Showing papers in "Composites Part B-engineering in 2002"

The revolutionary creation of new advanced materials - Carbon nanotube composites

Abstract: Since the discovery of carbon nanotubes at the beginning of the last decade, extensive research related to the nanotubes in the fields of chemistry, physics, materials science and engineering, and electrical and electronic engineering has been found increasingly. The nanotubes, having an extreme small physical size (diameter ≈1 nm) and many unique mechanical and electrical properties depending on its hexagonal lattice arrangement and chiral vector have been appreciated as ideal fibres for nanocomposite structures. It has been reported that the nanotubes own a remarkable mechanical properties with theoretical Young's modulus and tensile strength as high as 1 TPa and 200 GPa, respectively. Since the nanotubes are highly chemical insert and able to sustain a high strain (10–30%) without breakage, it could be foreseen that nanotube-related structures could be designed for nanoinstrument to create ultra-small electronic circuits and used as strong, light and high toughness fibres for nanocomposite structures. In this paper, recent researches and applications on carbon nanotubes and nanotube composites are reviewed. The interfacial bonding properties, mechanical performance and reliability of nanotube/polymer composites will be discussed.

Damage detection in composite materials using frequency response methods

Abstract: Cost-effective and reliable damage detection is critical for the utilization of composite materials. This paper presents part of an experimental and analytical survey of candidate methods for the in situ detection of damage in composite materials. The experimental results are presented for the application of modal analysis techniques applied to graphite/epoxy specimens containing representative damage modes. Changes in natural frequencies and modes were found using a laser vibrometer, and 2-D finite element models were created for comparison with the experimental results. The models accurately predicted the response of the specimens at low frequencies, but coalescence of higher frequency modes makes mode-dependant damage detection difficult for structural applications. The frequency response method was found to be reliable for detecting even small amounts of damage in a simple composite structure, however the potentially important information about damage type, size, location and orientation were lost using this method since several combinations of these variables can yield identical response signatures.

A modified pull-out test for bond of near-surface mounted FRP rods in concrete

Abstract: Among the strengthening techniques based on fiber-reinforced polymer (FRP) composites, the use of near-surface mounted (NSM) FRP rods is emerging as a promising technology for increasing flexural and shear strength of deficient concrete, masonry and timber members. In order for this technique to perform effectively, bond between the NSM reinforcement and the substrate material is a critical issue. Aim of this project was to investigate the mechanics of bond between NSM FRP rods and concrete, and to analyze the influence of the most critical parameters on the bond performance. Following up to previous investigations, a different type of specimen was designed in order to obtain a test procedure as efficient and reliable as possible. Among the investigated variables were: type of FRP rod (material and surface pattern), groove-filling material, bonded length, and groove size. Results of the first phase of the project are presented and discussed in this paper.

Strength prediction of bolted joints in graphite/epoxy composite laminates

Abstract: A parametric finite element analysis was conducted to investigate the effect of failure criteria and material property degradation rules on the tensile behaviour and strength of bolted joints in graphite/epoxy composite laminates. The analysis was based on a three-dimensional progressive damage model (PDM) developed earlier by the authors. The PDM comprises the components of stress analysis, failure analysis and material property degradation. The predicted load–displacement curves and failure loads of a single-lap single-bolt joint were compared with experimental data for different joint geometries and laminate stacking sequences. The stiffness of the joint was predicted with satisfactory accuracy for all configurations. The predicted failure load was significantly influenced by the combination of failure criteria and degradation rules used. A combination of failure criteria and material property degradation rules that leads to accurate strength prediction is proposed. For all the analyses performed, the macroscopic failure mechanism of the joint and the damage progression were also predicted.

The low velocity impact response of foam-based sandwich structures

Abstract: The low velocity impact response of a range of foam-based sandwich structures has been investigated using an instrumented falling-weight impact tower. Initially, the rate-sensitivity of the skin and core materials was investigated through a series of flexure and indentation tests. Here, it was shown that the flexural modulus of the skins and all 11 foam materials did not exhibit any sensitivity to crosshead displacement rate over the conditions studied here. In addition, it was shown that the indentation response of the sandwich structures could be modelled using a simple indentation law, the parameters of which did not exhibit any sensitivity to loading rate. Low velocity impact tests on the sandwich structures resulted in a number of different failure modes. Here, shear fracture was found to occur in the PVC/PUR systems based on brittle core materials. In contrast, buckling failures in the uppermost composite skin were observed in the intermediate modulus systems, whereas initial damage in the higher modulus PVC/PUR systems took the form of delamination within the top surface skin. It has been shown that a simple energy-balance model based on the dissipation of energy during the impact event can be used to successfully model the elastic response of foam-based sandwich structures. The energy-balance model is particularly useful since it can be used to establish the partition of energy during the impact process.

Free vibration analysis of composite sandwich plates based on Reddy's higher-order theory

Abstract: Two new C0 assumed strain finite element formulations of Reddy's higher-order theory are used to determine the natural frequencies of isotropic, orthotropic, and layered anisotropic composite and sandwich plates. The material properties typical of glass fibre polyester resins for the skin and HEREX C70 PVC (polyvinyl chloride) foam materials for the core are used to show the parametric effects of plate aspect ratio, length-to-thickness ratio, degree of orthotropy, number of layers and lamination scheme on the natural frequencies. A consistent mass matrix is adopted in the present formulation. The results presented in this investigation could be useful for a better understanding of the behaviour of sandwich laminates under free vibration conditions and potentially beneficial for designers of sandwich structures.

On the elastic modulus of hybrid particle/short-fiber/polymer composites

Abstract: In this investigation, the elastic modulus of hybrid particle/short-fiber/polymer composites was studied using the rule of hybrid mixtures (RoHM) equation and the laminate analogy approach (LAA). In the RoHM, such a hybrid composite was treated as a hybrid system consisting of two separate single systems, namely particle/polymer system and short-fiber/polymer system. Thus, no interaction between particles and short-fibers could be considered. Then, the elastic modulus of the hybrid composite was evaluated from that of the two single systems using the RoHM. In the LAA, particle-filled polymer was regarded as an effective-matrix. The interaction between particles and short-fibers was incarnated in a manner that short-fibers were incorporated into the effective-matrix containing particles. The modulus of the short-fiber reinforced effective-matrix composite was then estimated using the LAA. In addition, the two approaches were applied to previous experimental results. It was interestingly observed that the predicted values by the LAA and the experimental results for the elastic modulus of hybrid particle/short-fiber/polymer composites were consistently higher than those predicted by the RoHM, suggesting that the modulus of hybrid particle/short-fiber/polymer composites shows a positive hybrid effect.

A dual homogenization and finite element approach for material characterization of textile composites

Abstract: A novel procedure for predicting the effective nonlinear elastic moduli of textile composites through a combined approach of the homogenization method and the finite element formulation is presented. The homogenization method is first applied to investigate the meso-microscopic material behavior of a single fiber yarn based on the properties of the constituent phases. The obtained results are compared to existing analytical and experimental results to validate the homogenization method. Very good agreements have been obtained. A unit cell is then built to enclose the characteristic periodic pattern in the textile composites. Various numerical tests such as uni-axial and bi-axial extension and trellising tests are performed by 3D finite element analysis on the unit cell. Characteristic behaviors of force versus displacement are obtained. Meanwhile, trial mechanical elastic constants are imposed on a four-node shell element with the same outer size as the unit cell to match the force–displacement curves. The effective nonlinear mechanical stiffness tensor is thus obtained numerically as functions of elemental strains. The procedure is exemplified on a plain weave glass composite and is validated by comparing to experimental data. Using the proposed approach, the nonlinear behavior of textile composites can be anticipated accurately and efficiently.

The effects of geometric parameters on the failure strength for pin-loaded multi-directional fiber-glass reinforced epoxy laminate

Abstract: A numerical and experimental study was carried out to determine the failure of mechanically fastened fiber-reinforced laminated composite joints. E/glass–epoxy composites were manufactured to fabricate the specimens. Mechanical properties and strengths of the composite were obtained experimentally. Tests have been carried out on single pinned joints in [0/90/0]s and [90/0/90]s laminated composites. A parametric study considering geometries was performed to identify the failure characteristics of the pin-loaded laminated composite. Data obtained from pin-loaded laminate tests were compared with the ones calculated from a finite element model (PDNLPIN computer code). Damage accumulations in the laminates were evaluated by using Hashin's failure criteria combined with the proposed property degradation model. Based on the results, ply orientation and geometries of composites could be crucial for pinned laminated composite joints.

The effect of low temperatures on the intermediate and high velocity impact response of CFRPs

Abstract: The influence of low temperature on the damage produced on CFRPs by intermediate and high velocity impacts is analyzed. Spherical projectiles were launched against different carbon fiber/epoxy laminates (tape and woven). Experimental tests were done at temperatures ranging from 25 to −150 °C. The extension of the damage was measured by C-Scan. Results show a clear dependence of damage on temperature, impact velocity and the type of the laminate.

Finite element analysis of impact damage response of composite motorcycle safety helmets

Abstract: The energy absorption during impact provided by a motorcycle safety helmet is always of critical importance in order to protect the rider against head injury during an accident. In the present study, a parametric analysis has been performed in order to investigate the effect of the composite shell stiffness and the damage development during impact, on the dynamic response of a composite motorcycle safety helmet. This kind of parametric analysis may be used as a tool during helmet design for minimising testing needs. The LS-DYNA3D explicit hydrodynamic finite element code was used to analyse a detailed model of the helmet-headform system (composite shell/foam liner/metallic headform) and to simulate its dynamic response during impact. A significant part of the work was focused on the modelling of the mechanical behaviour of the composite materials, including damage and delamination development. The dynamic response of the different helmet-headform systems was judged in terms of the maximum acceleration monitored at the centre of gravity of the headform and the maximum value of head injury criterion. It was shown that composite shell systems exhibiting lower shear performance provide additional energy absorbing mechanisms and result to better crashworthiness helmet behaviour.

Monitoring of impact damages in composite laminates using wavelet transform

Abstract: Low-velocity impact damage is a major concern in the design of structures made of advanced laminated composites, because such damage is mostly hidden inside and cannot be detected by visual inspection. It is found that the acoustic emission (AE) waves generated by impact loads are undistinguishable from each mode and amount of damage by the conventional analysis methods in time or frequency domain. The wavelet transform (WT) can decompose the AE signals in time and wavelet scale domains, and catch the differences in these waves. It enables to distinguish the damage modes and size. This paper presents the application of the WT to detect the impact damage. As a fundamental approach, the characteristics of the AE signals due to matrix cracks and the evolution of free-edge delamination in [±452/02/902]S Gr/Ep laminates under tensile load were analyzed by the WT. Then the characteristics of impact damages of quasi-isotropic laminates were studied using the WT.

Development length of FRP straight rebars

Abstract: In recent years, some attempts have been performed to extend general design rules reported in the codes for steel reinforced concrete to Fiber Reinforced Polymer (FRP) materials; this is the case of relationships adopted in the evaluation of the development length clearly derived by extension of the formulations used for steel rebars. However, such relationships seem to be inappropriate for FRP reinforcing bars: in fact, experimental test results have shown that bond behaviour of FRP bars is different from that observed in case of deformed steel ones. As a consequence, a new procedure for the evaluation of development length based on an analytical approach is needed in order to directly account for the actual bond-slip constitutive law as obtained by experimental tests on different types of FRP reinforcing bars. An analytical solution of the problem of a FRP rebar embedded in a concrete block and pulled-out by means of a tensile force applied on the free end is presented herein. Such solution leads to an exact evaluation of the development length when splitting failure is prevented. Finally, based on the analytical approach, a limit state design procedure is suggested to evaluate the development length.

Nonlinear behavior of pultruded FRP composites

Abstract: Coupon tests are investigated and used to calibrate three-dimensional (3D) micromechanical models and to verify their prediction for the nonlinear elastic behavior of pultruded fiber reinforced plastic composites. The tested composite material system is made from E-glass/vinylester pultruded composite plate with both glass roving and continuous filament mat (CFM) layers. Tension, compression, and shear tests were performed, using off-axis coupons cut with different roving reinforcement orientations. The overall linear elastic properties are identified along with the nonlinear stress–strain behavior under in-plane multi-axial tension and compression loading. The tests were carried out for coupons with off-axis angles: 0, 15, 30,45, 60, and 90°, where each test was repeated three to five times. Finite element analyses are used to simulate the off-axis tests and examine the effects of coupon geometry, end-clamping condition, and off-axis orientation, on the spatial distribution of the axial strains at the center of the coupons. Lower initial elastic modulus and a softer nonlinear stress–strain responses were consistently observed in the tension tests compared to those in compression, for all off-axis (roving) orientations. The nonlinear behavior can be attributed to the relatively low overall fiber volume fractions (FVFs) in pultruded composites and the existence of manufacturing defects, such as voids and microcracks. It is also shown that the end-clamping effects for the tested geometry are relatively small at the center and allow extracting the nonlinear stress–strain response of the anisotropic material. The analytical part of this study includes two (3D) micromechanical models for the roving and CFM layers. Shear tests are used to calibrate the in situ nonlinear elastic properties of the matrix. Good prediction ability is shown by the proposed micromodels in capturing the stress–strain behavior in the off-axis tests.

A CDM-based failure model for predicting strength of notched composite laminates

Abstract: This paper investigates the ultimate tensile failure strength of laminated composites containing a central circular hole. Based on continuum damage mechanics, a Principal Damage Model is developed by combining the generalized standard material model with the Principal Damage concept of composite materials. Three in-plane failure modes: fiber breakage, matrix cracking, and fiber/matrix interface debonding are included in the present model. After obtaining material constants and damage relations from standard tensile tests, the material constitutive relations with damage model are implemented into commercial finite element code, abaqus . By comparing the predicted results with the experimental data, the proposed model has proven to be capable of predicting failure strength and load–deflection relations of notched laminated composites. The effects of hole size and specimen width are discussed in detail. In addition, the advantage of the present model is demonstrated through comparison with other existing models.

Modeling of functionally graded materials in dynamic analyses

Abstract: In this investigation, functionally graded material is modeled in several different ways. Five models are presented, two of which simulate fiber phases and three simulate particle phases. For fibers, there is a model in which the detailed micro-structure is simulated and one in which the material is represented by layers such that the volume fraction of the fibers in each layer changes. For the particles, a model with layers is employed and two models with continuously changing material parameters are presented. Four different dynamic input loads are applied to the detailed micro-structure to examine its effect. The finite element method is employed to determine the effective stress. Then one of the dynamic loads which simulates a step function is applied to all models. It is observed that there are no significant differences in the effective stresses at particular points within the time domain. The amplitude of the wave for each model is quite similar. The phase of the wave shifts as time increases. Thus, in the space domain, differences are observed in the effective stress at a particular time. As may be expected, the stresses are rather high within the fibers in the detailed micro-structural model. It is concluded that a continuously changing material model is a good candidate for carrying out dynamic analyses of functionally graded material.

Mechanical properties of Zylon/epoxy composite

Abstract: The Zylon/epoxy composite is formed by wet-winding Zylon fibre, poly( p -phenylene-2,6-benzobisoxazole) or PBO, with the epoxy (Stycast 1266). The effects of pre-stress on the distribution and the filling factor of the fibre are studied. It is shown that a Zylon/epoxy composite with a uniform fibre distribution and a very high filling factor is achievable. The mechanical properties of the Zylon/epoxy composite at room temperature and 77 K are investigated by uni-axial tensile tests and transverse compression tests. The results indicate that the ultimate tensile strength (UTS) of the Zylon/epoxy composite is mainly determined by the fraction of the Zylon fibre. The UTS of the Zylon fibres in the composite is found to be larger than 4.3 GPa. Due to the easy processing and the very high UTS, the Zylon/epoxy composite is suitable as reinforcement material for high-field magnet coils.

Strength and durability performance of concrete axially loaded members confined with AFRP composite sheets

Abstract: The performance of concrete columns externally wrapped with aramid fiber reinforced polymer composite sheets is presented in this paper. The confined and unconfined (control) specimens were loaded in uniaxial compression. Axial load and axial and hoop strains were measured in order to evaluate stress–strain behavior, ultimate strength, stiffness, and ductility of the wrapped specimens. Results show that external confinement of concrete by fiber reinforced polymer (FRP) composite sheets can significantly enhance strength, ductility and energy absorption capacity. An analytical model developed earlier by the author to predict the entire stress–strain response of concrete specimens wrapped with FRP composite sheets was applied. Comparison between the experimental and analytical results indicates that the model provides satisfactory predictions of the stress–strain response. The paper also presents the performance of the wrapped concrete specimens subjected to severe environmental conditions such as wet–dry and freeze–thaw cycles. The specimens were exposed to 300 cycles of wetting and drying using salt water. Results show that specimens wrapped with aramid fibers experienced no reduction in strength due to wet/dry exposure, but some reduction was observed due to freeze/thaw exposure.

The fatigue and final fracture behavior of SiC particle reinforced 7034 aluminum matrix composites

Abstract: In this research paper, the cyclic stress-amplitude-controlled fatigue response and fracture behavior of aluminum alloy 7034 discontinuously reinforced with silicon carbide particulates (SiCp) is presented. In view of the limited ambient temperature ductility, test specimens of the 7034/SiCp composite, in both the under-aged and peak-aged conditions, were cyclically deformed under stress-amplitude-control at an elevated temperature corresponding to the aging temperature of the alloy. The cyclic fatigue tests were conducted at two different load ratios with the objective of documenting the conjoint influences of intrinsic composite microstructural effects, nature of loading, and magnitude of cyclic stress amplitude on cyclic fatigue life and fracture characteristics. The final fracture behavior of the composite is discussed in light of the concurrent and mutually interactive influences of composite microstructural effects, deformation characteristics of the composite constituents, nature of loading, and resultant fatigue life.

An inverse procedure for identification of loads on composite laminates

Abstract: An inverse procedure is presented to determine the transient line loads on a surface of composite laminate. The procedure recovers the time history as well as the distribution functions of the line loads based on the displacement responses at one receiving point. It is assumed that the time and spatial dependencies of the loading function are separable. The hybrid numerical method is used to obtain two kernel functions, dynamic Green's function as well as the response function of Heaviside step excitation of the composite laminates. The displacement response to a load with an arbitrary force function is expressed in a form of convolution, where the continuous convolution functions are spatially and temporally discretized. The loading functions are recovered by optimizing a set of proposed error (objective) functions. Numerical verifications were performed to identify loads on composite laminate for both concentrated and extended cases. Very good agreements have been obtained in terms of both load distribution and its magnitude, where calculation converged within a small number of iterations.

Deformation and life analysis of composite flywheel disk systems

Abstract: In this study an attempt is made to put into perspective the problem of a rotating disk, be it a single disk or a number of concentric disks forming a unit. An analytical model capable of performing an elastic stress analysis for single/multiple, annular/solid, anisotropic/isotropic disk systems, subjected to pressure surface tractions, body forces (in the form of temperature-changes and rotation fields) and interfacial misfits is summarized. Results of an extensive parametric study are presented to clearly define the key design variables and their associated influence. In general the important parameters were identified as misfit, mean radius, thickness, material property and/or load gradation, and speed; all of which must be simultaneously optimized to achieve the ‘best’ and most reliable design. Also, the important issue of defining proper performance/merit indices (based on the specific stored energy), in the presence of multiaxiality and material anisotropy is addressed. These merit indices are then utilized to discuss the difference between flywheels made from PMC and TMC materials with either an annular or solid geometry. Finally two major aspects of failure analysis, that is the static and cyclic limit (burst) speeds are addressed. In the case of static limit loads, a lower (first fracture) bound for disks with constant thickness is presented. The results (interaction diagrams) are displayed graphically in designer friendly format. For the case of fatigue, a representative fatigue/life master curve is illustrated in which the normalized limit speed versus number of applied cycles is given for a cladded TMC disk application.

Shear strengthening effect by bonded composite fabrics on RC beams

Abstract: An experimental investigation was conducted to study the shear strengthening effect by bonded carbon fibre reinforced fabrics on the ultimate strength and behaviour of RC beams. A total of sixteen RC beams with or without the carbon fibre reinforced fabrics were tested in a load system by a point load at one-third of the length. The experimental results indicate that contribution of the composite fabrics on the shear capacity of the strengthened beam varies according to the applied composite fabric area, the spacing between the steel stirrups and the longitudinal steel bar diameter of the RC beam. The load–strain behaviour of the RC beam is improved by bonding the composite fabrics. The effect of the shear strengthening on the deflection, the longitudinal steel bar strains, the concrete strains, the stirrup and the composite fabric strains depends strongly on the strengthening area.

Cold-temperature and simultaneous aqueous environment related degradation of carbon/vinylester composites

Abstract: The effects of non-cryogenic, cold temperature conditions especially as related to freeze–thaw cycling both in the presence of aqueous solutions, and just ambient humidity, on carbon fiber composites is not well understood. Based on processing characteristics and lower cost, vinylester resin systems are increasingly being used in civil infrastructure and in off-shore applications. The use of these systems with carbon fibers causes some concern due to the current lack of appropriate finish on the fibers. In addition the use of ambient cure processes such as wet layup and resin infusion raise concerns of durability since these resin systems typically do not achieve full conversion and are thus susceptible to moisture induced degradation. This paper reports on results of short-term exposure of thin ambient cured carbon/vinylester specimens, processed following wet layup procedures similar to those used in rehabilitation of structural components, to freeze and freeze–thaw cycling. It is shown that freeze–thaw can cause significant reduction in mechanical properties and in glass-transition temperature with immersion in salt water having a larger effect on fiber-matrix bond deterioration and matrix cracking than other exposures.

Development of flexural capacity of a FRP prestressed beam with vertically distributed tendons

Abstract: FRP prestressed concrete beams with tendons vertically distributed throughout the section are susceptible to failure due to the progressive fracture of the most highly stressed tendons. This paper presents a theoretical approach to account for vertically distributed tendons, and discusses a design methodology to maximize the use of all tendons in the section. A discussion of full-scale tests associated with the theoretical development is included.

An analytical assessment of using the losipescu shear test for hybrid composites

Abstract: A theoretical evaluation of the applicability of the Iosipescu test (v-notch shear test) has been conducted for hybrid composites having unidirectional glass and carbon fiber tows that are intimately mixed, instead of being arranged in separate lamina. The v-notch specimen of hybrid composites was analyzed using the finite element method based on the fiber tow properties to evaluate the effect of varied microstructures in hybrids on the shear stress and strain states. The analyses were conducted to determine how closely the test would meet the requirement of an ideal shear test that there should be pure and uniform stress and strain distributions in the test region. The study shows that, theoretically, the v-notch test can be used to determine the shear modulus of the hybrid composites studied when it is correctly used. However, practically, premature failures caused by the stress concentrations near the notch roots can make the test undesirable for determining the shear strength of the hybrid composite.

A study of short-beam-shear and double-lap-shear specimens of glass fabric/epoxy composites

Abstract: Two experimental methods for determining the inter-laminar shear strength (ILSS), are compared: the short-beam-shear (SBS) and the double-lap-shear (DLS) test method. Specimens with a constant ply angle for all layers are considered. The experimental results show a significant difference (up to 50%) in the obtained ILSS. A finite element analysis shows that both test methods underestimate the real ILSS and demonstrate that the standardized ILSS evaluation procedures are more or less valid for the SBS test, but require modifications in the case of the DLS test. Acoustic emission measurements and numerical investigations were performed to determine the real ILSS from the DLS results. The real ILSS cannot be obtained from the SBS test without an extended analysis. It is, however, possible to determine bounds for the real ILSS from the SBS results.

On the properties of quasi-isotropic laminates

Abstract: The three-dimensional thermoelastic properties of quasi-isotropic laminates are derived in closed form equations, in terms of the principal lamina properties. The equations describe both the inplane and the out-of-plane properties, applying the Classical Laminate Theory and the averaging method proposed by Goetschel and Radford, J Adv Mater 28 (1997) 37. A worked example illustrates the results and indicates the limitations when bending plays a significant role.

Stiffening effects on the natural frequencies of laminated plates with piezoelectric actuators

Abstract: Piezoelectric actuators are usually mounted to the top and bottom surfaces of plates and may induce in-plane extension, bending and localized shear deformations at the structural element. The in-plane stresses may have a significant influence on the mechanical behavior of thin plates as initial and/or residual stresses affect the flexural stiffness and in turn the dynamic and stability characteristics of plates. In this work, the effect of the in-plane piezoelectric induced stresses on the natural frequencies of composite plates is numerically and experimentally investigated. A finite element formulation is presented for the analysis of laminated plates with an arbitrary number of piezoelectric actuators and/or sensors. Von Karman non-linear strain–displacement relations are used and ideal linear behavior is assumed for the piezoelectric actuation. The problem is decomposed into an in-plane problem where the strain field induced by the piezoelectric actuators is computed. The natural frequencies and vibration modes are then computed taking the stress stiffening effects of these piezoelectric stresses into account. A number of different configurations are numerically and experimentally analyzed to verify the proposed theory. The configurations use eight PZT actuators bonded to three layer glass fiber/epoxy plates. The plates are square and clamped along two opposing edges and free along the other two. Good agreement is obtained between the predicted and measured natural frequencies.

Static behavior of CFRPs at low temperatures

Abstract: This paper summarizes the results of the tests to determine the effect of the low temperature on the mechanical behavior of carbon fiber reinforced epoxy laminates. Tensile and bending static tests were carried out on two laminate lay-ups (quasi-isotropic and cross-ply laminates), determining properties such as the mechanical strength, stiffness and strain to failure. The results show the changes in the mechanical behavior of this material at different test temperatures (20, −60 and −150 °C).

Free vibration analysis of axially compressed laminated composite beam-columns with multiple delaminations

Abstract: In this work free vibration analysis is performed for multi-delaminated composite beam-columns subjected to axial compression load. In order to investigate the effects of multi-delaminations on the natural frequency and the elastic buckling load of multi-delaminated beam-columns, the general kinematic continuity conditions are derived from the assumption of constant slope and curvature at the multi-delamination tip. The characteristic equation of multi-delaminated beam-column is obtained by dividing the global multi-delaminated beam-columns into segments and by imposing recurrence relation from the continuity conditions on each sub-beam-column. The natural frequency and the elastic buckling load for multi-delaminated beam-columns are obtained in this work. The latter is based on the incremental load of axial compression, which is limited to the maximum elastic buckling load of the sound laminated beam-column. To verify the results of the present models, experimental results are obtained for isotropic single delaminated beam-columns. Comparisons are conducted between these experimental results and the present analysis. Good agreement is obtained from this comparison of results. It is found that the sizes, locations and numbers of multi-delaminations have significant effect on the natural frequency and the elastic buckling load, specifically the latter ones.