Weld detail fatigue life improvement techniques. Part 1: review

: Fatigue cracks in steel ships often occur at welded joints where stress concentrations due to the joint geometry are relatively high and the fatigue strength of the weld is reduced in comparison to that of the base metal. This becomes more critical in ships built of High Strength Steels (HSS) because the fatigue strength of steel in the a welded condition does not increase in proportion to the yield or tensile strength. In many cases, the fatigue performance of severely loaded details can be improved by employing good detail design practices, for example by upgrading the welded detail class to one having a higher fatigue strength. In some cases, however, there may be no better alternatives to the detail in question and modification of the detail may not be practicable. As an alternative to strengthening the structure at a considerable increase in costs, procedures which reduce the severity of the stress concentration at the weld, remove imperfections, and/or introduce local compressive stresses at the weld can be used for improvement of the fatigue life. Similarly, these fatigue improvement techniques can be applied as remedial measures to extend the fatigue life of critical welds that have failed prematurely and have been repaired. To date, weld fatigue life improvement techniques have been successfully applied in several industries. While there has been increasing interest in the application of fatigue life improvement techniques to ship structures, at present there is a lack of guidance on the use of such techniques for design, construction and repair. Hence the key elements of this project were to compile available data on fatigue life improvement techniques, assess the feasibility and practicality for their application to ship details, identify gaps in the technology, and finally to recommend design, construction and repair requirements.

Weld Detail Fatigue Life Improvement Techniques

The mixed-mode investigation of the fatigue crack in CTS metallic specimen

Fatigue strength improvement of longitudinal fillet welded out-of-plane gusset joints using air blast cleaning treatment

CHENG, PEI-YUAN. Influence of Residual Stress and Heat Affected Zone on Fatigue Failure of Welded Piping Joints. (Under the direction of Dr. Tasnim Hassan.) In recent decades, some unexpected fatigue failure occurred in welded joint of metal structures under cyclic loading. In many cases, the cause for the failure could not be detected. A study at NC State University revealed that welding procedure could be one of the factors that was not appropriately considered in current design methodologies. The welding procedure can influence the strain response near weld toe in two ways: one is by generating residual stress, and the other is changing material properties in the heat affected zone (HAZ). It was the primary objective in this investigation to determine the influence of these two factors on strain response by conducting experiments and performing numerical simulations of welded piping joints. In the experimental study a series of residual stress data were measured, using x-ray and neutron diffraction techniques, for welded piping joints. The measured results revealed that the initial maximum compressive residual stress of stainless steel piping joints is higher than the yield stress of base metal. Moreover, the axial residual stresses of stainless steel piping joints are mostly relaxed after 5 cycles. The change of mechanical material properties due to high temperature exposure was studied by conducting experiments on tubular specimens. It was obtained that some mechanical material properties changed after subjecting to high temperature cycles, and the changed material was correlated to the peak temperatures. A modified thermo-mechanical material heterogeneity model was then developed to improve the initial residual stress simulation at the weld toe. The heterogeneous material properties coupled with the Chaboche model were used for subsequent fatigue response simulation. Quarter-point elements were applied at the stress concentration locations. The analysis results showed that fatigue response and residual stress relaxation can be simulated well. Final two simulations in this research showed that the presence of initial residual stress influences strain amplitude and strain mean, both of which could influence the fatigue life of welded joints. Influence of Residual Stress and Heat Affected Zone on Fatigue Failure of Welded Piping Joints

Influence of Residual Stress and Heat Affected Zone on Fatigue Failure of Welded Piping Joints

The advantages provided by using high strength structural (HSS) steels for offshore construction might be limited by the fatigue resistance properties of the welded connections. Since fatigue crack propagation characteristics are not significantly different between low and high strength structural steels, the best way to improve the fatigue resistance of welded joints in HSS steels is to introduce a longer initiation period. For this purpose, we developed two complementary procedures: an improved weld procedure, and a global post weld improvement technique.

The overall geometry of the fillet weld is controlled to obtain the best stress flow and the lowest stress concentration in the weld toe region. Furthermore the weld toe run is done at the beginning of the welding. This improved weld procedure gives smaller global and local stress concentrations. The second step to improve the fatigue resistance of such welded joints is post weld shot peening. Such a technique has the advantage of being a “global treatment”, not only localized at the weld toe. The compressive residual stress field induced by shot peening has been studied at the surface of the specimen and in depth. By measuring the initial stress field (prior to fatigue testing) and the redistribution of these residual stresses during fatigue, optimum parameters of shot peening (ALMEN intensity, shot diameter, coverage rate, etc) may be chosen.

Fatigue tests were carried out on 30 mm thick T shape welded joints. Comparative results between as-welded and shot peened specimens show the efficiency of the improved weld procedure and of shot peening in improving the fatigue resistance of welded structures.

Improvement of the fatigue life for offshore welded connections

Study of residual welding stress using the step-by-step hole drilling and X-ray diffraction method

CET OUVRAGE RASSEMBLE LES PARTICIPATIONS SUIVANTES : PARTIE 1 - FATIGUE DES MATERIAUX ET DES STRUCTURES : LES PRINCIPAUX PARAMETRES DE RESISTANCE A LA FATIGUE DES ACIERS (LIEURADE,H-P) ; SUR LE CALCUL DE STRUCTURES SOUMISES A LA FATIGUE MULTIAXIALE (DANG VAN,K - PAPADOPOUPOS,Y - GRIVEAU,B - MESSAGE,O) ; TENUE A LA FATIGUE D'UN ACIER 35 NDC 16 GRENAILLE - EVOLUTION DES CONTRAINTES RESIDUELLES DE GRENAILLES EN FONCTION DU TYPE DE SOLLICITATION (BIGNONNET,A). PARTIE 2 - INFLUENCE DES CONTRAINTES RESIDUELLES DE COMPRESSION SUR LA TENUE A LA FATIGUE : INFLUENCE DU GRENAILLAGE DE PRECONTRAINTE SUR LA TENUE A LA FATIGUE D'ASSEMBLAGES SOUDES EN ACIER HLE (PICOUET,L - BIGNONNET,A - LIEURADE,HP - CASTEX,L - LEBRUN,J-L) ; CONTRAINTES RESIDUELLES ET FATIGUE DES ALLIAGES D'ALUMINIUM GRENAILLES (NIKU-LARI,A - GILLEREAU,D) ; INFLUENCE DES CONTRAINTES RESIDUELLES INTRODUITES LORS DU DRESSAGE SUR LE COMPORTEMENT A LA FATIGUE ET LA TENACITE DES RAILS (LIEURADE,H-P - DEROCHE,R-Y - MAILLARD-SALIN,C - KAPLAN,D) ; ESSAIS TURBOMECA RELATIFS AU GRENAILLAGE DE L'ALLIAGE BASE TITANE TA 6 V (BERTOLI,A). PARTIE 3 - MESURES DES CONTRAINTES RESIDUELLES PAR LES METHODES DE RELAXATION : LES METHODES DE MESURE DE CONTRAINTES RESIDUELLES PAR ENLEVEMENT DE COUCHES (KAPLAN,D) ; LES METHODES DE DETERMINATION DES CONTRAINTES RESIDUELLES FONDEES SUR LES ENLEVEMENTS DE COUCHES. EXEMPLES D'APPLICATION (VANNES,A - FOUGERES,R) ; MESURE DE LA DISTRIBUTION DES CONTRAINTES RESIDUELLES EN PROFONDEUR PAR LA METHODE DU TROU INCREMENTAL (NIKU-LARI,A - LU,J - FLAVENOT,J-F) ; MESURE DES CONTRAINTES RESIDUELLES SUR DES SURFACES PLANES (NIKU-LARI,A - FLAVENOT,J-F). PARTIE 4 - DETERMINATION DES CONTRAINTES RESIDUELLES PAR DES METHODES NON DESTRUCTIVES : ETUDE DES CISAILLEMENTS SUPERFICIELS OBSERVES EN ANALYSE DES CONTRAINTES PAR DIFFRACTOMETRIE X (CASTEX,L - BADAWI,K) ; DETERMINATION EXACTE DE L'ANGLE DE DIFFRACTION DANS LA MESURE DES CONTRAINTES RESIDUELLES PAR DIFFRACTION DES RAYONS X (BADAWI,K - CASTEX,L - SPRAUEL,J-M) ; LE BRUIT FERROMAGNETIQUE, UNE METHODE DE SUBSTITUTION AU TEST DE REFENDAGE (DEBUS,A - FLAMBART,C - VANNES,A). PARTIE 5 - PREVISION DES CONTRAINTES RESIDUELLES PAR CALCUL : PREVISION DES CONTRAINTES RESIDUELLES EN TRAITEMENTS THERMIQUES (SIMON,A - DENIS,S) ; LES CONTRAINTES RESIDUELLES DANS LES PIECES EN ALLIAGES D'ALUMINIUM - PREVISION PAR LA METHODE DES ELEMENTS FINIS - COMPARAISON AVEC L'EXPERIENCE (JEANMART,P - DUBOST,B - BOUVAIST,J). (INV. 19951 - DCMC/N - M.GOURMELON).

Materiaux et structures - fatigue et contraintes residuelles

Tensile tests were performed on two MMC materials: a recast Duralcan composite (A356+20% SiCp), and another composite elaborated by powder metallurgy (2124+15% SiCp). These composites increased their specific properties such as their Young's modulus, however these improvements were accompanied by a degradation of strain at failure, moreover σ-e curves were observed with a non linear behavior. In discontinuously reinforced metal manx composites, microvoid nucleation is the predominant fracture mode. The failure mechanism consist of void nucleation, growth and coalescence stages. The Rice-Tracey law used to illustrate growth stage was not checked by MMC materials, but Weibull's theory was successful; the very low Weibull's modulus confirmed the predominant stage of void nucleation by cracking particles. Nucleation of voids is studied numerically. Results of finite element calculation were compared with fracture surfaces. The microscopic observations confirmed the different initiation mechanisms predicted by modeling: cracking for particulate with sharp corner and microvoid in the matrix (A356+20% SiCp), and debonding for sphere (2124+15% SiCp). Acoustic emission tests were carried out simultaneously with tensile tests, and showed the damage appearation and his increasement

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H.-P. Lieurade

Papers

Improvement of the fatigue life for offshore welded connections

Study of residual welding stress using the step-by-step hole drilling and X-ray diffraction method

Materiaux et structures - fatigue et contraintes residuelles

Etude du comportement et de l'endommagement des composites à matrice aluminium et à renforts particulaires