Toughening high density polyethylene submitted to extreme ambient temperatures

The use of polyethylene is limited due to its low impact strength among other mechanical properties at extreme ambient temperatures, for example at ?46 °C and 66 °C. In this work, different polymer components, such as ultra-high molecular weight polyethylene (UHMWPE) and ethylene-vinyl acetate (EVA), were incorporated in high density polyethylene (HDPE) to test their ability to improve toughness of HDPE at extreme ambient temperatures. The polymer blends were processed by extrusion and injection molding and characterized by rotational rheometry, electron microscopy, thermal analysis, tensile, impact and dynamic mechanical tests. The results showed that low concentrations of EVA and UHMWPE in HDPE increased substantially the impact strength of HDPE at room temperature as well as in extreme ambient temperatures (?46 °C and 66 °C). This result indicates that these HDPE blends can be considered good candidates to replace pure HDPE in applications in which high values of toughness are required at extreme ambient temperatures.     

Effects of temperature and strain rate on the tensile behavior of short fiber reinforced polyamide-6

Tensile behavior of extruded short E-glass fiber reinforced polyamide-6 composite sheet has been determined at different temperatures (21.5°C, 50°C, 75°C, 100°C) and different strain rates (0.05/min, 0.5/min, 5/min). Experimental results show that this composite is a strain rate and temperature dependent material. Both elastic modulus and tensile strength of the composite increased with strain rate and decreased with temperature. Experimental results also show that strain rate sensitivity and temperature sensitivity of this composite change at a temperature between 25°C and 50°C as a result of the glass transition of the polyamide-6 matrix. Based on the experimental stress-strain curves, a two-parameter strain rate and temperature dependent constitutive model has been established to describe the tensile behavior of short fiber reinforced polyamide-6 composite. The parameters in this model are a stress exponent n and a stress coefficient σ*. It is shown that the stress exponent n, which controls the strain rate strengthening effect and the strain hardening effect of the composite, is not only strain rate independent but also temperature independent. The stress exponent σ*, on the other hand, varies with both strain rate and temperature.     

Effect of high loading rate and low temperature on mode I fracture toughness of ductile polyurethane adhesive

The mode I fracture toughness of an adhesive at low temperatures under high loading rates are studied experimentally. Typical R-curves of the polyurethane adhesive under different loading rates (0.5?mm/min, 50?mm/min, 500?mm/min) at different temperatures (room temperature, ?20?°C, ?40?°C) respectively are obtained. From the experimental results, the mode I fracture toughness of this adhesive is extremely sensitive to the high loading rates and low temperatures. With the increase of the loading rate and decrease of temperature, the mode I fracture toughness of this adhesive decreases significantly. Under the loading rate of 500?mm/min at ?40?°C, the mode I fracture toughness of adhesive is 15% of the value at room temperature (RT) under quasi-static conditions. Through the experiment, the relationship between mode I fracture toughness of this adhesive, nominal strain rate and temperature is obtained.     

Sintering effect on microstructural evolution and mechanical properties of spark plasma sintered Ti matrix composites reinforced by reduced graphene oxides

Ti matrix composites reinforced with 0.6?wt% reduced graphene oxide (rGO) sheets were fabricated using spark plasma sintering (SPS) technology at different sintering temperatures from 800?°C to 1100?°C. Effects of SPS sintering temperature on microstructural evolution and mechanical properties of rGO/Ti composites were studied. Results showed that with an increase in the sintering temperature, the relative density and densification of the composites were improved. The Ti grains were apparently refined owing to the presence of rGO. The optimum sintering temperature was found to be 1000?°C with a duration of 5?min under a pressure of 45?MPa in vacuum, and the structure of rGO was retained. At the same time, the reaction between Ti matrix and rGO at such high sintering temperatures resulted in uniform distribution of micro/nano TiC particle inside the rGO/Ti composites. The sintered rGO/Ti composites exhibited the best mechanical properties at the sintering temperature of 1000?°C, obtaining the values of micro-hardness, ultimate tensile strength, 0.2% yield strength of 224 HV, 535?MPa and 446?MPa, respectively. These are much higher than the composites sintered at the temperature of 900?°C. The fracture mode of the composites was found to change from a predominate trans-granular mode at low sintering temperatures to a ductile fracture mode with quasi-cleavage at higher temperatures, which is consistent with the theoretical calculations.     

Tensile properties of two-dimensional carbon fiber reinforced silicon carbide composites at temperatures up to 2300 °C

Carbon fiber reinforced silicon carbide (C/SiC) composites are of the few most promising materials for ultra-high-temperature structural applications. However, the existing studies are mainly conducted at room and moderate temperatures. In this work, the tensile properties of a two-dimensional plain-weave C/SiC composite are studied up to 2300 °C in inert atmosphere for the first time. The study shows that C/SiC composite firstly shows linear deformation behavior and then strong nonlinear characteristics at room temperature. The nonlinear deformation behavior rapidly reduces with temperature. The Young’s modulus increases up to 1000 °C and then decreases as temperature increases. The tensile strength increases up to 1000 °C firstly, followed by reduction to 1400 °C, then increases again to 1800 °C, and lastly decreases with increasing temperature. The failure mechanisms being responsible for the mechanical behavior are gained through macro and micro analysis. The results are useful for the applications of C/SiC composites in the thermal structure engineering.     

High‐temperature tensile behavior at different crosshead speeds during loading of glass fiber‐reinforced polymer composites

The present study evaluates on the static tensile behavior of glass fiber reinforced polymer (GFRP) composites at 50% and 70% volume fractions of reinforcement tested at room (25 °C), 70 °C, 90 °C, and 110 °C temperatures with 1, 10, 100, 500, and 1000 mm/min crosshead speeds to investigate the impact of high temperature on the mechanical properties and different dominating failures modes. The experimental results reveal that with increase in crosshead speeds the tensile strength of the composite is increasing. The effect of crosshead speeds and temperature with changing fiber volume fractions affects the GFRP composite. Although both the composite systems are found to be crosshead speed sensitive. Crosshead speed sensitivity seems to be more unpredictable at high temperature and at high crosshead speed. Furthermore, it appears to be more unprecedented nature of fluctuation with high fiber volume fraction. The crucial parameters required during the materials designing in various structural components were evaluated and modelled with the help of Weibull constitutive model. The fractography analyses were done to identify the various dominating failure modes in the GFRP composite. There was no significant change found in the glass transition temperatures (T_g) of both the composite system when exposed to different temperature environments. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44715.     

Materials characterisation and mechanical properties of Cf-UHTC powder composites

Ultra-high temperature ceramic composites based on carbon fibre, Cf, preforms impregnated with hafnium diboride, HfB₂, powder and then densified with carbon by chemical vapour infiltration, CVI, have been mechanically tested to measure the room temperature flexural, interlaminar shear, compressive and tensile strengths. The latter was also measured at 1000 °C. All the composites suffered a degree of delamination during the different mechanical tests but the strength values obtained were at least equal to, or better than, those previously reported in the literature for ultra-high temperature ceramic (UHTC)-based composites. Importantly, in spite of the oxidation of the tensile samples tested at 1000 °C, similar tensile strength values were obtained at both temperatures, suggesting that the materials can resist elevated temperatures. The samples tested at higher temperature did show greater evidence of fibre pull out, possibly due to a weaker fibre-matrix interface as a result of oxidative degradation. The results also suggested that the 0° orientation plies in the Cf preform structure offered greater resistance to mechanical stresses; this suggests that composites can now be designed to offer even greater strength values.     

Curing and constitutive relationships for polyester mortar

The compressive and tensile properties of polyester mortar were studied under various curing conditions, temperature, and strain rate. The curing temperature was varied from room temperature to 80°C. The behavior of polyester mortar was studied using a uniform sand with strain rate and temperature varied between 0.01 to 6 percent strain per minute and 22°C and 120°C, respectively. The strength, failure strain, modulus and stress-strain relationships of polyester mortar are influenced by the curing method, testing temperature, and strain rate to varying degrees. The influence of test variables on the mechanical properties of polyester mortar are quantified. Pretreating the aggregates with a silane coupling agent further enhances the compressive and tensile strength of the mortar. The compressive modulus and splitting tensile strength of polyester mortar are related to the compressive strength. A constitutive model is used to predict the compressive stress-strain behavior of polyester mortar.     

Modified double-notched specimen for ultra-high temperatures shear-strength testing of carbon/carbon composites

In this paper, a modified double-notched specimen (MDNS) is proposed to investigate shear strength of carbon/carbon composites at ultra-high temperatures. The effects of surplus notch length on distribution of shear stress in the gauge area are studied using finite element method. Both standard Iosipescu method and the MDNS method are used to test shear strength of 3D needled, 3D woven, and 4D woven C/C composites at room temperature. The results are in good agreement with each other. Uniform shear strain is observed using digital image correlation (DIC) technology to confirm the proposed method. Additionally, the shear strength of these three types C/C composites are measured at 2000 °C, 2400 °C, and 2800 °C using the designed testing system. The temperature field at 2000 °C is measured using a thermal imaging system to demonstrate uniform distribution of temperature. The failure mechanisms at ultra-high temperatures are also characterized via optical microscopy.     

High modulus semi-crystalline polymers by solid state rolling

Solid state rolling of semi-crystalline polymers is shown to be an effective method of producing high strength, high modulus tape at acceptable production rates. High density polyethylene tape was produced having a tensile strength exceeding 300 MPa and a tensile modulus of 8.7 GPa at production rates exceeding 8 m/min. A significant factor in producing highly oriented tape by the rolling process is roll temperature. Increasing the roll temperature from 25°C to 125°C not only increases the maximum extent of orientation achievable, but increases the mechanical properties at a given degree of thickness reduction. Internal frictional heat development limited the maximum thickness reduction ratio of polypropylene to 6.6:1. This reduction was reached by rolling at 150°C. The resultant tape had a tensile modulus of 5.1 GPa and a tensile strength of 300 MPa.     

Loading rate effects on mechanical properties of polymer composites at ultralow temperatures

E‐glass fibers of 55, 60, and 65 weight percentages were reinforced with epoxy matrix to prepare the laminated composites. They were exposed to ?40, ?60, and ?80°C temperatures for different times. The 3‐piont bend test was conducted on the conditioned samples at those temperatures. Mechanical test was carried out at 2 mm/min and 500 mm/min crosshead speeds. The main emphasis of the investigation was to evaluate the roles of percentage matrix phase and interfacial areas on the interlaminar shear failure mechanism of glass/epoxy composites at ultralow temperatures for different loading speeds. The mechanical performances of the laminated specimens at low temperatures were compared with room temperature property. The loading rate sensitivity of the polymer composites appeared to be inconsistent and contradictory at some points of conditioning time and as well as at a temperature of conditioning. This Phenomenon may be attributed to low‐temperature hardening, matrix cracking, misfit strain due to differential thermal coefficient of the constituent phases, and also to enhanced mechanical keying factor by compressive residual stresses at low temperatures. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2289–2292, 2006     

Structure and properties of iron containing glassy carbons

Glassy carbons containing iron were prepared from copolymers of furfuryl alcohol and ferrocene derivatives at heat-treatment temperatures from 500°C to 2500°C. The copolymerization produced a highly dispersed state of iron in carbonaceous matrices at least in the early stage of pyrolysis. Above 500°C, the homogeneously dispersed iron separated into irregularly spaced domains consisting of cementite, pure iron and iron compounds of unknown composition. Addition of iron resulted in a local graphitization of the glassy carbon at heat-treatment temperatures above 1000°C. At heat-treatment temperatures between 500°C and 800°C, electrical resistivities of the iron-doped carbons were much smaller than those of unmodified polyfurfuryl alcohol carbons but followed more or less the behavior of the latter for heat-treatment temperatures above 800°C.Measurements of mechanical properties indicated a remarkable increase in tensile strength of the low temperature carbons (500°C) with increasing iron content but the strength of the iron containing carbons decreased at higher carbonization temperatures.     

Effect of relative humidity on the mechanical properties of poly(1,4-butylene terephthalate)

Three grades of poly(1,4-butylene terephthalate) (PBT) were aged up to three years at 100, 75, 50, and 11% relative humidity and temperatures of 66–93°C. The decrease in mechanical properties, caused by hydrolysis, occurs rapidly at the higher temperatures and relative humidities and progressively slows as the temperature and/or humidity are decreased. Equations for making life-cycle predictions at any combination of temperature and humidity were derived from Arrhenius plots. If a 50% loss in tensile strength constitutes failure, then the PBT examined should be expected to last only three to four years at 50°C and 100% relative humidity. Reducing the humidity level to 50% extends the useful life at this temperature to 10–20 years. Predictions based on the tensile strength half-life should not be used where toughness or impact properties are important because PBT embrittles long before the tensile strength half-life is reached.     

Mechanical behavior of SiC joints brazed using an active Ag‐Cu‐In‐Ti braze at elevated temperatures

The behavior of SiC ceramic joints brazed with commercially available Incusil ABA (Ag‐32.25Cu‐12.5In‐1.25Ti, in wt.%) was characterized especially with respect to the mechanical performance at temperatures up to 550°C using four‐point bending and torsional shear tests. The failure mechanisms with changing temperature were investigated with the aid of fractography. Additionally, the microstructure of brazed specimens was characterized in detail by high resolution scanning electron microscopy. The test geometry and setup for the high temperature torsional shear test is presented. The change in mechanical behavior of the joints as a function of temperature is shown and discussed. The brazed joints interestingly showed that flexural bending strength was maintained with only a small decrease up to 300°C. Above 300°C, the bending strength decreased much faster. For the first time, this joint system was characterized in torsional shear test at temperatures up to 550°C to achieve the intrinsic shear strength values. Very strong joints were achieved, resulting in failure through the ceramic base materials up to (torsional shear) testing temperatures of 400°C. The results indicate that SiC joints brazed with Incusil ABA exhibit excellent mechanical performance for applications up to 300°C.     

Strength retention of self-reinforced,absorbable polyglycolide rods in hydrolytic environment

Absorbable polyglycolide suture fibers were sintered with the compression molding techniques to cylindrical rods at temperatures between 205°C and 232°C for 3–5 min with final pressures of 50–80 N/mm². The cylindrical rods had nominal diameters between 1.5–4.5 mm and a length of 50 mm. The initial bending moduli and the initial bending strengths of the rods were between 9–15 GPa and 220–430 MPa, respectively. The shear strengths of the rods were between 165–255 MPa. The hydrolytic loss of mechanical strength of the above self-reinforced, absorabable polyglycolide rods were studied in phosphate buffer at 37°C and 77°C. It was found that the rate of strength loss decreases with the increasing diameter of the rods. On the other hand, the rate of strength loss increases when the temperature of the buffer solution is raised. The strength, retention time at 37°C was between 7–10 weeks showing that the loss of mechanical strength of self-reinforced polyglycolide rods occurs more rapidly in vivo than in vitro.     

Mechanical and thermal properties of graphene oxide/phenolic resin composite

In this article, samples of phenolic resin reinforced with graphene oxide were prepared. Mechanical properties of the composite were tested. Dynamic mechanical analysis, Fourier transform infrared spectroscopy, Differential scanning calorimetry, and thermogravimetric analysis were employed to study the mechanical properties in varied temperatures to infer the influence of graphene oxide on thermal performance of phenolic resin. The results show that the addition of graphene oxide resulted in a significant improvement in the elastic modulus of phenolic resin; Young's modulus, and tensile strength increase along with the graphene oxide content. Esterification reaction happened between alcohol  OH groups on resole and carbonyl (sbond]COOH) groups on graphene oxide. Addition of graphene oxide improved residue rate of phenolic resin by 5.8%, whereas the temperature distribution of phenolic pyrolysis stay unchanged. The glass transition temperature of phenolic resin has been improved by 30°C. POLYM. COMPOS. 34:1245–1249, 2013. © 2013 Society of Plastics Engineers     

Microstructural Stability and Mechanical Properties of Directionally Solidified Alumina/YAG Eutectic Monofilaments

Fiber strength retention and creep currently limit the use of polycrystalline oxide fibers in ceramic matrix composites making it necessary to develop single crystal fibers. Two-phase alumina/YAG single crystal structures in the form of monofilaments show that the room temperature tensile strength increases according to the inverse square root of the microstructure size. Therefore, microstructure stability will play a significant role in determining the ‘use temperature’ of these fibers along with its creep resistance. In this work, the effects of temperature on microstructural stability and the creep behavior of directionally solidified alumina/YAG eutectic monofilaments were studied. Microstructural stability experiments were conducted in air from 1200 to 1500°C and creep tests at temperatures of 1400 to 1700°C. Inherent microstructure stability was found to be very good, however, extraneous impurity-induced heterogeneous coarsening was significant above 1400°C. The creep strength of monofilaments with aligned microstructures were superior to ones with low aspect ratio morphologies. Mechanisms for microstructural coarsening and creep behavior are discussed.     

Effect of strain rate and temperature on the performance of epoxy mortar

The behavior of epoxy mortar was studied under various curing conditions, temperature and strain rate. The effect of aggregate size and distribution on the mechanical properties of epoxy mortar was also studied. Epoxy mortar with a uniform fine sand was cured at various temperatures to determine the optimum curing condition. The strain rate was varied between 0.01 to 6 percent strain per minute and the testing temperature between 22°C and 80°C. The strength, modulus, and compressive strain-strain relationship of polymer mortar are influenced by the curing method, testing temperature, and strain rate to varying degrees. The influence of test variables on the mechanical properties of epoxy mortar are quantified. Compared to the uniformly graded fine aggregate fillers the gap-graded aggregates produced polymer mortar with better mechanical properties. The compressive modulus and splitting tensile strength of epoxy mortar are related to their compressive strength. A new nonlinear constitutive model is proposed to predict the complete compressive stress-strain behavior of epoxy mortar. The constitutive relationship parameters are also related to the testing temperature and logarithmic strain rate.     

In‐Situ Tensile Testing of Propellants in SEM: Influence of Temperature

A tensile module system placed within a Scanning Electron Microscope (SEM) was utilized to conduct in‐situ tensile testing of propellant samples. The tensile module system allows for real‐time in‐situ SEM analysis of the samples to determine the failure mechanism of the propellant material under tensile force. The focus of this study was to vary the experimental parameters of the tensile module system and analyze how they affect the failure mechanism of the samples. The experimental parameters varied included strain rate and sample temperature (−54, +25 and +40 °C). Stress‐strain diagrams were recorded during the in‐situ tensile tests, and these results were coupled with the in‐situ images and videos of the samples captured with SEM analysis. The experiments conducted at −54 °C showed a different failure behavior of the propellant sample due to its rigidity at this low temperature, while experiments conducted at +25 and +40 °C displayed a similar failure mechanism. For future testing using this tensile tester, special attention should be given to improved temperature control of the specimen, especially at low temperatures.     

The effects of preparation temperature on the SiCf/SiC 3D4d woven composite

Continuous silicon carbide fiber reinforced silicon carbide matrix (SiC_f/SiC) composites have promising applications in aero-engine due to their unique advantages, such as low density, high modulus and strength, outstanding high temperature resistance and oxidation resistance. As SiC fibers are main reinforcements in SiC_f/SiC composites, the crystallization rate and initial damage degree of SiC fibers are seriously influenced by preparation temperatures of SiC_f/SiC composites, namely mechanical properties of SiC fibers and SiC_f/SiC composites are influenced by preparation temperatures. In this paper, KD-II SiC fibers were woven into 3D4d preforms and SiC matrix was fabricated by PIP process at 1100 °C, 1200 °C, 1400 °C and 1600 °C. Digital image correlation (DIC) method was adopted to measure the uniaxial tensile properties of these SiC_f/SiC composites. In addition, finite element method (FEM) based on representative volume element (RVE) was adopted to predict the mechanical properties of SiC_f/SiC composites. The good agreements between numerical results and experimental results of uniaxial tensile tests verified the validity of the RVE. In last, the transverse tensile, transverse shear, uniaxial shear properties were predicted by this method. The predicted results illustrated that axial tensile, transverse tensile and axial shear properties were greatly influenced by the preparation temperatures of SiC_f/SiC composites while transverse shear properties were not significantly various. And the mechanical properties of SiC_f/SiC composites peaked at 1200 °C among these four temperatures while their values reached their lowest points at 1600 °C because of thermal damage and brittle failure of SiC_f/SiC composites.