Load-carrying Cruciform Welded Joints Research Articles

Comprehensive fatigue assessment for different types of aluminum fillet welded joints by local approaches

This paper investigates different types of MIG-welded fillet joints (lap welded joints, T welded joints, non-load-carrying cruciform welded joints, load-carrying cruciform welded joints) of 7N01 aluminum alloys under fully reversed pulsating axial loading. Fatigue assessments of the as-welded test results based on nominal, and local approaches are performed considering geometrical factors in different types of welded joints. The fatigue strength for these joints was compared with the international standard codes and those of other materials. Subsequently, the geometric parameters on the fatigue characteristic indicators in different types of welded joints were studied systematically by combining finite element calculated results and analytical methods, such as weld length, notch angles, incomplete penetration length, et al. The results show that the 7N01 lap welded joint owns the lowest fatigue strength (29 MPa) at 2*106 cycles. The corresponding geometrical analytical solutions of local fatigue indicator Strain Energy Density (SED) were established for these welded joints, which could quantitatively describe the fatigue characteristic relationships among different types of welded joints. The theoretical predictions based on SED notch geometric semi-analytical models are relatively higher than the recommended design curves of other local approaches.

Fatigue failure transition evaluation of load carrying cruciform welded joints by effective notch energy model

This paper investigates fatigue failure transition boundary of Load-carrying Cruciform Weld Joint (LCWJ) with different yield strength mismatch ratios and geometrical sizes of welded joints under cyclic loading conditions using a semi-empirical analytical method, which is based on the obtained fatigue test data and finite element analysis. To examine the fatigue failure mode transition relationship in CLWJs, different mismatch ratios and local geometries (plate thickness, weld size, and penetration ratio) were designed and fabricated to evaluate potential fatigue initiations (weld toe or weld root). The numerical simulations of cyclic responses at weld toe and weld root were conducted by material cyclic plastic properties from fatigue data of standard coupon specimens. A uniform effective notch energy indicator in the previous investigation was utilized to characterize the low and high cycle fatigue life by extending the SED method on the combination of generalized Neuber concept of Fictitious Notch Rounding (FNR). The related analytical formulations for potential failure points were used to predict the fatigue assessment indicator of LCWJ, considering the effects of plasticity and mechanical heterogeneity and geometry configurations. The effective notch energy relationship between the weld toe and weld root in LCWJ was determined by the established analytical solutions, verified by the fatigue data. The strategy is expected to provide some insights into assessing fatigue life for various types of weld joints for different potential failure locations.

Misalignment effect on the fatigue failure behavior of load-carrying cruciform welded joints

This paper presents an analytical model based on the Strain Energy Density (SED) method to predict the high cycle fatigue failure behavior of load-carrying cruciform welded joints (LCWJ) weakened by axial and angular misalignments. Compared with the Hot Spot Stress (HSS) and Effective Notch Stress (ENS) methods recommended in standard codes, the SED values at weld toe and weld root were carried out systematically considering the welded details and the types of misalignments. Moreover, the analytical model was applied to estimate the magnitudes of fatigue indicators and illustrated the effects of misalignment on the fatigue failure transition region in LCWJ. It is validated effectively as a supplementary analysis method for fatigue life prediction and fatigue design of welded joints compared with the experimental data and published reports.

Fatigue failure transition analysis of load carrying cruciform welded joints by extended local approach

This study was mainly concerned with the fatigue crack initiation points transition relationship between the weld toe and weld root in Load-carrying Cruciform Weld Joint (LCWJ) considering the mismatched filler material yield strength and geometrical sizes under cyclic loading conditions. The critical failure locations were investigated by the experimental tests and numerical simulations. To analyze the fatigue characteristic indicators of the potential fatigue initiations, different configurations of mismatch ratios and local geometries (plate thickness, weld size and penetration ratio) were designed to conduct the fatigue tests and cyclic calculations of CLWJ. Subsequently, New uniform effective notch energy indicator was proposed to characterize the low and high cycle fatigue life by extending the SED method. The related analytical formulations for weld toe and weld root were established to predict the low and high cycle fatigue assessment indicator of LCWJ considering the effects of plasticity and yield strength mismatch and geometry configurations. The effective notch energy relationship between the weld toe and weld root in LCWJ was estimated by the established analytical solutions.

Research on influencing factors of fatigue performance of cruciform welded joints on offshore platforms

In this study, the fatigue life of load-carrying cruciform welded joints under various plate thicknesses and degrees of penetration was performed. The fatigue assessment results showed a significant “thickness effect.” The fatigue limit of the 25 mm-thick sample was 13.4% lower than that of the 16 mm-thick fatigue sample. Essentially, the fatigue life of the weld toe does not change with the degree of penetration. Plate thickness is the key factor affecting the fatigue life. The thickness effect is determined by both the stress gradient along the plate thickness and the degree of surface stress concentration. The correction result based on the effective notch stress approach is more accurate than the thickness correction formula in the ABS standard, which underestimates the thickness effect of load-carrying cruciform welded joints and may lead to danger. The penetration ratio a/t affects the fatigue failure position of load-carrying cruciform welded joints. The fatigue design of load-carrying cruciform welded joints can be performed by appropriately increasing the degree of penetration while achieving the damage at the weld toe.

Notch energy-based low and high cycle fatigue assessment of load-carrying cruciform welded joints considering the strength mismatch

This study examines the Low and High Cycle Fatigue failure (LCF and HCF) of Load-carrying Cruciform Welded Joints (LCWJ) made of 10CrNi3MoV steel through a combined investigation of experimental, simulation, and analytical methods. LCF and HCF tests were conducted on LCWJ specimens with various weldment strength matching conditions and geometric configurations. Different fatigue failure modes can be observed from potential fracture points due to the discrepancy of strength and geometry in weldments, which exhibits the discrepancy of fatigue life. New uniform energy analytical formulations for Weld Toe (WT) and Weld Root (WR) are established to predict the LCF and HCF indicator of LCWJ considering the plasticity and mechanical heterogeneity effects of the weldments based on the generalized Neuber concept of Fictitious Notch Rounding (FNR). Furthermore, an effective notch energy indicator is introduced and implemented to evaluate fatigue behaviors of welded components in both LCF and HCF regimes. According to the proposed analytical solutions, experimental data from WT and WR failure under force and displacement-controlled cyclic loadings are evaluated and verified. The results show that the notch-energy indicator can express the unique relationship with fatigue life regardless of the strength mismatch ratios and geometry discrepancy.

Fatigue fracture assessment of 10CrNi3MoV welded load‐carrying cruciform joints considering mismatch effect

AbstractFatigue experiments and numerical simulations based on the linear elastic fracture mechanics (LEFM) theory were conducted on the evenmatched (EM) and undermatched (UM) 10CrNi3MoV load‐carrying cruciform welded joints (LCWJs). The study firstly experimentally investigated the fatigue crack growth rate (FCGR) of base metal, EM, and UM weldments. The corresponding Paris parameters as essential input data are provided to assess the fatigue crack propagation behavior for weld toe and weld root failure caused by notch stress concentration variations. On the one hand, the stress intensity factors (SIFs) at weld toe and weld root were calculated considering the effects of LCWJ specimen geometries, initial crack types, and sizes. The comparisons between simulated results and standards analytical solutions were executed, which exhibit good accordance. It proved that the fatigue fracture simulation procedure based on LEFM is appropriate for the fatigue assessment of LCWJs. Eventually, it conducted the parametric analysis by predicted S–N curves, which included in the weld length, initial crack shape, initial crack size, penetration length, and materials fracture parameter, to explore some safety assessment reference lines for both failure modes of LCWJ.

Effect of weld penetration on low-cycle fatigue strength of load-carrying cruciform joints

This study examined the low-cycle fatigue behavior of load-carrying cruciform-welded joints containing incomplete penetration. Five types of specimens with different sizes of weld penetrations and weld leg lengths were tested under cyclic plastic strains. During testing, cracking behavior was observed on the side surface of the specimen. The test results revealed that the fatigue strength and cracking patterns depend on the ratio of the weld penetration to the main plate thickness and the ratio of the weld leg length to the 5main plate thickness, respectively. Elasto-plastic finite element analyses including the effective notch concept were then performed. The results indicate that the fatigue strength and cracking pattern of the specimens can be correlated with the local strain distribution around the effective notch.

High cycle fatigue assessment of steel load-carrying cruciform welded joints: an overview of recent results

In this paper, high cycle fatigue failure behavior of Load-carrying Cruciform Welded Joints (LCWJ) is assessed using Strain Energy Density (SED) approach. Different analytical solutions are proposed and applied to illustrate the effects of LCWJ geometry under cycle tension and bending according to Finite Element (FE) calculation. The Notch Stress Intensity Factors (NSIF) according to the proposed analytical solutions are validated by comparing finite element data from several simulations in terms of LCWJ models, resulting in a good agreement. A bulk of experimental data taken from tests and the literature is assessed by the proposed solutions as the forms of SED, NSIF and peak stress. The results show that the SED-based analytical solutions for steel LCWJ are effective for high cycle fatigue failure analyses.

Fatigue assessment of steel load‐carrying cruciform welded joints by means of local approaches

AbstractIn this paper, local approaches are employed to analyse stress distributions, local fatigue indicators of weld toe, and weld root in load‐carrying cruciform welded joints (LCWJs). An extension of previous available equations is applied herein to calculate the approximate notch strain energy density (SED) based on the notch stress intensity factors (NSIFs) of LCWJ. The extending range of applicability is useful to explain various geometry effects comprehensively of LCWJ under cyclic tension and bending. By employing these extended equations, SED values can be rapidly evaluated without any finite element calculations. The NSIF and SED values obtained according to proposed analytical solutions are first validated by comparing finite element data from several stimulations in LCWJ models, resulting in a good agreement. Moreover, the failure transition relationship between weld toe and weld root is assured by SED values considering variations of notch angles. As a preliminary validation, the accuracy and reliability of proposed analytical model in this paper are checked by several experimental data generated from testing under fatigue loading. The good agreements between the experimental results and existing scatter bonds based on local NSIF, SED, and peak stress method fatigue design standards establish reliable basis for validation of the present analytical equations.

Strain energy density based fatigue cracking assessment of load-carrying cruciform welded joints

This paper presents fatigue failure crack initial points and the fatigue failure mode transition relationship between the weld toe and root of load-carrying cruciform welded joints on the basis of LEFM and Williams’ notch stress intensity theory. The notch stress intensity factors (NSIFs) of different failure locations were calculated quantitatively. The averaged strain energy density (SED) in a control volume near the notches is used to unify the scalar quantity and rectify inconsistencies of NSIFs units for the weld toe and root. The failure mode transition relations of different geometrical parametric models (such as main plate thickness, attachment plate thickness, weld length, and penetration size) were compared and analyzed based on the SED values from weld toe and root locations, and the effect of unequal main plate thickness ratio in a cruciform joint on failure mode was also investigated. The results show that SED approach can be used to accurately predict the transition region of equal base plate thickness. The weld length and penetration size are related to significant changes in the fatigue failure location of cruciform joints. The SED approach effectively reflects failure characteristics and can be used to assess fatigue failure behaviors for load-carrying cruciform welded joints.

Evaluation of Crack Initiation Points of Load-Carrying Cruciform Welded Joints Based on Effective Notch Stress Approach

For fatigue assessment, recently effective notch stress (ENS) approach is accepted as a valid alternative to fracture mechanics approaches in Japan. However, at least one study showed that the fatigue assessment at weld root of load-carrying cruciform welded joints based on ENS approach does not capture the influence of plate thickness, weld size and weld penetration. Consequently, there are still uncertainties about the fatigue assessment. This paper discusses the evaluation method of the joints based on ENS approach. Effects of weld size, weld penetration, and plate thickness on ENS are determined by using fine-meshed FEM analyses. Validity of this method is determined by comparing the evaluation results with fracture mechanics approach. It is shown that results from this method are comparable to those of the previous study and the results seem to be conservative when ratio of weld penetration to plate thickness becomes smaller and plate thickness becomes larger.

Fatigue strength assessment of load-carrying cruciform joints in low- and high-cycle fatigue region based on effective notch strain concept

This study newly defines the concept of effective notch strain and examines its applicability in elastic and elasto-plastic condition for load-carrying cruciform welded joints. Low- and high-cycle fatigue tests were performed on specimens. To calculate the effective notch strain, the reference radii of 0.3, 0.5, and 1.00 mm are assigned to the analysis model, elastic, and elasto-plastic analyses that were performed under the same condition as the experiments. By arranging the fatigue strength using the effective notch strain concept, it is possible to assess fatigue strength using FAT value of 200 from low- to high-cycle fatigue region. Furthermore, the simple method for estimating effective notch strain was proposed and verified by the experiments.

Comparison of the Stress Intensity Factor of Load-Carrying Cruciform Welded Joints with Different Geometries

The investigation of fatigue strength needs an accurate solution and reliable values of the stress intensity factor (SIF). In this study, SIF of load-carrying cruciform welded joints has been evaluated using finite element method (FEM), and compared with the available solutions from literature. Load-carrying cruciform welded joints with isosceles triangles and non-isosceles triangle fillet weld shapes were considered and have been analyzed by the FEM-based simulator FRANC2D program. Moreover, the effects of plate thickness and penetration depth have been considered. The aim of this work was to study the effects of these geometrical variables on fatigue SIF of the load-carrying welded joints with lack of penetration. The ability of FRANC2D to find an appropriate SIF solution is shown and compared with available solutions.

A boundary element analysis of misaligned load-carrying cruciform welded joints

Although it has been found that misalignment of the welded joints can reduce fatigue strength significantly, further studies are still required for comprehensive investigation, including varying geometry variables. In this study, boundary element technique and hypersingular boundary integral equation (HBIE) are employed to calculated the notch stress concentration factors and then predict the fatigue life as this method can lead to highly accurate and inexpensive results. The addressed variable is the root crack length, or partial penetration depth which can be more accurately modeled by the boundary element method. It is noted that the predicted fatigue life has a very good agreement with existing experimental results. The corresponding results, with different boundary conditions and root crack length, are presented. Finally, the effect of misalignment on the fatigue strength of load-carrying cruciform welded joints is discussed.