Key Engineering Materials Vols. 471-472

Abstract: The paper investigates the microwave properties of natural fiber reinforced biodegradable plastic composites in order to recognize their potential as alternatives to common printed circuit board (PCB) for electronic communication industries. Thus, the paper reports on measured dielectric properties for two new composites under study: Kenaf/Poly Lactic Acid (PLA) and rice husk/PLA and their results are compared. The sample is made from equal weight percentage loading (50%wt - 50%wt) of kenaf and PLA. Another sample has also equal weight percentage loading (50% wt - 50%wt) of rice husk and PLA. The complex dielectric permittivity (ε = ε' – jε'') and loss tangent (tan δ) of the two samples of natural fibers plastic composites have been studied in the frequency range of 500 MHz to 10 GHz. The dielectric permittivity is measured by scattering parameter (S-parameter) using a Vector Network Analyzer (VNA). The concentration dependence of permittivity and loss tangent is analyzed for each sample. It is observed that from 500 MHz to 3.32 GHz, real permittivity (ε') values are consistent throughout the wide frequency range, at approximately 3.3. However, the permittivity seems to decrease at higher frequencies starting from 3.35 GHz for both samples, down to 2.5 at 10 GHz. Measured results show that kenaf/PLA mixture has higher permittivity (ε') than the rice husk/PLA composite across the wide frequency range. Meanwhile, loss tangent (tan δ) is low and remains similar for both types of fiber compositions.

Abstract: Glass fiber reinforced materials are particularly attractive for transportation industries because of their high strength/mass ratio and their low cost compared to other materials. Therefore, the composite material system used in this study consists of polyester resin layers reinforced with E-glass fabrics. In this paper, the effect of mixed-mode loading on fracture parameters of glass reinforced polyester composite specimens is investigated both experimentally and numerically. Geometric factors were calculated for modified Arcan specimen using finite element analysis. The finite element results indicated that for loading angles close to pure mode-II loading, a high ratio of mode-II to mode-I fracture is dominant and there is an opposite trend for loading angles close to pure mode-I loading. According to experimentally measured fracture toughness for glass/polyester, the opening-mode and shearing mode interlaminar critical strain energy release rates, were found. Results indicated that the interlaminar cracked specimen is tougher in shear loading condition and weaker in tensile loading conditions.

Abstract: The present research examines analytically and experimentally the mode-I and mode-II and mixed mode-Interlaminar fracture toughness of PAN based carbon/epoxy composite. A modified Arcan fixture, well-suited for the study of the behavior of used composite assemblies, was developed in order to focus on the analysis of the fracture behavior of the material. The edge effects are minimized by using an appropriate design of the substrates so that experimental results give reliable data. Also the mode-I and mode-II stress intensity factors were computed for different crack lengths and load orientation angles using finite element analysis. The numerical results show that the modified Arcan specimen is able to provide pure mode-I, pure mode-II and any mixed mode loading conditions. It is shown that the results obtained from the fracture tests are consistent very well with mixed mode fracture theories. Obtained results indicated that fracture toughness and stress intensity factor for sliding mode enhanced up when the loading angle increased. Mechanism of fracture and toughening were examined by using scanning electron microscopy.

Abstract: The aim of this paper is to evaluate interlaminar fracture toughness and non dimensional stress intensity factors of woven Carbon-Polyester composite based on numerical and experimental methods. A modified version of Arcan specimen was employed to conduct a mixed-mode fracture test using a special loading device. By changing the loading angle, α, from 0° to 90°, mode-I, mode-II and all mixed-mode data were created. The finite element analysis was performed with Abaqus software. The interaction j-integral was used to separate the mixed mode stress intensity factors and energy release rate at the crack tip under different loading conditions and different thickness of specimens. The results of fracture toughness tests revealed that the interlaminar fracture of composite is strong under the shearing-mode loading but weaker to the opening- mode loading. It can be seen that by increasing the thickness of the composite specimen, non dimensional stress intensity factors for pure mode I (α=0°) and pure mode II (α=90° ) loading conditions were decreased.

Abstract: Material flaws, pre-cracks and crack initiation due to cyclic loading often lead to undetected crack propagation in commercial structures like aircraft components, automotive parts and computer motherboard. Cyclic loading can make the crack grow large into any shape with an arbitrary orientation, depending on the structure geometry, boundary and loading conditions. Since crack propagation in many cases may lead to catastrophic failure with human and monetary loss as a result, it is important to enable crack growth prediction at all stages of development and during maintenance in order to prevent such scenarios. Micro mechanical approach is used for modelling the crack in composite materials. Crack propagation in a single edge crack plate is carried out by using FEM analysis. 2D model is analysed to determine the crack growth. The crack propagation rate, stress intensity factor and strain energy release rate are predicted by varying the crack length in fiber reinforced epoxy composite using NISA/ENDURE.

Abstract: For many years, research was focused on developing a medical part of human body from polymer as to replace metal. In this study, the aim is to produce a Polyetheretherketone/Hydroxyapatite (PEEK/HA) composite which posses balance mechanical properties and good spreading of bioactive ceramic, hydroxyapatite. The composite consist of 10-30 wt% HA were compounded via nano-single screw extruder and sample for testing were produced by injection molding. Each formulation of HA was treated with (3-Aminopropyl)trimethoxysilanes coupling agent to compare with untreated HA. The result showed that the slight increasing value of Elastic modulus, flexural strength, tensile strength while decreasing flexural modulus for 10 and 20 wt% HA compared to untreated composite. The enhance of bioactivity has been proven with the incorporation of HA into PEEK. SEM-EDX image showed the bulk formation of apatite layers on the composite surface with 30 wt% HA after 3 days immersed in SBF solution. Finally, these composite be capable of being one of the biomedical part seing as the mechanical properties were found to be within the properties of human cortical and cancellous bone.

Abstract: There has been a growing interest in the use of composites especially in structural application ranging from aerospace to automotive and marine sectors. However, their performances under impact loading represent one of the major concerns as impacts may occur during manufacture, normal operations and maintenance. This paper presents two novel NDT techniques, thermosonics and digital shearography (DISH) to detect and assess barely visible impact damage (BVID) produced on a stiffened composite wing panel by unknown low energy impacts. Thermosonics is based on synchronized infrared imaging and ultrasonic excitation. Despite the apparent simplicity of the experimental setup, thermosonics involves a number of factors, e.g. acoustic horn location, horn crack proximity, horn-sample coupling etc., that significantly tend to influence both the degree and the period of the excitation. Then, a numerical-experimental procedure for the assessment of the size and depth of delamination by digital shearography (DISH) is proposed. The flaw detection capabilities of DISH have been evaluated by measuring the dynamic response of the delaminated area to applied stresses. The shearographic methodology is based on the recognition of the (0 1) resonance mode per defect. A simplified model of thin circular plate, idealized above each impacted area, is used to calculate the natural frequency of vibrating delamination. The numerical difference between experimental resonance frequencies and those computationally obtained is minimized using an unconstrained optimization algorithm in order to calculate the delamination depth. The results showed that thermosonics is a quick and effective method to detect and localize BVID damage while the combined shearography and optimization methodology was able to size and localize delamination due to low velocity impacts.

Abstract: This research work presents an in-situ imaging method for the localization of the impact point in complex anisotropic structures with diffuse field conditions, using only one passive transducer. The proposed technique is based on the time reversal approach applied to a number of waveforms stored into a database containing the experimental Green’s function of the medium. The present method exploits the benefits of multiple scattering, mode conversion and boundaries reflections to achieve the focusing of the source with high resolution. The optimal re-focusing of the back propagated wave field at the impact point is accomplished through a “virtual” imaging process, which does not require any iterative algorithms and a priori knowledge of the mechanical properties of the structure. The robustness of the time reversal method is experimentally demonstrated on a stiffened composite panel and the source position can be retrieved with a high level of accuracy (error less than 3%). The simple configuration, minimal processing requirements and computational time (less than 1 sec) make this method a valid alternative to the conventional imaging structural health monitoring systems for the acoustic emission source localization.

Abstract: A novel type of hybrid composite structure has been developed, experimentally investigated and used for many practical applications. The main supporting elements of composite structures are formed by the stamping process of partially cured and axially-oriented carbon fibre rods. This system can fill relatively thick parts of cross sections of beams without risk of delamination. Typical macroscopic sub-cells are formed in the transversal cross section of the part due to this technology. An advantage of this final 3D composite structure is its high shear strength and stiffness in comparison with thick unidirectional composite parts. To absorb the dynamic energy and increase the damping, a rubber-cork layer can be inserted during production, before the final pressing and curing of the whole part. The final stiffness property of the whole 3D composite is obtained from multiscale modeling. It is based on an averaging process and a homogenization technique in FEA. A parametric study was carried out to determine the influence of the size, orientation and thickness of the cell border winding layer on the components of the global elastic material matrix. A comparison of a numerical analysis prediction with experimental results shows acceptable agreement of the elastic modules. A mezzo scale model can be applied for designing a real part on a macro scale.

Abstract: For many years, precision investment casting foundries have periodically reported serious casting defects. One source is associated with the brittle ceramic shell mould which is very weak in tension and highly expose to the cracking mechanism. This situation not only lead to the appearance of defect in the end cast product but always attribute to the handling problem at the earlier stage in the investment casting industry. Due to this fact the strengthening mechanism of the brittle shell mold using reinforcement method was studied and investigated in this work. Rice husk was chosen as a reinforcement material as it contains silica element which can withstand high temperature of molten metal. Several testing procedures and characterization technique were carried out in order to measure the performance of the reinforced shell mould. Results show that the MOR value for the green reinforced shell mould is higher than the green body of non reinforced shell mould. Scanning electron microscope observations also show that the fiber alignment across the matrix structure increase the failure resistance and yet increase the strength of the shell mould green body. Furthermore, this reinforcement method using organic fiber leads to the pore enhancement and also leads to the new phase formation (zircon) in the fired reinforced body. Overall, this reinforcement method using rice husk increase the properties of the green and fired body of ceramic shell mould system.