Koji Gotoh

The Significance of Plastic Zone Growth under Cyclic Loading and Crack Opening/Closing Model in Fatigue Crack Propagation

Fatigue cracks remain closed at lower loading level during a part of load cycle even though a tension-to-tension loading is applied. The crack closure plays a role to obstruct the generation and growth of compressive plastic zone during unloading. Cyclic plastic work, which corresponds to an irreversible energy consumed in a cracked body is generated ahead of a crack, is required as a fatigue crack driving force. The amout of cyclic plasticity is reduced by a crack closure. The crack opening/closing model based on the Dugdale model under arbitrary stress distributions for a through thickness straight crack is proposed and the fatigue crack growth under various loadings is investigated.

A Simple Estimation Method of the Stress Distribution Normal to Cross Section at Weld Toe in Non-Load Carrying Welded Joints

Many fatigue damages are occurred in the welded built-up structures designed by the hot spot stress methodology, especially near a boxing fillet weld toe. These fatigue cracks usually initiate from the toe and propagate to the plate thickness direction. Although fatigue life is affected by the stress gradient working over crack propagation path, the effect of stress gradient in cross section is not considered in the hot spot stress methodology. Then, many attempts based on fracture mechanics for the improvement of fatigue life estimation are proposed. Whereas stress distributions along the fatigue crack path must be given in order to apply the methods based on fracture mechanics for the precise fatigue life prediction, no stress distribution along the path considering the stress concentration caused by weld toe shape is obtained in practical structural design stages because the shell elements are used in finite element analyses in the design stages. A simple estimation method of the stress distribution normal to cross section at weld toe in non-load carrying welded joints is proposed in this paper. Calculation results of finite element analysis with shell elements and geometrical conditions (radius and flank angle of fillet weld toe and plate thickness) are used as input data for the estimation. The validity of this method is confirmed by comparing estimation results with ones by finite element analysis with solid elements.Copyright

Numerical simulation of fatigue crack growth of a welded structural component under block program fatigue test

Numerical simulations of fatigue crack growth of welded structural component were performed under fatigue testing conditions with block loading and constant stress range and the fatigue crack growth profiles were measured under the same loading conditions. In the experiments, fatigue cracks developed at the boxing fillet welded toe and grew toward the top flange plates of the specimens.An advanced fracture mechanics approach based on the improved effective stress intensity factor range, which is the RPG (Re-tensile Plastic zone Generating) stress criterion, was applied to perform the numerical simulations of fatigue crack growth in the modeled components. A comparison of estimated fatigue crack growth profiles with measured ones verified that a reasonable estimation of fatigue crack growth can be estimated by applying the proposed numerical method.Copyright

Numerical simulation of fatigue crack propagation under simulated whipping loading arising in hull structures

Fatigue crack propagation behavior under simulated whipping loadings which contain two different frequency components is highlighted. Numerical simulation of fatigue crack propagation based on an advanced fracture mechanics approach using the RPG (Re-tensile Plastic zone Generating) stress criterion for fatigue crack propagation is improved to enable the extraction of the effective loading history. The critical value of the plastic hysteresis for the stress versus strain relationship occurring in the vicinity of a crack tip is defined as the control parameter for extracting the effective loading history. Comparison of fatigue crack propagation curves obtained from the improved numerical simulations with the measurements which were conducted by Sumi (2010) is performed. These comparisons show the validity of the proposed procedure for extracting the effective loading history from the superposed loading history containing different frequencies. In addition, imperfection of the S-N curves approach, which corresponds to the combination of the linear cumulative damage law and the loading cycle count by the rain flow method, is conducted.Copyright

A practical estimation method of the skin plate distortion caused by fillet weld

Development of a useful estimation method of the welding deformation about welded built-up structures is developed in this paper. Improvement of matching accuracy of each hull block element is essential to introduce the economical automatic welding machines in an assembly stage. Precise estimation method of the welding deformation is also necessary to achieve the aim. Equivalent inherent strain method is highlighted as a convenient estimation method about welding deformation in large welded built-up structures. Inherent strain can be seen as the deformation source generated by welding and derives from the thermal cycles under welding processes. This deformation source can be given if the amount of welding heat input is known. The deformation can be calculated by elastic finite element analysis. Shell element is applied to simplify the mesh idealization. However, this method did not give a good estimation result in case that out-of-plane deformation such as a skin plate distortion occurs. Improvement of equivalent inherent strain method to advance the accuracy of estimation about the skin plate distortion caused by fillet weld is performed in this paper. Beam element, which expresses the fillet weld bead and has inherent strain along the bead to generate tendon force, is incorporated with former equivalent inherent strain method. The skin plate distortion measured are compared with the calculated one by applying the proposed method. As a result, the validity of the improvement is confirmed.Copyright

A Study on the angular distortion in tee type full penetrated welded joint fabricated by the laser-arc hybrid welding

Abstract The reducing effect of the angular distortion in T-type full penetrated welded joints fabricated by laser-arc hybrid welding was investigated. The angular distortion caused by the laser-arc hybrid welding were measured and compared with one caused by CO2 gas shielded-arc welding. As a result, the amount of angular distortion by laser-arc hybrid welding was the same level as one in the case of forming a leg size of F1 grade regulated by the Common Structural Rules for Bulk Carriers by applying arc welding. In addition, numerical analysis of the welding deformation was carried out by thermal elastic–plastic finite element analysis and these results were generally consistent with the measured results. Moreover, the effect on the FE result of the penetrated shape is discussed.

Practical formula of the shape evolution of a surface crack under fatigue loading

Fatigue life estimation for planar cracks, e.g. part-through surface cracks or embedded cracks is very important because most of fatigue cracks found in welded built-up structures show planar crack morphologies. Although authors had proposed the estimation procedure of crack shape evolution for a planar crack based on the fracture mechanics approach, this method cannot apply if the values of stress intensity factor at the vertices of the surface crack approximated as an ellipse cannot calculated. Then, development of the shape evolution procedure of a planer crack under the stress field with arbitrary gradient, because fatigue cracks in welded built-up structures exist near the stress concentrated region. A Practical estimation formula the shape evolution of a surface crack under stress field with the gradient is proposed in this study. This formula is established by considering the stress field under no crack condition and some former proposed formulae under uniform and pure bending stress fields. The validity of the proposed formula are confirmed by comparing some measured surface crack shape evolutions under some stress gradient conditions.Copyright

Numerical Simulation of Fatigue Crack Growth Based on Strip Yield Model Considering Work Hardening of Materials

This paper presents the improved numerical simulation of fatigue crack growth considering the crack opening / closing behaviour based on the strip yield model with the stress intensity factor weight function. The mechanical property in the primitive model corresponds to rigid-plastic material and is replaced to the elastic - perfectly plastic material in order to describe the elastic behaviour of material around a crack tip during the unloading process. However, the simulation model based on the elastic - perfect plastic material gives poor growth estimations under rapidly changing of loading histories, e.g. the spike loading. The possibility is pointed out that insufficient considerations of work hardening effect of materials lead the excess crack closure in the numerical simulations. Authors propose the improved numerical simulation fatigue crack growth considering the work hardening effect of materials in this paper. Comparison of proposed simulation results with previous ones and with measured results confirms the primacy of proposed method over previous ones.Copyright

Low Cycle Fatigue Strength of X65 Pipeline Girth Welds

During pipe laying at large water depth using S-lay over a stinger, the pipe may be subjected to reversed plastic strains which could lead to low cycle fatigue failure. X65 pipeline girth welds with wall thickness 22mm were tested in cyclic strain control. Undermatched and overmatched welds were tested. Two types of cyclic strain programs were applied. In the first test series the welds were subjected to a tensile-tensile cyclic strain, simulating pipe going over the stinger, to validate that a pipe going over a stinger would not violate criteria for low cycle fatigue design. In the second test series, the specimens were initially pre-strained to 2% and subsequently fatigue-cycled with a maximum strain of 2% to obtain design criteria for a pipe laying stationary over the stinger for a period of time, subjected to cyclic loads due to waves. The results are presented in terms of Δe–N curves, with proposed design curves for the two types of welds.Copyright

Fatigue Crack Growth Behaviour of an Out-of-plane Gusset Welded Joints under In-plane Biaxial Tensile Loadings with Different Phases

Most of large frame-based welded structures are subjected to multiaxial cyclic loadings with different phases under in-service condition. However, the fatigue performance of these structures are evaluated according to the design codes based on theoretical and experimental investigations under a uniaxial loading condition. Most of these codes are based on the S-N curves approach. On the other hand, authors proposed the numerical simulation method of fatigue crack propagation histories of a cracked plate subjected in-plane biaxial loadings with phase difference of each loading component. Fracture mechanics approach was applied to establish our method. In this study, fatigue crack growth behaviour of an out-of-plane gusset welded joint under in-plane biaxial loading with two different phase conditions are investigated. The phase difference effect for fatigue crack shape evolution under in-plane biaxial loading is confirmed by measured ones. Besides, the numerical simulation method of fatigue crack growth is proposed and confirmed this method by comparisons of measured crack evolution with the numerical simulation results.