Analysis of adhesively bonded composite patches used in repair of aerospace structures

Bonded composite patches have been successfully used to restore damaged metallic airframes in both military as well as civil aircrafts and this technique has been recognized as one of the most effective methods for increasing the durability and damage tolerance of the repaired structure. These repairs alter the load path, bridge the damage and reduce stress intensity factors thereby retarding damage growth. Although much work has been done in the design of such patches, an important area, which has been neglected, is the design of composite repair patches taking into account the effect of temperature. The objective of this thesis is to analyze adhesively bonded composite patches for the repair of aluminum substrates subjected to thermal loads. The first paper included in this thesis presents the results of a parametric finite element analysis directed towards minimizing the thermal residual stresses present in the bonded composite repair. The second paper uses FEA to investigate the free-vibration and transient response of damaged substrate repaired by an adhesively bonded composite patch subjected to thermal loads. This thesis attempts to understand the effect of temperature on the bonded composite repair. The study includes the use of FEA for designing over incumbent thermal residual stresses and outlines the effect of temperature on the bonded composite joint. By studying the effect of temperature on such adhesively bonded composite repairs this study hopes to remove some of the obstacles faced during the application of such techniques in the repair and life enhancement of aging aircrafts --Abstract, page iv

Fatigue analysis of V-ribbed serpentine belt drive system

V-ribbed belt drive system provides an efficient means of transmitting power to front-end accessories common to modern automobiles. Fatigue failure and misalignment are the two primary issues that limit the performance and durability of V-ribbed belt drives. The focus of the present work is to develop a comprehensive fatigue crack growth model and to study the effect of misalignments --Abstract, page iii

Thermomechanical Analysis of Composite Cylinders for Hydrogen Storage

Safe installation and operation of lightweight composite hydrogen storage cylinders are of primary concern. Typically, the inner liner of the cylinder is made with a high molecular weight polymer or aluminum that serves as a hydrogen gas permeation barrier. A filament-wound, carbon/epoxy composite laminate placed over the liner provides the desired pressure load bearing capacity. In many current designs, a glass/epoxy layer or other material is placed over the carbon/epoxy laminate to provide impact and damage resistance. These cylinders also have pressure/thermal relief devices that are activated in case of an emergency. The difficulty in accurately analyzing the behavior of a filament wound composite storage cylinder derives form the continually varying orientation of the fibers. Most of the analysis reported in filament wound composite cylinders is based on simplifying assumptions and does not account for complexities like thermo-mechanical behavior and highly orthotropic nature of the material. In the present work, a comprehensive finite element simulation tool for the design of hydrogen storage cylinder system is developed. The structural response of the cylinder is analyzed using laminated shell theory accounting for transverse shear deformation and geometric nonlinearity. A composite failure model is used to predict the maximum burst pressure. Results for various thermomechanical loading cases are presented

Temperature Dependent Fatigue-Failure Analysis of V-Ribbed Serpentine Belts

The effect of temperature on fatigue life of V-ribbed serpentine belts is investigated. A predictive fatigue crack growth model to monitor progressive deterioration of initially small rib tip flaws subjected to thermal and mechanical loads is developed. The model is based on computational fracture mechanics and temperature dependent fatigue coupon tests. A global-local finite element strategy is used to compute the J-integral for a through-the-thickness crack in the rib tip. The finite element model accounts for thermal strains and temperature dependent properties of rubber. The three-dimensional global model is created with a coarse mesh using first order continuum elements while the local model for the rib crack is constructed with significantly finer mesh utilizing second order continuum elements. Maximum and minimum J-integrals calculated at two extreme configurations for a single belt running cycle is used to estimate the fatigue life of the belt. The J-integral and fatigue life estimates obtained from the analysis show that the life of the belt is significantly affected while operating at elevated temperatures

Finite Element Analysis of V-Ribbed Belt/Pulley System with Pulley Misalignment Using a Neural-Network-Based Material Model

Pulley misalignment limits the performance of V-ribbed belt/pulley system as it relates to rib load-sharing and contact pressure distribution for multiple rib belts required in high torque demands of modern automotive applications. In this paper, a three-dimensional dynamic finite element model is built to evaluate the effects of pulley misalignment. The model consists of a pulley and a segment of V-ribbed belt in contact with the pulley. A material model of belt, including rubber compound and reinforcing cord is developed. Multiple rubber layers are each considered hyperelastic with distinct material characterization parameters. A novel neural-network-based hyperelastic material model is implemented to represent properties of nonlinear elastic belt-rib compound. The models are implemented in the commercial code ABAQUS/ Explicit to simulate the misalignment of the belt- pulley system. The developed model is first validated by experimental measurements of pulley lateral force due to misalignment. Also, three common types of misalignment in the belt-pulley system are analyzed and results are presented

Analysis of Composite Hydrogen Storage Cylinders under Transient Thermal Loads

In order to ensure safe operation of hydrogen storage cylinders under adverse conditions, one should be able to predict the extremities under which these cylinders are capable of operating without failing catastrophically. It is therefore necessary to develop a comprehensive model which can predict the behavior and failure of composite storage cylinders when subjected to various types of loading conditions and operating environments. In the present work, a finite element model has been developed to analyze composite hydrogen storage cylinders subjected to transient localized thermal loads and internal pressure. The composite cylinder consists of an aluminum liner that serves as a hydrogen gas permeation barrier. A filament-wound, carbon/epoxy composite laminate placed over the liner provides the desired load bearing capacity. A glass/epoxy layer or other material is placed over the carbon/epoxy laminate to provide damage resistance for the carbon/epoxy laminates. A doubly curved composite shell element accounting for transverse shear deformation and geometric nonlinearity is used. A temperature dependent material model has been developed and implemented in ABAQUS using user subroutine. A failure model based on Hashin\u27s failure theory is used to predict the various types of failure in the cylinder. A progressive damage model has also been implemented to account for reduction in modulus due to failure. A sublaminate model has been developed to save computational time and reduce the complications in the analysis. A numerical study is conducted to analyze a typical hydrogen storage cylinder and possible failure trends due to localized thermal loading and internal pressure is presented

Impact Behavior of Fiber Reinforced Pultruded Soy-Epoxy Composites

Fiber reinforced composites in general are brittle and exhibit poor impact and damage resistance. In the present work, the impact response of an epoxy resin system was improved by addition of a soy-based resin system. The soy-based resin was prepared by the process of transesterfication and epoxidation of regular food grade soybean oil. The curing process of the soy epoxy resin system was studied using differential scanning calorimetry. Physicochemical properties of soy-based resin polymers were studied using dynamic mechanical analysis. Glass fiber reinforced composite panels were manufactured using pultrusion process. Low velocity impact tests were performed on the pultruded panels. For the samples impacted at 20 J, the soy based resin system absorbed 16.26 J while base Epon resin absorbed 14.06 J. The experimental results were compared with finite element simulations, which employed a three-dimensional finite element model developed for impact analysis using ABAQUS finite element code. The simulation results were in good agreement with the experimental findings. The soy based composites hold great potential as environmentally friendly and low cost alternatives for structural applications like shelters and affordable housing. Copyright © 2008 American Scientific Publishers. All rights reserved

Affordable Composites Using Renewable Materials

Bio-based composite products are finding widespread applications due to their low cost and environmental acceptability. Development of new bio-based raw material and automated composite manufacturing is the focus of the present study. Pultrusion is the fastest and the most cost-effective composite manufacturing processes, and is well suited for high volume production for structural applications. with growing opportunities to use pultruded composites, the development of cost effective pultrudable resin system is of great interest. A novel soy-based epoxy resin namely epoxidized allyl soyate is synthesized at the University of Missouri-Rolla. This resin forms co-polymers with the base Shell Epon epoxy resin in varied proportions to yield a family of polymeric networks. Glass fiber reinforced composite specimens are manufactured using a Durapul 6000 Labstar Pultrusion machine. The lubricity of soy-based resin significantly reduces the pull force. Mechanical tests show that pultruded composites with soy-based co-resin systems possess comparable or improved structural performance characteristics such as flexural strength, modulus and impact resistance

Analysis of Composite Hydrogen Storage Cylinders Subjected to Localized Flame Impingements

A comprehensive non-linear finite element model is developed for predicting the behavior of composite hydrogen storage cylinders subjected to high pressure and localized flame impingements. The model is formulated in an axi-symmetric coordinate system and incorporates with various sub-models to describe the behavior of the composite cylinder under extreme thermo-mechanical loadings. A heat transfer sub-model is employed to predict the temperature evolution of the composite cylinder wall and accounts for heat transport due to decomposition and mass loss. A composite decomposition sub-model described by Arrhenius\u27s law is implemented to predict the residual resin content of thermal damaged area. A sub-model for material degradation is implemented to account for the loss of mechanical properties. A progressive failure model is adopted to detect various types of mechanical failure. These sub-models are implemented in ABAQUS commercial finite element code using user subroutines. Numerical results are presented for thermal damage, residual properties and profile of resin content in the cylinder. The developed model provides a useful tool for safe design and structural assessment of high pressure composite hydrogen storage cylinders

Mode-I Fatigue Crack Growth Analysis of V-Ribbed Belts

One factor which is critical to the life of automotive serpentine belts is their failure due to propagation of initially small flaws in the elastomer located in the belt rib. This paper aims to develop a predictive fatigue crack growth model for serpentine belts with initially small rib tip flaws. The study combines the fatigue crack growth tests for belt rib material and computational fracture mechanics analysis for V-ribbed belts to predict fatigue life. The belt rib rubber material is tested for its fatigue behavior and the relationship between the fatigue crack growth rate and the tearing energy is constructed. A power law is employed to curve fit the fatigue crack growth rate curve. A global-local finite element analysis procedure is used to compute the J-integral for a through-the-thickness crack in the belt rib tip. The three-dimensional global model is created with relatively coarse mesh using first order continuum elements in ABAQUS. The three-dimensional local model for the rib crack is constructed with significantly finer mesh and employs the second order continuum elements. The boundary conditions for the local model are driven by the displacement solution of the global model. The maximum and minimum J-integrals are calculated for two extreme configurations in one belt running cycle. The range of the J-integral is input into the curve fitted power law to derive the fatigue crack growth rate and hence the fatigue life for the belt