拉拔速度对空拉拔锌合金微管的影响

运用deform-3D软件模拟了锌合金微管(Φ2.45 mm×0.13 mm)的空拉拔成形过程。研究了拉拔速度对锌合金微管表面质量、力学性能和尺寸精度的影响。结果表明:拉拔速度对微管的应变和应力影响非常大,进而直接影响其表面质量、力学性能和尺寸精度。适当的拉拔速度可使多道次空拉拔锌合金微管的应变较均匀、最大等效应力和损伤值较低,从而减少皱缩和拉拔裂纹等缺陷,降低断裂倾向。锌合金微管具有均匀应变、较小等效应力的拉拔速度为22 mm/s。实验结果很好地验证了模拟的结论:在模拟优化的拉拔速度下,空拉拔制备的锌合金微管具有较高的尺寸精度和力学性能,并且皱缩等缺陷较少。     

WC-8Co与Al2O3-TiC两种拉拔模具磨损行为研究

采用冷、热压烧结技术分别制备了WC-8Co和Al2O3-TiC两种拉拔模具.研究了该两种不同拉拔模具用于实际拉拔加工45#钢线时的摩擦磨损行为,并对其磨损机理进行了分析.结果表明:两种拉拔模具均具有较好的强度和硬度,能较好地进行拉拔加工,其内孔磨损在工作区及定径区最为严重,磨损面有金属脱落元素、模具颗粒脱落元素及润滑脂残留物.WC-8Co和Al2O3-TiC两种拉拔模具的磨损机理不尽相同,WC-8Co模具主要为粘着磨损和磨粒磨损,而Al2O3-TiC模具则以磨粒磨损和表面疲劳磨损为主.Al2O3-TiC拉拔模具更适合于45#钢线材的拉拨.     

强制润滑拉拔对包钢丝组织及性能的影响

采用强制润滑拉拔技术、金相观察和力学性能测试,研究了拉拔模角、道次变形程度等对铝包钢丝毛坯断面上各层间的变形分布和钢铝间结合性能的影响。在强制润滑拉拔的条件下,减小拉拔模角有利于降低不均匀变形程度;减小道次变形程度秀利于改善润滑条件,但易使变形集中在线材表层;合理的拉拔变形提高了变形均匀性和钢铝间的结合性能。     

强制润滑拉拔对铝包钢丝组织及性能的影响

采用强制润滑拉拔技术、金相观察和力学性能测试,研究了拉拔模角、道次变形程度等对铝包钢丝毛坯断面上各层间的变形分布和钢铝间结合性能的影响.在强制润滑拉拔的条件下,减小拉拔模角有利于降低不均匀变形程度;减小道次变形程度有利于改善润滑条件,但易使变形集中在线材表层;合理的拉拔变形提高了变形均匀性和钢铝间的结合性能.     

AZ31镁合金管材游动芯头拉拔有限元模拟

运用Deform软件模拟了AZ31镁合金管材游动芯头拉拔过程,并对比研究了模具锥角、定径带长度、变径带长度对拉拔管材成形性的影响。结果表明:在拉拔AZ31管材时,最大压应力出现在外模变径段与定径段过渡区,合适的模角配合不仅能降低拉拔力,还能提高尺寸精度。模具定径段长度对管材变形时的均匀性和拉拔力的影响较大,变径段长度对管材拉拔的影响较小。最终通过实验验证了模拟结果,制备出了尺寸精度高的AZ31镁合金管材。     

异型钢纤维拉拔界面模型

主要提出了一种新型的异型纤维拉拔界面模型,完成了整个拉拔过程中拉拔荷载与拉拔位移的对应计算。同传统的异型纤维拉拔计算模型相比,本模型不仅考虑到了拉拔过程中由于纤维的塑性变形而且考虑到了界面压力变化导致的界面摩擦变化所引起的荷载阻抗,此外,模型中还量化模拟了基体材料的剥落损伤行为及其对载荷释放的影响。最后从计算模拟结果与试验结果的对比来看,本模型具有较好的适应性。     

高强度镀锌钢丝表面硬度与扭转性能关系研究

对高强度镀锌钢丝的扭转性能和拉拔钢丝的力学性能及微观组织进行研究。力学性能分析表明:扭转性能好的拉拔钢丝的强度和塑性相比扭转性能差的拉拔钢丝无较大差别,然而扭转性能好的拉拔钢丝表面显微硬度比扭转性能差的拉拔钢丝低,波动相对也较平稳。微观组织研究表明:扭转性能差的拉拔钢丝表层组织无半脱碳是其表面硬度较高的主要原因。     

多芯复合超导线拉拔的有限元模拟

利用商业有限元软件DEFORM-3D对多芯复合超导线材的拉拔过程进行有限元模拟,研究变形过程中复合超导线各亚组元的变形情况,以及拉拔速度对超导线成形的影响.分析超导线拉拔过程中等效应力、等效应变的分布规律;并且结合韧性断裂准则来预测不同位置开裂产生的可能性.总结出拉拔速度对拉拔过程的影响规律,确定了拉拔过程中的危险区域.     

硅青铜线材无模拉拔工艺与组织性能关系

在进料速度0.5mm.s-1、拉拔速度0.67～1.00mm.s-1、加热温度600～800℃的条件下,对QSi3-1硅青铜线材进行了无模拉拔实验,研究了无模拉拔工艺与QSi3-1硅青铜的显微组织和力学性能的关系。结果表明:在拉拔速度和断面收缩率相同的条件下,加热温度越高,无模拉拔成形结束后硅青铜晶粒的平均尺寸越大;当加热温度一定时,拉拔速度越高,硅青铜晶粒就越细小。无模拉拔成形硅青铜的硬度及抗拉强度随着加热温度的升高而逐渐降低,随着拉拔速度的升高而略有上升;在加热温度较低的条件下,抗拉强度下降明显,当加热温度升高到一定程度后,抗拉强度下降缓慢。无模拉拔成形过程中QSi3-1硅青铜的晶粒经历了变形细化及后续高温状态下的晶粒相互吞并长大两个过程。     

钢丝拉拔过程中的硬脆现象及消除硬脆的拉拔工艺

钢丝拉拔一段时间出现的硬脆现象,进一步拉拔出现拉拔困难甚至脆性断裂。本文将探讨钢丝拉拔过程中硬脆现象,并制定合理的拉拔工艺消除硬脆现象。     

Steel–concrete bond strength of lightweight self-consolidating concrete

The bond behavior of lightweight self-consolidating concrete (LWSCC) must be understood in order to use this type of high performance concrete in structural members. The objective of this research program is to assess the bond behavior of reinforcing steel bars embedded in LWSCC members. Three different classes of LWSCC mixtures were developed with two different types of lightweight aggregates. In addition, one normal weight SCC (NWSCC) was developed and used as a control mixture. A total of twenty four pullout tests were conducted on deformed reinforcing bars with an embedded length of either 100 or 200 mm and the load-slip responses, failure modes and bond strengths of LWSCC and NWSCC were compared. Based on the results of this study, the bond strength of deformed bars for LWSCCs are found to be less (between 16 and 38%) as compared with NWSCC. Under the conditions of equivalent workability properties and compressive strength, bond slip properties were shown to be significantly influenced by the type of lightweight aggregate used. In this study, the use of expanded shale in the production of LWSCC significantly enhanced the pullout strength when compared with lightweight slag aggregate.     

Pullout tests of epoxy-coated reinforcement in concrete

The bond strength and slip of epoxy-coated reinforcing bars in concrete have been evaluated by carrying out single pullout and double pullout tests. In extended single pullout tests, slip measurements were made while tensile force was applied to reinforcing bars embedded in concrete. In double pullout tests, 20 cycles of load were applied at levels of steel stress between zero and 0·5 times characteristic steel strength. Strains were measured by electrical resistance strain gauges glued inside the bars. Both epoxy-coated and uncoated bars were used in the investigation, to obtain comparative results. The strain gradient along the bar was found to be less for the coated reinforcement. In general, the epoxy coating was found to increase slip in bond and thereby reduce the bond performance of coated bars.     

Geometrical and mechanical aspects of fabric bonding and pullout in cement composites

Fabric reinforced cement composites are a new class of cementitious materials with enhanced tensile strength and ductility. The reinforcing mechanisms of 2-D fabric structures in cement matrix are studied using a fabric pullout model based on nonlinear finite difference method. Three main aspects of the composite are evaluated: nonlinear bond slip characteristic at interface; slack in longitudinal warp yarns, and mechanical anchorage provided by cross yarn junctions. Parametric studies of these key parameters indicate that an increase in the interfacial bond strength directly increases the pullout strength. Grid structures offering mechanical anchorage at cross yarn junctions can substantially increase the pullout resistance. Presence of slack in the yarn geometry causes an apparently weaker and more compliant pullout response. The model was calibrated using a variety of test data on the experimental pullout response of AR-Glass specimens, manufactured by different techniques to investigate the relative force contribution from bond at interface and from cross yarn junctions of alkaline resistant glass fabric reinforced cement composites.     

Study of the Effect of Tool Wear on Hole Quality in Drilling CFRP to Select a Suitable Drill for Multi-Criteria Hole Quality

In recent years the use of carbon fiber composites in aerospace industries has been on the rise due to their unique properties like high specific strength, high stiffness, and fatigue characteristics. The machining behavior of these materials differs from machining homogenous metals because the drill bit encounters fiber and matrix alternatively which have widely different properties. Among the various machining operations, drilling is very common to facilitate assembly using fasteners to others parts in the structure. Rapid wear of the tool due to the abrasive carbon fibers is an important reason for damage occurrence. This results in frequent drill changes affecting the production cycle and increasing the final cost. Drill geometry is an important factor which decides the quality of the drilled hole. Hole quality is decided by a number of parameters like fiber pullout, delamination, surface finish, etc., among others. It is therefore necessary to evaluate the performance of the different drills available based on the quality of the holes they produce and drill life time. Three tool geometries are evaluated for thrust force, tool wear, surface roughness, fiber pullout, hole oversize, and eccentricity. Recommendations are given to aid in the selection of appropriate drill for the desired hole quality parameter.     

Experimental study on z-pin bridging law by pullout test

This paper presents an experimental study on the evaluation of bridging law for a z-pin. The relationship between the z-pin bridging force and its displacement was measured by z-pin pullout tests. The tests were carried out using three types of samples with: single small pin; 3 × 3 small-pins (three columns × three rows) and 3 × 3 big-pins. For 3 × 3 small-pins samples, a typical pullout curve with initial bonding, debonding and frictional sliding was obtained. A high peak value of the debonding force was reached before z-pin debonding started. After debonding was initiated, the pullout force dropped rapidly to a lower value, the pins were then pulled out steadily against friction. However, for samples with 3 × 3 big-pins, it was difficult to discern the peak debonding force. The major results of this study are expected to provide a better physical understanding of the mechanics and mechanisms of z-pin bridging, aside from an efficient and accurate methodology to measure the crack-bridging law.     

Investigation of fiber configurations of chafer cuticle by SEM, mechanical modeling and test of pullout forces

Insect cuticle as a natural biocomposite includes many favored microstructures which have been refined over centuries and endow the cuticle eminent mechanical properties. This paper first studies the microstructures of chafer cuticle through SEM observations. Several peculiar fiber configurations and fiber-ply arrangements such as branched fiber, acanth-fiber and helicoid plies are observed. These microstructures are useful for man-made fiber-reinforced composites to improve their mechanical properties. Then, a special configuration of the branched fiber found in chafer cuticle is in details analyzed through a mechanical model and experimental verification. The pullout force of fibers as an index is firstly studied through parameter study. The factors, which can improve the pullout forces, are identified. Finally, the maximal pullout force of the branched fiber is experimentally tested and compared with that of plain straight fiber. It is proved that the maximal pullout force of branched fibers is obviously greater than that of the plain straight fibers.     

Investigation of welded structures on mechanical properties of 304L welded tube-to-tubesheet joints

This paper presents an investigation on effects of welded structures on pullout force and fatigue life of welded joints of 304L heat exchanger tube-to-tubesheet. Welded structures including inner-hole with a groove of 45°, extension, inner-hole with a groove of U-shape and plain-end were discussed. Full-size test blocks covering four types of welded structures were used to conduct pullout test and low cycle fatigue test. Furthermore, optical microscopy (OM) and scanning electron microscopy (SEM) were utilized to analyze the failure mechanism. The results show that the joints of the extension welded structure have the largest pullout force and fatigue life because of its largest fracture length, and the undeformed welded zone revealed favorable quality of weld. The cracks originate at the outer surface of tube and striations propagate at an average rate of 50 μm for a loading cycle, finally, the separation commences at the inner surface of the tube, exhibiting equiaxed or hemispheroidal dimples.     

Evaluation of the bond behavior of steel reinforcing bars in recycled fine aggregate concrete

This paper describes pullout test results on deformed reinforcing bars in natural and recycled fine aggregate (RFA) concrete. The effects of bar location and RFA grade on bond strength between reinforcing bar and recycled aggregate concrete (RAC) were evaluated through the experimental program. A total of 150 pullout specimens were fabricated for the experiment. Two reinforcing bar orientations were considered with respect to the casting direction; vertical bars and horizontal bars, the latter of which was prepared to evaluate top-bar effect. Considered variables included four RFA replacement ratios (RFArs), two water-absorption grades (RFA-A: 5.83%, RFA-B: 7.95%) of RFA and three reinforcing bar locations (75, 225 and 375 mm height from the bottom of the casting mold). In addition, to evaluate the thermal and aging effect on bond behavior between the reinforcing bar and RFA concrete, some parts of pullout specimens had exposed to rapid freeze–thaw environment or been cured at air during 28 or 730 days. Test results demonstrated that bond strength does not seem to be affected by the RFAr for higher RFA grades (RFA-A), at least up to 60% RFAr. In contrast, the RAC including lower RFA grade (RFA-B) showed clear decreases in bond strength with increasing RFAr, similar to the trend observed for compressive strength. For horizontal pullout specimens, RFA concrete specimens showed higher bond strength gap between top and bottom bars than natural aggregate concrete (NAC) specimens. Bond strengths of the horizontally cast pullout specimens were affected by the flowability of concrete rather than the RFAr or RFA grade. No noticeable degradation occurred during freeze–thaw cycling of the RAC specimens, indicating that the RFA used in this study is appropriate for use in freeze–thaw environments.     

Effect of shrinkage reducing agent on pullout resistance of high-strength steel fibers embedded in ultra-high-performance concrete

The interfacial bond strength of long high-strength steel fibers embedded in ultra-high-performance concrete (UHPC) reinforced with short steel microfibers was investigated by conducting single-fiber pullout tests. In particular, the influence of the addition of a shrinkage-reducing to a UHPC matrix on the pullout resistance of high-strength steel fibers was investigated. The addition of a shrinkage-reducing agent produced a noticeable reduction in the fiber pullout resistance owing to the lower matrix shrinkage, although the reduction of pullout resistance differed according to the type of fiber. Long smooth and twisted steel fibers were highly sensitive to the addition of the shrinkage-reducing agent whereas hooked fibers were not. Among the various high-strength steel fibers tested, twisted steel macrofibers showed the highest interfacial bond resistance, although twisted fibers embedded in UHPC showed slip softening pullout behavior rather than the typical slip hardening behavior observed in mortar.     

Effect of loading rates on pullout behavior of high strength steel fibers embedded in ultra-high performance concrete

In this paper single fiber pull-out performance of high strength steel fibers embedded in ultra-high performance concrete (UHPC) is investigated. The research emphasis is placed on the experimental performance at various pullout rates to better understand the dynamic tensile behavior of ultra-high performance fiber reinforced concrete (UHP-FRC). Based on the knowledge that crack formation is strain rate sensitive, it is hypothesized that the formation of micro-splitting cracks and the damage of cement-based matrix in the fiber tunnel are mainly attributing to the rate sensitivity. Hereby, different pull-out mechanisms of straight and mechanically bonded fibers will be examined more closely. The experimental investigation considers four types of high strength steel fibers as follows: straight smooth brass-coated with a diameter of 0.2 mm and 0.38 mm, half end hooked with a diameter of 0.38 mm and twisted fibers with an equivalent diameter of 0.3 mm. Four different pull out loading rates were applied ranging from 0.025 mm/s to 25 mm/s. The loading rate effects on maximum fiber tensile stress, use of material, pullout energy, equivalent bond strength, and average bond strength are characterized and analyzed. The test results indicate that half-hooked fibers exhibit highest loading rate sensitivity of all fibers used in this research, which might be attributed to potential matrix split cracking. Furthermore, the effect of fiber embedment angles on the loading rate sensitivity of fiber pullout behavior is investigated. Three fiber embedment angles, 0°, 20°, and 45°, are considered. The results reveal that there is a correlation between fiber embedment angle and loading rate sensitivity of fiber pullout behavior.