Durability of bonded crack retarders for aerospace

The increase in demand for aircraft focuses the structural designers and manufacturers towards the reduction of manufacturing cost and structural weight while maintaining high safety, damage tolerance, and structural integrity. These weight savings can be achieved by using large integral structures. However, large integral structures show reduced performance with respect to damage tolerant design due to lack of physical barriers that can arrest a growing crack such as presently exist in structures joined with rivets and bolts. Fibre metal laminates such as Glass Laminated Aluminium Reinforced Epoxy (GLARE) have been proven effective as bonded crack retarders (BCR) in reducing the fatigue crack growth rate and improving the life of metallic aircraft structures. A major problem associated with bonded crack retarders is the development of thermal residual stresses which may have negative impact on the performance of the structure. Hence, the objectives of this research are to investigate the thermal residual stress developed during the strap bonding process and the fatigue durability of bonded crack retarders. Extensive research performed in this dissertation covers the detailed investigation of thermal residual stresses and the fatigue durability of GLARE bonded crack retarders when incorporated onto different structural coupons and on an aircraft mock-up panel. Thermal residual stresses developed during the strap bonding process are very low and the application of a bonded crack retarder improved the fatigue performance of the specimen. The experimental data on residual stress measurements and fatigue testing provides information for researchers and aircraft structural designers to improve the performance and life of critical aircraft structures and the possibilities of incorporating the bonded crack retarder concept in the initial design and fabrication stages

Bond-slip Relationship of Carbon Fiber Reinforced Polymer (CFRP) Plated Steel Member under Fatigue Loading

This paper intended to discuss in depth the provision of the groundwork for the development of bond-slip relationship of carbon fiber reinforced polymer (CFRP) plated steel member under fatigue loading. The bond-slip characteristics of the adhesive joint between the CFRP and steel have been studied under monotonic load so that debonding does not occur whilst the members are in service. A typical bond-slip relationship is assumed to be bilinear consisting of an elastic branch which peaks at τmax and the softening branch up to δmax. However there are relatively few studies on the bond-slip relationship due to fatigue loading. Thus this research focuses to study the behavior of this composite system by thoroughly examining the shear stress distribution along the bonded length. Experimental program using single lap pull test subjected to monotonic loading is carried out using CFRP plated steel block. Later, fatigue life prediction of the composite system is done using stress-life approach in order to come up with a suitable fatigue loading program. The output of this research will be a firm base for a good formulation of the bond-slip relationship under fatigue loading and therefore will enhance the knowledge of time-dependant behavior for steel bridges, steel jetty and offshore platforms retrofitting as well as the development of design standard for fatigue conditions

An experimental and numerical study of repairs on composite substrates with composite and aluminum doublers using riveted, bonded, and hybrid joints

In this work, experimental and numerical analyses of repairs on carbon fiber reinforced epoxy (CFRE) substrates, with CFRE and aluminum alloy doublers typical of aircraft structures, are presented. The substrates have a bridge gap of 12.7 mm (simulated crack), repaired with twin doublers joined with riveted, adhesive bonded, and hybrid joints. The performance of the repairs using different doubler materials and joining techniques are compared under static loading. The experimental results show that riveted joints have the lowest strength, while adhesive bonded joints have the highest strength, irrespective of the doubler material. Finite element analysis (FEA) of the studied joints is also performed using commercial FEA tool Abaqus. In the FEA model, point-based fasteners are used for the rivets, and a cohesive zone contact model is used to simulate the adhesive bond. The FEA results indicate that the riveted joints have higher tensile stresses on the metal doublers compared to the composite doublers. As per the failure modes, interestingly, for hybrid joints using composite doublers, the doublers fail due to net-section failure, while, for hybrid joints using metal doublers, it is the composite substrate that fails due to net-section failure. This suggests vulnerability of the composite structures to mechanical fastener holes. Lastly, the Autodesk Helius composite tool is used for prediction of first-ply failure and ply load distribution, and for progressive failure analysis of the composite substrate.Peer ReviewedPostprint (published version

Video and laser microscopy investigation of adhesive joint fracture in fiber reinforced composite materials

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1996.Includes bibliographical references (p. 141-142).by David H. Walworth.M.S

The Working Principles of a Multifunctional Bondline with Disbond Stopping and Health Monitoring Features for Composite Structures

In comparison to bolted joints, structural bonds are the desirable joining method for light-weight composite structures. To achieve a broad implementation of this technology in safety critical structures, the issues of structural bonds due to their complex and often unpredictable failure mechanisms have to be overcome. The proposed multifunctional bondline approach aims at solving this by adding two safety mechanisms to structural bondlines. These are a design feature for limiting damages to a certain size and a structural health monitoring system for damage detection. The key question is whether or not the implementation of both safety features without deteriorating the strength in comparison to a healthy conventional bondline is possible. In previous studies on the hybrid bondline, a design feature for damage limitations in bondlines by means of disbond stopping features was already developed. Thus, the approach to evolve the hybrid bondline to a multifunctional one is followed. A thorough analysis of the shear stress and tensile strain distribution within the hybrid bondline demonstrates the feasibility to access the status of the bondline by monitoring either of these quantities. Moreover, the results indicate that it is sufficient to place sensors within the disbond stopping feature only and not throughout the entire bondline. Based on these findings, the three main working principles of the multifunctional are stated. Finally, two initial concepts for a novel multifunctional disbond arrest feature are derived for testing the fundamental hypothesis that the integration of micro sensors into the disbond stopping feature only enables the crack arrest and the health monitoring functions, while reaching the mechanical strength of a conventional healthy epoxy bondline. This work therefore provides the fundamentals for future investigations in the scope of the multifunctional bondline

Distributed Optical Sensing in Adhesively Bonded Joints and Polymer Matrix Composite Laminates

As the use of polymer matrix composites for structures increases, there is a growing need for monitoring these structures. Distributed strain sensing using optical fibers shows promise for monitoring composite structures due to optical fiber\u27s small size, light weight, and ability to obtain continuously distributed strain data. This study investigates the feasibility of using embedded optical fibers using two case studies: embedding the fibers in the adhesive layer of double lap shear composite specimens, and within composite end-notched flexure specimens to locate a growing crack front. To establish a repeatable fabrication methodology, manufacturing techniques for embedding the optical fibers were investigated. The measured strain distribution from the optical fibers compares well with data obtained from finite element analyses for both the double lap shear and end-notch flexure specimens. Additionally, the embedded optical fibers do not seem to impact the failure loads or fracture behavior of the specimens

Axially Loaded Steel Columns Strengthened with CFRP

During the recent decades, Carbon Fiber-Reinforced Polymer (CFRP) composite materials have proven valuable properties and suitability to be used in the construction of new buildings and in upgrading the existing ones. The objective of this paper is to review the previous work in this area and compare it with the current experimental results to show the design equations for CFRP strengthened steel structural elements. Research findings have shown that CFRP sheets and strips are not only effective in restoring the lost capacity of a damaged steel section, but are also quite effective in strengthening of steel sections to resist higher loads, extend their fatigue life and reduce crack propagation