Crystallographic data on axially compressed Kevlar 49 fibres

Axially compressed Kevlar 49 fibres have been examined by X-ray diffraction methods. The most prominent effect of axial compression is the anisotropic deformation of the unit cell. Whereas thec-axial length, which corresponds to the chain axis, undergoes contraction, the basal plane dimensions manifest enlargement. The deformations increase with the extent of axial compression. The half-widths and the azimuthal spread of reflections also exhibit changes. The compression induced structural changes provide qualitative support to the experimentally observed reduction in tensile strength and modulus. Paper presented at the 5th IUMRS-ICA 98 International conference held at Bangalore, October 1998.     

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.     

Very low temperature thermal conductivity of Kevlar 49

We measured the thermal conductivity of a Kevlar 49 in the temperature range. The data were fitted with a power-law: . Kevlar 49 is a candidate material for the supports of CUORE experiment.     

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.     

Decomposition behaviour of Kevlar 49 fibres: Part I. AtT≈T d

The structural changes which accompany decomposition of Kevlar 49 fibres atT=500°C and 550°C, respectively, have been elucidated. At both these temperatures, cumulative thermal exposures of specific durations,t _cum(T), are required to result in decomposition. The conspicuous features which characterize isothermal decomposition of the fibres are: (i) progressive reduction and an eventual total loss in fibre crystallinity, (ii) progressive loss in weight, (iii) introduction of surface damages, (iv) introduction of hollowness, and (v) deterioration in tensile properties.     

Fracto-emission from single fibres of Kevlar

Fracto-emission (FE) is the emission of particles (e.g. electrons, ions and photons) during and following fracture. In this paper, we present data on electron emission (EE) and positive ion emission (PIE) from the tensile fracture of Kevlar single fibres. The fibres were initially fractured in pure tension, where a stranded form of fracture was observed, often with multiple peaks spread over several hundred microseconds. The loading condition was then changed by stretching and breaking the fibres over a dull metal edge. With this change in the loading, different forms of fracture were observed, each with distinctive forms of emission curves. When fracture was accompanied by extensive fibril formation, total emission was high and both EE and PIE decay times were long relative to fractures in which little fibril formation occurred. The results of this study suggest that FE has some applicability as a tool for the detection of fracture mechanisms of single fibres.     

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.     

Properties of cement composites reinforced with Kevlar fibres

The organic polyamide fibre, Kevlar, is promising as an efficient reinforcement for cementitious matrices. For cement boards, in which chopped fibres are distributed randomly in two dimensions, typical mechanical properties obtained with 1.9 vol% fibre addition are as follows: ultimate tensile strength (UTS) 16 MN m^–2; MOR 44 MIN m^–2; impact strength 17 kJ m^–2. The composite material can be produced by autoclaving if desired and at ambient temperatures they are expected to be durable in most environments. The relatively low decomposition point of Kevlar (as opposed to glass fibres or steel) is a disadvantage for its use in building components which may come into contact with high temperatures, as in a fire. It should be noted that a solvent which is used in the manufacture of the fibre and remains in the fibre in minute quantities has been found to produce cancer in rats. There is no evidence of it causing cancer in humans but the significance of this in terms of a possible health risk, if any, will need to be assessed by the appropriate medical authorities in relation to any applications.     

Finite element modeling of ballistic impact on multi-layer Kevlar 49 fabrics

This paper presents a material model suitable for simulating the behavior of dry fabrics subjected to ballistic impact. The developed material model is implemented in a commercial explicit finite element (FE) software LS-DYNA through a user defined material subroutine (UMAT). The constitutive model is developed using data from uniaxial quasi-static and high strain rate tension tests, picture frame tests and friction tests. Different finite element modeling schemes using shell finite elements are used to study efficiency and accuracy issues. First, single FE layer (SL) and multiple FE layers (ML) were used to simulate the ballistic tests conducted at NASA Glenn Research Center (NASA-GRC). Second, in the multiple layer configuration, a new modeling approach called Spiral Modeling Scheme (SMS) was tried and compared to the existing Concentric Modeling Scheme (CMS). Regression analyses were used to fill missing experimental data – the shear properties of the fabric, damping coefficient and the parameters used in Cowper-Symonds (CS) model which account for strain rate effect on material properties, in order to achieve close match between FE simulations and experimental data. The difference in absorbed energy by the fabric after impact, displacement of fabric near point of impact, and extent of damage were used as metrics for evaluating the material model. In addition, the ballistic limits of the multi-layer fabrics for various configurations were also determined.     

Lifetime statistics for single Kevlar 49 filaments in creep-rupture

Experimental data are presented for the lifetime of single Kevlar 49 filaments under moderate to high stress levels at standard ambient conditions (21°C, 65% r.h.). Filaments were drawn from two spools, A and B, taken from the same production lot. Previously we found that filaments from spool A were 7% lower in mean strength but much less variable in diameter than filaments from spool B; however, the respective variabilities in failure stress were equivalent. The lifetime data were interpreted in light of a previously developed kinetic model embodying Weibull failure statistics and power law dependence of lifetime on stress level. As predicted, lifetime data at each stress level generally followed a two-parameter Weibull distribution with a shape parameter value near 0.2. Based on absolute stress levels, the filaments drawn from spool B had a Weibull scale parameter for lifetime about ten times greater than those from spool A; however, when the stress-levels were normalized by the respective Weibull scale parameters for short-term strength, these differences disappeared. With respect to power law dependence of lifetime on stress level, three distinct time domains emerged, each marked by a different power law exponent. Similar behaviour was observed earlier for preproduction Kevlar 49/epoxy strands, and the values for the power law exponents for the filaments agree closely with those for the strands.     

The ultraviolet sensitivity of Kevlar 149 and Technora fibres

The effect of ultraviolet radiation on the tensile properties of two engineering fibres, namely Kevlar 149 and Technora, is reported. Both fibre types show considerable loss in strength, but the former also exhibits a significant and unexpected fall in modulus. Electron microscope examination of the virgin and extended fibres revealed the structural reasons for the observed mechanical behaviour.     

Effect of thermal spikes on the structural characteristics of Kevlar fibres

The effect of upto six cumulative exposures to thermal spikes, each of 10 s duration, on Kevlar 49 fibres has been analysed. X-ray data show that exposures to spikes corresponding to T's 400°C cause changes at the level of the crystal lattice. At and above 500°C, severe surface damages such as introduction of longitudinal openings, peel-offs and extraneous material are found to occur. The tensile properties of the spike-exposed fibres manifest changes which conform well with the structural changes. As in the case of prolonged thermal exposures, the spike induced effects are also controlled by two parameters, viz., the temperature and the duration of the cumulative exposure. The data from spike exposed fibres indicate that the thermally induced changes in the structural and tensile characteristics get initiated at the very early stages of thermal exposure viz., of the order of 10 s.     

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.     

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。      

Strain rate and gage length effects on tensile behavior of Kevlar 49 single yarn

In the present paper, Kevlar® 49 single yarns with different gage lengths were tested under both quasi-static loading at a strain rate of 4.2 × 10^?4 s^?1 using a MTS load frame and dynamic tensile loading over a strain rate range of 20–100 s^?1 using a servo-hydraulic high-rate testing system. The experimental results showed that the material mechanical properties are dependent on gage length and strain rate. Young’s modulus, tensile strength, maximum strain and toughness increase with increasing strain rate under dynamic loading; however the tensile strength decreases with increasing gage length under quasi-static loading. Weibull statistics were used to quantify the degree of variability in yarn strength at different gage lengths and strain rates. This data was then used to build an analytical model simulating the stress–strain response of single yarn under dynamic loading. The model predictions agree reasonably well with the experimental data.     

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.