fracture toughness values were obtained for weldments, as opposed to a unique value of J_IC and a unique J-R curve typically obtained for homogeneous materials. Detailed observations of the crack path within the weldments were made to understand the wide scatter in the fracture toughness behavior. The yield strength mismatch between the base metal and the weld metal was found to directly influence the stable crack path, and hence the fracture toughness behavior. The denomination of ‘apparent fracture toughness’ was used to describe the variability of the fracture toughness in the weld region due to microstructure and mechanical property gradients. The apparent fracture toughness exhibited a minima at a fixed distance from the fusion line for a specific weld. The relative position of the fatigue precrack with respect to the fusion line and the region of low fracture toughness was also shown to influence the measured fracture toughness behavior of the specimen. A frame-work is provided for representing the weld fracture toughness behavior and the associated variability due to microstructural gradients. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ductile fracture mechanisms in shielded metal-arc and gas tungsten-arc welds of Type 347 stainless steels

The ductile fracture behavior of two different welds of Type 347 stainless steel, which are made by SMAW (shielded metal arc welding) and GTAW (gas tungsten arc welding) processes was characterized by J-integral testing and microstructural evaluation techniques. Both welds by SMAW and GTAW processes showed significantly low fracture toughness compared with that of the base metal. Metallographic and fractographic examinations revealed that different micromechanisms are operative in the fracture process of the two welds. In the SMAW weld, the fracture was dominated by void initiation and growth at the inclusions that are homogeneously distributed in the matrix. On the other hand, in the GTAW weld, a large number of Nb(CN) particles precipitated on the austenite/ferrite interface as long rod shapes and the fracture proceeded by void initiation at these particles and accompanying decohesion of the interface. It is recommended that the C and Nb contents be reduced in weld metal itself as well and that the welding atmosphere be controlled.     

The fracture behaviour of intermetallic TiAl alloys with and without warm pre-stressing

The fracture behaviour of near- (NG) and fully lamellar (DFL) intermetallic TiAl alloys with and without warm pre-stressing (WPS) is investigated by testing three point bending specimens with fatigue pre-cracks at room temperature and at 700 °C. Detailed fractographic observations and FEM calculations are carried out to find the critical step of cleavage fracture. The results show that the cleavage fracture is induced by direct propagation of the pre-crack and that the stress at the crack tip is the decisive controlling factor. The WPS process improves the fracture initiation toughness of both the NG and the DFL microstructure, but deteriorates slightly the increase of the K-resistance curves, especially for the DFL microstructure. The compressive residual stress induced by WPS plays the main role in improving the fracture initiation toughness.     

Mechanism of effects of warm prestressing (WPS) on apparent toughness of notched steel specimens Part II: Calculations and analyses

Based on the experimental results of Part I of present work, this paper describes results of FEM calculations and analyses in details which identified that the effect of tensile-warm pre-stressing (WPS) on improvement of the apparent toughness of notched specimens results from three factors i.e. the residual compressive stress, macroscopic blunting of the original notch, and prestrain-deactivating cleavage initiation. The effects of three factors are separated and is effective for each at various extents of prestressing specified with a prestress-ratio, P₀/P_gy, defining the prestressing load P₀ as a fraction of general yield load P_gy. For values of prestress-ratio lower than 1.0, the residual compressive stress acts as the main factor. Between 1.0 to 1.5 of prestress-ratio values, in addition to the residual compressive stress the macroscopic blunting plays increasing role. The effect of the prestrain-deactivating cleavage initiation presents at the prestress-ratio P₀/P_gy1.2. In the case of compressive-warm prestressing, the apparent toughness is deteriorated due to the residual tensile stress. The effects of complex cycles of WPS, with various steps of loading and unloading different in signs, are determined mainly by the loading step just before the fracturing step.     

热输入对E36钢SAW焊缝组织及韧性的影响

为提高建造海洋采油平台的效率、减少生产周期,进而为实际生产提供数据支持,采用3种不同热输入对海洋采油平台用E36钢进行埋弧焊焊接,通过光学显微镜(OM)、透射电镜(TEM)、扫描电镜(SEM)和电子背散射衍射技术(EBSD)对焊缝微观组织及夹杂物形貌进行了观察,研究了不同热输入对焊缝组织及韧性的影响,并分析了不同热输入对焊缝夹杂物尺寸分布和成分的影响.结果表明:热输入为50 k J/cm时,焊缝金属韧性较好;随着焊接热输入的增加,焊缝的冲击韧性降低,但仍能满足性能指标,焊缝金属中夹杂物成分相差较大,有效夹杂物数量减少,焊缝金属中大角度晶界比例减少;对于E36钢,热输入为160 k J/cm时不仅能使韧性符合要求还能提高生产效率.     

INVESTIGATION OF THE ROLE OF RESIDUAL STRESSES IN THE WARM PRESTRESS (WPS) EFFECT. PART I—EXPERIMENTAL

Abstract— The role of residual stresses in the warm prestress (WPS) effect has been investigated. Three types of specimen have been tested in this investigation: smooth uniaxial tensile specimens, blunt notched single edge notched bend (SENB) specimens and sharply precracked SENB specimens. Room temperature prestraining of uniaxial tensile specimens leads to a dramatic decrease in the measured nominal fracture stress at — 196°C. Such an embrittling effect may be expected to reduce the beneficial increase in subsequent fracture toughness commonly observed in WPS sequences. The blunt-notched specimens were prestressed in tension and compression. Compressive prestressing was found to lead to a decrease in subsequent fracture load whereas tensile prestressing leads to an increase. The load decrease following a compressive WPS was greater than the load increase following a tensile WPS. Various sequences of loading, unloading and cooling have been investigated and the differences in the subsequent fracture behaviour of specimens have been explained qualitatively by superposition arguments. The theories of Chell and Curry have been supported by the general trend of results.     

Effect of laser beam welding on fracture toughness of a Ti-6.5Al-2Zr-1Mo-1V alloy sheet

Investigation of fracture toughness on Ti-6.5Al-2Zr-1Mo-1V alloy thin sheet and its laser-welded joints has been carried out. In the test compact tension (CT) specimens and single specimen technology were used. In addition, hardness distribution and microstructure of the welded joints were examined. Fracture test indicates that brittle unstable fracture occurs after slow crack propagation for all the specimens, except that one heat affected zone (HAZ) specimen is brittle crack initiation. It is found that rolling directions have no obvious effect on fracture toughness of base metal. Moreover, fracture toughness of weld metal is obviously decreased in comparison with base metal whatever in as-welded condition or in stress relief condition. Post-weld heat treatment (PWHT) leads to fracture toughness of the welds further decreasing. Fractography observation shows that the fracture mode is predominantly dimpled in base metal. However, there exists intergranular fracture in the weld metal. Thus, the transition of fracture mode from both base metal and HAZ to weld metal may lead to dramatic decrease in fracture toughness. Microstructure examination reveals that the microstructure of weld metal consists of large grains with fine acicular structure. The formation of fine α acicular structure is due to rapid cooling during laser welding. After PWHT, the acicular structure is coarsened.     

BT20钛合金激光焊接接头的断裂韧性研究

对BT20钛合金及其激光焊接接头的断裂韧性进行了研究.同时分析了合金及激光焊接接头的硬度分布及显微组织.断裂实验表明,除了一个焊接接头紧凑拉伸(CT)试样是脆性启裂外,其它CT试样均在裂纹延性启裂并缓慢扩展后,发生脆性失稳断裂.母材的断裂韧性明显高于焊接接头,轧制方向对母材断裂韧性的影响不明显.焊接热影响区的断裂韧性介于母材和焊缝金属之间.本研究采用的焊后热处理没有改善焊接接头的断裂韧性,还有进一步恶化的趋势.添加活性剂对焊缝金属的断裂韧性没有明显作用,但对延性裂纹扩展长度有所改善.     

PREDICTION OF THE COMPETITION BETWEEN SURFACE AND INTERNAL FATIGUE CRACK INITIATION IN PM ALLOYS

Abstract— Low cycle fatigue (LCF) testing of powder metallurgy (PM) superalloys has determined that LCF cracks initiate from defects such as pores and ceramic inclusions located either at the surface or internally. The ratio of surface-to-internal crack initiation sites depends upon experimental conditions. In the current work, a probabilistic model is presented that describes the competition between these two failure mechanisms from a single experimental parameter, μ. The model predicts the size distribution of defects which initiate LCF cracks, as well as the proportion of specimens which fail as a result of internally located defects. Model predictions were found to correlate well with experimental LCF data obtained from a N18 PM Ni-base superalloy. The model further enables evaluation of a "size effect", as it predicts that internal crack initiation is favoured for larger components/specimens.     

A notched cross weld tensile testing method for determining true stress-strain curves for weldments

Cross weld tensile testing is widely used in the industry to qualify welds. In these conventional testing fracture load is measured and the location of fracture (weld metal, base metal or heat affected zone) is evaluated. Because the load-elongation curve depends on the location of fracture and the initial gauge length, it cannot be utilized in the failure assessment of weldments. Failure assessment of weldments requires input of true stress-strain behaviour for each material zone. In this paper, a notched cross weld tensile testing method is proposed for determining the true stress-strain curve for each material zone of a weldment. In the proposed method, cylindrical cross weld tensile specimens, with a notch located either in the weld metal, base metal or possibly heat affected zone are applied. Due to the notch, plastic deformation is forced to develop in the notched region. A load versus diameter reduction curve is recorded. It has been shown that the true strain at maximum load is independent of the notch geometry. Furthermore, the materials true stress-strain curve can be determined from the recorded load versus diameter reduction curve of a notched cross weld tensile specimen by dividing a geometry-factor G, which is approximated by a quadratic function of the specimen diameter to notch radius ratio and a linear function of the true strain at the maximum load. It is found that G is independent of the material zone length when the homogenous material length is larger or equal to the minimum diameter.     

Low cycle fatigue behaviour of particulate reinforced metal matrix composites

The low cycle fatigue (LCF) resistance of two different 6061 Al/20 vol% alumina particulate metal matrix composites (MMCs) in a peaked-aged condition has been evaluated under fully reversed strain control testing. Test results were combined with scanning electron and optical microscopy investigations to determine the effects of reinforcement particles and strain amplitude on the LCF behaviour of these MMCs. Both materials show three stages of response to LCF: initial fast hardening or softening in the first few cycles; gradual softening for most of the fatigue life; and a rapid drop in the stress carrying capability prior to failure. Both MMCs exhibit short LCF life which follows a Coffin-Manson relationship. All tested specimens demonstrate ductile fracture morphology at final failure. The experimental results are discussed in respect of strain amplitude, matrix composition and reinforcement shape and crack initiation.     

Non-metallic inclusions and their relation to the J-integral, Ji, phys, at physical crack initiation for different steels and weld metals

For two types of steel, the heat-resistant 12% Cr–steel X 20 CrMoV 12 1 and the fine-grained pressure vessel steel 20 MnMoNi 5 5 (A 508 cl.3), a wide range of toughness values in the upper shelf was realized by different sulfur contents and the inclusion of similar weld metals. Additionally, the pressure vessel steel 22 NiMoCr 3 7 (A 508 cl.2) was also investigated. Tensile, fracture mechanics and Charpy V-notch impact tests, as well as detailed microstructural investigations with respect to the size distribution and density of non-metallic inclusions and precipitates, were carried out. In order to ensure ductile behaviour, a test temperature of 150 °C was chosen. The relevance of two quantitative relations available for the calculation of the J-integral, Ji, phys, at physical crack initiation using tensile test data and microstructural parameters, were examined by comparison with the corresponding experimental Ji, phys-values. Only one quantitative relation was able to give good agreement between calculated and experimental Ji, phys-values. This holds not only for the base materials but also for the weld metals. The importance of the size and density of the non-metallic inclusions became quantitatively obvious with the consequence that their size times density is a decisive parameter for toughness. Observations of void initiation, growth and coalescence illustrate the fracture process. © 1998 Chapman & Hall