Tubular Joints Research Articles

Modelling of Harmonic Response of Scarf Adhesive Joints

Adhesives are used for attaching structures to various spacers such as tubular and scarf joints to replace the traditional joining methods of welding, brazing, soldering, etc. Many of the unique features associated with adhesives are low manufacturing cost, long component life, and lightweight. Accordingly, a considerable amount of study has been carried out and is still underway to develop. The main objective of this study is to investigate the response of the scarf joints when exposed to a harmonic load. The finite element method (FEM) using ANSYS is availed to study and analysis the natural frequencies, mode shape, and frequency response. Additionally, the influence of the adhesion angle is examined.

Modelling of Harmonic Response of Scarf Adhesive Joints

Adhesives are used for attaching structures to various spacers such as tubular and scarf joints to replace the traditional joining methods of welding, brazing, soldering, etc. Many of the unique features associated with adhesives are low manufacturing cost, long component life, and lightweight. Accordingly, a considerable amount of study has been carried out and is still underway to develop. The main objective of this study is to investigate the response of the scarf joints when exposed to a harmonic load. The finite element method (FEM) using ANSYS is availed to study and analysis the natural frequencies, mode shape, and frequency response. Additionally, the influence of the adhesion angle is examined.

Numerical Analysis and Experimental Study of Carbon Fiber Reinforced Composite Joints under Bending Load

Three dimensional finite element models of composite joints were established to investigate the load-displacement behavior, failure mode of multi-axial tubular joints under bending load, and stress-strain relationship in some key positions. The joints were prepared by plain weave fabric. The effective elastic constants of fabric composite were calculated using meso-mechanics theory. A progressive failure analysis was performed using ABAQUS software to obtain the ultimate strength and failure mode of the sample. In addition, the damage process, failure mode and damage position was further studied. The bending properties of the joints were also presented by quasi-static load test using a three-point bending test device. Results of the ultimate load and damage analyses are compared to experimental data. The accuracy of the method was proved by the consistency of the relation between the load displacement curve trend and the correlation of the damage position and failure pattern.

Static load-bearing capacity formulation for steel tubular T/Y-joints strengthened with GFRP and CFRP

Based on full-scale experiments on steel tubular T/Y-joints wrapped with FRP, nonlinear finite element (FE) models were verified and calibrated for prediction of the static load-bearing capacity (SLC) of FRP-strengthened joints. The verified FE methodology was utilized to perform parametric studies. Due to the fact that local buckling of the chord member is the governing mode of failure in tubular joints capacity, FRP strengthening on the chord is determining while brace strengthening showed negligible effects. Increasing the brace-to-chord diameter ratio (β), chord diameter-to-thickness ratio (γ), and brace inclination angle (θ) enhanced the joint capacity but decreased the FRP effectiveness on the joint capacity improvement. Based on the results of the experimentally-verified numerical analyses and parametric studies, three indices ψ, λ and δ were introduced as strengthening index, FRP fibers orientation and the stacking sequence, to represent the effective parameters respectively. Multiplying these indices by the unstrengthened joint capacity constitutes a practical and efficient formulation for estimating the SLC of FRP strengthened joints. Since a large number of FE models were generated and analyzed, the presented formulas can be employed for FRP optimal application on tubular joints, while providing a guideline for the assessment of the strengthened joint SLC.

Accelerated curing of adhesively bonded G-FRP tube connections — Part I: Experiments

In the wake of the industry-wide trend towards lightweight construction, fibre-reinforced polymers (FRP) gradually established as an alternative to traditional materials. They are used in particular when low weight, high strength or high corrosion resistance is required. For joining FRP components, adhesive bonding is increasingly preferred to mechanical fasteners as poorer transfer of loads resulting from interrupted fibres represents problematic. Despite all the technical advantages, adhesive bonding remains a comparatively slow joining process that is usually bound to limited manufacturing temperatures. This article shows, by means of experimental investigations performed on bonded FRP tubes, that curing by inductive heating of the adhesive bond can compensate for both disadvantages. For that, particles susceptible to electromagnetic fields (EMF) were added to two adhesives, a two component epoxy (2 K-EPX) and polyurethane (2 K-PUR), and the effect thereof on the adhesive was assessed. The particles consisted of Curie material (Curie particles, CP), which relieved all requirements regarding temperature control during the induction process. Subsequently, G-FRP single lap tubular joints (SLTJ) were assembled, cured following different protocols, and subjected to tensile tests. During the process, curing temperatures were monitored using appropriate measuring techniques. Mechanical testing showed that inductively cured SLTJ achieved equal or higher shear strengths, if compared to reference sets cured following the instructions given by the adhesive manufacturers. The results showed that adhesive type as well as curing conditions, in most cases had no significant influence on joint strength, which was verified by means of an analysis of variance. Summing up, inductive heating using CP proved effective in controlling temperatures without any external control, and yielded very high joint strengths with a reduction in curing time by the factor of 100 for the epoxy and 1400 for the polyurethane.

Numerical investigation and design rules for stress concentration factors of stainless-steel hybrid tubular joints

This paper describes a numerical study on stress concentration factors (SCFs) of stainless-steel hybrid tubular T-, Y- and X-joints with circular hollow sections (CHSs) and with square hollow sections (SHSs). The finite element models (FE models) were developed and validated against the corresponding test results available in the literature. The validated FE models were then utilized to perform an extensive parametric study containing 47 FE models for T-joints, 47 FE models for Y-joints and 47 FE models for X-joints. In total, 141 FE models were analyzed in the parametric study. The brace diameter to chord width ratio (β=d1∕b0), brace to chord thickness ratio (τ=t1∕t0) and chord width to thickness ratio (2γ=b0∕t0) were selected as the key geometric parameters in the parametric study. The SCFs at the hot spot locations obtained from the parametric study were compared against those calculated from the design formulae specified in the current design guidelines of the International Committee for the Development and Study of Tubular Structures (CIDECT). The comparison results show that the design formulae given in the CIDECT specification are not appropriate for predicting the SCFs of stainless-steel CHS-to-SHS hybrid tubular T-, Y- and X-joints. Therefore, new design equations were proposed in this study. It is shown that the values of SCFs calculated from the proposed design equations are generally in good agreement with the values obtained from the FEA.

Stress concentration factors of multi-planar tubular KT-joints subjected to in-plane bending moments

Numerical research on the stress concentration factors (SCFs) in uniplanar tubular joints is abundantly reported in literature. However, it has been shown that SCF equations for uniplanar joints can lead to both under- or overestimation of the SCFs in multi-planar joints. In this paper, a parametric finite element study of 81 different three-planar KT joints subjected to five different in-plane bending loading conditions is performed. The effects of different non-dimensional geometrical parameters on the SCF values at the crown locations of the central and outer braces are studied. Based on nonlinear regression analyses of the finite element results, a new set of SCF equations is developed and presented.

A multi-axial fatigue-oriented strategy for fatigue damage monitoring and assessment of tubular joints

In this paper, a structural health monitoring strategy that performs in-time multi-axial fatigue damage evaluations for welded tubular joints is proposed. First, we developed a stress monitoring program that obtains the reference for the multi-axial fatigue strength assessment, namely, the components of the hot-spot stress and zero point structural stress, from a sensor arrangement strategy representing explicit physical meaning for tubular T-joints. Then, we established a programmable procedure for multi-axial fatigue damage evaluation based on stress monitoring, which is verified through a finite element simulation. Taking practical interests into consideration, we suggest that the proposed strategy is suitable for in-field engineering practice.

Buckling Behavior of Non-Retrofitted and FRP-Retrofitted Steel CHS T-Joints

This paper aims to investigate the buckling behavior of circular hollow section (CHS) T-joints in retrofitted and non-retrofitted states under axial brace compressive loading. For this purpose, two types of analysis are carried out. The first one is evaluating the critical buckling load in various tubular joints, and the other one is investigating the post-buckling behavior after each buckling mode. More than 180 CHS T-joints with various normalized geometric properties were numerically modeled in non-retrofitted state to compute their governing buckling mode, i.e., chord ovalization, brace local, or global buckling. Then three joints with different buckling modes were selected to be retrofitted by fiber-reinforced polymer (FRP) patches to illustrate the improving effect of the FRP wrapping on the post-buckling performance of the retrofitted joints. In addition, FRP composite failures were investigated. The results indicate that the FRP retrofitting is able to prevent the brace local buckling, and that matrix failure is the most common composite failure in the retrofitted joints.

Experimental and numerical research on fire resistance of stainless steel tubular X-joints

The fire test was carried out on six stainless steel square hollow section (SHS) tubular X-joints to investigate the influences of axial loading ratio in brace member, brace to chord width ratio and inclined angle between brace and chord member on the fire resistance performance of joints. The furnace temperature-time curve, surface temperature-time curve, vertical deformation-time curve, lateral deformation of chord web-time curve and failure mode for all specimens were obtained by the fire test. The test results show that, the failure modes of stainless steel SHS tubular X-joints at elevated temperatures can be divided to three types: plastic failure of chord flange, plastic failure of chord flange and web, and buckling failure of chord web. Then, the failure criterion of stainless steel SHS tubular X-joints under fire conditions was proposed: the joint can be considered to be damaged when its concave deformation of chord flange reaches 0.03 times of chord width. The finite element models of stainless steel SHS tubular X-joints were established and validated against the test data. Finally, based on the validated models, a parametric analysis was performed on the influencing factors of the fire resistance of tubular joints, and the results show that the failure temperature of stainless steel SHS tubular X-joints will be enhanced with the increase of brace to chord width ratio and chord width to thickness ratio, or with the decrease of axial loading ratio in brace member and chord preload ratio.

Structural behaviour of thin-walled CHS-RHS T-joints: Experimental and numerical assessment

The design of tubular joints with slender cross-sections is currently limited by the validity ranges of established international design standards. Aiming to assess the structural behaviour of tubular joints characterised by thin-walled chord cross-sections, this paper initially reports, an experimental investigation into tubular T-joints consisted of CHS braces welded to thin-walled cold-formed RHS chords, which slenderness values were outside of the current design standard limits. The experimental programme was carried out on 12 cold-formed steel slender cross-section prototypes where the braces were subjected to axial compressive loading. The geometric parameters, including chord and brace thickness and brace diameter were varied. The connected chord face failure was the failure mode observed in all tests. The results of the experimental tests were thoroughly discussed. A second step was used to calibrate a numerical model that was the basis for a parametric analysis involving the most relevant parameters affecting the joint's behaviour. The obtained joint's strengths were compared with those forecasted by ISO 14346 developed for compact cross-sections modifying the chord stress function concerning the elastic cross-section modulus and effective cross-section area when slender cross-section chords are concerned. However, this approach leads to scattered predictions, roughly too conservative. Finally, this paper proposes an alternative design approach for the studied joints that are more accurate in predicting their strength when compared to the presented experimental and numerical results.

Computation of stress concentration factor of tubular joints for the fatigue analysis of steel structures

The offshore structures are welded tubular structures which are continuously subjected to cyclic environmental loads like waves, winds, currents, earthquakes etc. These cyclic environmental loads (predominantly wave loads) will induce time varying cyclic stresses on the offshore structures causing fatigue damages even at very low nominal stress levels. Fatigue failure is defined as the tendency of a material to fracture by means of progressive brittle cracking under repeated alternating or cyclic stresses of intensity considerably below the normal strength. Fatigue is very critical at the welded joints, because the stresses at the welded regions are several times higher than the nominal stress. Stress Concentration Factors (SCFs) converts the nominal stress into higher stresses known as hot spot stresses. These SCFs are very sensitive in the computation of fatigue life of steel structures. A small difference in SCFs can cause a huge difference in fatigue life of steel structures. Different empirical methods give different values of SCFs for the same element, hence different fatigue life. This study aims to investigate the stress concentration factor of different configurations of tubular joints numerically using Finite Element Analysis (ANSYS) under certain boundary conditions and compare the result with the SCF obtained from empirical method (using parametric equations).

Experimental and numerical study on ultra low cycle fatigue fracture of X steel tubular joints with CHS braces to SHS chord

Steel tubular joints in steel structures may suffer from ultra low cycle fatigue fracture under strong seismic action. This paper aims to investigate the ultra low cycle fatigue behavior of X steel tubular joints with CHS braces to SHS chord by means of experiments and numerical simulation. Firstly, three X-joints with different constructions were tested. The experimental results indicate that the X-joint with fillet weld or penetration weld exhibits punching failure mode, and the weld forms have no obvious effect on the deformation ability and fatigue life of the joint. As for the X-joint with internal stiffener in the chord, it shows tensile fracture failure characteristic and has low deformation capacity, and there is no obvious surface crack before sudden fracture failure. Afterwards, the fracture failure of the X-joints were numerically simulated by using the cyclic void growth model. It shows that the crack growth rate is greatly underestimated by the model, and the predicted fatigue life of the X-joints was obviously longer than that of the tests, thus the evaluation on fatigue life of X-joints tended to be unsafe. A Lode parameter enhanced cyclic void growth model (referred to as LCVGM) was thus developed to consider shear effect on the fracture prediction, and the LCVGM parameters of base metal and weld metal were determined based on the experimental results of the pure shear specimens. Finally, the LCVGM was applied to simulate the ultra low cycle fatigue fracture of the X-joints, and it is shown that the model can well predict the whole fracture process and fatigue life of the joints.

Relationship among joined tubular material properties, joining behavior and performance by elastomeric swaging

Joining by elastomeric swaging (ES) is a deformation-based way to manufacture tubular form-fit joints composed of aluminum alloy (AA) tubes and high-strength steel sleeve-fittings. To explore the relationship among the joined tubular material properties, joining behavior and performance of tubular form-fit joints fabricated by elastomeric swaging (ES), this paper took 6061-T4 and 5052-O AA tubes as material cases and carried out a comprehensive investigation by modeling and experiments. In terms of these two AA tubes, their microstructure was characterized by means of EBSD; their mechanical properties were characterized by tension tests based on digital image correlation and Visco-plasticity Self Consistent modeling. The characterization results demonstrate that the 6061-T4 tube has precipitation strengthening mechanism, texture unfavorable to shear, no Portevin–Le Chatelier (PLC) effect, larger strain hardening exponent and higher strength. In view of process, based on the material modeling for anisotropic AA tubes, hyperelastic polyurethane elastomer hose and 15-5PH sleeve, implicit axisymmetric numerical models of ES joining process with loading and unloading were established and validated. The simulation results show that there is a larger normal deformation in the 5052-O tube under the same extrusion depth and rate, and at the same bulging heights as 5052-O, the 6061-T4 tubular joints have larger residual contact shear stress at the undercuts. As for service performance, the ES joining tests and joining performance verification experiments were carried out. The experimental results show that the 6061-T4 tubular joining components have higher pressure resistance, larger pull-out force and better fatigue endurance. In conclusion, the joined tubes with texture unfavorable to shear, higher strength, larger strain hardening exponent and no PLC effect are beneficial to forming form-fit joints.

Experimental and numerical investigation of ultimate strength of ring-stiffened tubular T-joints under axial compression

The design procedure based on empirical equation for computing the chord strength of tubular joints in offshore structures has been established by various researchers and adopted by American Petroleum Institute and other organizations. Local stiffening of chord may be required to carry the loads from braces from various directions. One such method is to introduce internal circular rings at the brace-chord interface. In the present study experimental and numerical investigation on a ring-stiffened tubular joint have been carried out using 1:8 scale model of commonly adopted configuration used in the industry. Experiments have been conducted to measure the failure loads of stiffened and unstiffened tubular joints of (T-geometry) with plain and flanged ring-stiffeners fitted inside the chord. Models were fabricated using steel material and tests were conducted to measure the chord deformation using displacement transducers. First peak in load-displacement relationship has been used to determine the ultimate capacity. Numerical simulations have been carried out using ABAQUS software based on nonlinear stress-strain relationship of steel. The load-displacement characteristics for stiffened and unstiffened joints have been established. Results obtained from numerical simulation have been compared with that obtained from the experimental studies. Strength enhancement factors for stiffened joints have been calculated and compared with that of the unstiffened joints. The enhancement factors for plain and flanged internal ring-stiffener are noted to be 1.66 and 2.03 respectively.

Experimental investigation on the seismic performance of concrete-filled steel tubular joints in diagrid structures

Concrete-filled steel tubular (CFST) joints are widely used as a common joint form in diagrid structure systems. However, previous studies on CFST diagrid joints have mainly focused on the compressive bearing capacity, and they lack relevant experimental studies on the seismic performance of diagrid structure joints. In this paper, five CFST diagrid joints are tested under cycle loading to study the failure mode and hysteretic characteristics. In addition, the influence of the intersection angle, structural forms and the wall thickness of the steel tube on the seismic performance of the joints, such as the bearing capacity, ductility, and energy dissipation, are analysed. Three CFST joints are mainly damaged by shear cracking in the centre of the joints, and the other two specimens are damaged by buckling of the inclined column. Test results showed that the increase in the intersection angle reduces the bearing capacity of the specimen and changes the distribution of the main energy dissipation; the increase in the wall thickness of the steel tube can effectively improve the bearing capacity and energy dissipation capacity of the specimen. The joint reinforcing ring can effectively improve the ductility, energy consumption and other seismic performance of the joint. In the CFST diagrid joints, the weak parts along the horizontal intersecting line of the joint and the intersection between the joint and inclined column need to be strengthened in the actual structure.

Finite element modelling and proposed design rules of stainless steel hybrid tubular joints with square braces and circular chord

This paper presents a finite element (FE) study to investigate the ultimate strength of stainless steel hybrid tubular X-, T- and Y-joints with square braces and circular chord. The accuracy of FE models was verified against the test data reported in Feng et al. [6]. Using the verified FE model, a parametric study was conducted including 288 FE models, which includes 144 models for T/Y-joints and the remaining 144 models for X-joints. In the parametric study, critical geometric parameters of the brace width-chord diameter ratio (β = b1/d0), the brace thickness-chord thickness ratio (τ = t1/t0), the chord diameter-chord thickness ratio (2γ = d0/t0) and the angle from the centre lines of brace to chord (θ) were varied. It was concluded from parametric studies that the load-carrying capacities of tubular joints were influenced by the geometric parameters, being especially sensitive to the value of β. Furthermore, the parametric study results were compared with the current design predictions determined from the International Committee for the Development and Study of Tubular Structures (CIDECT), Eurocode 3 (EC3), Chinese Code (GB50017) and Australian/New Zealand Standard (AS/NZS). The comparison results demonstrate that the current design codes are inaccurate to determine the strength of stainless steel SHS-to-CHS hybrid tubular T/Y- and X-joints. Therefore, improved design equations were proposed based on the current design guidelines of CIDECT, which could closely match the test and FEA results.

A numerical study and proposed design rules for stress concentration factors of stainless steel hybrid tubular K-joints

This paper presents a finite-element analysis (FEA) of the stress concentration factors (SCFs) of stainless steel hybrid tubular K-joints with square braces and circular chord and K-joints with circular braces and square chord. The developed finite-element (FE) models include material non-linearities and are validated against corresponding experimental results available in the literature. The validated FE models were used to conduct an extensive parametric study comprising 324 FE models. The critical geometric parameters of the parametric study include: brace width-to-chord diameter ratio (β1 = b1/d0), brace-to-chord thickness ratio (τ = t1/t0), chord diameter-to-thickness ratio (2γ1 = d0/t0), brace diameter-to-chord width ratio (β2 = d1/b0), chord width-to-thickness ratio (2γ2 = b0/t0), eccentricity (e), gap distance (g) of K-joints with gap, and overlap ratio (Ov) of overlapped K-joints. The results of the parametric study show that the effect of β on the SCF varies for different types of K-joints. For all hybrid tubular K-joints, the values of SCFs increases with an increase in 2γ value. For gapped K-joints, the values of SCFs increases with an increase in e value. An attempt was made to compare the numerical results against the SCFs calculated in accordance with International Committee for the Development and Study of Tubular Structures (CIDECT) (2001) for both the stainless steel hybrid tubular K-joints with square braces and circular chord and K-joints with circular braces and square chord. This comparison reveals how the design equations available in CIDECT (2001) for pure circular and square tubular joints are not suitable for predicting the SCFs of both the stainless steel hybrid tubular K-joints with square braces and circular chord and K-joints with circular braces and square chord. Therefore, improved design equations are proposed in this paper for predicting accurately the SCFs of stainless steel hybrid tubular K-joints with square braces and circular chord, as well as circular braces and square chord.

Development of parametric equations for ultimate capacity of internally ring-stiffened tubular T/Y-joints under axial and moment load

ABSTRACT Tubular joints in offshore structures is being designed using semi-empirical equations developed based on experimental and numerical studies and same were recommend for use in API-RP-2A and other codes. The parametric equation does not cover the tubular joints reinforced with internal ring-stiffeners. Hence, an attempt is made in the present study to develop parametric equations as a capacity enhancement factor (Qr ) to ultimate strength equation recommended for unstiffened joints by API-RP-2A. The development of Qr for T/Y-joints under axial and moment load has been carried out using non-linear finite element. Numerical model has been validated using measured results from a scale model test conducted for this purpose and matches reasonably well. Parametric studies have been performed for various geometric non-dimensional parameters. The proposed parametric equations for ring-stiffeners in axial and moment load works well with the unstiffened equations specified in API-RP-2A enabling easy design procedure for ring-stiffened joints.

Interference-fit joining of Cu-SS composite tubes by electromagnetic crimping for different surface profiles

This paper explores the possibility of producing an improved interference-fit tubular joint between pure copper and stainless steel tube by electromagnetic crimping (EMC). Successful joints are obtained with optimal parameters consisting of the outer surface profile of the inner tube and discharge energy. Joints exceeding the strength of the parent tube are formed without any metallic bond formation. Three destructive testings, namely, pull-out, compression and torsion tests, have confirmed the successful joint formation as the failure occurs in the copper base tube. Analysis of failure mechanisms has revealed joint behaviour at various discharge energies and surface profiles. Among smooth, knurled and threaded surface profiles with different discharge energy levels, knurled surface provides best joint strength at 6.2 kJ, which has shown a failure in the parent Cu tube during all three destructive testings. Radial deformation of the tubes is measured and compared for different discharge energy and surface profile. No metallic bond formation and the wavy interface are observed between the joining partners during microstructural analysis. Furthermore, no elemental overlapping is observed during energy dispersive spectroscopy mapping analysis indicating an absence of diffusion. Higher micro-hardness is observed near the Cu-SS tubular joint interface due to strain hardening caused by high-velocity impact.