Effect of temperature on the compressive stress-strain properties of polystyrene

The compressive stress-strain behavior of a commercial polystyrene has been studied and the effect of deformation temperature on modulus, yield stress, percent yield strain and yield energy was determined. Yield energy is the only one of these parameters that is linear with temperature in the ductile region. A change in the mode of failure from ductile to brittle occurs between 5–30°C at a strain rate of O.1/in./in./min. At all temperatures studied, the yield or fracture stress varied linearly with the rate of deformation for strain rates ranging from 0.1 to 1.0 in./in./min. The yield data as a function of temperature were analyzed via a rate expression modified to incorporate the Coulomb-Navier yield criterion, Activation energy was found to be a function of deformation temperature with a change in slope occurring near the β transition. Activation volume increased linearly with deformation temperature, for the range studied. Agreement of dynamic mechanical and yield activation energies imply that the type of motion and the height of the energy barrier are similar for both. However, an increase in activation volume for stressed vs unstressed conditions suggests that a greater number of chain segments move as a result of stress biasing. Also the increase of both activation volume and activation energy with temperature implies that the correlated length of chain movement increases as temperature is increased. Similar to activation energy, yield stress exhibits a change in temperature dependence near the β transition. Data on other glassy polymers suggest that the highest temperature sub-T_g, transition is related to the change in the temperature dependence of yield stress.     

The impact endurance of polycrystalline graphite

Repeated impact tests have been carried out on a wide range of polycrystalline graphites. Two modes of test were employed using centrally impacted rods and discs with the rods supported horizontally at their ends and the discs supported around the circumference. The resulting impact endurance curves for all the different graphites under repeated impacts of constant energy were found to have a substantially common shape in both the disc and the rod tests. The absolute levels of the endurance curves differ considerably and correlate well with other mechanical properties of graphites, in particular the strain energy density at failure in bend. Measurement of impact forces on the single impact failure of graphite rods supports this correlation by showing that the dynamic stresses generated at failure in a single impact are the same as the corresponding static 3-point bend strengths in the same test mode. Measurement of impact forces at energies less than those required to cause failure in a single impact show that the fraction of energy absorbed as specimen strain energy is dependent on specimen size and shape but is not very sensitive to impact energy. A fracture mechanics model based on incremental crack growth and previously used to interpret stress-cycling fatigue data for graphite is proposed to describe also the endurance of polycrystalline graphite under repeated impacts. The model describes available experimental data obtained under both impact and fatigue conditions. On this model, the difference between the two cyclic stressing modes is the rate of crack growth per stress cycle, this being greater under repeated impacts than under fatigue cycles of the same stress amplitude.     

Influence of fiber orientation,temperature and relative humidity on the long-term performance of short glass fiber reinforced polyamide 6

As a result of processing of short fiber reinforced thermoplastics, the fiber orientation varies throughout a product giving rise to a pronounced anisotropic mechanical response. Different flow conditions in a product result in spatial variation in both short- and long-term mechanical properties. In this study, a modeling approach is presented to evaluate the lifetime of short fiber reinforced polyamide 6, both in plasticity- and crack growth controlled regions of failure. In the plasticity-controlled region, a viscoplastic model based on separation of the load angle (by means of Hill's equivalent stress formulation) and time dependence of the yield stress is used in the form of an associative flow rule. The influence of temperature and relative humidity on the magnitude of the plastic flow rate is described by using an apparent temperature approach combined with a Ree-Eyring formulation. The depression of the glass transition temperature in the polyamide 6 matrix with increasing amount of absorbed moisture was used to predict the anisotropic deformation kinetics in a humid environment. Similar to the plasticity controlled failure, in slow crack growth controlled failure region the effect of temperature, relative humidity, and load angle on the lifetime under a fatigue load is investigated. The apparent temperature approach could also be successfully applied to predict the slow crack growth failure, while the load angle dependence is shown to scale similar to the plasticity-controlled failure with the Hill's equivalent stress.     

Contact Damage in Plasma-Sprayed Alumina-Based Coatings

A study of Hertzian contact damage in plasma-sprayed alumina-based ceramic coatings on steel substrates has been made. Presectioned specimens are used to identify subsurface micromechanical damage processes within the coating and substrate layers as a function of increasing contact load, from both postcontact and in situ observations. Damage occurs principally by cracking in the ceramic coating and plastic deformation in the metal substrate, along with delamination at the coating/substrate interface. Coating thickness, cycling loading (fatigue), and processing history (coating microstructure) are shown to be important factors in the damage patterns and ensuing modes of failure. Indentation stress-strain curves are used to measure macroscopic mechanical responses, to quantify the maximum sustainable contact stresses and to determine the relative roles of coating and substrate in the net deformation.     

Fatigue mechanism of fiber-reinforced polypropylene swollen by oil

Fatigue mechanism was investigated for the fiber-reinforced polypropylene (FRPP) swollen by a small amount of oil. The curves of applied cyclic stress (S) vs. logarithm of cycles to failure (N) shifted into smaller values of S and N, respectively, by adding oil into the polymer and with increasing a test temperature. The endurance limiting stress (σ_e) for the swollen FRPP, defined as the stress at a long lifetime, became much the same as the one for the swollen unfilled polypropylene, though the former was considerably larger than the latter for the breaking strength measured in standard bending test. The activation energy and the activated volume, which have been determined by the Eyring's model from the S–N curves at higher stress levels, suggest that the introduction of a small amount of oil into the polymer lowers a motional unit associated with a fatigue process from 30 to several repeat units. The major decrease in σ_e by swelling can be explained in terms of the crack–nucleation theory. It is indicated that this decrease yields from the change in the stress–concentration factor.     

Evaluating the fatigue resistance of notched specimens of polyethylene

The fatigue life of a linear low density polyethylene was measured as a function of stress, notch depth., and temperature under plane strain conditions. The fatigue life was precisely related to the stress intensity. The temperature dependence of the fatigue life corresponded to an activation energy of 72.5 KJ which was independent of stress level. A general equation for the fatigue lifetime was proposed for polyethylenes in terms of the material parameters, the mechanical parameters such as stress intensity, stress amplitude, frequency, and waveform.     

对焦炭塔塔鼓变形失效的机理分析

焦炭塔是延迟焦化反应的反应釜,是延迟焦化装置的重要组成部分,其长期安全的运行是炼油企业取得高效益的前提和保障.但由于工作条件的恶劣,焦炭塔普遍存在着塔体变形、裙座及塔体焊接开裂等问题,严重影响着焦炭塔的安全运行.究其原因目前主要有以下几种情况:高温蠕变的结果;低周热疲劳的结果;高温蠕变与低周热疲劳共同作用的结果冷;急热温差热应力引起的局部塑性变形.通过对高温蠕变与低周热疲劳的产生条件以及损坏特征的仔细研究,并辅助以各种试验的结果,用排除法确定了焦炭塔的腰鼓变形失效的原因为急冷、急热温差热应力导致的局部塑性变形,并且总结出了其变形失效的规律和防治的具体方法.     

Tensile properties and stress relaxation of polypropylene at elevated temperatures

Samples of isotactic polypropylene(PP) were subjected to stress-relaxation experiments after simple tensile tests at four strain rates and at different levels of temperature. The relaxation moduli were determined in the range of temperature between 20 and 80°C with a relaxation period of 1200s duration. The activation energy value of the shift factor was determined using the time-temperature superposition principle. The calculated stress-strain curves and stress-relaxation curves were obtained from constitutive equations based on an overstress theory in which the temperature dependence of viscosity and the activation energy were considered. The temperature dependence of viscosity was amenable to an Arrhenius type equation. The quasi-equilibrium stress depends on both the current strain and the temperature. The calculated results were obtained by the proposed constitutive equation and compared with the data. The proposed constitutive equations based on the overstress model explain well the viscoelastic-plastic behavior of PP samples.     

低温下泡沫混凝土的动态力学性能

为了研究低温下泡沫混凝土的动态力学性能,采用φ100 mm的铝制分离式霍普金森压杆(SHPB)装置对不同温度下泡沫混凝土试件进行冲击压缩试验,得到了不同温度、应变率下的泡沫混凝土的应力应变曲线、能量参数、破碎形态等。结果表明:当应变率在62.59 s^-1以下时,泡沫混凝土应力应变曲线分为线弹性阶段、屈服阶段、破坏阶段;当应变率超过62.59 s^-1时,其应力应变曲线分为线弹性阶段、屈服阶段、局部失稳、应力平台、破坏阶段;常温、0 ℃、-10 ℃、-20 ℃和-30 ℃下,泡沫混凝土的动态抗压强度以及吸收能在对应的应变率分界点前表现出显著的应变率效应,但超出该分界点后,应变率效应便不再明显;泡沫混凝土的动态峰值抗压强度与吸收能随温度的降低而提高,但峰值应变随温度降低而降低;泡沫混凝土冲击破碎后的块度随着温度的降低逐渐变大。在低温环境下泡沫混凝土的抗动载设计中,对于泡沫混凝土的峰值抗压强度、吸收能,应优先考虑温度效应的影响,而对于其峰值应变,应优先考虑应变率效应。     

Fatigue behavior of polyamide 66/glass fiber under various kinds of applied load

In this study, the fatigue behavior of polyamide 66 reinforced with short glass fibers and especially the role of glass fibers has been investigated under two kinds of cyclic loading. tension–tension fatigue tests with stress controlled and alternative flexural fatigue test with strain controlled were carried out. The main topics include microscope damage observation, described by fiber/matrix debonding and interfacial failure, endurance limit with Wohler curves, effect of self‐heating temperature. For both tests, the surface temperature increases with an increasing applied load. The results show that the self‐heating has an important effect in the failure point where the Wohler curves join each other. The fracture surface was analyzed by scanning electron microscope for both applied loads. The stress ratio is −1 for alternative flexural fatigue test and 0.1 and 0.3 for tension–tension fatigue test ones at frequencies ranging 2–60 Hz. POLYM. COMPOS.,, 2012. © 2012 Society of Plastics Engineers     

Ce-TZP陶瓷在应力作用下的相变特性

本文研究了不同CeO_2含量,不同烧结制度的Ce-TZP陶瓷的应力-应变曲线,并同时测定在不同应力作用下材料中t-ZrO_2和m-ZrO_2相含量的变化规律。试验结果表明Ce-TZP陶瓷在应力作用下产生屈服和塑性变形。同的还发现这种陶瓷的屈服现象、形状记忆效应和织构特性都与材料中的t-ZrO_2(?)m-ZrO_2相变有关。本文详细讨论了Ce-TZP陶瓷产生屈服的原因和条件,探讨了应力、应变、温度和相变的内在关系。     

Impact behavior and modeling of engineering polymers

Because of their many unique and desirable properties, engineering polymers have increasingly been applied in applications where impact behavior is of primary concern. In this paper, the impact behavior of a glassy polymer acrylonitrile‐butadiene‐styrene (ABS) and a semicrystalline polymer alloy of polycarbonate and polybutylene‐terephthalates (PBT) are obtained as a function of impact velocity and temperature from the standard ASTM D3763 multiaxial impact test. As computer simulation of destructive impact events requires two material models, a constitutive model and a failure model, uniaxial mechanical tests of the two polymers are carried out to obtain true stress vs, true strain curves at various temperatures and strain rates. The generalized DSGZ constitutive model, previously developed by the authors to uniformly describe the entire range of deformation behavior of glassy and semicrystalline polymers for any monotonic loading modes, is calibrated and applied. The thermomechanical coupling phenomenon of polymers during high strain rate plastic deformation is considered and modeled. A failure criterion based on maximum plastic strain is proposed. Finally, the generalized DSGZ model, the thermomechanical coupling model, and the failure criterion are integrated into the commercial finite element analysis package ABAQUS/Explicit through a user material subroutine to simulate the multiaxial impact behavior of the two polymers ABS and PBT. Impact load vs. striker displacement curves and impact energy vs. striker displacement curves from computer simulation are compared with multiaxial impact test data and were found to be in good agreement.     

Investigation of nonelastic response of semicrystalline polymers at high strain levels

The nonelastic behavior at high strains of three semicrystalline polymers [i.e., nylon‐6, poly(ethylene terephthalate), and poly(ethylene 2,6‐naphthalenedicarboxylate] was investigated. For all materials, room temperature tensile strain recovery tests revealed the existence of two components of nonelastic deformation: a fast‐relaxing component (called anelastic) and a slow‐relaxing component (usually called plastic). A strain recovery master curve could be constructed for each material from the strain recovery data obtained at various temperatures. The shift factors versus temperature relationship for the strain recovery master curves allowed us to evaluate an activation energy for the nonelastic strain recovery process. These data were then compared with the activation energy for the glass‐transition process evaluated by dynamic mechanical measurements at low strain. The aim of this comparison was to investigate the influence of viscoelasticity on the nonelastic deformation recovery. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1664–1670, 2000     

Effect of extrusion temperature on elongational flow behavior of PVC melt

The effect of the sample extrusion temperature between 160° and 180°C on the elongational flow properties of a low molecular weight suspension PVC (unplasticized) has been studied with the Rheometrics Extensional Rheometer. The results of both the tensile creep measurements at a constant stress of 24 KPa and the stretching experiment at a constant strain rate of 0.01 sec^?1 indicated the existence of a rheological transition at 185°C marked by the dual valued flow activation energy and also by the temperature dependence of the tensile stress-strain curves. Increasing extrusion temperature increased both the flow activation energy and the extensional viscosity below the transition temperature.     

Mechanical Properties of Polycrystalline β-Sic

The mechanical properties of chemically vapor-deposited β-Sic were measured in bending between room temperature and 1400°C. Material with grain diameters from less than 1 to 15 μm was tested. No grain-size dependence of the bend strength of dense (<99% of theoretical) Sic was observed at any test temperature. The fracture strength of dense Sic remained approximately constant between room temperature and about 900°C and then increased sharply up to the maximum test temperature of 1215° to 1400°C. This increase in fracture stress coincided with the onset of plastic yielding detectable in the stress-strain curves. The fracture mode of this material was transgranular cleavage at all test temperatures. The fracture stress of Sic of lower density, which was characterized by the presence of grain boundary flaws, decreased slightly at high temperature. The fracture mode of the low-density (3.17 g/cm³) β-Sic underwent a transition from predominantly transgranular at room temperature to predominantly intergranular at high temperature.     

Evaluation of the Strength and Creep–Fatigue Behavior of Hot Isostatically Pressed Silicon Nitride

The strength of a commericially available hot isostatically pressed silicon nitride was measured as a function of temperature. To evaluate long-term mechanical reliability of this material, the tensile creep and fatigue behavior was measured at 1150°, 1260°, and 1370°C. The stress and temperature sensitivities of the secondary (or minimum) creep strain rate were used to estimate the stress exponent and activation energy associated with the dominant creep mechanism. The fatigue characteristics were evaluated by allowing individual creep tests to continue until specimen failure. The applicability of the four-point load geometry to the study of strength and creep behavior was also determined by conducting a limited number of flexural creep tests. The tensile fatigue data revealed two distinct failure mechanisms. At 1150°C, failure was controlled by a slow crack growth mechanism. At 1260° and 1370°C, the accumulation of creep damage in the form of grain boundary cavities and cracks dominated the fatigue behavior. In this temperature regime, the fatigue life was controlled by the secondary (or minimum) creep strain rate in accordance with the Monkman–Grant relation.     

Indentation of an oriented transparent polyamide

In this study, the effect of orientation on the indentation hardness and energy absorption of an oriented transparent Trogamid polyamide was investigated with a spherical indentation methodology. It was found that the orientation significantly improved the indentation hardness and energy absorbed by plastic deformation. From the indentation hardness measurement, the elastic modulus, yield stress, and strain hardening exponent were derived from both the elastic and plastic regions of the indentation load–displacement curves. The elastic modulus was found to remain the same with orientation; the yield stress and the strain hardening exponent increased with orientation. The increase in the strain hardening exponent was the primary reason for the improved indentation hardness and energy absorption in the oriented samples. The mechanical properties from indentation measurements were compared to values obtained from tension true stress/true strain measurements. Good agreement was observed between the results from the indentation and tension tests. The effect of temperature on the mechanical properties was also studied. It was found that the modulus and yield stress were higher at a lower temperature; however, the strain hardening exponent remained unaffected. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Static fatigue behavior of cracked piezoelectric ceramics in three-point bending under electric fields

This paper describes an experimental and analytical study on the static fatigue behavior of piezoelectric ceramics under electromechanical loading. Static fatigue tests were carried out in three-point bending with the single-edge precracked-beam specimens. The crack was created perpendicular to the poling direction. Time-to-failure under different mechanical loads and dc electric fields were obtained from the experiment. Microscopic examination of the fracture surface of the piezoelectric ceramics was performed as well. A finite element analysis was also made, and the applied energy release rate for the permeable crack model was calculated. The effect of applied dc electric fields on the energy release rate versus lifetime curve is examined. The most important conclusion we reach is that the lifetimes for the piezoelectric specimens under a positive electric field are much shorter than the failure times of specimens under a negative electric field for the same mechanical load level.