Fatigue Behavior of Open-Holed CFRP Laminates with Initially Cut Fibers

Carbon fiber-reinforced plastic (CFRP) laminates with initially cut fibers (ICFs) have good formability without large degradation of static strength;however, their fatigue behavior has not been investigated thus far. In this paper, we investigated fatigue behavior and damage progress of open-holed CFRP laminates with ICFs having interlayers. Three types of CFRP laminates were employed: a laminate without ICF fabricated using an autoclave (Continuous-A), a laminate with ICF fabricated using an autoclave (ICF-A) and a laminate with ICF fabricated using press molding (ICF-P). First, fatigue test was conducted to obtain S (maximum stress)-N (the number of cycles to failure) curves in order to reveal fatigue strength. The fatigue tests for several specimens were interrupted at three prescribed numbers of cycles to observe damage progress. It is found that the Continuous-A laminate shows little strength degradation in the S-N curve while fatigue strength in both ICF laminates is decreased by approximately 30% at N of 106. In contrast, the damage progress of the ICF-P laminate is the least among the three laminates while the delamination progress at both edges and around the hole in the Continuous-A laminate is the most prominent.

Ballistic penetration damages of hybrid plain-woven laminates with carbon,Kevlar and UHMWPE fibers in different stacking sequences

Hybrid composite materials combine different fibers in preform and take advantages of different mechanical behaviors for improving ballistic impact damage tolerances.Here we report ballistic impact damages of plain-woven laminates with different hybrids and stacking sequences.Three kinds of hybrid laminates,i.e.,carbon/Kevlar,carbon/ultra-high molecular weight polyethylene(UHMWPE),and UHMWPE/Kevlar,had been prepared and tested in ballistic penetration with fragment simulating projectiles(FSP).The residual velocities of the projectiles and impact damage morphologies of the laminates have been obtained to show impact energy absorptions for the different hybrid schemes.A microstructural model of the hybrid laminates had also been established to show impact damage mechanisms with finite element analysis(FEA).We found that the UHMWPE/Kevlar hybrid laminates with Kevlar layers as the front face have the highest energy absorption capacity,followed by the carbon/Kevlar hybrid laminates with carbon layers as the front face.The main damage modes are fiber breakages,matrix crack and interlayer delamination.The ballistic damage evolutions from the FEA results show that the major damage is shear failure for front layers,while tension failure for the back layers.We expect that the ballistic impact performance could be improved from the different hybrid schemes.

Multi-stage penetration characteristics of thick ultra-high molecular weight polyethylene laminates

To further reveal the failure mechanisms of thick ultra-high molecular weight polyethylene(UHMWPE)laminates,field firing tests were conducted for 10-,20-,and 30-mm thick laminates against 12.7-mm calibre wedge-shaped fragment simulated projectiles at high velocities between 450 and 1200 m/s.The ballistic performance,deformation process,and staged failure characteristics of the laminates with different thicknesses were compared and analysed.The results demonstrate that the ballistic limits of the UHMWPE laminates increase almost linearly with laminate thickness.The 10-mm thick laminate generally experiences two-stage failure characteristics,whereas three-staged failure occurs in the 20-and 30-mm thick laminates and the progressive delamination is evident.The energy limit concept representing the maximum energy absorption efficiency and the idea of reuse of the thick UHMWPE laminates are proposed in this study.The findings of this research will be useful in the design of flexible and effective UHMWPE-based protective equipment.

Effects of Sinusoidal Vibration of Crystallization Roller on Composite Microstructure of Ti/Al Laminated Composites by Twin-Roll Casting

A new,innovative vibration cast-rolling technology of “electromagnetic stirring+dendrite breaking+asynchronous rolling” was proposed with the adoption of sinusoidal vibration of crystallization roller to prepare Ti/Al laminated composites,and the effect of sinusoidal vibration of crystallization roller on composite microstructure was investigated in detail.The results show that the metallurgical bonding of titanium and aluminum is realized by mesh interweaving and mosaic meshing,instead of transition bonding by forming metal compound layer.The meshing depth between titanium and aluminum layers (6.6μm) of cast-rolling materials with strong vibration of crystallization roller (amplitude 0.87 mm,vibration frequency 25 Hz) is doubled compared with that of traditional cast-rolling materials (3.1μm),and the composite interfacial strength(27.0 N/mm) is twice as high as that of traditional cast-rolling materials (14.9 N/mm).This is because with the action of high-speed superposition of strong tension along the rolling direction,strong pressure along the width direction and rolling force,the composite linearity evolves from "straight line" with traditional casting-rolling to "curved line",and the depth and number of cracks in the interface increases greatly compared with those with traditional cast-rolling,which leads to the deep expansion of the meshing area between interfacial layers and promotes the stable enhancement of composite quality.

Theoretical and experimental investigation of the resonance responses and chaotic dynamics of a bistable laminated composite shell in the dynamic snap-through mode

The dynamic model of a bistable laminated composite shell simply supported by four corners is further developed to investigate the resonance responses and chaotic behaviors.The existence of the 1:1 resonance relationship between two order vibration modes of the system is verified.The resonance response of this class of bistable structures in the dynamic snap-through mode is investigated,and the four-dimensional(4D)nonlinear modulation equations are derived based on the 1:1 internal resonance relationship by means of the multiple scales method.The Hopf bifurcation and instability interval of the amplitude frequency and force amplitude curves are analyzed.The discussion focuses on investigating the effects of key parameters,e.g.,excitation amplitude,damping coefficient,and detuning parameters,on the resonance responses.The numerical simulations show that the foundation excitation and the degree of coupling between the vibration modes exert a substantial effect on the chaotic dynamics of the system.Furthermore,the significant motions under particular excitation conditions are visualized by bifurcation diagrams,time histories,phase portraits,three-dimensional(3D)phase portraits,and Poincare maps.Finally,the vibration experiment is carried out to study the amplitude frequency responses and bifurcation characteristics for the bistable laminated composite shell,yielding results that are qualitatively consistent with the theoretical results.

Snap-through behaviors and nonlinear vibrations of a bistable composite laminated cantilever shell:an experimental and numerical study

The snap-through behaviors and nonlinear vibrations are investigated for a bistable composite laminated cantilever shell subjected to transversal foundation excitation based on experimental and theoretical approaches.An improved experimental specimen is designed in order to satisfy the cantilever support boundary condition,which is composed of an asymmetric region and a symmetric region.The symmetric region of the experimental specimen is entirely clamped,which is rigidly connected to an electromagnetic shaker,while the asymmetric region remains free of constraint.Different motion paths are realized for the bistable cantilever shell by changing the input signal levels of the electromagnetic shaker,and the displacement responses of the shell are collected by the laser displacement sensors.The numerical simulation is conducted based on the established theoretical model of the bistable composite laminated cantilever shell,and an off-axis three-dimensional dynamic snap-through domain is obtained.The numerical solutions are in good agreement with the experimental results.The nonlinear stiffness characteristics,dynamic snap-through domain,and chaos and bifurcation behaviors of the shell are quantitatively analyzed.Due to the asymmetry of the boundary condition and the shell,the upper stable-state of the shell exhibits an obvious soft spring stiffness characteristic,and the lower stable-state shows a linear stiffness characteristic of the shell.

Study of damage behavior and repair effectiveness of patch repaired carbon fiber laminate under quasi-static indentation loading

Damage caused due to low-velocity impacts in composites leads to substantial deterioration in their residual strength and eventually provokes structural failure.This work presents an experimental investigation on the effects of different patch and parent laminate stacking sequences on the enhancement of impact strength of Carbon Fiber Reinforced Polymers(CFRP)composites by utilising the adhesively bonded external patch repair technique.Damage evolution study is also performed with the aid of Acoustic Emission(AE).Two different quasi-isotropic configurations were selected for the parent laminate,viz.,[45°/45°/0°/0°]s and[45°/0°/45°/0°]s.Quasi Static Indentation(QSI)test was performed on both the pristine laminates,and damage areas were detected by using the C-scan inspection technique.Damaged laminates were repaired by using a single-sided patch of two different configurations,viz.,[45°/45°/45°/45°]and[45°/0°/0°/45°],and employing a circular plug to fill the damaged hole.Four different combinations of repaired laminates with two configurations of each parent and patch laminate were produced,which were further subjected to the QSI test.The results reveal the effectiveness of the repair method,as all the repaired laminates show higher impact resistance compared to the respective pristine laminates.Patches of[45°/0°/0°/45°]configuration when repaired by taking[45°/45°/0°/0°]s and[45°/0°/45°/0°]s as parents exhibited 68%and 73%higher peak loads,respectively,than the respective pristine laminates.Furthermore,parent and patch of configuration[45°/0°/45°/0°]s and[45°/0°/0°/45°],respectively,attain the highest peak load,whereas[45°/45°/0°/0°]s and[45°/45°/45°/45°]combinations possess the most gradual decrease in the load.

Dynamic and electrical responses of a curved sandwich beam with glass reinforced laminate layers and a pliable core in the presence of a piezoelectric layer under low-velocity impact

The dynamic responses and generated voltage in a curved sandwich beam with glass reinforced laminate(GRL)layers and a pliable core in the presence of a piezoelectric layer under low-velocity impact(LVI)are investigated.The current study aims to carry out a dynamic analysis on the sandwich beam when the impactor hits the top face sheet with an initial velocity.For the layer analysis,the high-order shear deformation theory(HSDT)and Frostig's second model for the displacement fields of the core layer are used.The classical non-adhesive elastic contact theory and Hunter's principle are used to calculate the dynamic responses in terms of time.In order to validate the analytical method,the outcomes of the current investigation are compared with those gained by the experimental tests carried out by other researchers for a rectangular composite plate subject to the LVI.Finite element(FE)simulations are conducted by means of the ABAQUS software.The effects of the parameters such as foam modulus,layer material,fiber angle,impactor mass,and its velocity on the generated voltage are reviewed.

ON THE CALCULATION OF ENERGY RELEASE RATE FOR VISCOELASTIC CRACKED LAMINATES

The energy release rate(ERR) of crack growth as the energy change at the same time t between the two states of the structure is redefined, one is with crack length a under the loading σ(t), the other is the state with crack length a+ Δ a under the same loading condition. Thus the defined energy release rate corresponds to the released energy when a crack grows from a to a+ Δ a in an infinitesimal time. It is found that under a given loading history, the ERR is a function of time, and its maximum value should correspond with the critical state for delamination to propagate. Following William’s work, the explicit expressions of ERR for DCB experimental configurations to measure the interfacial fracture toughness have been obtained through the classical beam assumption.

Two-dimensional analyses of delamination buckling of symmetrically cross-ply rectangular laminates

The conventional approach to analysis the buckling of rectangular laminates containing an embedded delamination subjected to the in-plane loading is to simplify the laminate as a beam-plate from which the predicted buckling load decreases as the length of the laminate increases. Two-dimensional analyses are employed in this paper by extending the one-dimensional model to take into consideration of the influence of the delamination width on the buckling performance of the laminates. The laminate is simply supported containing a through width delamination. A new parameterβ defined as the ratio of delamination length to delamination width is introduced with an emphasis on the influence of the delamination size. It is found that （i） when the ratio β is greater than one snap-through buckling prevails, the buckling load is determined by the delamination size and depth only; （ii） as the ratio β continues to increase, the buckling load will approach to a constant value. Solutions are verified with the well established results and are found in good agreement with the latter.

Nonlinear progressive damage model for composite laminates used for low-velocity impact

In order to effectively describe the progressively intralaminar and interlam- inar damage for composite laminates, a three dimensional progressive damage model for composite laminates to be used for low-velocity impact is presented. Being applied to three-dimensional （3D） solid elements and cohesive elements, the nonlinear damage model can be used to analyze the dynamic performance of composite structure and its failure be- havior. For the intralaminar damage, as a function of the energy release rate, the damage model in an exponential function can describe progressive development of the damage. For the interlaminar damage, the damage evolution is described by the framework of the continuum mechanics through cohesive elements. Coding the user subroutine VUMAT of the finite element software ABAQUS/Explicit, the model is applied to an example, i.e., carbon fiber reinforced epoxy composite laminates under low-velocity impact. It is shown that the prediction of damage and deformation agrees well with the experimental results.

DAMAGE PROGRESSIVE MODEL OF COMPRESSION OFCOMPOSITE LAMINATES AFTER LOW VELOCITY IMPACT

Compressive properties of composite laminates after low velocity impact are one of the most serious circumstances that must be taken into account in damage tolerance design of composite structures. In order to investigate compressive properties of composite laminates after low velocity impact, three dimensional dynamic finite element method (FEM) was used to simulate low-velocity impact damage of 2 kinds of composite laminates firstly. Damage distributions and projective damage areas of the laminates were predicted under two impact energy levels. The analyzed damage after impact was considered to be the initial damage of the laminates under compressive loads. Then three dimensional static FEM was used to simulate the compressive failure process and to calculate residual compressive strengths of the impact damaged laminates. It is achieved to simulate the whole process from initial low-velocity impact damage to final compressive failure of composite laminates. Compared with experimental results, it shows that the numerical predicting results agree with the test results fairly well.

Low velocity impact studies of E-glass/epoxy composite laminates at different thicknesses and temperatures

Low velocity impact experiments were carried out on E-glass/epoxy composite laminates having varying thicknesses at sub zero and elevated temperatures using hemi spherical steel impactor of 16 mm diameter with impact energies in the rage of 50-150 J.The performance of the laminates was assessed in terms of energy absorption,maximum displacement,peak force and failure behaviour.Results indicated that the effect of temperature on energy absorption of the laminate is negligible although the laminates are embrittling at sub zero temperatures.However it has influence on failure behaviour and displacement.Peak force has increased linearly with increase in laminate thickness from 5 to 10 mm.However it got reduced by 25% when temperature was increased from-20℃ to 100℃,Based on experimental results,laminate perforation energies were predicted using curve fitting equations.Statistical analysis was carried out using Taguchi method to identify the global effects of various parameters on laminate performance and confirmed that the laminate thickness has significant influence as compared to temperature,for the studied range.

Damping and Mechanical Properties of Cocured Composite Laminates with Embedded Perforate Viscoelastic Layer

The composite laminates with embedded acrylonitrile butadiene rubber （NBR） layer were fabricated by cocuring process. The embedded layers were perforated with a series of small holes to allow resin to flow through the damping layer and completely couple the structure to improve bending stiffness and interlaminar shearing strength of these cocured composite laminates. The damping, bending stiffness and shearing strength of these composite laminates with different perforation diameters were investigated. The experimental results show that increasing the perforation diameter leads to significant decreases in damping and significant increase in bending stiffness up to an area ratio of 7.065%. The area ratio here is defined as the ratio of perforation area to the total damping area. Beyond the area ratio of 7.065%, increasing the diameter to an area ratio of 50.24% results in only a slight variation in damping and bending stiffness. Moreover, increasing the perforation diameter does not always increase the shearing strength of the embedded viscoelastic layer. The shearing strength of embedded viscoelastic layer increases only when the area ratio is greater than 19.625%; instead, it will decrease.

Damage Detection and Material Property Reconstruction of Composite Laminates Using Laser Ultrasonic Technique

Laser ultrasonic technique has received increasing attentions in the past decade due to its contactless nature and a wide range of applications have been reported. In this review,applications of laser ultrasonic technique developed at Nanjing University of Aeronautics and Astronautics(NUAA)as well as elsewhere for non-destructive testing in composite laminates are presented. The principles of generating and detecting in a laser ultrasonic system are introduced,three different system configurations are also introduced with each configuration's advantages and disadvantages explained. More importantly,two major applications developed at NUAA for composite laminates are presented including damage detection,stiffness reconstruction and fatigue life prediction. Both applications are realized by a fixed-point PZT sensor and scanning pulse laser based on the linear reciprocal theorem. Analytical method and numerical models are employed and developed to realize the functionalities.

FATIGUE LIFE PREDICTION THEORY OF COMPOSITE LAMINATES AND EXPERIMENTAL VERIFICATION

According to traditional phenomenological fatigue methodology and moderncontinuum damage mechanics theory, dual fatigue cumulative damage rules to predict fatigue damageformation and propagation lives of the notched composite laminates are presented. A 3-dimensionaldamage constitutive equation of anisotropic composites is also established. Damage strain energyrelease rate is interpreted as a driving force of the fatigue delamination damage propagation. A newdamage evolution equation and a damage propagation σ_a-σ_m-N~* surface (stress amplitude-meanstress-life surface) are derived. Hence, using the method above, the fatigue life of compositecomponents can be predicted. Finally, theoretically predicted results are compared with experimentaldata. It is found that the deviation of theoretic prediction from experimental results is about22%.

A new higher-order shear deformation theory and refined beam element of composite laminates

A new higher-order shear deformation theory based on global-local superposition technique is developed. The theory satisfies the free surface conditions and the geometric and stress continuity conditions at interfaces. The global displacement components are of the Reddy theory and local components are of the internal first to third-order terms in each layer. A two-node beam element based on this theory is proposed. The solutions are compared with 3D-elasticity solutions. Numerical results show that present beam element has higher computational efficiency and higher accuracy.

Multi-objective Optimization of Co-cured Composite Laminates with Embedded Viscoelastic Damping Layer

Presented herein is a methodology for the multi-objective optimization of damping and bending stiffness of cocoured composite laminates with embedded viscoelastic damping layer. The embedded viscoelastic damping layer is perforated with a series of small holes, and the ratio of the perforation area to the total damping area is the design variable of the methodology. The multi-objective optimization is converted into a single-objective problem by an evaluation function which is a liner weigh sum of the two sub-objective functions. The proposed methodology was carried out to determine the optimal perforation area ratios of two viscoelstic layers with different perforation distance embedded in two composite plates. Both the optimal perforation area ratios are approximate to 2.2%. However, the objective value of the plate with greater perforation distance in embedded viscoelatic layer is much greater.

Effects of Cure Pressure Induced Voids on the Mechanical Strength of Carbon/Epoxy Laminates

This work aims at designing a set of curing pressure routes to produce laminates with various void contents. The effects of various consolidation pressures resulting in different void contents on mechanical strength of carbon/epoxy laminates have been examined. Characterization of the voids, in terms of void volume fraction, void distribution, size, and shape, was performed by standard test, ultrasonic inspection and metallographic analysis. The interlaminar shear strength was measured by the short-beam method. An empirical model was used to predict the strength vs porosity. The predicted strengths conform well with the experimental data and voids were found to be uniformly distributed throughout the laminate.

Extended Kantorovich method for local stresses in composite laminates upon polynomial stress functions

The extended Kantorovich method is employed to study the local stress concentrations at the vicinity of free edges in symmetrically layered composite laminates subjected to uniaxial tensile load upon polynomial stress functions. The stress fields are initially assumed by means of the Lekhnitskii stress functions under the plane strain state. Applying the principle of complementary virtual work,the coupled ordinary differential equations are obtained in which the solutions can be obtained by solving a generalized eigenvalue problem. Then an iterative procedure is established to achieve convergent stress distributions. It should be noted that the stress function based extended Kantorovich method can satisfy both the traction-free and free edge stress boundary conditions during the iterative processes. The stress components near the free edges and in the interior regions are calculated and compared with those obtained results by finite element method（FEM）. The convergent stresses have good agreements with those results obtained by three dimensional（3D） FEM. For generality, various layup configurations are considered for the numerical analysis. The results show that the proposed polynomial stress function based extended Kantorovich method is accurate and efficient in predicting the local stresses in composite laminates and computationally much more efficient than the 3D FEM.