Composite Material : Carbon Fiber Reinforced Epoxy Matrix Material

 Preamble

A composite material is a synthetic material made from a polymer matrix reinforced with fiber, which remains separate and distinct on a macroscopic level while forming a single component.  Composites essentially combine the strength and stiffness of metals and the lightweight, flexibility and corrosion resistance of plastic. They offer the ability for lighter weight materials with fewer corrosion problems. The fibers are usually fiber glass, carbon, or aramid, while the polymer is usually an epoxy, vinyl ester or polyester thermosetting plastic. Today, composite materials find a wide range of applications in many industrial sectors like the aerospace, automotive, marine, and construction industries due to their high strength and lightness (i.e., strength-weight ratio).

The complex manufacturing process of composite materials causes relatively frequent occurrence of damages both in the composite matrix and on the ply interfaces. Since the material is subjected to high loads, ballistic impact or continuous cyclic loads in the process of operation, the presence of even small damage may lead to the crack growth and total destruction of the component.

The nature of failure in composite materials is far more complex than in homogeneous material like aluminum. Unlike most metals where failure is usually related to yielding and cracking, composite materials begin to fail from internal cracks on the fiber component which is the load bearing material within the composite. If an accurate predication can be made about the reliability of a composite material system before it has been damaged, then proper maintenance can be administered.

As various load conditions are applied to a composite material, the internal makeup can become damaged with failure mechanisms such as matrix cracking, delamination, fiber breakage, and local buckling. Primary matrix failure modes are characterized by cracks that run parallel to the fiber in plies that are not aligned with the principal tensile loading direction. Secondary matrix failure causes cracks that extend into adjacent plies, thus initiating delamination. It has been observed that delamination only occurs in the presence of matrix cracks. As delamination damage accumulates the material characteristics change until ultimately the system fails in the form of fiber breakage, but in reality the failure is progressive as mechanisms such as matrix cracking occur gradually rather than suddenly.

The fiber fracture is modeled using an increment approach and MATLAB will be used as the computational tool for the simulation of the crack growth and the graphs obtained to predict the reliability of the system.

Objective of study

The objective of this work is to;

·     Predict the intensity of damage caused by cracks in carbon fiber materials before the system fails.

·         Estimate the crack paths in the fibers of the composite material.

Scope and Limitation

The scope of this work covers only on the central through crack pre-existing in a carbon fiber reinforced epoxy matrix material and we were limited with many factors like temperature changes and cost that stalled the work at different occasions.

LITERATURE REVIEW

Background

Cracks occur in many structural and engineering components as the material is subjected to high loads or continuous cyclic loads in the process of operation. A review of past assessment of cracks in composite materials and their methods of simulation through theoretical research is being presented below.

Early Developments

Itou (1981) solved within the two-dimensional linearized couple stress theory, a problem of a uniformly propagating finite crack in an infinite medium. The self equilibrated system of pressure is applied to the crack surfaces and the problem is reduced to dual integral equations and solved by a series expansion method. The crack propagates only to the right, maintaining its constant length and affects the magnitude of the local stress field to some extent, but does not alter the qualitative features of the stress solution.

Ye (1992) characterized the inter-laminar crack growth in composite materials. Mode-I inter-laminar crack growth in a unidirectional carbon-fiber/epoxy composite was studied. A simple model is proposed to estimate the orthotropic correction factor of the stress-intensity factor for the inter-laminar fracture mode. It was found that, for composite materials (involving thermoset and thermoplastic composites), the interrelationship between the inter-laminar fracture toughness can be correlated on the basis of linear-elastic-fracture mechanics, although the special inter-laminar fracture mechanisms can disturb the relationship.

Huston (1994) used results obtained from repeated tension fatigue tests on unidirectional carbon fiber reinforced epoxy material to test the residual strength and residual stiffness models. The fatigue tests were also carried out under spectrum loadings so that the results were correlated with the cumulative damage predicted by the residual strength model. Fatigue theories based on damage mechanism modeled the intrinsic defects in the matrix as small as cracks parallel to the fibers in the composite material, and the propagation of these cracks where predicted by linear fracture mechanism analysis using the Monte Carlo method for simulation.

Seale and Others (1998) demonstrated that lamb waves as a non-destructive evaluation (NDE) is an excellent tool to monitor damage in composite materials. They described two studies which monitor fatigue damage and two studies which monitor thermal degradation in composite materials using ultrasonic lamb waves. The lamb wave velocity was monitored along with the crack density which provided information about the in-plane properties of the composite material because the lamb wave propagation travel perpendicular to the crack direction. The lamb wave velocity exhibited a decrease with increasing crack density as the number of fatigue cycles increased.

Patel (1999) used the critical element paradigm to predict the combined effects of alternating environmental (temperature and moisture) conditions imposed during fatigue testing on composite materials. Moisture uptake model for laminates containing cracks in directions parallel and transverse to the loading direction was evaluated to determine the damage and failure modes through fatigue testing, residual strength testing and non-destructive evaluation. It was observed that damage progression during fatigue occurred in the following general order: transverse micro-cracking and delamination around the fiber bundle undulation regions, longitudinal micro-cracking around fiber bundle undulation regions, growth of transverse cracks across the entire height of the transverse fiber bundle, and cross-ply like growth of edge and inter-ply delamination. The rate at which the damage progression occurred generally depended on the maximum fatigue stress amplitude and the corresponding fatigue life.

Recent Developments

Kessler and others (2002) investigates the feasibility of modal evaluation techniques in detecting damage for health monitoring of composite systems. Characteristics examined include the method’s ability to detect various types of damage, their precision in determining the damage location, their sensitivity to sensor density and the impact of conformability for system for system implementation. Finite element models were created to validate results and perform trade studied. 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.

Wenk (2003) used finite element model with material properties and dynamic characteristics to generate simulations. It was beneficial to evaluating the response of the model using information about the future dynamic loadings to predict the distributions of internal stress in the composite material. By using stress as a measure of the load carrying path, predictions were made about where the damage will grow. The effects of the force location and size were found to have a significant impact on the response of the composite material. The finite element model used as means of evaluating the change in stress distribution due to damage in multiple locations within the composite material, also affects the application of load in different locations to provide valuable insight to the levels of stress in the composite material at different frequencies. The load applied at the center of the damage location generated the largest stress in each of the damage.

Avadutala (2005) investigated a more universal approach “time - domain” technique and plots which accommodates the diversity of failure modes exhibited by systems for various types of damages in composite and homogeneous materials. The analytical predictions made were by calculating fracture parameters such as stress intensity factors in the crack region which is used to estimate the crack growth rate. A finite element model (modal analysis) is used to determine the vibration characteristics and frequency response of the system while it is being designed.

Xie and others (2006) analyzed crack growth in composite by a strategy of two independent steps based on material strength theory and fracture mechanics. These were used to predict the damage mechanism, damage pattern, crack growth path and to determine the failure loads. In this analysis, failures such as fiber breaking, fiber kinking, matrix cracking, and fiber/matrix interface debonding were detected based on different failure criteria. The advantage of the material strength theory is that it does not require the predefined crack path. In removing the damaged elements it creates stress singularity, therefore, fracture mechanics theory was performed to improve the prediction capability for the failure loads by embedding node pairs along the crack path. Finite element analysis (FEA) in conjunction with discrete cohesive zone model (DCZM) was used to control the crack growth.

Nakamura and others (2007) studied fatigue growth of fiber reinforced composite laminate which was characterized under a combined experimental and computational investigation. The crack growth monitored under variable amplitude measured data to determine the relationship between temperature and growth rate. Three-dimensional finite element analysis were performed to obtain energy release rate and mixed mode stress intensity factors. The crack growth rate was correlated with range of energy release rate using a power law relation. While many studies on carbon fiber reinforced epoxy laminates might base their work on two-dimensional computational analyses for fatigue crack growth characterization, the study revealed that three-dimensional effects play a significant role on the interface crack propagation behavior in laminated composites.

Miklashevich (2009) investigated the influence of the discrete interlock bonds breaking on the crack development at different levels of the system. The investigation further differentiated between two stages of fracture interpreted as the indentation of the influence zone into undisturbed material under the action of an end load. The principal difference of the considered processes is in the fact that the shock acts not along the beam axis and that the system loses its stability not as a result of shock load but as a result of a quasi-static load under conditions of parametrical fracture of an elementary cell.

Prasad and Kumar (2009) investigated the accuracy of predicting the dynamic response by finite element modeling of systems with cracks. Composite materials subjected to various types of damages, mostly cracks and delamination results in local changes of stiffness of the materials and their dynamic characteristics. The stress pattern at the local regions of crack were modeled for stress and dynamic analysis, to study the influence of mesh and spring stiffness values on the error of dynamic and modal frequency values of a beam. Several models of cracked structure were described with challenges in the development of structural stiffness loss due to damage.

Camanho and others (2010) proposed to measure the crack resistance curves associated with fiber dominated failure modes in polymer matrix composites by the identification of the crack tip location using the digital image correlation (DIC) system and the calculation of the J-integral directly from the test data. They also cited that the finite element method (FEM) based technique used for the numerical calculation of the J-integral and to extract crack resistance curves in compact compression tests was inadequate. Though some difficulties were encountered in the identification of the crack tip in the presence of delamination which will still need to be addressed, the displacement and strain fields obtained using DIC during the compact tension and compact compression tests of composite laminates serve as the basis for the determination of the location of crack tips and for automatic calculation of the J-integral.

Karmazin and others (2010) analyzed the lamb wave’s propagation as a method of non destructive testing for monitoring of structural health and was implemented for the online monitoring in the automatic operation mode. The method makes it possible to locate cracks efficiently. The crack size however, can only be identified roughly. The advantage of this approach lies in the simplicity of the method and the fact that it is not necessary to model the defect itself, since the method can be used for defects of any type.

Patricio and Mattheij (2010) described an algorithm to predict the path of pre-existing cracks in homogeneous and heterogeneous materials based on an incremental approach. The simulation of the cracks growth was based on the static stress intensity factors, and dynamic effects such as wave propagation were not taken into account. The crack path predicted for the heterogeneous materials assumed that the loading is sufficiently large to ensure crack growth, and a hybrid approach was employed to solve the elasticity problems in predicting the crack propagation in composite materials. They also noted that for a microscopic resolution, homogenization may not be employed in the vicinity of the crack tip.

Based on their finding (Patricio and Mattheij, 2010) which predicted the path of pre-existing cracks in homogeneous and heterogeneous materials, this work will emphasize on the effects of cracks on a carbon fiber / epoxy composite material, the intensity of damage caused by cracks and the crack growth rate, and will use MATLAB as a computational tool for incremental approach used to simulate model to generate crack curves.

Knowledge Gaps Based On Literature Reviewed

This work will predict the crack path in the fibers of AS4/3501-6 Carbon fiber/Epoxy composite materials using a step by step incremental approach and also the intensity of damage caused by cracks on the carbon fiber composite material before failure and will compare its findings with the results of other researchers.