Interface stress relaxation in magnesium matrix composites studied by mechanical spectroscopy

Abstract

Mechanical spectroscopy has been used to study magnesium matrix composites reinforced either by long SiC fibres or randomly distributed Al₂O₃ (Saffil) short fibres. It is well known that, in metal matrix composites, thermal stresses can be built up at the interface due to the mismatch between the thermal expansion coefficients of matrix and reinforcements. In magnesium matrix composites, these thermal stresses are relaxed by dislocation motion in the matrix. This mechanism of thermal stress relaxation yields an extra transient component in the mechanical loss spectrum, which depends on the heating/cooling rate and disappears in isothermal condition in the behaviour of the shear modulus G with temperature has been observed during thermal cycling between 100 K and 500 K. The intensity of this phenomenondepends on the spatial distribution of the reinforcements in the matrix. In particular, composites reinforced with long fibres exhibit a more pronounced anomaly. This is interpreted by the modification of the interface strength when temperature is changed.     

Thermal stress relaxation in magnesium matrix composites controlled by dislocation breakaway

In metal matrix composites (MMCs) thermal stress relaxation can be achieved either by interface debonding, crack propagation or by dislocation motion. The present paper shows that in the case of magnesium matrix, interface thermal stresses are relaxed by dislocation motion. Moreover the results obtained by mechanical spectroscopy prove that this dislocation motion is controlled by a solid friction mechanism, which is not thermally activated. This point is very interesting for the development of MMCs, which exhibit a high damping capacity over a wide frequency range. Dislocation hysteretic motion in the magnesium matrix is evidenced by the dependence of the mechanical loss on the stress amplitude. The obtained relationship obeys perfectly to the Granato–Lücke model for dislocation breakaway.     

Thermal expansion and stress relaxation of metal-matrix composites

The coefficient of thermal expansion (CTE) of a series of Al-6%Si matrix samples, with reinforcements of carbon, SiC, Al₂O₃, or boron fibres, cloths, or ceramic particles was measured in the range 60°–220°C with a dilatometer. The anisotropy of the CTE was measured and found to be very large for specimens unidirectionally reinforced with carbon fibre. Relaxation of stresses due to the different thermal expansion of matrix and reinforcement was studied by using the bending of asymmetrically reinforced samples and the magnitude of the stresses evaluated using bending beam theory.     

GH4090合金圆柱螺旋弹簧应力松弛性能研究

以航空发动机用GH4090合金圆柱螺旋弹簧为研究对象,研究了不同温度下弹簧的静态和动态抗松弛性能。结果表明,静态抗松弛实验下,时效态GH4090合金弹簧在室温和300℃下无松弛、无蠕变;350和400℃下应力松弛率分别为1.08%和1.09%。动态抗松弛实验下,时效态GH4090合金弹簧在室温下无松弛,350℃下应力松弛率为3.23%;变形态GH4090合金弹簧在室温和350℃下应力松弛率分别为3.30%和16.85%。变形态和时效态GH4090合金弹簧的松弛率均随温度的升高而增大,且变形态松弛率变化尤为明显。     

Viscoplastic analysis of structural polymer composites using stress relaxation and creep data

A structural carbon based composite material has been investigated for its high temperature viscoplastic properties using a model based on an overbearing stress concept and using the data obtained from load relaxation and creep. The time dependent viscoplastic properties were obtained at several load and temperature levels. An elastic–viscoplastic constitutive model (proposed by Gates) was used for the modeling efforts. The model is based on an overstress concept appropriate to inelastic properties of composites. The materials parameters for the model are obtained from a set of load relaxation experiments. The model predictions have been compared to the results of creep tests. The results show that the model is capable of predicting the creep behavior at shorter time periods and lower temperatures. As the temperature is increased or as the creep is prolonged the model predictions deviate from the experimental results.     

Maxwell-Wagner relaxation in magnetoelectric composites

Giant Maxwell-Wagner relaxation of the magnetoelectric susceptibility and the magnetoelectric voltage coefficient was observed in a ferrite-piezoelectric composite. The relaxation frequency of the magnetoelectric susceptibility of this material can be controlled within broad limits by varying the volume fractions of the components and by modifying their properties. The maximum value of the magnetoelectric susceptibility of the composite studied exceeds the values known in other materials.     

Interface diffusion-induced creep and stress relaxation in unidirectional metal matrix composites under biaxial loading

Analytical solutions are developed for interface diffusion-induced creep and stress relaxation in unidirectional metal matrix composites under biaxial transverse loading. The driving force for the interface diffusion is the normal stress acting on the interface, which is obtained from rigorous Eshelby inclusion theory. The solutions are a function of the applied stress, volume fraction and radius of the reinforced-fiber, the modulus ratio between the fiber and the matrix, specially, exhibit a strong dependence of creep rate and stress relaxation behavior on the biaxial stress ratio. Moreover, the solution for the interface stress presented in this study also gives some insight into the relationship between the interface diffusion and interface slip. For the application of the solutions in the realistic composites, the scale effect is taken into account by detailed finite element analysis based on a unit cell model.     

Piezoelectric relaxation in polymer and ferroelectric composites

Recently, fairly large numbers of studies have been reported on piezoelectric relaxations of polymers. The piezoelectric constants of polymer and ferroelectric composites also show such relaxations. It has been found that both polymer and polymer and ferroelectric composities exhibit typical Debye-type piezoelectric dispersion. Cole-Cole diagrams for the piezoelectric properties of these materials have been drawn and an attempt has been made to interpret the results.     

Modelling the viscoelastic stress relaxation of glass fibre reinforcements under constant compaction strain during composites manufacturing

Viscoelastic stress relaxation of glass fibre reinforcements is commonly encountered in the manufacture of glass fibre reinforced polymer composites. A better understanding of the phenomenon, coupled with an ability to predict this behaviour, will aid improved manufacturing process control and tooling design. Finished product quality may also be bettered by virtue of increased knowledge of stresses acting within the composite product. This paper presents a simple Maxwell element-based model to both simulate and help explain the viscoelastic stress relaxation of glass fibre reinforcements under compressive strain compaction of layers during composites manufacturing. The model was validated against experimental data for reinforcement materials of different architecture, and good-to-reasonable predictions of the stress relaxation response were obtained.     

柱面SH波作用下管道的动应力集中研究

在地下工程爆破开挖过程中,爆破地震波对管道安全的影响是十分重要的。为了更好的研究柱面SH波作用下管道的动应力集中情况,以爆破地震波中的柱面SH波作为研究对象,推导了柱面SH波作用下管道内壁上动应力集中系数(DSCF)的解析解。通过宝通禅寺地下通道爆破开挖工程,讨论了归一化爆心距r^*和入射波频率f对DSCF的影响。结果表明,由于管道与周围土层剪切模量差异较大,管道上的动应力集中系数普遍较大;从DSCF分布的角度来讲,当r*和入射波频率f对DSCF的影响。结果表明,由于管道与周围土层剪切模量差异较大,管道上的动应力集中系数普遍较大;从DSCF分布的角度来讲,当r^*<5时,柱面波与平面波的差异较大,但当r*<5时,柱面波与平面波的差异较大,但当r^*>5时,柱面SH波基本可以认为与平面SH波等价;低频入射波对管道的安全更为显著,特别是入射波频率与管道自振频率接近时,管道受到的影响最为严重。     

WJ-8扣件系统中弹性垫板的应力松弛性能研究

对高速铁路轨道WJ-8扣件中的聚氨酯弹性垫板分别进行长时限压缩15％、20％、25％试验,并测定应力松弛过程.应用五元件广义Maxwell为模型,对应力松弛过程进行了数学回归.结果表明,五元件Maxwell模型可以完美拟合弹性垫板的应力松弛过程.随着应变增加,松弛时间呈下降趋势,即垫板抗形变能力减弱,垫板恢复原有形状能力下降.为WJ-8扣件承载限量预测、模拟工作特性研究和垫板分子结构设计提供理论依据.     

Thermal stress hysteresis and stress relaxation in an epoxy film

Thermal cycling of an epoxy coating on silicon through the glass transition temperature (T _g) revealed a large stress hysteresis on the first thermal cycle through T _g and a change in the stress–temperature slope at T _g resulting from the change in the epoxy elastic properties due to the glass transition. This stress hysteresis was not observed on subsequent thermal cycles through T _g. However, after the coating was annealed (aged) below T _g (for hours or longer)—during which the stress relaxed exponentially with time—the stress hysteresis returned. The magnitude of stress hysteresis, on cycling through T _g, was found to correlate to the magnitude of long-time relaxation that occurred during annealing at temperatures below T _g.     

Effect of mean stress on residual stress relaxation in steel specimens

Abstract

Residual stress relaxation was investigated by subjecting specimens with various mean stress levels to strain controlled cyclic loading. The material studied was mild steel in three different conditions. The mean stress levels ranged from 100 to 200 MPa, and two strain amplitudes, 0·05 and 0·06%, were studied in detail. The residual stresses in the specimens were measured before and after mean stress relaxation experiments. It was found that experimental factors such as temperature variations and crack growth have a significant influence on the results. Based on the experimental results, it is proposed that the mean stress relaxation exponent should be divided into two parts: mean stress dependent and mean stress independent. The first includes the contribution of quasi-static relaxation, i.e. mean stress dependent plastic deformation. The second part includes the contribution of cycle dependent mean stress relaxation, which does not depend on the mean stress.     

Experimentally-based relaxation modulus of polyurea and its composites

Polyurea is a block copolymer that has been widely used in the coating industry as an abrasion-resistant and energy-dissipative material. Its mechanical properties can be tuned by choosing different variations of diamines and diisocyanates as well as by adding various nano- and micro-inclusions to create polyurea-based composites. Our aim here is to provide the necessary experimentally-based viscoelastic constitutive relations for polyurea and its composites in a format convenient to support computational studies. The polyurea used in this research is synthesized by the reaction of Versalink P-1000 (Air Products) and Isonate 143L (Dow Chemicals). Samples of pure polyurea and polyurea composites are fabricated and then characterized using dynamic mechanical analysis (DMA). Based on the DMA data, master curves of storage and loss moduli are developed using time–temperature superposition. The quality of the master curves is carefully assessed by comparing with the ultrasonic wave measurements and by Kramers–Kronig relations. Based on the master curves, continuous relaxation spectra are calculated, then the time-domain relaxation moduli are approximated from the relaxation spectra. Prony series of desired number of terms for the frequency ranges of interest are extracted from the relaxation modulus. This method for developing cost efficient Prony series has been proven to be effective and efficient for numerous DMA test results of many polyurea/polyurea-based material systems, including pure polyurea with various stoichiometric ratios, polyurea with milled glass inclusions, polyurea with hybrid nano-particles and polyurea with phenolic microbubbles. The resulting viscoelastic models are customized for the frequency ranges of interest, reference temperature and desired number of Prony terms, achieving both computational accuracy and low cost. The method is not limited to polyurea-based systems. It can be applied to other similar polymers systems.