Failure of composite cylinders under combined external pressure and axial loading

An experimental and theoretical investigation was carried out into the collapse behaviour of filament wound glass fibre/epoxy cylinders under combinations of external pressure and axial loading in the third quadrant of the stress plane. Samples were tested with length-to-diameter ratios from 2·5 to 20 and diameter-to-thickness ratios in the approximate range of 20 to 40. Four ratios of hoop to axial stress were employed: ∞, 2, 1 and 0·5. The theoretical study employed a special purpose finite element program to calculate first ply failure (FPF) and buckling loads for shells of revolution made from multi-layered orthotropic materials. In all cases the experimental collapse pressure was strongly influenced by the predicted buckling failure mode. For those samples predicted to fail by buckling, agreement between the model and the experimental results was excellent. With the samples predicted to undergo FPF prior to buckling it was found that the residual strength was often sufficient to permit the buckling load to be approached.     

Residual strength of composite pipes under fire. Part 1

Fiber reinforced polymer composite structures have valuable characteristics for its use in marine and off‐shore applications. On the other hand, a potential problem with these structures is their behavior in the presence of fire. Most polymer matrix systems soften when the glass transition temperature is approached, and decompose at higher temperatures. The present paper describes preliminary research addressed to determine the behavior of water filled composite pipes under the effect of high temperatures. Approximate procedures are reviewed for the determination of the temperature‐time history and the corresponding decrement in strength. Then, an alternative procedure using thermal and viscoelastic numerical analyses is proposed.     

Procedure for strength calculation of laminated composite materials under thermomechanical loading conditions

We analyze the procedure for evaluating the load-carrying capacity of structures using complex-reinforced composite materials at the stages of design and checking calculations for strength. A general procedure for predicting the strength characteristics of laminated composite materials is developed that takes into account the temperature effect. The numerical examples of the implementation of this procedure are provided.     

爆炸荷载作用下悬索桥竖弯响应的数值模拟

针对大跨度悬索桥可能遭受的爆炸袭击,采用数值模拟方法研究了空中爆炸冲击波作用下悬索桥竖向弯曲响应。研究表明：悬索桥的竖弯响应过程可分为非稳态和稳态两阶段,所有构件的最大内力均发生在非稳态阶段。非稳态阶段相联构件间相互作用强烈,构件内力变化大。稳态阶段构件间的相互作用减弱,构件内力绕恒载值小幅波动。装药水平位置对加劲梁和吊杆最大内力影响显著,但对主索最大内力的影响不明显;加劲梁的最不利荷载位置在跨端,吊杆的最不利荷载位置在跨中     

Dynamic micromechanical modeling of textile composite strength under impact and multi-axial loading

Micromechanical finite element modeling has been employed to define the failure behavior of S2 glass/BMI textile composite materials under impact loading. Dynamic explicit analysis of a representative volume element (RVE) has been performed to explore dynamic behavior and failure modes including strain rate effects, damage localization, and impedance mismatch effects. For accurate reflection of strain rate effects, differences between an applied nominal strain rate across a representative volume element (RVE) and the true realized local strain rates in regions of failure are investigated. To this end, contour plots of strain rate, as well as classical stress contours, are developed during progressive failure. Using a previously developed cohesive element failure model, interfacial failure between tow and matrix phases is considered, as well as classical failure modes such as fiber breakage and matrix microcracking. In-plane compressive and tensile loading have been investigated, including multi-axial loading cases. Highly refined meshes have been employed to ensure convergence and accuracy in such load cases which exhibit large stress gradients across the textile RVE. The effect of strain rate and phase interfacial strength have been included to develop macro-level material failure envelopes for a 2D plain weave and 3D orthogonal microgeometry.     

Failure and strength of 2D-C/SiC composite under in-plane shear loading at elevated temperatures

In-plane shear strength (IPSS) of a two-dimensional carbon fiber reinforced silicon carbide composite (C/SiC) was measured by compression of double-notched specimens (DNS) from room temperature to 1873 K. The result indicates that the compression of DNS is an effective method to measure IPSS of C/SiC. A significant dependence of IPSS on temperature was found for C/SiC. IPSS increases with increasing temperature up to 1273 K, and then decreases when the temperature was higher. The main failure modes under the shear loading contain matrix cracking, delamination and pullout of fibers and fiber bundles.     

Hygrothermomechanical evaluation of porous media under finite deformation. Part I - finite element formulations

The coupled thermomechanical responses of fluid-saturated porous continua subjected to finite deformation are investigated. Field equations governing the transient response of the media are derived from a continuum thermodynamics mixture theory based on mass balance, momentum balance and energy balance laws as well as the Clausius-Duhem inequality. Finite element procedures for the two-dimensional response, employing updated Lagrangian formulations for the solid skeleton deformation and the weak formulations for fluid and thermal transport equations, are implemented in a fully implicit form. Temperature-dependent mechanical properties for the non-linear solid matrix, characterized by Perzyna's viscoplastic model, are assumed. An iterative scheme based on the full Newton-Raphson method is presented for simultaneously solving the coupled non-linear equations.     

Fatigue resistance of high-temperature alloys under cyclic temperature variation. Part 2. Method of strength and service life prediction under two-frequency nonisothermal loading

We propose a criterion of the limiting state of a metal under two-frequency nonisothermal loading. The fatigue curves that are calculated using the linear damage summation hypothesis for the studied materials and thermocycling regimes lie above the experimental curves, while the curves calculated using the proposed technique are in good agreement with experiment. Analysis shows that the described technique for calculating the service life and the fatigue limits of materials under cyclic nonisothermal loading is universal, effective, and fully adequate for engineering calculations.Translated from Problemy Prochnosti, No. 4, pp. 16–22, April, 1994.     

Damage evaluation and strain monitoring for composite cylinders using tin-coated FBG sensors under low-velocity impacts

Background/purposeThe impact-induced damage of composite structures induced by low-velocity impacts were evaluated to verify the damage evaluation concept using the “memory effects” of tin-coated FBG sensors.MethodsLow-velocity impact tests for the composite cylinder with tin-coated FBG sensors were performed at three impact energies. Hoop ring tests for the composite cylinder including impact-induced damage were additionally undertaken in order to measure the burst pressure and to study the parameter correlations. The test results were compared with the numerical results obtained by a finite element analysis (FEA) based on a continuum damage mechanics (CDM) considering damage model. The parameter correlations among the impact parameters and the residual strains induced by tin-coated FBG sensors were investigated based on the tests results.ResultsImpact behaviors obtained by the tests and the numerical simulation were agreed well. It was found that tin-coated FBG sensors can monitor the strain of the composite cylinder under low-velocity impacts and their strain monitoring capability is comparable to that of normally used FBG sensors. The residual strains of tin-coated FBG sensors were correlated with the impact parameters such as the impact energy, the sensing position of the sensors, and the burst pressure of the composite cylinder.ConclusionThe correlations among the residual strains and the parameters proved the damage evaluation concept for composite cylinders using the “memory effects” of tin-coated FBG sensors under low-velocity impact conditions; that is, the impact-induced damage, impact location, and burst pressure can be inversely evaluated by referring to the correlations.     

Modeling for fracture in materials under long-term static creep loading and neutron irradiation. Part 2. Prediction of creep rupture strength for austenitic materials

We present some results of prediction of creep rupture strength and plasticity for austenitic materials prior to and after irradiation with variable neutron flux rates, based on physicomechanical model as outlined in Part 1. The calculated results are compared with the available experimental data. __________ Translated from Problemy Prochnosti, No. 5, pp. 5–15, September–October, 2006.     

Numerical evaluation of J2-integral for rubbery materials under dynamic loads with finite element method

The J₂-integral for rubbery material problems with a stationary crack under dynamic large deformation is calculated with finite element solutions. General 2-D problems, including plane stress, plane strain, and axisymmetric cases, are considered in the formulation. The numerical procedure is verified to be path-independent in a generalized sense, and the near-tip region is hence always included in the calculation. A numerical treatment for simulation of the integration around the near-tip area is therefore proposed. It is demonstrated numerically that, with this treatment, the calculation is very insensitive to the crack tip finite element grid. In fact, no detailed information on the complicated asymptotic dynamic singular behavior for rubbery materials is required in the calculation.     

Buckling loads of CFRP composite cylinders under combined axial and torsion loading – experiments and computations

During the years 1994 through 1999, a European research project under the title “Design and Validation of Imperfection-Tolerant Laminated Shells” (DEVILS) was carried out. In this project, 11 European partners were involved. A goal of the project was an analytical and experimental study of the buckling behavior of thin-walled carbon fibre reinforced polymer (CFRP) laminated shells under combined axial and torsion loading. An additional aim was to compose a guideline for the dimensioning of such shells. This paper deals with the experimental and the analytical work conducted by DLR (Institute of Structural Mechanics, Braunschweig), ETH Zürich (former Institute of Lightweight Structures and Ropeways) and EMPA Dübendorf (Department of Polymers/Composites) in that project. The study was aimed at the determination of buckling loads of circular cylindrical shells of different laminate lay-ups. Nine shells were tested at DLR in Braunschweig for axial compression and at EMPA in Dübendorf under axial load and under combined axial compression and superimposed torsion. To determine the geometrical quality, the internal and the external surfaces of the specimens were mapped. ETH used photogrammetry and laser scanning prior to loading, while EMPA applied coordinate measurements for the unloaded shells and Moiré projection to monitor the lateral deflection of the cylindrical wall during loading and after buckling. At DLR, strain measurements were performed to assess regularity of the load distribution throughout the loading. The investigation showed that buckling loads of cylinders which are imperfection-sensitive under axial loading may not be so sensitive to combined loads. Furthermore, it was found that the stiffness eccentricity of the laminate played a significant role on the magnitude of the axial buckling load, while for combined loads this effect was somewhat reduced. This paper contains the results of those tests and also the comparison with results of analytical investigations and FE modeling; the obtained data can be used as benchmark reference.     

Fatigue strength of tubular structural elements under bending oscillations. Report 2. Fatigue strength of tubular structural members under programmed loading

Programs are presented for block loading with determined and random alternation of stress amplitude simulating the service loading spectrum. The results of fatigue tests of straight and bent tubular structural members are presented. It is concluded that low fatigue strength of bent tubular structural members is caused by the unfavorable technological effects of bending these pipes and by warping of the cross section during testing.Translated from Problemy Prochnosti, No. 4, pp. 89–93, April, 1994.     

面外载荷下含孔复合材料点阵夹层结构弯曲强度数值计算方法

通过实验手段和有限元方法针对含孔碳纤维增强树脂基复合材料(Carbon Fiber Reinforced Polymer,CFRP)点阵夹层结构在面外载荷作用下的失效模式及其影响因素进行了研究。首先通过实验获得了含孔CFRP点阵夹层结构的失效模式,其次建立了其有限元渐进损伤失效分析模型,基于该模型对开孔形状、开孔率及开孔位置对结构弯曲强度的影响进行了探讨。结果表明,当面板较厚时,含孔CFRP点阵夹层结构的主要失效模式为节点脱粘和面板皱曲;有限元计算结果和实验结果吻合较好,极限承载力的误差约为9.1%;当开孔率φ1.3%时,CFRP点阵夹层结构的弯曲承载能力与开孔形状基本无关;当开孔率1.3%≤φ8.5%时,含圆形孔夹层结构的弯曲承载能力较大;当开孔率φ8.5%时,含方孔夹层结构的弯曲承载能力较大;当开孔位于点阵夹层结构的几何中心或边缘时,对弯曲承载能力影响较大。     

Analysis of multi-layered filament-wound composite pipes under combined internal pressure and thermomechanical loading with thermal variations

The composite pipes manufactured by filament winding technology have anisotropic behavior owing to different reinforced ply angles. Composite pipes can be exposed to the thermomechanical loading due to hot fluid that flows into them. In this paper, based on the three-dimensional anisotropic elasticity, an exact elastic solution for thermal stresses and deformations of the pipes under internal pressure and a temperature gradient has been studied. Giving heat convection conditions the variation of temperature field within the pipe is obtained by solving the conduction equation at the wall. The influence of temperature field in the governing equations of thermoelasticity has been considered via a constitutive law. The shear extension coupling is also considered because of lay-up angles. Stress, strain and deformation distributions for different angle-ply pipe designs are investigated using the present theory.     

Numerical simulation of injection/compression liquid composite molding. Part 2: preform compression

In the injection/compression liquid composite molding process (I/C-LCM), a liquid polymer resin is injected into a partially open mold, which contains a preform of reinforcing fibers. After some or all of the resin has been injected, the mold is closed, compressing the preform and causing additional resin flow. This paper addresses compression of the preform, with particular emphasis on modeling three-dimensional mold geometries and multi-layer preforms in which the layers have different mechanical responses. First, a new constitutive relation is developed to model the mechanical response of fiber mats during compression. We introduce a new form of nonlinear elasticity for transversely isotropic materials. A special case of this form is chosen that includes the compressive stress generated by changes in mat thickness, but suppresses all other responses. This avoids the need to model slip of the preform along the mold surface. Second, a finite element method, based on the principle of virtual displacement, is developed to solve for the deformation of the preform at any stage of mold closing. The formulation includes both geometric and material nonlinearities, and uses a full Newton–Raphson iteration in the solution. An open gap above the preform can be incorporated by treating the gap as a distinct material layer with a very small stiffness. Examples show that this approach successfully predicts compression in dry preforms for three-dimensional I/C-LCM molds.