冻融循环冻土的冲击动态力学性能

以典型冻土为研究对象,通过不同冻融循环次数的冻融循环实验、不同温度的冻结实验以及不同应变率的冲击动态实验,综合研究了冻融循环冻土的冲击动态力学性能。结果表明,冻土存在冻融循环效应,随着冻融循环次数的增加,冻土的峰值应力有一定程度的降低,但在达到临界冻融循环次数后,峰值应力将维持稳定;同时,冻土表现出明显的应变率效应和温度效应,其峰值应力随应变率的增加或温度的降低而增加。通过定义冻融损伤因子,推导满足Weibull分布的冲击损伤,提出了一个基于Z-W-T方程的损伤黏弹性本构模型。该模型可较好地描述冻融循环后冻土的冲击动态力学行为,为研究季节性冻土区冻土的冲击动态破坏提供参考。     

热处理对TC4钛合金动态力学性能和微观组织的影响

为揭示热处理对TC4钛合金动态力学性能及微观组织的影响，选取2种典型热处理方式和5种加载应变率开展了TC4钛合金试样的动态力学性能实验，获取了动态应力-应变数据，并进行了试样的XRD和金相分析。结果表明：高应变率下TC4钛合金应变率强化效应显著。时效处理后，TC4钛合金流动应力、屈服强度及抗压强度得到提升，而固溶时效处理后上述性能降低。时效处理和未热处理试样应力-应变曲线均具有弹性、屈服和塑性阶段，而固溶时效处理后无明显弹性和屈服阶段。固溶时效处理后流动应力随应变率增加而增加，时效处理和未热处理试样流动应力无明显变化。时效处理后试样等轴初生α相显著增大且β相含量较低，固溶时效处理后α相晶界增大且含有针状α的β转变基体，TC4钛合金力学性能与β相和亚稳β相的马氏体转变有关。     

高温高应变率下异种不锈钢激光焊接件的力学性能

利用改进的SHTB实验设备,对316L和304不锈钢焊接结构的动态力学性能进行了实验.提出了一种适用于焊接结构件的新型的SHTB夹持装置,并在应变率约103 s-1、温度为25～500℃的环境下获得了焊接结构件动态应力-应变曲线.研究表明:随应变率的升高,结构件的屈服强度和抗拉强度随应变率的增大而增大,随温度的升高而降...     

YB-2航空有机玻璃的应变率和温度敏感性及其本构模型

为了理解和评价YB-2航空有机玻璃在极端环境下的动态力学性能,采用电子万能试验机和分离式Hopkinson压杆对YB-2航空有机玻璃在218~373 K温度范围、10^-3~3 000 s^-1应变率范围内的压缩力学行为进行了研究,得到了材料的应力应变曲线。结果表明：随着温度的升高,材料的流动应力逐渐减小而破坏应变呈现增大的趋势;温度相同时,材料的流动应力随应变率的增加而增大,破坏应变随应变率的增加而减小。随着应变率的提高,材料的应变软化效应更加剧烈。基于朱-王-唐(ZWT)本构模型,得到了考虑温度效应的本构参数。结果显示,在8%应变范围内,改进的考虑温度效应的本构模型可以较为理想地表征该材料的应力应变响应。

     

重组竹顺纹冲击力学性能研究

重组竹是一种新型竹基复合材料,其力学性能优于落叶松等木材。为评价重组竹在动态加载下的顺纹抗冲击力学性能,以密度1.06 g/cm3、含水率8.52%、龄期3～5年的毛竹基重组竹为研究对象,通过准静态单轴压缩和循环加卸载以及动态加载实验,研究了重组竹加载变形过程、各项力学性能指标以及对应变率的敏感性。结果表明:重组竹顺纹压缩过程可以分为弹性变形和弹塑性变形阶段,破坏类型为延性破坏,其各项强度指标随应变率的提高而提高,动态增长因子与应变率之间呈现线性关系,斜率为0.0024;重组竹压缩过程中的应变比能与应变之间呈线性关系,且随应变率的增长而增大,证明其吸能能力随着应变率的增大而提高。实验结果证明,重组竹顺纹具有良好的抗冲击力学性能和显著的应变率效应。     

软质聚氨酯泡沫的动态压缩力学性能和本构模型

采用Instron 9350落锤试验机研究了中低应变率下软质聚氨酯泡沫的动态压缩力学性能,分析了其应力-应变响应特征和应变率敏感性,讨论了应变率对材料应变率敏感性指数和能量吸收特性的影响,并基于实验结果建立了可准确描述其压缩力学响应的率相关本构模型。结果表明,软质聚氨酯泡沫的静动态压缩应力-应变响应具有典型的三阶段特征,且呈现出明显的应变率强化效应。准静态加载下,材料具有较高的吸能效率但能量吸收值较小,应变率对最大吸能效率和比吸能的影响较小;动态加载下,随着应变率的增加,最大吸能效率显著减小而比吸能明显增大。考虑应变率影响的修正Sherwood-Frost模型和修正Avalle模型都能够很好地表征软质聚氨酯泡沫的静动态压缩应力-应变响应,但修正Avalle模型的参数较少,更便于工程应用。研究结果可为软质聚氨酯泡沫抗冲击结构的设计和优化提供指导。     

高温后钢管活性粉末混凝土的动态力学性能

采用霍普金森压杆装置对高温后钢管活性粉末混凝土(reactive powder concrete-filled steel tube,RPC-FST)进行冲击压缩实验,分析了应变率效应及温度效应对试件动态力学性能的影响。结果表明:高温(200、300 ℃)后RPC-FST仍具有较好的抗冲击能力、延性和完整性;冲击荷载作用下,RPC-FST的应变率效应明显弱于RPC的应变率效应;随着过火温度的提高,RPC-FST的峰值应力逐渐增大,变形能力增强,抗冲击能力提高。动力提高系数随过火温度的提高而增大,说明高温后RPC-FST的应变率效应更显著。     

极端环境下连续碳纤维增韧的陶瓷基复合材料的力学行为

连续纤维增韧的碳化硅复合材料(以下简称C/SiC),作为超高速飞行器热结构使用时,有可能在高温环境下受到高速撞击的作用,因此,掌握其在极端环境(高温、高应变率)下的力学性能是进行结构安全设计的基础。本文采用具有高温实验能力的分离式Hopkinson杆,在293~1273K温度范围内进行了动态压缩力学性能测试,研究了环境温度和加载速率对材料力学性能的影响。结果表明:C/SiC复合材料的高温压缩力学性能主要受应力氧化损伤和残余应力的共同影响。实验温度低于873K时,应力氧化损伤的影响很小,而由于增强纤维和基体界面残余应力的释放使界面结合强度增大,复合材料的压缩强度随温度的升高而增大;当实验温度高于873K时,应力氧化损伤加剧,其对压缩强度的削弱超过残余应力释放对强度的贡献,材料的压缩强度随温度的升高逐渐降低。由于应力氧化损伤受应变率的影响很大,当温度由873K升高至1273K时,高应变率下压缩强度降低的程度要比应变率为0.0001/s时低得多。     

一种基于电磁霍普金森杆的材料动态包辛格效应测试装置及方法

金属材料在复杂载荷条件下的动态力学行为研究一直备受关注,但受限于实验设备,金属材料的动态包辛格效应响应一直都难以获得。为了探究金属材料的包辛格效应与应变率效应之间的关系,本文中提出一种基于电磁霍普金森杆(electromagnetic split Hopkinson bar,ESHB) 的非同步加载实验技术,为测试金属材料在高应变率加载下的包辛格效应提供了一种有效的实验方法。本文中,首先介绍了非同步加载装置的主要特点,即可以用两列由脉冲发生器产生的应力波对受载试样进行连续的一次动态拉-压循环加载,且加载过程保证了应力波的一致性。分析了应力波对试样加载过程中的波传播历程,确保了加载过程的连续性。随后介绍了动态加载过程,数据处理方法和波形分离手段,并对动态加载过程进行应力平衡性分析,论证了实验装置的可靠性。最后采用该方法测试了5%预应变下6061铝合金动态压缩-动态拉伸的包辛格效应,并与准静态下的实验结果进行对比。实验结果表明,该材料单轴压缩没有明显的应变率效应,但其包辛格效应具有应变率依赖性,高应变率下材料的包辛格应力影响因子由0.07增大至0.17,具有显著的提升,这对传统意义上铝合金材料应变率不敏感的结论提出了挑战。

     

宽应变率范围下2A16-T4铝合金动态力学性能

为了研究2A16-T4铝合金的动态力学性能,利用电子万能试验机、高速液压伺服试验机及霍普金森压杆(SHPB)装置进行常温下准静态、中应变率和高应变率的动态力学性能实验,得到不同应变率下的应力应变曲线,基于修正的Johnson-Cook本构模型对它进行拟合,并分析材料中应变率力学特性对模型应变率敏感参量的影响。结果表明:2A16-T4铝合金在应变率10-4~102 s-1范围内应变率敏感性较弱,而在102~103 s-1范围内应变率敏感性较强,且应变率强化效应随塑性应变的增大而减小;同时,在10-4~103 s-1范围内具有较强的应变硬化效应,且应变硬化效应随应变率的增大而减小;此外,修正Johnson-Cook本构模型的拟合结果与实验结果吻合很好,能够很好表征材料的动态力学行为,且考虑材料中应变率力学特性可提高本构模型参量的准确性。     

Dynamic behavior of frozen soil under uniaxial strain and stress conditions

The split Hopkinson pressure bar (SHPB) method is used to investigate the dynamic behavior of the artificial frozen soil under the nearly uniaxial strain and uniaxial stress conditions. The tests are conducted at the temperatures of −3^?C, −8^?C, −13^?C, −17^?C, −23^?C, and −28^?C and with the strain rates from 900 s⁻¹ to 1 500 s⁻¹. The nearly uniaxial stress-strain curves exhibit an elastic-plastic behavior, whereas the uniaxial stress-strain curves show a brittle behavior. The compressive strength of the frozen soil exhibits the positive strain rate and negative temperature sensitivity, and the final strain of the frozen soil shows the positive strain rate sensitivity. The strength of the frozen soil under the nearly uniaxial strain is greater than that under the uniaxial stress. After the negative confinement tests, the specimens are compressed, and the visible cracks are not observed. However, the specimens are catastrophically damaged after the uniaxial SHPB tests. A phenomenological model with the thermal sensitivity is established to describe the dynamic behavior of the confined frozen soil.     

冻结黏土力学特性分析及改进S-M蠕变模型1)

为合理描述人工冻结法施工的矿井土层力学特性,对某矿井土样的重塑土进行单轴抗压强度实验和单轴压缩蠕变实验,得到重塑冻结黏土在不同温度及载荷加载等级下的应力-应变曲线和蠕变曲线。实验结果表明：冻结温度越低,土样的单轴强度越大;相同冻结温度下,土样的蠕变变形随着应力水平的升高而增大。单轴压缩蠕变的等时应力-应变曲线随时间发展向应变轴靠拢;土样经历初始蠕变和等速蠕变两个阶段,在较高应力水平下有进入加速蠕变的趋势;对S-M模型中各参数的意义进行修正并考虑温度的影响,得到人工冻结黏土改进S-M蠕变显式模型,然后采用粒子群算法对人工冻结黏土S-M蠕变模型参数进行优化。改进S-M蠕变显式模型理论计算值与实验值吻合良好,表明改进S-M蠕变显式模型能较好模拟人工冻结黏土的蠕变特性。改进S-M蠕变显式模型为人工冻土蠕变计算提供一种新方法。     

Experimental analysis of dynamic mechanical properties for artificially frozen clay by the split Hopkinson pressure bar

A device for impact compression experiments is the split Hopkinson pressure bar with a refrigerating attemperator. Data for incident and reflected waves are obtained by the measuring technique with strain gauges, and data for transmitted waves are obtained by the measuring technique with semiconductor gauges. Static compression tests of frozen clay are conducted at an identical temperature and different strain rates of 0.001 and 0.01 sec ⁻¹ . Dynamic stress-strain curves are obtained at strain rates of 360–1470 sec ⁻¹ . The low and high temperatures correspond to high and low strain rates, respectively. It is shown that both the temperature and strain rate affect the frozen soil deformation process. Different dynamic stress-strain curves obtained at the same temperature but different strain rates are found to converge. The test results indicate that frozen soil has both temperature-brittleness and impact-brittleness.     

冻土动态力学性能的实验研究

利用分离式霍布金森压杆(SHPB)对4种低温下的冻土进行了4种高应变 率的动态压缩实验. 实验结果表明：冻土不仅具有温度效应,还具有应变率效应,两种效应 反映出冻土材料的时温等效性. 另外这种时温等效性在分析冻土材料的破坏过程时还体现在 它的冻脆性和动脆性. 冻土材料动态应力应变曲线的汇聚现象和振荡现象均起源于这种冻脆 性和动脆性.     

High-rate Characterization of 304L Stainless Steel at Elevated Temperatures for Recrystallization Investigation

An integrated experimental technique was developed for high-rate mechanical characterization of 304L stainless steel at elevated temperatures by using a modified split Hopkinson pressure bar (SHPB). A sandwich structure consisting of two platens and the specimen in between was heated before mechanical loading while the bars were maintained at room temperature to eliminate the temperature gradient effect on the wave propagation in the bars. Upon contacting the cold bars, temperature gradients form in the platens, leaving the temperature in specimen constant and uniform. Pulse shaping techniques were employed to maintain constant strain-rate deformation and dynamic stress equilibrium in the specimen. Dynamic compressive stress-strain curves at elevated temperatures for the 304L stainless steel were obtained. To relate recrystallization to impact loading, a momentum trapping system was employed to apply a single loading on the specimen during one dynamic experiment. We also controlled the quenching time to study its effect on recrystallization.     

考虑围压效应的冻结砂土动态本构模型研究

为描述主动围压作用下冻结砂土的动态力学特性,通过在朱-王-唐模型的非线性体上串联塑性体,建立了能够考虑围压效应的冻结砂土动态损伤本构模型;分析了损伤参数对应力-应变曲线特征、屈服点、峰值应力和峰值应变的影响规律,基于冻结砂土动力学试验数据确定了模型参数;通过将模型和试验数据进行对比,并对不同试验条件下模型的预测误差进行分析,验证了模型的适用性和准确性。结果表明,损伤参数对应力-应变曲线弹性阶段和屈服点无明显影响,而对塑性阶段和破坏阶段的影响较为显著,本构模型预测的应力-应变曲线与试验结果具有较好的一致性。模型能够预测围压引起冻结砂土塑性阶段占比大和屈服点明显的特征,且能够描述围压对冻结砂土动态强度的增强效应;不同负温和主动围压条件下,模型对峰值应力和屈服强度的预测效果优于峰值应变和屈服应变。     

Strain-rate history and temperature effects on the torsional-shear behavior of a mild steel

Previous investigations on the effects of strain-rate and temperature histories on the mechanical behavior of steel are briefly reviewed. A study is presented on the influence of strain rate and strain-rate history on the shear behavior of a mild steel, over a wide range of temperature Experiments were performed on thin-walled tubular specimens of short gage length, using a torsional split-Hopkinson-bar apparatus adapted to permit quasi-static as well as dynamic straining at different temperatures. The constant-rate behavior was first measured at nominal strain rates of 10^?3 and 10³ s^?1 for ?150, ?100, ?50, 20, 200 and 400°C. Tests were then carried out, at the same temperatures, in which the strain rate was suddenly increased during deformation from the lower to the higher rate at various large values of plastic strain. The increase in rate occurred in a time of the order of 20 μs so that relatively little change of strain took place during the jump. The low strain-rate results show a well-defined elastic limit but no yield drop, a small yield plateau is found at room temperature. The subsequent strain hardening shows a maximum at 200°C, when serrated flow occurs and the ductility is reduced. The high strain-rate results show a considerable drop of stress at yield. The post-yield flow stress decreases steadily with increasing temperature, throughout the temperature range investigated. At room temperature and below, the strain-hardening rate becomes negative at large strains. The adiabatic temperature rise in the dynamic tests was computed on the assumption that the plastic work is entirely converted to heat. This enabled the isothermal dynamic stress-strain curves to be calculated, and showed that considerable thermal softening took place. The initial response to a strain-rate jump is approximately elastic, and has a magnitude which increases with decrease of testing temperature; it is little affected by the amount of prestrain. At 200 and 400° C, a yield drop occurs after the initial stress increment. The post-jump flow stress is always greater than that for the same strain in a constant-rate dynamic test, the strain-hardening rate becoming negative at large strains or low testing temperature. This observed effect of strain-rate history cannot be explained by the thermal softening accompanying dynamic deformation. These and other results concerning total ductility under various strain-rate and temperature conditions show that strain-rate history strongly affects the mechanical behavior of the mild steel tested and, hence, should be taken into account in the formulation of constitutive equations for that material.     

Mechanical behavior of metals under tension- compression loading at high strain rate

By using a new technique based on a split Hopkinson pressure bar method, a sequenced reverse test (quasi-static tensile prestress, followed by dynamic compression and then followed by dynamic tension) at high strain rate was performed and tension-compression stress-strain relations were derived by using one-dimensional stress wave analysis. Three materials, 2017 aluminium alloy, 0.45% carbon steel, and pure aluminium, were investigated at low and high strain rates, and the strain rate effect on the reverse loading stress-strain curves was compared to that on the loading stress-strain curves. It was found that reduction of yield stress is always associated with load reversals, and the strain rate effect on the reverse loading (tension) is almost the same as that during loading (compression) at higher values of reverse deformation.     

碳布叠层/碳复合材料动态压缩性能测试研究

利用带有波形整形器的Split Hopkinson Pressure Bar(SHPB)技术测试了碳布叠层/碳复合材料在应变率为500、1 500 s-1时的动态压缩性能。研究结果表明:利用轧制紫铜作为整形器材料不仅可以有效地实现对碳布叠层/碳复合材料的常应变率压缩加载，而且有助于改善试样两端的应力平衡，从而保证测试数据的可靠性；此外，与准静态压缩相比较，在动态压缩载荷下，碳布叠层/碳复合材料的压缩强度有较强的应变率效应，且复合材料压缩强度的动态增加函数可以用Cowper-Symonds幂函数的形式来表示。