Cruciform Joints Research Articles

Fatigue life prediction of gas metal arc welded crucifrom joints of AA7075 aluminium alloy failing from root region

Empirical relationship was developed to predict the fatigue life of gas metal arc welded (GMAW) cruciform joints failing from root region. High strength, age hardenable aluminium alloy of AA7075-T6 grade was used as the base material. The design of experiments concept was used to optimize the required number of fatigue testing experiments. Fatigue experiment was conducted in a servo hydraulic controlled fatigue testing machine under constant amplitude loading. The empirical relationship was developed. By using the developed empirical relationship, the fatigue life of GMAW cruciform joints failing from root region was predicted at 95% confidence level. The effect of cruciform joint dimensions on fatigue life was discussed in detail.

Study on the preciseness of hot spot stress of web-stiffened cruciform welded joints derived from shell finite element analyses

A new simple and accurate shell FE-based structural Hot Spot Stress (HSS) determination method for web-stiffened cruciform joints has been proposed. Local stress of welded joints in full-scale bulk carrier (BC)’s lower stool models subject to bending and pressure loadings are examined. HSSs determined by the proposed method are compared with those derived by Lotsberg’s method and the conventional 0.5 t–1.5 t extrapolation. As results, following are found: (1) The local stress of full-scale BC lower stool models with various stool angle and plate thickness can be calculated accurately solely from shell FE results by means of the proposed method. (2) HSSs of welded joints in full-scale BC lower stool models subject to bending and pressure loadings derived by the proposed method show good agreement with the target HSSs determined from fine solid models. This demonstrates the validity of the proposed method for actual ship structures under the real load. (3) The excessive safety allowance of HSS determined by the conventional 0.5 t–1.5 t extrapolation can be reduced substantially by adopting Lotsberg’s method or our proposal. The proposed HSS determination method gives more accurate estimates compared to Lotsberg’s method under the conditions chosen, and the application range of the proposed method is equal to or wider than Lotsberg’s method.

Evaluations of mixed-mode stress intensity factors for load-carrying fillet welded cruciform joints using the least-squares method

This study presents the numerical and experimental applications of using the least-squares method (LSM) to evaluate the mixed-mode stress intensity factors (SIFs) of load-carrying fillet welded cruciform joints. In the numerical application, fillet welded cruciform joints of different combinations of geometric parameters (including weld leg size, initial lack of penetration size, and plate thickness) were systematically studied using the finite element method and the LSM. The derived SIF results were used to develop the general formulae of the opening mode (mode-I) and shearing mode (mode-II) SIFs, K I and K II, respectively. To validate the numerical results, five fillet welded cruciform joint specimens were fabricated and tested by a non-contact optical experiment using a commercial digital camera and the digital image correlation technique for image processing. The numerical and experimental mode-I SIF results were in good agreement with the results from the existing BS 7910 formula for fillet welded cruciform joints. In addition, the suggested non-contact optical experiment is simple and portable, making it well-suited for measuring mixed-mode SIFs directly from the welded details on site. This is very useful for fracture and fatigue evaluation and analysis of weld in existing or newly designed structures, such as steel bridges and offshore structures.

Fatigue Behavior Analysis of Cruciform Welded Joints by Infrared Thermographic Method

Fatigue tests were carried out at different stress levels on cruciform welded joints made from mastensitic stainless steel. The purpose of the present paper was to verify the validity of the thermographic method and to extend its capability on welded structural evaluation, considering the real operating situations. Due to limitations of the traditional fatigue test, the infrared thermographic technique was developed to predict and assess the fatigue limit and the entire S-N (Stress-Life) curve of cruciform welded joints. Through the comparison, the predictions of the fatigue behavior by the thermographic method (TM) manifested good agreement with the traditional method. The present research paper concludes that the TM as a promising method enables us to rapidly obtain reliable fatigue characteristics of welded structural components.

Fatigue strength investigations of welded details of stiffened plate structures in steel ships

Ships are prone to fatigue due to high cyclic loads mainly caused by waves and changing loading conditions. Therefore, fatigue is an important criterion during design. Different approaches are applied to the fatigue strength assessment. However, the results are varying and the validity of results from small-scale fatigue tests for real structures is sometimes unclear. Therefore, deeper fatigue strength investigations were performed within a German industry-wide joint research project aiming at the harmonization of the approaches. Regarding ship structures, two types of structures were selected for the investigations. The first concerns web frame corners being typical for ro/ro ships from which three models were tested. The associated small-scale specimen is a cruciform joint. The second type is the intersection between longitudinals and transverse web frames, which recently showed fatigue failures in containerships. Five models were tested, three under constant and two under variable amplitude loading. The associated small-scale specimen is a longitudinal attachment on a stiffener top. All large-scale tests showed a relatively long crack propagation phase after first cracks had appeared, calling for a reasonable failure criterion. For the numerical analysis, the nominal, the structural hot-spot as well as the effective notch stress approach have been applied. The latter allows the consideration of the weld shape which could partly explain differences in the observed and calculated failure behaviour. The applicability of the different approaches is quite good if some specific aspects are observed. Insofar the investigations give a good insight into the strength behaviour of complex welded structures and into current problems and opportunities offered by numerical analyses.

Experimental study of load-carrying cruciform joints Containing Incomplete Penetration and Strength Under-matching in Low and High Cycle Fatigue Regions

Fatigue behaviors of load-carrying cruciform joints containing incomplete penetration and strength under-matching between base metal and weld deposit were studied. Low and high cycle fatigue tests were performed on specimens with five matching conditions and two sizes of incomplete penetration. Observation of the specimens revealed that crack propagation paths differ by low and high cycle loading conditions and that failure life was dominated by crack propagation. Besides, it was found that the effect of the strength under matching on the fatigue strength becomes large in low cycle fatigue region by significantly reducing fatigue life of the specimen. Moreover, local strain obtained from analysis model can be governing parameter to fatigue life of specimens.

Study on the Preciseness of Hot Spot Stress of Web-Stiffened Cruciform Welded Joints Derived by Shell Finite Element Analyses

A new simple and accurate shell FE-based structural Hot Spot Stress (HSS) determination method for web-stiffened cruciform joints has been proposed. Local stress of welded joints in full-scale BC lower stool models subject to bending and pressure loadings are examined. HSSs determined by the proposed method are compared with those derived by Lotsberg's method (Lotsberg et al., 2007) and the conventional 0.5t-1.5t extrapolation. It is shown that the local stress of full-scale BC lower stool models with various stool angle and plate thickness can be calculated accurately solely from shell FE results by following the proposed method. HSSs of welded joints in BC lower stool models subject to pressure loadings derived by the proposed method show good agreement with the target HSSs. This demonstrates the validity of the proposed method for actual ship structures under the real load. The proposed HSS determination method has advantage over Lotsberg's method in the accuracy and application range.

Fatigue Evaluation on AZ31B Magnesium Alloy Weld Joints by Hot Spot Stress Approach

To study the fatigue properties of magnesium alloy weld structures,the fatigue strength of butt joint and non-load-carrying longitudinal cruciform joint of AZ31B magnesium alloy with 10 mm thickness are evaluated by hot spot stress approach.The test data is statistically treated with the method recommended by International Institute of Welding(IIW).The hot spot stress S-N curves are obtained and compared with the nominal stress S-N curves.The test analysis shows that the fatigue data of two kinds of joints is dispersed greatly when it is evaluated by nominal stress approach,and its correlation coefficient is 0.916,the test fatigue level are respectively FAT35 and FAT25.In comparison with nominal stress approach,the dispersibility evaluated by hot spot stress is reduced obviously.Correlation coefficient is 0.948,which improves about 4.48 %,thus the fatigue strength can be expressed a single one S-N curve,and fatigue level is FAT33.The test fatigue level of these two evaluation methods is higher than the fatigue level obtained by using 3 recommended by IIW,which indicates there is a higher safety factor in weld structure design if 3 is applied.

未溶着部を有する軟質溶接継手の低サイクル疲労特性

Low cycle fatigue strengths of load-carrying cruciform joints with incomplete penetration and strength under-match between base metal and deposit metal were studied. Fatigue tests were performed on specimens with five matching conditions and two sizes of incomplete penetration. Test results revealed that failure life was governed by crack propagation, and that crack propagation paths differed by matching and loading conditions. Also, fatigue strength of the joints was compared regarding the degree of strength matching and size of incomplete penetration. It was found that the effect of the strength matching on the fatigue strength becomes large in low cycle fatigue region by significantly reducing fatigue life of the specimen in the case of under-matched joints.

Fatigue Design of Welded Components of Railway Vehicles — Influence of Manufacturing Conditions and Weld Quality

The weld quality has a significant influence on the fatigue strength of welded joints and components. In the industrial field, there are different manufacturing conditions and as a consequence of this, the weld quality can vary considerably. Therefore, fatigue tests of different joint types were performed, analysing the influence of manufacturing conditions and of weld quality on the fatigue strength of welded components of railway vehicles. For these fatigue tests, typical welded joints with the significantly different notch effect were selected, i.e. butt weld specimens with relatively low notch effect and cruciform joints with relatively high notch effect. These specimens were made under typical industrial conditions by three different companies from the railway industry (Bombardier, Siemens, and ALSTOM). Thus, they represented the weld quality level of this industrial field. Prior to the fatigue testing of these specimens, the weld quality was checked in accordance with the criteria of the DIN EN 15085, which is based upon the general weld quality standard DIN EN ISO 5817. On the basis of the result of this weld quality inspection, most specimens have the quality level B relating to DIN EN ISO 5817. The fatigue tests under axial loading show a significantly higher fatigue strength level for the nominal approach in comparison with IIW Recommendations. However, the determined fatigue stress range of both joint types investigated is conservative, in comparison with the allowable fatigue strength of the actual guidelines or standards applied in the railway industry. For both joint types investigated the relevant notch stress range was determined based on the results for nominal fatigue strength. The fatigue strength differences between the notch stress range of the test results and the notch fatigue resistance of the IIW Recommendations are significantly less than the differences in the nominal strength comparison. The determined nominal fatigue strength values of the investigated welded joints correspond to the weld quality level B, as defined by DIN EN ISO 5817, and additionally, to the requirement of angular distortion ≤ 1°. Furthermore, the determined nominal fatigue strength values of the cruciform joints is related to the throat thickness ≥ 0.75 × plate thickness and to the root face length ≤ 0.75 × plate thickness. In summary, this investigation shows the fatigue strength potential for welded components in the field of the railway industry. The results provide useful information for the economical and safe fatigue design of welded components, depending on the manufacturing conditions and the real weld quality.

십자형 용접부 피로강도 산정을 위한 국부응력법의 비교연구

The notch effects on the fatigue strength of welded joints are both stress concentration and fatigue strength reduction. In the notch stress approach, the notch effects are usually approximated by introducing weld-bead parameters for the local detailed weld joints. In this paper, well-known notch stress approaches - critical distance method, area method and fictitious rounding method are presented for the fatigue strength of cruciform joints. The estimated results of the present methods are applied to the experiments performed in this study and reported in the references. The results of the application show that the fatigue-life scatterness of the experimental data expressed in the nominal stress is significantly reduced by introducing the effective fatigue stress of the present study.

An Effect of Finishing Welded Parts on Fatigue Strength of Cruciform Joints

An Effect of Finishing Welded Parts on Fatigue Strength of Cruciform Joints

Fatigue assessment of naval welded assemblies

Ship structures are commonly assembled by using welding process and they are submitted to some variable and complex loadings. Moreover, near the weld toe, some local stress concentrations are generated. Accordingly, welded joints could be a critical area regarding fatigue damage. In a previous work, a methodology to predict fatigue life has been developed and tested on butt-welded joints. The present work focuses on more complex assemblies in order to be able to estimate the fatigue life of representative naval structures. The strategy could be split into two steps. First, finite element calculation is performed with constant or variable amplitude loadings in order to analyze the elastic shakedown of the structure. To characterize the material heterogeneity of the welded joint, experimental tests on simulated heat affected zone and some micro-hardness measurements have been conducted. If the structure shakedowns, a post-treatment has been developed to predict the fatigue crack initiation which is based on a two-scale damage model initially developed by Lemaitre et al., using again the heterogeneity of fatigue material properties, in order to obtain an accurate numerical predicted fatigue life. To validate this methodology, some experimental tests have been performed on various welded structures (cruciform joints type representative of naval assemblies) and comparisons between experimental and numerical fatigue life are encouraging.

Experimental and Numerical Analyses of Fatigue Behavior of Welded Cruciform Joints

Abstract Ship structures are commonly assembled by using welding process. Due to the swell, all ships are submitted to some variable and complex loadings. The welding process creates specific geometries at the weld toe where local stress concentrations are generated and also creates various mechanical properties in the heat affected zone. Accordingly, welded joints could be a critical area for fatigue damage. In a previous work, a methodology to predict fatigue life has been developed and tested on butt-welded joints. To go further, the present work focuses on more complex assemblies in order to validate this strategy and be able to estimate the fatigue life of representative naval structures. First, the methodology consists of the elastic shakedown study and then of a post-treatment which predicts the fatigue crack initiation. A comparison between experimental and numerical results is proposed in order to present the accuracy of the proposed strategy to predict fatigue life time of welded assemblies typical of shipbuilding applications.

溶接施工に及ぼす静的荷重の影響

The experiments under the static loads were carried out in order to investigate the possibility of repair/reinforcement by welding. Preventing the influence on the experimental devices due to the welding current by isolating between the jigs and the experimental devices, the stable experiments could be carried out. For the cruciform joints and I-shape joints, semi-auto CO2 gas welding was carried out under compressive static loads on the assumption of the dead load. As the experimental results, any cracks were not found. In the cruciform joints with multi-pass welding, there was a tendency that the longer weld length became, the larger the magnitude of out-of-plane displacement became. In the actual construction, it was verified that repair welding under static loads could be performed without welding defects by confirming repair weld length, heat input and inter-pass temperature and so on.

Experimental Investigation of Fillet-Welded Joints Subjected to Out-of-Plane Eccentric Loads

The current AISC design specification for welded connections does not make a distinction between joints subjected to eccentric loads in the plane of the weld group, and those subjected to eccentric loads not in the plane of the weld group. To address this issue, results from 60 tests on cruciform joint specimens are presented to examine the effect of combined shear and out-of-plane bending on the strength of fillet welded joints. All specimens are loaded in a three point bending configuration, such that the out-of-plane bending is resisted through a combination of tensile weld stresses and compressive bearing stresses between the connected plates. Two welding filler metals (flux-cored wires, toughness rated E70T7-K2 and non-toughness rated E70T-7), two nominal weld sizes (5/16 inch and 1/2 inch), three nominal load eccentricity ratios (0.75, 1.375 and 2.125) and three plate bearing widths (1.25 inch, 1.75 inch and 2.5 inch) are investigated. Analysis of the test data, in addition to similar data available in the literature, reveals that the current (13th Edition Steel Construction Manual) AISC design tables for eccentrically loaded welds are highly conservative (i.e. test-to-predicted load ratios are, on average, 1.75; with a coefficient of variation = 0.25) for joints with out-of-plane eccentricity. This conservatism is attributed to the disregard of plate bearing stresses that significantly alter the stress distribution in the joint. An alternate approach that explicitly incorporates this bearing effect is proposed, and the resulting strength predictions are determined to be significantly less conservative when compared to the current design standards. Limitations of the research and future work are outlined.

Fatigue Capacity of Load Carrying Fillet-Welded Connections Subjected to Axial and Shear Loading

The status on current design recommendations concerning the fatigue capacity of load carrying fillet welds was presented by Maddox (Maddox, S., 2006, “Status Review on Fatigue Performance of Fillet Welds,” Proceedings of the OMAE Conference, Hamburg, Germany, Jun., Paper No. OMAE2006-92314) based on a literature survey. In order to examine the validity of the recommendations and to supplement the fatigue test database, a test matrix with 33 specimens was developed. This included 8 simple fillet-welded cruciform joints that were subjected to axial loading and 25 fillet-welded tubular specimens that were subjected to axial load and/or torsion for simulation of a combined stress condition in the fillet weld. The data obtained from these fatigue tests are presented in this paper. The test data are also compared with design guidance from IIW (1996, Fatigue Design of Welded Joints and Components: Recommendations of IIW Joint Working Group XIII-XV, A. Hobbacher, ed., Abington Publishing, Cambridge), Eurocode 3 (1993, Eurocode 3: Design of Steel Structures—Part 1–1: General Rules and Rules for Buildings), and DNV-RP-C203 (DNV, 2005, DNV-RP-C203, Fatigue Strength Analysis of Offshore Steel Structures).

응력확대계수를 이용한 하중 전달형 필릿 용접부의 피로강도 평가에 관한 연구

It is well known that there exist two typical fatigue crack initiation locations in ship structures: one is the weld toe and the other is the weld root where partial penetration weld is performed. In particular, it is important for fillet weldments to avoid weld root cracking in order to prevent catastrophic failure particularly in ship structures. Therefore detail considerations are required for cruciform joints with partial penetration when there is a possibility of weld root crack initiation. For these reasons, fatigue tests on welded joints were performed in this study. Concept of stress intensity factor(SIF) by means of fracture mechanics is applied for predicting fatigue life of fillet welded joints.

The use of the J V parameter in welded joints: Stress analysis and fatigue assessment

This paper investigates the use of the J V parameter, a path-independent integral, for the evaluation of the elastic local stress parameters in welded details and for the estimation of their fatigue life. First, the stress intensity factors (SIF) of an embedded crack lying along the bisector of a sharp V-notch is calculated by means of the J V without modelling the crack and by keeping the same external load and boundary conditions of the cracked model. Furthermore, the notch stress intensity factors (NSIFs) of the welds can be calculated after a post processing procedure of FE analysis with the advantage of using coarse meshes. Second, a correlation between the fatigue life of welded details and the J V parameter is shown. In fact, careful analysis of the fracture surface of fillet welds taken from literature and of new fatigued joints indicates that the first stage of fatigue crack propagation follows the bisector line of the local V-notch as only mode I would be present. Therefore, since the J V evaluated on a suitable integration path represents the SIF of an embedded crack lying along the bisector, the J V is used for the fatigue life assessment of welded details. The critical characteristic length of a suitable integration path for welded joints made of steel and aluminium alloys has been calculated. These critical characteristic lengths were used for the evaluation of two fatigue general scatter bands, mainly based on fatigue data of non-load-carrying cruciform joints characterised by a V-notch angle of 135°. Further, fatigue life data of steel and aluminium alloy welded connections have also been investigated when both mode I and mode II loadings are singular.

Effect of weld geometry on the fatigue life of non-load-carrying fillet welded cruciform joints

In the present study, the effect of weld geometry on the fatigue life of non-load-carrying fillet welded cruciform joints was experimentally investigated. The weld geometry of the cruciform specimens was intentionally varied, and fatigue tests were carried out on various weld geometry configurations. The configurations included weld flank angle, weld toe radius and weld throat thickness. Constant amplitude fatigue tests with stress ratio, R = 0 were carried out using 250 kN servo-hydraulic MTS universal testing machine with a frequency of 5 Hz. From the experimental results, correlation between the weld geometry and the fatigue life of non-load-carrying fillet welded cruciform joints was analyzed in detail.