Manufacturing and mechanical properties of thermoplastic hybrid laminates based on DP500 steel

This paper investigates the manufacturing and mechanical properties of thermoplastic composite-metal hybrid laminates based on DP500 steel sheets bonded to a glass fiber-reinforced polypropylene (PP) or to a self-reinforced PP composite materials. Single cantilever beam (SCB) tests revealed that a good level of adhesion can be achieved by using a hot dip zinc surface treatment on the DP500 steel and incorporating an interlayer based on modified polypropylene at the interface between the composite and metal plies. In addition, SCB tests have shown that the interfacial fracture energy of the hybrid laminates increased with increasing crosshead displacement rate. Additional testing of a number of laminates has shown that the tensile properties of these composite-metal hybrid laminates are strongly dependant on the tensile properties of the constituent materials and that a modified rule of mixtures could be used to predict such properties.     

FRP strengthening of steel and steel-concrete composite structures: an analytical approach

A recent technique for strengthening steel and steel-concrete composite structures by the use of externally bonded Fiber Reinforced Polymer (FRP) sheets, to increase the flexural capacity of the structural element, is described. Several researches developed FRP strengthening of reinforced concrete and masonry structures, but few experimental studies about steel and steel-concrete composite elements are available. Some examples of guidelines for the design and construction of externally bonded FRP systems for strengthening existing metal structures are available, but the method used to predict the flexural behaviour of FRP strengthened elements is usually based on the hypothesis of elastic behaviour of materials and FRP laminate is mainly considered only under the tensile flange. In this paper, an analytical procedure to predict the flexural behaviour of FRP strengthened steel and steel-concrete composite elements, based on cross-sectional behaviour and taking into account the non-linear behaviour of the materials with any configuration of FRP reinforcement, is given. Analytical predictions are compared with some experimental results available in the literature on the flexural behaviour of FRP strengthened steel and steel-concrete composite elements, showing good agreement of the results, even in the non-linear phase, until failure.     

Fatigue behaviour of dissimilar friction stir spot welds between A6061‐T6 and low carbon steel sheets welded by a scroll grooved tool without probe

A6061 and low carbon steel sheets, whose thicknesses were 2 mm, were welded by a friction stir spot welding (FSSW) technique using a scroll grooved tool without probe (scroll tool). Tensile‐shear fatigue tests were performed using lap‐shear specimens at a stress ratio R = 0.1, and the fatigue behaviour of dissimilar welds was discussed. Tensile‐shear force of the dissimilar welds was higher than that of the A6061 similar ones. Furthermore, the dissimilar welds exhibited nearly the same fatigue strengths as the A6061 similar ones, indicating FSSW by a scroll tool was effective technique for joining aluminium to steel sheet. Fatigue fracture modes of the dissimilar welds were dependent on load levels, where shear fracture through the interface between A6061 and steel occurred at high load levels, while crack grew through A6061 sheet at low load level.     

Similar and dissimilar RSW of low carbon and austenitic stainless steels: effect of weld microstructure and hardness profile on failure mode

Abstract

In this paper, the failure behaviour of similar and dissimilar resistance spot welded joints of low carbon and austenitic stainless steel sheets was studied under tensile shear test with attention focused on the failure mode. Results showed that the microstructure of the fusion zone and the hardness distribution across the weld have a profound effect on the failure behaviour. Similar spot welds of stainless steel sheets exhibit the highest tendency to fail in interfacial failure mode, compared to low carbon steel similar spot welds and dissimilar low carbon and stainless steel spot welds. This behaviour is explained by the consideration of pullout failure location and hardness profile characteristics of each joint. It was shown that the failure mode transition is controlled by the hardness ratio of the fusion zone and the pullout failure location. In the case of dissimilar resistance spot welding, the hardness of the fusion zone which is governed by the dilution between two base metals, and the fusion zone size of the low carbon steel side are the dominant factors determining the failure mode of the joint.     

强动载荷下焊接钢板力学性能及本构模型研究

为研究强动载荷下船用焊接钢板的力学性能。开展了典型船用焊接钢板母材、焊缝和热影响区的准静态拉伸试验、高温拉伸试验及SHPB动态压缩试验,分析了焊接钢板材料在不同应力状态下的力学行为,基于力学性能试验结果拟合了焊接钢板母材、焊缝和热影响区材料的本构模型。结果表明:准静态条件下,与母材相比,焊缝和热影响区材料的屈服强度与抗拉强度偏大,延伸率偏小;高应变率下,热影响区材料抵抗塑性变形的能力明显强于其他两种材料,且随着应变率的增加抵抗塑性变形的能力呈增强趋势;焊接板母材、焊缝与热影响区材料均表现出应变率效应和温度效应;热影响区是焊接板抗冲击性能相对薄弱的区域。建立的Johnson-Cook模型可以描述强动载荷下焊接钢板的力学性能。     

Fatigue behaviour of friction stir welds without neither welding flash nor flaw in several aluminium alloys

Fully reversed axial fatigue tests have been performed in order to investigate the fatigue behaviour in the friction stir welds of 1050-O, 5083-O, 6061-T6 and 7075-T6 aluminium alloys. In all alloys, the comparative studies on the fatigue behaviour between parent materials and welds have been done. The fatigue behaviour of the welds was sensitive to the microstructures such as stir zone, thermo-mechanically affected zone and heat affected zone. The fatigue strengths of the welds are comparative to or lower than those of the parent materials. The observed fatigue strengths were discussed based on the microstructure and crack initiation behaviour.     

CREEP BEHAVIOUR OF FULL SIZE WELDED JOINTS: DESIGN IMPROVEMENTS

In order to ascertain the validity of creep strength reduction factors for welds in real components, creep tests were performed at 600 °C on large specimens made of welded 316  L (N) stainless steel plates. Manual Metal Arc (MMA) welds as well as Tungsten Inert Gas (TIG) welds were investigated. The overall, and in some cases, local deformations of the weld were recorded. The experimental results were compared with two-dimensional, and in some cases, three-dimensional finite elements computations using the creep behaviour of the materials as determined by testing small standard specimens. The times to rupture were compared with predictions from design codes and other simplified engineering methods.     

Improving the ballistic immunity of armour steel weldments by plasma transferred arc (PTA) hardfacing

This investigation describes about improving the ballistic immunity of armour steel joints which are fabricated by sandwiching of plasma transferred arc (PTA) hardfaced interlayers in between soft austenitic stainless steel (ASS) welds. From the results, the welds with sandwiched interlayer stopped all the projectiles successfully, irrespective of processes used, whereas welds without sandwiched interlayer were failed. In order to know the cause of failure, a detailed metallographic examination was carried out. The variation in microstructure and hardness at various zones of the weld are discussed. For the first time, it was found that the armour steel could be hardfaced by the PTA process with tungsten carbide powder.     

Microstructural and Mechanical Observations of Galvanized TRIP Steel after Friction Stir Spot Welding

Friction stir spot welding was done in transformation-induced plasticity steel sheets coated with zinc. The influence of tool rotational speed and dwell time on the microstructure and mechanical properties of lap-joints were investigated. After processing, different zones were formed in the joints. Microstructures in each zone depended on the welding conditions employed. Higher dwell time coupled with higher rotational speed promoted the deposition of a large amount of allotriomorphic ferrite beside the keyhole left by the pin. Coalesced bainite formation was stimulated by the deformation. Mechanical and chemical stabilization of the austenite occurred in different welding zones. Some zinc from the coating remained in the joint, in the stirring zone, representing a partial bonding between the steel sheets. The strength of the welds depended on a complex interaction between geometrical features, such as bonding ligament length and distance between the zinc and the keyhole left by the pin and the resultant microstructure in the stirring zone. The highest joint strength was observed for the “lowest tool rotational speed–highest dwell time” combination of welding parameters.     

Finite element method simulation of residual stresses in Al2O3–AISI 304 steel joints

Abstract

Thermal residual stresses are very detrimental to the mechanical resistance of metal–ceramic joints and thin metallic foils acting as stress relieving interlayers have been used to reduce their effect. The present work presents finite element method simulations of the residual stress field in Al₂O₃–AISI 304 steel joints using interlayers. Different interlayer materials (Ti, Ni, Mo, and Cu) were considered, either separately or in combination. Calculations show that among the different interlayer materials considered, Cu and Ti/Cu are most effective in reducing the thermal stresses and that this role is determined mainly by the ductility of the interlayer material. The calculated results were validated by shear tests performed on real joints obtained by diffusion bonding and it was concluded that residual stresses control the mechanical resistance of the joints.     

Mechanical Properties and Corrosion Resistance of Nitrided or Oxinitrided,and Powder Painted Regular and Interstitial Free (IF) Drawing Steel Sheet

Specimens of 0.8 mm thick regular and interstitial free (IF) drawing steel sheet have been nitrided in fluidised bed for 2 hours at 620 °C and 560 °C with and without a post‐oxidation, and slow and accelerated cooling. As a result, surface hardness, yield and tensile strength of the sheets increased considerably without a critical loss of ductility. Resistance welds between the sheets did not lose their original strength after nitriding‐oxinitriding. Nitrided‐oxinitrided at 620 °C and then powder painted sheets, as compared with powder painted raw sheets, were more corrosion resistant in neutral salt spray and climatic tests. Some mechanical and anticorrosion properties of the IF steel sheet that had undergone the nitriding‐oxinitriding processes were definitely better than those of equally processed regular steel sheet.     

Structural and Mechanical Features of Laser-Welded Joints of Zinc-Coated Advanced Steel Sheets

Materials Science - We analyze the quality of laser-welded bodywork steel sheets. The welding was done by a CO2 laser. We studied the welds made on steel sheets of advanced grades: high-strength...     

Interface structure and bonding in abrasion circle friction stir spot welding: A novel approach for rapid welding aluminium alloy to steel automotive sheet

Aluminium alloy 6111-T4 and steel DC04 1 mm sheets have been successfully welded with a cycle time <1 s by “Abrasion circle friction spot welding”, a novel approach to joining dissimilar materials. This was achieved by using a probe tool translated through a circular path to abrade the steel sheet. It is shown that successful welds can be produced between these two weld members with a cycle time of less than one second, that exhibit very high failure loads and a nugget pullout fracture mode desired by industry. Transmission electron microscopy investigation of the joint interface revealed no intermetallic reaction layer. The weld formation mechanisms are discussed.     

Softened zone formation and joint strength properties in dissimilar friction welds

The mechanical properties of dissimilar MMC/AISI 304 stainless steel friction welds with and without silver interlayers were examined. The notch tensile strengths of MMC/AISI 304 stainless steel and MMC/Ag/AISI 304 stainless steel friction welds increased when high friction pressures were applied during the joining operation. The higher notch tensile strengths of dissimilar MMC/AISI and MMC/Ag/AISI 304 stainless steel friction welds resulted from the formation of narrow softened zones in MMC material immediately adjacent to the bondline. The influence of softened zone width and hardness (yield strength) on the notch tensile strengths of dissimilar welds was analysed using finite element modelling (FEM). FEM in combination with the assumption of a ductile failure criterion was used to calculate the notch tensile strengths of dissimilar joints. The key assumption in this work is that dissimilar weld failure wholly depended on the characteristics (mechanical properties and dimensions) of the softened zone formed in MMC material immediately adjacent to the bondline. The modelling results produced based on this assumption closely correspond with the actual notch tensile strengths of dissimilar MMC/Ag/AISI 304 stainless steel and MMC/Ag/AISI 304 stainless steel friction welds.     

Interfacial bonding mechanism in Al/coated steel dissimilar refill friction stir spot welds

Defect-free dissimilar Al/zinc coated steel and Al/AlSi coated steel welds were successfully fabricated by refill friction stir spot welding. However, Al alloy and uncoated steel could not be welded under the same welding condition. Al-Zn eutectic layer formed at the Al/zinc coated steel interface showed non-uniformity in thickness and nanoscale intermetallic (IMC) produced was discontinuous. The bonding formation between the Al-Zn layer and the surrounding materials was attributed to a liquid/solid reaction mechanism. Bonding formation at Al alloy and AlSi coated steel interface was attributed to a solid/solid reaction mechanism, as the joining process did not involve with melting of base metals or AlSi coating materials. Kissing bond formed at the weld boundary acted as a crack initiation and propagation site, and the present study showed that weld strength of Al 5754/AlSi coated steel was greatly influenced by properties of original IMC layer.     

基于微观机理的Q460钢材角焊缝搭接接头延性断裂研究

该文基于微观断裂力学的方法,针对Q460钢材角焊缝搭接接头的断裂行为进行了研究。分别通过对接焊缝试件的静力拉伸试验得到了焊缝材料的本构关系,11个开圆弧形平滑槽圆形截面拉伸试件标定了焊缝材料微孔扩张模型VGM和应力修正临界应变模型SMCS的断裂参量η、α,5个正面角焊缝搭接接头拉伸试验获得了焊接接头的荷载-变形曲线并进行了数值模拟。试验结果表明,E55-D2型焊材的角焊缝搭接接头拉伸破坏形式呈延性破坏。断裂参量结果分析表明,焊材的化学成分会导致焊缝金属断裂性能的差异。该文研究成果为焊缝材料建立了准确的数值模型,同时为高强度钢材焊缝的设计和焊接结构的破坏预测提供了一种有效的方法。     

Microstructural and mechanical characterisation of laser-welded lap joints with linear and circular beads in thin low carbon steel sheets

This paper presents a microstructural and mechanical characterisation of laser-welded lap joints in low carbon steel thin sheets. Different combinations of steel types (DC05, S355MC) and thickness values are used to assemble welded specimens with linear and circular weld bead. Metallurgical observations and micro-hardness tests are used to characterise the weld microstructure. Mechanical response in tensile test is then used to evaluate the static strength, rotation angle of weld bead and failure mode of welded specimens. Lap-joints with circular weld showed a lower rotation angle compared to linear welds. The fracture in all tested specimens occurred at the base metal, far away from the weld. A simplified mechanical model is finally proposed to derive theoretical formulae for estimating the tensile strength of welded joints as a function of material properties and weld geometry. The analytical results are in good agreement with experimental findings and they estimate an increased strength for circular welds, compared to linear weld with same lateral width. A design chart is also derived to allow a design of laser-welded joints with virtually equal strength of base metal and weld zone.     

Fatigue performance of butt welds between cast steel joint and steel tubular members

Cast steel joints have gained increasing popularity for use in engineering. The fatigue performance of butt welds between cast steel joint and steel tubular members, however, is not yet well characterized. A series of fatigue tests were conducted on a new type of welding detail, cast steel joint with sloped end and integrated backing ring (Type C), for butt welds between cast steel joint and steel tubular members. Fatigue failure mechanism and S–N curves obtained from the tests were compared with those of Type A connection with backing ring and Type B cast steel joint with integrated nose obtained in literature. Fatigue behaviours of all three welding details were successfully predicted by local strain approach, the results of which are consistent with test results, especially for Types A and B welding details. According to the results of tests and analysis, fatigue cracking of butt welds between cast steel joint and steel tubular members always initiates from the bottom of the weld root, and the fatigue performance of Type C welding detail is better than those of Types A and B because of its lower stress concentration level. A simplified fatigue design equation was then proposed based on the results of local strain approach, in which the fatigue notch factor was adopted in fatigue behaviour evaluation. The accuracy of this simplified fatigue design equation was verified for Type C welding details with various geometry configurations.     

THE EFFECTS OF CRACK DEPTH ON THE J-INTEGRAL AND CTOD FRACTURE TOUGHNESS FOR WELDED BEND SPECIMENS

This work deals with the influence of crack depth on the fracture toughness at initiation of crack growth and the constraint factor in relationship between the J-integral and the crack tip opening displacement (CTOD). A series of tests were performed on high strength low alloyed HT80 steel welds, and the critical J-integral and CTOD were determined using the load versus load point displacement record from three-point bend specimens with 0.05 < a/W < 0.5. It was found that the fracture toughness for shallow cracks at the onset of crack growth was larger than that for deep cracks for the steel welds tested, but it is felt that there is no fixed relationship between these values in the welds tested. The constraint factor is also a function of crack depth, and values of the factor increase from 0.5 to 1.5 when a/W increases from about 0.05 to 0.5. The factors are not very sensitive to the crack tip materials (HAZ or weld metal) in the welds tested.     

Finite element simulation of the micromechanics of interlayered polymer/fibre composites: A study of the interactions between the reinforcing phases

Finite element analysis has been applied to study the mechanical behaviour of composites with ductile thermoplastic and rubbery interlayers between fibres and matrix. The stress distribution in the transverse direction and the interactions between fibres was investigated with particular emphasis on the effect of varying the modulus and thickness of the interlayers. The results of the analysis show that the use of interlayers is more beneficial at higher fibre volume fractions in enhancing the energy absorbing capabilities of a polymer/fibre composite. In particular, ductile interlayers are clearly shown to have a less detrimental effect than rubbery interlayers on the modulus of coated fibre composites.