Application of time–temperature–stress superposition on creep of wood–plastic composites

Time–temperature–stress superposition principle (TTSSP) was widely applied in studies of viscoelastic properties of materials. It involves shifting curves at various conditions to construct master curves. To extend the application of this principle, a temperature–stress hybrid shift factor and a modified Williams–Landel–Ferry (WLF) equation that incorporated variables of stress and temperature for the shift factor fitting were studied. A wood–plastic composite (WPC) was selected as the test subject to conduct a series of short-term creep tests. The results indicate that the WPC were rheologically simple materials and merely a horizontal shift was needed for the time–temperature superposition, whereas vertical shifting would be needed for time–stress superposition. The shift factor was independent of the stress for horizontal shifts in time–temperature superposition. In addition, the temperature- and stress-shift factors used to construct master curves were well fitted with the WLF equation. Furthermore, the parameters of the modified WLF equation were also successfully calibrated. The application of this method and equation can be extended to curve shifting that involves the effects of both temperature and stress simultaneously.     

Micromechanical modeling and experimental characterization on viscoelastic behavior of 1–3 active composites

A study on the temperature-dependent viscoelastic behavior of (1–3 active composites) 1–3 piezocomposites and bulk piezoceramic subjected to electromechanical loading is carried out. The temperature-dependent effective properties are obtained experimentally using resonance based measurement technique. Experiments are also preformed for various fiber volume fractions of 1–3 piezocomposites subjected to constant compressive prestress and cyclic electric field at elevated temperature to understand the time-dependent behavior. Based on the measurements it is observed that the viscoelastic behavior has a significant influence on the electromechanical responses of 1–3 piezocomposites. Hence a viscoelastic based numerical model (unit cell approach) is proposed to predict the time-dependent effective properties of 1–3 piezocomposites. The evaluated effective properties are incorporated in a finite element based 3-D micromechanical model to predict the time-dependent thermo-electro-mechanical behavior of 1–3 piezocomposites and compared with the experimental observations.     

A modeling study on residual stress-induced interfacial debonding and stress–strain behavior of weakly bonded UD composites

Residual stress-induced interfacial debonding and its influence on stress–strain behavior of unidirectional fiber-reinforced brittle matrix composites with weak interface were studied using mini-composite model by means of the two-dimensional shear lag analysis combined with a Monte Carlo method. Damages (fracture of fiber, matrix and interface) were accumulated intermittently, resulting in serrated stress–strain curve. In this process, the residual stresses changed the strain, order and location of occurrence of damages, and consequently the shape of stress–strain curve and strength of composite. Under the existence of compressive and tensile axial residual stresses in fiber and matrix, respectively, the fracture of the matrix and the debonding from the fracture-ends of matrix were enhanced, while the fracture of fiber and the debonding from the fracture-ends of fiber were suppressed. The residual stress-induced premature fracture of the matrix, followed by debonding, reduced the strength of composite.     

Prediction of color changes in acetaminophen solution using the time–temperature superposition principle

A prediction method for color changes based on the time–temperature superposition principle (TTSP) was developed for acetaminophen solution. Color changes of acetaminophen solution are caused by the degradation of acetaminophen, such as hydrolysis and oxidation. In principle, the TTSP can be applied to only thermal aging. Therefore, the impact of oxidation on the color changes of acetaminophen solution was verified. The results of our experiment suggested that the oxidation products enhanced the color changes in acetaminophen solution. Next, the color changes of acetaminophen solution samples of the same head space volume after accelerated aging at various temperatures were investigated using the Commission Internationale de l’Eclairage (CIE) LAB color space (a*, b*, L* and ΔE*ab), following which the TTSP was adopted to kinetic analysis of the color changes. The apparent activation energies using the time–temperature shift factor of a*, b*, L* and ΔE*ab were calculated as 72.4, 69.2, 72.3 and 70.9 (kJ/mol), respectively, which are similar to the values for acetaminophen hydrolysis reported in the literature. The predicted values of a*, b*, L* and ΔE*ab at 40?°C were obtained by calculation using Arrhenius plots. A comparison between the experimental and predicted values for each color parameter revealed sufficiently high R² values (>0.98), suggesting the high reliability of the prediction. The kinetic analysis using TTSP was successfully applied to predicting the color changes under the controlled oxygen amount at any temperature and for any length of time.     

Validation of the time–temperature superposition principle for crack propagation in bituminous mixtures

The time–temperature superposition principle (TTSP) is known to be valid in the small strain domain where the behaviour of bituminous mixtures is linear viscoelastic (LVE). The behaviour is then called thermorheologically simple. In this work, an experimental campaign was performed at University of Lyon/ENTPE (France) to check the validity of the TTSP in the linear domain in the tridimensional case and also when cracks occur and propagate in bituminous mixture. A four-point bending test, which has been designed at University of Lyon/ENTPE, was used as crack propagation test. First, a complex modulus test is performed on cylindrical specimen in the LVE domain. Then, a series of crack propagation tests are carried out at different temperatures and different imposed displacement rates. The same shift factors obtained for master curve of complex modulus is also applied for the crack propagation tests analysis. The results allow obtaining a unique curve, for identical loadings when plotting as a function of reduced time. This result confirms that the TTSP is also valid for crack propagation in bituminous mixtures.     

Review: mechanical behavior of metal/ceramic interfaces in nanolayered composites—experiments and modeling

Recent experimental and modeling studies in nanolayered metal/ceramic composites are reviewed, with focus on the mechanical behaviors of metal/nitrides interfaces. The experimental and modeling studies of the slip systems in bulk TiN are reviewed first. Then, the experimental studies of interfaces, including co-deformation mechanism by micropillar compression tests, in situ TEM straining tests for the dynamic process of the co-deformation, thickness-dependent fracture behavior, and interrelationship among the interfacial bonding, microstructure, and mechanical response, are reviewed for the specific material systems of Al/TiN and Cu/TiN multilayers at nanoscale. The modeling studies reviewed cover first-principles density functional theory-based modeling, atomistic molecular dynamics simulations, and mesoscale modeling of nanolayered composites using discrete dislocation dynamics. The phase transformation between zinc-blende and wurtzite AlN phases in Al/AlN multilayers at nanoscale is also reviewed. Finally, a summary and perspective of possible research directions and challenges are given.     

Dilatometric behavior and microstructure of sintered Fe–NbC and Fe–TaC composites

Fe-reinforced composites were manufactured by the addition of 10–20 wt.% NbC or TaC particles aiming at improved mechanical and wear behavior. Two varieties of Fe powders from Hoeganaes Corp. were used, Ancorsteel 1000B and 45P. Composites produced using the former variety included a small amount of Fe₃P to induce liquid-phase sintering whereas 45P powder was pre-alloyed with P by the manufacturer. The hardness of the matrix was adjusted adding carbon to the composite mixture. The powders were milled for different times and annealed prior to pressing. A dilatometric study was carried out under hydrogen to establish optimum sintering profiles. Relative densities up to 97% TD were achieved. Both microstructure and density of the sintered pellets were evaluated in order to establish correlations involving composition, processing parameters and microstructure of the composite.     

Effect of stress ratios on tension–tension fatigue behavior and micro-damage evolution of basalt fiber-reinforced epoxy polymer composites

The tension–tension fatigue behavior and damage mechanism of basalt fiber-reinforced epoxy polymer (BFRP) composites at different stress ratios are studied in this paper. The fatigue experiments were performed under stress ratios, R?=?σ_min/σ_max of 0.1 and 0.5, while the lifetime and the stiffness degradation were monitored and analyzed to investigate the effect of stress ratios. The damage propagation during fatigue loading was periodically monitored by using an in situ scanning electron microscope (SEM). The results show that the fatigue life decreases and the fatigue life degradation rate increases with the decrease of stress ratio for examined BFRP composites. The stiffness degradation is also sensitive to different stress ratios, showing a greater stiffness loss before failure at lower stress ratio. From the SEM images, it is indicated that the micro-damage mode shifts from interface debonding and matrix cracking into fiber breaking with decreasing stress ratios.     

Novel starch thermoplastic/Bioglass® composites: Mechanical properties, degradation behavior and in-vitro bioactivity

The present research aims to evaluate the possibility of creating new degradable, stiff and highly bioactive composites based on a biodegradable thermoplastic starch-based polymeric blend and a Bioglass® filler. Such combination should allow for the development of bioactive and degradable composites with a great potential for a range of temporary applications. A blend of starch with ethylene–vinyl alcohol copolymer (SEVA-C) was reinforced with a 45S5 Bioglass® powder presenting a granulometric distribution between 38 and 53 m. Composites with 10 and 40 wt % of 45S5 Bioglass® were compounded by twin-screw extrusion (TSE) and subsequently injection molded under optimized conditions. The mechanical properties of the composites were evaluated by tensile testing, and their bioactivity assessed by immersion in a simulated body fluid (SBF) for different periods of time. The biodegradability of these composites was also monitored after several immersion periods in an isotonic saline solution. The tensile tests results obtained indicated that SEVA-C/Bioglass® composites present a slightly higher stiffness and strength (a modulus of 3.8 GPa and UTS of 38.6 MPa) than previously developed SEVA-C/Hydroxylapatite (HA) composites. The bioactivity of SEVA-C composites becomes relevant for 45S5 amounts of only 10 wt %. This was observed by scanning electron microscopy (SEM) and confirmed for immersion periods up to 30 days by both thin-film X-ray diffraction (TF-XRD) (where HA typical peaks are clearly observed) and induced coupled plasma emission (ICP) spectroscopy used to follow the elemental composition of the SBF as function of time. Additionally, it was observed that the composites are biodegradable being the results correlated with the correspondent materials composition.     

Compressive and impression creep behavior of a cast Mg–Al–Zn–Si alloy

Creep behavior of an Mg–6Al–1Zn–0.7Si cast alloy was investigated by compression and impression creep test methods in order to evaluate the correspondence of impression creep results and creep mechanisms with conventional compression test. All creep tests were carried out in the temperature range 423–523 K and under normal stresses in the range 50–300 MPa for the compression creep and 150–650 MPa for impression creep tests. The microstructure of the AZ61–0.7Si alloy consists of β-Mg₁₇Al₁₂ and Mg₂Si intermetallic phases in the α-Mg matrix. The softening of the former at high temperatures is compensated by the strengthening effect of the latter, which acts as a barrier opposing recovery processes. The impression results were in good agreement with those of the conventional compressive creep tests. The creep behavior can be divided into two stress regimes, with a change from the low-stress regime to the high-stress regime occurring, depending on the test temperature, around 0.009 < (σ/G) < 0.015 and 0.021 < (σ_imp/G) < 0.033 for the compressive and impression creep tests, respectively. Based on the steady-state power-law creep relationship, the stress exponents of about 4–5 and 10–12 were obtained at low and high stresses, respectively. The low-stress regime activation energies of about 90 kJ mol⁻¹, which are close to that for dislocation pipe diffusion in the Mg, and stress exponents in the range of 4–5 suggest that the operative creep mechanism is pipe-diffusion-controlled dislocation viscous glide. This behavior is in contrast to the high-stress regime, in which the stress exponents of 10–12 and activation energies of about 141 kJ mol⁻¹ are indicative of a dislocation climb mechanism similar to those noted in dispersion strengthening mechanisms.     

Applicability of empirical models for evaluation of stress ratio effect on the durability of fiber-reinforced creep rupture–susceptible composites

Fiber-reinforced polymer–matrix composites are known to exhibit loading rate- and time-dependent mechanical response. Their fatigue strength is determined by a complex interaction of damage processes governed by loading duration and cycle number. Apart from mechanistic approaches, a number of empirical models of various sophistication have been proposed to predict the durability of composites, differing in the amount of experimental data needed for their application. The accuracy of several such models is evaluated by comparing the prediction to the experimentally determined stress ratio effect on fatigue life of glass fiber-reinforced polyester–matrix composite. It is found that the accuracy of prediction generally improves with increasing the amount of test data needed for model calibration. However, the most accurate method of fatigue life estimation, among the selected ones, is by the modified Goodman diagram.     

Effects of environment on creep behavior of two oxide/oxide ceramic–matrix composites at 1200 °C

The tensile creep behavior of two oxide/oxide ceramic–matrix composites (CMCs) was investigated at 1200 °C in laboratory air, in steam, and in argon. The composites consist of a porous oxide matrix reinforced with laminated, woven mullite/alumina (Nextel™720) fibers, have no interface between the fiber and matrix, and rely on the porous matrix for flaw tolerance. The matrix materials were alumina and aluminosilicate. The tensile stress–strain behavior was investigated and the tensile properties were measured at 1200 °C. Tensile creep behavior of both CMCs was examined for creep stresses in the 80–150 MPa range. Creep run-out defined as 100 h at creep stress was achieved in air and in argon for stress levels ≤100 MPa for both composites. The retained strength and modulus of all specimens that achieved run-out were evaluated. The presence of steam accelerated creep rates and reduced creep life of both CMCs. In the case of the composite with the aluminosilicate matrix, no-load exposure in steam at 1200 °C caused severe degradation of tensile strength. Composite microstructure, as well as damage and failure mechanisms were investigated. Poor creep performance of both composites in steam is attributed to the degradation of the fibers and densification of the matrix. Results indicate that the aluminosilicate matrix is considerably more susceptible to densification and coarsening of the porosity than the alumina matrix. The views expressed are those of the authors and do not reflect the official policy or position of the United States Air Force, Department of Defense or the U.S. Government.     

Space–time FSI modeling and dynamical analysis of spacecraft parachutes and parachute clusters

Computer modeling of spacecraft parachutes, which are quite often used in clusters of two or three large parachutes, involves fluid–structure interaction (FSI) between the parachute canopy and the air, geometric complexities created by the construction of the parachute from “rings” and “sails” with hundreds of gaps and slits, and the contact between the parachutes. The Team for Advanced Flow Simulation and Modeling (T \bigstar AFSM){({{\rm T} \bigstar {\rm AFSM}})} has successfully addressed the computational challenges related to the FSI and geometric complexities, and recently started addressing the challenges related to the contact between the parachutes of a cluster. The core numerical technology is the stabilized space–time FSI technique developed and improved over the years by the T \bigstar AFSM{{{\rm T} \bigstar {\rm AFSM}}} . The special technique used in dealing with the geometric complexities is the Homogenized Modeling of Geometric Porosity, which was also developed and improved in recent years by the T \bigstar AFSM{{{\rm T} \bigstar {\rm AFSM}}} . In this paper we describe the technique developed by the T \bigstar AFSM{{{\rm T} \bigstar {\rm AFSM}}} for modeling, in the context of an FSI problem, the contact between two structural surfaces. We show how we use this technique in dealing with the contact between parachutes. We present the results obtained with the FSI computation of parachute clusters, the related dynamical analysis, and a special decomposition technique for parachute descent speed to make that analysis more informative. We also present a special technique for extracting from a parachute FSI computation model parameters, such as added mass, that can be used in fast, approximate engineering analysis models for parachute dynamics.     

Densification behavior and mechanical properties of spark plasma-sintered ZrC–TiC and ZrC–TiC–CNT composites

Titanium carbide (TiC) and carbon nanotubes (CNTs) were introduced into zirconium carbide (ZrC) ceramics to improve the fracture toughness. ZrC–TiC and ZrC–TiC–CNT composites containing 0–30 vol.% TiC and 0.25–1 mass% CNT were prepared by spark plasma sintering at temperatures of 1750–1850 °C for 300 s under a pressure of 40 MPa. Densification behavior, microstructure, and mechanical properties of the ZrC-based composites were investigated. Fully dense ZrC–TiC and ZrC–TiC–CNT composites with a relative density of more than 98 % were obtained. Vickers hardness of ZrC-based composites increased with increasing TiC content and the highest hardness was achieved with the addition of 20 vol.% TiC. Addition of CNTs up to 0.5 wt% significantly increased the fracture toughness of ZrC-based composites, whereas the addition of TiC did not have this effect.     

Effects of steam environment on creep behavior of Nextel™720/alumina–mullite ceramic composite at elevated temperature

The tensile creep behavior of an oxide–oxide continuous fiber ceramic composite was investigated at 1000 and 1100 °C in laboratory air and in steam. The composite consists of a porous alumina–mullite matrix reinforced with laminated, woven mullite/alumina (Nextel?720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. The tensile stress–strain behavior was investigated and the tensile properties measured. Tensile creep behavior was examined for creep stresses in the 70–140 MPa range. The presence of steam accelerated creep rates and dramatically reduced creep lifetimes. The degrading effects of steam become more pronounced with increasing temperature. At 1000 °C, creep run-out (set to 100 h) was achieved in all tests. At 1100 °C, creep run-out was achieved in all tests in air and only in the 87.5 MPa test in steam. Composite microstructure, as well as damage and failure mechanisms were investigated.