Boxing Fillet Research Articles

New LTT Welding Material and All-Position Repair Welding with Elongated Bead for Super-Long Fatigue Life of Boxing Fillet Joints

New LTT Welding Material and All-Position Repair Welding with Elongated Bead for Super-Long Fatigue Life of Boxing Fillet Joints

Isogeometric Analysis and Bayesian Optimization on Efficient Weld Geometry Design for Remarkable Stress Concentration Reduction

Shape optimization combined with Isogeometric Analysis will be an efficient design method since exact Computer Aided Design geometries can be used for numerical analysis thanks to the same basis functions. In this study, authors developed an in-house research software JWRIAN-IGA and applied it to optimize the weld geometries of two models, namely a T-joint fillet weld and an elongated boxing fillet weld to reduce stress concentration. A gradient-free stochastic global optimization algorithm, Bayesian Optimization was used to sought optimal geometry parameters. It was shown that the optimized geometry was efficiently designed and the stress concentration in the welds for both cases was greatly reduced.

Efficient analysis of welding thermal conduction using the Newton\u2013Raphson method, implicit method, and their combination

Finite element analysis is commonly used to investigate the thermal-mechanical phenomena during welding. To improve the computing efficiency of finite element analysis for welding thermal conduction, a novel Newton–Raphson method (NRM) without the computation of inverse matrix and a hybrid method combing the NRM and conventional implicit method (IMP) were developed. Comparison of computing time between the hybrid method implemented in an in-house software JWRIAN and the IMP used in a commercial software ABAQUS indicated that the computing speed of the former was about 4.5 times faster than that of the latter. Additionally, compared to the conventional IMP, the NRM exhibited higher computing efficiency in the analysis of transient thermal conduction during the welding heating process. Meanwhile, a combined hybrid method of the NRM and IMP was verified to be more efficient in analyzing the welding thermal conduction throughout the heating and cooling processes. Moreover, the thermal cycles computed by the hybrid method were consistent with those from experimental measurement, indicating the high accuracy of the hybrid method. Furthermore, the hybrid method was used to predict the temperature field of the corner boxing fillet joint welded by a low transformation temperature weld metal for generation of compressive residual stress.

A design method of tensile triangles and low transformation temperature weld metal for reduction of stress concentration and residual stress of welded joints

Stress concentration and residual stress have a significant influence on fatigue life of welded joints. In order to reduce the stress concentration of welded joints, a mathematical design method of tensile triangles (MTT) based on bionics was applied to weld shape design. Accordingly, the stress concentration of various weld beads in the corner boxing welded joint and the fillet welded T-joint was dissected using our in-house FEM software JWRIAN. It was found that there existed a large stress concentration in the conventional welded joints, whereas those welded joints with elongated weld bead were accompanied by a lower stress concentration, especially for elongated weld bead with MTT design. Furthermore, among the weld shapes of the corner boxing fillet welded joint, the rectangle shape of weld bead had the minimum stress concentration factor (1.05). For the fillet welded T-joint with MTT design, the stress concentration of weld toe decreased dramatically with the increase of the index of designed shape, but there was a minor difference of stress concentration at weld root between the weld beads with MTT design. In addition, application of low transformation temperature (LTT) weld metal utilizing martensitic transformation to the fillet welded T-joints can produce compressive residual stress at weld toe.

New LTT Welding Material and All-Position Repair Welding with Elongated Bead for Super-Long Fatigue Life of Boxing Fillet Joints

New LTT Welding Material and All-Position Repair Welding with Elongated Bead for Super-Long Fatigue Life of Boxing Fillet Joints

Suggestion of welding procedure for compressive residual stress induction by using low transformation temperature welding materials

ABSTRACTTo understand the cause of compressive residual stress in welded joints, we analysed by numerical analysis the effect of welding pass sequence using low transformation temperature (LTT) welding materials on residual stress around the weld toe of boxing fillet welded joints. It was determined by numerical analysis that the produced compressive residual stress and the influence of the stiffeners are reduced in the equivalent position of the weld toe in a fillet welded joint because of the influence on the behaviour of the stiffener in the weld being due to residual stress distribution around the weld toe. The residual stress reduction method of extending the length of the welded bead and releasing the weld toe from the stiffener, similar to the concept of discarding a bead to reduce tensile residual stress, was effective in fillet welded joints. Numerical analysis of the relationship between residual stress around the weld toe and width of the weld bead in the bead-on-plate welding model clarified that compressive residual stress can be introduced around the weld toe by having a wide width weld bead. In addition, a fully penetrated welded joint was very effective for causing compressive residual stress around the weld toe.

Elongated bead weld method for improvement of fatigue properties in welded joints of ship hull structures using low transformation temperature welding materials

To improve the fatigue properties in the welded joints of high strength steels, we developed an elongated bead method for low transformation temperature (LTT) welding materials. The proposed method intensifies the compressive residual stress and reduces the stress concentration. We investigated the length of the elongated bead in the boxing fillet portion of the gusset front, the effectiveness of inexpensive LTT wires with low Ni content and the potential applicability to repair welding. The most effective welding method, overlay of the elongated bead LTT weld metal on the conventional boxing fillet weld metal, extended the fatigue lifetime of the boxing fillet by seven to ten times. Residual stress distributions in joints welded with low-Ni metals of different martensite start temperatures (Ms) as well as the stress concentration in weld joints formed from elongated beads with different lengths were investigated in finite element simulations. Residual stress of weld and its toe was decreasing as the Ms of weld metal was lowering and the most compressive residual stress was obtained for weld metal with Ms temperature of 184 °C. The stress concentration reduces by about 30% of fatigue load, and the residual stress decreases down to compressive one of −685 MPa for the LTT overlay welding elongated bead treatment.

Formulation of CTOD design curve considering the yield to tensile ratio

The Crack Tip Opening Displacement (CTOD) design curve approach is one of the fracture assessment procedures for estimating CTOD as an elastic-plastic fracture driving force, and is used for the assessment of flaws in welded structures. The Japan Welding Engineering Society (JWES) has produced a semi-empirical CTOD design curve, where CTOD is calculated from the local strain acting on the assumed crack, the crack size and the yield strain of the material. CTOD should be estimated by using all the factors influencing the crack driving force in the welded joint, but the yield to tensile ratio, Y/T, which relates to strain hardening behavior, is not used for the CTOD design curve. On the other hand, the new CTOD calculation formula adopted by the JWES CTOD fracture toughness testing standard uses Y/T, and derived CTOD values from the formula are affected by Y/T. Consequently, there is a mismatch in CTOD calculation between the fracture driving force of welded structures and the fracture toughness of standard specimens. In this study, Y/T was considered for formulating a new CTOD design curve. CTOD estimated by the new design curve was compared with CTOD obtained by the finite element analysis of a corner boxing fillet welded joint. The new CTOD design curve contributes to a good CTOD estimation for different Y/T welded joint models.

Fatigue improvement in high-strength steel welded joints with compressive residual stress

In order to improve fatigue properties of gusset fillet welded joints in high-strength steels, intensification of compressive residual stress around the weld toe was studied using computer simulations of welded joints with low transformation temperature (LTT) weld metal. A boxing fillet weld metal of elongated bead shape is most effective for the formation of compressive residual stress. An elongated bead is also preferable for the reduction of stress concentration at the weld toe. In the case of a 70-mm-long elongated bead formed using LTT weld metal with martensite start temperature (Ms) of 150 °C (indicated by Ms 150 °C), a residual compressive stress of about −450 MPa was formed at the weld toe position, whereas in the case of conventional boxing fillet welding using conventional weld metal (Ms 500 °C), a residual tensile stress of 450 MPa was formed at the weld toe. Fatigue measurements of gusset fillet welded joints with an elongated bead were conducted using 800-MPa grade steel plates. Results showed that the elongated bead welding method achieved a significant improvement in fatigue properties. It is equal to the fatigue lifetime of welded joints treated using high-frequency peening methods such as high-frequency impact treatment and ultrasonic impact treatment.

Analysis of martensite transformation behaviour in welded joint using low transformation temperature welding wire

Low transformation temperature welding (LTTW) wire has been found to improve the fatigue strength in welded joints. In the present study, the temperature dependence of the mechanical properties in a dual phase microstructure of austenite and martensite was estimated using the properties of full austenite and full martensite in numerical analyses. A welding method effective for residual stress reduction and fatigue strength improvement was shown by applying a calculation method under transformation superplasticity and transformation induced plasticity in high strength steel welded joints. With this method, the influence of the welding pass sequence on the residual stress distribution and fatigue strength was examined in a boxing fillet welded joint using LTTW. The transformation tensile residual stress in the weld toe was decreased by sectioned welding, and the fatigue limit by sectioned welding with LTTW improved in comparison with the fatigue limit of a joint welded with conventional wire in the same process.

A Simple Estimation Method of the Normal Stress over the Cross-Section of a Plate at Weld Toe

Many fatigue damages in large welded built-up structures are found near a boxing fillet weld toe. These fatigue cracks usually initiate from the toe and propagate in the cross-section of wall thickness as surface cracks.Many attempts based on fracture mechanics for the improvement of fatigue life estimation are proposed, because current practical methods based on the S-N curves and the cumulative damage rules cannot estimate the accurate fatigue life under in-service structures. Whereas stress distributions along the fatigue crack path must be given in order to apply fracture mechanics methods for the precise fatigue life estimation, no stress distribuiton along the path is calculated because the shell type elements are used in finite element analyses in practical structural design stages.A simple estimation method of the stress distribution over the cross-section of a plate at weld toe, which corresponds to the supposed fatigue crack path in the case mentioned above, is proposed in this paper. The validity of this method is confirmed by comparing estimation results with ones by finite element analyses in which solid type elements are used.

Improvement of Prediction Ability of Fatigue Crack Growth Analysis

In this study, the authors have developed a software system based on influence function method using 3 D-FEM input data and 3 D-FEM result to improve prediction ability of fatigue crack growth analysis of surface cracks which initiates from root of boxing fillet welded joints of ship structures. The software system developed in this study uses the database of influence coefficients, Kij, developed by one of the authors, Shiratori, for various types of surface crack through a series of FE-analyses. It is shown that crack growth result based on influence function method using 3 D-FEM result is more accurate for experimental data than the result by a method based on linear approximation of arbitrary distributed stress. And further study, a simple approximate equation of arbitrary distributed stress in the cross section of virtual crack surface are determined to simplify crack growth analysis based on influence function method. Surface crack growth analysis based on influence function method using the approximate equation gives almost the same result of that based on influence function method using 3 D FEM result.

Corrosion Fatigue Strength Evaluation for Ship Hull Structure

Corrosion fatigue tests of the three types of specimen (small size elementary component, middle size elementary component and structural component) with boxing fillet welded joint of TMCP steel (KA 32) were conducted in synthetic sea water at 40°C which is nearly the same environments as actual ballast tanks. As a result of testing and fatigue life prediction analysis, the following conclusions were obtained.(1) Corrosion fatigue life in 40°C sea water shows 50% reduction comparing with the life in air (corrosion factor for strength (Kc) is 1.3). This environment factor is the same for three configurations of specimen.(2) Fatigue life at failure of small size specimen is equal to the number of cycles in which the crack in structural component specimen propagates to 30 mm length. This phenomenon is shown in both air and 40°C sea water.(3) Fatigue life prediction of ship hull structure is simplified by adopting the proposed evaluation methods that replace the complicated small crack growth analysis at the toe area of welded joint by the failure life in small size specimen.

Corrosion Fatigue Strength of Welded Joints

The fatigue crack propagation and fatigue life tests were performed in sour crude oil containing H2S of 400 ppm and in air to study the corrosion fatigue strength of the welded jouints of ship hull structural plate which is essential for the evaluation of fatigue life of the crude oil tank components of VLCC's. The test specimens were machined out of the CO2 welded joints and boxing fillet welded joints of a 25 mm thick, ship hull structural plate KA36 (TMCP). The main results obtained follow : 1) The fatigue crack propagation rate was much higher in sour crude oil than in air. It became smaller in the order of base metal, heat affected zone (HAZ) and weld metal (WM) both in sour crude oil and in air.2) The fatigue crack propagation in HAZ and WM in sour crude oil consisted of three stages. They are Stage 1 where the fatigue crack propagation rate (da/dN) decreases with the increase in the range of stress intensity factor (ΔK) as a result of the protection of crack tip by corrosion product, Stage 2 where da/dN increases with ΔK as sour crude oil gradually reaches the crack tip through broken corrosion product, and Stage 3 where da/dN increases with ΔK in a manner of typical fatigue crack propagation in corrosive environment.3) Brittle striations were observed on the fracture surface of the specimen tested in sour crude oil particularly in the fatigue crack propagation accelerating region.4) The sour crude oil environment did not deteriorate the fatigue strength of the boxing fillet welded joint at lower stress levels.

付加物継手の疲労強度におよぼす継手端部形状の影響

Gusset welded joint is known as having the lowest fatigue strength among other types of welded joints. In order to improve the fatigue strength of gusset welded joints, the effect of the shape of weld ends on fatigue strength was investigated for three types of gusset welded joints using mild steel (SS41). Non-load carrying and load carrying gusset welded joints were examined, which were boxing fillet welding (type A and A'), side fillet welding (type B and B') and lengthened side fillet welding (type C and C') types.The test result indicates that the fatigue strength increases in the following order, type A and A'<type B and B'<type C and C' and that type C and C' are about 22% stronger than type A and A' at N=2×106. This result can be discussed in terms of difference in notch effect at the weld ends and in crack propagation process. The effect of residual stress may be insignificant.The effect of toe grinding was also investigated. Toe grinding increased fatigue strength of type A' and did not increase that of type C' while it changed location and appearance of the fracture.