超轻便通用起复设备研究

伴随着我国经济的快速发展,以铁路运输为代表的现代化物流方式迎来了空前绝后的发展机遇,进入了快速发展阶段。为了确保铁路运输安全高效,在车辆发生脱线事故时,需要对车辆进行快速的起升复轨作业,清除路线障碍,恢复线路的正常运行。超轻便通用起复救援设备大部分组件为碳纤维复合材料,具有质量轻、韧度强、便于迅速组装、操作迅捷、作业安全等特点,能够在车辆脱轨后快速实施救援,适应范围广泛,为快速起复救援作业奠定了充分的技术保证。

Static mechanical properties and ductility of biomedical ultrafine-grained commercially pure titanium produced by ECAP process

Equal channel angular pressing(ECAP)is one of the most effective processes to produce ultra-fine grain(UFG)and nanocrystalline(NC)materials.Because the commercially pure titanium exhibits excellent biocompatibility properties,it has a significant potential to be utilized as an implant material.The low static and dynamic strengths of the pure titanium are one of the weaknesses of this material.This defect can be removed by applying the ECAP process on the pure titanium.In this work,the commercially pure titanium Grade2(CP-Ti of Grade2)was pressed at room temperature by the ECAP process via a channel angle of135°for3passes.The microstructural analysis and mechanical tests such as tensile test,hardness test,three-point bending test and Charpy impact test were all carried out on the ECAPed CP-Ti through3passes.The microstructural evolution reveals that by applying the ECAP process,coarse grain(CG)structure develops to UFG/NC structure.Moreover,the results of the mechanical tests show that the process significantly increases the yield and ultimate tensile strengths,bending strength,hardness and fracture toughness of the commercially pure titanium so that it can be used as a replacement for metallic alloys used as biomaterials.

Stress-strain behavior of plastic concrete using monotonic triaxial compression tests

The mechanical behavior of plastic concrete used in the cut-off walls of earth dams has been studied. Triaxial compression tests on the specimens in various ages and mix designs under different confining pressures have been done and the stress-strain behavior of such materials and their strength parameter changes have been experimentally investigated. It has been observed that increasing the confining pressures applied on the specimens causes the material behavior to be alike the more ductile materials and the compressive strength increases considerably as well. Moreover, a parametric study has been carded out to investigate the influence of essential parameters on the shear strength parameters of these materials. According to the research, increasing the coarse to fine aggregates ratio leads to the increase of compressive strength of the specimens as well as the increase of the cohesion and internal friction angle of the materials. Furthermore, the bentonite content decrease and the cement factor increase result in an increase of the cohesion parameter of plastic concretes and decrease of the internal friction angle of such materials.

Defect assessment of welded specimen considering weld induced residual stresses using SINTAP procedure and FEA

The defect assessment in butt-welded joint of ASTM A36 steel plates and 7075-T7351 aluminum alloy plates containing transverse through thickness crack was analyzed using SINTAP procedure and FEA incorporating weld induced residual stresses. Weld induced longitudinal residual stress profile can be obtained through SINTAP procedure, FEA or experimental analysis. This residual stress profile can be fitted with the trapezoidal residual stress profile available in SINTAP. For three different cases, crack length and residual stress intensity factor （SIF） are calculated and its comparison with the results obtained through FEA is plotted with respect to crack length. The stress intensity factor for mechanical loading is also plotted in the same graph. Using this graphical plot, the total SIF, including residual stress and mechanical loading, can be calculated for any particular crack size. The total SIF can be compared with the fracture toughness of the material for damage tolerance analysis. Also a failure assessment diagram is drawn for welded 7075-T7351 aluminum alloy plates with different crack sizes for as-welded （only residual stress） and mechanical loading along with the existing weld induced residual stresses to show the safety level for a particular crack size and mechanical loading.

Flexural response of reinforced concrete beams strengthened with post-poured ultra high toughness cementitious composites layer

Four-point bending tests were conducted up to failure on eleven reinforced concrete (RC) beams and strengthening beams to study the effectiveness of externally pouring ultra high toughness cementitious composites (UHTCC) on improving the flexural behavior of existing RC beams.The strengthening materials included UHTCC and high strength grade concrete.The parameters,such as thickness and length of strengthening layer and reinforcement in post-poured layer,were analyzed.The flexural behavior,failure mode and crack propagation of composite beams were investigated.The test results show that the strengthening layer improves the cracking and ultimate load by increasing the cross section area.Introducing UHTCC material into strengthening not only improves the bearing capacity of the original specimens,but also disperses larger cracks in upper concrete into multiple tightly-spaced fine cracks,thus prolonging the appearance of harm surface cracks and increasing the durability of existing structures.Compared with post-poured concrete,UHTCC is more suitable for working together with reinforcement.The load?deflection plots obtained from three-dimensional finite-element model (FEM) analyses are compared with those obtained from the experimental results,and show close correlation.

Effects of submicron-MgO and nano-MgO on the expansion and microscopic properties of high-performance concrete

MgO-series expansive agents can effectively compensate for the shrinkage and deformation of concrete structures.However,few experimental studies have been conducted on MgO expansive agents,particularly concerning the difference between and effects of submicron-MgO and nano-MgO in high-performance concrete(HPC)with a low water-cement ratio,thereby limiting their application in practical engineering.To clarify the expansion effect and expansion mechanism of MgO expansive agents in HPC,the effects of submicron-MgO and nano-MgO on the strength,toughness,and expansion characteristics of HPC were examined.The test results showed that submicron-MgO and nano-MgO continued to hydrate in the cement environment to produce Mg(OH)_(2),thus improving the structural compactness and structural strength of HPC.Nano-MgO concrete was found to have more stable mechanical properties and better structural deformability than submicron-MgO concrete.This study provides effective data support and theoretical reference concerning the hydration expansion mechanisms and engineering applications of nano-expanded materials.

Study on equivalent strength for crack directors of RCC arch dam based on size effect rule

The size effect rule of roller compacted concrete (RCC) fracture toughness was reached on the analysis of fracture toughness of RCC specimens, which have been done by project team. And then the rule was applied to the calculation formula of equivalent strength of crack director in the RCC arch dam, thus a simple and useful formula was reached. The study shows that the equivalent strength of crack directors increases with the increasing intensity of concrete, but the surplus rate of strength of crack directors section decreases with the increasing intensity of concrete and the distance between centers of adjacent crack directors, and that bilateral interval crack directors are more efficient in weakening the strength of section than unidirectional interval crack directors in the case of the same distance between adjacent crack director centers. A good design for crack directors of RCC arch dam is proposed via the rule.

The impact toughness of a nitride-strengthened martensitic heat resistant steel

The nitride-strengthened martensitic heat resistant steel is precipitation strengthened only by nitrides.In the present work,the effect of nitride precipitation behavior on the impact toughness of an experimental steel was investigated.Nitrides could hardly be observed when the steel was tempered at 650℃.When the tempering temperature was increased to 700℃ and 750℃,a large amount of nitrides were observed in the matrix.It was surprising to reveal that the impact energy of the half-size samples greatly increased from several Joules to nearly a hundred Joules.The ductile-brittle transition temperature(DBTT) was also discovered to decrease from room temperature to 50℃ when the tempering temperature was increased from 650℃ to 750℃.The nitride precipitation with increasing tempering temperature was revealed to be responsible for the improved impact toughness.

Austenite/martensite structure and corresponding ultrahigh strength and high ductility of steels processed by Q&P techniques

The microstructure of steels treated by Q&P(quenching and partitioning) process was characterized,a method of controlling retained austenite fraction based on inhomogeneous martensitic transformation was proposed,and the mechanical properties of steels treated by Q&P process were measured.The results show that the microstructure of the studied steels is mainly composed of initial martensite,fresh martensite and retained austenite.The initial marteniste formed at the first quenching step is easily etched;the fresh martensite formed at the final quenching step looks like 'blocky' type phase,and the retained austenite is mainly located on the packet boundary and initial austenite grain boundary.The inhomogeneous microstructure causes the experimental optimum quenching temperature corresponding to maximum retained austenite fraction to be higher than the calculation based on CPE(constrained paraequilibrium) model.The product of tensile strength and total tensile elongation is 47.5 GPa%,and tensile strength of 1760 MPa was obtained for the steel with carbon content of 0.51 wt%.The TRIP(transformation induced plasticity) effects of the large fractioned metastable austenite make a main contribution to the high ductility improvement,and the martensitic matrix provides high strength.

Experimental study on mechanical behaviors of pseudo-ductile cementitious composites under biaxial compression

Experimental investigation was conducted to characterize the responses of pseudo-ductile cementitious composites (PDCCs) when subjected to uniaxial and biaxial compression.The PDCCs is a class of fiber reinforced cementitious composites with ultra-high ductility by using a low volume fraction (2%) of polyvinyl alcohol (PVA) fiber.Two different strength grades of PDCC were examined with cubic specimen size of 100 mm in the tests.The specimens were loaded with a servo-hydraulic jack at different stress ratios.The principle stresses and strains of the specimens were recorded,and the failure modes with various stress states were examined.The test results indicated that the ultimate strength of PDCCs increased due to the lateral confinement in the other principal stress direction,and the maximum ultimate strength occurred at the biaxial stress ratio of 0.25,which was very different from common concrete material.For the PDCC specimens,the biaxial strength may be lower than the uniaxial strength when subjected to biaxial compression with the stress ratio of 1.0,and the failure mode showed a shear-type failure because of the bridging effect of fibers.Finally,a failure criterion was proposed for PDCCs under biaxial compression.

Microstructure and mechanical properties of liquid phase sintered silicon carbide composites

Silicon carbide (SiC) composites were prepared by hot-press sintering from α-SiC starting powders with BaAl2Si2O8 (BAS). The effects of additives on densification, microstructure, flexural strength, and fracture behavior of the liquid phase sintered (LPS) SiC composites were investigated. The results show that the served BAS effectively promotes the densification of SiC composites. The flexural strength and fracture toughness of the SiC composites can reach a maximum value of 454 MPa and 5.1 MPa·m1/2, respectively, for 40% (w/w) BAS/SiC composites. SiC grain pullout, crack deflection, and crack bridging were main toughening mechanisms for the sintered composites.

Effect of composition on flexural behavior of engineered cementitious composites

In this paper, four point bending tests were carried out to investigate the flexural properties of PVA fiber reinforced engineered cementitious composites (ECC) with different mix proportions. Based on the test results, the flexural toughness was evaluated with the methods of JSCE and post crack strength method (PCSm), respectively. Several parameters such as amount of water reducer, amount of sand, and fiber volume fraction were investigated to study their effects on the flexural toughness of ECC beams. According to the test results, superfluous water reducing additions can cause adverse effect on strength of the matrix and interfacial bond between fibers and the matrix, resulting in decreased bending strength and flexural toughness of ECC beams. Increase of the fiber volume fraction can result in increased flexural strength and toughness due to enhanced bridging effect between fibers and cementitious matrix. High amount of sand can reduce ductility and strain hardening behavior of ECC material, and better flexural toughness can be achieved when the amount of sand by weight is set to 0.2 for current water/cement ratio.

Inter-zone constraint modifies the stress-strain response of the constituent layer in gradient structure

How an individual constituent zone behaves during the deformation of a heterostructured metallic material is a fundamental issue for understanding heterostructure deformation, but it remains a challenge to experimentally observe it. Here we report a study on the stress-strain behavior of the nanostructured gradient layer(NGL) in an integrated gradient specimen that consists of a coarse-grained(CG)central layer sandwiched between two NGLs. Constraint from the CG central layer led to the formation of dense and dispersed stable strain bands(SBs) in the NGL, which regained dislocation hardening after initial recovery and grain coarsening. Consequently, the NGL exhibited a transient plateau of flow stress after yielding, and then regained extra strain hardening to achieve excellent uniform elongation. These unique behaviors are dramatically different from those of a freestanding NGL, indicating a fundamentally different deformation principle that is intrinsic to heterostructures, i.e.,inter-zone constraint modifies the constitutive behavior of constituent zones.

Third generation high strength low alloy steels with improved toughness

On-line thermo mechanical controlled processing(TMCP) was conducted to develop the third generation high strength low alloy(HSLA) steel with high toughness economically.The ultra-low carbon content ensured a high level of upper shelf energy while ultrafine lath martensitic structure transformed from super-thin pancaked austenite during controlled rolling and cooling.The reduction of martensite block size decreased ductile-to-brittle transition temperature(DBTT) and compensated the strength loss due to carbon reduction.Consequently,the excellent balance of strength and toughness values was obtained as 950-1060 MPa for yield strength,180 J for Charpy V-notch impact absorbed energy at 30℃,which is much superior to that of traditional martensitic steel.Two mechanisms for the refinement of lath martensite block were proposed:One is the austenite grain refinement in the direction of thickness,and the other is the reduction in the fraction of sub-block boundaries with small misorientation and the increase in the fraction of block boundaries with large misorientation,possibly due to austenite hardening.

Determination of mixed-mode interfacial fracture toughness for thermal barrier coatings

In this work, a test method was developed to determine the interfacial fracture toughness of the air plasma sprayed （APS） thermal barrier coatings （TBCs） over a wide range of mode mixities. For this mixed-mode test method, the analytical expres- sions for the energy release rate and stress intensity factors were derived based on the energy theory and the concept of ＂equi- valence＂. The fidelity of these expressions was affirmed by selected finite element analysis. The experimental results showed that the critical energy release rate increased with the increase of the positive mode mixity, which was mainly due to the increase in contact/friction effect and plastic work dissipation with increasing shear mode loading. Furthermore, an elliptical interfacial failure criterion in terms of the stress intensity factors was proposed. The agreement between the experimental results in the literature and those in our work indicated that our test method and the corresponding analytical solutions can well determine the interfaeial fracture toughness of the TBCs over a wide range of mode mixities.

Brittle-Ductile Transition of Ferroelectric Ceramics Induced by Thermally Activated Dislocation Emission

Thermally activated dislocation emission in high-temperature ferroelectric ceramics is investigated through an assumption of thermal stability and a novel analytical method. The stress intensity factor (SIF) arising from domain switching is evaluated by using a Green's function method, and the critical applied electric field intensity factor (CAEFIF) for brittle fracture at room temperature is obtained. Besides, the lowest temperature for single dislocation emission before brittle fracture is also obtained by constructing an energy balance. The multi-scale analysis of facture toughness of the ferroelectric ceramics at high temperature is carried out. Through the analysis, the CAEFIF for crack extension is recalculated. The results show that the competition and interaction effects between dislocation emission and brittle fracture are very obvious. Besides, the higher critical activation temperature, the more columns of obstacles will be overcome. Additionally, the shielding effect arising from thermally activated dislocations is remarkable, thus, the brittle-ductile transition can promote the fracture toughness of high-temperature ferroelectric ceramics.