Papers by Keyword: Prestrain

Abstract: The effect of thermo-mechanical treatment on the microstructural evolution of low carbon micro-alloyed high strength steel was studied by combining prestrain with tempering (PST) in this paper. It was found that the prestrain causes the dislocation to plug up around the grain boundary and carbide, resulting in carbide boundary fragmentation. Moreover, it breaks the thermo-dynamic equilibrium between the matrix and carbide, induces the dissolution of carbon in the high energy state, and then changes the distribution of carbon in the matrix. In the subsequent tempering process, the precipitation regularity of carbide was changed, which promoted the precipitation carbide at low temperature. The influence of carbide precipitation on dislocation can be divided into two stages: the first stage was precipitation induced creep, which promoted stress relaxation; the second stage was precipitation pinning dislocation, which improved material strength and inhibited stress relaxation.

Abstract: It is impossible to keep pipelines free from defects in the manufacturing, installation and servicing processes. In this paper, pre-tension deformation of X60 pipeline steel was employed to experimentally simulate the influence of dents and the fatigue fracture surfaces of X60 pipeline steel after per-tension deformation under cyclic loading were investigated. The results indicate that the static state tensile mechanics is the dominant factor in the fracture mechanisms at high stress ratio. With the decrease of the stress ratio R, the fractograph presents evident fatigue facture characteristics. The amount and intensity of MnS inclusion evidently increase with the loading frequency decrease and the typical cleavage fracture characteristics have been found.

Abstract: Considering the advantages of steel strands in mechanical properties, a new form of shape memory alloy (SMA) wire, SMA strands, is designed in this study. To compare the recovery properties of martensitic SMA strands and SMA wire, the recovery stress of which with different prestrains were tested by using a material testing machine with a temperature-controlled cabinet. In this experiment, the SMA strands and the SMA wire have the same composition and the same cross-sectional area. The experimental results show that the maximum recovery stresses of the SMA strands with different prestrain are almost the same, which illustrates that the prestrain has no much effect on the maximum recovery stress of the SMA strands. Moreover, the recovery stress curve in heating stage does not coincide with that in cooling stage. Furthermore, the residual recovery stress of the SMA strands in cooling stage is also smaller than that of the SMA wire. This demonstrates the SMA strands can recovery initial state better.

Abstract: It is impossible to keep pipelines free from defects in the manufacturing, installation and servicing processes. In this paper, pre-tension deformation was used to imitate the defects of the part of material and the U-notch specimens were used to investigate the fatigue crack initiation life of X60 pipeline steel after pre-tension straining with ductile deformation of 0,5%,10% and 15%. Test results show that the prestrain had apparent straining hardening effects, but decreased the plasticity of pipeline steel. The prestrain prolonged the fatigue crack initiation life, and increased resistance coefficient and equivalent stress threshold of the fatigue crack initiation.

Abstract: The paper presents a new design concept for evaluating the mechanical strength of unidirectional composite material with shape memory alloy (SMA) fiber in the presence of matrix crack. NiTi SMA fiber with both ends knotted is used to actively control the composite strength instead of straight fiber for purpose of better ensuring stress transfer from the matrix to the fiber. Experiment is conducted to verify the effectiveness of this new design concept.

Abstract: Prestrain affects the performance of dielectric elastomer actuators so much that rare actuators is used before pre-stretching. In this article, the author made a series of circular actuators with acrylic VHB4910 and graphite mixture electrodes, and performed contrast experiments, under non-prestrain, uniaxial prestrain and equibiaxial prestrain, respectively, to figure out the changing of area strain against prestrain. Result of experiment indicates that the film actuator obtains the highest output strain while applying equibiaxial prestrain. An optimum prestrain ratio of actuators, in this research, occurs between 150%~200% under equibiaxial prestrain condition. This result has practical value in the future design and use of dielectric elastomer actuators.

Abstract: Dual phase (DP) steel is increasingly utilized in automotive industry to match the requirement of both performance and lightweight of autobody for economical and ecological considerations. Most of autobody structures are manufactured by stamping process which imports prestrain effects into original material and alters its static and cyclic mechanical behaviors. In this study, mechanical behavior of prestrained DP steel was researched. Firstly, microstructure of commercial DP steel with different prestrain levels was examined with optical microscope of 500X magnification. Then, static tensile experiments and strain-controlled fatigue experiments were carried out at room temperature. Test results of differenet prestrain levels were compared. Static stress-strain relationship and local strain-life model were determined using linear regression method. Furthermore, fatigue performance and notch fatigue resistance capability were compared with prestrain level based on Neuber factor. Finally, explanation of the change with prestrain was presented with crack closure theory.

Abstract: The tension, Charpy notch impact and fracture toughness tests on structural steels Q235B and Q345 were carried out under different prestrains. Ductile to brittle transition curves are investigated using Charpy impact tests. The results indicate that the prestrain decreases the impact toughness of both base metal (BM) and weld metal (WM); and increases the ductile-brittle transition temperature as well as the risk of fracture. The prestrain is also found to have a more significant effect in a less ductile material. River marking, the characteristic of cleavage fracture, is observed in the Q235B WM specimens 10% prestrain conditions at 0°C. The fracture surfaces of Q345 WM specimens are characterized by dimple-like structures. The depth of these dimples is decreased under prestrain applied before welding. The CTOD-prestrain curves are obtained using Boltzmann relation.

Abstract: In this paper, the experimental investigation into the enhancement of mechanical strength in shape memory alloy (SMA) fiber composites is made by using knotted fiber at the two ends instead of straight fiber. TiNi SMA fiber with both ends knotted is used for purpose of better ensuring stress transfer from the matrix to the fiber than straight fiber. Tension test is carried out above the austenitic finish temperature in air. Specimens are heated by means of electrical resistive lamplight heating. The results indicate that the mechanical strength is larger in the knotted fiber composite than in the straight fiber composite. Knotted fiber exerts the superiority of TiNi SMA fiber composite.

Abstract: In this study, the effect of thermomechanical treatment (TMT) on the aging and mechanical behavior of Mg-Al-Zn alloys has been investigated. Three Mg-Al alloys AZ31, AZ61, and AZ91 were mold cast into a block under a CO2 and SF6 gas mixture atmosphere. The cast specimens were hot-swaged to a rod after homogenizing treatment at 400°C for 4 hours. The prestrain was applied by cold swaging up to 10% RA after solution treatment. From the aging curves obtained at 150°C, it was identified that the hardness of TMT-processed AZ31 and AZ61 did not increase during aging, while that of AZ91 remarkably increased. The tensile strength and elongation of the TMT-processed AZ31 and AZ61 remarkably increased depending on the amount of applied prestrains, indicating the introduction of dislocations prior to aging significantly improved the mechanical properties.