Effect Of Strength Mismatch Research Articles

Characterisation of electron beam welded aluminium alloys

Electron beam (EB) welding was performed on three different aluminium alloys, namely alloys 2024, 5005, and 6061 (plate thickness 5 mm except alloy 5005 which was 3 mm in thickness), to establish the local microstructure–property relationships that would satisfy the service requirements for an electron beam welded aluminium alloy component with weld zone strength undermatching. Microstructural characterisation of the weld metals was carried out by optical and scanning electron microscopy. A very low level of porosity was observed in all EB welds owing to surface cleaning before welding and the vacuum environment of the EB welding process. Extensive microhardness measurements were also conducted in the weld regions of the joints. Global tensile properties and fracture toughness properties (in terms of crack tip opening displacement, CTOD) of the EB joints were determined at room temperature. The effects of strength mismatch and local microstructure on fracture toughness of the EB joints are discussed. The purpose of the present paper is to report the partial results of the European Brite–Euram project ASPOW (assessment of quality of power beam weld joints; BRPR–CT95–0021), which has been undertaken predominantly by industrial companies to establish a European framework for destructive and non-destructive testing and assessment criteria for laser and electron beam welds of over 20 metallic materials.

Application of local approach to inhomogeneous welds. Influence of crack position and strength mismatch

In steel welds there is often a large variation in fracture toughness and mechanical properties between the weld metal, base material and the various heat affected zone (HAZ) microstructures. The stress field in front of a crack in a weldment can be noticeably affected by the strength mismatch between the weld metal, HAZ and the base material. The crack position relative to the various microstructures will clearly influence the strength mismatch effect. In this paper the influence of crack tip positioning on the fracture performance of strength mismatched steel welds has been studied both experimentally and by FEM analysis. For a mismatched weld with local brittle zones small changes in crack tip location can give considerable changes in the fracture performance of a CTOD specimen. A high degree of strength mismatch increases the effect of crack positioning. Weld metal overmatch increases the stress level in the heat affected zone due to material constraint and thereby reduces the cleavage fracture resistance of the weldment when the coarse grained HAZ (CGHAZ) controls the fracture. The detrimental effect of high overmatch is most pronounced for specimens with notch position at fusion line and a short distance into the brittle CGHAZ. The Weibull stress has been shown to be a suitable fracture parameter in the case where one microstructure clearly controls the cleavage fracture and the calculation of the Weibull stress therefore can be limited to this zone.

The Effect of Strength Mis-Match on Mechanical Performance of Weld Joints

This paper presents numerical studies on strength mis-match effects in welded joints. Crack growth in a mis-matched single edge notched specimen under pure bending, with a crack lying at the center line of the weld metal, is simulated via a two-dimensional plane strain finite element analysis (FEA). The fracture process is modeled using a cohesive zone model (CZM). The work is focussed on the effects of yield strength mis-match as well as thickness of the weld metal on fracture resistance and load-deformation for both under- and overmatched specimens. Weld metal mis-match is achieved by keeping the same weld metal and changing the strength of the base metal.

Effect of strength mismatch on fully plastic fields in dissimilar joints under combined loading

Via detailed finite clement limit analyses, plastic limit loads, rotation factors, and crack-tip stress field are investigated for a combined tension and bending of a plance strain single-edge-cracked bimaterial specimen. Limiting bimaterial specimens are considered, consisting of an clastic/perfectly plastic material bonded to an elastic material having the same elastic properties. The limit loads of bimaterial specimens are shown to be very close to those of homogeneous specimens, so that limit load information for homogeneous specimens can be used even for bimaterial specimens. A tractable, approximate elliptical yield locus is proposed, which first the FE. results within 1% for all ranges of tension-to-bending ratios. The plastic rotation factor of bimaterial specimens can be higher than that of homogeneous specimens as much as 25%, when the specimen is subject to small tensile forces. Results from the present analysis is applied to the analysis of typical fracture testing specimens such as compact tension specimens. For both homogeneous and bimaterial specimens, larger tensile forces are associated with substantial loss of crack-tip constrait. Bimaterial specimens have as much as 2 times higher constraint than homogenous specimens, due to plastic strength mismatch. Tractable closed form approximations for crack-tip stresses are proposed in terms of tension-to-bending ratio.

Geometry effect of welded joints on failure assessment curves

Abstract In the present work effect of weld width parameter, h c , on the J -integral and the failure assessment diagram (FAD) has been investigated. The FAD in this studies is based on the CEGB R6 procedure option 3. In the investigation center-cracked tensile specimens with mismatched welds were used. Crack size parameter, a W , was equal to 0.1 and 0.5. The values of h c was changed from 0.1 to 0.5. Weld strength mismatch factor, M , was selected to be 0.8, 1.0 and 1.2. The base material investigated was A508C13 steel. Numerical computation was performed using an ABAQUS two-dimensional elastic-plastic finite element analysis code. The results have indicated that the weld width parameter has evident influence on the J -integral and the FAD in the elastic-plastic loading condition. With the increase in the weld width parameter, the J -integral of undermatched joints are increased, while the J -integral of overmatched joints are decreased. In the meantime, with the increase of h c , the safety region of the FAD is reduced for the undermatched joints and expanded for the overmatched joints. Moreover, the results display the effects of strength mismatch and crack depth on the FAD. The safety zone of the FAD clearly expands for the overmatched joints and contracts for the undermatched joints. No matter what mismatched joints or homogeneous materials, the safety margin of the FAD is expanded for the specimens with deep crack. Thus, the weld geometry should be considered, when one aims to construct the expression of J -integral estimation or to carry out the integrity assessment of welded structures using the FAD.

Stress/strain behaviour at stable crack tips and effect of strength mismatch on crack growth resistance: Study on evaluation of stable crack growth resistance of mismatched welded joints (1st Report)

Stable crack resistance (R) curves are frequently used to evaluate the integrity of structures containing defects in various loading regimes. The R curves are treated as being inherent characteristics of the materials themselves, though they are known to be affected in practice by a variety of factors. This paper describes an investigation of effect of strength mismatch on stable crack growth resistance. To compare the R curves of bimaterial specimens with those of homogeneous ones, compact tests were conducted. The results of the experiments were used to perform three-dimensional FEM analyses with regard to stable crack growth. In the overmatched specimens (high-strength region including a notch surrounded by a low-strength region), the dCTOD/da value is greater than that of the homogeneous specimens. In the undermatched specimens (opposite of the previous case), the dCTOD/da value is smaller than that of the homogeneous specimens. The strength mismatch affects the CTOD(J)-R curves. The equivalent plastic strain at the stable crack tip, however, is shown by the three-dimensional elastic-plastic analyses to be generally independent of the strength mismatch during the ductile fracture process.

Study of stable crack growth resistance based on local displacement at the crack tip: Study of evaluation of stable crack growth resistance of mismatched welded joints (3rd Report)

Summary The previously reported numerical results of stable crack growth simulations (three-dimensional FEM analysis) using equivalent plastic strain show good agreement with the experimental results despite heterogeneous specimens being tested. The results show that the equivalent plastic strain is a characteristic value not susceptible to the effect of strength mismatch in the ductile fracture process. To determine the equivalent plastic strain, however, it is necessary to perform three-dimensional FEM calculations, whereas the local displacement is satisfactorily obtainable by experimental measurements regardless of numerical simulations. The local displacement is affected by the measurement distance from the initial crack tip. The value of the local displacement at the shifted location of the crack tip does not change with an increase in stable crack growth. The authors also specify a similar relationship between the equivalent plastic strain and stable crack growth in a previous publication. The local displacement measured at the mid-thickness is not susceptible to the effect of strength mismatch, but such measurements are never easy to perform. When, for practical purposes, the local displacement is measured on the specimen surface, the effect of strength mismatch on the ratio of the local displacement at the specimen surface to its value at the mid-thickness should be determined. The strain estimated by the local displacement can be used as a substitute for the equivalent plastic strain. Moreover, it expresses the stable crack growth resistance of mismatched joints in terms of the local displacement measured on the specimen surface.

Local Approach to Strength Mis-Match Effect on Cleavage Fracture of Notched Material

This paper discusses the strength mis-match effect on cleavage fracture of notched materials based on the local approach. The aim of the study is to verify the independence of the critical Weibull stress at fracture on the strength mis-match condition. Diffusion bonded joints were made with two materials S and H different in strength level. Two types of 3-point bend specimen were extracted. One had a notch in the lower strength material S near the bonded interface, and another in the higher strength material H. Homogeneous specimens were also made for each material. The critical CTOD value at cleavage fracture for the overmatched specimen with a notch in the material S was smaller than that of the homogeneous specimen of the material S. By contrast, the undermatched specimen with a notch in the material H showed apparently larger critical CTOD value than the homogeneous specimen of the material H. The strength mis-match effect on the CTOD results was explained in terms of the constraint effect on the near-tip stress field. Namely, the near-tip stress is elevated by strength overmatching and relaxed by strength undermatching. The Weibull stresses for the homogeneous and mis-matched specimens were evaluated in the light of the near-tip stress fields. The critical Weibull stress at fracture did not depend on the strength mis-match condition near the notch. By using the critical Weibull stress obtained for the homogeneous specimen, the strength mis-match effect on the CTOD results can be predicted. Close agreement was observed between the critical CTOD value predicted and that obtained by the experiment.

Equivalent CTOD Concept Based on the Local Approach and Its Application to Fracture Performance Evaluation of Welded Joints

In this study, an equivalent CTOD concept was proposed on the basis of the local approach. This concept was born to consider the transferability of 3-point bend CTOD results to fracture performance evaluation of structural components. The equivalent CTOD was defined as the CTOD at which the 3 -point bend specimen and the structural component provide the compatible Weibull stress. The fracture performance of a wide plate with a surface notch predicted by the equivalent CTOD concept was consistent with the experimental result. On the other hand, the analysis based on the conventional CTOD concept gave very conservative estimation of the fracture performance of the wide plate.A new procedure for fracture toughness requirement was presented based on the equivalent CTOD concept. It was pointed out that the equivalent bend CTODδR3P to meet the required deformability of the wide plate was much smaller than the required CTOD δRWPfor the wide plate. The equivalent CTOD concept was effective to quantify the strength mis-match effect between the base and weld metals on the required fracture toughness of the weld metals. The required weld metal CTOD δδRWP for the wide plate increased with decreasing the yield strength mis-match ratio σWY/σBY between the base and weld metals. By contrast, the required bend CTOD δR3Pof the weld metal was not always affected very much by the mis-match condition in welds.

Effect of Strength Mis-Match on Fracture Mechanical Behavior of HAZ-Notched Weld Joint.

Strength mis-matching between the base and weld metals exerts a marked influence on the deformation behavior as well as on the fracture resistance of the weld HAZ. HAZ-notched CTOD test results for high strength steel welds were discussed with attention to mis-matching effect. Stress fields in the HAZ were addressed by 3D FEM with a model having a notch at the HAZ/weld metal boundary. Strength overmatching of the weld metal brings about different effects depending on the toughness level of the HAZ. In the case of the HAZ with moderate toughness, strength overmatching results in crack path deviation toward a softer material due to asymmetrical plastic deformation. On the other hand, for the embrittled HAZ, the overmatch condition lowers the critical CTOD value of the HAZ. This is attributed to elevation of the local stress in the HAZ by constraint effect of the overmatched weld metal. The geometry of the test specimen also affects the degree of crack tip constraint. The deep notch bend specimen produces the largest geometrical constraint. The fracture resistance of the weld HAZ is controlled by the combined effects of strength mis-matching and the specimen geometry. The geometrical constraint in the deep notch bend specimen was more significant than the constraint by 25% strength overmatching in the tension specimen.

Fracture toughness evaluation of multipass weld HAZ considering the strength mismatch effect

Summary This paper describes HAZ‐notched CTOD tests of multipass welds in SMYS = 420–460 MPa class high‐strength steels for offshore structural applications. The weld metal strength overmatch causes different fracture behaviour depending on the actual CGHAZ toughness. When the CGHAZ is completely embrittled, the weld metal strength overmatch leads to the lower bound critical CTOD value. This is due to elevation of the local stress in the CGHAZ caused by the restraint effect of the overmatched weld metal. The fracture surface is generally flat, and brittle fracture originates from the CGHAZ sampled by the fatigue crack front. A larger fraction of the CGHAZ along the crack front gives a smaller critical CTOD value. When the CGHAZ has moderate toughness, however, the weld metal strength overmatch may produce a higher critical CTOD value at brittle fracture initiation. This is due to crack growth path deviation towards the base metal. Plastic deformation preferentially accumulates to a greater extent on the s...

Effect of Strength Mismatch on Crack Tip Stress Fields of HAZ-notched Joints Subjected to Bending and Tension

A numerical investigation for a crack located in the boundary between the weld metal and the heat affected zone (HAZ) of a welded joint is presented. Three-dimensional finite element analyses are performed on a center cracked panel (CCP) subjected to tension and single edge notch bend (SENB) specimens. The numerical results show that the stresses that develop in the HAZ region of the overmatched welded joint are more intense than those in the undermatch and evenmatch cases. It is noted that highly stressed near-tip zones are detrimental to fracture resistance of HAZ regions containing local brittle zones. A discussion also follows on the significance of weld strength overmatch on fracture behavior of HAZ-notched welded joints, which is based on the local approach. The analyses indicate that the relative load-carrying capacity of a overmatched joint may be reduced, especially in the case of the high constrained deep notch bend specimen. Such features may be particularly deleterious to cleavage fracture resistance of welded joints containing cracks close to the HAZ region.

Vibration platform modified

In steel welds there is often a large variation in fracture toughness and mechanical properties between the weld metal, base material and the various heat affected zone (HAZ) microstructures. The stress field in front of a crack in a weldment can be noticeably affected by the strength mismatch between the weld metal, HAZ and the base material. The crack position relative to the various microstructures will clearly influence the strength mismatch effect. In this paper the influence of crack tip positioning on the fracture performance of strength mismatched steel welds has been studied both experimentally and by FEM analysis. For a mismatched weld with local brittle zones small changes in crack tip location can give considerable changes in the fracture performance of a CTOD specimen. A high degree of strength mismatch increases the effect of crack positioning. Weld metal overmatch increases the stress level in the heat affected zone due to material constraint and thereby reduces the cleavage fracture resistance of the weldment when the coarse grained HAZ (CGHAZ) controls the fracture. The detrimental effect of high overmatch is most pronounced for specimens with notch position at fusion line and a short distance into the brittle CGHAZ. The Weibull stress has been shown to be a suitable fracture parameter in the case where one microstructure clearly controls the cleavage fracture and the calculation of the Weibull stress therefore can be limited to this zone.