Cruciform Joints Research Articles

Fatigue life prediction of shielded metal arc welded cruciform joints containing LOP defects by a mathematical model

A new mathematical model is developed to predict the fatigue life of Shielded Metal Arc Welded (SMAW) cruciform joints containing Lack of Penetration (LOP) defect. High strength, quenched and tempered steel (ASTM 517 ‘F’ Grade) is used as the base material throughout the investigation. Four factors, five level, central composite, rotatable design matrix is used to optimise the required number of experiments. Fatigue experiments have been conducted in a mechanical resonance pulsator, under constant amplitude loading. The model is developed by Response Surface Method (RSM). Analysis of Variance (ANOVA) technique is applied to check the validity of the model. Student’s t-test is utilised to find out the significant factors. The effect of joint dimensions on fatigue life have been analysed in detail.

Optimising the shielded metal arc welded cruciform joint dimensions of ASTM 517 F grade steels containing LOP defects

A new method was developed to optimise the dimensions of shielded metal arc welded (SMAW) cruciform joints, containing lack of penetration (LOP) defects, to attain maximum fatigue lives. High strength, quenched and tempered steel (ASTM 517 F grade) was used as the base material throughout the investigation. Design of experiments (DoE) concept was used to optimise the required number of experiments. Fatigue experiments were conducted in a mechanical resonance pulsator, under constant amplitude loading. The optimisation procedure was incorporated by (i) response table and response graph, (ii) normal probability plot, and (iii) analysis of variance (ANOVA) techniques.

Vorhersage der Schwingfestigkeit von Schweißverbindungen auf der Basis des statistischen Größeneinflusses

Die Schwingfestigkeit von Schweißstößen nimmt mit zunehmender Nahtlänge ab. Dieser Effekt wird durch Versuche an Stumpf- und Doppel-T-Stößen nachgewiesen und auf der Basis der Theorie des statistischen Größeneinflusses quantitativ beschrieben. Es wird gezeigt, daß der Einfluß der Nahtlänge auf die Schwingfestigkeit von der Streuung der Kenngrößen für die Nahtgeometrie abhängt. Das Konzept des statistischen Größeneinflusses bewährte sich bei der Anpassung und Vorhersage aller an den Proben gefundenen Ergebnisse. Das Verhalten der geschweißten Platten wird immer dann gut vorhergesagt, wenn die Schwingfestigkeit nicht von weiteren Einflüssen, wie z. B. Eigenspannungen beeinträchtigt wird. Zusätzlich wird ein Verfahren zur Berechnung des Rißfortschritts beschrieben, das Winkel- und Kantenversatz sowie die lokalen Schwankungen der Nahtgeometrie berücksichtigt. Prediction of the fatigue strength of welded joints based on the statistical size effect With increasing length of welding seams the fatigue strength decreases. This effect is confirmed in fatigue tests on butt joints and cruciform joints. The influence of the length of the welding seam on the fatigue strength depends on the scatter of the parameters describing the geometry of the welding seam. The theory of the statistical size effect worked well in fitting and predicting the test results. The fatigue strength of welded joints is well predicted, if no other influences like e. g. residual stresses change the fatigue strength. Additionally a method for the calculation of crack propagation is proposed, which takes in account angular and linear misalignement as well as local changes of the geometry of the welding seam.

突合せ十字継手の塑性変形を伴う脆性破断に関する研究 １ 単調荷重下における溶接十字継手の脆性破断発生と伸び能力

In The Great Hanshin-Awaji Earthquake Disaster, brittle fractures with plastic strain were observed in beam-column connections of steel building frames. It is considered that the mechanical properties of weld metal, especially the ductility of weld metal, have a substantial effect on the fracture.In this paper we describe bi-axial loading test results using cruciform butt specimens under monotonic loading. The specimens are modeled on a cruciform joint by taking out the part of the beam-flange to column-flange connection and welded by two types of welding consumable. So we use two series of specimen that have different ductility. The purpose of this paper is to examine the effect of the ductility of weld metal on fractures under monotonic loading at room temperature and -40°C The high strain rates during the earthquake are substituted for static tests at -40°C.The main results under monotonic loading are summarized as follows. 1) The fracture-surface appearance mainly depends on the specimen temperature and hardly depends on the ductility of the weld metal. 2) In the case of the ductile weld metal specimen, it presented a brittle fracture-surface at -40°C, but the elongation is almost equal to that at room temperature which presented a ductile fracture-surface. 3) In the case of the brittle weld metal specimen, the elongation at -40°C fell to less than 1/2 of that at room temperature, mainly because the brittle fracture occurred after the plastic deformation. 4) In the case of the brittle weld metal specimen, the elongation in the bi-axial tests tends to become large with specimen temperature rising. 5) In the range of this experiment, only the absolute values of reduction of area in tensile test correspond to the elongation capacity in the bi-axial loading test using cruciform butt specimens.

Fatigue crack initiation point in load carrying fillet-welded cruciform joints

Summary Load carrying fillet-welded cruciform joints have two fatigue crack initiation points. They are located in the weld toes and weld roots where high stress concentrations occur. Which of the two serves as the initiation point depends on the weld leg length, plate thickness, and weld penetration, and the mechanism concerned is easily understood. Some experimental results indicate that the fatigue crack initiation point is also affected by the magnitude of the stress range. The reason for this phenomenon remains rather obscure. To establish this reason, the present research study is intended to clarify the relationship between the stress range level and fatigue crack initiation point changes and to quantify the effects of the weld leg length and penetration depth on the fatigue crack initiation point. Fatigue tests of welded joint specimens, thermal elastic-plastic finite element method (FEM) analyses of welding residual stresses and fatigue crack propagation analyses considering the residual stress are performed.

荷重伝達型十字すみ肉溶接継手の疲労破壊起点の検討

Load carrying fillet welded cruciform joints have two fatigue crack initiating points. They are weld toes and weld roots at which high stress concentrations occur. Which of the two is to be the initiating point depends on weld size, plate thickness and weld penetration, and this mechanism can be easily understood. Some experimental results indicated that fatigue crack originating point was also influenced by the magnitude of stress range. The reason for this phenomenon, however, is not clear yet. In this study, in order to make clear the reason, fatigue tests on the welded joint specimens, analyses of welding residual stresses with the aid of the thermo-elastic-plastic finite element method and fatigue crack propagation analyses in considera-tion of the residual stresses are carried out.

Effect of L/ Tp ratio on fatigue life prediction of SMAW cruciform joints of ASTM 517 ‘F’ Grade steels

New fracture mechanics equations have been developed to predict the fatigue life of shielded metal arc welded (SMAW) cruciform joints of ASTM 517 ‘F’ Grade steels, containing lack of penetration (LOP) defects. These equations have been developed by combining the Paris law with the Δ K i-endurance equation The initiation life ( N i) and the propagation life ( N p) of the joints were accounted for, to obtain the total fatigue life ( N f). The initiation life was obtained experimentally using crack initiation criteria. The propagation life was evaluated using the numerically developed equations. The accuracy of the developed equations were tested by comparing the predicted data with the experimental data and it was found that the correlation is quite good. Further, the effect of L/ T p ratio (weld size) on fatigue life prediction was analysed in detail.

Influence of flux cored arc welded cruciform joint dimensions on fatigue life of ASTM 517 `F' grade steels

A mathematical model has been developed to predict the fatigue life of flux-cored arc-welded (FCAW) cruciform joints containing lack of penetration (LOP) defects. High strength, quenched and tempered steel (ASTM 517 `F' grade) has been used as the base material throughout the investigation. The design of experiments (DoE) concept has been used to optimize the required number of experiments. Fatigue experiments have been conducted in a mechanical resonance pulsator, under constant amplitude loading. The model has been developed by use of the Response Surface Method (RSM). Analysis of variance (ANOVA) techniques have been applied to find the significant factors. The adequacy of the developed model has been checked by calculating the correlation coefficient. By using the developed model, the fatigue life of FCAW cruciform joints containing LOP defects can be predicted at a 95% confidence level. The influence of cruciform joint dimensions on fatigue life have been discussed.

Low cycle fatigue strength assessment of cruciform welded joints

This paper presents the results of an experimental study on the low cycle fatigue strength of cruciform welded joints. Tests were performed on 22 specimens of three different categories, namely full-penetration, partial-penetration and fillet welded cruciform joints. These tests consisted basically of imposing on each specimen a constant amplitude cyclic displacement history in the plastic range of the material behaviour. The experimental testing procedure is described and the results are presented. The unified approach for low and high cycle fatigue strength assessment proposed by Ballio and Castiglioni [1]is applied to the test results. Such approach leads to the conclusion that the same set of Wöhler ( S– N) lines [2]usually given in recommendations [3]for high cycle fatigue can be also applicable in the low cycle fatigue range. It is demonstrated that the proposed damage model allows prediction of the number of cycles to failure for the specimens, at each given cycle amplitude, with a notable accuracy.

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.

A new model to predict the fatigue life of flux cored arc welded cruciform joints containing LOP defects

A mathematical model has been developed to predict the fatigue life of Flux Cored Arc Welded (FCAW) cruciform joints containing Lack of Penetration (LOP) defect. High strength, Quenched and Tempered steel ASTM 517 ‘F’ Grade has been used as the base material throughout the investigation. Two level, full factorial technique has been applied to design the experiments. The model has been developed by Response Surface Method (RSM). The adequacy of the model have been checked by ANOVA technique and correlation co-efficient. By using the developed model, fatigue life of flux cored arc welded cruciform joints, containing LOP defects, can be predicted at 95% confidence level.

A longitudinal study of resin-bonded prostheses : Hansson, O. and Bergstrom, B. Journal of Prosthetic Denstistry (1996) 76, 132–139

This paper presents a method for extending the structural stress method as defined in design guidance documents for fatigue assessment of welded plate-type structures. The conventional structural stress approach is not sensitive to plate thickness and an empirical thickness correction factor is recommended. Also, the traditional method specifies alternate fatigue design curves depending on whether the welded joint is generally classified as load carrying or as non-load carrying. The proposed modified structural stress improves both of these limitations by defining a multi-linear stress distribution through the plate thickness beneath the weld toe. The linear structural stress is modified by taking into account the nominal stress in the weld. This modified structural stress better captures the local effect of the weld size and weld stress. Simple equations are presented for determining the proposed stress distribution which, depending on the geometry of the structure, is composed of two or three linear segments. The proposed modified structural stress distribution is easily determined based on the through-thickness stresses computed using finite element analysis. The method is presented and illustrated in the case of a symmetric welded cover plate subject to both axial loading and bending loading. The method has been verified using both fracture mechanic calculations and published experimental results for cruciform joints with varying weld sizes, base plate thicknesses and attachment sizes.

Prediction of Fatigue Life of MIG Welded Cruciform Joints Failing from the Root LOP Region

Prediction of Fatigue Life of MIG Welded Cruciform Joints Failing from the Root LOP Region

Effect of welding process on fatigue crack growth behaviour of austenitic stainless steel welds in a low alloy (Q & T) steel

Fatigue crack growth behaviour from a lack of penetration (LOP) defect in austenitic stainless steel weld metals of cruciform joints made of a low alloy high strength (Q & T) steel has been studied to understand the effect of two welding processes, namely, shielded metal arc welding (SMAW) and flux cored arc welding (FCAW). Fatigue crack growth studies were carried out at a stress ratio of R = 0 and a frequency of 90 to 110 Hz in a resonant testing equipment (Rumul, Model:8601). Crack growth rates were relatively lower in the weld metal obtained by flux cored arc welding process. Microstructural features observed revealed marked difference in the morphology of delta ferrite for the welded joints obtained from the above two welding processes. Long streaks of delta ferrite in austenite matrix were found in case of SMAW-weld metal which seem to have lowered the resistance to the fatigue crack propagation. A discontinuous network of delta ferrite found in austenite matrix in the case of FCAW-weld metal seems to have contributed to slower propagation of fatigue crack. Fractographic features also substantiate the observed trends in the fatigue crack growth behaviour.

Morphological aspects of fatigue crack propagation Part I—Computational procedure

In the present paper a simulation method is proposed for the evaluation of paths and lives of fatigue cracks. The simulation is based on an incremental crack extension procedure. At each increment the stress analysis ahead of a crack tip is carried out by the finite element method, and the next incremental crack-growth path is predicted by the first order perturbation method with the use of the local symmetry criterion. From the computational viewpoint, the step-by-step rezoning of finite element mesh subdivision is one of the most difficult processes of the simulation procedure. In order to overcome this difficulty, we shall use the modified quadtree method as an automatic mesh generation technique. Considerations are made for the proper mesh arrangement in the vicinity of a crack tip, where a special fine mesh pattern is embedded so that mixed mode stress intensity factors and the higher order coefficients of the near tip stress field parameters can accurately be obtained. Using the proposed method, we simulate the branched and curved fatigue crack growth in three-point-bending specimens. They show fairly good agreement with the experimental results. The simulation procedure is also applied to biaxially loaded cruciform joints.

EARLY DEVELOPMENT OF FATIGUE CRACKING AT MANUAL FILLET WELDS

Abstract— An experimental study within the Canadian Offshore Corrosion Fatigue Research Programme was performed on the early development of fatigue cracking along the wavy toe of manual fillet welds between structural steel plates. Stress relieved and as‐welded cruciform joints were tested under R =−1 and R= 0 loading at different stress amplitudes. The depth and the opening level of cracks as small as 10–20 μm were monitored using miniature strain gauges installed along the toe apex, in combination with beach marking. Most of the “initiation life” (25% to 50% of total life), conventionally defined by a crack depth of 0.5 mm, is consumed in short crack propagation. Three types of short crack development for different combinations of local mean stress and stress range are identified and analyzed. Growth rates in as‐welded specimens are faster than in stress relieved specimens, which results in shorter “initiation lives”. This is associated with a higher effective stress range, particularly under R = ‐ 1 loading where cracks are open over nearly the full stress range. The V‐notch stress intensity factor is a promising parameter to rationalize the crack “initiation life”. It takes into account the thickness effect experimentally observed. Under R = ‐ 1 loading of as‐welded joints, using R = 0 data and taking the whole stress range gives a reasonably conservative approximation of the crack “initiation life”.

THE EFFECT OF MISALIGNMENT ON THE FATIGUE STRENGTH OF WELDED CRUCIFORM JOINTS

Abstract— The effect of axial misalignment on the fatigue strength of load‐carrying transverse cruciform welded joints was investigated using experimental and fracture mechanics methods. Where failure occurred by cracking from the weld toe, misalignment significantly reduced the fatigue strength. The reduction could be predicted using a nominal stress concentration factor (SCF). Misalignment had less effect where failure was due to cracking through the weld metal; an expression was deduced for the SCF in this case. For fracture mechanics assessments, an expression for an effective stress intensity factor using the SCF and stress intensity factors for aligned welds was shown to agree with the finite element (FE) results. Predictions of the effect of misalignment using the FE results agreed with experimental data. Misaligned transverse load‐carrying cruciform joints should be assessed for fatigue failure from the toe using the same SCF as for a butt weld with the same misalignment. For failure through the throat, an alternative expression for the SCF is recommended. Fracture mechanics assessments of misaligned joints should be carried out using an effective stress intensity factor derived from the SCF and stress intensity factors for aligned joints. These recommendations are now incorporated in British Standard PD 6493:1991.

A new fracture mechanics method to predict the fatigue life of welded cruciform joints

The fatigue life of MIG welded cruciform joint failing from root (LOP) region was successfully predicted using new fracture mechanics equations. These equations were developed by combining Paris' law and ΔK i-endurance equation and incorporating an integral factor ( I p) obtained by integrating Paris' equation. In the numerical formulation of the equation, the initiation life ( N i) and propagation life ( N p) of the joint were accounted for to obtain the total fatigue life ( N f). The initiation life was obtained experimentally using crack “Initiation Criteria”. The propagation life was evaluated using the numerically developed equations. The method outlined successfully predicted the fatigue life of the joint from its crack growth parameters. To test the accuracy of the method, the predicted data were compared with the experimental data. It was found that the predicted data compared quite well with the experimental data. This method is simple and can be applied easily for other joint geometries.

A fracture mechanics based fatigue life prediction for welded joints of square tubes

Square tubes of mild and low alloy steels are extensively used in vehicle structures and offshore platforms. Despite this fact the fatigue behaviour of welded nodes of these tubes is not sufficiently known, especially in thin sections. The paper presents the results of fatigue life predictions based on fracture mechanics, in non-load carrying T and also load carrying cruciform joints. The fatigue life prediction models include crack initiation and crack propagation. The stress intensity factor formulations were obtained using the weight function solution for semi-elliptical surface cracks with the stresses computed using a three-dimensional finite element program. The prediction results are compared with experimental results and a good agreement was obtained. The tube material was mild steel St 37 (DIN 17100). The loading modes were in-plane three point bending and cantilever bending or tension.

Analysis of fatigue strength on non-load-carrying and load-carrying fillet welded joints

An attempt has been made to study, theoretically, the difference in the fatigue behaviour of non-load-carrying and load-carrying fillet welded cruciform joints. In the former case, the pre-existing flaws produced during the welding process will always propagate from the weld toe. Accurate stress analysis using the boundary element method on the load-carrying fillet welded cruciform joints shows that the weld throat, h, should not be less than 1.414 T, or the weld leg length, w, should not be less than 2.0 T, where T is the main plate thickness, for the cracks to initiate from the weld toe. Furthermore, the values of Mk, the correction factor to take into account the presence of the weld, are found to decrease when the weld size is increased for the load-carrying fillet welded joints. The Paris crack propagation law is then computed numerically to obtain the S/N (signal-to-noise) curves for the two welded joints. The non-load-carrying fillet welded joints give 5 per cent higher fatigue life than the load-carrying ones for an applied stress range of 100 N/mm2.