Elasto-plastic behavior of thermoplastic composite laminates under cyclic loading

An experimental and numerical study of the elasto-plastic behavior of thermoplastic matrix composite laminates under static and cyclic loads is presented. Off-axis and angle ply specimens cut from laminates of poly(ether ether ketone) (PEEK) reinforced with continuous carbon fibers have been tested under cyclic sinusoidal tensile loads and the hysteresis loops have been monitored. A micro mechanical model, which includes a parabolic criteria based on the plastic behavior of the matrix, has been adopted to study the composite non-linear behavior and a correlation between plastic deformation and a strong rise of damping and temperature at high stresses is outlined. Good agreement is shown between theory and experimental results. The mathematical mdoel presented here can be used to predict the visco-elastic-plastic response of the material at high stresses and its influence in the fatigue damage.     

About the influence of temperature and matrix ductility on the behavior of carbon woven-ply PPS or epoxy laminates: Notched and unnotched laminates

Could thermoplastic-based composites be used to replace thermosetting-based composites in high-temperature secondary aircraft structures? The purpose of this work is to establish the ability of a material system to be used in aircraft engine nacelles when subjected to static loadings, with a key upper temperature of 120 °C. In order to provide answers to this question, the thermo-mechanical behaviors of carbon fiber fabric reinforced PPS or epoxy laminates have been compared specifically within the temperature change with 120 °C at the upper bound. The temperature-dependent ductile behavior of laminates is more or less exacerbated, depending on polymers glass transition temperature, and laminates stacking sequence. For both materials, the degree of retention of tensile mechanical properties is quite high in notched and unnotched quasi-isotropic laminates. A Digital Image Correlation technique has been used in order to understand the influence of temperature and matrix ductility on the mechanisms of overstresses accommodation near the hole. In fabric reinforced laminates, the high-temperature results suggest a competition between the mechanisms of damage, and the mechanism of plasticization, enhanced in angle-ply lay-ups. Thus, the highly ductile behavior of TP-based laminates, at temperatures higher than their T_g, is very effective to accommodate the overstresses near the hole.     

About the creep-fatigue interaction on the fatigue behaviour of off-axis woven-ply thermoplastic laminates at temperatures higher than Tg

The influence of plasticity and viscous effects on the fatigue behaviour of off-axis C/PPS laminates was investigated at temperatures higher than glass transition temperature. The obtained results clearly show that creep and fatigue are mutually influencing phenomena. Compared to the reference fatigue behaviour (with no prior loading), the fatigue life can be significantly extended with prior creep depending on loading conditions. Indeed, the strain accumulation seems to slow down after a long time creep preload, as if the time-dependent mechanisms were “evacuated” during this preload. The same conclusion can be drawn for the damage accumulation when the prior creep stresses are higher than the damage threshold or when the hold time is long enough, inducing significant plastic deformations. In angle-ply laminates, such deformations are associated with the reorientation of fibres. They contribute to the reduction of stress intensities, which results in increasing both fatigue life and maximum strain ɛ_max at failure during fatigue loadings.     

Physical aging in the creep behavior of thermosetting and thermoplastic composites

The creep behavior of a series of fiber-reinforced plastics (FRP) and corresponding resins has been studied, with emphasis on elucidating the role of physical aging effects on FRP viscoelastic behavior. Thermosetting and thermoplastic composites were studied, representing semicrystalline, amorphous, and highly filled amorphous polymer matrix FRPs. It was found that physical aging effects are operative for all FRPs, including the semicrystalline systems. Time/ aging-time and time/temperature superposition are found to be valid procedures for short-term creep behaviour; they cannot be applied to long-term creep behavior. However, long-term creep can be satisfactorily predicted from momentary creep by using an effective time theory. Evidence of a universal, temperature shift factor temperature dependence is presented.     

Investigations on stamping of C/PEEK laminates: Influence on meso-structure and macroscopic mechanical properties under severe environmental conditions

This study was aimed at addressing the influence of stamping on the mechanical performance (tensile, in-plane shear and inter-laminar shear) of fabric reinforced thermoplastic laminates under severe conditions. The effects of processing have been discussed at different levels: influence on the micro-structure (porosity and mean free path) and meso-structure (reinforcement and matrix distribution), changes in the matrix properties as well as in the fiber/matrix interface. The obtained results and the SEM observations suggest that these changes are closely associated with the macroscopic mechanical behavior of laminates. Stamping proved to be a re-consolidation process, and the high stamping pressure promotes two primary mechanisms: re-compaction of the fiber network and migration of melted matrix. These mechanisms significantly influence the meso-structure properties (better interlaminar adhesion and fiber/matrix bonding), resulting in the improvement of the material properties.     

Influence of stamping on the compressive behavior and the damage mechanisms of C/PEEK laminates bolted joints under severe conditions

Bolted joint tests have been performed in order to evaluate the influence of stamping on the behavior of thermoplastic-based woven-ply laminates subjected to structural loadings under severe service conditions (120 °C after hygrothermal aging). Compressive tests have been carried out on carbon fabrics reinforced PolyEtherEtherKetone (PEEK) laminates to investigate fibers buckling due to changes induced by stamping on the non-planar interply structure of woven-ply laminates. As compressive strength decreases by 13% in stamped laminates, it facilitates the plastic buckling of 0° and ±45° oriented fibers due to compressive loads in bolted joints. Contrary to double-lap joints, stamping does not affect the strength of single-lap joints, as the geometry of single-lap joints is non-symmetric. Stamping modifies the damage mechanisms of PEEK-based laminates under bolt-bearing loadings, such as the failure of stamped bolted joints is dominated by bearing failure mode.     

Determination of the damage threshold in woven-ply thermoplastic laminates at T > Tg: Acoustic emission and microscopic damage analysis

An original in situ measurement of acoustic emission (AE) was applied to monitor damage progress in discrete steps during gradual load/unload tensile tests on [±45°]₇ C/PPS laminates at temperatures T > Tg, when matrix ductility is enhanced. In order to understand the specific damage behavior of such materials under severe environmental conditions, AE analysis was accompanied by microscopic observations to detect the damage initiation threshold as well as the damage mechanisms within the composite material. Once the AE source mechanisms have been separated into classes thanks to the pattern recognition software Noesis, they have been identified to match physical phenomena. Earliest cracks events occur at the crimps where the rotation of warp/weft fibres takes place, followed by the intra-bundles splitting on free surface. It is observed that the onset of intralaminar cracking and debonding is affected by the presence of matrix-rich regions between the plies, because of an extensive plasticization of the PPS matrix. The study of the specific acoustic activity of neat PPS resin specimens confirms that the local plastic deformation in matrix-rich areas contributes to delay the initiation of damage, and subsequent AE signals. Finally, AE proved to be a relevant technique to investigate damage mechanisms and to determine accurately the damage threshold in TP-based composites to be used in aeronautical applications at T > Tg.     

Fatigue damage accumulation in notched woven-ply thermoplastic and thermoset laminates at high-temperature: Influence of matrix ductility and fatigue life prediction

This work was aimed at investigating the influence of matrix ductility on the high-temperature tensile fatigue behaviour in notched and unnotched C/PPS (thermoplastic) and C/Epoxy (thermoset) laminates. Damage mechanisms and overstress accommodation near the hole have been discussed by means of X-rays observations and fractography analysis. In order to quantitatively evaluate the fatigue damage within notched and unnotched laminates as a function of the cycles, a damage variable based on a mean strain calculated on each cycle from the experimental stress–strain loops has been used. Finally, a simple analytical model was applied to test its predictive capabilities to evaluate the fatigue damage accumulation in both materials. This model proved to be relevant to predict the evolution of fatigue damage in notched C/PPS composites but not in C/Epoxy laminates.     

Numerical simulation of the damage behavior of bidirectionally reinforced ceramic cross-ply laminates

The behavior of bidirectionally reinforced SiC/SiC cross-ply laminates is studied with the help of numerical simulations based on the finite element method (FEM). Within the presented model the composite is regarded on the layer scale considering each layer as homogeneous with `layer properties'. Brittle cracking as well as damage effects can appear within each layer, which is why both a damage and a fracture model for the plies is derived. The damage model is based on damage variables depending on the strain state. Fracture is checked using a fracture criterion, a crack and a post-failure model. In this way fracture can be considered for multiaxial stress states and the statistical distribution of strength values as well as load transfer effects after crack initiation can be taken into account. By subjecting the structure to a cooling down process before mechanical loading in one of the fiber directions the residual thermal stresses within the layers can also be regarded. The purpose of the simulations is to indicate the influence of important parameters on the composite behavior.     

Quasi-static penetration resistance behavior of glass fiber reinforced thermoplastic composites

Quasi-static penetration resistance of a composite structure represents the energy dissipating capacity of the structure under transverse loading without dynamic and rate effects. In this paper, a comparative study of the quasi-static penetration resistance behavior of S-2 Glass/SC-15, S-2 Glass/HDPE and E-Glass/HDPE composite systems with varying thicknesses, i.e., 1.4–8.4-mm, is presented using the Quasi-Static Punch Shear Test (QS-PST) methodology developed earlier. The penetration resistance behavior is usually presented by a series of force–displacement graphs at different support conditions, the integral of which is the energy dissipated by the composite during the quasi-static penetration at corresponding support conditions. The penetration energy varies with the diameter of the support span which is usually higher than the punch diameter, and also with the thickness of the composite laminate. During QS-PST experiments, a flat punch of diameter 7.6-mm with a range of support spans 8.89–50.8-mm has been used to obtain varying support span to punch diameter ratios (i.e., SPR = D_S/D_P = 1.16, 1.33, 1.67, 2.00, 2.33, 2.67, etc.). In order to compare the penetration resistance behavior of three different material systems, the S-2 Glass/SC-15, S-2 Glass/HDPE and E-Glass/HDPE composites of identical layer counts are used and the S-2 Glass/SC15 composite system is considered as the baseline. Composite plate specimens are sectioned after the test and then dipped into an ink–alcohol solution to study the damage mechanisms at different SPRs. Non-linear penetration stiffness and an average penetration resistance force are defined to quantify the average penetration resistance of each material. S-2 Glass and E-Glass reinforced HDPE composite material showed lower stiffness, lower peak force, higher deflection, lower damage area, and lower energy dissipation as compared to the baseline. A detailed comparison of results is presented.     

Effect of strain rate on tensile behavior of carbon fiber reinforced aluminum laminates

In this paper, the tensile behavior of carbon fiber reinforced aluminum laminates (CRALL) has been determined at a strain rate range from 0.001 s^− 1 to 1200 s^− 1. Experimental results show that CRALL composite is a strain rate sensitive material, and the tensile strength and failure strain both increased with increasing strain rate. A linear strain hardening model has been combined with Weibull distribution function to establish a constitutive equation for CRALL at different strain rates. The analysis of the model shows that the Weibull scale parameter, σ₀, increased with increasing strain rate, but Weibull shape parameter, β, can be regarded as a constant.     

Duration of the viscofluid state of thermosetting plastics under arbitrary heating conditions

A method for estimating the duration of the viscofluid state of a thermosetting material under nonisothermal heating during injection molding is developed on the basis of the generalized Arrhenius equation.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 58, No. 2, pp. 250–254, February, 1990.     

Response of fibre reinforced aluminium–lithium laminates to different fatigue conditions

Under fatigue conditions fibre reinforced aluminium–lithium laminates do not respond in the same manner as monolithic aluminium alloys. The variation of fatigue crack growth rates with initial loading condition has been examined for both carbon and glass fibre reinforced laminates, and compared with the behaviour of unreinforced 8090 aluminium–lithium alloy for a range of conditions (different initial nominal stress intensity factor range, load range and reversed loading). During fatigue, cracks grow in the metal layers of these laminates whilst the fibres in the crack wake remain intact, bridging the crack faces. The fibre bridging mechanism, inherent in this laminate system, reduces the fatigue crack growth rate. The magnitude of the bridging effect appears to be inversely related to the applied load range. This relationship can account for the behaviour observed in the performed experiments. This revised version was published online in November 2006 with corrections to the Cover Date.     

Study of reinforced concrete slabs after three years of cathodic protection under severe conditions

The objective of this study is to determine the influence of cathodic protection on the properties of concrete cover in concrete slabs. To accelerate deterioration, the slabs were subjected to freezing and thawing cycles and sprayed with sodium chloride solution. Cathodic protection was applied to the reinforcements for three years. Then, the test pieces underwent physico-chemical analysis. Scanning electron microscopy and X-ray spectrometry were used to study the reinforcement-concrete interface, in particular for detecting alkali-silica reaction due to the alkalization. Carbonation depths, chloride penetration profiles and examinations of the reinforcements showed the good condition of the concrete cover and the beneficial effect of the cathodic protection of embedded steel.     

Experimental study of thermo-mechanical behavior of a thermosetting shape-memory polymer

Mechanics of Time-Dependent Materials - The thermo-mechanical behavior of shape-memory polymers (SMPs) serves for the engineering applications of SMPs. Therefore the understanding of...     

Experimental study on mechanical behavior of laminates at low temperature

A series of tensile tests were conducted to research the mechanical behavior of the glass fiber and the carbon fiber reinforced epoxy laminates at low temperature (77 K). The specimens of laminates contained various stacking sequences (angle-ply) and notch geometry (central and edge notch). The curves of loading vs displacement of cross-head were recorded and the curves of stress-strain were got. The strengths of the various laminates were given too. Using a microscope-CCD imaging system, the growth of damage area was imaged. The concept of the energy dissipation density of laminates was presented and the data of the energy dissipation density for some laminates were got. The results showed that the strength and the energy dissipation density of laminates at 77 K are higher than those at 296 K.     

纤维增强热塑性树脂基复合材料界面研究进展

纤维增强热塑性树脂基复合材料具有优良的耐化学药品性、生产周期短、可二次加工等特点,克服了热固性树脂基复合材料韧性差,断裂延伸率低,易发生早期应力开裂等缺点,可在使用环境苛刻,承载能力要求高的场合得到应用.本文从复合材料界面设计思想入手,评述了纤维/热塑性复合材料界面的最新研究进展,并结合剪滞法对微复合材料的界面测试方法进行了分析讨论.     

石墨烯及碳化硅增强铝基复合材料的冲击力学行为

采用微机控制电子万能实验机和分离式霍普金森压杆(SHPB)对石墨烯增强的铝基复合材料和碳化硅增强的铝基复合材料进行准静态压缩实验和动态冲击实验,研究石墨烯增强铝基复合材料在不同应变率下的冲击力学性能,采用SEM扫描电镜研究石墨烯增强的铝基复合材料和碳化硅增强的铝基复合材料的形貌特征。结果表明:在各个应变率载荷下,添加石墨烯和添加碳化硅都增强了铝合金的屈服强度,其中,添加石墨烯对铝合金的屈服强度提升更加明显,但不影响材料的应变硬化率;相较于在材料中添加碳化硅,添加石墨烯弱化了材料的应变率效应,在高应变率条件下,添加石墨烯降低了材料的强度极限;选取部分实验数据,拟合确定了添加石墨烯和添加碳化硅两种复合材料的J-C和Z-A本构方程的参数,并比较了两种本构模型的预测能力,对于本工作所研究的复合材料,J-C模型的预测能力更好。     

Characterization of fatigue behavior of long fiber reinforced thermoplastic (LFT) composites

Fatigue behavior of long fiber reinforced thermoplastic composites (polypropylene/20 vol.% E-glass fiber) is presented in terms of stress – number of cycles to failure curves. Samples tested along longitudinal direction showed a higher fatigue life than the transverse samples which can be explained by the preferential orientation of the fibers along the longitudinal direction developed during the processing. Fatigue life decreased with increase in frequency. Hysteretic loss and temperature rise were measured; they depended on the stress amplitude as well as the cyclic frequency. Long fiber reinforced thermoplastic composite showed a lower temperature rise compared to unreinforced PP because long fiber reinforced thermoplastic has higher thermal conductivity than unreinforced PP and thus faster heat dissipation to the surroundings occur. The hysteretic heating also led to decrease in the modulus of long fiber reinforced thermoplastic as a function of number of cycles due to the softening of the matrix during fatigue cycling and depended on stress amplitude and frequency of the test.     

不同环境下宽径比对碳纤维增强环氧树脂复合材料开孔层压板压缩失效的影响

复合材料结构强度的参数影响研究是结构设计的必要内容,然而还缺乏在不同湿热环境条件下结构尺寸对其强度的影响研究。采用数值和试验方法研究了宽径比（W/D）对不同湿度、温度时T800/X850碳纤维增强环氧树脂复合材料（CF/EP）开孔层压板压缩强度的影响。设计了参数影响研究试件,通过试验获得了不同湿热条件下的开孔层压板压缩失效结果;并利用现有的考虑湿热影响的复合材料渐进损伤方法,建立了湿热及几何参数影响的渐进损伤模型,通过将预测结果与试验结果对比验证了模型正确性。进一步结合试验和数值分析,揭示了不同湿热条件下几何参数的影响规律。研究表明:湿热环境对T800/X850 CF/EP开孔板的压缩失效载荷影响显著,相比于室温干态（RTD）,室温湿态（RTW）和高温湿态（ETW）压缩失效载荷分别下降了7.75%和14.68%; RTW和RTD失效形式接近,ETW失效形式不同且失效面积更大; RTW和RTD时压缩失效强度随W/D的增大而增大,增大速度相似,ETW增大速度比前两者慢。