Investigating direct tensile behavior of high performance fiber reinforced cementitious composites at high strain rates

This research reported the process of measuring direct tensile stress versus strain response of high performance fiber reinforced cementitious composites (HPFRCCs) at high strain rates between 10 s^− 1 and 40 s^− 1. High rate tensile tests were performed using a strain energy frame impact machine (SEFIM) built by authors. The stress history of HPFRCC at high rates was estimated from two strain gauges attached on two sides of a transmitter bar while the strain history was obtained by analyzing the sequential images recorded using a high speed camera. HPFRCCs exhibited strong rate sensitivity, i.e., their tensile parameters, including post cracking strength, strain capacity, peak toughness, and number of cracks, were significantly enhanced as the strain rate increased although the enhancement was different according to the types of fiber. The source of the dynamic enhancements in the tensile parameters of HPFRCCs was discussed.     

Direct tensile behavior of ultra high performance fiber reinforced concrete (UHP-FRC) at high strain rates

This experimental study investigates the direct tensile behavior of ultra-high performance fiber reinforced concrete (UHP-FRC) at strain rates ranging from 90 to 146/s. The tests are conducted using a recently developed impact testing system that uses suddenly released strain energy to generate an impact pulse. Three fiber types were considered, a twisted fiber and two other types of straight fibers. Specimen impact response was evaluated in terms of first cracking strength, post-cracking strength, energy absorption capacity and strain capacity. The test results indicate that specimens with twisted fibers generally exhibit somewhat better mechanical properties than specimens with straight fibers for the range of strain rates considered. All UHP-FRC series tested showed exceptional rate sensitivities in energy absorption capacity, generally becoming much more energy dissipative under increasing strain rates. This characteristic highlights the potential of UHP-FRC as a promising cement based material for impact- and blast-resistant applications.     

Characterization and modeling of tensile properties of continuous fiber reinforced C/C-SiC composite at high temperatures

In order to study the effects of temperature on the material behavior of Liquid Silicon Infiltration (LSI) based continuous carbon fiber reinforced silicon carbide (C/C-SiC), the mechanical properties at room temperature (RT) in in-plane and out-of-plane directions are summarized and the tensile properties of C/C-SiC were then determined at high temperature (HT) 1200 °C and 1400 °C under quasi static and compliance loading. The stress-strain response of both HT tests is similar and almost no permanent strain can be observed compared to the RT, which can be explained through the relaxation of residual thermal stresses and the crack distribution under various states. The different fracture mechanisms are confirmed by the analysis of fracture surface. Furthermore, based on the analysis of hysteresis measurements at RT, a modeling approach for the prediction of material behavior at HT has been developed and a good agreement between test and modeling results can be observed.     

The hierarchical structure and flexure behavior of woven carbon fiber epoxy composite

The hierarchical structure and flexure behavior of woven carbon fibers epoxy composite were investigated in this work. First, the hierarchical structure of the composite is characterized on three levels: composite, ply, and yarn. Structura imperfections,

1 In this paper, “structural imperfections” is used to describe the inherent structural characteristics of the actual composite, which deviate from the theoretically ideal and perfect composites, control, that are used in composite theories.

such as, ply‐ply misalignment, ply‐ply offset, and resin pockets, are identified and described. Second, a four‐point bending arrangement is used to study the flexure properties of the composite. Additionally, in‐situ traveling microscope and acoustic emission (AE) techniques are utilized to gain insight to the failure proceses during flexure test. AE showed early stages of matrix cracking before visual observation, which makes it a valuable tool for early failure detection.     

Study of SiC fiber axial compressive behavior using tensile recoil measurement

SiC fibers have been widely investigated as reinforcements for advanced ceramic matrix composites owing to their excellent high-temperature properties. However, the axial compressive strength of SiC fibers has not been thoroughly studied. In this study, the compressive behavior of two SiC fiber types containing different compositions and thermal degradation were characterized by tensile recoil measurements. Results illustrated that the SiC fiber compressive strength was 30%–50% of its tensile strength, after heat treatment at 1200℃–1800℃ for 0.5 h in argon. The fiber compressive failure mechanism was studied, and a “shear-bending-cleavage” model was proposed for the recoil compression fracture of pristine SiC fibers. The average compressive and tensile strengths of the pristine SiC-II fiber were 1.37 and 3.08 GPa, respectively. After treatment at 1800℃ for 0.5 h in argon, the SiC-II fiber compressive strength decreased to 0.42 GPa, whereas the tensile strength reduced to 1.47 GPa. The mechanical properties of the fibers degraded after high-temperature treatment. This could be attributed to SiC grain coarsening and SiC_xO_y phase decomposition.     

Crystallization of Nylon 11 under compressive high strain rates

The mechanical behavior of semicrystalline Nylon 11 was studied at strain rates between 10^?3 and 8800 s^?1. X‐ray diffraction and DSC were employed to examine the crystal structure and the crystallinity content. The as‐received material comprised a mixed structure of a predominately triclinic (α) form. DSC revealed that the material gave rise to two melting peaks. The compressive flow stress of Nylon 11 experienced a large increase at 1200 s^?1 and decreased at higher strain rates. The maximum level of the flow stress corresponded with a higher level of crystallinity and a structure mainly of a pseudohexagonal form. The subsequent drop in stress at higher rates was associated with a decrease in the crystallinity content and a mixed crystal structure, different from that observed in the as‐received material. After compression, the low melting peak disappeared and the material melted over an increased temperature range. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 2031–2038, 2001     

The deformation behavior of polypropylene film under high strain rates

The relationship between stress and strain for polypropylene film was studied under strain rates from 0.13 to 5.21 s^?1 in order to study the deformation behavior of film under higher strain rates than previous studies. Uniform thickness was obtained in the strain rates from 2.08 to 5.21 s^?1 at 435 K, or from 2.08 to 3.13 s^?1 at 437 K. The temperature rise of film due to the generation of heat from plastic strain influenced the relationship between stress and strain, in particular, at high strain rates and low temperature. Material constants for the constitutive equation of film were determined using the measurements from 2.08 to 5.21 s^?1 at 435 K and from 2.08 to 3.13 s^?1 at 437 K. Film thicknesses during and after transverse direction stretching were successfully predicted by applying the material constants obtained. The authors concluded that the material constants should be determined by applying the stretching conditions, under which there is little or no effect from heat generation and under which film can be stretched uniformly in thickness. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

The tensile behavior of carbon fibers at high temperatures up to 2400 °C

The tensile behavior of four different brands of carbon fibers (a rayon-based, a PAN-based, and 2 pitch-based fibers) has been investigated at various temperatures up to 2400 °C. The tests were carried out using an original fiber testing apparatus. Various mechanical properties including strength and Young's modulus, as well as Weibull statistical parameters were extracted from test data. Typical tensile behaviors were evidenced such as an essentially linear elastic behavior at room temperature and intermediate temperatures up to 1400-1800 °C, then a nonlinear elastic delayed response at higher temperatures and ultimately an inelastic response with permanent deformations at very high temperatures. Such unusual nonlinear responses for homogeneous materials were related to structure and texture features at the nanometer scale, that were described through an X-ray diffraction technique.     

Compressive behavior of biaxial spacer weft knitted fabric reinforced composite at various strain rates

The in‐plane and out‐of‐plane compressive properties of biaxial weft knitted E‐glass fabric reinforced vinyl ester composite at quasi‐static strain rate of 0.001/s and high strain rates from 700/s to 2200/s were tested to investigate the strain rate effect on the compressive behavior. The compressive tests were conducted on split Hopkinson pressure bar at high strain rate and on MTS 810.23 system at quasi‐static state. The experimental results indicated the strain rate sensitivity of compressive stiffness, failure stress, and strain of the composite in both out‐of‐plane and in‐plane compressive direction. The compressive stiffness and failure stress linearly increased with the increase of strain rate. The failure strain linearly decreased with the increase of strain rate. As the strain rate increased, the main failure mode at out‐of‐plane compression is the interlaminar shear failure and at in‐plane direction is the delamination. At the high strain rate of 2200/s, the composite coupon was compressed into debris with the shear or delamination failure. POLYM. COMPOS., 28:224–232, 2007. © 2007 Society of Plastics Engineers     

Laminated connections under tensile load at different temperatures and strain rates

In the last years, a novel typology of adhesive connections for structural glass application has emerged, known as laminated adhesive connections, which makes use of the transparent ionomer SentryGlas® (SG) from Kuraray and the Transparent Structural Silicon Adhesive (TSSA) from Dow Corning. Despite being used in several projects, limited information is available in literature on their mechanical behaviour and on the effects of strain rate and temperature. In this work the behaviour of laminated connections under tensile loading is studied by means of experimental, analytical and numerical analyses. The experimental investigations show that temperature and strain rate variations have important effects on the mechanical response of the connections. Two main interesting phenomena are also observed: the whitening phenomenon in TSSA and the development of bubble within the SG adhesive. The analytical studies of the stress state show that confinement state of the adhesive induces a non-uniform three-dimensional stress distribution in the adhesive with a dominant hydrostatic component of the stress tensor, which is observed to be in agreement with the experimental results. Three-dimensional finite numerical analyses show that the stress field deviates from the uniform distribution with a large gradient of hydrostatic and deviatoric stresses over the adhesive area. The output of the finite numerical model are then compared with the observations of the experimental campaigns. Herein, the full set of numerical results is synthetized by the definition of so-called stress factors. The latter allow to derive the three-dimensional stress state in the adhesive at different temperatures and to compute the stress peak in the non-linear stress field distribution. Finally, prediction models are proposed for the tensile resistance of TSSA and SG laminated connections. A logarithmic law is proposed for the strain rate effects for both TSSA and SG connections. Linear and inverse hyperbolic-tangent-based laws are instead proposed for the TSSA and SG temperature effects, respectively.     

A new test method to obtain biaxial tensile behaviors of solid propellant at high strain rates

A new test method was proposed and applied for studying the biaxial tensile behaviors of hydroxyl-terminated polybutadiene (HTPB) propellant at high strain rates. The biaxial tensile stress responses of the propellant at room temperature and at different strain rates (0.40–85.71 s^?1) were obtained through the use of biaxial tensile strip samples, a new designed aluminum apparatus and a uniaxial Instron testing machine. A high-speed camera and scanning electron microscop (SEM) were employed to observe the biaxial tensile deformation and the damage of HTPB propellant under the test conditions. The results indicated that strain rate could remarkably influence the biaxial tensile behaviors of HTPB propellant. The effect of strain rate on the characteristics of stress–strain curves, mechanical properties and fracture mechanisms was consistent with that in uniaxial tension. However, the biaxial weakening of HTPB propellant was obvious. The strain at biaxial maximum tensile stress was between 10 and 30 % lower than that at the corresponding uniaxial case. Finally, the correlations between the fracture mechanisms and the mechanical properties of HTPB propellant, stress state and the damage of HTPB propellant were discussed. The damage of the propellant under the biaxial tensile test was less serious than that under uniaxial tension at the same strain rate. In addition, continuously increasing strain rate could change the fracture mechanism of the propellant under the biaxial and uniaxial tensile tests. In this investigation, the dominating fracture mechanism of HTPB propellant changed from the dewetting and matrix tearing at lower strain rate to the particles fracture at higher strain rate.     

Compressive behavior of C/SiC composites over a wide range of strain rates and temperatures

The mechanical behavior of two-dimensional (2D) carbon fiber reinforced silicon carbide (C/SiC) composites is investigated at both quasi-static and dynamic uniaxial compression under temperatures ranging from 293 to 1273 K. Experimental results show that temperature and strain rate dramatically affect the compressive behavior of 2D C/SiC composites. If the temperature is below 873 K, the compressive strength increases with rising temperature. The reason is that the release of thermal residual stress enhances the compressive strength and this enhancement is more significant than the strength degradation due to the high temperature induced oxidation. In contrast, when the temperature rises above 873 K, the compressive strength decreases as temperature rises due to the stronger effect of oxidation induced strength degradation. Moreover, the degradation of compressive strength at strain rate of 10⁻⁴/s and temperatures above 873 K is much more obvious than those at higher strain rates, and the strain rate sensitivity factor of compressive strength increases remarkably at temperature above 873 K. Post-deformation observation shows that failure angles and fracture surfaces are also strongly dependent on testing temperature and strain rate. The change of interfacial strength at high strain rate or high temperature is responsible for the variations.     

Stress-strain behavior of a polyurea and a polyurethane from low to high strain rates

The large deformation stress-strain behavior of thermoplastic-elastomeric polyurethanes and elastomeric-thermoset polyureas is strongly dependent on strain rate. Their mechanical behavior at very high strain rates is of particular interest due to their role as a protective coating on structures to enhance structural survivability during high rate loading events. Here we report on the uniaxial compression stress-strain behavior of a representative polyurea and a representative polyurethane over a wide range in strain rates, from 0.001 s⁻¹ to 10,000 s⁻¹, successively marching through each order of magnitude in strain rate using equipment relevant for testing at each particular rate. These results are further analyzed in association with recently reported compressive data on the same materials by Yi et al. [Polymer 2006;47(1):319-29] and intermediate rate tensile data on the same polyurea by Roland et al. [Polymer 2007;48(2):574-8]. The polyurea tested is seen to undergo transition from a rubbery-regime behavior at low rates to a leathery-regime behavior at the highest rates, consistent with the earlier compression study as well as the recent tension study; the polyurethane tested is observed to undergo transition from a rubbery-regime behavior at the low rates to a glassy behavior at the highest rates. The uniaxial compression data for the polyurea are found to be fully consistent with the recently reported uniaxial tension data over the range of rates studied, demonstrating the consistency and complementary aspects of testing at high rates in both compression and tension.     

Effects of cyclic tensile loading on the rupture behavior of orthogonal 3-D woven SiC fiber/SiC matrix composites at elevated temperatures in air

This study examined the rupture mechanisms of an orthogonal 3D woven SiC fiber/BN interface/SiC matrix composite under combination of constant and cyclic tensile loading at elevated temperature in air. Monotonic tensile testing, constant tensile load testing, and tension–tension fatigue testing were conducted at 1100 °C. A rectangular waveform was used for fatigue testing to assess effects of unloading on the damage and failure behavior. Microscopic observation and single-fiber push-out tests were conducted to reveal the rupture mechanisms. Results show that both oxidative matrix crack propagation attributable to oxidation of the fiber–matrix interface and the decrease in the interfacial shear stress (IFSS) at the fiber–matrix interface significantly affect the lifetime of the SiC/SiC composites. A rupture strength degradation model was proposed using the combination of the oxidative matrix crack growth model and the IFSS degradation model. The prediction roughly agreed with the experimentally obtained results.     

High strain rate compressive response of the Cf/SiC composite

Carbon fiber reinforced ceramic owns the properties of lightweight, high fracture toughness, excellent shock resistance, and thus overcomes ceramic's brittleness. The researches on the advanced structure of astronautics, marine have exclusively evaluated the quasi-static mechanical response of carbon fiber reinforced ceramics, while few investigations are available in the open literature regarding elastodynamics. This paper reports the dynamic compressive responses of a carbon fiber reinforced silicon carbide (C_f/SiC) composite (CFCMC) tested by the material test system 801 machine (MTS) and the split Hopkinson pressure bar (SHPB). These tests were to determine the rate dependent compression response and high strain rate failure mechanism of the C_f/SiC composite in in-plane and out-plane directions. The in-plane compressive strain rates are from 0.001 to 2200?s^?1, and that of the out-plane direction are from 0.001 to 2400?s^?1. The compressive stress-strain curves show the C_f/SiC composite has a property of strain rate sensitivity in both directions while under high strain rate loadings. Its compressive stiffness, compressive stress, and corresponding strain are also strain rate sensitive. The compressive damage morphologies after high strain rate impacting show different failure modes for each loading direction. This study provides knowledge about elastodynamics of fiber-reinforced ceramics and extends their design criterion with a reliable evaluation while applying in the scenario of loading high strain rate.     

Analysis of the tensile stress-strain behavior of elastomers at constant strain rates. I. Criteria for separability of the time and strain effects

Considering the case where the relaxation time spectrum is preserved at finite deformations, a theoretical analysis of the tensile stress-strain relation of elastomers at constant strain rates has been carried out. The finite strain effect is taken into account by replacing the Cauchy strain by a general strain function, ?(?), in the Boltzmann superposition integral. The analysis shows that there are two cases where the time and strain effects are separable when: (1) the segment of the stress relaxation modulus which coincides with the experimental time of stretching can be represented by a single power law; and (2) the general strain function, ?(?), is linearly proportional to the Cauchy strain. Separability of the time and strain effects, therefore, can be achieved by adjusting the stretching time (or strain) and temperature, if the relaxation time spectrum remains unchanged by the deformation. The tensile stress-strain relations derived from the theoretical analysis were applied to analyze data on a crosslinked styrene butadiene rubber obtained in the temperature range ?40 to 60°C. Γ(?), which describes the strain dependence of tensile stress, B_?, the ratio of isochronal stresses at different strains, and a_i, slope of a segment of the relaxation modulus E_i(t) on log t plot, were obtained directly from the experiment. Values of Γ(?), B_? and a_i obtained at ?40°C are quite different from those obtained at ?30°C or higher. Results obtained from our analysis are generally in agreement with those obtained by an empirical method for analyzing the experimental data.     

Stress-strain behaviour of concrete and mortar at high rates of tensile loading

The Split-Hopkinson-Bar technique was used in the investigation on tensile stress-strain behaviour of concrete and mortar at high stress rates (5–30 N/mm²ms).The single loading tests showed that the impact tensile strength was higher than the static one, and that impact strains at the maximum stress were larger than static strains.The impact fatique tensile tests indicated an increase of strains in the course of fatigue loading and increasing fatique life with decreasing maximum stress level.These phenomena are discussed with the aid of fracture mechanics concepts and explanations for differences in the behaviour of concrete and mortar are suggested.     

Model for tensile fracture of concrete at high rates of loading

Mechanisms of tensile fracture of concrete are described. A model is developed for an idealized material. The amount of simultaneous cracking and the path of each crack depend on the rate of stressing. The fracture energy and the tensile strength have been determined as functions of the rate of loading. The results of earlier experiments on concrete under impact tensile loading can be explained by this model.     

Fatigue and fracture behavior in notched specimens of C/C composite with fine-woven carbon fiber laminates

Fatigue and fracture behavior of C/C composite with fine-woven carbon fiber laminates was investigated in several schemes of notched specimens. Slits and holes were cut in the specimens as notches, and the effect of fiber orientation and notch configuration on the fatigue behavior was examined. The fatigue limit was defined by a simple method that used incremental applied stress and number of cycles. The fatigue limit was dependent on fiber orientation, notch configuration and stress ratio (ratio of minimum cyclic stress to maximum cyclic stress). It was found that the fatigue limit was determined by the true stress rather than by the stress concentration in the present material and was not affected by variations in slit depth. Also, the fracture behavior was controlled by shear deformation.     

The preparation of high performance silk fiber/fibroin composite

An all-silk composite, in which uniaxially-aligned and continuous-typed Bombyx mori silk fibers were embedded in a matrix of silk protein (fibroin), was successfully prepared via a solution casting process. The structure, morphology, mechanical and thermal properties of such silk fiber/fibroin composites were investigated with X-ray diffraction, scanning electron microscopy, tensile and compression tests, dynamic mechanical analysis and thermogravimetric analysis. The results demonstrated that the interface adhesion between silk fiber and the fibroin matrix was enhanced by controlling the fiber dissolution through 6 mol L⁻¹ LiBr aqueous solution. Compare to those of the pure fibroin counterparts, the overall mechanical properties as well as the thermal stability of such silk fiber/fibroin composites were significantly improved. For example, the composite with 25 wt% fibers showed a breaking stress of 151 MPa and a breaking elongation of 27.1% in the direction parallel to the fiber array, and a compression modulus of 1.1 GPa in the perpendicular direction. The pure fibroin matrix (film), on the other hand, typically had a breaking stress of 60 MPa, a breaking elongation of 2.1% and a compression modulus of 0.5 GPa, respectively. This work suggests that such a controllable technique may help in the preparation of animal silk based materials with promising properties for various applications.