The key challenge of such predictions using fracture. Oto 1999022 fatigue life implications for design and. These inconsistencies in scf derivation are reflected in the hotspot sn curves used to. Stress concentration factors of cold formed stainless steel tubular x joints r feng a and b young b a school of civil engineering, hefei university of technology, hefei, anhui, china b department of civil engineering, the university of hong kong, pokfulam road, hong kong abstract. A wave histogram has to be obtained for each direction around the platform. G also known as the hotspot stressstructural stress, is used to calculate the fatigue life of a tubularnontubular joints. Consequently, the ability to assess the magnitude of the stress concentration is a prerequisite to dealing with the fatigue problem of tubular joints. Youve probably felt your joints crack or pop when you exercise. An extensive parametric study is presented of the distributions of stress concentration factors in tubular k joints commonly found in offshore platforms. Significant stress concentrations at the vicinity of the welds are highly detrimental to the fatigue life of a tubular connection. Structural failure due to scc can be very unpredictablefailure could occur after as little as a few hours of exposure, or the equipment could continue to function normally for.
Stress concentration factors for the fatigue design of. Jiki studied the stress concentrations in gapped tubular k joint using 20noded. Stress concentration factors of cold formed stainless. I have been tested for lyme through igenex which i should get todaythe thing is i live in sf so its likely not that. Stress concentrations in tubular joints welded from one side. The aim of this paper is to study the stress concentration factors of doubler plate reinforced tubular t joints subjected to various types of basic loading such as axial tension, axial compression, and in plane and outofplane bending. Simple joints complex joint figure 4 types of joints the term simple joint is deceptive, because even the simplest tubular joint has a complex stress pattern for the most basic loads. Stress concentrations reduce the fatigue behavior of welded. Stress intensity factors of tubular ty joints subjected to three basic loading 111 define the joint can be derived from figure 1. Stress concentration factors for simple tubular joints. Single sided welds commonly used for connecting braces to chords in tubular joints for offshore structures are known to have failed from cracks initiated at the root of the weld. Proceedings of the institution of civil engineers structures and buildings, volume.
The influence of longitudinal stiffeners on weld toe stress concentrations in multi planar tubular kk joints p. Stress intensity factor for a tubular tjoint with grouted. Experiment investigation of stress concentration factor of. The fatigue life of the joint depends very much on the stress concentration factor, scf. General requirements for details in tubular joint nodes. Stress corrosion cracking scc is a progressive fracturing that occurs in metals as a result of the combined influence of tensile stress and a corrosive environment. These joints are differentiated by the mode of load transfer exhibited. This paper reports a study on the determination of stress intensity factors in tubular joints in offshore jacket structures. Degree of bending in axially loaded tubular ktjoints of. Scfs are used for the prediction of the hotspot stress, in order to assess joint structural integrity against fatigue. Grouted t joint has less stress concentration but lower degree of bending dob. The definition of hotspot stress 14, as the region where fatigue cracking is most likely to initiate due to a stress concentration, proposed linear extrapolation of the. Stress concentrations due to misalignment at butt welds in plated structures and at girth welds in tubulars. Stress concentration factor of doubler plate reinforced.
Although creaky joints are unnerving, in most cases they are not cause for worry. Proceedings of the institution of civil engineers structures and buildings. The research focusses on failure modes, an understanding of stress concentrations on the welded tubeplate or tubetube interface using experimental or numerical methods. My joints are cracking and popping when exercising. Finally, a methodology for predicting the hotspot stress and the fatigue life of tubular dt joints is presented. Stress concentrations in tubular joints welded from one. Weld toe stress concentrations in multiplanar stiffened. Prediction of stress concentrations and degrees of bending. This fatigue crack is always initiated at the weld toe of the brace to chord intersection areas in the tubular joints where high lever stress concentration is induced by the structural discontinuity. Stress concentration factors for the fatigue design of tubular flange connections. Stress concentration and fatigue of welded details, ph. Emphasis is also placed on the complexity of capturing different types of stresses in tubular non tubular joints for estimation of scf. The surface crack is located at the weld toe on the chord side of the hot spot stress location, and its value is obtained using the scf equations recommended in apirp2a 19. This paper presents a comprehensive study on stress concentration in welded tubular steel n joints.
Stress concentration factors scfs for partial joint. These joints are subjected to axial load, by use of nonlinear least squares fitter, the above results are derived a new set of parametric equations, in terms of several nondimensional geometric ratios. Hot spot stress determination for a tubular tjoint under. Note that the stress concentration factor is a function of the geometry of a crack. Stress concentration factors of multiplanar tubular xt. This paper concerns the calculation of stress concentration factors scfs in steel multiplanar welded tubular double t dt connections, caused by axial loads. Stress concentration and fatigue of welded details by nicholas zettlemoyer jfritz engineering laboratory llibrary a dissertation presented to the graduate committee of lehigh university in candidacy for the degree of doctor of philosophy in the department of civil engineering lehigh university bethlehem, pennsylvania october 1976. Apr 15, 2014 in the design of recently constructed steelconcrete composite bridges using cfchs truss girders for the main load carrying structure, the fatigue verification of the tubular spatial truss joints is a main issue. Role of stress concentration on fatigue of welded joints.
Therefore, an experimental study has been carried out on tubular joints in order to characterize the 3d residual stress field, using mainly the neutron diffraction method. Large stress concentrations arise1 due to the abrupt geometric discontinuities at the intersections of these welded tubular members, called joints or nodes. The main components of steel offshore structures, whether fixed or floating, are generally tubular members. As the radius of curvature approaches zero, the maximum stress approaches infinity. Experimental study on the fatigue behaviour of welded tubular k. Single sided welding presupposes that the fatigue crack always initiates at the weld toe. Highlights fracture mechanics study is conducted on grouted and aswelded tubular t joints. Correlation of stress concentration factors for twelded. These inconsistencies in scf derivation are reflected in. Jan 01, 2009 read weld toe stress concentrations in multiplanar stiffened tubular kk joints, international journal of fatigue on deepdyve, the largest online rental service for scholarly research with thousands of academic publications available at your fingertips. Although the tubular x joints are commonly found in offshore jacket.
Stress concentration factors for simple tubular joints 3. Estimation of stress concentrations in singlesided welds. Fatigue damage appears in the form of semielliptical surface cracks. The influence of longitudinal stiffeners on weld toe. This relates the stress at the critical location to the nominal stress. Traditional design of welded joints against fatigue is made according to codes such as eurocode iii 9. Tubular joint, stress concentration, stiffener, fatigue life, literature. Abstract fatigue cracking caused by stress concentrations in tubular joints has.
This study covers a comprehensive range of geometric joint parameters under balanced axial loading. Stress concentration factors of coldformed stainless steel. This could be an immense contribution to the increase in fatigue life of the joint, but other hot spot sites are introduced into the joint. Nominal stress ranges in braces and chords are calculated by a global stress analyses. Stress concentration factor for some dtk joints based on. Stress corrosion cracking scc is the growth of crack formation in a corrosive environment. I have thought of fibro and am considering that, i hear stress can bring it one. Perovi c department of mechanical engineering, university of montenegro george washington sn, 8 podgorica montenegro abstract the assessment of beneficial effects of weld geometry modification methods is considered in this paper. The equations used for the determination of the tubular joint scf 3 4 are based on the equations developed by efthymiou 5. The highest scfs are usually found for stainless steel tubular xjoints with medium. Nominal stress approaches are based on extensive tests of welded joints and connections.
Stress concentration factors in frp strengthened tubular t. Stress concentration factors of doubler plate reinforced. This uniaxial approach is useful to many applications. The kk joint finds application in the leg structure of offshore oil and gas jackup platforms. Read behaviour of tubular t joints subjected to combined loadings, journal of constructional steel research on deepdyve, the largest online rental service for scholarly research with thousands of academic publications available at your fingertips. Fatigue cracking caused by stress concentrations in tubular joints has been observed in some fixed platforms installed in hostile environments. Calculation of joint fatigue lives using sn curves for tubular members at joints. Location results for brace spacing in gap tubular k joints. Stress and strain concentration factors of completely. It is generally assumed that fatigue cracks will develop where the peak stresses. The rack plate works as a stiffener which reduces the stress concentration at the bracechord intersection. The current practice is to use semiempirical formulas proposed by various investigators to determine scf. Stress concentrations in tubular dtjoints for fatigue. G also known as the hotspot stress structural stress, is used to calculate the fatigue life of a tubular nontubular joints.
An extensive parametric study is presented on stress concentrations in tubular k joints calculated using thin shell finite elements. As long as the cracking is not accompanied by pain or swelling, there is no reason for immediate concern. Read weld toe stress concentrations in multiplanar stiffened tubular kk joints, international journal of fatigue on deepdyve, the largest online rental service for scholarly research with thousands of academic publications available at your fingertips. In this paper parametric equations are presented for estimating stress concentration factors scfs in tubular ty and k joints with emphasis on overlapped joints where no parametric equations are as yet available. The program included fem analysis of 9 k joints and comparison of the results to other known data. Corrosionpedia what is stress corrosion cracking scc. Finite element analysis of a tubular tjoint subjected to various loading conditions including pure axial loading, pure in plane bending ipb. This is due to global and local stress concentrations, presence of welding defects and residual stresses introduced by welding process. Structures of offshore platforms are generally subjected to high loads. Bending part of stress intensity concentration factor. The repeated damages in tubular joints have clearly shown that the safety of these structures depends on the safe fatigue design of the joints. Weld toes stress concentrations in multi planar stiffened. The fatigue analysis of offshore platforms requires accurate knowledge of the stress concentrations at tubular intersections.
Stress concentrations caused by the above three basic load cases have been extensively studied. A finite element study of stress fields and stress. Stress concentration, especially in the welded joints of these structures, is an important design consideration. A synthesis based on the strain energy density over a control volume.
In the design of recently constructed steelconcrete composite bridges using cfchs truss girders for the main load carrying structure, the fatigue verification of the tubular spatial truss joints is a main issue. The present paper offers a comprehensive study on stress concentrations in welded tubular steel gap k joints and its implications on fatigue design. Using finite elements, information on stress concentration factors and throughthickness stress distributions was first obtained from uncracked geometries. Dec 18, 2018 hearing a crackle or pop while youre exercising can be alarming. Stress concentration factors in frp strengthened tubular t joints under brace in plane bending and outofplane bending moments 2 investigated by jiao and zhao 7. New parametric equations for stress concentrations in. The results of this investigation were used to assess the existing parametric equations of efthymiou proc. Significant stress concentrations at the vicinity of the welds are highly detrimental to the fatigue life. A stress concentration factor is the ratio of the highest stress s max to a reference stress s of the gross crosssection. Stress concentration factors of cold formed stainless steel. Weld joints are classified by type, loading and shape. The varying environmental loads acting on these joints cause fatigue crack initiation, growth and their final catastrophic failure. To investigate the acceptability of considering only the outside of single sided welds in rather stiff tubular joints, an analysis program of k joints has been performed.
Simple tubular joint scf stress concentration factors. Flange joints are used for base supports and continuity joints of tubular members in a variety of. Corrosion fatigue of welded highstrength cast and structural. The pain feels like its sharp hits to my boes joints not my muscles. Joint stress fda prescribing information, side effects and uses. Tubular structures are widely used in offshore installations, trusses, high rise buildings, towers for wind turbines, skilift installations, lightning, road pole signals etc.
Due to the difference in deformations between the brace and chord member of a joint, the tube wall tries to bend to maintain the compatibility and therefore, giving rise to. For simple tubular joints several anomalies have been identified between measured scfs. Stress concentration factors of njoints of concretefilled. Therefore, an in depth stress analysis of a tubular joint always requires a finite element analysis, or alternatively, stress determination by experimental methods. Hot spot stress determination for a tubular tjoint under combined axial and bending loading m. Probabilistic assessment of degree of bending in tubular x. L 8 axial outofplane bending antisymmetrical inplane bending js symmetrical a b fig. In simple tubular joints, these cracks usually initiate from the weld toe and then propagate into the tubular chord wall. Stress concentrations due to misalignment at butt welds in. The stress intensity factors sifs determined agree with the fatigue test results. Twodimensional stress distribution cannot be easily represented by a polynomial. Evaluation of stress concentrations in simple and complex nedes.
According to astm d883, stress cracking is defined as an external or internal crack in a plastic caused by tensile stresses less than its shortterm mechanical. The rack plate is a dual purpose element of the joint because it firstly functions as a stiffener which reduces the stress concentration at. The fatigue life of tubular joints commonly found in offshore industry is not only dependent on the value of hotspot stress hss, but is also significantly influenced by the throughthethickness stress distribution characterized by the degree of bending dob. The stress intensity factor range ak for an applied cyclic stress range at is given by ak yatrx 6 prediction of stress concentrations and bending in tubular k joints 89 the crack growth rate may be described by the pads equation 20 da cakm 7 dn for the region where the crack growth rate is approximately linear. These differences led to inconsistencies both in the measured scfs themselves, and also in the scf parametric formulae based on these measured scf values. The main issue with single sided welding is the occurrence. Improved formula for the stress intensity factor of semi. The dob exhibits considerable scatter calling for greater emphasis in accurate determination of its governing probability distribution which is a key input for the fatigue reliability analysis of a tubular joint. Due to the difference in deformations between the brace and chord member of a joint, the tube wall tries to bend to maintain the compatibility and therefore, giving rise to geometric stress. Prediction of fatigue crack growth behaviour of tubular joints used. Read prediction of stress concentrations and degrees of bending in axially loaded tubular k joints, journal of constructional steel research on deepdyve, the largest online rental service for scholarly research with thousands of academic publications available at your fingertips. Stress concentration factors in uniplanar tubular kt. Formulas for variation of stress concentration around. A tubular joint is made up of a chord element of largest diameter and one or more braces.
Stress concentrations in simple tubular joints introduction typical offshore structures are composed of three dimensional frames fabricated from steel tubulars. Stress concentration, especially in the welded joints of these structures, is an. Fracture mechanics is a useful approach in which the fatigue life of the welded joint can be predicted. Stress concentrations in tubular joints welded from one side authors morten baerheim. In twelded joints, fatigue cracking initiates at the weld toe, which acts as a raiser. For some joint configurations the addition of ringstiffeners can lead to an increase in the hotspot stress. Fatigue life of tubular joints in offshore structures is significantly influenced by the degree of bending dob. Significant stress concentrations at the vicinity of the welds are highly detrimental to the fatigue life of a tubular. The finiteelement method is adopted in the numerical parametric studies to evaluate the stress distribution.
The connections between the different tubular members are denoted tubular joints. The aim of this paper is to study the stress concentration factors of doubler plate reinforced tubular t joints subjected to various types of basic loading such as axial tension, axial compression. Stress concentration factors for simple tubular joints hse. Fatigue failure could occur even when the stress level is much lower than the ultimate stress of the material. The design principle is a specific fatigue reference curve for each class of weld excluding the influence of material properties. This is because the tubular joints are subjected to loadings caused by alternating wave forces. This method of fabrication, sometimes referred to as pointtopoint construction, is cheaper and quicker than the alternative of separately fabricating nodal joints with brace stubs connected by double sided welding.
Single sided welding welding from the outside only is used as the principal method for connecting braces to chords for tubular joints in offshore structures in areas of the world other than the north sea. Some of the key principles on which these efforts were based include. For these reasons tubular joints are widely used for jackets in offshore platforms, shown respectively in figure 1 and figure 2. Stress and strain concentration factors of completely overlapped tubular kn joints article in journal of structural engineering 1291 january 2003 with 27 reads how we measure reads. Stress concentration factors in uniplanar tubular ktjoints of. Fatigue strength analysis of offshore tubular welded. Prev next stress concentrations of k tubular joints subjected to basic and combined loadings. There is currently no common industry standard to assess the fatigue performance of singlesided welds and hence guidelines on this issue are urgently needed. Feb, 2017 welded joints are prone to fatigue cracking with the existence of welding defects and bending stress. One of the main causes of damage to offshore structures is fatigue failure of the tubular joints.
There are a few reasons why this happens, but unless theres pain or swelling, you have nothing to worry about. Fatigue cracks generally occur on the chord in the region of high local stresses. Combined with high stress concentrations, tensile residual stress is a relevant factor in fatigue crack development. These structures are mainly fabricated from steel with welded complex tubular steel nodes where fatigue cracking occurs in the weld toe, weld undercut or in the weld due to stress concentrations. A comprehensive comparison of tubular joint scf parameteric equations can be found in stress concentration factors for simple tubular joints, assessment of existing and development of new parametric formulae 6. However, the particular problem of stress concentra tions in lapped and gapped tubular joints with respect to effective location of the spacing of the braces has not hitherto been properly addressed. The effect of low dob should be considered for current hot spot stress sn method. Prediction of fatigue crack growth rate in welded tubular. Evaluation of stress concentration for planar tubular joints. Measured hotspot stresses in the joints were compared with hotspot stresses calculated using. Contraction joints must be deep enough to ensure they are truly weakened planes that crack before random or outof joint cracking occurs. By research of simple tubular joints, thinshell element analyses have been conducted for thousands of dtk joints frequently used in offshore structures in this paper. Stress concentration factors in tubular kjoints under in. Environmental stress cracking esc is one of the most common causes of unexpected brittle failure of thermoplastic especially amorphous polymers known at present.
The weld profile effect on stress concentration factors in weldments zoran d. Residual stress effects in tubular kjoints crack growth. This paper reports a parametric stress analysis of various configurations of rack plate stiffened multiplanar welded kk joints using the finite element method. It can lead to unexpected sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. Stress concentration factors of cold formed stainless steel tubular x joints r feng a. Tubular joints are subjected to cyclic loads induced by sea waves and hence they are susceptible to fatigue damage due to the formation and propagation of cracks. Aging is a common cause of cracking and popping joints during exercise, as is weight gain.
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