Understanding the behavior of a defect in a welded structure subjected to loading is an important aspect of its integrity assessment. Additionally, local strength and toughness variations (heterogeneity) present within a weld region complicate the assessment. Single specimen Single Edge notched Tension (SE(T)) testing has become a mainstream method to characterize the ductile tearing resistance of metals under conditions of low crack tip constraint. The post-processing of single specimen SE(T) test results requires calculation of crack driving force and crack extension as the test progresses. Various methods have been investigated and were shown to be reliable for testing of homogeneous materials. Published data, however, indicate that the accuracy of these methods may degrade when testing welded joints. This paper reports on an SE(T) test program in which several weld configurations (including severely heterogeneous welds) were tested. Direct Current Potential Drop (DCPD) method is adopted to assess the crack extension and is compared with the Normalization Data Reduction (NDR) technique and nine point method. Based on the results, it can be observed that DCPD is a robust technique to understand the crack behaviour in the heterogeneous region as it indicates the variations in resistance curves and is also an effective method to predict final crack extension. The NDR method, albeit more straightforward to apply, does not reveal the irregular shape of tearing resistance curves observed for heterogeneous welds. In presence of crack path deviation, DCPD analysis is related to crack extension projected in the through-thickness direction, rather than the extension along its actual trajectory.
COBISS.SI-ID: 22171670
Flawed welds 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 structural components with defects under operating conditions. Adding to the complexity of a weld ECA is the occurrence of local constitutive property variations in the weldment (‘weld heterogeneity’). Their quantification allows for a more accurate assessment compared to common (standardized) practice, which assumes welds to be homogeneous. Hardness measurements allow to quantify weld strength heterogeneity given their theoretical relation with ultimate tensile strength. However, various standards and procedures report a wide variety of relations (‘transfer functions’) between hardness and strength, and recognize substantial scatter in hardness based predictions of strength. Within this context, this paper investigates the suitability of Vickers hardness mapping to perform an accurate weld ECA for high strength low alloy steel. To overcome the scatter associated with standardized transfer functions, this paper suggests an experimental calibration procedure based on all weld metal tensile tests. Finite Element (FE) analysis has been conducted on welds originating from steels to simulate their crack driving force response in 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 transfer function calibrated by all weld metal tensile tests yields a better agreement with experimental load-CTOD curves than transfer functions mentioned in standards and codes. Finally, a step-by-step procedure facilitates a practical adoption of the methodology.
COBISS.SI-ID: 21624854
The integrity assessment of repaired welds is dependent on accurate characterization of their fracture behaviour and limit load estimation. The final weld consists of multiple microstructures due to different weld consumables used for repair welding. As a result, a large degree of heterogeneity is to be expected. The variation of mechanical and fracture properties within the weld influences the fracture behaviour and limit load capacity of repaired weld. This motivated the authors of this work to adapt existing testing methods in order to characterize the fracture behaviour of repair welds and to develop limit load solutions which include the effects of weld heterogeneity. This work focuses on the idea of implementing T-stress in characterization of fracture toughness at the onset of crack tip blunting. By normalizing J-integral by stress biaxiality coefficient ß, obtained from T-stress solution, a new fracture toughness parameter is derived which tends to be dependent only on the distance from crack tip to interface between two mismatched weld materials.
COBISS.SI-ID: 22047254
The integrity assessment of defected welds is dependent on accurate estimations of their load carrying capacity. As welds consist of variable microstructures, a large degree of heterogeneity is to be expected. The variation of constitutive properties within the weld influences the deformation patterns around the crack and, as a consequence, the load bearing capacity of the joint. Constitutive heterogeneity is simplified in standardized assessments in order to facilitate the analysis and reduce the complexity of its required input. However, these weld simplifications may lead to inaccurate assessments with unknown errors. This motivates the work of the authors, which aims to include the effects of weld heterogeneity into integrity assessment procedures. The presented paper focuses on the prediction of limit load, which allows to calculate the structure’s proximity to plastic collapse. Simplified theorems have been developed to identify lower and upper bound values of limit load. This work explores the predictive accuracy of various methods to estimate the limit load of heterogeneous welds, including lower and upper bound theorems. A parametric study involves 2D plane strain analyses of single-edge notched tension (SE(T)) simulations. Welds consisting of two regions of different material properties (at the root and at the cap) are introduced. The obtained estimations of limit load are then compared against the simulated limit loads, and against published limit load solutions for mismatched welded joints.
COBISS.SI-ID: 21537046
The heterogeneity of arc-welded connections is often ignored in structural assessments, giving rise to inaccuracies. Improved assessments taking into account heterogeneity require the characterization of local constitutive properties. We have compared two methods to do this: Vickers-hardness mapping and miniature tensile testing. Whereas the former is more straightforward to apply, the latter provides full-range stress-strain data. This paper discusses an experimental comparison of both methods on a heterogeneous arc weld. Miniature tensile tests were performed, using digital image correlation to measure the strain. The specimens were indented to compare their stress-strain response with Vickers hardness. Notwithstanding that small natural flaws invalidated some tests, reliable stress-strain curves were obtained. Vickers hardness testing is a convenient alternative if the yield and ultimate tensile strength are the only points of interest and the corresponding conversion inaccuracy is acceptable.
COBISS.SI-ID: 19697686