Nonlinear Response of Laminated Cylindrical Shell Panels Subjected to Thermomechanical Loads

The nonlinear response of multi-layered composite cylindrical shell panels subjected to thermomechanical loads are studied in this article. The structural model is based on the first order shear deformation theory incorporating geometric nonlinearities. The nonlinear equilibrium paths are traced using the arc-length control algorithm within the framework of finite element method. Hashin’s failure criterion has been adopted to predict the first-ply failure of cylindrical laminates. Both temperature independent and temperature dependent elastic properties are considered in the analysis. Specific numerical results are reported to show the effect of radius-to-span ratio, thickness-to-span ratio, laminate stacking sequence, and boundary condition on stability characteristics of laminated cylindrical shell panels subjected to combined thermal and mechanical transverse loads.     

Behavior of Composite Unreinforced Masonry–Fiber-Reinforced Polymer Wall Assemblages Under In-Plane Loading

An experimental investigation was conducted to study the in-plane behavior of face shell mortar bedded unreinforced masonry (URM) wall assemblages retrofitted with fiber-reinforced polymer (FRP) laminates. Forty-two URM assemblages were tested under different stress conditions present in masonry shear and infill walls. Tests included prisms loaded in compression with different bed joint orientation (on/off-axis compression), diagonal tension specimens, and specimens loaded under joint shear. The behavior of each specimen type is discussed with emphasis on modes of failure, strength and deformation characteristics. Results showed that the application of FRP laminates on URM has a great influence on strength, postpeak behavior, as well as altering failure modes and maintaining the specimen integrity. The retrofitted specimens reached compressive strength of 1.62–5.64 times that of their unretrofitted counterparts, depending on the bed joint orientation, and joint shear strength increased by eightfold.     

复合材料层板冲击破碎实验 与数值评估方法

纤维增强复合材料层板是电路板基板的重要组成部分, 预测其破碎过程是废旧电路板破碎机设计的重要依据.首先, 基于实验测得复合材料层板的冲击能量, 对比面内和面外两种冲击破碎方式.然后, 基于Hashin损伤理论建立复合材料层板冲击破碎预测模型, 预测的冲击消耗能量与冲击速度与实验结果吻合度高, 表明Hashin损伤理论可以应用于玻璃纤维复合材料层板冲击破碎问题.通过复合材料层板的面内和面外冲击损伤过程对比分析, 发现面内冲击破碎可以避免层间开裂造成的额外能耗, 所以沿平面方向的冲击破碎效果要好于沿厚度方向的破碎效果.当前冲击损伤预测模型可进一步应用于预测电路板基板的破碎过程, 也可用于指导电路板破碎机传动系统设计.      

薄铺层复合材料薄壁管轴压屈曲行为研究

在薄壁结构的应用中,屈曲稳定性是影响其承载性能的关键因素,为研究减薄铺层厚度对复合材料薄壁结构局部屈曲行为的影响,本文采用不同厚度(0.125、0.055和0.020 mm)的预浸料制备复合材料薄壁管,实验测试了其在轴压下的局部屈曲行为.实验结果表明,随着铺层厚度减薄,实验采用的正交和均衡两种铺层方式的复合材料薄壁管局部屈曲载荷均随之提高,而屈曲失效模式没有发生改变.力学分析表明,铺层厚度减薄后,管壁弯曲刚度的改变和层间剪切应力分布对薄壁管局部屈曲载荷提高有重要影响.采用薄铺层制备复合材料薄壁结构件能够有效提高其局部屈曲能力.      

Delamination Analysis for Laminated Composites. I: Fundamentals

Analytical models for the delamination of a class of composite laminates are developed. Two approaches are used in deriving the governing equations. The first approach follows a refined engineering bending theory that stems from the premise that the statically equivalent stresses obtained from classical theory can be used to estimate the strains ignored in the classical theory. The second follows a modified Donnell approach in which the stresses obtained from the classical engineering theory are improved by adding a series of corrections. These corrections are determined by satisfying the stress equilibrium and compatibility conditions of two‐dimensional elasticity theory. A comparison of the derived governing equations is provided in order to assess the consistency and accuracy of the engineering approach. The deformation modes associated with each model are identified. Interlaminar stresses predicted by the developed models for a quasi‐isotropic double cracked‐lap‐shear laminate are compared with finite‐element results.     

Interfacial Behavior and Debonding Failures in FRP-Strengthened Concrete Slabs

It has been demonstrated, through laboratory investigations and various field projects, that the external bonding of fiber- reinforced polymer (FRP) laminates is an effective technique for the structural enhancement of reinforced concrete slabs. In such applications, failure is generally governed by debonding of the FRP laminate. Nevertheless, numerical simulations to date of FRP-strengthened slabs have usually been based on the assumption of full bond between the concrete and FRP. In this study, the interfacial behavior between the FRP laminates and the concrete substrate is accounted for by introducing appropriate bond-slip models for the interface in a nonlinear finite-element analysis of FRP-strengthened two-way slabs. The numerical model is capable of simulating slabs strengthened in shear or in flexure; it can be applied to arbitrary FRP configurations, and can also accommodate both passive as well as prestressed FRP strengthening schemes. Results are presented in terms of load-deflection relationships, ultimate load capacities, failure modes, and interfacial slip and stress distributions. When compared to test results reported in the literature, the analysis is shown to lead to excellent predictions in that, for the entire set of FRP-strengthened specimens considered, the average of the numerical-to-experimental load capacity ratios is 0.966, with a standard deviation of 0.066. Furthermore, in all cases when FRP debonding was observed experimentally, the analysis correctly predicted the mode of failure.     

Failure mechanisms of laminated carbon-carbon composites—II. Under shear loads

Failure mechanisms under both interlaminar and in-plane shear loading are determined for two-dimensional carbon-carbon composites by using a direct shear set-up. This set-up is applicable for both types of shear loading, “as manufactured” laminate thickness can be tested without the need to make long samples by gluing different pieces together. A detailed finite element analysis, which considers the microstructure of the composite shows that for woven laminates, the initial crimp angle morphology does not allow the composite to deform in a state of simple shear. In fact, normal tensile and compressive stresses of almost twice the magnitude of the peak shear stress are produced in the vicinity of the crimped bundles. Consistent with these predictions, we observed the shear fault following the crimp boundaries in 0°/90° and quasi-isotropic laminates. Therefore, experimental techniques which can secure a state of pure shear stress in aligned, unkinked, uniaxial fiber composites cannot do so in woven laminated composites.     

Investigation of the load sequence effects in fatigue of damaged composite laminate & characterization of fatigue damage

Carbon fiber reinforced composites are widely used today in various areas and specially in aerospace industry for structural applications. This investigation focuses on the effect of different load sequencing and impact damage on the fatigue behaviour of CFC laminates. The specimens made from plain CFC laminates and low energy impact damaged CFC laminates were subjected to a typical flight block loading sequence and the fatigue strength degradation was monitored through stiffness measurement using load displacement data obtained during block loading. Three different stress/strain levels were used in testing. All the tests were performed using a computer controlled 100 kN servo-hydraulic test machine in load mode at room temperature and in lab air atmosphere on undamaged and low energy impact damaged composite laminates. Fatigue tests were performed with a sinusoidal waveform at 3 Hz. It was observed that lower strain levels did not show any significant effect on the fatigue properties in both the type of loading i.e. low to high and in high to low block loading in case of both the undamaged and impact damaged CFC specimens. Significant.reduction in stiffness was seen at higher strain level i.e. 6500me in both the undamaged and impact damaged CFC specimens. The low energy impact damaged specimens showed early failure at higher strain levels compared to undamaged specimens. The specimens were observed to have delaminated in the high stress fatigue cycling. The observed stiffness reduction due to fatigue cycling and the presence of delamination provide a means of macroscopic identification of fatigue strength degradation in composite materials. The energy plots appear useful tool to assess the damage growth.     

Model for Reinforced Concrete Members under Torsion, Bending, and Shear. I: Theory

A model has been developed that can predict the load-deformation response of a reinforced concrete (RC) member subjected to torsion combined with bending and shear to spalling or ultimate capacity. The model can also be used to create interaction surfaces to predict the failure of a member subjected to different ratios of applied torsion, bending, and shear. The model idealizes the sides of an reinforced concrete member as shear “wall panels.” The applied loads are distributed to the wall panels as uniform normal stresses and uniform shear stresses. The shear stress due to an applied torsional moment and shear force are summed over the thickness of the shear flow zone. Stress-strain relationships are adopted for tension stiffening and softened concrete in compression. The crack alignment rotates to remain normal to the principal tensile stress and the contribution of concrete in shear is neglected. The model has been validated by comparing the predicted and experimental behavior of members loaded under torsion combined with different ratios of bending and shear. The torque-twist behavior, reinforcement stress, and concrete surface strain predicted by the model were in agreement with experimental results.     

Evaluation of the MMCLIFE 3.0 code in predicting crack growth in titanium aluminide composites

Crack growth and fatigue life predictions made with the MMCLIFE 3.0 code are compared to test data for unidirectional, continuously reinforced SCS-6/Ti-14Al-21Nb (wt pct) composite laminates. The MMCLIFE 3.0 analysis package is a design tool capable of predicting strength and fatigue performance in metal matrix composite (MMC) laminates. The code uses a combination of micromechanic lamina and macromechanic laminate analyses to predict stresses and uses linear elastic fracture mechanics to predict crack growth. The crack growth analysis includes a fiber bridging model to predict the growth of matrix flaws in 0-deg laminates and is capable of predicting the effects of interfacial shear stress and thermal residual stresses. The code has also been modified to include edge-notch flaws in addition to center-notch flaws. The model was correlated with constant amplitude, isothermal data from crack growth tests conducted on 0- and 90-deg SCS-6/Ti-14-21 laminates. Spectrum fatigue tests were conducted, which included dwell times and frequency effects. Strengths and areas for improvement for the analysis are discussed. This article is based on a presentation made in the symposium “Fatigue and Creep of Composite Materials” presented at the TMS Fall Meeting in Indianapolis, Indiana, September 14–18, 1997, under the auspices of the TMS/ASM Composite Materials Committee.     

Prediction Models for Debonding Failure Loads of Carbon Fiber Reinforced Polymer Retrofitted Reinforced Concrete Beams

This study focuses on debonding failure in reinforced concrete beams with carbon fiber reinforced polymer composite bonded on the soffit using the wet lay-up method. An experimental study, which involved 26 tests, was carried out. The experiments showed two failure modes: Intermediate span debond and end debond. The first failure is the result of the high bond stress near the tip of a flexure-shear crack, whereas the second type of failure is due to the high shear stress developed in the weakest concrete layer at the tension reinforcement level. The experiments have shown that U-straps can be effective in preventing intermediate span and end debond. Based on experimental observations, two simple and practical theoretical models were developed and verified with the experimental data, together with a large database of other existing tests.     

Fatigue Behavior for Composite Laminates with Circular Hole

Based on the fatigue model of exponential function and WN criterion of static strength for the composite material laminates with a circular hole, the stress correct factorβ is presented. In order to gain the factorβ, the fatigue experiments of laminates with holes in different diameters and the same ratio of width and diameter. The fatigue behavior is usually accompanied with extensive damages. Those damages can affect composite materials in their strength and stiffness. The new model based on damage theory and strain equivalent hypothesis meets engineering requirement.T300/KH304, which is recently studied, is a high capability composite material. The fatigue analysis and tests of laminates with a hole in diameter of 5 mm are carried under difference stress levels. The simple, prompt and practical method was provided for the predication of fatigue life of composite material plate with a circular hole.     

Performance of a Three-Dimensional Hvorslev–Modified Cam Clay Model for Overconsolidated Clay

It is well established that critical state soil mechanics provides a useful theoretical framework for constitutive modeling of soil. Most of the critical state models, including the popular modified Cam clay (MCC) model, predict soil behavior in the subcritical region fairly well. However, the predictions for heavily overconsolidated soils, in the supercritical region, are not so satisfactory. Furthermore, the critical state models were developed from triaxial test data and extension of these models into three-dimensional (3D) stress space has not been investigated thoroughly. In the present work, experiments were carried out to obtain stress–strain behavior of overconsolidated soil in triaxial compression, extension, and plane strain conditions. A novel biaxial device has been developed to conduct the plane strain tests. The experimental results were used to formulate Hvorslev–MCC model which has MCC features in the subcritical region and Hvorslev surface in the supercritical region. The model was generalized to 3D stress space using the Mohr–Coulomb failure criterion. A comparison of the model predictions with test results has indicated that the Hvorslev–MCC model performs fairly well up to the peak supercritical point, during which deformations are fairly uniform and the specimens remain reasonably intact. Limitations of this simple model in predicting postpeak localization are also discussed. The model’s predictions for volumetric response in different shear modes seem to agree reasonably well with test results.     

Delamination Analysis for Laminated Composites. II: Numerical Verification

A validation of the delamination analysis models developed in a companion paper is provided through comparisons of predictions with finite‐element and elasticity solutions. The models are applied to the analysis of composite compression specimens reinforced with end tabs. An elasticity solution for the gage section of the specimens is developed. A comparison of the characteristic roots shows that the predictions of the models include the material and geometric parameters that control the behavior, and the roots corresponding to the basic stretching and bending modes are accurately predicted. The stress distribution at the interface between tabs and specimen is in good agreement with a finite‐element simulation. The interlaminar shear and peel stresses show an exponential increase with a maximum intensity at the free edges of the tabs. The behavior of previously tested specimens is explained; and practical guidelines for specimen design are provided to avoid unwanted extraneous modes of failure. The influence of the deformation modes associated with each model is investigated. An assessment of the accuracy and level of complexity is presented.     

Contribution to Shear Wrinkling of GFRP Webs in Cell-Core Sandwiches

Glass-fiber-reinforced polymer (GFRP) cell-core sandwiches are composed of outer GFRP face sheets, a foam core, and a grid of GFRP webs integrated into the core to reinforce the shear load capacity. One of the critical failure modes of cell-core sandwich structures is shear wrinkling, a local buckling failure in the sandwich webs because of shear loading. The shear wrinkling behavior of GFRP laminates with different laminate sequences, stabilized by a polyurethane foam core, was experimentally and numerically investigated. Shear wrinkling was simulated by a biaxial compression–tension setup. The results show that an increasing transverse tension load significantly decreases the wrinkling load. The decreasing effect of tension is explained by the lateral contraction because of Poisson’s effect, which causes an increase in the initial imperfections and subsequent accelerated bending.     

The multiple yield phenomenon in single-fiber composites of iron in copper

Repeated yielding has been observed in single fiber composites of iron in copper. During each yield event, a Luders band forms in the iron, propagates a short distance, and stops. The number of separate yield events decreases with increasing volume fraction of fiber until a critical volume fraction, above which only one yield event per specimen occurs. A model based on interfacial shear and a consequent reduction in Luders band propagation stress has been developed to explain this behavior. The results of the proposed model agree well with the observations. Finally, it is shown that the theories of the volume fraction dependence of composite strength after yield point drops and after fiber failure are quite similar in nature.     

Flexural Reliability of Reinforced Concrete Bridge Girders Strengthened with Carbon Fiber-Reinforced Polymer Laminates

This paper presents the development of a resistance model for reinforced concrete bridge girders flexurally strengthened with externally bonded carbon fiber-reinforced polymer (CFRP) laminates. The resistance model is limited to pure flexural failure and does not address shear failure, laminate debonding, or delamination. The resistance model is used to calculate the probability of failure and reliability index of CFRP-strengthened cross sections. The first-order reliability method is employed to calibrate the flexural resistance factor for a broad range of design variables. The study shows that the addition of CFRP improves reliability somewhat because the strength of CFRP laminates has a lower coefficient of variation than steel or concrete. However, the brittle nature of CFRP laminates necessitates a reliability index that is greater than that generally implied in the AASHTO LRFD for 1998. This leads to a lower resistance factor than is currently accepted for reinforced concrete sections in flexure.     

Impact Fatigue Damage of GFRP Materials Due to Repeated Raindrop Collisions

An impact fatigue study has been conducted for GFRP composite laminates to investigate failure mechanisms. A nylon bead with diameter of 4 mm was used as an impactor to simulate raindrop impact. Various specimen thicknesses of 3.0, 4.0 and 5.0 mm were used during experiment. Incident impact velocity of nylon bead ranged between 100 to 220 m/s. Optical microscopic observations were conducted to evaluate the damage at specimen center part of front and back surfaces. SEM investigations were made on the cross-section of damaged specimen. In conclusion, there are three damage modes were found to appear: debonding, matrix cracking, and delamination. Debonding occurred inside specimen at an early stage. Matrix cracking at front speciemens surface was ring crack, and that at back specimen surface was star crack. Delamination was resulted by repeated impacts. Initiation life for each damage mode depends on incident impact energy expressed as an (E–N) diagram of impact fatigue.     

Effect of Progressive Failure on Measured Shear Strength of Geomembrane/GCL Interface

This paper presents experimental data and numerical modeling results that illustrate the effects of progressive failure on the measured shear strength of a textured geomembrane/geosynthetic clay liner (GMX/GCL) interface. Large direct shear tests were conducted using different specimen gripping/clamping systems to isolate the effects of progressive failure. These tests indicate that progressive failure causes a reduction in measured peak shear strength, an increase in the displacement at peak, an increase in large displacement shear strength, and significant distortion of the shear stress–displacement relationship. A numerical model was developed to simulate progressive failure of a GMX/GCL interface. Measured and simulated shear stress–displacement relationships are in good-to-excellent agreement at four normal stress levels. The model was then used to investigate mechanisms of progressive interface failure and factors that control its significance. The results indicate that accurate measurements of shear stress–displacement behavior and strength are obtained when gripping surfaces prevent slippage of the test specimen and the intended failure surface has the lowest shear resistance of all possible sliding surfaces. The use of proper gripping surfaces is expected to reduce difficulties in test data interpretation and to increase the accuracy and reproducibility of test results.