Coupled strain rate and temperature effects on the tensile behavior of strain-hardening cement-based composites (SHCC) with PVA fibers

This paper reports the experimental findings on the tensile behavior of Strain-hardening cement-based composites (SHCC) subjected to elevated temperatures and different strain rates and to combinations of these parameters. Uniaxial tension tests with in-situ temperature control were performed at 22 °C, 60 °C, 100 °C and 150 °C. In addition, the effect of loading rate was investigated using the strain rates of 10^? 5 s^? 1, 3 × 10^? 4 s^? 1 and 10^? 2 s^? 1 at all four temperatures considered. It was shown that tensile strength decreases both with an increase in temperature and with a decrease in the strain rate. The strain capacity increases with decreasing strain rate at temperatures of 22 °C and 60 °C, but for the temperature of 100 °C this material property increases when the strain rate increases. At 150 °C the investigated SHCC loses its ductility and no noticeable strain rate effect can be observed. Furthermore, the residual properties of SHCC were evaluated using uniaxial tensile tests at room temperature on the specimens which were previously heated to 60 °C, 100 °C or 150 °C. The residual tests showed that the strength, strain capacity, and work-to-fracture decrease with increasing pre-treatment temperature. However, in comparison with the results of the in-situ tests with elevated in-situ temperatures, the residual tests on SHCC yielded higher tensile strength and lower ductility. These results and possible mechanisms leading to changes in mechanical performance are discussed on the basis of the observed crack patterns on the specimens' surfaces as well as the microscopic investigations of the condition of fibers on fracture surfaces.     

Strain rate-dependent tensile properties and dynamic electromechanical response of carbon nanotube fibers

This investigation into the rate-dependent tensile behavior of carbon nanotube (CNT) fibers provides insight into the role of strain rate and specimen gage length on tensile strength. Chemical vapor produced CNT continuous fibers made of single and dual wall CNTs are evaluated and the potential for fiber improvement by post-process stretching to improve alignment is explored. Post-processed CNT fibers exhibit significantly higher strengths (3–5 GPa) and moduli (80–200 GPa) than untreated fibers. During dynamic tension evaluation, real-time electrical measurements provide correlations between high rate deformation/damage mechanical behavior and electrical resistance of the fiber specimens. Furthermore, this first look into the dynamic tensile behavior of CNT fibers demonstrates their potential to serve as sensors in high rate applications.     

Dynamic fracture of C/SiC composites under high strain-rate loading: microstructures and mechanisms

We investigate dynamic fracture of C/SiC composites under high strain-rate compression or tension with split Hopkinson pressure bar (SHPB) and gas gun loading. Components of the as-fabricated composites are mapped and quantified with X-ray computed tomography, including C fibers and fiber bundles, SiC matrix, and inter- and intrabundle voids. Compression loading is applied along the out-of- and in-plane directions by SHPB at strain rates of 10²–10³ s⁻¹ along with in situ X-ray phase contrast imaging. Out-of-plane direction compression and tension are examined with gas gun impact at strain rates 10⁴–10⁵ s⁻¹. For the out-of-plane loading, compression induces fracture via void collapse and shear damage banding, while delamination dominates fracture for the in-plane direction compression. With increasing strain rates, the compression failure modes transit from interbundle to intrabundle fracture of SiC, and then to fiber and bundle breaking. Tensile failure involves delamination, fiber pullout and fiber breaking. In contrary to normal solids, dynamic tensile or spall strength decreases with increasing impact velocities, owing to compression-induced predamage before subsequent tensile loading.     

The effective interfacial shear strength of carbon nanotube fibers in an epoxy matrix characterized by a microdroplet test

The tensile properties of continuous carbon nanotube (CNT) fibers spun from a CNT carpet consisting of mainly double- and triple-walled tubes, and their interfacial properties in an epoxy matrix, are investigated by single fiber tensile tests and microdroplet tests, respectively. The average CNT fiber strength, modulus and strain to failure are 1.2 ± 0.3 GPa, 43.3 ± 7.4 GPa and 2.7 ± 0.5%, respectively. A detailed study of strength distribution of CNT fiber has been carried out. Statistical analysis shows that the CNT fiber strength is less scattered than those of MWCNTs as well as commercial carbon and glass fibers without surface treatment. The effective CNT fiber/epoxy interfacial shear strength is 14.4 MPa. Unlike traditional fiber-reinforced composites, the interfacial shear sliding occurs along the interface between regions with and without resin infiltration in the CNT fiber. Guidelines for microdroplet experiments are established through probability analysis of variables basic to specimen design.     

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.     

Tailoring engineered cementitious composites for impact resistance

This paper presents results of deliberate tailoring of engineered cementitious composites (ECC) for impact resistance. Microstructure control involving fiber, matrix and fiber/matrix interface was based on steady-state dynamic crack growth analyses accounting for rate dependence of composite phases. Uniaxial tensile stress–strain curves of the resulting impact resistant ECC were experimentally determined for strain rates ranging from 10^? 5 s^? 1 to 10^? 1 s^? 1. Low speed drop weight tower test on ECC panels and beams was also conducted. Damage characteristics, load and energy dissipation capacities, and response to repeated impacts, were studied.     

Microstructure and mechanical behaviour of a Nextel™610/alumina weak matrix composite subjected to tensile and compressive loadings

The present study was aimed at determining the mechanical behaviour of a weak matrix oxide/oxide CMC subjected to tensile and compressive loadings in the fibre direction and at identifying the damage mechanisms. The material consisted of Nextel™610 fibres (8 HSW) embedded in an alumina matrix, with a 49 % fibre volume fraction and 24 ± 2 % total porosity. The average ultimate tensile stress and strain of the material were, respectively, 260 ± 37 MPa and 0.3 ± 0.09 % under tensile loading and −261 ± 69 MPa and −0.19 ± 0.04 % under compressive loading. Three types of pores were differentiated within the material: nanopores (13 ± 1 %), micropores (6 ± 2 %) and macropores (5 ± 1 %). The latter appear to be the most detrimental for the material, enhancing delamination. The damage mechanisms of the material were assessed through SEM examination and in situ tensile tests.     

A new model for the cracking process and tensile ductility of Strain Hardening Cementitious Composites (SHCC)

Strain Hardening Cementitious Composites (SHCC) are materials exhibiting tensile hardening behavior up to several percent strain accompanied by the formation of fine multiple cracks. Their tensile ductility is governed by the spacing and opening of cracks, which depend on the stress transfer between the fibers and the matrix. In this article, a new analytic model which takes into consideration the effects of non-uniform matrix strength, post-cracking increase in fiber bridging stress and fiber rupture on stress transfer and multiple cracking behavior of SHCC is developed. Using material parameters within the range reported in the literature, simulation results can reach reasonable agreement with test data on SHCC for two different fiber contents. The effect of fiber length on tensile behavior of SHCC is then simulated to illustrate the applicability of the model to material design. The new model should be helpful to the micromechanics-based design of SHCC for various ductility requirements.     

Carbon fibers from dry-spinning of acetylated softwood kraft lignin

An acetylated softwood kraft lignin was dry-spun into precursor fibers and successfully processed into carbon fibers with a tensile strength exceeding most values reported in prior studies on lignin-based carbon fibers. Limited acetylation of lignin hydroxyl groups enabled dry-spinning of the precursor using acetone (solvent) followed by thermo-oxidative stabilization. Resulting carbon fibers (∼7 μm diameter) displayed a tensile modulus, strength, and strain-to-failure values of 52 ± 2 GPa, 1.04 ± 0.10 GPa, and 2.0 ± 0.2%, respectively. Because of solvent diffusion during dry-spinning, fibers displayed a crenulated surface that can provide a larger specific interfacial area for enhanced fiber/matrix bonding in composite applications.     

In situ transmission electron microscope tensile testing reveals structure–property relationships in carbon nanofibers

Tensile tests were performed on carbon nanofibers in situ a transmission electron microscope (TEM) using a microelectromechanical system (MEMS) tensile testing device. The carbon nanofibers tested in this study were produced via the electrospinning of polyacrylonitrile (PAN) into fibers, which are subsequently stabilized in an oxygen environment at 270 °C and carbonized in nitrogen at 800 °C. To investigate the relationship between the fiber molecular structure, diameter, and mechanical properties, nanofibers with diameters ranging from ∼100 to 300 nm were mounted onto a MEMS device using nanomanipulation inside the chamber of a Scanning Electron Microscope, and subsequently tested in tension in situ a TEM. The results show the dependence of strength and modulus on diameter, with a maximum modulus of 262 GPa and strength of 7.3 GPa measured for a 108 nm diameter fiber. In particular, through TEM evaluation of the structure of each individual nanofiber immediately prior to testing, we elucidate a dependence of mechanical properties on the molecular orientation of the graphitic structure: the strength and stiffness of the fibers increases with a higher degree of orientation of the 0 0 2 graphitic planes along the fiber axis, which coincides with decreasing fiber diameter.     

Preparation of carbon fibers from linear low density polyethylene

Carbon fibers were produced from linear low density polyethylene (LLDPE) instead of commonly used precursors, such as viscose rayon, mesophase pitch and polyacrylonitrile (PAN). Cross-linked fibers were produced at various temperatures, times and stress conditions during a sulfuric acid treatment using LLDPE fibers obtained from dry-wet spinning. The effects of cross-linking were analyzed using a range of characterization techniques, such as differential scanning calorimetry, color change, fourier transform infrared spectroscopy, elemental analysis, density, scanning electron microscopy, and single filament mechanical properties. The carbonization process of cross-linked fibers was carried out at 950 °C for 5 min in a nitrogen atmosphere. The carbon fibers with the best mechanical properties were obtained from the cross-linked fiber with the highest tensile modulus. In particular, the carbon fibers with the best mechanical properties (tensile strength and tensile modulus of 1.65 GPa and 110 GPa, respectively), similar to commercial-grade carbon fiber, were obtained from the cross-linked fiber that had undergone a carbonization process with a stress of 0.25 MPa after an acid treatment for 150 min at 140 °C and a stress of 0.26 MPa.     

High strength and high modulus carbon fibers

Carbon fibers have been processed from gel spun polyacrylonitrile copolymer on a continuous carbonization line at Georgia Tech (GT) with a tensile strength in the range of 5.5–5.8 GPa, and tensile modulus in the range of 354–375 GPa. This combination of strength and modulus is the highest for any continuous fiber reported to date, and the gel spinning route provides a pathway for further improvements in strength and modulus for mass production of carbon fibers. At short gauge length, fiber tensile strength was as high as 12.1 GPa, which is the highest value ever reported for a PAN based carbon fiber. Structure analysis shows random flaws of about 2 nm size, which results in limiting tensile strength of higher than 20 GPa. Inter-planar turbostratic graphite shear modulus in high strength carbon fibers is 30 GPa, while in graphite the corresponding value is only 4 GPa.     

Electromagnetic characteristics and microstructure stability of Nextel 610 fiber after heat treatment

The thermal and microstructure stability of Nextel 610 fibers has great influence on high-temperature application of Nextel 610 fiber-reinforced ceramic matrix composites. In this work, Nextel 610 fibers were heat treated at 500–700 °C in vacuum and 800–1100 °C in Ar atmosphere, respectively. The sizing agent on Nextel 610 fiber surface could be decomposed into pyrolytic carbon, SiC and gaseous little molecules at lower temperatures, otherwise it was decomposed mainly in the form of gaseous little molecules at higher temperatures, so that the complex permittivity firstly increased and then decreased with the increasing of temperatures. The results showed that the annealed Nextel 610 fiber (T>900 °C) could be regarded as electromagnetic wave transparent fibers, while the tensile strength had declined by half when the temperature increased to 1100 °C. Therefore, Nextel 610 fibers after being annealed at higher temperatures could be further used as reinforcement to prepare high temperature ceramic matrix composites for electromagnetic wave absorption and transparent applications.     

Grafting of epoxy chains onto graphene oxide for epoxy composites with improved mechanical and thermal properties

Epoxy composites filled with both graphene oxide (GO) and diglycidyl ether of bisphenol-A functionalized GO (DGEBA–f–GO) sheets were prepared at different filler loading levels. The correlations between surface modification, morphology, dispersion/exfoliation and interfacial interaction of sheets and the corresponding mechanical and thermal properties of the composites were systematically investigated. The surface functionalization of DGEBA layer was found to effectively improve the compatibility and dispersion of GO sheets in epoxy matrix. The tensile test indicated that the DGEBA–f–GO/epoxy composites showed higher tensile modulus and strength than either the neat epoxy or the GO/epoxy composites. For epoxy composite with 0.25 wt% DGEBA–f–GO, the tensile modulus and strength increased from 3.15 ± 0.11 to 3.56 ± 0.08 GPa (∼13%) and 52.98 ± 5.82 to 92.94 ± 5.03 MPa (∼75%), respectively, compared to the neat epoxy resin. Furthermore, enhanced quasi-static fracture toughness (K_IC) was measured in case of the surface functionalization. The GO and DGEBA–f–GO at 0.25 wt% loading produced ∼26% and ∼41% improvements in K_IC values of epoxy composites, respectively. Fracture surface analysis revealed improved interfacial interaction between DGEBA–f–GO and matrix. Moreover, increased glass transition temperature and thermal stability of the DGEBA–f–GO/epoxy composites were also observed in the dynamic mechanical properties and thermo-gravimetric analysis compared to those of the GO/epoxy composites.     

Mechanical behavior of ferroelastic LaAlO3

The present study investigates the mechanical deformation response of lanthanum aluminate (LaAlO₃) under various loading rates and temperatures. Ferroelastic domains were observed in samples of different porosity. Uniaxial compression tests were performed at room temperature and different loading rates ranging from 0.03 to 5.75 MPa/s. Temperature variation experiments were performed at 93 K, 193 K, 293 K, 393 K, and 553 K. LaAlO₃ shows non-elastic stress-strain behavior in which hysteresis loops are observed during loading–unloading cycles owing to ferroelasticity. The slope of the stress-strain curve became steeper with increasing loading rate and temperature. After unloading, remnant strain was stored in the material owing to ferroelastic domain switching.     

Strength of cylindrical butt joints bonded with epoxy adhesives under combined static or high-rate loading

The influence of loading rates and the combined stress states of tension and shearing on the strength, strain, and absorbed energy of an adhesively bonded joint was experimentally investigated. Cylindrical butt joint specimens were prepared and strength tests were performed on the specimens with a servo-controlled hydraulic testing machine that combined tension and torsion loading. Two types of epoxy adhesives, ductile and brittle, were applied to the specimens. The tests were performed under a quasi-static condition of 6.67×10⁻² MPa/s and a high-rate loading condition of 1.00×10³ MPa/s. The results of the combined loading tests showed that the states of the fractured surfaces were not affected by the loading rates. As for the ratio of tensile and shear loading, adhesive failure tended to partially occur when the ratio of shear loading was very high. The strength points for the specimens bonded with each adhesive were distributed in a stress plane of tension and shearing and could be fitted with a curve that was described by an equation with exponential parameters that were not influenced by the strain rate; however, other parameters that described the intercepts were influenced. The failure strains and absorbed energies for the brittle adhesive were slightly dependent on the strain rate, but this dependency was unclear for the ductile adhesive.     

The use of a carbon nanotube layer on a polyurethane multifilament substrate for monitoring strains as large as 400%

A noninvasive approach is used to fabricate electronically conductive and flexible polymer fibers by fixing carbon nanotube (CNT) networks as a thin layer on thermoplastic polyurethane (TPU) multifilaments. The anchoring of the CNT layer is achieved by partially embedding or penetrating CNTs from the dispersion into the swollen multifilament surface. Thus a stable and high conductivity (up to 10² S/m at 10 wt.% CNT loading) of the resulting CNTs–TPU fibers is realized while the mechanical properties of the TPU multifilament, especially the strain to failure of >1500%, are not affected by increasing the thickness of the CNT layer. Real time analysis of the resistance of the CNTs–TPU fibers during incremental tensile loading tests reveal that the increase of resistance as a function of the strain is attributed to stretching-induced deformation, alignment, and, at high strains, destruction of the conducting network. Moreover, the changes in resistance are highly reversible under cyclic stretching up to a strain deformation of 400%.     

Influence of the carbonization temperature on the mechanical properties of thermoplastic polymer derived C/C-SiC composites

Carbon/Carbon (C/C) composites derived from the thermoplastic polymer polyetherimide (PEI) were pyrolized up to 1000 °C, subsequently carbonized in inert atmosphere up to 2200 °C and afterwards infiltrated with liquid silicon. The investigation of fibers and matrix with Raman microspectroscopy revealed, that an increased carbonization temperature leads to an increased carbon order as well as an incipient stress-induced graphitization of the carbon matrix close to the fiber surfaces at 2200 °C. The derived C/C-SiC samples show a maximum flexural strength of 180 MPa with C/C composites treated at 2000 °C and monotonically increasing Young’s moduli ranging from 49 GPa with C/C preforms treated at 1600 °C up to 59 GPa after carbonization at 2200 °C. The carbon fiber strength was evaluated with a single fiber tensile test, which showed a monotonically increased Young’s modulus and a decrease of the strength after carbonization at 2200 °C.     

Determination of the impact tensile strength of structural adhesive butt joints with a modified split Hopkinson pressure bar

The impact tensile strength of structural adhesive butt joints was determined with a modified split Hopkinson pressure bar using hat-shaped specimens. A typical two-part structural epoxy adhesive (Scotch weld^® DP-460) and two different adherend materials (Al alloy 7075-T6 and commercially pure titanium) were used in the adhesion tests. The impact tensile strength of adhesive butt joints with similar adherends was evaluated from the peak value of the applied tensile stress history. The corresponding static tensile strengths were measured on an Instron testing machine using joint specimens of the same geometry as those used in the impact tests. An axisymmetric finite element analysis was performed to investigate the static elastic stress distributions in the adhesive layer of the joint specimens. The effects of loading rate, adherend material and adhesive thickness on the joint tensile strength were examined. The joint tensile strength was clearly observed to increase with the loading rate up to an order of 10⁶ MPa/s, and decrease gradually with the adhesive thickness up to nearly 180 μm, depending on the adherend materials used. The loading rate dependence of the tensile strength was herein discussed in terms of the dominant failure modes in the joint specimens after static and impact testing.     

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.