GLARE层板挤压渐进损伤分析

对GLARE36/5层板进行挤压性能试验研究,采用超声C扫描、断口微距拍摄和扫描电子显微镜等方法观测GLARE层板挤压渐进损伤过程和最终破坏模式。结果表明:GLARE层板挤压起始损伤为铝合金塑性变形;损伤扩展阶段,0°纤维主要承受挤压正应力,铝合金塑性变形增大,铺层间分层起始并扩展;0°纤维屈曲折断后层内纤维基体损伤和分层损伤急剧扩展,层板最终发生挤压破坏。将GLARE层板挤压失效分为层内失效和层间失效,采用应变描述的Hashin准则和界面单元方法并引入金属塑性建立GLARE层板挤压渐进损伤数值模型,数值模型对层板损伤起始位置、分层产生位置、损伤演化过程、最终破坏模式及破坏载荷进行了预测,计算结果与试验结果吻合较好,说明该计算方法能够有效模拟GLARE层板挤压渐进损伤性能。      

The behaviour of Kevlar fibre-epoxy laminates under static and fatigue loadings. Part I—experimental

Kevlar 49 fibre and unidirectional Kevlar fibre reinforced plastic (KFRP) laminates both show an increase in stiffness under monotonic tensile loading. This stiffening effect is time-dependent and is reversible once the load is removed. In contrast, the modulus of a cross-ply KFRP laminate is affected primarily by matrix cracking of the transverse (90°) ply, and is sensitive to strain-rate and temperature. In cyclic (tensile) loading, however, the modulus of the cross-ply laminate depends on a combination of the fibre stiffening effect and transverse matrix cracking.     

Damage response of stitched cross-ply laminates under impact loadings

The impact response of stitched graphite/epoxy laminates was examined with the aim of evaluating the efficiency of stitching as a reinforcing mechanism able to improve the delamination resistance of laminates. The investigation, which focussed on two classes of cross-ply stacking sequences ([0₃/90₃]_s and [0/90]_3s), showed that the role of stitches in controlling damage progression of laminates and their capability to reduce the impact sensitivity of specimens are greatly dependent on the impact behaviour of base (unstitched) laminates. In [0₃/90₃]_s laminates, in particular, stitching is able to reduce damage area, on condition that the impact energy is higher than a threshold level and delaminations are sufficiently developed. In [0/90]_3s laminates, on the other hand, stress concentration regions generated by the stitching process appear to promote the initiation and propagation of fibre fractures, thereby inducing a decrease in the penetration resistance of the laminate.     

Post-impact static and cyclic flexural characterisation of hemp fibre reinforced laminates

The suitability for use of plant fibre composites after an impact event is still an open issue in literature. In this work, hemp fibre reinforced laminates have been subjected to cyclic flexural tests following falling weight impact at 12, 16 and 20 J. At these energies, still quite far from penetration, which was at about 40 J, damage starts to be increasingly apparent on both laminate surfaces. Post-impact flexural tests have been monitored using acoustic emission. Laminates with a sufficiently strong fibre–matrix interface have been obtained, as revealed from the impact hysteresis cycles and electron microscopy damage characterisation. However, a quite significant decrease of flexural properties and an increase of unrecoverable deformation after cyclical loading were also revealed with growing impact energies, in particular passing from 12 to 16 J.     

Damage characterization in thin-ply composite laminates under out-of-plane transverse loadings

Composite laminates with thin-ply layers are expected to exhibit superior damage resistance to the standard composite laminates. This study investigated the damage characteristics of carbon fiber/toughened epoxy thin-ply laminates subjected to transverse loadings. Quasi-isotropic laminates were prepared using both standard prepregs and thin-ply prepregs in order to examine the effect of ply thickness on the damage accumulation processes. Clear difference on damage accumulation process between standard laminates and thin-ply laminates was identified; fiber fractures were susceptible to formation in thin-ply laminates. Finally, the reason of the difference on damage process was investigated using finite element analyses, and it was clarified that the accumulated delamination position has a significant effect on the fiber fractures during the indentation.     

Strength and damage accumulation of carbon fabric composites with a cross-linked NBR modified epoxy under static and cyclic loadings

An epoxy matrix of carbon fabric composites (CFC) was modified using sub-micron cross-linked acrylonitrile butadiene rubber (CNBR) particles. The static tensile strength was improved by rubber modification by more than 15% in comparison with the strength of unmodified CFC when the rubber content was 10 wt%. The Young's modulus was little changed due to CNBR modification of the matrix. The interlaminar strength under Mode I loading increased more than 200% due to CNBR modification. AE observation reveals that damage accumulation is very different between unmodified and CNBR modified CFCs. Less fiber breaks for CNBR modified CFC occurred until final failure. Fatigue lives were also extended by CNBR modification. Although the slope of the S-N line of CNBR modified CFC is almost the same as that of unmodified CFC, the fatigue strength is much improved in comparison with the fatigue strength of unmodified CFC. The fatigue limit of 10% CNBR modified CFC is around 480 MPa while unmodified CFC fails at this cyclic stress within 10⁵ cycles. Differences in fractured surface and internal damage accumulation process between two CFCs are found in the fatigue case.     

Durability of hygrothermally aged graphite/epoxy woven composite under combined hygrothermal conditions

The objective of this study was to examine the effects of hygrothermal aging on the durability of a graphite/epoxy woven composite material system. The study was part of a larger project in which the objective was to evaluate and model the effects of moisture, temperature, and combined hygrothermal conditions on the strength and life of a graphite/epoxy woven composite material system. The results presented here represent an extension of the work by Patel and Case (Int. J. Fatigue 22 (2000) 809).

The hygrothermal aging consisted of cyclical temperature and moisture variations which were meant to simulate mission conditions for an advanced subsonic aircraft. Durability studies were carried out on the aged material system in the form of fatigue and residual strength testing under humid and elevated temperature environments. Damage mechanisms and failure modes were determined through fatigue testing, residual strength testing, and nondestructive evaluation.

Changes in physical appearance, thermal analysis results, fracture surfaces, and moisture diffusion behavior all supported the idea that the material was affected by the aging process. However, experimental testing also showed that the initial and residual tensile properties of the aged material were virtually unaffected by the imposed environmental aging (as compared to unaged material testing results), except when tested at elevated temperature. At elevated temperature, both the dynamic stiffness and residual strength were noticeably reduced from that at room temperature.     

Crack resistance curves of GLARE laminates by elastic compliance

The objective of this work was to study the applicability of the elastic compliance technique for crack resistance curves evaluation of commercial GLARE laminates using small SE(B) and C(T) specimens. The experimental evaluation of R-curves of 25.0 mm wide SE(B) specimens of unidirectional GLARE 1 3/2 0.3 and 50.0 mm wide C(T) specimens of bidirectional GLARE 3 5/4 0.3 was performed. Fracture toughness was measured through a recently proposed experimental methodology based on standardized specimens and elastic-plastic methodologies (J-integral and CTOD δ₅), whereas crack growth was measured optically and estimated by elastic compliance. According to the results the elastic compliance technique seemed to be applicable to GLARE laminates, accurately predicting stable crack growth during the tests.     

Characterization of magnesium spot welds under tensile and cyclic loadings

Resistance spot welds of a magnesium alloy were characterized in terms of microstructure, hardness and monotonic and cyclic properties. Microstructural features in base metal and different zones in the weld region were discussed and the mechanical behavior of spot welds in tensile–shear configuration was studied. Effects of welding parameters were investigated on the micro- and macro-scale characteristics of magnesium spot welds. To this end, five sets of spot weld specimens were prepared, utilizing different welding parameters. The effect of cyclic loading was studied on microstructure and hardness of the base metal and the weld region, and it was shown that microstructural features do not change remarkably under cyclic loading. Fatigue crack initiation and propagation behavior was discussed for different specimen sets under both low and high cyclic loads. Fatigue cracks under high cyclic loading initiated close to the nugget edge, and decreasing the cyclic load nucleated the cracks farther from the nugget.     

圆孔对GLARE层合板抗冲击性能的影响

为获得圆孔对玻璃纤维增强铝合金（GLARE）层合板抗冲击性能的影响规律,采用40 J的冲击能量对无孔和含圆孔GLARE层合板进行了落锤低速冲击试验,获得了冲击载荷、挠度和能量-时间曲线。应用ABAQUS/Explicit有限元分析软件对试验进行模拟,并预测了圆孔直径对GLARE层合板抗冲击性能的影响。结果显示:在低速冲击下,GLARE层合板纤维层的失效模式以分层损伤和纤维断裂为主;随着圆孔边缘至冲击中心距离的增加,层合板的冲击载荷峰值提高,而挠度峰值减小;数值模拟结果与试验结果的比较验证了模型的合理性;随着圆孔直径的增大,GLARE层合板的抗冲击性能逐步劣化。      

Alumina carbon/epoxy laminates under cyclic loading

The behaviour of alumina carbon/epoxy laminate under cyclic loading was investigated. The laminate was constructed by alternating dense alumina thin plates with unidirectional carbon/epoxy (C/E) prepreg tapes. Several cyclic load amplitudes were applied in unidirectional tension, corresponding to the stresses at onset of cracks in the alumina layers. The experimental results revealed high threshold stresses before damage occurred. These threshold stresses are matched with the stresses at onset of cracks in the alumina layers at static tensile tests. When the maximum stresses exceed this threshold, a very rapid stiffness reduction follows. The rate of loss of stiffness was examined. The short range rate was varied as a function of the maximum stress amplitude, but the long-term rate of loss of stiffness was found to be independent of the maximum stresses. A plastic shakedown mechanism was evident for cracked system undergoing high number of cycles, and is attributed to the nearly elastic plastic feature of the epoxy, the bonding agent between the alumina and the C/E layers. This revised version was published online in November 2006 with corrections to the Cover Date.     

Bearing strength of carbon epoxy laminates under static and dynamic loading

An experimental study has been carried out to investigate both the static and dynamic bearing strengths of a pinned-joint carbon epoxy composite plate with [0°/45°/−45°/90°]_S and [90°/45°/−45°/0°]_S stacking configurations. The static and dynamic experiments have been carried out according to the ASTM D953 standards and ASTM STP 749, respectively [ASTM D 953-D, Standard Test method for Bearing Strength of Plastics, ASTM Designation. 342; Joining of Composite Materials, ASTM STP 749, American Society for Testing and Materials (1981) 131]. The ratio of the edge distance to the pin diameter (E/D), and that of the width to the pin diameter (W/D) of the specimens were varied to obtain the static bearing strength and the S–N fatigue curve. The experiments show that the static bearing strengths reach their upper limit when E/D and W/D ratios are equal to or greater than 4 for both [0°/45°/−45°/90°]_S and [90°/45°/−45°/0°]_S stacking sequences. The fatigue strength, on the other hand, reduces by up to 65% as E/D and W/D ratios increase for both stacking configurations.     

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.     

Specificity of matrix cracking development in CFRP laminates under mechanical or thermal loadings

Fatigue tests at room temperature and thermal cycling experiments have been performed on carbon/epoxy laminates of cross-ply and complex stacking sequences. An ‘equivalent’ fatigue loading level has been evaluated in order to impose on the 90° plies the same amplitude of transverse stress σ₂₂ than in thermal cycling. A comparison of the matrix crack development throughout both types of tests has been undertaken: they have been found analogous, but with very far much faster kinetics under thermal cycling. Moreover, the fatigue test frequency has a significant influence on crack onset and development. However, it seems that the parameters ‘time’ and ‘transverse ply stress amplitude’ are not sufficient to completely explain the very fast cracking kinetics observed under thermal cycling.     

A generalized damage model for woven ply laminates under static and fatigue loading conditions

A generalized non-linear cumulative damage model for woven ply laminates subjected to static and fatigue loading is developed in this paper. The damage, consisting of small cracks running parallel to the fibers, leads to a loss of stiffness in the warp, weft and shear directions. The model presented here describes the evolution of the damage up to failure of the first ply. By replacing the woven ply by two stacked unidirectional plies corresponding to the warp and weft thicknesses, this general model is extended to cover a broad range of plies, from quasi-unidirectional to balanced woven plies. A continuum damage approach (CDM) is then used to define the behaviour of the two virtual unidirectional plies under static and fatigue loading conditions. The model is applied here to an unbalanced woven ply with glass reinforcement and the results of the simulations are compared with experimental data.     

A theoretical and experimental study on corrugated paperboard crushing under quasi‐static loadings

This paper presents 2 mathematical models to predict the initial peak stress and the plateau stress of corrugated paperboard which is simplified and regarded as the orthotropic plate under longitudinal compression. The resultant stress‐strain curve exhibits initial stiffening stage and a long plateau stage, where the initial peak stress determining the edgewise crush resistance of the board and most impact energy will be absorbed by the long plateau stage. By analyzing the elastic buckling of the board wall, a model of predicting the initial peak stress was obtained, while the plateau stress model was developed based on the gradual process of compression for corrugated board and energy conservation principle. Moreover, experiments were carried out to corroborate the presented model by comparing the predicted value with that by experiments, showing overall good agreement. It can be concluded that the proposed models can be applied for design of corrugated containers and cushioning packaging by corrugated paperboard.     

Damage modelling of laminated carbon/epoxy composites under static and fatigue loadings

This study deals with the modelling of the behaviour of laminated carbon/epoxy composites under static and fatigue loading. The non-linear cumulative damage model developed is written on the scale of the plies. It is based on a multi-criterion approach: brittle behaviour in the fibre direction, elastoplastic damage behaviour under shear and transverse tension stress, and elastoplastic behaviour under transverse compression loading. The range of validity of the model described here is limited to the first intra-laminar macro-crack, and it does not account for the delamination processes. This first-ply failure criteria model provides a conservative approach, which should be useful in some industrial context where the highest safety is required. Static tests results obtained on plate samples are presented [±45°]_3s and hybrid glass/carbon [0,90]_s are presented: they justify the modelling adopted here. The first tension/tension fatigue test results obtained on plate samples [±45°]_3s are presented: they are used to identify the parameters of the model contributing to the shear behaviour.     

Exact buckling solutions for composite laminates: proper free edge conditions under in-plane loadings

Summary This paper considers the elastic buckling of symmetric cross-ply laminated rectangular plates with two parallel edges simply supported, one edge free and the remaining edge free, simply supported or clamped. The first-order shear deformation plate theory is used in the analysis. An error apparently made by previous researchers for boundary conditions at free edges subjected to in-plane loads is corrected. Closed-form buckling factors are obtained using a generalised Levy-type solution method to solve the differential equations which govern the buckling behaviour of the laminates. Comparisons are made with previously published results, and the differences between buckling factors obtained with the appropriate and inappropriate free edge conditions are examined. The variation of buckling factors with plate aspect ratio, thickness ratio and the number of layers is investigated. Sets of first-known buckling solutions for cross-ply laminates are reported in design charts and tables.     

Localized fracture phenomena in thermo-visco-plastic flow processes under cyclic dynamic loadings

Summary The main objective of the paper is the investigation of localized fatigue fracture phenomena in thermo-viscoplastic flow processes under cyclic dynamic loadings. Recent experimental observations for cycle fatigue damage mechanics at high temperature and dynamic loadings of metals suggest that the intrinsic microdamage process does very much depend on the strain rate and the wave shape effects and is mostly developed in the regions where the plastic deformation is localized. The microdamage kinetics interacts with thermal and load changes to make failure of solids a highly rate, temperature and history dependent, nonlinear process.A general constitutive model of elasto-viscoplastic damaged polycrystalline solids developed within the thermodynamic framework of the rate type covariance structure with a finite set of the internal state variables is used (cf. Dornowski and Perzyna [16], [17], [18]). A set of the internal state variables is assumed and interpreted such that the theory developed takes account of the effects as follows: (i) plastic nonnormality; (ii) plastic strain induced anisotropy (kinematic hardening); (iii) softening generated by microdamage mechanisms (nucleation, growth and coalescence of microcracks); (iv) thermomechanical coupling (thermal plastic softening and thermal expansion); (v) rate sensitivity; (vi) plastic spin.To describe suitably the time and temperature dependent effects observed experimentally and the accumulation of the plastic deformation and damage during a dynamic cyclic loading process the kinetics of microdamage and the kinematic hardening law have been modified. The relaxation time is used as a regularization parameter. By assuming that the relaxation time tends to zero, the rate independent elasticplastic response can be obtained. The viscoplastic regularization procedure assures the stable integration algorithm by using the finite difference method. Particular attention is focussed on the well-posedness of the evolution problem (the initial-boundary value problem) as well as on its numerical solutions. The Lax-Richtmyer equivalence theorem is formulated, and conditions under which this theory is valid are examined. Utilizing the finite difference method for a regularized elasto-viscoplastic model, the numerical investigation of the three-dimensional dynamic adiabatic deformation in a particular body under cyclic loading condition is presented.Particular examples have been considered, namely a dynamic adiabatic cyclic loading process for a thin plate with sharp notch. To the upper edge of the plate is applied a cyclic constraint realized by rigid rotation of the edge of the plate while the lower edge is supported rigidly. A small localized region, distributed asymmetrically near the tip of the notch, which undergoes significant deformation and temperature rise, has been determined. Its evolution until occurrence of fatigue fracture has been simulated.The propagation of the macroscopic fatigue damage crack within the material of the plate is investigated. It has been found that the length of the macroscopic fatigue damage crack distinctly depends on the wave shape of the assumed loading cycle.