Creep Of Composites Research Articles

Mechanical Properties and Creep Behavior of Carbon Fiber Nano-Composites

Recently, it has been observed that surface modification of carbon nanotubes（CNTs）influences on CNT’s distribution among epoxy resin and affects the mechanical properties and electrical conductivities of CNTs. Owing to above-mentioned effects, carbon nanotubes treated with oxidizing in organic acids, a kind of surface modification, generates functional groups on the surface of CNTs taht is a major investigation in this study to enhance mechanical properties and electrical conductivities of CNTs. The influence of the different proportion contents of CNTs added into epoxy resin on mechanical properties and electrical conductivities of composites was investigated, and strength of material tested under different temperature environments was observed. Moreover, the creep behavior of carbon fiber（CF）/epoxy resin thermosetting composites tested under different temperature and stress were also concerned to be analyzed. The resulting creep behavior consists of only two stages, including primary creep and steady-state creep. The effects of creep stress, creep time, different humidity treatment on the various temperature creep of composites containing different proportion contents of CNTs were investigated. It is believed that the increased creep strains can be attributed to higher applied stresses, longer creep times, higher humidity, higher temperature and lower hardnesses. The test results also exhibit that mechanical strength and electrical conductivity increase with the increase of CNTs content added into composites. In the influence of temperature effect on specimen, because of different coefficient of expansion among matrix, fiber and CNTs, the overexpansion of matrix caused by high temperature results in crack occurred among matrix. From observation of the fracture surface by SEM image, the debonding occurs and longitudinal fibers are pulled out due to poor interfacial bonding of fiber and matrix, which also results in entire strength degeneration.

Structural characterization and high-temperature compressive creep of PTFE-based composites filled with inorganic nanoparticles

The PTFE-based nanocomposites with various contents of inorganic nanoparticles (n-AlN and n-Si3N4) were prepared by cold compaction followed by free sintering. The results of scanning electron microscopy, differential scanning calorimetry, and X-ray diffraction show that PTFE spherulite formed in the nanocomposites. When 2 wt% inorganic nanoparticles were added into the PTFE matrix, the crystallinity increased from 34.3% to 42.1% and 43.2%, respectively. Moreover, the interplanar distances for each crystal plane were enlarged and the grain sizes were smaller than that of pure PTFE. In addition, the mechanical and high-temperature compressive creep properties were investigated. The results indicate that the introduction of inorganic nanoparticles largely increased the high-temperature compressive creep resistance, and the maximal reduction of percentage of creep strain was up to 68%. The tensile strengths of the nanocomposites increased with increasing filler content when it was no more than 2%, and then decreased with the further increase of the filler content, whereas the elongations at break showed a reverse tendency. Copyright © 2011 John Wiley & Sons, Ltd.

Mechanical and tribological behaviors of UHMWPE composites filled with basalt fibers

In this article, the basalt fibers were used to fill the UHMWPE because of their outstanding mechanical and tribological performances. The UHMWPE composites with the filler content of 5—20 wt.% were prepared. Then, the mechanical and tribological performances of the composites were tested. Also, the wear mechanism was analyzed. The results indicated that the UHMWPE composites’ impact toughness was decreased when filled with the basalt fibers, but the strength, hardness, and anti-indentation creep of the UHMWPE composites were improved. Under the same experimental conditions, the friction coefficient of the UHMWPE composites filled with basalt fibers was decreased to some extent, and the wear loss of the composites was significantly reduced. The analysis showed that the wear type of the UHMWPE composites was changed when filled with the basalt fibers. So, the wear-resistant performance of the UHMWPE composites was effectively enhanced.

Parametric study of the effect of microstructure on creep of plain weave composites

While the effect of microstructure on elastic properties and failure behavior of woven composites has been studied before, its effect on creep of woven composites has not been studied. Hence, a parametric study of the effect of microstructure on tensile creep of plain weave composites was pursued. The microstructure of F263/T300 plain weave composites was characterized experimentally and the range of values for various parameters that define the microstructure of the plain weave composite was obtained. Using these values, experimental creep data for unidirectional lamina, and a modified equivalent laminate model (MELM), tensile creep of woven composite, under on-axis and off-axis loading, was predicted and evaluated. For the range of crimp angle (15.2–19.5°) encountered in this composite, an increase in crimp angle increased both the instantaneous compliance and the magnitude of creep. The effect of crimp angle on creep was more under on-axis than under off-axis loading.

Study on the Mechanical Properties and Creep Behaviour of Carbon Fiber Nano-Composites

ABSTRACTThe surface modification of carbon nanotubes（CNTs）has been recently observed to influence the distribution of CNTs in epoxy resin and the mechanical properties and electrical conductivities of these CNTs. Accordingly, the treatment of CNTs with organic acids to oxidize them generates functional groups on the surface of CNTs. This investigation studies the consequent enhancement of the mechanical properties and electrical conductivities of CNTs. The influence of adding various proportions of CNTs to the epoxy resin on the mechanical properties and electrical conductivities of the composites thus formed is investigated, and the strength of the material is tested at different temperatures. Moreover, the creep behavior of carbon fiber （CF）/epoxy resin thermosetting composites was tested and analyzed at different temperatures and stresses. The creep exhibits only two stages- primary creep and steady-state creep. The effects of creep stress, creep time, and humidity on the creep of composites that contain various proportion of CNTs were investigated at various temperatures.Creep strain is believed to increase with applied stress, creep time, humidity, and temperature. It also increases as hardness decrease. The test results also indicate that mechanical strength and electrical conductivity increase with the amount of CNTs added to the composites. Different coefficients of expansion of the matrix, fiber and CNTs, are such that overexpansion of the matrix at high temperature results in cracking in it. An SEM image of the fracture surface reveals debonding and the pulling out of longitudinal fibers because of poor interfacial bonding between fiber and matrix, which reduce overall strength.

Effect of Molecular Weight, Particle Size and Ringer’s Solution on Mechanical Properties of Surface-treated Polypropylene-Hydroxyapatite Biocomposites

Bovine bone is burned at 400°C and the resulting ash is heat treated at 900°C for 90 min in order to obtain a white powder. Chemical analysis shows that this white powder is mainly hydroxyapatite (HA). Three different average particle sizes of this powder are produced by milling at three different times. The produced HA is surface treated with a silane coupling agent. Composite samples with 40% (by volume) of HA are produced by hot press molding, followed by mechanical testing in air and in Ringer’s solution. The results show that by increasing the molecular weight of polypropylene (PP), the creep strain of composites decreases proportionally, and the ultimate tensile strength (UTS) and impact resistance increase dramatically. In addition, the effect of testing in Ringer’s solution is to decrease UTS, creep, and impact properties of PP-HA composites. Furthermore, by decreasing the HA particle size, the creep strain decreases, while the UTS and impact resistance increase for the composites tested in air. Vice versa, in a wet condition decreasing of HA particle size causes UTS, creep, and impact resistance to fall. Fracture surfaces of the samples after impact tests are analyzed by a scanning electron microscope.

Creep of plain weave polymer matrix composites under on-axis and off-axis loading

Despite the increasing use of polymer woven composites in load-bearing structural applications, published research on their long-term durability is very limited. Hence, creep of plain weave polymer woven composites was investigated experimentally and analytically, using Hexcel Corporation’s F263/T300 2-harness (HS) composite material under on-axis (0°) and off-axis (45°) loading. Constant load creep experiments were conducted over a wide range of temperatures (80–240 °C) and stresses (1–70% of the ultimate tensile strength). Time–temperature-superposition-principle (TTSP) was used to obtain master creep curves at each stress level for a time period beyond the experimental time window. A modified equivalent laminate model (MELM) was developed to predict the creep compliance of plain weave composites for any load orientation with respect to fiber axis, using experimental creep compliance of unidirectional polymer composites. The model predictions were found to be in good agreement with the experimental results, within as well as beyond the experimental time window.

Flexural creep of long fiber-reinforced thermoplastic composites: Effect of processing-dependent fiber variables on creep response

Flexural creep properties were studied as a function of fiber weight fraction and processing-induced fiber alignment in extrusion/compression-molded, long fiber-reinforced thermoplastic (LFT) nylon 6/6, polypropylene, and high-density polyethylene and their 10 wt.% and 40 wt.% E-glass fiber reinforced LFT composites. The residual fiber lengths and probability distribution parameters were near-equal, regardless of the initial fiber length and processing. Creep compliances decreased with increasing fiber weight fraction, and clear influence of fiber alignment was found in model parameters. Processing-induced fiber alignment imaged using X-ray radiography, was correlated with the creep compliances of strategically sectioned specimens, and tested as per ASTM D-2990. Longitudinal fibers aided in lowering the creep compliance, and the range in compliance decreased with lower preferential fiber alignment. Creep compliances from flexural creep tests and dynamic mechanical analysis/static creep tests were combined using time–temperature–stress superposition (TTSSP) to construct long-term master curves that correlated closely with long-term tests.

A mean-field micromechanical study on thermal-cycling creep of short fiber-reinforced metal matrix composites: Al–Al3Ni eutectic composites

In this study, a unified mean-field micromechanical model was applied to analysis of thermal-cycling creep of short fiber-reinforced metal matrix composites subject to a wide range of external loads from compression to tension. The model considered the rate process of the local mass transfer by diffusion along the fiber/matrix interface. The thermal-cycling creep of the composites can be divided into three groups observed in high compressive stress regime, low compressive and tensile stress regime, and high tensile stress regime based on the micromechanical examination of the deformation mechanism. Numerical and experimental studies were systematically conducted with directionally solidified Al–Al3Ni eutectic composites. These studies demonstrated that even though compressive loads were applied, the elongate creep deformation occurred under given thermal-cycling conditions, and thermal-cycling creep was largely affected by the length of the fiber and temperature profile.

High Temperature Properties of Ti<sub>3</sub>SiC<sub>2</sub>-SiC Composites

Fully dense Ti3SiC2-64vol.%SiC(0.5m) composite materials are successfully fabricated by in situ synthesis under hot isostatic pressing. The high temperature flexure, tension and creep of Ti3SiC2-SiC composites were studied, and the results indicated that the composite had a high tensile strength under high temperature. Above 1000°C the composite were displayed a good high temperature plastic. The creep behavior is characterized by two regimes: an initial transient regime, where έ(dε/dt) decreases with t (time); a secondary creep regime in which έ is more or less constant with time.

Tensile creep and rupture of 2D-woven SiC/SiC composites for high temperature applications

The tensile creep and rupture behavior of 2D-woven SiC fiber-reinforced SiC matrix composites with potential for advanced high temperature structural applications was determined in air at 1315 °C. The results are compared to similar SiC/SiC data in the literature in order to understand the underlying creep and rupture mechanisms. Focus was placed on three different near-stoichiometric SiC fiber-types and three SiC-based matrix systems produced by different process routes. In general, the creep and rupture properties of the tested composites were primarily dictated by the creep resistance of the fiber-type, with the Sylramic-iBN fiber typically showing the best behavior. However, the type of matrix did have an effect on the composite creep and rupture lives due to load-sharing differences for the different matrix types and due to stoichiometry in the case of chemical vapor infiltration SiC matrices.

Prediction of Long-term Creep of Composites from Doubly-shifted Polymer Creep Data

Environmental effects on the creep response of composites are of great interest because these materials are very sensitive to temperature and other environmental conditions. Short and long-term creep effects have been studied for composites under various environmental conditions, but most studies rely on experimental testing of particular laminates. Here, a novel micro-mechanical/laminate model is developed to predict the creep response of laminated polymer matrix composites from matrix creep data. Furthermore, a novel two-parameter superposition method is used to predict the long-term response of the polymer matrix as a function of two environmental parameters: moisture and temperature. The procedure used to compute the two shift factors is described. The resulting master curve can be used to predict the long-term properties of the matrix and for any temperature and humidity conditions. Once the matrix behavior is known, the micro-mechanical/ laminate model allows for accurate prediction of long-term properties of arbitrary laminates for the same conditions.

Thresholds in creep of magnesium alloy AX41 composites reinforced with short fibers

Abstract Creep behavior of four magnesium Mg-4Al-1Ca alloy-based composites reinforced with short alumina or carbon fibers was investigated at temperatures 423, 473, 523, and 623 K. The threshold creep behavior was observed in all composites as well as in the matrix alloy AX41. Both types of fibers strongly increase creep resistance of the composites. A stronger strengthening effect was observed in composites with alumina fibers. The applied stress dependences of the minimum creep rate of all investigated materials were well described by Garofalo' sinh relationship modified for threshold concept. Threshold stresses and their temperature dependences were evaluated from the stress dependences and discussed in detail from the viewpoint of contemporary knowledge of mechanisms governing the creep behavior of metal-matrix composites.

Creep mechanisms of laminated alumina/zirconia-toughened alumina composites

High-temperature plastic deformation of laminar composites containing alternate layers of Al 2O 3 and a mixture of 60 vol.% Al 2O 3 + 40 vol.% 3 mol% Y 2O 3-stabilized tetragonal ZrO 2 (ZTA) produced by tape casting is investigated in isostrain compression testing at temperatures between 1400 and 1500 °C. The stress exponent n and the creep activation energy Q are close to 1 and 700 kJ/mol, respectively. Microstructual observations reveal the lack of differential features in the ZTA layers and a general creep damage of the Al 2O 3 layers, with little microcracking by cavity coalescence even up to strains of 30%. The layer interfaces maintain their initial structural integrity after testing. An isostrain composite creep model predicts correctly the overall mechanical behavior of the laminates, which is dictated by the alumina phase via diffusional creep controlled by oxygen grain boundary diffusion.

Load partitioning during creep of powder metallurgy metal matrix composites and Shear-Lag model predictions

It was shown in a previous work that the load transfer mechanism plays a relevant role during the high temperature deformation of discontinuously reinforced metal matrix composites, MMCs. This idea emerged from the comparison of the creep data of a powder metallurgy, PM, 6061Al–15vol%SiC w composite and the corresponding un-reinforced 6061Al alloy. The idea was further supported by a qualitative analysis of the creep data of MMCs from a number of investigations reported in the literature, particularly of PM composites. In the present work a quantitative and more thorough study of the creep data of these PM composites is presented. Specifically, a well-known Shear-Lag model is used to compare the composites creep strength increment and the predicted load transferred to the reinforcement. These new results sustain more thoroughly the relevance of the load transfer mechanism during creep of MMCs.

Tensile creep and fatigue of Sylramic-iBN melt-infiltrated SiC matrix composites: Retained properties, damage development, and failure mechanisms

An understanding of the elevated temperature tensile creep, fatigue, rupture, and retained properties of ceramic matrix composites (CMC) envisioned for use in gas turbine engine applications is essential for component design and life-prediction. In order to quantify the effect of stress, time, temperature, and oxidation for a state-of-the-art composite system, a wide variety of tensile creep, dwell fatigue, and cyclic fatigue experiments were performed in air at 1204 °C for the SiC/SiC CMC system consisting of Sylramic-iBN SiC fibers, BN fiber interphase coating, and slurry-cast melt-infiltrated (MI) SiC-based matrix. Tests were either taken to failure or interrupted. Interrupted tests were then mechanically tested at room temperature to determine the residual properties. The retained properties of most of the composites subjected to tensile creep or fatigue were usually within 20% of the as-produced strength and 10% of the as-produced elastic modulus. It was observed that during creep, residual stresses in the composite are altered to some extent which results in an increased compressive stress in the matrix upon cooling and a subsequent increased stress required to form matrix cracks. Microscopy of polished sections and the fracture surfaces of specimens which failed during stressed-oxidation or after the room-temperature retained property test was performed on some of the specimens in order to quantify the nature and extent of damage accumulation that occurred during the test. It was discovered that the distribution of stress-dependent matrix cracking at 1204 °C was similar to the as-produced composites at room temperature; however, matrix crack growth occurred over time and typically did not appear to propagate through-the-thickness except at the final failure crack. Failure of the composites was due to either oxidation-induced unbridged crack growth, which dominated the higher stress regime (⩾179 MPa) or controlled by degradation of the fibers, probably caused by intrinsic creep-induced flaw growth of the fibers or internal attack of the fibers via Si diffusion through the CVI SiC and/or microcracks at the lower stress regime (⩽165 MPa).

Additivity of reinforcing mechanisms during creep of metal matrix composites: Role of the microstructure and the processing route

During decades, great efforts have been devoted to understand and predict the creep behavior of discontinuously reinforced metal matrix composites, particularly aluminum alloy matrix materials. As a result of all these investigations, however, a confuse panorama of the precise role of the reinforcing particles on the enhanced creep response of these materials rules the present understanding. Here, an analysis of the reasons that have driven to this situation is made. Also, a simple and generalized framework of the relevant mechanisms required to understand this behavior is proposed. This view is based on previous work by these authors on 6061Al alloy, prepared by ingot and powder metallurgy, and 6061Al–15 vol.% SiC w composite, as well as a data analysis of published investigations aimed at clarifying the above situation. The additivity of the proposed contributions and the potential damage mechanism are discussed in the context of the processing route employed.

Creep and fatigue lifetime prediction of polymer matrix composites based on simple cumulative damage laws

Abstract Presently, due to the energy saving concerns, there is an increase demand for lightweight structures for transportation industry, electric power wind generators among others. In this context design and maintenance of these civil engineering structures has put a stress on durability issues. The time scale for aeronautical applications is different for the rest of civil engineering structural applications therefore the durability experience accumulated on aeronautics is of little use. Usually civil engineering structures must remain in service for 50 years or more. Therefore the needs for real long-term mechanical performance prediction become quite important. Presently, for instance, the standards for buried GRP (Glass Fiber Reinforced Polymer Composites) pipes demand more than 10,000 h of creep tests in order to extrapolate data for two decades (in the time log scale) with a high level of confidence. The accelerated methodology proposed by Miyano et al. [Miyano Y, Nakada M, Sekine N. Accelerated testing for long-term durability of FRP laminates for marine use. J Composite Mater 2005;39:5–20] on Carbon and Glass Fiber Reinforced Polymers (CFRP and GFRP) laminates, rests on the fact that the time-superposition principle is the same for static, creep and fatigue strengths. Although not universally verified, it has been used successfully to predict fatigue lifetimes of many typical composites. In this context time-dependent failure criteria for viscoelastic materials is reviewed. Comparison between creep failure and static failure under constant stress/strain rate tests are analyzed. Cumulative damage laws, including the classical Linear Cumulative Damage law (LCD), are introduced to enlighten these comparisons. Finally some applications to lifetime prediction under variable amplitude fatigue loading are presented. The empirical LCD law performance is compared against the other physical based cumulative damage laws using fatigue loading sequences invented to expose the incoherencies of LCD lifetime predictions.

Formulation of Long-term Creep and Fatigue Strengths of Polymer Composites Based on Accelerated Testing Methodology

The applicability of our developed accelerated testing methodology (ATM) based on the time—temperature superposition principle (TTSP) and others for the prediction of long-term fatigue life of polymer matrix composites is performed experimentally and theoretically. A formulation method of master curves of creep and fatigue strengths for polymer matrix composites is introduced based on ATM. The master curves of creep and fatigue strengths for the typical three directions of unidirectional CFRP, that is the longitudinal tensile and compressive loadings and the transverse tensile loading, are formulated using the data measured based on ATM.

Effect of thermal shock cycling on the creep behavior of glass-epoxy composites

Abstract The aim of the present work is to investigate the effect of thermal fatigue on the creep and recovery behavior of polymer-matrix composites (ET441 epoxy resin, reinforced with woven E-glass fibers). For this purpose, 3 h creep followed by 3 h recovery tests were performed to specimens, which have previously been subjected to a certain number of thermal cycles (from +50 °C to −27 °C). It was found that the variation of the reduced creep compliance of the materials considered with the number of cycles follows an exponential decay law which was independent on the time period over which the load was applied. In order to better understand such a behavior, a four elements viscoelastic model was applied and the variation of all the four elements with the number of cycles gave a better insight into the observed behavior. In addition, the viscoelastic behavior observed was correlated with the value of the degree of damage and its variation with the number of cycles. It was found that there is a damage threshold corresponding to a certain number of cycles above which damage increases rapidly reaching a plateau where saturation of micro-damage is attained.