Transient out-of-plane distortion of multi-pass fillet welded tube to pipe T-joints

The out-of-plane distortion induced in a multi-pass circumferential fillet welding of tube to pipe under different weld sequences and directions was studied using Finite Element Method(FEM) based Sysweld software and verified experimentally. The FEM analyses consisted of thermal and mechanical analyses.Thermal analysis was validated with experimental transient temperature measurements. In the mechanical analysis, three different weld sequences and directions were considered to understand the mechanism of out-of-plane distortion in the tube to pipe T-joints. It was learnt that the welding direction plays a major role in minimizing the out-of-plane distortion. Further, during circumferential fillet welding of the tube to pipe component, the out-of-plane distortion generated in the x direction was primarily influenced by heat input due to the start and stop points, whereas the distortion in the z direction was influenced by time lag and welding direction. The FEM predicted distortion was compared with experimental measurements and the mechanism of out-of-plane distortion was confirmed.

Generation and Application of A Standardized Load-Time History to Tubular T-joints in Offshore Platforms

Marine structures are mostly made of metals and always experience complex random loading during their service periods. The fatigue crack growth behaviors of metal materials have been proved from laboratory tests to be sensitive to the loading sequence encountered. In order to take account of the loading sequence effect, fatigue life prediction should be based on fatigue crack propagation（FCP） theory rather than the currently used cumulative fatigue damage（CFD） theory. A unified fatigue life prediction（UFLP） method for marine structures has been proposed by the authors＇ group. In order to apply the UFLP method for newly designed structures, authorities such as the classification societies should provide a standardized load-time history（SLH） such as the TWIST and FALSTAFF sequences for transport and fighter aircraft. This paper mainly aims at proposing a procedure to generate the SLHs for marine structures based on a short-term loading sample and to provide an illustration on how to use the presented SLH to a typical tubular T-joint in an offshore platform based on the UFLP method.

Hail impact responses and residual tensile properties of CFRP T-joints

Hail impact is a major challenge encountered by aircraft in flight,and thus is a key concern in the design of damage-tolerant composite T-joints in aviation.The study uses the Z-pinning technique(the pre-hole insertion technology)in combination with fillets of two radiuses to manufacture four types of Carbon Fiber Reinforced Polymers(CFRP)T-joints.The T-joints are then subjected to hail impact tests at two energy levels,as well as post-impact quasi-static tensile tests.The results show that increasing the size of the deltoid and Z-pin-induced reinforcement has a limited influence on the responses of the T-joints to hail impact,but a significant influence on their residual tensile strength after impact.These influences are not only dependent on the structural characteristics of the T-joints,but are also closely related to the degree of discrete damage within the T-joints caused by hail impact.Acoustic Emission(AE)technology is used to monitor the quantitative evolution of damage to the T-joints in a timely manner during the loading process,and helps characterize the characteristics of evolution of various types of damage over different periods.The research here can help design lightweight and damage-tolerant T-joints.

Distribution Features of the Residual Stress For T-Joints by Laser Welding

The residual stress of T-joints for SUS304 stainless steel by YAG laser welding was measured by the method of hole-drilling,and the effects of heat treatment and hardness distribution of weld joints on the distribution of residual stress for T-joints were analyzed.The results showed that the maximum of longitudinal residual tensile stress for T-joints was about 140MPa.The maximum of residual stress was not more than 40MPa after heat treatment,the peak of residual stress reduced obviously.The gradient of residual stress distribution was also reduced significantly.Strain hardening phenomenon occurred for T-joints,and the hardness of weld and heat affected zone was both higher than the hardness of base metal,the peak of hardness occurred in the fusion line.The effect of strain hardening phenomenon of weld and heat affected zone on the residual stress distribution of T-joints for SUS304 stainless steel was obvious,which made longitudinal residual tensile stress become higher.

Biomimetic Composite Structural T-joints

Biological structural fixed joints exhibit unique attributes, including highly optimized fiber paths which minimize stress concentrations. In addition, since the joints consist of continuous, uncut fiber architectures, the joints enable the organism to transport information and chemicals from one part of the body to the other. To the contrary, sections of man-made composite material structures are often joined using bolted or bonded joints, which involve low strength and high stress concentrations. These methods are also expensive to achieve. Additional functions such as fluid transport, electrical signal delivery, and thermal conductivity across the joints typically require parasitic tubes, wires, and attachment clips. By using the biomimetic methods, we seek to overcome the limitations which are present in the conventional methods. In the present work, biomimetic co-cured composite sandwich T-joints were constructed using unidirectional glass fiber, epoxy resin, and structural foam. The joints were fabricated using the wet lay-up vacuum bag resin infusion method. Foam sandwich T-joints with multiple continuous fiber architectures and sandwich foam thickness were prepared. The designs were tested in quasi-static bending using a mechanical load frame. The significant weight savings using the biomimetic approaches is discussed, as well as a comparison of failure modes versus architecture is described.

Multi-objective optimization of gas metal arc welding parameters and sequences for low-carbon steel (Q345D) T-joints

Q345D high-quality low-carbon steel has been extensively employed in structures with stringent weld- ing quality requirements. A multi-objective optimization of welding stress and deformation was presented to design reasonable values of gas metal arc welding parameters and sequences of Q345D T-joints. The optimized factors included continuous variables （welding current （I）, welding voltage （U） ahd welding speed （V）） and discrete variables （welding sequence （S） and welding direc- tion （D））. The concepts of the pointer and stack in Visual Basic （VB） and the interpolation method were introduced to optimize the variables. The optimization objectives included the different combina- tions of the angular distortion and transverse welding stress along the transverse and longitudinal dis- tributions. Based on the design of experiments （DOE） and the polynomial regression （PR） model, the finite element （FE） results of the T-joint were used to establish the mathematical models. The Pareto front and the compromise solutions were obtained by using a multi-objective particle swarm optimization （MOPSO） algorithm. The optimal results were validated by the corresponding results of the FE method, and the error between the FE results and the two-objective results as well as that be-tween the FE results and the three-objective optimization results were less than 17.2% and 21.5%, respectively. The influence and setting regularity of different factors were discussed according to the compromise solutions.

Experimental and numerical investigations of the compressive behavior of carbon fiber-reinforced polymer-strengthened tubular steel T-joints

A method for strengthening damaged tubular steel T-joints under axial compression by wrapping them with carbon fiber-reinforced polymer(CFRP)sheets was proposed and evaluated.The influence of the CFRP strengthening on the failure mode and load capacity of T-joints with different degrees of damage was investigated using experiments and finite element analyses.Five T-joints were physically tested:one bare joint to obtain the peak load and corresponding displacement(D1m),two reinforced joints to provide a reference,and two pre-damaged then retrofitted joints to serve as the primary research objects.The ratio of the pre-loaded specimen chord displacement to the value of D1m was considered to be the degree of damage of the two retrofitted joints,and was set to 0.80 and 1.20.The results demonstrate that the maximum capacity of the retrofitted specimen was increased by 0.83%–15.06%over the corresponding unreinforced specimens.However,the capacity of the retrofitted specimen was 2.51%–22.77%lesser compared with that of the directly reinforced specimens.Next,111 numerical analysis models(0.63≤b≤0.76,9.70≤g≤16.92)were established to parametrically evaluate the effects of different geometric and strengthening parameters on the load capacity of strengthened tubular T-joints under different degrees of damage.The numerical analysis results revealed that the development of equivalent plastic strain at the selected measuring points was moderated by strengthening with CFRP wrapping,and indicated the optimal CFRP strengthening thickness and wrapping orientation according to tubular T-joint parameters.Finally,reasonable equations for calculating the load capacity of CFRP-strengthened joints were proposed and demonstrated to provide accurate results.The findings of this study can be used to inform improved CFRP strengthening of damaged tubular steel structures.

Numerical simulation of temperature fields for T-joint during TIG welding of titanium alloy

Three-dimensional finite element model was established to simulate temperature fields of T-joint titanium sheets during TIG welding with finite element method （FEM） software. Temperature dependent material properties and the effect of latent heat were considered. A technique of element birth and death was used to simulate the process of welded metal filling. Dynamic variation process of temperature fields during T1G welding was achieved. The simulated results agreed well with the measured results.

Numerical analysis of temperature profile and weld dimensions in laser ＋ GMAW hybrid welding of aluminum alloy T-joint

The temperature fields in the transient state and weld dimensions in laser ＋ gas metal arc welding （GMAW） hybrid welding of aluminum alloy T-joint for different welding conditions were calculated using the developed heat source model, and the effect of welding speed on them was analyzed. The results show that the temperature field for the first weld pass only shows the feature of GMAW and the one for the second weld pass has the characteristics of both laser welding and GMAW. Welding speed can affect greatly weld dimensions and temperature distribution. When welding speed reaches 3.5 m/min, the fusion zones of two weld passes are separated and the maximum peak temperature of thermal cycle on the workpiece surface decreases largely.

Progressive Failure Evaluation of Composite Skin-Stiffener Joints Using Node to Surface Interactions and CZM

T shaped skin-stiffener joint are one of the most commonly used structures in aerospace components.It has been proven in various studies that these joints are susceptible to failure when loaded in pull out conditions however,in specific applications these joints undergo pull loading.De-lamination/de-bond nucleation and its growth is one of the most common failure mechanisms in a fiber reinforced composite structure.Crack growth takes place due to the induced interlaminar normal and shear stresses between different structural constituents when a load is applied.In this study,Finite Element Analysis has been performed using cohesive contact interactions on a composite T-joint to simulate the pull out test conditions.A simplified shell based model coupled with CZM is proposed,which can evaluate the failure initiation and progression accurately with lesser computational efforts.The final failure occurred at a displacement of 4.71 mm at the computed failure load of 472.57 kgf for basic configuration.Computed Failure load for the padded configuration is 672.8 kgf and corresponding displacement is 4.6 mm.The results obtained by the proposed numerical model are validated by experimental results and it is observed that predicted failure displacements and failure load calculated were correlating reasonably well with the experiment.

Mechanical Behavior of Tubular T-joint after Fire

This paper presents experimental results of the post-fire behavior of tubular T-joints.The research aims at the failure modes and the residual strengths of the T-joints after fire.Three tests of full-scale tubular T-joints are conducted.The first one is carried out to study the mechanical behavior of T-joints under ambient temperature.The other two tests are performed to study the influence of pre-load,heating and cooling phase on residual load-bearing capacity of the tubular T-joints.The test results show that the sustained axial load on the brace has remarkable influence on the residual deflection of the T-joints which is cooled down to room temperature.The results of the experiments also indicate that the axial load level and heating and cooling history have more significant effects on the compressive stiffness of the T-joints than the residual strength.In the numerical study,the result of finite element model agrees well with the test result.The work in this paper provides a basis for further parametric analysis and theoretical study on the structural evaluation after fire.

Numerical Analysis of Welding Residual Stress and Distortion in Laser+GMAW Hybrid Welding of Aluminum Alloy T-Joint

A 3-D finite element model is developed to predict the temperature field and thermally induced residual stress and distortion in laser＋GMAW hybrid welding of 6061-T6 aluminum alloy T-joint. And the characteristics of residual stress distribution and deformation are numerically investigated. In the simulation, the heat source model takes into account the effect of joint geometric shape and welding torch slant on the heat flux distribution and a sequentially coupled thermo-mechanical method is used. The calculated results show that higher residual stress is distributed in and surround the weld zone. Its peak value is very close to the yield strength of base metal. Besides, a large deformation appears in the middle and rear part of the weldment.

Hybrid laser/arc welding of thick high-strength steel in different configurations

In this investigation, hybrid laser/arc welding （HLAW） was employed to join 8-mm-thick high-strength quenched and tempered steel （HSQTS） plates in the butt- and T-joint configurations. The influences of welding parame- ters, such as laser power, welding speed, stand-off distance （SD） between the arc of gas metal arc welding, and the laser heat source on the weld quality and mechanical properties of joints, were studied to obtain non-porous and crack-free fully-penetrated welds. The weld microstructure, cross- section, and mechanical properties were evaluated by an optical microscope, and microhardness and tensile tests. In addition, a finite element model was developed to investigate the thermal history and molten pool geometry of the HLAW process to join the HSQTS. The numerical study demon- strated that the SD had a paramount role in good synergy between the heat sources and the stability of the keyhole. For the butt-joint configuration, the results showed that, at a higher welding speed （35 mm/s） and optimum SD between the arc and laser, a fully-penetrated sound weld could be achieved. A non-porous weld in the T-joint configuration was obtained at a lower welding speed （10 mm/s）. Microstructural evaluations indicated that the formation of residual austenite and the continuous network of martensitic structure along the grain boundary through the heat affected zone were the primary reasons of the softening behavior of this area. This was confirmed by the sharp hardness reduction and failure behavior of the tensile coupons in this area.