Experimental and theoretical study on the strain rate and temperature dependence of mechanical behaviour of Kevlar fibre

The tensile impact experiments on Kevlar 49 fibre bundles were carried out at strain rates 140, 440, 1350 s⁻¹ and at temperatures −60, −20, 15, 50 and 90°C. It was found that the tensile mechanical properties of Kevlar 49 fibre bundles depend both upon the strain rate and the temperature. A bimodal Weibull distribution statistical model of the strain rate and temperature dependence of fibres, and a test method of determining mechanical properties and Weibull parameters of fibres from the fibre bundle test were adopted to characterize the combined effects of strain rate and temperature on Kevlar 49 fibre strength distribution. The simulated results are in good agreement with the experimental data, which proves that the bimodal Weibull distribution function is suitable to represent the statistical strength distribution of Kevlar 49 fibre which has peculiar ‘skin–core’ physical structure, and that the test method is valid and reliable.     

Investigating the transverse behavior of Glass–Epoxy composites under intermediate strain rates

In this study, the strain rate effects on transverse tensile and compressive properties of unidirectional Glass fiber reinforced polymeric composites are investigated. To demonstrate strain rate effects, the tensile and compressive composite specimens with identical configuration are fabricated and tested to failure in the transverse direction at quasi-static strain rate of approximately 0.001 s⁻¹ and intermediate strain rates of 1–100 s⁻¹. The tensile and compressive tests are performed using a servo-hydraulic testing apparatus equipped with strain rate increasing mechanisms. For performing the practical tests, a jig and a fixture and other test supplies are designed and manufactured. The performance of the test jig is evaluated and showed that it is adequate for composites testing under tension and compression loads. The effects of strain rate on mechanical properties (maximum strength, modulus, and strain to failure) are considered. The characteristic results for the transverse properties indicate that damage evolution is strain-rate-dependent for the examined material. Also, a strain-rate-dependent empirical material model associated with different regression constants is proposed based on the experimental results obtained to characterize the rate dependent behavior of Glass/Epoxy composite material.     

Dynamic mechanical properties of fabric armour

Twaron^®, a fabric made from aramid fibres and somewhat similar to the commonly known Kevlar^®, is also often used in flexible armour applications and hence subjected to high rates of loading. The dynamic mechanical properties of Twaron^® fabric are examined via high-speed tensile tests on specimens using a split Hopkinson bar. The load-deformation and failure characteristics at different rates of stretching are determined, from which constitutive equations representing its viscoelasticity and strain-rate dependence are formulated. This facilitates modelling of the material response to impact and perforation. Experimental results indicate that Twaron^® is highly strain-rate dependent; the tensile strength and modulus increase with strain rate while the failure strain decreases. Twaron^® specimens are also observed to fail in a more brittle fashion as the strain rate increases; this phenomenon significantly reduces the amount of energy absorbed at high strain rates. An analysis based on the morphology and fracture mechanisms of poly(p-phenylene-terephthalamide) (PPTA) fibres, the main constituent of Twaron^®, is formulated to account for the experimental observations. The proposed constitutive equation, based on a three-element linear viscoelastic model is able to describe reasonably accurately the experimental stress–strain response over a range of strain rates.     

Strain rate dependence of HPFRCC cylinders in monotonic tension

High-Performance Fiber-Reinforced Cementitious Composite (HPFRCC) materials exhibit strain hardening in uniaxial, monotonic tension accompanied by multiple cracking. The durability of HPFRCC materials under repeated loading makes them potentially suitable for seismic design applications. In this paper, the strain rate dependence of tensile properties of two HPFRCC materials in cylindrical specimens is reported from a larger study on strain rate effects in tension, compression and cyclic tension–compression loading. The cylindrical specimens were loaded in monotonic tension at strain rates ranging from quasi-static to 0.2 s⁻¹. To evaluate the impact of specimen geometry on tensile response, coupon specimens loaded in monotonic tension under a quasi-static strain rate were compared to corresponding cylindrical specimens made from the same batch of material. Tensile strength and ductility of the HPFRCC materials were significantly reduced with increasing strain rate. Multiple cracking, strain hardening, strain capacity, and the shape of the stress–strain response were found to be dependent on specimen geometry. SEM images taken of the fracture plane of several specimens indicated that pullout and fracture of the fibers occurred for both HPFRCC materials studied here.     

Derived Dynamic Ply Properties from Test Data on Angle Ply Laminates

Dynamic unidirectional tensile ply properties were extracted from the results of burst tests on angle ply filament wound GRP and KRP tubes under internal hoop loading. The extracted longitudinal and transverse tensile strengths as well as transverse tensile moduli exhibited strain-rate sensitivities. Shear properties were derived from test results on 55° and 65° tube angles. Derived shear stress-strain curves and shear strength values are presented at different strain rates; again clearly demonstrating rate effects on these properties. Complete sets of strain rate dependent lamina tensile properties are presented for GRP and KRP covering the strain rate regime of 1 to 10² sec^-1.     

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.     

The static and high strain rate behaviour of a commingled E-glass/polypropylene woven fabric composite

In this paper the effect of strain rate on the tensile, shear and compression behaviour of a commingled E-glass/polypropylene woven fabric composite over a strain rate range of 10⁻³–10² s⁻¹ is reported. The quasi-static tests were conducted on an electro-mechanical universal test machine and a modified instrumented falling weight drop tower was used for high strain rate characterisation. The tensile and compression modulus and strength increased with increasing strain rate. However, the shear modulus and strength were seen to decrease with increasing strain rate. Strain rate constants for use in finite element analyses are derived from the data. The observed failure mechanisms deduced from a microscopic study of the fractured specimens are presented.     

Temperature and loading rate effects on tensile properties of kenaf bast fiber bundles and composites

This paper presents extensive experiments and micromechanics-based modeling to evaluate systematically the tensile properties of kenaf bast fibers bundle (KBFB) and kenaf bast fiber-reinforced epoxy strands. Uniaxial tension behaviors of KBFBs and KBFB-reinforced epoxy strands were evaluated statistically using large sample sets. The elastic modulus, tensile strength, as well as failure strains of KBFBs, displayed large scatter statistically ranging from 10% to 30%. The loading rate-dependency was evaluated at three strain rates ranging from approximately 10^?4 ～ 10^?2/s. The tensile strength increases gradually as the loading rate increases, while the tensile modulus almost remains the same as the loading rate increases until the loading rate reaches 10^?2/s, at which a much higher modulus was presented. The high temperatures (170–180 °C), possibly subjected during fiber processing and composite fabrication, do not impose significant effects on the tensile properties of KBFBs if the duration is less than 1-h. The tensile properties of KBFB were not affected by the conditioning at 130 °C for 24-h, which mimics the severe service temperature of automotive front-end components. KBFB-epoxy composite strands were further evaluated at various loading rates. A micromechanics-based Mori–Tanaka model was implemented to predict the anisotropic elastic moduli of KBFB and KBFB-epoxy composite strands based on the microstructural compositions.     

Diametral compression of pultruded composite rods

Diametral compression tests were performed on pultruded composite rods comprised of unidirectional glass or carbon fibers in a common matrix. During compression tests, acoustic emission (AE) activity was recorded and images were acquired from the sample for analysis by digital image correlation (DIC). In both composite systems, localized tensile strain developed in the transverse plane under the load platens prior to failure, producing non-linearity in the load–displacement curve and AE signals. In situ SEM diametral compression tests revealed the development of matrix microcracking and debonding in regions of localized strain, perpendicular to the tensile strain direction (parallel to the load axis). Comparison of linear finite element simulations and experimental results showed a deviation from linear elastic behavior in the load displacement curve. The apparent transverse modulus, in plane shear modulus, and transverse tensile strength of the GF rod was greater than that of the CF rod, and fracture surfaces indicated greater fiber/matrix adhesion in the GF system compared to the CF system. A mixed mode fracture surface showed that two failure modes were active – matrix tensile failure and matrix compression failure by shear near the loading edge.     

Effect of oxygen plasma treatment of PPTA and PBO fibers on the interfacial properties of single fiber/epoxy composites studied by Raman spectroscopy

The interfacial micromechanics of single poly(p-phenylene terephthalamide (PPTA) and poly(p-phenylene benzobisoxazole (PBO) fibers embedded in an epoxy resin has been investigated by determining the interfacial shear stress distributions along the fiber length. The effects of an oxygen plasma treatment on the interfacial shear stress of the fiber-epoxy systems are analyzed. Raman spectroscopy was used to map the stress distributions along the fiber when the composite is subjected to a small axial tensile strain (3.5% for PPTA and 2.5% for PBO). The quality of the interface or adhesion was improved after the surface treatment, supporting the ability of plasma oxidation to enhance the adhesion of high-performance fibers to epoxy resins. The tensile behavior of fiber-reinforced systems was different in each case. PPTA reinforcements underwent fragmentation, likely by fiber microfailure, whereas debonding or bridging is the most probable fragmentation mechanism in the case of PBO.     

Synergistic response of blending fibers in ultra-high-performance concrete under high rate tensile loads

This study investigated the synergistic tensile response of blending 1% long and 0.5% short steel fibers in ultra-high-performance concrete (UHPC) at high strain rates of 16–37 s⁻¹. Three ultra-high-performance hybrid-fiber-reinforced concretes (UHP-HFRCs) containing twisted, hooked, or smooth long (30 mm) fibers blended with short (13 mm) smooth fibers, as well as one sample (LS10MS05) blending long and medium (19 mm) smooth fibers, were examined. The blending of long and shorter steel fibers in UHPC generated high synergy in the tensile responses of the UHP-HFRCs, especially at high strain rates. Synergies were significant for strain capacity and peak toughness, but not for post-cracking strength and softening fracture energy. Among the long fibers, the hooked fibers generated the highest synergy at high strain rates, but smooth fibers produced the highest rate sensitivity in UHPC. Consequently, the LS10MS05 sample demonstrated the highest tensile resistance at high strain rates.     

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.     

Tensile behavior of carbon fiber bundles at different strain rates

Quasi-static and high strain rate tensile tests have been performed on T700 carbon fiber bundles and complete stress-strain curves at the strain rate range of 0.001 s^− 1 to 1300 s^− 1 were obtained. Results show that strain rate has negligible effect on both ultimate strength and failure strain, and T700 carbon fiber can be regarded as strain rate insensitive materials. On the basis of the fiber bundles model and the statistic theory of fiber strength, a damage constitutive model based on Weibull distribution function has been developed to describe tensile behavior of T700 fiber bundles. And the method to determine the statistic parameters of fibers by tensile tests of fiber bundles is established, too.     

Influence of Shock Pre‐Compression Stress and Tensile Strain Rate on the Spall Behaviour of Mild Steel

Abstract: A series of plate‐impact spall experiments were conducted to investigate the influence of shock pre‐compression stress and tensile strain rates on the dynamic tensile fracture (or spall) behaviour of shocked mild steel. The shock pre‐compression stress amplitude and tensile strain rate were controlled independently to ensure that only one single‐loading parameter varied for each experiment. A push–pull type velocity interferometer system for any reflector (VISAR) was used to measure the free surface velocity profiles of samples. It is observed from experimental results that the influence of shock pre‐compression stress amplitude on the spall strength is less significant in the range attained in these experiments, whereas with increasing tensile strain rate, an evident 65% increase of spall strength is determined in the present tensile strain rate range of 10⁴ to 10⁶ s^?1. VISAR data are compared with finite‐difference calculations employing a modified damage function model with a percolation–relaxation function, and a good agreement between the calculation and the experiments was obtained. Preliminary simulation results also revealed that a critical damage exists, which physically corresponds to the critical intervoid ligament distance for triggering the onset of void coalescence, and may be regarded as a material parameter for describing the dynamic tensile fracture and independent of the loading conditions.     

The influence of martensite,bainite and ferrite on the as-quenched constitutive response of simultaneously quenched and deformed boron steel – Experiments and model

This paper examines the relationship between as-formed microstructure and mechanical properties of a hot stamped boron steel used in automotive structural applications. Boron steel sheet metal blanks were austenized and quenched at cooling rates of 30 °C/s, 15 °C/s and 10 °C/s within a Gleeble thermal–mechanical simulator. For each cooling rate condition, the blanks were simultaneously deformed at temperatures of 600 °C and 800 °C. A strain of approximately 0.20 was imposed in the middle of the blanks, from which miniature tensile specimens were extracted. Depending on the cooling rate and deformation temperature imposed on the specimens, some of the as-quenched microstructures consisted of predominantly martensite and bainite, while others consisted of martensite, bainite and ferrite. Optical and SEM metallographraphic techniques were used to quantify the area fractions of the phases present and quasi-static (0.003 s⁻¹) uniaxial tests were conducted on the miniature tensile specimens. The results revealed that an area fraction of ferrite greater than 6% led to an increased uniform elongation and an increase in n-value without affecting the strength of the material for equivalent hardness levels. This finding resulted in improved energy absorption due to the presence of ferrite and showed that a material with a predominantly bainitic microstructure containing 16% ferrite (with 257 HV) resulted in a 28% increase in energy absorption when compared to a material condition that was fully bainitic with a hardness of 268 HV. Elevated strain rate tension tests were also conducted at 10 s⁻¹ and 80 s⁻¹ and the effect of strain rate on the ultimate tensile strength (σ_UTS) and yield strength (σ_Y) was shown to be moderate for all of the conditions. The true stress versus effective plastic strain (flow stress) curves generated from the tensile tests were used to develop the “Tailored Crash Model II” (TCM II) which is a strain rate sensitive constitutive model that is a function of effective plastic strain, true strain rate and area fraction of martensite, bainite and ferrite. The model was shown to accurately capture the hardening behaviour and strain rate sensitivity of the multiphase material conditions examined.     

Effect of strain rate on quasistatic tensile flow behaviour of solution annealed 304 austenitic stainless steel at room temperature

Room temperature tensile test results of solution annealed 304 stainless steel at strain rates ranging between 5 × 10⁻⁴ and 1 × 10⁻¹ s⁻¹ reveal that with increase in strain rate yield strength increases and tensile strength decreases, both maintaining power–law relationships with strain rate. The decrease in tensile strength with increasing strain rate is attributed to the lesser amount of deformation-induced martensite formation and greater role of thermal softening over work hardening at higher strain rates. Tensile deformation of the steel is found to occur in three stages. The deformation transition strains are found to depend on strain rate in such a manner that Stage-I deformation (planar slip) is favoured at lower strain rate. A continuously decreasing linear function of strain rate sensitivity with true strain has been observed. Reasonably good estimation for the stress exponent relating dislocation velocity and stress has been made. The linear plot of reciprocal of strain rate sensitivity with true strain suggests that after some critical amount of deformation the increased dislocation density in austenite due to the formation of some critical amount of deformation-induced martensite plays important role in carrying out the imposed strain rate.     

Mechanical properties of polypropylene hybrid fiber-reinforced concrete

This paper investigates the mechanical properties of polypropylene hybrid fiber-reinforced concrete. There are two forms of polypropylene fibers including coarse monofilament, and staple fibers. The content of the former is at 3 kg/m³, 6 kg/m³, and 9 kg/m³, and the content of the latter is at 0.6 kg/m³. The experimental results show that the compressive strength, splitting tensile strength, and flexural properties of the polypropylene hybrid fiber-reinforced concrete are better than the properties of single fiber-reinforced concrete. These two forms of fibers work complementarily. The staple fibers have good fineness and dispersion so they can restrain the cracks in primary stage. The monofilament fibers have high elastic modulus and stiffness. When the monofilament fiber content is high enough, it is similar to the function of steel fiber. Therefore, they can take more stress during destruction. In addition, hybrid fibers disperse throughout concrete, and they are bond with mixture well, so the polypropylene hybrid fiber-reinforced concrete can effectively decrease drying shrinkage strain.     

Dynamic fracture of bovine bone

True clinical fracture of bones in bovine, race horses or humans occur predominantly during impact loading (e.g. car accidents, falls or physical violence). Although static fracture tests provide an estimate of fracture toughness or R-curve behavior in bones, the static toughness values may be ill suited for predicting failure under dynamic loading conditions due to the visco-elastic response of bone (i.e. strain rate dependent properties). Despite decades of the study on deformation rate dependency of bone properties such as compression and fracture toughness, high-quality dynamic fracture data remain limited. Preliminary tests (compression and fracture toughness) have been conducted on dry and wet bovine bone under both static and dynamic loading conditions. While compression tests have been conducted with loading direction parallel and perpendicular to the bone axis (longitudinal and transverse, respectively), fracture tests were performed only in the transverse direction. The strain rate in compression tests varied between 10^− 3 and 10³ s^− 1, and the stress intensity rate varied between ∼10^− 3 and 10⁵ MPa√m/s. While low strain rate tests were conducted on conventional mechanical testing machines, high strain rate experiments were conducted on a split-Hopkinson bar under compression and a novel three-point bend configuration. The fracture morphology and the extent of damage of bone in each case were characterized using SEM, and an attempt is made to relate these to the rate dependent fracture toughness of the bone. It is believed that such understanding is crucial for mechanistic interpretation of bone fracture phenomenon and eventually for predicting bone failure reliably.     

An experimental method for dynamic tensile testing of concrete by spalling

A new application of the spalling phenomenon in long specimens is reported in this paper. The new experimental technique is based on an experimental setup which consists of an air launcher of cylindrical projectiles with a Hopkinson bar as a measuring tool and a relatively long concrete specimen in contact with the bar. The incident compression wave transmitted by the Hopkinson bar into the specimen is reflected as a tensile wave causing spalling. Although such configurations have been reported in the past, the main advantage of the present approach lies in the application of the detailed analysis, based on the wave mechanics with dispersion, to extract the specimen behaviour. Such an approach leads to an exact estimation of the local failure stress in tension at high strain rates, even above 100 s⁻¹. This paper demonstrates, using two series of tests on concrete, that this experimental setup can cover one decimal order of strain rates, from ∼10 to ∼120 s⁻¹. The tests performed at high strain rates on wet and dry concrete have indicated that the tensile strength is substantially influenced by the loading rate or strain rate. The absolute value of the failure stress for wet and dry concrete is almost the same for a particular strain rate, which does not occur when subject to low strain rates in tension or compression. A brief discussion is offered on a high rate sensitivity of concrete strength in tension at high strain rates.     

Tensile and torsional shear strength of the bone implant interface of titanium implants in the rabbit

The effect of three different titanium plasma flame spray coatings on the tensile strength and the effect of macrostructures on the torsional shear strength of the bone implant interface was studied. Titanium cylinders, of 8 mm length and 4 mm diameter, were implanted into distal rabbit femurs. For tensile testing, two porous titanium plasma flame spray coatings, Plasmapore^®, fine-grain Plasmapore^®, 1 dense, unporous coating, Plasmapore^® fine on cylinders with axial grooves, and corundum blasted specimens as control group were used. For torsional loading smooth, and macrostructured cylinders with axial grooves, both with Plasmapore^® fine-coating, were used. After 168 days the implant-bone interface was biomechanically tested. A tensile test and a torsional shear test was performed. The results indicated, that the titanium plasma flame spray coatings did not differ in their tensile interface strength, but yielded a stronger interface as sandblasted surfaces and that the macrostructures did not influence the torsional shear strength.