基于时间响应函数的结构阻尼识别方法比较

研究了3种基于时间响应函数的结构阻尼识别方法, 包括对数衰减法、希尔 伯特方法和小波方法. 给出了3种方法的实现算法, 分析了对密集模态的识别能力. 构造仿真算例, 采用3种方法识别了5{\%}, 10{\%}和30{\%}噪声条件下的模态阻尼. 结果表明, 小波方法比对数衰减法和希尔伯特方法具有更好的噪声鲁棒性. 采用小波方法分析了润扬大桥结构健康监测系统获得的实测数据, 识别出了润扬大桥悬索桥前6阶模态参数, 第2阶和第3阶模态频率相差仅为0.015\,Hz. 研究表明, 小波方法具备噪声条件下密集模态的识别能力, 是工程中阻尼识别的优选方法.     

横摆振荡翼型动态气动掠效应试验研究

大型风力机设计对获取翼型更加全面、准确的动态载荷提出更高要求, 研究翼型横摆振荡动态气动特性具有重要意义. 借助"电子凸轮"技术和动态数据同步采集手段, 针对翼型动态“掠效应”首次开展了横摆振荡风洞试验研究, 研究表明: 横摆振荡翼型的气动曲线存在明显迟滞效应, 吸力面压力周期性波动是主要诱因, 且随着振荡频率、初始迎角和振幅的增大, 气动迟滞特性均增强; 升力和压差阻力随横摆角变化的迟滞回线呈"W"形, 俯仰力矩迟滞回线呈"M"形, 升力差量迟滞回线呈"$\infty$"形; 负行程下翼型气动力相对于正行程下的更高, 且负行程下翼型气动力随振荡频率的增大而略有增大, 正行程下则明显减小; 升力系数功率谱密度分布在振荡频率倍频处的能量集中的幅值随着振荡频率增大有增大趋势; 吸力面1.2%和40%弦长处压力的滞回特性较强, 是由于翼面剪切层涡和动态分离涡周期性发展、运动、破裂和重建; 振幅为$10^{\circ}$时, 升力迟滞曲线呈"$^{\wedge}$"形, 振幅为$30^{\circ}$ 时, 升力迟滞曲线呈"$^{\wedge\wedge\wedge}$"形.      

汽车薄钢板应力应变曲线及屈服轨迹的研究

采用十字形双向拉伸的实验方法对两种汽车用薄钢板BH220和SPEN进行了不同 加载路径下的双向拉伸试验，得到了不同应力状态下的应力应变关系曲线，同时，根据单位 体积塑性功相等的原则，确定了两种钢板等效塑性应变从0.2\%$\sim$2\%的实验屈服轨迹. 结果分析表明：不同加载路径下板料的应力应变关系不同，随着加载比例由单拉到等双拉状 态，板料的硬化指数逐步增大；实验屈服轨迹呈外凸性，且以等双拉为界的上下部分屈服轨 迹不对称，随着变形程度的增加，屈服轨迹向外扩大，但单拉时强化程度最小，而等双拉 时最大. 对BH220和SPEN钢板的实验屈服轨迹与几种常用理论屈服轨迹的比较发现，Hosford各向 异性屈服准则的理论轨迹与实验结果最为接近，Hill48准则与实验结果相差最大，此外一 向被视为只适用于各向同性材料的Mises准则与实验结果也较为接近，其他几个屈服准则的 理论屈服轨迹与实验点相差较大.     

The study on the compressive behavior of ptfe/al energetic composite

金属/氟聚合物含能复合材料是一类新型的高级含能材料. 研究了室温下Al含量和应变 率对PTFE/Al含能复合材料压缩性能和反应性能的影响,所加载的应变率为6\times 10^{-3}s^{-1}\sim8\times 10^{3}s^{ -1}. 材料压缩性能的应变 率效应明显：与静态加载相比,动态加载下材料模量和强度明显提 高,但应变降低. 材料的损伤过程主要包括塑性变形、开裂和反应3部分. 随着Al含量的增 加,材料准静态和动态压缩强度均呈先升后降的趋势,在 Al含量为35\%时达到 最高值102.6 MPa和154 MPa; 引发反应所需加载的应变率增加,但对应的应力值 差别不明显,基本在165 MPa左右, 材料引发后反应完全性降低.     

应变率对SiC颗粒增强铝基复合材料拉伸性能的影响

本文利用岛津试验机和自行研制的冲击拉伸试验装置,对体积含量为10%的SiC颗粒增强铝基复合材料进行了准静态的拉伸试验、冲击拉伸试验和冲击拉伸加卸载试验,获得了复合材料在应变率为0.002s^-1-1000s^-1范围内从弹塑性变形直至断裂的完整应力应变曲线。试验结果表明,随着应变速率的提高,复合屈服应力,拉伸强度以及破坏应变均相应提高,具有明显的应变率强化效应和高速韧性现象;同时,由于冲击拉伸试验过程中热力耦合效应的影响,准静态加载下复合材料的应力指数与冲击拉伸加载下复合材料的应力指数相比降低了17.8%;在用冲击拉伸复元试验解耦出热力耦合效应的影响后,材料的静、动态等温应力应变曲线具有相同的应变硬化规律。最后,根据复合材料在不同应变率下的试验结果和Eshelby‘s等效夹杂理论,本文建立了一个计及应变率强化效应的弹塑性自洽模型,模型拟合结果与试验结果吻合得很好。     

混凝土一维应力层裂实验的全场DIC分析

基于74mm直径分离式Hopkinson杆(SHPB)实验平台进行了混凝土杆的一维应力层裂实验.采用超高速相机(采样频率:2 $\mu$s/frame)结合数字图像相关法(DIC),记录混凝土试件中的动态位移场实时变化情况,探讨了混凝土在拉伸断裂过程中的表面位移场及速度场演化规律.针对实验中出现的多重层裂现象,基于一维应力波传播理论,指出各个位置在发生层裂时,其最大拉应力均由透射压缩波与反射拉伸波叠加而成,各处层裂发生时均处于一维应力状态.并提出了根据层裂位置左右两点速度趋势变化判断层裂发生时刻的判据.该判据可以给出所有层裂的起裂时间,结合DIC分析直接给出了混凝土多重层裂应变.结果显示混凝土的拉伸强度具有明显的应变率效应,在30 s$^{-1}$的应变率下,其拉伸强度的动态增强因子(DIF)可以达到5.与传统的波叠加法和自由面速度回跳法相比,DIC全场分析法不受加载波形限制,可以精确给出每个层裂的位置和起裂时间,从而得到试件在高应变率加载下不同位置处的断裂应变、拉伸强度及相应应变率,提高了测量效率.      

ARALL 材料拉伸力学性能的试验研究

利用自行研制的旋转盘式间接杆杆型冲击拉伸试验装置对ＡＲＡＬＬ材料以及施加预应力的ＡＲＡＬＬ材料进行了３个应变率（２００、５００、１３００ｓ－１）的冲击拉伸试验，得到了两种材料在不同应变率下的完整的应力应变曲线。结果表明ＡＲＡＬＬ材料在高速加载条件下的变形可以分为弹性变形、塑性变形和材料失稳后的残余变形三个部分，每个部分都有不同的变形机理。结果还表明，随着应变率的增加两种材料的屈服应力、失稳应力以及失稳应变均相应增加，表现出明显的应变率强化和动态韧性现象。最后根据材料在不同应变率下的试验结果，建立了ＡＲＡＬＬ材料计及应变率影响的三段线性本构模型。     

超细晶D6A钢动态拉伸力学特性实验研究

为了推进超细晶D6A钢在半穿甲战斗部壳体上的应用,研究了动态加载下其宏观力学行为和细观变形机理。运用旋转盘式Hopkinson拉杆技术,开展了超细晶D6A低合金钢（平均晶粒尺寸为510 nm）的动态拉伸实验,获得了不同应变率（500～1000 s?1）下超细晶钢的应力-应变曲线。运用TEM观测微观形貌,从细观层次研究了高应变率拉伸作用下超细晶钢的动态力学特性。结果表明,超细晶D6A钢具有较高的动态拉伸强度和良好的延展性。并且,晶粒细化和纳米析出相（渗碳体）是超细晶钢同时拥有高强度和较好韧性的重要因素;在动态拉伸过程中析出的大量纳米级渗碳体,与高密度晶界共同作用限制了位错运动,从而产生额外的塑性变形抗力,有效提升了超细晶钢的强度;在塑性变形阶段超细晶钢出现的明显应力下降现象,是可动位错密度增高的结果。     

对称拉压循环对HRB400E钢弹塑性行为影响的试验研究

对HRB400E钢在单向拉伸、拉压循环和循环后拉伸载荷作用下的力学行为进行了试验研究,采用0.3%、0.4%、0.5%、0.6%、0.7%、0.8%、0.9%和1%八种对称应变幅对HRB400E薄壁圆管试件施加100周次拉压循环载荷,循环后施加单轴拉伸载荷直至试件断裂。试验数据分析表明,循环加载过程中HRB400E钢前10周应力应变曲线曲率明显变小,峰值应力明显下降,呈现循环软化特性,随着应变循环幅值的增大,循环滞回曲线稳定所需循环周次逐渐减小。HRB400E钢在不同应变幅循环后的拉伸弹性模量均出现下降,下降幅度约为3%～8%,屈服应力会随着预循环应变幅的增大而增大,随着预循环应变幅增大材料的硬化效应有所减弱。与单轴拉伸相比,循环后拉伸的强度极限和断裂应变无明显变化,塑性变形能力没有明显下降,破坏为典型的塑性剪切断裂模式,薄壁圆管试件断口中的断裂纤维区和剪切唇区位于接近管壁处,断面径向中部的断裂扩展区中,在大韧窝周围存在多个较小韧窝。     

基于声发射矩张量分析混凝土破坏的裂纹运动

从细观上看, 混凝土是一种由骨料、水泥浆基体、裂纹等组成的非均匀复合材料. 单轴准静态加载条件下, 应力应变曲线表现出明显的准脆性特征. 其变形破坏过程实质上是内部微裂纹产生、扩展和汇合的过程, 研究细观尺度的裂纹扩展演化将有助于深入了解混凝土的变形和破坏过程. 声发射作为一种物理检测方法可以获取材料内部细观损伤演化的物理信息. 本文基于声发射技术, 结合改进的时差定位算法和矩张量理论对声发射信号进行分析, 反演了混凝土巴西劈裂破坏中裂纹位置、裂纹类型以及裂纹面运动方向, 揭示了混凝土宏观拉伸破坏的细观裂纹扩展机制. 结果表明: 裂纹运动过程清晰地显示了混凝土内裂纹源首先在试件与载荷接触面附近产生, 之后聚集形成局部损伤区域, 并沿轴线向中心扩展(加载平面)以及裂纹从试件中间向表面扩展的动态过程(厚度方向); 裂纹运动体积可以作为裂纹形成、扩展过程中弹性能释放的度量, 初始裂纹成核时体积参数较小, 峰值载荷时, 裂纹运动体积最大达到$5.93\times10^{-4}$ mm$^{3}$; 混凝土宏观尺度的拉伸破坏在细观尺度上存在有拉伸裂纹、混合裂纹以及剪切裂纹; 拉伸裂纹最多, 占裂纹总数约为60%, 剪切裂纹最少, 约占裂纹总数的10%; 拉伸裂纹运动主导了试件的宏观劈裂破坏.      

Several contributions to soil compactibility induced by cyclic loading test

To investigate the soil compactibility during the cyclic loading, three different kinds of paddy field soils were prepared so as to clarify interrelationships among stress amplitude ratio, bulk density, soil water content and pore water pressure. The presetting values of specimen include the soil water content(percent dry basis) and bulk density of 25% d.b. and 1.1 Mg/m³ respectively. The relation between the number of cyclic loading and axial strain exhibited an asymptotically increasing trend, converging toward a specific value for each experimental condition. Possible effect caused by elastic–plastic characteristics could be recognized, when axial strain for 0.5 Hz excitation becomes greater than the one of 1.0 Hz under same stress amplitude ratio. When the stress amplitude ratio took 0.1, the absolute value of axial strain of 0.5 Hz was greater than the one of 1.0 Hz, whereas its decreasing trend was recognized in the sequence of silt, clay and silty sand. The qualitative relations between pore water pressure and number of cyclic loading were also examined to scrutinize the effect of effective water pressure to the soil compactibility.     

Attenuating performance of a polymer layer coated onto floating structures subjected to water blasts

The water blast attenuating performance of a polymer layer coated onto floating structures has been experimentally investigated. A series of contrastive live fire tests were conducted on a rectangular metal box before and after coating a layer of polymer on its outer hull. The blast responses of the box and water pressure near the wet surface were monitored and analyzed. Test results show that the transmitted impulse at the initial FSI stage can be reduced by almost 50%, owing to the rubber wall flexibility. Consequently, both acceleration and strain peaks of the box induced by the first shock wave can be effectively reduced. Further shock response spectrum analysis indicates that the coating performs like a low pass filter with cutoff frequency at about 50 Hz. Response components higher than 50 Hz can be attenuated effectively, but for components lower than the cutoff frequency, the attenuating effect is limited.     

Fatigue characterization of tire rubber

An investigation of the fatigue behavior of tire rubber was undertaken. The tire rubber used in this work is a blend of natural rubber and polybutadiene. It is vulcanized and filled with carbon black. Fatigue tests were conducted on edge-notched prismatic specimens subjected to tension–tension loading in a displacement-controlled machine. The maximum strain was 8% and the minimum strain was 0.8%. The waveform of the fatigue cycle was sinusoidal at a frequency of 10 Hz. Initial cracks of various lengths were cut into the edge of the rubber specimens with a sharp razor blade. The tearing energy was calculated during the test. A power law relation between the crack growth rate and tearing energy was obtained. The fatigue life of tire rubber was estimated based on the integration of the power law relations and the tearing energy versus fatigue cycles curves in conjunction with the value of the critical tearing energy at crack growth initiation.     

Exponential shear flow of branched polymer melts

  The behavior of a low-density polyethylene melt in exponential shear strain histories is examined and compared to its behavior in constant rate planar elongation. A new set of shear stress and first normal stress difference data in exponential shear are presented and used in several different material functions that have been previously proposed. Viscosities composed of principal stress differences for the two flows showed no correspondence suggesting that, contrary to previous assertions, exponential shear and constant rate planar elongation flows are fundamentally different. It is further suggested that the presence of vorticity makes exponential shear a weak, rather than strong, flow. Received: 5 March 1999/Accepted: 1 September 1999     

High-rate decremental-strain-rate test

A modified torsional split-Hopkinson bar is intoduced and used to study material response associated with a sudden reduction of stain rate during high-rate plastic deformation. In tests on 1100-0 aluminum iniial deformation at a strain rate of approximately 2400 s⁻¹ is reduced by a factor of 15 after 200 μs of high-rate deformation. After the reduction, the deformation continues at the low rate for additional 550 μs. The change in the strain rate is obtained by using a stepped input bar. The results for 1100-0 aluminum show a decrease in the flow stress following the reduction in the strain rate. A short delay exists between the beginning of the strain-rate reduction and the response of the stress. The magnitude of the drop in the stress agrees with the difference in flow stress expected in constant-strain-rate tests in the corresponding high- and low-strain rates. Following the stress reduction. The stress remains essentially constant with no hardening during the subsequent deformation at the low rate.     

Response of foil strain gages to hydraulic pressure

A series of tests to determine the effect that hydraulic pressure has on the output of electrical-resistance foil strain gages is described. Tests were conducted on Micro-Measurements gages, type EA-13-125BT-120 for pressures up to 20.7 MPa and frequencies up to 150 Hz, both for purely dilatational strain as well as various combinations of dilatational and distortional strains. The results showed that gage performance due to the combined effects of hydrostatic pressure and distortion of the structure differed from the condition predicted when pressure and distortion were considered separately. Rate of pressurization (and, hence, strain rate) had no measurable effect within practical limits.     

A Technique for Dynamic Tensile Testing of Human Cervical Spine Ligaments

An experimental study is undertaken to examine the dynamic stress–strain characteristics of ligaments from the human cervical spine (neck). Tests were conducted using a tensile split Hopkinson bar device and the engineering strain rates imposed were of the order of 10²∼10³/s. As ligaments are extremely soft and pliable, specialized test protocols applicable to Hopkinson bar testing were developed to facilitate acquisition of reliable and accurate data. Seven primary ligaments types from the cervical spines of three male cadavers were subjected to mechanical tests. These yielded dynamic stress–strain curves which could be approximated by empirical equations. The dynamic failure stress/load, failure stain/deformation, modulus/stiffness, as well as energy absorption capacity, were obtained for the various ligaments and classified according to their location, the strain rate imposed and the cadaveric source. Compared with static responses, the overall average dynamic stress–strain behavior foreach type of ligament exhibited an elevation in strength but reduced elongation.     

Experimental observations of evolving yield loci in biaxially strained AA5754-O

Experimental measurement of the plastic biaxial mechanical response for an aluminum alloy (AA5754-O) sheet metal is presented. Traditional methods of multiaxial sheet metal testing require the use of finite element analysis (FEA) or other assumptions of material response to determine the multiaxial true stress versus true strain behavior of the as-received sheet material. The method used here strives to produce less ambiguous measurements of data for a larger strain range than previously possible, through a combination of the Marciniak flat bottom ram test and an X-ray diffraction technique for stress measurement. The study is performed in conjunction with a study of the microstructural changes that occur during deformation, and these microstructural results are briefly mentioned in this work. Issues of calibration and applicability are discussed, and results are presented for uniaxial (U), plane strain (PS), and balanced biaxial (BB) extension. The results show repeatable behavior (within quantified uncertainties) for U to 20%, PS to almost 15%, and BB to above 20% in-plane strains. The results are first compared with three common yield locus models (von Mises’, Hill’48, and Hosford’79), and show some unexpected results in the shape change of the yield locus at high strain levels (>5% strain). These changes include the rotation of the locus toward the von Mises surface and elongation in the balanced biaxial direction. Comparison with a more complex yield locus model (Yld2000-2d with eight adjustable parameters) showed that the locus elongation in the biaxial direction could be fit well (for a specific level of work), but at the detriment of fit to the plane strain data. Artificially large plastic strain ratios would be needed to match both the biaxial and plane strain behavior even with this more complex model.     

火灾下无粘结预应力筋应力-应变全过程分析

火灾下预应力筋的膨胀应变、高温蠕变较大,对其受力的影响也较大。提出了通过对火灾下各时间段、各微段无粘结筋膨胀应变、高温蠕变以及无粘结筋总伸长量的分析来得到无粘结筋应力以及应力引起的应变,从而完成无粘结筋应力一应变全过程分析的方法。计算借助编程完成,将火灾下无粘结筋应力变化的计算值与相关的试验数据进行比较,吻合程度较好。...     

Experimental determination of strain-induced anisotropy during nonproportional straining of an A1/Mg alloy at room temperature

A servohydraulic, computer controlled MTSn axial-torsion testing machine with a bi-axial clip-on extensometer is used to test thin-walled tubes of an A1/Mg alloy under strain control. A plastic offset strain of 10⁻⁴ determines the yield surfaces. Straining and yield surface probing is governed by a computer program which also controls digital data acquisition. Yield surfaces in stress and in strain space as well as the axial and shear stress-strain diagrams can be reconstructed from the digitally recorded data. The specimens were subjected to a strain path in the form of a regular 16-sided polygon which was followed on some specimens by a square path. The total inelastic strain path length can exceed 15% while the equivalent strain excursion is less than 2%. It is shown that yield surfaces measured on specimens withclose initial stress-strain diagrams are very consistent and that yield surface probing has an insignificant effect on subsequent yield surfaces. Yield surfaces are shown to translate, changein shape and size and to exhibit a cross effect. A post processor which includes a least square smoothing routine calculates the area and the centroid of each yield surface. The size increase is initially rapid but the rate of increase decreases as a saturation is approached. After strining for less than 1% in a fixed direction a characteristic yield surface shape is established. Yield surfaces obtained at the same point in strain space with identical prestrain direction of at least 1% but with increased amounts of accumulated plastic strain have the same shape but show an increase in size. The yield surfaces differ in shape and size when the same strain point is reached from different directions. The centroid of the yield surface in stress space moves almost in a circular path for a polygonal strain path. All stress space yield surfaces contain the origin but this is not the case for the surfaces in strain space.