Sensitivity study of crack driving force predictions in heterogeneous welds using Vickers hardness maps

Weld flaws often require an engineering critical assessment (ECA) to judge on the necessity for weld repair. ECA is a fracture mechanics based prediction of the integrity of welds under operating conditions. Adding to the complexity of an ECA is the occurrence of local constitutive property variations in the weldment (‘weld heterogeneity’). Their quantification is important to allow for an accurate assessment. Hereto, hardness measurements are widely adopted given their theoretical relation with ultimate tensile strength. However, various standards and procedures report a wide variety of different hardness transfer functions and additionally recognize substantial scatter in predictions of strength. Within this context, this paper investigates the suitability of hardness mapping to perform an accurate weld ECA. A finite element analysis has been conducted on welds originating from steel pipelines to simulate their crack driving force response using single-edge notched tension (SE(T)) specimens. Vickers hardness maps and hardness transfer functions are combined to assign element-specific constitutive properties to the model. The resulting crack driving force curves are probed against experimental results. The variable agreement between simulations and experiments highlights the need for further research into the characterization of local constitutive properties of heterogeneous welds. A hardness transfer procedure based on all weld metal tensile testing appears to be particularly promising

Evaluation of weld homogenization schemes based on plastic loading of single edge notched tension (SE(T)) tests

Engineering Critical Assessment (ECA) of welds is the process of predicting the structural integrity of a structure in the presence of a weld defect under specified loading conditions. Standardized ECA techniques consider the weld to be equal in properties when compared with base metal or use the concept of ‘strength mismatch’ to distinguish the weld from the base metal. In both cases, the weld region is homogeneous. This is a severe approximation from reality, as welds show complex strength heterogeneity patterns. The authors are concerned with techniques to simplify welds in such way that the structural response of the weld is similar to that of the idealized, homogeneous weld. Two approaches are considered: (a) integrating properties along assumed slip lines originating from the defect tip, and (b) assigning All Weld Metal Tensile Tests (AWMTT) to the entire weld region. A plastic analysis procedure suggested by the ASME BP&V code (‘Twice Elastic Slope method’) is adopted to estimate Plastic Load, whose values are compared for the heterogeneous and equivalent homogeneous welds. Finite Element (FE) simulations were performed for Single Edge notched Tensile (SE(T)) specimens. The results put forward the possibilities of weld homogenization while showing its limitations. This will assist in further improvement of weld ECA

Evaluation of a finite element model for SENT testing of welded connections

The SENT test has recently gained popularity for the characterization of the ductile tearing resistance of welded connections under low crack tip constraint. In addition to this practical purpose, Soete Laboratory adopts the SENT test as a tool to investigate effects of weld strength heterogeneity on the crack driving force response of weld defects. The numerical aspect of this investigation relies on a finite element model of a SENT specimen in which the heterogeneous strength properties of the weld region are defined on the basis of an imported hardness map. This paper evaluates the model in two respects. First, crack driving force response is validated on the basis of an experimental SENT test result of a non-welded specimen. Second, the potential effect of the transfer function between hardness and constitutive properties is illustrated. It is concluded that more work is required to improve the feasibility of weld hardness data as a means to characterize effects of weld strength heterogeneity

Calibration of hardness transfer functions based on micro tensile and all weld metal tensile tests of heterogeneous welds

In order to assess the integrity of welded structures, it is important to accurately know the material characteristics of the weld regions. A weldment is heterogeneous i.e. strength properties vary at different locations within the weld. This influences the behavior of the structure when it is subjected to loading. Hence, the evaluation of material properties within the weld region plays a pivotal role in structural integrity assessment. Traditionally, tensile tests provide constitutive properties like tensile strength and yield strength along with stress (σ) – strain (ε) curves. Alternatively, hardness indentations are also used to procure strength properties of a material. Several transfer functions have been formulated to convert hardness values to strength properties. The validity of these transfer functions with the presence of strength variations is questionable, as these relations do not consider the aspect of heterogeneity. Accordingly, in this research, a heterogeneous weld was considered to assess the relation between Vickers hardness (HV5) and strength properties. Two tensile test configurations were considered – All Weld Metal Tensile Tests (AWMTT) and Micro Tensile Tests (MTT). While AWMTT provides average weld stress-strain properties, MTT provide local properties. These results help to validate the hardness transfer functions and thus calibrate them appropriately. Hardness maps were obtained on polished weld macrographs. The material properties obtained from three methods were compared and significant variations were observed. Based on these differences, an experimentally calibrated transfer function is implemented. With this relation, it is possible to predict weld behavior more accurately and appropriately using hardness maps and tensile tests

Experimental and numerical analysis of deformation patterns in notched heterogeneous welds

Standardized weld flaw assessment techniques assume the weld region to be homogeneous which is a strong idealisation of reality. Characterising the effects of heterogeneous properties of welds through the analysis of deformation patterns and slip lines is the major concern of this research. It is the goal to investigate which effects these variations in properties within the weld material have on the propagation of cracks within the weld material. Performed experiments are SENT tests on strongly heterogeneous welded connections. The same material is also simulated with a weld heterogenisation model in ABAQUS®. Results from both experiments and simulations are discussed and compared. It is shown that slip lines tend to avoid zones of high hardness in a way that a path of least resistance is found. Related to this, it is seen that the slip line angles deviate from the theoretical 45° for homogeneous material. Obtained results validate the numerical model used

Crack tip constraint analysis in welded joints with pronounced strength and toughness heterogeneity

Application of standardized fracture testing methods in heterogeneous welded joints might lead to overestimation or underestimation of fracture toughness and consequentially to inaccurate estimation of loading capacity of welded structure. Combining experimental testing and numerical modelling of double mismatched welded joints provided a view on influence of pronounced strength heterogeneity on fracture behaviour under monotonous loading. Investigated double mismatched welded joints had a fatigue crack located in one mismatched weld material region. The fatigue crack plane was perpendicular to the interface of two distinctive mismatched weld material regions, while the stable crack extension was estimated to be from one weld material region to other, crossing the interface. Single edge notched bend (SE(B)) specimens were used to investigate double mismatched welded joint fracture behaviour under high constraint conditions. Crack extension has been evaluated using normalization data reduction technique and crack driving force has been evaluated as specified in ASTM E1820 standard. Stress triaxiality has been implemented as a second fracture parameter, evaluating level of constraint. Obtained results show that combination of strength overmatched and undermatched weld material has a significant effect on the weld load capacity and stress triaxiality ahead of the crack tip. The latter leads to reduced or increased fracture toughness of the weld as the crack tip is closer to the fusion line between overmatched and undermatched weld material. Finally, computational simulations revealed how double mismatched weld configuration alters the stress field near the crack tip and corresponding values of J-integral. This implies that standard fracture test overestimate or underestimate fracture toughness of the double mismatched welded joint