Constraint loss correction for assessment of CTOD fracture toughness under welding residual stress. Part I: Methodology using the equivalent CTOD ratio

This part I of a two-part paper presents a method of assessing the effects of welding residual stress and constraint loss on the cleavage fracture of a wide plate subjected to membrane stress based on the Weibull stress criterion. It has been found that the Weibull stress criterion is efficient for evaluating the fracture instability of wide plates with and without a welding residual stress field. The concept of an equivalent crack-tip opening displacement (CTOD) ratio β_r under a welding residual stress field is introduced for assessment of constraint loss effects on CTOD fracture toughness of wide plates. The equivalent CTOD ratio β_r is defined as the ratio of the CTOD in the standard fracture toughness specimen to the CTOD in a wide plate with a welding residual stress at the same level of the Weibull stress. Fracture assessment procedures using β_r for wide plates are shown within the framework of the failure assessment diagram. It has been found that the excessive conservatism observed in the conventional procedure can be reduced reasonably by applying the proposed method. The companion part II of the paper presents applications of the CTOD toughness correction method using β_r to the fracture test data of welded joints and verifies the proposed method in the ductile-brittle transition temperature region.     

Method of constraint loss correction of CTOD fracture toughness for fracture assessment of steel components

This paper presents a procedure for transferring the CTOD fracture toughness obtained from laboratory specimens to an equivalent CTOD for structural components, taking constraint loss into account. The Weibull stress criterion is applied to correct the CTOD for constraint loss, which leads to an equivalent CTOD ratio, β, defined as β = δ/δ_WP, where δ and δ_WP are CTODs of the standard fracture toughness specimen and the structural component, respectively, at the same level of the Weibull stress. The CTOD ratio β is intended to apply to the fracture assessment of ferritic steel components to stress levels beyond small-scale yielding. Nomographs are given to determine the β-value as a function of the crack type and size in the component, the yield-to-tensile ratio of the material and the Weibull shape parameter m. Examples of the fracture assessment using β are shown within the context of a failure assessment diagram (FAD). An excessive conservatism observed in the conventional procedure is reduced reasonably by applying the equivalent CTOD ratio, β.     

Part-through fracture toughness (KIe) and crack-tip opening displacement (CTOD) of welded joints of AA2014-T6 at low temperatures

The part-through fracture toughness (K_Ie) and crack-tip opening displacement (CTOD, δ_m) of welded joints of aluminum alloy (AA) 20l4-T6, including the weld metal, the fusion zone (FZ), the heat-affected zone (HAZ), and the base material, were investigated at both liquid nitrogen temperature and liquid helium temperature with surface-crack tension (SCT) specimen and single-edge-notched bend (SENB) specimen respectively. Results indicate a conventional fusion welding process leads to formation of second-phase precipitations and inclusions, which cause significant reduction of fracture resistance at the weld metal and the FZ by fractographic analyses of fractured surface.     

Fracture toughness of Friction Hydro-Pillar Processing welding in C–Mn steel

This investigation focuses on the analysis of fracture toughness characteristics of Friction Hydro-Pillar Process (FHPP) welded joints in C–Mn steels. Crack tip opening displacement (CTOD) tests were performed to analyze the influence of axial loads applied during welding on fracture toughness. For this evaluation compact tension C(T) specimens were used with notches located at the bonding line of the welded specimens. The welding process produced a significant reduction of toughness independent of the axial force applied. Fractographic analysis indicated that the fracture process occurred by microvoid coalescence. The variation in the morphology of the prior inclusions resulting from welding was responsible for the observed decrease in toughness.     

Fracture toughness of ISO 3183 X80M (API 5L X80) steel friction stir welds

Friction stir welding (FSW) is a solid-state joining process with numerous advantages such as good dimensional stability and repeatability, which is widely used Al alloys and with a great potential for critical joining applications involving high melting temperature alloys. Twelve millimeter thick plates of ISO 3183 X80M (API 5L X80) steel was friction stir welded using two passes on both sides of the plate using ceramic tools. Different heat inputs were obtained using a fix travel (welding) speed in combination with several spindle speeds. The fracture toughness of the two-pass joints was evaluated at 25 °C using the critical crack tip opening displacement (CTOD_m), revealing that joints produced with lower spindle speeds presented higher toughness at the heat-affected zone (HAZ) and stir zone (SZ), which are comparable with the base metal (BM) toughness. On the other hand, joints produced using higher spindle speeds presented low fracture toughness at the SZ and elevated CTOD_m toughness at the HAZ. The joints produced with low spindle speeds showed CTOD_m-values above the offshore standard (DNV-OS-F101) requirements.     

高强船体钢双丝埋弧焊焊接接头CTOD试验研究

根据GB／T2358—1994标准,对16ram厚的高强船体钢焊接接头的焊缝中心-40℃的裂纹尖端张开位移（CTOD）进行测试,绝大部分试样的断裂韧性值是有效合格的,其CTOD值大于0．15mm,符合DNV验收标准。试验结果表明,在给定的双丝埋弧焊接工艺下,该钢种焊缝低温韧性好。该钢种中等厚度双丝埋弧焊焊接接头可以在不进行焊后热处理的情况下使用。     

Fracture toughness of unidirectional fiber-metal laminates: crack orientation effect

Fiber-metal laminates (FMLs) are structural composites developed for aeronautical applications. The application of FMLs to structures demands a deep knowledge of a wide set of properties, including fracture toughness. The objective of this work was to evaluate the effect of crack orientation on the fracture toughness (critical J-integral and CTOD δ₅) of unidirectional FMLs. Small C(T) and SE(B) specimens with notches parallel and perpendicular to the fibers direction were tested. A study of the relation and equivalence between J_C and δ_5C, which heavily depend on the yield strength and on the stress state, was performed motivated by apparently contradictory experimental results. These results can be explained by the direction-dependent yielding properties of unidirectional FMLs. The best overall equivalence between J_C and δ_5C was obtained considering plane stress state and using the effective yield strength, both for unidirectional FMLs notched parallel and perpendicular to the fibers direction.     

Fracture toughness and microstructure of multipass welds in a Q.T. C-Mn microalloyed steel

An assessment of the fracture toughness above the ductile-brittle transition temperature and microstructure was made for welded joints in a Q.T. C-Mn steel in the “as welded” condition. J integral and COD values were determined for two different M.M.A. electrodes and for a wire-flux combination for S.A. welding. A comparison was made between J integral results obtained using the standard ASTM and Sumpter-Turner procedures and J-COD relationships were also analyzed.     

Analysis of the geometry dependence of fracture toughness at cracking initiation by comparison of circumferentially cracked round bars and SENB tests on Copper

In the first part of the paper, the use of circumferentially cracked round bars (CRB geometry) for characterizing fracture toughness of a ductile material, namely copper, is assessed experimentally through a comparison with the single edge notched bend (SENB) geometry. The J _R curve method with multiple-specimens was applied, but, as unstable cracking appeared very early in the CRB specimen, an engineering definition of fracture toughness was not pertinent. Unloaded specimens were analyzed metallographically to determine the CTOD at physical cracking initiation. The fracture toughness measured using the CRB geometry was 50% larger than using the SENB geometry. The second part of the paper aims at justifying this difference of fracture toughness at cracking initiation. Finite element simulations revealed a slightly higher constraint in the SENB specimens. The main difference between the two specimen geometries lies in a 50% larger extension of the finite strain zone with respect to the CTOD in the case of the SENB specimens. Based on the observation that, in the studied material, the critical CTOD is one order of magnitude larger than the void spacing, we conclude that the geometry dependence of the fracture toughness is caused by the difference in the finite strain zone extension rather than by a stress triaxiality effect.     

Recent developments in local concepts of fatigue assessment of welded joints

Several lately proposed modifications or variants of the structural stress or strain concepts, of the notch stress or strain concepts (also termed ‘local stress or strain concepts’) and of the fracture mechanics concepts of fatigue assessment of welded joints are reviewed, whereas the wider context is presented in a recently republished and actualised standard work. The structural stress concepts described first are based on a linearisation of the stress distribution across the plate thickness or along the anticipated crack path and, alternatively, on the structural stress 1 mm in depth below the weld toe. The structural stress is defined and set against design S–N curves. A further structural stress concept is presented for welded joints in thin-sheet steels and aluminium alloys. Among the elastic notch stress concepts, the variant with the reference notch radius, ρ_r = 1 mm, recently verified also for welded joints in aluminium alloys with plate thicknesses t ? 5 mm and the variant with a small-size reference notch radius, ρ_r = 0.05 mm, applicable to welded joints in thin-sheet materials, are outlined. The elastic–plastic notch strain concept is applied to a spot-welded tensile-shear specimen starting from a small-size keyhole notch at the nugget edge. The novel notch stress intensity factor (NSIF) approach relating to crack initiation and extrapolated to final fracture of seam-welded joints in steels and in aluminium alloys is reviewed. A more recently developed crack propagation approach for spot welds is finally described.     

Measurement of fracture initiation toughness and crack resistance in instrumented Charpy impact testing

A new method has been developed involving direct measurement of the load-line displacement during instrumented Charpy testing. The method uses a laser interferometer to measure displacement in addition to the load-line displacement derived from the load signal. Tests were conducted using fatigue precracked and V-notched test pieces in the temperature range +23°C to −80°C on a conventional ship grade steel, a pressure vessel steel and two welded joints. Good correlation was found between the J_0.2 initiation fracture toughness determined by the multi-specimen method and the J_i fracture toughness determined from single specimens using the new method to detect ductile fracture initiation.     

Fatigue life prediction based on crack closure for 6156 Al-alloy laser welded joints under variable amplitude loading

A fatigue prediction approach is proposed using fracture mechanics for laser beam welded Al-alloy joints under stationary variable amplitude loading. The proposed approach was based on the constant crack open stress intensity factor in each loading block for stationary variable amplitude loading. The influence of welding residual stress on fatigue life under stationary variable amplitude was taken into account by the change of crack open stress intensity factor in each loading block. The residual stress relaxation coefficient β = 0.5 was proposed to consider the residual stress relaxation for the laser beam welded Al-alloy joints during the fatigue crack growth process. Fatigue life prediction results showed that a very good agreement between experimental and estimated results was obtained.     

A Ductile Tear Fracture Analysis of Lap Welded Joints

Numerical analysis of the ductile fracture of lap welded joints has been performed for a standard CT specimen. Mono-, bi- and trimetallic CT configurations were studied in order to compare the J-integral and CTOD global approaches with a local approach (Rice and Tracey model). It is found that the above change in configuration has no impact on evolution of the global parameters, while the void growth ratio R/R₀ is very sensitive to the stress and strain fields around the crack tip. Furthermore, using the parameter R/R₀ distribution at the crack tip, predicted the crack propagation direction in the case of lap welded joints.     

Structure parameters governing fracture toughness of engineering materials

Conclusions 1. Of the two concepts of brittle fracture of engineering materials based on the decohesion and coalescence fracture mechanisms, respectively, the first reflects the threshold fracture toughness for materials of perfect structure, the second — the minimal fracture toughness of the material of a real structure. Since the coalescence fracture mechanism is observed with most engineering materials and requires higher fracture energy and the decohesion mechanism is a part of the coalescence mechanism, it is necessary to investigate both of them in order to study the nature of the fracture process and to optimize the material structure. 2. The model parameters on which the coalescence fracture mechanism is based, namely, characteristic distance X_c and microcleavage stress _f ^* , are directly related to the material minimal fracture toughness and are defined by the weakest elements of its microstructure. 3. Rigorous physical interpretation of the characteristic distance and microcleavage stress requires statistical (dimensional and orientation) consideration, yet modeling of the fracture process in mean values of the above parameters seems to be useful. 4. Fracture toughness dependences on the temperature and loading rate both for a number of ceramic materials and for steels in the brittle-to-ductile transition region have much in common. For this reason, it is possible to use some fracture models, and, in particular, the K_µ-model, to analyze fracture of ceramic materials and to optimize their structure. 5. The main ways of enhancing fracture toughness of engineering materials are associated not only with the plasticization of the latter but also with the creation of such structures that would contribute to an increase of their minimal fracture toughness values. This can be achieved by increasing each of the two fundamental parameters of the material fracture micromechanism: characteristic distance and cleavage stress.Institute of Strength Problems, Ukraine Academy of Sciences, Kiev. Université de Metz. Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 29, No. 3, pp. 113–123, May–June, 1993.     

Quantitative Bewertung von Werkstoffinhomogenitäten auf der Basis der Bruchmechanik

Today it is impossible to quantitatively judge defects of welded joints, because a definite connection between defect and failurestress does not exist. Based on fracture mechanics a theoretical connection is produced between the portion of defects in a welded joint and the failurestress of such a welded component (plate or pipe). It Proved right that the failure-stress is dependent upon both the ratio K_c/σ* (K_c = fracture toughness; σ* = flow stress) and the component-size, whereas the allowable portion of defects rise with increasing K_c/σ* However, the allowable portion of defects decrease with increasing component-size.     

Microstructural and mechanical characterization of electron beam welded Al-alloy 7020

The electron beam (EB) welding process is used to weld any metal that can be arc welded with equal or superior weld quality. EB welding is carried out in a high-purity vacuum environment, which results in freedom from impurities such as oxides and nitrides. Thus, pore-free joints can readily be achieved in metallic materials, such as Al-alloys and Ti-alloys. However, autogenous EB welding of some aluminium alloys leads to a significant strength reduction (undermatching) in the fusion zone due to the loss of strengthening phases. For such Al-alloys, the local microstructure-property relationships should be established to satisfy the service requirement of a welded component with strength undermatching. Autogenous EB welding was performed on 5 mm thick aluminium alloy 7020 plate. Microstructural characterization of the weld metals was made by optical and scanning electron microscopy. Extensive microhardness measurements were conducted in the weld regions of the joints which exhibited a hardness loss in the fusion zone due to the loss of strengthening phases. Tensile properties of the joints were determined by testing flat transverse tensile specimens at room temperature without machining the weld profiles. Furthermore, elastic-plastic fracture toughness tests (CTOD) were carried out on the base material and welded joints at room temperature.     

Predicting fatigue crack growth rate in a welded butt joint: The role of effective R ratio in accounting for residual stress effect

A simple and efficient method is presented in this paper for predicting fatigue crack growth rate in welded butt joints. Three well-known empirical crack growth laws are employed using the material constants that were obtained from the base material coupon tests. Based on the superposition rule of the linear elastic fracture mechanics, welding residual stress effect is accounted for by replacing the nominal stress ratio (R) in the empirical laws by the effective stress intensity factor ratio (R_eff). The key part of the analysis process is to calculate the stress intensity factor due to the initial residual stress field and also the stress relaxation and redistribution due to crack growth. The finite element method in conjunction with the modified virtual crack closure technique was used for this analysis. Fatigue crack growth rates were then calculated by the empirical laws and comparisons were made among these predictions as well as against published experimental tests, which were conducted under either constant amplitude load or constant stress intensity factor range. Test samples were M(T) geometry made of aluminium alloy 2024-T351 with a longitudinal weld by the variable polarity plasma arc welding process. Good agreement was achieved.     

Fracture toughness analysis of laser-beam-welded superalloys Inconel 718 and 625

In this study, two 3.2‐mm thick Ni‐base superalloys, Inconel 718 and 625, have been laser‐beam‐welded by a 6‐kW CO₂ laser and their room temperature fracture toughness properties have been investigated. Fracture toughness behaviour of the base metal (BM), fusion zone (FZ) and heat affected zone (HAZ) regions was determined in terms of crack tip opening displacement (CTOD) using compact tension‐type (C(T)) specimens. Laser‐beam‐weld regions showed no significant strength overmatching in both alloys. Ductile crack growth analysis (R‐curve) also showed that both materials exhibited similar behaviour. Compared to the BM there is a slight decrease in fracture toughness of the fusion and the HAZ.     

海管焊接接头全壁厚裂纹尖端张开位移试验

裂纹尖端张开位移(CTOD)试验是深海管线管的一个重要试验项目,试验通常使用矩形横截面试样,由于该试样是截取焊接接头的一部分,并不能很好地反映整个焊接接头的断裂韧度。根据BS 7448:Part2:1997的要求制备全壁厚试样并在-10℃下做低温CTOD试验,结果与矩形横截面试样的结果作比较,发现全壁厚试样的断裂韧度明显优于矩形横截面试样的断裂韧度,分析发现结果与焊缝的应力状态、化学成分和显微组织的影响有关。全壁厚试样能更真实地反映焊接接头的真实断裂韧度,更好地指导工程设计。