Undermatched Specimens Research Articles

The effect of the weld mismatch on the yield load of C(T) and SE(B) fracture specimens

The yield load values of two-dimensional C(T) and SE(B) specimens with various degrees of mismatch, were numerically evaluated. The yield load values were normalized using the common format in order to analyse the effect of the weld, using C(T) and SE(B) specimens made only of base metal (BM) as reference material. In undermatched specimens (the weld metal elastic limit is lower than that of the BM), the normalized yield load decreases compared to the elastic limit of the BM. In overmatched specimens (the weld metal elastic limit is larger than that of the BM), the yield load increases. This variation of the yield load depends on the degree of mismatch and on the relation between weld size and crack length. The numerical results were compared with experimental values obtained with mismatched weld C(T) specimens using an ASTM A 588 steel as BM.

El efecto de la heterogeneidad de las propiedades mecánicas de una unión soldada sobre la carga de plastificación de probetas de fractura tipo C(T) Y SE(B)

The yield load values of two-dimensional C(T) and SE(B) specimens with various degrees of mismatch, were numerically evaluated. The yield load values were normalized using the Common Format in order to analyze the effect of the weld, using as reference material C(T) and SE(B) specimens made only of base metal (BM). In undermatched specimens (the weld metal elastic limit is lower than that of BM) the normalized yield load decreases compared to the elastic limit of BM. In overmatched specimens (the weld metal elastic limit is larger than that of BM), the yield load increases. This variation of the yield load depends on the degree of mismatch, and on the relation between weld size and crack length. The numerical results were compared with experimental values obtained with mismatched weld C(T) specimens using an ASTM A 588 steel as base metal.

Effect of three-dimensional deformation on the limit load of highly weld strength undermatched specimens under tension

In the case of welded structures with cracks, a number of parameters, such as those with units of length, makes it difficult to present the results of numerical solutions in a form convenient for direct engineering applications, such as flaw assessment procedures. For centre-cracked components under tension, it is shown in the current article that an effect of crack length on the limit load can be taken into account by means of an upper bound limit load for the corresponding structure with no crack without any additional numerical treatment. Using this result, it is sufficient to find an upper bound limit load for the structure with no crack and then to apply the analytical formulae for finding the corresponding limit load for the structure of interest. Welding of some aluminium alloys for structural applications usually leads to a significantly lower strength (undermatched) weld joint. The approach proposed is used to demonstrate an effect of three-dimensional deformation on an upper bound limit load for such highly strength undermatched centre-cracked welded specimens under tension. This result can also be used in metal-forming applications where upper bound solutions are more useful than lower bound solutions.

The influence of constraint on crack tip stress fields in strength mismatched welded joints

This paper describes the results of a series of two-dimensional, plane strain, finite element analyses which were performed using the modified boundary layer formulation to model two material idealisations of welded joints containing cracks. The cracks were assumed to be contained within the weld material and ran parallel to the material interface. The effects of changes in geometrical constraint on the crack tip stress fields were modelled by varying the magnitude of the T-stress term at the crack tip, and the effects of constraint due to strength mismatching were investigated by changing the strength of the base material outside the weld. The results show that constraint due to material mismatching depends on the level of strength mismatching, the applied load, and the width of the weld material. A normalised load parameter, J hσ Yw , scales the size of the plastic zone with the width of the weld material, and can be used to quantify the level of constraint for a given degree of mismatching. Any two specimens loaded to the same value of this parameter will have the same crack tip stress field, regardless of the size of weld material. The effect of geometrical constraint on overmatched welds, applied through the T-stress term was similar to that observed in homogeneous materials. For undermatched specimens the influence of the T-stress term on the stress field depends on the normalised load parameter. A level of constraint can be obtained in undermatched specimens which is higher than that obtainable in homogeneous laboratory specimens. This paper discusses the implications of this on the fracture resistance of undermatched welds.

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.

不均質材の延性き裂進展抵抗評価に関する研究（第１報） 延性き裂先端の応力・ひずみ挙動とき裂進展抵抗に及ぼす強度不均一の影響

Stable crack resistance (R) curves were frequently used for assessing the integrity of structures containing defects in loading regimes. In that case, R curves are treated as characteristics of materials, but it is known that R curves are actually affected by various factors.The effect of strength mis-match on stable crack growth resistance is discussed in this paper. In order to compare the R curve of bimaterial materials with that of homogeneous ones, compact type tests are carried out. 3-dimensional FEM analyses taking into account the stable crack growth are also executed using the result of performed experiments. In the case of over-matched specimens (high strength region including a notch are surrounded with low strength region), dCTOD/da apparently increases comparing with homogeneity. In the case of under-matched specimens (opposite of the previous case), dCTOD/da apparently decreases comparing with homogeneity. Strength mis-match affects on CTOD(J)-R curves. On the other hand, the equivalent plastic strain at stable crack tip obtained from 3D analyses may be generally independent of the strength mis-match during ductile fracture process.