A study on residual fatigue bending strength and damage behavior of CFRP composites subjected to impact loadings

This paper evaluates the static and fatigue bending strengths of CFRP (carbon-fiber reinforced plastic) laminates having impact damages, e.g., foreign object damages (FOD). Composite laminates used in this experiment are CF/EPOXY and CF/PEEK orthotropy laminated plates with two-interfaces [0°₄/90°₄]_s A steel ball launched by an air gun collides against the CFRP laminates to generate impact damages. The damage growth during a bending fatigue test is observed by a scanning acoustic microscope (SAM). When the impacted side is compressed, the residual fatigue bending strength of CF/PEEK specimen P is greater than that of CF/EPOXY specimen B. On the other hand, when the impacted side is in tension, the residual fatigue bending strength of CF/PEEK specimen P is smaller than that of CF/EPOXY specimen B. In the case of impacted-side compression, the fracture is propagated from the transverse crack generated near the impact point. In the case of impacted-side tension, however, the fracture develops toward the impact point from the edge of interface-B delamination.     

Impact damage and hygrothermal effects on fatigue bending strength of orthotropy composite laminates

This paper focuses on fracture mechanisms experimentally based on the scanning acoustic microscope (SAM) when subjected to impact damages, i. e., foreign object damages (FOD), and also on the influence of impact damages and hygrothermals on residual fatigue bending strength of CFRP laminates. Composite laminates used in the experiments are CF/Epoxy orthotropy laminated plates, [0₄/90₄]_s. A steel ball launched by an air gun impacts on CFRP laminates to generate impact damages. Bending fatigue tests are periodically interrupted for a nondestructive evaluation (NDE) measurement of the progressive damages to build the fracture mechanism based on impact damages, and three-point fatigue bending tests are carried out to investigate the influence of hygrothermals on the effect of the residual bending fatigue strength of CFRP laminates.     

Nonlinear bending response of laminated composite spherical shell panel with system randomness subjected to hygro-thermo-mechanical loading

In the present paper, the effect of random system properties on transverse nonlinear central deflection of laminated composite spherical shell panel subjected to hygro-thermo-mechanical loading is investigated. System properties such as material properties, thermal expansion coefficients, hygro-contraction coefficients, load intensity and lamina plate thickness are taken as independent random variables. The higher order shear deformation theory and von-Karman nonlinear kinematics are used for basic mathematical formulation. The elastic and hygrothermal properties of the composite material, which are considered to be dependent on temperature and moisture concentration, have been obtained based on micromechanical modeling. A direct iterative based C⁰ nonlinear finite element method combined with mean centered first-order perturbation technique (FOPT) proposed by present authors for the plate is extended for the spherical shell panel subjected to hygro-thermo-mechanical loading. The influences of random system properties with plate geometry, stacking sequences, support conditions, fiber volume fraction and temperature, and moisture distributions on the response of laminated spherical shell panel are examined in detail. The performance of the proposed approach is validated through comparison with those available in the literature and independent Monte Carlo simulation (MCS).     

Limit loading of cylindrical shells subjected to local circumferential bending moments

Experiments have been carried out on twelve mild steel cylindrical shells each of which had a rigid radial square bar attached at its mid length. Strain and deformation patterns in the shell were measured when a hoop bending moment was applied to it through the attachment. Though some elastic stress distributions were obtained, the observations were mainly concerned with plastic deformations, the determination of plastic limit moments and the onset of instability. Measured deformation patterns were used to suggest a collapse mechanism from which a theoretical upper bound for the plastic limit moment was determined. Theoretical and experimental limit moments were compared for all specimens which had ratios varying between 13·75 and 67·5, and attachment half width/cylinder radius ratios varying between and .     

Fatigue strength and fracture behaviour of CHS-to-RHS T-joints subjected to out-of-plane bending

The fatigue behaviour of six different hollow section T-joints subjected to out-of-plane bending moment was investigated experimentally using scaled steel models. The joints had circular brace members and rectangular chord members. Hot spot stresses and the stress concentration factors (SCFs) were determined experimentally. Fatigue testing was carried out under constant amplitude loading in air. The test results have been statistically evaluated, and show that the experimental SCF values for circular-to-rectangular (CHS-to-RHS) hollow section joints were found to be below those of circular-to-circular (CHS-to-CHS) hollow section joints. The fatigue strength, referred to experimental hot spot stress, was in reasonably good agreement with referred fatigue design codes for tubular joints.     

Estimation of residual strength of components of composite constructions after low-speed impact

A methodology of computing the damageability of composite materials at low-speed impact is suggested. The methodology is based on applying a fracture criterion of the monolayer and following computation of material damageability taking the interaction of different damage evolution mechanisms into account. This approach allows us to obtain the distribution of damageability parameters in the surface of the monolayer and the width of material at an arbitrary moment of time until complete destruction of the construction component, computing the degradation of elastic properties, and the position and sizes of delamination regions after the impact. Applying this methodology for estimating residual strength permits the avoidance of excessive conservatism of the projected strength of construction components from composite materials which have incurred impact damage.     

Ultimate strength of composite laminates with free-edge delamination

The present paper deals with the free edge effect of composite laminates by using a generalized quasi-three dimensional analysis and experimental verification of an analysis performed for laminates with Teflon inserted on interfaces to simulate initial free-edge delamination. We performed tensile tests for laminates [30₂/?30₂/90]_s carbon-epoxy laminates with free-edge delamination under uniaxial tension. The experiment reveals that extensional stiffness of the laminate decreases by the initiation of the delamination, and that strength of the laminate without delamination is smaller than that of the laminates with delamination. Generalized quasi-three dimensional finite element technique, which employs energy release rate and maximum stress criteria, is developed to estimate behavior of the laminate after initial delamination. The numerical result by use of this technique predicts the ultimate strength of the laminates with sufficient accuracy according as the comparison with an experimental stress-strain curve. In the experiment conducted both for the laminate with initial delamination and the laminate without initial delamination, an unexpected results were obtained that is the ultimate load of the laminate without initial delamination was lower than that of the laminate with initial delamination. We presented clear explanation on the phenomenon occurred and developed the method to predict the nonlinear behavior of the laminate with or without initial delamination.     

Computer simulation of the human leg subjected to impact loading

One of the most important tasks in clinical practice is to determine accurately the forces and displacements of the various anatomical structural components of the human leg, especially when subjected to impact loading. This paper therefore develops a computer simulation which describes the biodynamic response of the human lower extremity for realistic activities such as kicking by foot etc. Firstly, the paper derives the nonlinear equations of motion and nonlinear constraint equation governing the sagittal plane response of the human leg subjected to impact loading. The equations are then solved using numerical techniques and the results are presented. Special attention is given to the ligament, contact and muscle group forces, the location of contact points, anterior-posterior displacements and the motion trajectories of the femur and the tibia.     

Some observations on the interlaminar strength of composite laminates

Definition of the influence of interlaminar stresses on the failure characteristics of composite laminates may require the development of novel experimental characterization procedures. The so-called free edge problem in laminate elasticity offers a cnovenient mechanism to accomplish this purpose because of the high interlaminar stresses in the neighbourhood of a free boundary. The detailed design of a laminate specimen which can exhibit catastrophic delamination induced by interlaminar tension is presented, along with a preliminary failure hypothesis to characterize this mode of rupture and the associated experimental demonstration of the phenomena.     

Evaluation of material properties and internal damage within CFRP and CFRP/Al sandwich panels due to impact loading

In the present paper, damage development within Carbon fiber reinforced plastic (CFRP) laminates and CFRP/Aluminum (Al) honeycomb core sandwich panels by impact loading was evaluated, and change in material properties due to the damage development was investigated. Falling weight impact tests, 3-point bending tests and cross-sectional observation were carried out. As results, it is found that falling rate of bending elastic modulus due to internal damage in the laminate only is lower than that in the upper face-sheet of the sandwich panel, and that difference in the falling rate between them becomes maximum at the impact energy of 5.1 J. As a result of investigating the relationship between reduction in bending elastic modulus and internal damage development, the reduction is caused by delamination within CFRP laminates mainly. Since total length of cracks in CFRP/Al honeycomb core sandwich panels is smaller than that in the laminates only, the sandwich panels have high impact tolerance because of absorption of impact energy by damaging Al honeycomb core.     

Terahertz spectroscopy approach of the fiber orientation influence on CFRP composite solid laminates

Terahertz ray (T-ray) imaging applications have provided one of the most promising new powerful nondestructive evaluation techniques, and new application systems are under process development for area applications. Detecting flaws and defects in fiber reinforced plastic (FRP) composite laminates due to flaws in FRP composite laminate that affect laminate properties, including stiffness, strength, and thermal behavior, is very important. In this study, a new time-domain spectroscopy system was utilized for detecting and evaluating the flaws in FRP solid composite laminates. Extensive experimental measurements in reflection mode were made to map out T-ray images. In particular, electromagnetic properties, such as refractive index, were estimated in this characterization procedure. The estimates of properties were in good agreement with known data. Using these characteristic material properties, we successfully demonstrated the characteristics of the T-ray behavior propagating through FRP composites. Furthermore, layup effect and flaws of FRP composite laminates were observed in reflection mode, and limitations were discussed in the T-ray processing.     

Low velocity impact and axial loading response of hybrid fibre aluminium laminates

Journal of Mechanical Science and Technology - Fibre metal laminates (FMLs) are layered materials based on stacking arrangements of metals and fibre reinforced plastic (FRP) composite laminates....     

复合材料层合板首末层失效强度的预测方法研究

建立复合材料的三维有限元模型,该模型能有效计算自由边界区的应力以及层间应力,与二维模型相比提高了求解精度;采用Tsai-Wu张量理论作为单元失效的判断依据,引入安全系数计算首层失效,然后使用增量法求解复合材料最终失效强度,使得求解速度加快的同时又不影响求解精度;使用正交实验方法研究不同刚度缩减系数( SCR)对首末层失效强度的影响,发现在进行渐进失效分析时,有限元模型是否合理应该根据首层失效强度与实验值进行比较,不能只考虑最终失效强度与实验值的差异.     

Modelling of cross-ply piezoelectric composite laminates in cylindrical bending with interfacial shear slip

An approximate theory for cross-ply piezoelectric composite laminates in cylindrical bending with interfacial shear slip is developed. This theory uses only 4 displacement and potential variables, the number of which is independent of the number of layers involved. The displacement and electric potential fields are depicted by the displacement and electric potential distribution functions through thickness, respectively. The two functions are formulated according to particular solutions to the three-dimensional elasticity equilibrium equations and electrostatics charge equation. In this shear slip modelling interfacial opening is neglected. The interfacial bonding conditions are characterised by a linear slip law and an electrically permeable assumption. A corresponding finite element is also developed to deal with piezoelectric laminates with local shear slip. The accuracy and effectiveness of the present theory are demonstrated in numerical examples.     

On impact damage detection and quantification for CFRP laminates using structural response data only

The overall purpose of the research is to detect and attempt to quantify impact damage in structures made from composite materials. A study that uses simplified coupon specimens made from a Carbon Fibre-Reinforced Polymer (CFRP) prepreg with 11, 12 and 13 plies is presented. PZT sensors were placed at three separate locations in each test specimen to record the responses from impact events. To perform damaging impact tests, an instrumented drop-test machine was used and the impact energy was set to cover a range of 0.37–41.72 J. The response signals captured from each sensor were recorded by a data acquisition system for subsequent evaluation. The impacted specimens were examined with an X-ray technique to determine the extent of the damaged areas and it was found that the apparent damaged area grew monotonically with impact energy. A number of simple univariate and multivariate features were extracted from the sensor signals recorded during impact by computing their spectra and calculating frequency centroids. The concept of discordancy from the statistical discipline of outlier analysis is employed in order to separate the responses from non-damaging and damaging impacts. The results show that the potential damage indices introduced here provide a means of identifying damaging impacts from the response data alone.     

Effect of prestress level on the strength of CFRP composite laminate

The effect of fiber prestressing has been investigated on the tensile strength, modulus strength, flexural properties and residual stresses of Carbon fiber reinforced polymer (CFRP) composites. Unidirectional carbon fiber in an epoxy resin has been examined under different prestressing levels (0, 5, 10, 15, 20, 30, 45, 60, 80 MPa) at ambient temperature and 50 % fiber volume fraction. A new method was implemented to produce the prestressed laminates for several standard tests. The results showed that prestressing on 3-ply unidirectional carbon fibers improved the fiber tensile strength to 99 % and the tensile modulus to 31 %. Similarly on 8-ply unidirectional carbon fibers has improved the fiber flexural strength to 63 % and flexural modulus to 81 %. A new technique was used to measure the residual stresses and tensile modulus of the composite laminate by recording the final extension and the remaining load directly after the curing process and releasing the applied load.

     

Fatigue damage of quasi-isotropic composite laminates under tensile loading in different directions

The purpose of this work is to investigate fatigue damage of quasi-isotropic laminates under tensile loading in different directions. Low cycle fatigue tests of [0/−60/60]_s laminates and [30/−30/90]_s laminates were carried out. Material systems used are AS4/Epoxy and AS4/PEEK. The fatigue damage of [30/−30/90]_s is very different from that of [0/−60/60]_s. The experimental results are compared with the result obtained from the method for determining strain energy release rate components proposed by the authors. The analytical results were in good agreement with the experimental results. It is proved that the failure criterion based on the strain energy release rate is an appropriate approach to predict the initiation and growth of delaminations under cyclic loading.     

Geometrically nonlinear bending analysis of Metal-Ceramic composite beams under thermomechanical loading

A new method is developed to derive equilibrium equations of Metal-Ceramic beams based on first order shear deformation plate theory which is named first order shear deformation beam theory2(FSDBT2). Equilibrium equations obtained from conventional method (FSDBT1) is compared with FSDBT2 and the case of cylindrical bending of Metal-Ceramic composite plates for non-linear thermomechanical deformations and various loadings and boundary conditions. These equations are solved by using three different methods (analytical, perturbation technique and finite element solution). The through-thickness variation of the volume fraction of the ceramic phase in a Metal-Ceramic beam is assumed to be given by a power-law type function. The non-linear strain-displacement relations in the von-Kármán sense are used to study the effect of geometric non-linearity. Also, four other representative averaging estimation methods, the linear rule, Mori-Tanaka, Self-Consistent and Wakashima-Tsukamoto schemes, by comparing with the power-law type function are also investigated. Temperature distribution through the thickness of the beams in thermal loadings is obtained by solving the one-dimensional heat transfer equation. Finally it is concluded that for Metal-Ceramic composites, these two theories result in identical static responses. Also the displacement field and equilibrium equations in the case of cylindrical bending of Metal-Ceramic plates are the same as those supposed in FSDBT2.     

Measurement of residual stress in thick section composite laminates using the deep-hole method

The deep-hole method is a method of measuring residual stress in large metallic components. In this paper, an extension to the deep-hole method is described to allow the residual stresses in thick section composite laminated plates to be evaluated. The method involves first drilling a small hole through the laminate perpendicular to the surface. The material around the hole is then machined away, resulting in a change in diameter of the hole due to the release of residual stress. This change in diameter is measured and used to calculate the residual stress. The calculation requires the evaluation of coefficients that depend on the properties of the composite. In this work, the finite element method is used to evaluate these coefficients. Using this method, the residual stresses in a 22 mm thick carbon/epoxy composite plate are measured and reported.     

Modelling the evolution of residual stresses during tensile testing of elastoplastic wires subjected to a previous bending operation

During coiling operations high residual stresses are frequently developed in steel wire. In this paper the stress distribution in wires during coiling, unwinding and subsequent tensile testing is modelled for numerous bending degrees, assuming perfect Voce plastic deformation and linear elastic behaviour. The influence of such residual stresses on the observed tensile test data can be deduced. It is shown that coiling with spool radii as used today industrially can lead to measurement of wire properties deviating significantly from the “true” properties of a properly coiled wire. Also, a method is proposed to deduce the original flow behaviour of coiled samples from tensile test curves, hence filtering the effect of the residual stresses.