Applied Composite Materials (v.17, #5)

Guest Editorial by Hany A. El Kadi (415-416).

Natural Weathering and Sea Water Effects on the Durability of Glass Fiber Reinforced Vinylester: Fractographic Analysis by Nesar Merah; Seyed Nizamuddin; Zafarullah Khan; Faleh Al-Sulaiman; Moeid Mehdi (417-426).
This paper presents a study of the effects of harsh outdoor weather and warm sea water on the tensile behavior of Glass-Fiber Reinforced Vinylester (GFRV) pipe materials destined for sea water handling and transportation. The effect of Dhahran’s outdoor weather for exposure periods ranging from 3 to 36 months revealed an improvement in tensile strength when compared with the as received GFRV sample. A significant increasing trend of tensile strength from 3 to 12 months was noted. This is attributed mainly to the post curing effects resulting in higher cross linking density. After 12 months of exposure the tensile strength showed a decreasing trend, but remaining still higher than the average tensile strength of as received (baseline) GFRV sample. Similar results of enhanced tensile strength were noted after immersion of GFRV pipes in warm Gulf sea water for 12 months. Fractographic analysis was performed on the tensile tested GFRV samples using optical microscope followed by scanning electron microscope (SEM). The characterization of the controlling failure mechanisms involved from fracture initiation to fracture propagation through the gage section of the specimen were predicted and were justified by correlating the optical and SEM pictures.
Keywords: Durability; Environment; Curing; Tensile; GFRV

Micro-Macro Analysis of Viscoelastic Unidirectional Laminated Composite Plates Using DR Method by Seyed Reza Falahatgar; Manouchehr Salehi; Mohammad Mohammadi Aghdam (427-440).
The Dynamic Relaxation (DR) technique together with finite difference discritization is used to study the bending behavior of Mindlin composite plate including geometric nonlinearity. The overall behavior of the unidirectional composite is obtained from a three-dimensional (3D) micromechanical model, in any combination of normal and shear loading conditions, based on the assumptions of Simplified Unit Cell Method (SUCM). The composite system consists of nonlinear viscoelastic matrix reinforced by transversely isotropic elastic fibers. A recursive formulation for the hereditary integral of the Schapery viscoelastic constitutive equation in multiaxial stress state is used to model the nonlinear viscoelastic matrix material in the material level. The creep tests data is used for verification of the predicted response of the current approach. Under uniform lateral pressure, the laminated plate deformation with clamped and hinged edged constraints is predicted for various time steps.
Keywords: Micromechanics; Nonlinear viscoelastic; DR method; Composite plate

In this article, initiation and propagation of delamination of a double cantilevered beam (DCB) is studied. The delamination of DCB specimen occurs between 0 o and θ 0 layers. Due to damage induced, during the loading, in the matrix of θ 0 layer, virtual crack closure technique (VCCT) as a well known method for simulation of initiation and propagation of the delamination for DCB is not able to model the propagation of delamination properly. To overcome this shortcoming, the stiffness of the damaged θ 0 layer is decreased with a developed technique in this study. The stiffness reduction technique (SR) is introduced in this paper to model the induced cracks in the matrix of the θ 0 layer. Then the stiffness reduction technique is coupled with the critical length parameter. The presented method is called stiffness reduction—critical length (SR-CL) method. By using SR-CL method, initiation and propagation of delamination for a $$ left[ {{{left( {0_2^o/{{90}^o}} ight)}_6}/0_2^o//{ heta^o}/{{left( {0_2^o/{{90}^o}} ight)}_6}/0_2^o} ight] $$ configuration in which θ = 0o, 22.5o, 45o, 67.5o, 90o and for a cross ply laminate (0°/90°) 12 are modeled. The obtained results are in very good agreements with experimental data.
Keywords: Critical length method; Stiffness reduction technique; DCB; Delamination

A methodology for stiffness improvement by optimal orientation of fibers placed using fiber steering techniques of composite plates has been developed and is described here. A genetic algorithm is employed to determine the optimal orientation of the tow fibers and, in addition, once the plate has been divided up into cells in order to apply the technique, the orientation gradient between adjacent cells is capped. The finite element method (FEM) is used to determine the fitness of each design candidate. The approach developed also differs from existing ones by having a more sophisticated chromosome string. By relying on the algorithm for the calculation of the fiber orientation in a specific cell, a relatively short and rapid convergence string is assembled. The numerical results obtained show a significant improvement in stiffness when the fiber orientation angle is allowed to vary spatially throughout the ply.
Keywords: Fiber steering; Composite laminates; Optimization; Genetic algorithm

Impact Response Characterization in Composite Plates—Experimental Validation by Andreas P. Christoforou; Ahmet S. Yigit; Wesley J. Cantwell; Fanjing Yang (463-472).
This paper presents an experimental study for the normalized low-velocity impact response of composite plates. It is demonstrated that a characterization diagram that shows the relationship of three non-dimensional parameters with the normalized maximum impact force can be used to fully characterize the response. With the governing non-dimensional parameters obtained experimentally, it is shown that impact tests having the same non-dimensional parameters, have dynamic similarity and the same non-dimensional response. Furthermore, the experiments can be placed in appropriate dynamic regions in which simplified dynamic models can be used to predict the response.
Keywords: Impact response; Composites; Characterization; Experimental

The advent of carbon loaded composite materials gave a boost to many industries. This is because of their light weight, durability and strength. As new structures utilizing carbon loaded composites are built, the need for a reliable nondestructive testing technique increases. A carbon-loaded composite testing poses a challenge to most nondestructive testing and evaluation (NDT&E) techniques. Microwave NDT&I techniques main challenge is the lossy nature of carbon, especially at high microwave frequencies. Lower frequencies penetrate deeper in carbon-loaded composites, however, to operate at lower frequencies the size of the waveguide probe increases significantly which degrades the resolution rapidly. Open-ended rectangular waveguide sensors filled with a dielectric material will be used to inspect carbon-loaded composites. The filling of the waveguide reduces the frequency of operation and keeps the small size of the waveguide (i.e. increases the penetration depth and maintains the resolution). However, varying the waveguide filling material dielectric properties will have an impact on the measurement parameters optimization process and consequently on the detection sensitivity. In this paper, the use of the waveguide filling material as an optimization parameter will be investigated. Carbon-loaded composites with disbonds will be inspected and the variation of the dielectric properties of the loading material of rectangular waveguide probes for carbon loaded composites inspection will be assessed.
Keywords: Near-field microwave inspection; Carbon loaded composites; Loaded open-ended rectangular waveguide probes

The objective of this work was to image the presence of impact damage by monitoring the nonlinear response of damaged carbon/epoxy composite samples. The presence of microcracks, debonding, delamination, etc… induce the material to behave in a nonlinear elastic fashion highlighted by the presence and amplitude of harmonics in the spectrum of the received signal when the sample is periodically excited at one of its resonance frequencies. The sensitivity of a second harmonic imaging technique (SEHIT) based on material nonlinear elastic effect known as second harmonic generation (SHG) was investigated. The proposed imaging process was used to detect barely visible impact damage (BVID) due to low velocity impact (<12 J). The results showed that the SEHIT methods appear to be highly accurate in assessing the presence and magnitude of damage with a very promising future for both NDT and possibly structural health monitoring (SHM) applications. Moreover the technique was validated with two conventional NDT techniques: pulse thermography and thermosonics. The first NDT method failed in detecting the damage on the impacted face. The second technique was capable of localising and quantifying the damage on the impacted surface agreeing well with the results obtained using the proposed nonlinear imaging method.
Keywords: Non linear elastic material; Barely visible impact damage; Imaging technique

Finite Element Dynamic Analysis of Laminated Viscoelastic Structures by Naser Al-Huniti; Fadi Al-Faqs; Osama Abu Zaid (489-498).
This work is concerned with the dynamic behavior of laminated beam, plate and shell structures consisting of a viscoelastic damping layer constrained between two structural layers. Finite element models for modal, harmonic and transient analyses are developed. The dynamic interlaminar shear stresses are determined and presented under harmonic and transient loads. The effect of the damping ratio of the viscoelastic material is investigated. It is found that the viscoelastic material damping reduces the interlaminar stresses. The results also show the dependency of the viscoelastic material on frequency, hence, the effect of the viscoelastic material appears significantly under harmonic loading. In transient analysis, the importance of the viscoelastic material is observed in absorbing the impact and returning the structure to its original configuration.
Keywords: Finite element; Viscoelastic; Laminated beam; Plate and shell; Dynamic interlaminar stresses

Effects of Boundary Conditions in Laminated Composite Plates Using Higher Order Shear Deformation Theory by Pervez Tasneem; Al-Zebdeh Khalid; Farooq K. S. Al-Jahwari (499-514).
This paper extends the applicability of a modified higher order shear deformation theory to accurately determine the in-plane and transverse shear stress distributions in an orthotropic laminated composite plate subjected to different boundary conditions. A simpler, two-dimensional, shear deformable, plate theory accompanied with an appropriate set of through-thickness variations, is used to accurately predict transverse shear stresses. A finite element code was developed based on a higher order shear deformation theory to study the effects of boundary conditions on the behavior of thin-to-thick anisotropic laminated composite plates. The code was verified against three dimensional elasticity results. The study also compared the stresses and deformation results of higher order theory with those obtained using commercial software such as LUSAS, ANSYS and ALGOR. The commercial software are heavily used by designers to design various components/products made of composites. Various combinations of fixed, clamped and simply supported boundary conditions were used to verify a large class of anticipated applications. Results obtained from software are in good agreement for some cases and significantly differ for others. It was found that LUSAS and ANSYS yield better results for transverse deflection and in-plane stresses. But for transverse shear stresses, it is highly dependent on boundary conditions.
Keywords: Laminated composite plates; Higher order shear deformation theory; Comparative study; Boundary condition; Finite element analysis

In this study, an efficient method is developed to investigate the compressive large deflection behavior of unsymmetric composite laminates with multiple through-the-width delaminations. The analytical method is based on the first order shear deformation theory (FSDT) and its formulation is developed on the basis of the Rayleigh-Ritz approximation technique by the implementation of the simple and complete polynomial series. The method can handle both local deflection of the delaminated sublaminate and global deflection of the whole plate. Also, the contact among sublaminates is investigated. The three-dimensional finite element analysis is performed by using ANSYS5.4 general purpose commercial software, and the results are compared with those obtained by the analytical model.
Keywords: Structures; Composites; Large deflection; Delamination; Contact

A Viscoelastic Sandwich Finite Element Model for the Analysis of Passive, Active and Hybrid Structures by Aurelio L. Araújo; Cristovao M. Mota Soares; Carlos A. Mota Soares (529-542).
In this paper we present a finite element model for the analysis of active sandwich laminated plates with a viscoelastic core and laminated anisotropic face layers, as well as piezoelectric sensor and actuator layers. The model is formulated using a mixed layerwise approach, by considering a higher order shear deformation theory (HSDT) to represent the displacement field of the viscoelastic core and a first order shear deformation theory (FSDT) for the displacement field of the adjacent laminated anisotropic face layers and exterior piezoelectric layers. The dynamic problem is solved in the frequency domain with viscoelastic frequency dependent material properties for the core. Control laws are also implemented for the piezoelectric sensors and actuators. The model behaviour in dynamics is assessed with the few solutions found in the literature, including experimental data, and a laminated composite active sandwich application is proposed. In this numerical application, velocity feedback control law is implemented for active control, using co-located piezoelectric patch sensors and actuators.
Keywords: Sandwich structures; Active-passive damping; Co-located control; Velocity feedback; Viscoelastic materials; Finite element method

Characterisation by Inverse Techniques of Elastic, Viscoelastic and Piezoelectric Properties of Anisotropic Sandwich Adaptive Structures by Aurelio L. Araújo; Cristovao M. Mota Soares; Carlos A. Mota Soares; Jose Herskovits (543-556).
In this article we present recent developments regarding parameter estimation in sandwich structures with viscoelastic frequency dependent core and elastic laminated skin layers, with piezoelectric patch sensors and actuators bonded to the exterior surfaces of the sandwich. The frequency dependent viscoelastic properties of the core material are modelled using fractional derivative models, with unknown parameters that are to be estimated by an inverse technique, using experimentally measured natural frequencies and associated modal loss factors. The inverse problem is formulated as a constrained minimisation problem, and gradient based optimization techniques are employed. Applications are presented and discussed, focused on the identification of viscoelastic frequency dependent core material properties.
Keywords: Inverse problems; Optimization; Sandwich structures; Hybrid damping

Effect of Seawater and Warm Environment on Glass/Epoxy and Glass/Polyurethane Composites by Abdel-Hamid I. Mourad; Beckry Mohamed Abdel-Magid; Tamer El-Maaddawy; Maryam E. Grami (557-573).
A study of the durability of fiber reinforced polymer (FRP) materials in seawater and warm environment is presented in this paper. The major objective of the study is to evaluate the effects of seawater and temperature on the structural properties of glass/epoxy and glass/polyurethane composite materials. These effects were studied in terms of seawater absorption, permeation of salt and contaminants, chemical and physical bonds at the interface, degradation in mechanical properties, and failure mechanisms. Test parameters included immersion time, ranging from 3 months to 1 year, and temperature including room temperature and 65°C. Seawater absorption increased with immersion time and with temperature. The matrix in both composites was efficient in protecting the fibers from corrosive elements in seawater; however moisture creates a dual mechanism of stress relaxation—swelling—mechanical adhesion, and breakdown of chemical bonds between fiber and matrix at the interface. It is observed that high temperature accelerates the degradation mechanism in the glass/polyurethane composite. No significant changes were observed in tensile strength of glass/epoxy and in the modulus of both glass/epoxy and glass/polyurethane composites. However, the tensile strength of the glass/polyurethane composite decreased by 19% after 1 year of exposure to seawater at room temperature and by 31% after 1 year of exposure at 65°C. Plasticization due to moisture absorption leads to ductile failure in the matrix, but this can be reversed in glass/polyurethane composites after extended exposure to seawater at high temperature where brittle failure of matrix and fiber were observed.
Keywords: Composite materials; Durability; Fiber reinforced polymers; Mechanical properties; Epoxy; Polyurethane