标距和应变率对Kevlar49单束拉伸力学性能的影响

为探究Kevlar 49单束的尺寸效应及应变率敏感性, 首先, 利用MTS万能试验机对不同标距(25、50、100、150、200和300 mm)的Kevlar 49单束进行了准静态(应变率为1/600 s-1)拉伸测试; 然后, 利用Instron落锤冲击系统对标距为25 mm的试样进行了动态(应变率为40~160 s-1)拉伸测试; 最后, 利用Weibull模型进行统计分析, 量化了不同标距和应变率下Kevlar 49单束拉伸强度的随机变化程度。结果表明: Kevlar 49单束的拉伸力学性能与标距和应变率有相关性; 拉伸强度随标距的增加而减小, 但随应变率的增加而增大; 峰值应变和韧性均随标距和应变率的增加而减小; 提取的Weibull参数可用于数值模拟及工程应用。      

Kevlar49单丝和单束的随机破坏及有限元模拟

为研究不同尺度下Kevlar49单丝(微观)和单束(细观)的拉伸力学性能,首先,分别采用MTI微型拉伸试验机和MTS微机控制电子万能试验机对纤维丝和纤维束进行了单向拉伸试验,发现纤维丝和纤维束的力学性能与试样的标距及结构尺度存在很大相关性,试样的拉伸强度会随着其标距的增加和结构尺度从纤维丝增大到纤维束而降低;随后,按照Weibull分布对试验数据进行统计分析,量化了不同标距下纤维丝和纤维束拉伸强度的随机变化程度;接着,考虑到纤维丝的拉伸强度符合Weibull分布随机破坏,利用ANSYS中的用户自定义子程序(USERMAT)建立了纤维丝本构模型;最后,采用纤维丝本构模型模拟纤维束的拉伸破坏行为,并讨论了关键参数对纤维束拉伸破坏行为的影响。结果表明:纤维束的模拟结果与试验结果吻合程度较好,所建模型可以较准确地预测纤维束的拉伸性能。      

Creep of Kevlar 49 fibre and a Kevlar 49-cement composite

The creep strain responses of Kevlar 49 fibres and a Kevlar 49 — cement mortar composite board to sustained stresses have been studied over an extended period in excess of four years at ambient temperature. Single filaments of Kevlar 49, 900 mm in length, were stressed in tension in the range 830 to 1830 MPa. The relationship between creep and elapsed time is represented by the power functionAt ⁿ whereA is a function of stress andn is a constant. The creep strain in Kevlar 49 was low compared with other polymers. For example after 1000 days at a stress of 1830 MPa the creep strain was 13% of the initial elastic strain and is predicted by the power function to increase to 14.6% after 4000 days. The Kevlar 49 — mortar composite was subjected to bending stresses in the range 6 to 35 MPa and the creep deflection was monitored. The relationship between creep and time could again be represented by the power functionAt ⁿ withA dependent on stress andn constant. The creep was similar to that expected from the matrix alone. The ratio of the creep deflection to the initial deflection after 1000 days at a stress of 6.15 MPa (well below the matrix cracking stress) was 1.31 and at 23.5 MPa (well above the matrix cracking stress) was 1.63.     

动态拉伸载荷下应变率和温度对Kevlar 49芳纶纤维布增强环氧树脂复合材料力学性能的影响

为研究Kevlar 49芳纶纤维布增强环氧树脂复合材料在中等应变率和不同温度耦合作用下的力学响应和断裂行为,首先,利用MTS液压伺服高速机在不同初始应变率(25、50、100、200 s-1)和温度(-25、0、25、50、100℃)下对芳纶纤维增强复合材料(AFRP)进行单向动态拉伸测试;然后,采用Weibull分析模型量化了拉伸强度在不同应变率和温度下的离散程度。结果表明:在相同温度(25℃)下,随着应变率的增加,弹性模量和拉伸强度均先增大(初始应变率介于25~50 s-1范围内)后减小(初始应变率介于50~200 s-1范围内),极限应变则呈现出相反的变化趋势,而韧性随应变率的变化幅度不大;在相同初始应变率(25 s-1)下,与在25℃下的情况相比,温度的升高或降低均会造成弹性模量的降低,在温度为100℃时,极限应变显著增加,而拉伸强度和韧性均不会随温度的变化而发生明显改变。对AFRP断裂形态进行的对比分析表明不同试验条件下AFRP的断裂形态基本相同,均呈现出较为平整的断裂面。所得结论可为AFRP在极端载荷和环境作用下的理论研究和应用提供依据。      

黄麻纤维束力学性能的尺寸和应变率效应

为了探究黄麻纤维束的尺寸效应和应变率敏感性,利用C43电子式万能试验机和CEAST 9340落锤试验冲击系统分别在静动载条件下对黄麻纤维束进行测试,获得了杨氏模量、强度、峰值应变和韧性随标距和应变率的变化关系静载试验在1/600s-1应变率条件下进行,测试了6组不同标距(25、50、100、150、200和300mm)的试件;动载试验以应变率为变量,在4组不同的应变率(40、80、120和160s-1)条件下进行了测试,试件标距均为25mm。测试结果表明:随着试件标距增大,杨氏模量初始增大,当标距大于100mm时趋于稳定;强度、峰值应变和韧性均减小。随着应变率增大,杨氏模量和强度均增大;峰值应变初始减小后趋于稳定;韧性先减小后增大。鉴于植物纤维束材料较大的性能离散性,采用Weibull分布对试验数据进行拟合,获得了黄麻纤维束强度在不同试验条件(标距和应变率)下的分布规律。     

Strain rate effects on tensile deformation behavior of Fe-3.5Mn-0.3C-5Al ferritic based lightweight steel

Tensile deformation behavior of Fe-3.5Mn-0.3C-5Al ferritic based lightweight steel was studied in a large range of strain rate (0.001 s⁻¹–1200 s⁻¹). Microstructures of the steel before and after tension were observed. The results show that Fe-3.5Mn-0.3C-5Al lightweight steel has a good strength (820 MPa) and plasticity (40 %) and exhibits excellent combinations of specific strength and ductility (>32000 MPa %) at the strain-rate of 0.001 s⁻¹ after annealing at 850 °C for 5 minutes then directly quenching into water. The austenite in the steel tested was transformed into α′-martensite during the tensile deformation process. With an increase in strain rate from 0.001 s⁻¹ to 1200 s⁻¹, tensile strength of the steel investigated increased from 820 MPa to 932 MPa, while its elongation first decreased from 40 % to 15 %, and then increased from 15 % to 29 %. At the strain rate of 1200 s⁻¹, adiabatic heating resulted in temperature rising in matrix, suppressed the transformation of austenite to α′-martensite. Comparing with transformation induced plasticity steel, the austenite in 3.5Mn lightweight steel is obviously unstable and cannot provide progressive phase transition.     

Temperature dependence of lifetime statistics for single Kevlar 49 filaments in creep-rupture

Experimental data are presented for the strength and lifetime under constant stress of single Kevlar 49 aramid filaments at two elevated temperatures, 80 and 130° C. As seen in previously published work performed at room temperature (21 °C), the strength data could be fitted to a two-parameter Weibull distribution; increasing the temperature caused a decrease in the Weibull scale parameter while the shape parameter remained relatively constant, indicating a decrease in the mean strength but no change in strength variability. Lifetime experiments at both 80 and 130°C were performed at different filament stress levels, ranging from 55 to 92.5% of the Weibull scale parameter for short-term strength at that temperature. These data were fitted to a two-parameter Weibull distribution with large variability (scale parameter values 1), and evaluated using an exponential kinetic breakdown model in the spirit of Eyring and Zhurkov. Using this model, activation energies in the neighbourhood of 80 kcal mol^–1 (3.35 × 105 J mol^–1 ) were obtained, suggesting that scission of the C-N bond plays the dominant role in fibre failure at longer times under constant stress.Kevlar and Kevlar 49 are registered tardemarks of E. I. du Pont de Nemours & Co., Inc.     

Effect of strain rate on the tensile behavior of a single crystal nickel-base superalloy

The effect of various strain rates on the tensile behavior of a single crystal nickel-base superalloy was studied. Single crystals with 0 0 1 crystal orientation were tested at 800 and 1000 °C under three kinds of strain rate of 10⁻³, 10⁻⁴ and 6 × 10⁻⁵ s⁻¹. The yield strength increased with the increase of strain rate, while the configuration of the stress–strain curves was independent of strain rate. Additionally, fracture surface was related to strain rate at two temperatures. At 800 °C the amount of cleavage surface was different at three strain rates, which resulted from the difference of activated slip systems. The elongation increased with the decrease of strain rate, which was influenced by the heterogeneous ductile deformation. At 1000 °C the difference of fracture surface was attributed to the microvoid at higher strain rate, while the γ/γ′ interfaces also played an important role at lower strain rate; elongation rate was independent of strain rate.     

Transverse compression behavior of Kevlar KM2 single fiber

This paper investigates the quasi static transverse compression behavior of Kevlar KM2 single fiber widely used in high velocity impact (HVI) applications. The nominal stress–strain response of single fibers exhibits nonlinear inelastic behavior under transverse compression. The nonlinearity is due to both geometric and material nonlinearities. The inelastic behavior is attributed to plastic deformation and microstructural damage resulting from fibrillation and micro cracking. The experimental set up allows for the observation and measurement of compressed width in real time. An experimental methodology is presented to determine the fiber material constitutive behavior by removing the geometric nonlinearity due to the growing contact area. Results from finite element model of the test method are correlated with the experimental results to assess the accuracy of the constitutive model.     

Strain rate effects on the mechanical behavior of nanocrystalline Au films

The effect of fabrication, film thickness, and strain rate on the mechanical behavior of Au films with 100 nm (evaporated gold) and 200 nm (electroplated gold) average grain sizes was investigated. Uniaxial tension was imposed at 10^− 3-10^− 6 s^− 1 strain rates on evaporated 0.5 μm and 0.65 μm thick Au specimens, and at 10^− 2-10^− 5 s^− 1 on electroplated 2.8 μm thick Au specimens. Strain rates between 10^− 3 and 10^− 5 s^− 1 had a marked impact on the ultimate strain of evaporated films and less significant effect on their yield and saturation stress. The ductility increased with decreasing strain rate and it varied between 2-4.5% for 500-650 nm thick films and 3.4-10.6% for 2.8 μm thick films. When compared at the same strain rate, the thick electroplated films were more ductile than the thin evaporated films, but their yield and saturation stresses were lower, possibly due to their larger grain size. Qualitatively, the stress-strain behavior was consistent at all rates except at the slowest that resulted in significantly different trends. A marked decrease of the maximum strength, effective Young's modulus, and yield strength occurred at 10^− 6 s^− 1 for thin, and at 10^− 5 s^− 1 for thick films, while for 500 nm thin films multiple stress localizations per stress-strain curve were recorded. Because of temperature, applied stress, and grain size considerations this behavior was attributed to dislocation creep taking place at a strain rate comparable to the applied strain rate.     

Compressive behaviour of Kevlar 49 fibres and composites

The low compressive strength of Kevlar 49^® unidirectional composites cannot be satisfactorily explained in terms of current theories which assume that failure is due to the matrix material. For a given matrix, Kevlar 49 composites are considerably weaker in compression than those based on other comparable high strength, high modulus filaments. Fracture is found to occur before any plastic deformation of the matrix is observed.This behaviour can be explained in terms of the very low compressive yield strength of the Kevlar 49 fibres themselves. Elastica loop tests show that non-Hookean deformation of the fibres occurs at quite low stresses corresponding to values of the order of those at which fracture takes place in the composite. This deformation is plastic in nature.Buckled areas on the compression side of the elastica loop can be seen in the optical and scanning electron microscopes. It is suggested that the buckling follows from the separation of microfibrils under compression.     

Compressive and torsional behaviour of Kevlar 49 fibre

The mechanical anisotropy of an aromatic polyamide fibre, Kevlar 49, was studied in tension, compression and torsion. A new technique involved applying small and defined compressive strains to filaments by bonding them to one side of a beam which is subsequently bent to compress the fibres. Using scanning electron and optical microscopy, fibres were shown to form regularly-spaced helical kink bands at 50 to 60° to the fibre axis after the application of small axial compressive strains. Tensile tests of previously-compressed fibres revealed only a 10% loss in tensile strength, after application of as much as 3% compressive strain. A torsion pendulum apparatus was used to measure the shear modulus and an apparent shear strength of fibres. A loss of tensile strength after the application of large (> 10%) torsional shear strains coincided with a loss in recoverable shear strain due to longitudinal fibre splitting. Ratios of tensile-to-compressive strength, tensile-to-shear strength and tensile-to-shear moduli of 51, 171, and 701, respectively, were measured for Kevlar 49.     

High strain rate effects on direct tensile behavior of high performance fiber reinforced cementitious composites

Direct tensile behavior of high performance fiber reinforced cementitious composites (HPFRCCs) at high strain rates between 10 s⁻¹ and 30 s⁻¹ was investigated using strain energy frame impact machine (SEFIM) built by authors. Six series of HPFRCC combining three variables including two types of fiber, hooked (H) and twisted (T) steel fiber, two fiber volume contents, 1% and 1.5%, and two matrix strengths, 56 MPa and 81 MPa, were investigated. The influence of these three variables on the high strain rate effects on the direct tensile behavior of HPFRCCs was analyzed based on the test results. All series of HPFRCCs showed strongly sensitive tensile behavior at high strain rates, i.e., much higher post cracking strength, strain capacity, and energy absorption capacity at high strain rates than at static rate. However, the enhancement was different according to the types of fiber, fiber volume content and matrix strength: HPFRCCs with T-fibers produced higher impact resistance than those with H-fibers; and matrix strength was more influential, than fiber contents, for the high strain rate sensitivity. In addition, an attempt to predict the dynamic increase factor (DIF) of post cracking strength for HPFRCCs considering the influences of fiber type and matrix strength was made.     

Orientation and strain rate dependent tensile behavior of single crystal titanium nanowires by molecular dynamics simulations

Molecular dynamics simulation was employed to study the tensile behavior of single crystal titanium nanowires(NWs)with1120,1100and[0001]orientations at different strain rates from 10~8s~(-1)to10~(11)s~(-1).When strain rates are above 10~(10)s~(-1),the state transformation from HCP structure to amorphous state leads to super plasticity of Ti NWs,which is similar to FCC NWs.When strain rates are below 10~(10)s~(-1),deformation mechanisms of Ti NWs show strong dependence on orientation.For1120orientated NW,1011compression twins(CTs)and the frequently activated transformation between CTs and deformation faults lead to higher plasticity than the other two orientated NWs.Besides,tensile deformation process along1120orientation is insensitive to strain rate.For 1100orientated NW,prismaticaslip is the main deformation mode at 10~8s~(-1).As the strain rate increases,more types of dislocations are activated during plastic deformation process.For[0001]orientated NW,1012extension twinning is the main deformation mechanism,inducing the yield stress of[0001]orientated NW,which has the highest strain rate sensitivity.The number of initial nucleated twins increases while the saturation twin volume fraction decreases nonlinearly with increasing strain rate.     

Thermal- and stress-induced lattice distortions in a single Kevlar49 fibre studied by microfocus X-ray diffraction

Understanding the macroscopic physical and mechanical properties of poly(p-phenylene terephthalamide) (PpTA) fibres as a function of temperature requires an understanding of how temperature influences its microscopic structure. This study investigates lattice distortions in single PpTA fibres using the high brilliance of a synchrotron radiation microbeam. Lattice distortions are studied over a temperature range of 110–350 K and the influence of tensile deformation is also considered. The results reveal linear thermal expansion behaviour for all unit cell axes, in general agreement with literature. Expansion/contraction is greatest along the [100] direction whilst being reduced along [010] by inter-chain hydrogen bonding. During macroscopic deformation, longitudinal crystal strain dominates with respect to axial lattice distortions induced by temperature changes. There is only a small change in the [100] coefficient of thermal expansion, with the [010] and [001] directions being largely unaffected.     

Strain rate sensitivity of titanium matrix composites under tensile loading

Titanium matrix composites continuously reinforced by silicon carbide fibres are considered as key technology for a new generation of aero engines. High specific strength and stiffness promise significant weight savings and performance improvements. Impact loading may be one critical aspect in the design of rotating compressor components with titanium matrix composites. On the other hand the strength increases at high strain rates. Considering this additional loading capability enables a design closer to the edge. Therefore, the strength to strain rate relation needs to be examined. This paper presents results of high strain rate tests at the fly wheel device. Loadings resulting in strain rates from quasistatic loading to 740 1/s were applied. The results are discussed and the mechanism of energy consumption was investigated by determination of the plastic deformed length by scanning electron microscopy.     

Strain rate effects on dynamic fracture and strength

An experimental procedure and accompanying theoretical analysis is presented to produce a well-characterized technique for quantifying dynamic fracture properties of quasi-brittle materials. An analytical and experimental investigation of mode I fracture of concrete was conducted under the dynamic loading of a split Hopkinson pressure bar. Fracture specimens in the form of notched-cavity splitting tension cylinders were subjected to stress wave loading that produced strain rates nearing 10/s. Fracture parameters were extracted by the application of the two-parameter fracture model, a nonlinear fracture model for quasi-brittle materials. Finite element analysis verified the experimental configuration and addressed inertial contributions within the dynamic environment. Ultra-high-speed digital photography was synchronized with the fracture process to provide additional validation and insight to the experimental technique. Results show that the effective fracture toughness and specimen strength both increase significantly with loading rate. The numeric and photographic results validate the experimental technique as a new tool in determining rate dependent material properties.     

Thermal conductivity of Kevlar 49 between 7 and 290 K

We measured the thermal conductivity of a Kevlar 49 in the 7–290 K temperature range. With a maximum error of 4%, our data are fitted by the simple formula: Kevlar 49 is a candidate material for the supports of CUORE experiment.