Composite Materials have Transformed the Modern Aerospace Industry - Engineering Assignment Help

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Composite materials have transformed the modern aerospace industry. It’s high strength-to-weight characteristics combined with higher fatigue performance have led to the development of lighter and larger aircrafts for the commercial and the defense industry. Though composites fair better than most traditional materials in their current applications, they are considered to be weak and fragile under several environmental and operational conditions. This project aims to study the effect of moisture on composites and predict its residual strength under operational conditions such as compression. The proposal presents the current research and knowledge on the study of composites under various operational and environmental conditions. The major research gaps are identified from the available literature to set the baseline for the research under this project proposal.
IntroductionCarbon fiber composites have found extensive application in commercial aerospace, marine and automobile industry in the past decade. This demand for composites is generated due to the high specific strength, superior fatigue performance, light weight characteristics and high specific stiffness. The defense industry had an early beginning with the composites such as carbon fiber in aerospace applications, fiber glass in naval/marine vehicles and Kevlar in armored vehicles and safety equipment. However, the development of specialized composites has enhanced the utility of composites in both commercial and defense applications. Earlier composites were limited to secondary control surfaces. Modern day commercial airliners such as Airbus A350 XWB and Boeing 787 Dreamliner have peaked the utilization of composites and over 50 percent of the weight of the aircraft is contributed by composite materials CITATION Par19 \l 1033 (Park, et al., 2019).

Figure SEQ Figure \* ARABIC 1: Aircraft Composite Content CITATION Smi13 \l 1033 (Smith, 2013)Composite materials tend to have superior properties compared to traditional materials in terms of weight, strength, stiffness and endurance. However, these properties are severely dependent on the arrangement of fibers in the composite and the matrix holding the fibers. Thus, the strength of composites in through-thickness direction are considerably inferior and prone to impact damages. Such out-of-plane and low velocity impacts can cause undetectable internal damages and or total failure of the component. Internal damages in composites lead to delamination and effectively weaken the components’ ability to perform its function CITATION Ali21 \l 1033 (Alizadeh & Soares, 2021). Delamination in composites is more severe for structures performing under cycling loading and failure of such components during the course of flight could lead to a major catastrophe.
Composites with the polymer matrix could be categorized as thermoplastic and thermosetting. This characterization of composites is based on the type of resin employed as the matrix. The thermoplastic composites are manufactured under high temperature and high-pressure conditions in order to bring the resin to liquid state and impregnate the fibers with the resin. Due to such extreme manufacturing conditions and exposure to heat and moisture the structural properties of the composite are impacted CITATION MaY18 \l 1033 (Ma, et al., 2018). The fiber-matrix interaction is critical for the composites to facilitate the transmission of stresses from the fibers on to the matrix. This interface determines the structural properties of the composites and ultimately impacts its operational performance CITATION Won85 \l 1033 (Wong & Broutman, 1985). The impregnation of moisture in the layers can produce chemical changes such as relaxation and oxidation in the composite material. Such cyclic exposure to moisture could lead to irreversible damages such as the debonding of the fiber from the matrix, degradation of matrix and causing it to crack CITATION MaY18 \l 1033 (Ma, et al., 2018).
The ingress of moisture into the matrix polymer could be attributed to the polymer microstructure and the chemical kinetics characterized by the moisture attraction towards the polymer molecules. Epoxy resins are the primary matrix polymer for the Carbon fiber reinforced plastics (CFRP). The molecular topology (nano-voids) of the epoxy resin permits free volume route for the moisture impregnation in the polymer resulting in interaction of the moisture with the polar groups in the epoxy resin. The presence of moisture in the epoxy matrix causes degradation of mechanical properties of the composite which may adversely result in catastrophic failure or depletion in service-life of the composite. The extent of moisture ingress into the polymer is a function of its crystallinity. The level of moisture ingress is proportional to the crystallinity of the polymer, thus increase in crystallinity leads to the increase in moisture ingress CITATION Ala18 \l 1033 (Alam, et al., 2018).
Literature ReviewThe utilization of CFRPs in commercial transport aircrafts has seen significant increase owing to its high tailorability along with other mechanical properties. But the response of composites in certain environmental and operational conditions are points of concern for engineers. Park, et al., 2019, in their paper “An experiment study into aging unidirectional carbon fiber epoxy composite under thermal cycling and moisture absorption”, have studied the environmental degradation of the the unidirectional CFRP. The experimentation for this study was performed using thermoset epoxy resin (Cycom5276-1), Polyacrylonitrile based intermediate modulus carbon fiber (G40-800-24K) and, unidirectional prepreg tape. The test samples were exposed to hygrothermal conditions to determine the effective degardation. The samples were exposed to extreme temperature ranging from -55 C (Tmin) to 70 C (Tmax) for different time cycles. The moisture exposure experiment were conducted at isothermal conditions, 70 C and relative humidity of 98 percent at regular time intervals for different tme cycles. The weight of the sample was continuously monitired between each step of the experiment. The results of the experimentation were compared with the properties of the baseline composite laminates and compared with Cross-ply and Quasi-isotropic composites. As per the results, the unidirectional fiber composite sample lose weight due to thermo-oxidation of the epoxy-resin when exposed to extreme temperatures. Thus, extreme temperatures cause the degradation of the epoxy resin causing the breakage of the polymer bonds and adversely impacting its mechanical properties. The moisture absorption of the sample followed the Fick’s diffusion law, the transient moisture absorption (Mt) increases lineary with exposure time (t) and subsequently levels off.


Figure SEQ Figure \* ARABIC 2: Normalized weight change curves (a) thermal cycling; and (b) moisture absorptionThe mechanical properties of composites greatly depend upon the exposure to environmental conditions such as moisture. This exposure may be during the manufacturing of the composite or during the service-life. The study by Chouhan, et al., focuses on the compressive high strain rate properties characterization of Ultra-High-Molecular -Weight-Polyethylene (UHMWPE) composite. For the experimentation, two different gardes of UHMWPE composite prepreg were used to create three sets of specimen, i.e., fresh dry, wet, and forced dried. For the high-strain testing of the samples, a compressive split Hopkinson pressure bar (SHPB) test appartus was configured. The test samples were created using 16 layers of prepregs at 34.5 2 ?r pressure at two different temperature ranges of 116 C and 127 C. The three sets of test specimen were exposed to environemnt conditions as described by their names. The difference in weight of each of the specimen were noted after the environmental conditioning. The rate-dependent behavious of the two UHMWPE composites showed that the moisture ingress has resulted in the stress loss of 52.2% and 24.6%, toughness loss of 66.5% and 24.7% respectively. Expected results were attained in case of moisture egress such that, the stress loss under the HSR loading decreased by 14.7% and 6.5% while the toughness loss were down by 33.6% and 24.9%. The results proved the detoriation of properties due to ingress of mositure in composites and the importance of the polymer selection process.
The effect of moisture diffusion in woven carbon/epoxy composites were presnted by Sinchuk, et al., in their paper. The reasearch utilized -Computed Tomography image-based finite element (FE) models to analyse swelling and stressed induced in the composites due to mositure diffusion. Analytical models are created of composite with voids and without viods. The diffusion simulation is performed on both the models to calculate diffusion induced swelling and stress in the composite. The diffusion properties of the composite are derived using FE analsyis and periodic boundary conditions.
Impact damages on composites cause serious threat to the operational performance of aircrafts. Low-velocity impacts with no exterior damages are difficult to locate and thus could lead to catastrophic failures. Jean-St-Laurent, et al., in this paper have studied the behviour of carbon/epoxy composites under compression, in case of impact at extremly low temperattures. Five impact conditions were considered for the experimentation progressing from barely visible to total perfroation of the test samples. The experimentation results presented three failure modes as given in the Figure 1 below. First local buckling, second crack propogation from the point of impact or damge towards the specimens outer edges and third, a hybrid failure mode comprising of both crack propagation and the local bucking. It also confirmed that the impact damage reduces the residual compressive strength of the composites and crack propagation is the primary failure condition at all the temperatures.

Figure SEQ Figure \* ARABIC 3: Failure modes observed for specimens under in-plane compression after impactApart from CFRP, glass-reinforced composites are another major class of composites. The “Influence of long-term moisture exposure and impact damage on the residual compressive strength of glass-reinforced vinylester” were researched by Alizadeh & Soares. Similar to the CFRP, the glass-vinylester composite face deteriation of its mechnaical properties on exposure to moisture. The researchers conducted Compression-after-impact test on the specimen to determine the residual strength of glass-fiber composite at room temperature post-moisture exposure. Unlike the CFRP, the glass-vinylester composte in this experiment do not confer to the Fick’s absorption behaviour. The compressive failure behvaviour indicates fiber failure to be the primary cause of total failure of the composite.
Research GapThe research gaps identified from the literature review in Section 2 are described below:
Most studies focus on the general characteristics or response of the composites to hygrothermal conditions and do not consider the specific loading conditions.
Lack of focused studies on CFRP used in aerospace components and operational conditions.
The influence of moisture ingress on the growth of delamination in CFRP has not been investigated yet.
The lack of in-depth research in determining the residual strength of CFRP exposed to moisture.
ObjectivesThe main objectives of the project determined from the research gaps identified in Section 3, are as follows:
1. To develop linear model of CFRP and analyze the effect of moisture under different loading conditions.
2. To investigate the effect of moisture on delamination of CFRP and its propagation within it to predict its residual strength.
Research QuestionsThe research questions addressing the project objectives are as follows:
1. How does moisture effect the bonding between the polymer matrix and fiber in case of CFRP?
2. Which loading condition would have the maximum decrease in residual strength of CFRP due to moisture?
Expected OutcomesThe results of this project would explain the interaction between the constituents of CFRP composite in the presence of moisture. Moreover, the adverse impact on the mechanical properties and subsequent damage in form of delamination could be explained. The modelling and analysis would present the way to determine the residual strength in CFRP composites under loading condition of interest. This project would also corroborate some of the existing research in understanding the effect of moisture on composites.
Research MethodologyThe research would be based on modelling and simulation using Abaqus. For this project, microscopic modelling of the composite would be performed as to model both matrix and the fiber reinforcement material separately. The finite element simulations would be modelled as layered-shells and layered-solids to study the effect and propagation of delamination in every layer. The models would also entail progressive damage and failure prediction for fiber reinforcement and the matrix.
 

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