分离式霍普金森压杆试验中工程材料端面摩擦模型的确定

分离式霍普金森压杆(SHPB)是一种最常用于工程材料高应变率下动态压缩力学性能的技术。在SHPB试样与压杆的接触界面间不可避免地存在端面摩擦效应,这将导致SHPB试样内的应力状态可能偏离一维应力状态,而SHPB试样内的一维应力状态是决定SHPB试验结果准确性的基本假设之一。基于动摩擦试验结果,建立了端面动摩擦模型以描述端面动摩擦系数随SHPB试样与钢压杆接触界面间最大径向相对滑动速度的关系。结果发现本文提出的端面动摩擦模型可很好地拟合     

冲击荷载作用下早强EPS混凝土的力学性能

采用&;#61542;100mm分离式霍普金森压杆（split Hopkinson pressure bar,简称SHPB）试验装置,对养护龄期分别为12h、24h和36h的早强聚苯乙烯混凝土（EPS）进行了冲击压缩试验,得到了相应的应力-应变曲线,并与养护龄期为28d的聚苯乙烯混凝土（EPS）的应力-应变曲线进行比较。结果表明：养护龄期为36h、28d的EPS混凝土随着应变率的增加,其冲击压缩强度也相应增加;养护龄期为12h、24h的EPS混凝土随着应变率的增加,其冲击压缩强度变化不明显。另外,还研究了试件动态抗压强度与平均应变率的关系和养护龄期对动态抗压强度的影响,证明了EPS混凝土的抗冲击性能随养护龄期的增长而增加。     

闭孔泡沫铝的动态压缩性能试验研究

为了研究闭孔泡沫铝动态压缩性能的应变率效应,采用改进的INSTRON高速动力加载系统,对不同应变率下闭孔泡沫铝试件进行动态压缩试验研究。首先利用正向试验和反向试验技术对不同厚度的闭孔泡沫铝试件在同一加载速率下的动态压缩性能进行了研究,得到了在一定速率下消除泡沫铝动态压缩试验中惯性效应的合理试件厚度。进一步开展了闭孔泡沫铝试件在不同加载速率下的高速压缩试验,研究了其动态压缩性能随应变率的变化规律。结果表明在高速压缩下,闭孔泡沫铝的应力-应变曲线与准静态条件相同,具有明显的弹性段、平台段及压实段的3阶段特征。闭孔泡沫铝的平台应力具有明显的应变率效应,而致密应变在不同的应变率下表现出了不同的变化趋势,初步解释为泡沫铝孔壁塑性变形机制的改变以及波动效应的相互影响。闭孔泡沫铝的吸能能力随应变率的增加而明显提升。     

UHMWPE/乙烯基酯2.5D角联锁机织复合材料动态压缩性能实验研究

采用分离式霍普金森压杆(SHPB)动态测试系统和岛津材料试验机,对材料厚度方向的压缩性能进行实验研究,得到了三种高应变率(360,710s-1和1120s-1)和准静态下的压缩应力-应变关系,压缩强度和冲击吸能,通过冲击破坏形貌探讨材料的动态压缩破坏模式和吸能机理.结果表明:UHMWPE/乙烯基酯2.5D角联锁机织复合...     

UHMWPE纤维混凝土动态压缩力学性能研究

试验研究了一种捻制超高分子量聚乙烯(UHMWPE)纤维增强的新型纤维混凝土动态压缩力学性能。研制了4种纤维体积掺量(0.3%、0.5%、0.7%、1.0%)的C70等级纤维混凝土,采用Φ100 mm分离式霍普金森压杆进行冲击压缩试验,研究了纤维混凝土在140~255 s~(-1)应变率下的动态压缩力学性能。试验结果表明:UHMWPE纤维混凝土抗压强度、峰值应变和弹性模量具有明显的应变率敏感性;纤维混凝土抗压强度应变率敏感性弱于素混凝土,但其弹性模量应变率敏感性强于素混凝土;动态强度增长因子与应变率对数呈线性关系,具体关系与纤维掺量相关。     

混凝土材料的动态压缩破坏机理及本构关系

为研究混凝土材料的动态性能,利用直径φ100 mm的SHPB(Split Hopkinson Pressure Bar)装置对骨料尺寸为15 mm～20 mm的混凝土材料试样进行了应变率范围30s(-1)～180s(-1)的动态压缩试验,并借助高速摄影装置获得了试样的变形与破坏过程,结果表明:在动态压缩强度附近应力区,材料表面先出现一条沿试样轴向的可见宏观裂纹,而多条主裂纹的形成与扩展才导致材料的最终破坏;建立了改进的ZWT模型,模型预测结果与试验结果较吻合.     

主动围压下岩石的冲击力学性能试验研究

利用具有主动围压加载装置的直径为100 mm分离式Hopkinson压杆（SHPB）试验装置和薄圆形紫铜片作为波形整形器,研究了斜长角闪岩在不同围压等级（0~6 MPa）、不同应变速率（50~170 s-1）下的动态力学性能,并对试验有效性进行了分析。试验结果表明：斜长角闪岩的动态强度增长因子与应变率的对数呈近似线性关系,强度与比能量吸收随应变率的增加而近似线性增加,体现了显著的应变率相关性;在同等级应变率范围内,随着围压的增加,岩石的增强效果与增韧效果逐渐增强;同时发现,在围压作用下,岩石的破坏由拉伸破坏向压剪破坏逐渐过渡和发展。SHPB试验中,近似恒应变率加载时间比例约为69.5 %,能够较好地满足应力均匀分布及近似恒应变率加载要求,表明SHPB试验的有效性和结果的可靠性     

三轴SHPB加载下砂岩力学特性和破坏模式的试验研究

利用改造的三轴SHPB动静组合加载实验装置,对均质砂岩进行了不同围压和不同应变率下的三轴冲击压缩试验,作为对比利用RMT-150C试验机也进行了部分准静态下的三轴压缩实验。根据实验结果,分析了围压对砂岩动态冲击性能的影响,并重点讨论了冲击过程中岩石的破坏模式。研究结果表明,在围压一定的情况下,岩石的动态压缩强度随应变率的提高而提高;在应变率相同的情况下,岩石的动态压缩强度和弹性模量会随着围压的增大而增大。岩石发生破坏的临界入射能,随着围压的增大而增大。岩石单位体积吸收能与应变率之间呈线性递增关系,而且递增的程度随着围压的增加而增加。三轴冲击加载下,应变率较低时岩石内部形成压剪破裂面但整体不失稳,应变率很大时岩石破碎形成锥形块体形式。     

氧化锌晶须辅助增强复合材料压缩性能

采用万能实验机和分离式Hopkinson压杆装置(SHPB)为加载手段,对5种不同质量分数的ZnO晶须辅助增强玻璃纤维/环氧树脂复合材料层合板厚度和平面内2 个方向的准静态和动态压缩性能进行了实验研究,分别得到了5种材料的压缩应力应变曲线;采用SEM对材料断口进行了宏、细观观察,对其破坏机制进行了初步研究。结果表明：该类材料具有明显的非线性特征的本构关系和应变率强化效应,在厚度和平面内2个方向的压缩破坏也具有完全不同的复杂的破坏机制,同时还发现试样的压缩强度和失效应变与氧化锌晶须的质量分数有关。     

钢/聚氨酯夹层结构动态压缩力学性能与本构模型研究

基于霍普金森压杆(SHPB)装置,对高应变率下的钢/聚氨酯夹层结构进行了动态压缩实验,通过对不同厚度比的试样进行比较,建立了夹层结构的动态本构模型。实验结果表明,钢/聚氨酯夹层结构对应变率较为敏感,其屈服强度随应变率的增大而增大,且屈服强度的增幅随应变率的提高会有一定幅度的上升。基于Johnson-Cook模型,考虑厚度比对结果的影响,建立适用于钢/聚氨酯夹层结构的动态本构模型,并通过试验分析,确定了模型参数。利用实测应力应变曲线拟合的方法得到的钢/聚氨酯夹层结构的动态压缩本构模型,能描述不同厚度比的钢/聚氨酯夹层结构在高应变率下的应力应变关系,具有较高的精度。     

粉煤灰聚氨酯复合泡沫静动态力学特性实验研究

以大尺寸粉煤灰漂珠为主要组分,以硬质聚氨酯泡沫为黏结剂制备了一种具有多尺度胞孔形态的复合泡沫,对其准静态压缩和动态冲击下的力学性能和变形机制进行研究。结果表明:①该复合泡沫应力应变曲线具有典型的线弹性、塑性平台和致密化三个特征阶段且具有相对稳定的平台应力;在密度0.45~0.6 g/cm^3,复合泡沫平台应力(6.5~18 MPa)和到压实应变处吸收的能量(3.42~8.9 MJ/m 3)随密度增大而提高,且平台应力与相对密度之间满足幂函数关系;②采用铝蜂窝为增强相可使同密度下复合泡沫抗压强度和平台应力分别提升约20%~45%和10%~25%,准静态下复合泡沫主要发生剪切失效,增强泡沫的主要失效形式则转变为轴向压缩失效。③在0.001~1500 s^-1应变率范围内,复合泡沫抗压强度有明显的应变率效应但平台应力并未随应变率的增大而提高。增强复合泡沫的强度和平台应力均呈现出明显的应变率效应,采用铝蜂窝不仅能提高复合泡沫力学性能,还能够改善其力学行为,使材料具有更优异的动力学特性;研究为工业固废粉煤灰的综合利用提供新思路。     

Ratcheting response of nylon fiber reinforced natural rubber/styrene butadiene rubber composites under uniaxial stress cycles: Experimental studies

The present study intends to study the ratcheting response of nylon fiber reinforced natural rubber (NR)‐styrene butadiene rubber (SBR) composite samples under asymmetric stress cycles. Uniaxial tests conducted on composite samples have shown how influential the weight fraction of short nylon fibers on the stress‐strain curves/loops under monotonic and cyclic loads is. NR/SBR composite samples with various fiber contents of 0, 10, 20, 30, and 40 per hundred rubber (phr) were tested under asymmetric stress cycles. In these tests, stress‐strain hysteresis loops were progressively shifted over stress cycles resulting in progressive plastic strain accumulation. Over stress cycles, ratcheting strain progressed within the first few cycles with a relatively high rate, and as the number of cycles increased, this rate decayed resulting in a plateau in strain accumulation (shakedown). The ratcheting strain rate and magnitude resulting in shakedown were highly affected by the nylon fiber content. The experimental observations showed that this plateau (shakedown) occurred after a number of cycles in NR/SBR composite samples where the widths of hysteresis loops stayed unchanged. Samples with no fiber and that with 10 phr fiber content possessed high ratcheting rates leading samples to failure after a few stress cycles. Fracture surfaces in these samples were further analyzed through SEM investigation.     

圆形孔洞结构岩体冲击响应识别实验模拟研究

孔洞是地质和岩石工程中常见的结构,为了模拟孔洞结构岩体在高速冲击荷载下的响应特征,设计和加工了孔洞结构的花岗岩体试样,采用改进的冲击动力学测试模拟系统,在不同冲击能下,在垂直和平行孔洞轴线方向对孔洞结构花岗岩体进行渐进式循环冲击模拟试验,对实验杆入/反射应力波进行测量。研究表明,峰值反射应力比与冲击能大小无关,但反射应变比能比随冲击能的增大而降低;在低冲击能下,孔洞结构对反射应力波的响应影响不大;随着冲击能的增大,其影响增大;岩体沿孔洞结构的轴线方向发生开裂破坏,与冲击位置无关;在试样开裂后,应变比能显著增高但峰值应力比的变化不大,说明应变比能比可作为岩体结构破坏指示的响应指标。模拟实验为孔洞类岩石结构的钻进识别以及防护设计提供了理论依据。     

Effects of temperature and pressure on failure and post-failure behavior of Westerly granite

Failure and post-failure behavior of Westerly granite were investigated at pressures up to 400 MPa and temperatures up to 700°C in an internally heated gas apparatus. 95 experiments were performed. For dry samples, the effects of temperature and strain rate on the failure stress are not significant relative to that of pressure. Some preliminary data indicate that the effect of water can be more pronounced at elevated temperature than at room temperature. At pressures above 80 MPa, crack morphology changes induced by thermal cracking have no effect on fracture strength.Wawersik's manual control technique was adapted to high temperature, and a range of class I and II post-failure behavior was observed. Temperature was shown to be a stabilizing factor. Preliminary post-creep failure experiments show that the post-failure behavior depends on loading history. The behavior is more stable at a lower differential stress and a slower strain rate. The data are analyzed in terms of subcritical crack growth by stress corrosion. In light of previous room temperature post-failure and creep results and our analysis, we suggest that details of the post-failure curves are sensitive to statistical variation among samples.     

杨木静动态压缩本构模型研究

目的 以杨木为研究对象,研究其静动态压缩载荷作用下应力-应变曲线的变化特征,建立适合的本构模型,并对其进行描述。方法 对杨木试件进行静动态压缩加载试验,分析静动态压缩载荷作用下杨木应力-应变曲线的变化特征,构建适用于静动态压缩载荷作用下杨木的本构模型。结果 静态压缩加载杨木的应力-应变曲线分为线弹性阶段、屈服阶段和密实化阶段等3个部分,动态压缩加载杨木的应力-应变曲线分为线弹性阶段和屈服阶段等2个部分;静态压缩加载时,杨木轴向屈服应力最大,分别是径向和弦向的5.70倍和7.75倍;动态压缩加载时,当应变率从400 s-1增加到1000 s-1时,径向、弦向和轴向的屈服应力分别增加了1.51,1.59,3.12倍,杨木的屈服应力具有应变率敏感性;采用包含应变率影响的本构方程来描述杨木在静动态压缩载荷作用下的本构关系是比较合适的。结论 杨木是一种应变率敏感材料,静动态压缩载荷作用下杨木的应力-应变曲线均表现出多孔材料的特征,将多孔材料本构模型应用于木材是可行的。     

Influence of temperature on the low cycle fatigue behaviour of a modified 9Cr–1Mo ferritic steel

Low cycle fatigue (LCF) behaviour of a modified 9Cr–1Mo steel under normalized and tempered conditions is reported. The alloy was normalized at 1313K for 1 h followed by tempering at 1033K for 1 h, which resulted in a tempered martensitic structure. Total axial strain controlled LCF tests were conducted at a constant strain rate of 3×10⁻³ s⁻¹ at different strain amplitudes varying from ±0.25 to ±1.0% in the temperature range of 300–873K. The cyclic stress response behaviour, in general, showed an initial brief hardening for the first few cycles, followed by a continuous and gradual softening regime that ended in a stress plateau that continued up to the specimen failure. The fatigue life decreased as the temperature increased. The temperature effect on life was more pronounced at low strain amplitudes. The metallography of the failed samples revealed that the fatigue failure at high amplitudes of testing was marked by extensive crack branching and the formation of secondary cracks. Oxidation was found to exert major influence on LCF life reduction at 873K.     

In-plane Compressive Behaviors of 3-D Textile Composites at Various Strain Rates

The in-plane compressive behaviors of 3-D textile composites, which including 3-D woven composite, multi-axial multi-layer warp knitted (MMWK) composite and 3-D braided composite, were studied at quasi-static and high strain rate compression loading. The compression behaviors at high strain rates (600∼2,500/s) were tested with split Hopkinson pressure bar (SHPB). The quasi-static compressive tests were conducted on a MTS 810.23 tester and compared with the results at high strain rates. The comparisons indicate that the compression stiffness, failure stress and failure strain for the three kinds of 3-D textile composites are sensitive to strain rate. The MMWK composite has higher failure stress than the 3-D woven composite and 3-D braided composite at the same strain rate; however, the failure strain of the 3-D braided composites is higher than that of the 3-D woven composite and 3-D knitted composite at quasi-static compression because of the quasi-isotropic structure feature in the 3-D braided composite. The compressive failure modes of the 3-D woven composite, MMWK composite and 3-D braided composite are totally different because of the different preform structure.     

循环荷载作用下钢桥面铺装疲劳损伤失效行为研究

针对钢桥面铺装工程中普遍采用的改性沥青(Stone Matrix Asphalt,SMA)、浇筑式沥青(Guss asphalt,GA)、环氧沥青(Epoxy asphalt,EP)混合料双层铺装结构,进行了循环车载作用下钢桥面与沥青混凝土铺装疲劳损伤特性理论分析与试验研究。基于疲劳损伤度,研究了钢桥面铺装疲劳损伤失效行为和疲劳开裂过程中损伤场、应力和应变场动态演变机制,推导出疲劳失效时的损伤场、应力和应变场计算表达式,并给出钢桥面铺装疲劳寿命理论公式。以三座钢箱梁桥桥面铺装(润扬长江大桥2005,南京长江三桥2005,苏通大桥2008)为例,对不同铺装结构组合方案下的复合梁进行疲劳试验分析和使用寿命理论预测。实例研究结果表明,钢桥面铺装疲劳损伤失效行为预估模型合理可行;相较于改性沥青、浇筑式沥青,环氧沥青混合料具有较强高的强度低变形能力,更适合于大跨径钢桥面铺装抗疲劳的设计要求;由环氧沥青混合料组合而成的“双层环氧沥青混凝土”和“浇注式沥青混凝土(下层)+环氧沥青混凝土(上层)”的抗疲劳性能优于其它沥青混合料铺装结构组合方案,同等厚度组合情况下疲劳使用寿命可延长1倍~2倍以上;“双层环氧沥青混凝土”已应用于润扬长江大桥、南京长江三桥和苏通长江大桥钢桥面工程,并已成功运行10年以上,其跟踪观测结果良好。     

冰球冲击试验的近场动力学方法数值模拟

为了研究近场动力学方法在模拟冰体高应变率破坏行为的可靠性,基于近场动力学理论通过自编程实现对冰球高速冲击力学行为的数值模拟,将计算结果和试验结果进行对比。结果发现:近场动力学方法能准确模拟冰球冲击过程下高应变率破坏的完整过程,对球体表面裂纹扩散以及冰球整体裂解的细节模拟十分到位,且在20~60 m/s速度范围内对冲击载荷预报的幅值最大误差18.2%,均值最大误差18.4%;同时,发现在20~60 ms/中低速冲击过程中,冰球破坏以半球切面为界限,靠近冲击面的下半球体呈粉碎性破坏,上半球体呈碎片状破坏,当冲击速度大于60 m/s后,继续增大速度会使上半球体破坏程度剧烈增大;当冲击速度大于100 m/s,上下半球都呈粉碎状破坏。     

High strain rate behavior of 4-step 3D braided composites under compressive failure

The out-of-plane and in-plane compressive failure behavior of 4-step 3D braided composite materials was investigated at quasi-static and high strain rates. The out-of-plane and in-plane direction compressive tests at high strain rates from 800/s to 3,500/s were tested with the split Hopkinson pressure bar (SHPB) technique. The quasi-static compressive tests were conducted on a MTS 810.23 tester and compared with those at high strain rates. The comparisons indicate that the failure stress, failure strain and compressive stiffness both for out-of-plane and in-plane loading directions are rate sensitive. For example, the failure stress, failure strain and stiffness are 55.19 MPa, 6.70% and 1.35 GPa respectively as opposed to 145.00 MPa, 1.21% and 13.50 GPa respectively for strain rate of 2,500 s⁻¹ under in-plane compression. The 3D braided composites have higher values of failure stress and strain for out-of-plane than for in-plane compression at the same strain rate; however, the in-plane compression stiffness is higher than that of out-of-plane compression at high strain rates. The compressive failure mode of 3D braided composites in the out-of-plane direction is mainly shear failure at various strain rates, while for the in-plane direction it is mainly cracking of matrix.