Microstructure and mechanical properties of laser welded dissimilar DP600/DP980 dual-phase steel joints

The use of dual phase (DP) steels in the automobile industry unavoidably involves welding and dynamic loading. The aim of this investigation was to evaluate the microstructural change and mechanical properties of laser welded dissimilar DP600/DP980 steel joints. The dissimilar joints showed a significant microstructural change from nearly full martensite in the fusion zone (FZ) to the unchanged ferrite-martensite dual-phase microstructure in the base metal. The welding resulted in a significant hardness increase in the FZ but the formation of a soft zone in the heat-affected zone (HAZ). The dissimilar welded joints were observed to exhibit a distinctive unsymmetrical hardness profile, yield-point-like phenomenon, and single-stage work hardening characteristic, with yield strength and work hardening rate lying in-between those of DP600 and DP980 base metals, and ultimate tensile strength equivalent to that of DP600 base metal. Although the welded joints showed a lower fatigue limit than the base metals, the fatigue life of the welded joints at higher stress amplitudes was almost the same as that of the DP600 base metal. The welded joints failed in the soft zone at the DP600 side under tensile loading and fatigue loading at the higher stress amplitudes. Fatigue crack initiation occurred from the specimen surface and crack propagation was characterized by typical fatigue striation together with secondary cracks.     

Effect of solution treatment on the strength and fracture toughness of aluminum alloy 7050

The effect of the solution treatment on the tensile property and fracture toughness of aluminum alloy 7050 were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), tensile test and the plane-strain fracture toughness test. The results show that with increasing single-stage solution temperature, the volume fraction of the residual phase decreases, but the volume fraction of the recrystallized grains and the size of the sub-grains increase. Thus, the strength and fracture toughness of the single-stage solution treated samples increase first and then decrease. The enhanced solution treated samples result in an improved dissolution of the residual phase, a lower recrystallized grains fraction and smaller sub-grains, which leads to a higher strength and fracture toughness than that of the single-stage solution treated samples. The grain structure of the high temperature pre-precipitation treated samples is similar to that of the enhanced solution treated samples. However, the high temperature pre-precipitation treated samples exhibit a lower strength and fracture toughness, due to a mass of AlZnMgCu phase precipitating from the matrix.     

国产X100钢级螺旋缝埋弧焊管的力学性能分析

研究了国产X100钢级Φ1016 mm×15.3 mm螺旋缝埋弧焊管采用不同的试样类型以及在线模拟防腐加热试验对钢管拉伸性能的影响.试验发现,X100钢级板状试样与圆棒试样拉伸性能的差异没有明显的规律性,在线的涂敷加热试验温度对此种焊管拉伸性能的影响比较有限.     

Mechanical Behavior of Nanostructured and Microstructured Explosives

Nanostructured hexolites (40/60), (60/40), (80/20) and microstructured hexolite (60/40) powders are pressed by uniaxial compression to obtain explosive charges. This kind of composition is often used for the synthesis of detonation nanodiamonds. The morphology, density and cohesion of the resulting pellets are analyzed in the light of the different used compression parameters. This study allows optimizing the compression parameters to obtain well suited explosive charges from nanostructured explosive components. A good cohesion of the nanostructured explosive pellets could be obtained with increasing the temperature used for the compression. Another very important point is that the nanostructuring of the composites is maintained for every compression.     

Experimental study and code predictions of fibre reinforced polymer reinforced concrete (FRP RC) tensile members

Due to their different mechanical properties, cracking and deformability behaviour of FRP reinforced concrete (FRP RC) members is quite different from traditional steel reinforced concrete (SRC) having great incidence on their serviceability design. This paper presents and discusses the results of an experimental programme concerning concrete tension members reinforced with glass fibre reinforced polymer (GFRP) bars. The main aim of the study is to evaluate the response of GFRP reinforced concrete (GFRP RC) tension members in terms of cracking and deformations. The results show the dependence of load-deformation response and crack spacing on the reinforcement ratio. The experimental results are compared to prediction models from codes and guidelines (ACI and Eurocode 2) and the suitability of the different approaches for predicting the behaviour of tensile members is analysed and discussed.     

Optimization of infrared radiation cure process parameters for glass fiber reinforced polymer composites

Elevated temperature post curing is one of the most critical step in the processing of polymer composites. It ensures that the complete cross-linking takes place to produce the targeted properties of composites. In this work infrared radiation (IR) post curing process for glass fiber reinforced polymer composite laminates is studied as an alternative to conventional thermal cure. Distance from the IR source, curing schedule and volume of the composite were selected as the IR cure parameters for optimization. Design of experiments (DOE) approach was adopted for conducting the experiments. Tensile strength and flexural strength of the composite laminate were the responses measured to select the final cure parameters. Analysis of variance (ANOVA), surface plots and contour plots clearly demonstrate that the distance from the IR source and volume of the composite contribute nearly 70% to the response functions. This establishes that polymer composites cured using IR technique can achieve the same properties using only 25% of the total time compared to that of conventional thermal curing.     

Woven hybrid composites: Tensile and flexural properties of oil palm-woven jute fibres based epoxy composites

In this research, tensile and flexural performance of tri layer oil palm empty fruit bunches (EFB)/woven jute (Jw) fibre reinforced epoxy hybrid composites subjected to layering pattern has been experimentally investigated. Sandwich composites were fabricated by hand lay-up technique in a mould and cured with 105 °C temperatures for 1 h by using hot press. Pure EFB and woven jute composites were also fabricate for comparison purpose. Results showed that tensile and flexural properties of pure EFB composite can be improved by hybridization with woven jute fibre as extreme woven jute fibre mat. It was found that tensile and flexural properties of hybrid composite is higher than that of EFB composite but less than woven jute composite. Statistical analysis of composites done by ANOVA-one way, it showed significant differences between the results obtained. The fracture surface morphology of the tensile samples of the hybrid composites was performed by using scanning electron microscopy.     

Effect of direct quenching on the microstructure and mechanical properties of the lean-chemistry HSLA-100 steel plates

The influence of direct quenching on structure-property behavior of lean chemistry HSLA-100 steels was studied. Two laboratory heats, one containing Cu and Nb (C:0.052, Mn:0.99, Cu:1.08, Nb:0.043, Cr:0.57, Ni:1.76, Mo:0.55 pct) and the other containing Cu, Nb and B (C:0.04, Mn:1.02, Cu:1.06, Nb:0.036, Cr:0.87, Ni:1.32, Mo:0.41, B:0.002 percent) were hot-rolled into 25 and 12.5 mm thick plates by varying finish-rolling temperatures. The plates were heat-treated by conventional reheat quenching and tempering (RQT), as well as by direct quenching and tempering (DQT) techniques. In general, direct-quench and tempered plates of Nb-Cu heat exhibited good strength (yield strength ∼ 900 MPa) and low-temperature impact toughness (average: 74 J at −85 °C); the Charpy V-notch impact energies were marginally lower than conventional HSLA-100 steel. In Nb-Cu-B heat, impact toughness at low-temperature was inferior owing to boron segregation at grain boundaries. Transmission electron microscopy (TEM) and scanning auger microprobe (SAM) analysis confirmed existence of borocarbides at grain boundaries in this steel. In general, for both the steels, the mechanical properties of the direct-quench and tempered plates were found to be superior to reheat quench and tempered plates. A detailed transmission electron microscopy study revealed presence of fine Cu and Nb (C, N) precipitates in these steels. It was also observed that smaller martensite inter-lath spacing, finer grains and precipitates in direct-quench and tempered plates compared to the reheat quench and tempered plates resulted in their superior strength and good impact toughness.     

SiC fibers with controllable thickness of carbon layer prepared directly by preceramic polymer pyrolysis routes

Continuous SiC fibers with different thickness of carbon layer were prepared through three preceramic polymer pyrolysis routes. To make the carbon layer thickness controllable, a simple improvement by using a ceramic bushing was adopted to retard the deposition of the pyrolytic carbons. Auger electron spectroscopy (AES) analysis reveals that the carbon layer thickness varies from less than 5 nm to 40 nm. The specific resistivity of the fibers increases by 5 orders of magnitude as the carbon layer thickness decreases. All of the fibers exhibit a tensile strength of around 1.8 GPa which is independent of the carbon layer thickness. The formation process of the carbon layer is discussed in three steps: the decomposition, the carbonization and the deposition. The as-received fibers have a potential application as the reinforcement of functional materials.     

科卡金矿边坡岩石力学试验

通过岩石单轴压缩试验,获得岩石的基本力学性质指标,分析岩石的破坏形式,利用声发射装置探讨岩石试件在压缩过程中内部能量的变化及破坏过程。结果显示,边坡岩石属于硬质岩,在外荷载达到抗压强度时,瞬间发生破坏,破坏后岩石呈碎块状,表明该岩石破坏类型为脆性破坏。