Fr Mashiri

Effects of weld profile and undercut on fatigue crack propagation life of thin-walled cruciform joint

Fatigue may occur in undercarriages and support systems of trailers, haymakers, graders and swing-ploughs made up of thin-walled tubular sections with wall thicknesses less than 4 mm. Little research has been done on the fatigue of thin-walled tubular sections below 4 mm thickness. The weld profile and weld undercut may affect the fatigue crack propagation life of welded joints especially for thin-walled sections. Numerical analysis of 2-dimensional non-load carrying thin-walled cruciform joints was performed to determine the effect of weld profile and weld undercut on fatigue crack propagation life under cyclic tensile loading. The cruciform joints analysed are made up of 3 mm thick plates, joined by fillet welds. The Boundary Element Analysis System Software (BEASY) is used, which uses fracture mechanics theory to carry out crack propagation analysis. The weld profiles and weld undercuts were measured from welded connections in thin-walled fillet welded sections. The measured weld profiles and undercut were used in the analysis. The results are compared with those of a similar study to determine the comparative reduction in fatigue crack propagation life between thin-walled cruciform joints (T=3 mm) and thicker walled cruciform joints (T=20 mm). This paper provides an understanding of the impact which weld toe undercut has on the fatigue crack propagation life of thin-walled cruciform joints. The presence of undercuts may reduce the benefits of the thickness effect in thin-walled joints of thicknesses less than 4 mm.

Fatigue design of welded very thin-walled SHS-to-plate joints under in-plane bending

The existing fatigue design S-N curve for SHS-to-plate T-joints under in-plane bending is given in the Canadian Standard, CAN/CSA-S16.1-M89, in terms of the classification method. That S-N curve is however based on the class of longitudinally loaded plates with welded non-load carrying attachments, which are different from the SHS-to-plate T-joints. The increased use of welded thin-walled (t<4 mm) tubular joints in the road transport and agricultural industry for applications such as lighting poles, traffic sign supports, truck trailers, swing ploughs, haymakers and linkage graders, means that there is a need to develop fatigue design curves for tubular joints where the tube wall thickness is less than 4 mm. This paper aims to determine fatigue design curves for SHS-to-plate T-joints where the thin-walled tubes have a thickness of less than 4 mm. Tube-to-plate T-joints, made up by welding a square hollow section tube to a plate, are tested under fatigue loading. Constant stress-amplitude cyclic loading is applied to these connections as in-plane bending load. Stress concentration factors (SCFs) have been determined from strain distributions obtained using strain gauge measurements. Analysis of the fatigue test data using least squares method is carried out to determine the design curves of the tube-to-plate T-joints under in-plane bending, for both the classification method and the hot spot stress method. A class of 44 is recommended for the classification method. An S rhs -N curve is proposed, with a recommended SCF of 2.0 for the hot spot stress method.

Fatigue Behavior of Welded T-Joints with a CHS Brace and CFCHS Chord under Axial Loading in the Brace

AbstractThe welded truss composed of circular hollow section (CHS) braces and concrete-filled circular hollow-section (CFCHS) chords is a new kind of structural system that has been increasingly applied in large span arch bridges in China. It is necessary to have a good knowledge of fatigue strength of the welded CHS-to-CFCHS joints for the design of this kind of composite bridge. This paper reports on a series of tests on welded CHS-to-CFCHS T-joints subjected to axial cyclic fatigue loading in the brace. Eleven joints were designed to investigate various influence factors such as different nondimensional geometric parameters of circular hollow sections and different concrete strength grades. The quality of welds connecting brace and chord members were examined using the magnetic particle and radiographic inspection methods. The conditions of hot spot stress at both the crown and saddle positions in brace and chord members were determined by means of linear and nonlinear extrapolation methods. During the...

Stress Concentration Factors and Fatigue Failure of Welded T-Connections in Circular Hollow Sections under In-Plane Bending

The fatigue behavior of welded thin-walled T-joints made up of both circular hollow section (CHS) braces and chords, subjected to cyclic in-plane bending, is described in this paper. CHS chords and braces are of thicknesses less than 4 mm. Current fatigue design guidelines show that the design of welded tubular nodal joints is restricted to thicknesses greater than or equal to 4 mm. The increased availability and use of thin-walled (t<4 mm) tubes of high-strength steels in recent years, in structures subjected to cyclic loading, means that it is important to study the fatigue behavior of welded thin-walled tubular nodal joints. In this paper, welded thin-walled CHS-CHS T-joints subjected to constant-stress-amplitude cyclic in-plane bending range are studied. The stress concentration factors (SCFs) determined experimentally at the brace and chord crown positions are shown to be about 30% and 40% respectively of the SCFs determined using parametric equations in existing fatigue design guidelines. The fatigue tests showed that in welded thin-walled CHS-CHS T-joints, a through-thickness crack occurs when the surface crack length along the weld toes in the chord has grown to a length equal to about 40% of the circumference of the brace member. An end of test failure criterion was proposed as an alternative to the through-thickness failure criterion, in obtaining data that is suitable for determining fatigue design S-N curves.

Stress-concentration factors in circular hollow section and square hollow section T-connections : experiments, finite-element analysis, and formulas

AbstractNodal T-connections made up of circular hollow section (CHS) braces and square hollow section (SHS) chords (CHS-SHS T-connections) have the advantage that they do not require complex brace end preparation when compared with CHS-CHS T-connections. The stress-concentration factors (SCFs) in CHS-SHS T-connections have also been found to be lower than those in SHS-SHS T-connections by previous researchers. At present, no parametric equations have been developed for determination of SCFs for the design of CHS-SHS T-connections. In this investigation, eight CHS-SHS T-connections with unique nondimensional parameters were strain gauged for determination of strain-concentration factors (SNCFs) and therefore SCFs. A three-dimensional finite-element model was then developed using the ANSYS software to simulate the stress distribution at the brace-chord welded interface under axial force and in-plane bending in the brace. Validation of the model was carried out by comparing the SNCFs determined from the expe...

Size effect of welded thin-walled tubular joints

This paper clarifies the terminologies used to describe the size effect on fatigue behavior of welded joints. It summarizes the existing research on size effect in the perspective of newly defined terminologies. It identifies knowledge gaps in designing tubular joints using the hot spot stress method, i.e. thin-walled tubular joints with wall thickness less than 4 mm and thick-walled tubular joints with wall thickness larger than 50 mm, or diameter to thickness ratio less than 24. It is the thin-walled tubular joints that are addressed in this paper. It is found that thin-walled tube-plate T-joints do not follow the conventional trend: the thinner the section is, the higher the fatigue life. It is also found that simple extrapolation of existing fatigue design curves may result in unsafe design of thin-walled tube–tube T-joints. The effect of chord stiffness on fatigue behavior of thin-walled tubular T-joints is also discussed.

Behaviour and design of high strength steel-concrete filled columns

Publisher Summary This chapter deals with the behavior and design of high strength steel–concrete filled box columns. A set of design procedures is outlined for short concrete filled high strength steel box column, which considers the inherent differences between mild and high strength steel. It develops a cross-sectional analysis procedure to study the ductility of such columns. Thrust-moment-curvature relationships are developed and a set of strength interaction diagrams are obtained, which are useful in the design of short columns for strength. Issues, such as increased lettable floor area and savings in weight are calculated and discussed as being major advantages of the implementation and use of high strength steel in concrete filled box columns for use in the construction of tall buildings. .

Structural health monitoring of a dragline cluster using the hot spot stress method

‘Hot spot stress’ is an approach often used to consider fatigue loadings in heavily welded tubular joints. This article reports the determination of hot spot stresses in mining dragline booms, which are often ≥100 m in length, using strain gage measurements and finite element analysis (FEA) modeling as part of a structural health monitoring concept. Strain gages were installed on a typical A11 cluster for estimating hot spot stresses, as recommended in the existing fatigue design guidelines by the International Institute of Welding (IIW) and the International Committee for the Development and Study of Tubular Construction (CIDECT). The results from the experimental measurements and the FEA were found to be comparable to a large measure. It was concluded that while hot spot stresses were high enough at the weld toes to cause cracking, they could not explain the cracking that occurs at the welds in the main chords on their own. Issues in comparing theoretical and experimental measurements are discussed.

High-Cycle Fatigue Behaviour of Welded Thin SHS-CHS T-Joints under In-Plane Bending

Static tests of welded thin walled square hollow sections (SHS) and circular hollow sections (CHS) T-joints, under in-plane bending, have been used to produce load-deformation curves from which the maximum linear response moment of connection has been determined. The loads corresponding to the maximum linear response moment and below have been applied during cyclic loading and found to produce a high-cycle fatigue response in the SHS-CHS joints. A formula has been derived for predicting the ratio between maximum linear response moment and static strength as a function of the nondimensionaL parameters. The fatigue data of the welded thin-walled SHS-CHS T-joints was found to lie within the same scatter band as the S-N data of thin-walled SHS-CHS T-joints. An analysis of the S-N data of the thin-walled SHS-CHS T-joints shows that there is a minimal difference in the design curve obtained when the inherent scatter in fatigue data is taken into account.

ULTIMATE STRENGTH OF WELDED THIN-WALLED SHS-CHS T-JOINTS UNDER IN-PLANE BENDING

Publisher Summary Welded thin-walled T-joints made up of square hollow section (SHS) chords and circular hollow (CHS) section braces are tested under static in-plane bending load. The hollow sections are cold formed and have thicknesses less than 4 mm. The SHS–CHS T-joints are used in building the undercarriages and structural supports of equipment and structural systems used in the road transport and agricultural industries. Failure in the SHS–CHS T-joints is observed to occur as a result of chordface yielding. Chord cracking is also observed after large deformations, resulting in a peak load being attained in these joints. In this chapter, load versus chord flange indentation graphs for the SHSCHS T-joints are used to determine the deformation limit that can be used in defining the ultimate strength of the joints. The deformed shape of the chord observed from experimental tests is used to create a yield line model. A formula is derived for the ultimate strength of the SHS chord and CHS brace vierendeel connections based on a plastic mechanism analysis using yield line theory. The ultimate strength determined through the use of the deformation limit criteria is compared to the ultimate strength calculated using the formula obtained from yield line theory.