A Practical Tool for the Preliminary Design of Bonded Composite Repairs

Composite structures are increasingly finding more applications in the aeronautical field as well as the automotive one, thanks to their low weight – performance ratios, in terms of strength and stiffness. However, composite materials, as well known, are characterized by a critical behavior in terms of detectability of damage and performances of damaged components. A critical aspect related to damaged composite structures is, for sure, the repair aimed to restore the original stiffness and strength characteristics of the component depending on the damage typology and location. In this paper, a preliminary repair design tool is presented. The tool is aimed to help the designer by suggesting different repair typologies and proper repair size. This tool, by means of optimization analyses can provide the best repair solution with minimal adhesive shear stress and size of the repair patch. The tool has been tested against a literature case study on multistep composite-metal joints.     

Fracture Analysis of Double-Side Adhesively Bonded Composite Repairs to Cracked Aluminium Plate Using Line Spring Model

A line spring model is developed for analyzing the fracture problem of cracked metallic plate repaired with the double-sided adhesively bonded composite patch. The restraining action of the bonded patch is modeled as continuous distributed linear springs bridging the crack faces provided that the cracked plate is subjected to extensional load. The effective spring constant is determined from 1-D bonded joint theory. The hyper-singular integral equation (HSIE), which can be solved using the second kind Chebyshev polynomial expansion method, is applied to determine the crack opening displacements (COD) and the crack tip stress intensity factors (SIF) of the repaired cracked plate. The numerical result of SIF for the crack-tip correlates very well with the finite element (FE) computations based on the virtual crack closure technique (VCCT). The present analysis approaches and mathematical techniques are critical to the successful design, analysis and implementation of crack patching.     

Analysis of Multiple Bonded Patch Interaction Simple Design Guidelines for Multiple Bonded Repairs in Close Proximity

The use of bonded composite patches to extend the life of aging metal airframes continues to grow as air forces and airline companies operate larger fleets beyond their original design lives. The crack-slowing power of bonded composite patches is well-known; often the process can be accomplished at a few percent of the cost of replacing the damaged component. However, care must be taken in the design of repairs to multiple site fatigue and corrosion damage so the repairs in close proximity do not create additional structural problems by inducing high stresses into the area of the repair.The current paper discusses recent modeling efforts to quantify the effects of multiple bonded composite patches in close proximity. It is well-established that a single structural repair attracts load into the locally stiffened area around the patch; further, single-sided repairs induce significant bending in the skin. The paper addresses the situation when two or more structural repairs occur near each other, and illustrates with simple design rules the acceptable practices for this case.     

航空结构用增韧复合材料的研究与开发

先进复合材料国防科技重点实验室在拥有自主知识产权的"离位"增韧新概念基础上,成功开发了兼具优异工艺性和高抗冲击损伤性的航空级"离位"增韧的热压罐固化复合材料体系和液态成型(RTM及RFI等)复合材料体系.这些增韧的材料体系包括高性能的环氧树脂体系和双马来酰亚胺体系的复合材料.经试验验证,"离位"增韧可在显著提高复合材料层问韧性、冲击损伤阻抗和容限的同时,保持材料原有优异的成型工艺性和湿热性能.     

Comparison of Double- and Single-Bonded Repairs to Symmetrical Composite Structures

Journal of Failure Analysis and Prevention - The object of this study is an experimental analysis of the repair of a laminate composite damaged by central circular notch. The effects of the repair...     

Design of Emergency Aircraft Repairs Using Composite Patches

The increasing demand for fatigue life extension of both military and civilian aircraft has led to advances in repair technology for cracked metallic structures. Conventional structural repairs may significantly degrade the aircraft fatigue life and lower its aerodynamic performance. Adhesively bonded composite reinforcement is a new technology of great importance due to the remarkable advantages obtained, such as mechanical efficiency and repair time and cost reduction. In this article, bonded composite patch repairs were designed for quick application to aircraft under emergency conditions, such as aircraft battle damage repair (ABDR). A formulated method was developed, to be applied when damage has to be restored quickly, without restrictions to safety of flight. Different damage cases were investigated using finite-element analysis (FEA), taking into account specific parameters of the structure under repair. Based on the FEA results, a quick design procedure using composite patch repairs for the most frequent damage cases is proposed.     

复合材料胶接修复受损结构力学建模研究进展

复合材料胶接修复结构是一个包含损伤母板、聚合物胶粘剂层及复合材料补片的多层复杂体系,其力学模型主要有2种,即解析模型与数值模型.详细阐述了2种力学模型研究的发展过程及现状,介绍了国内外对该项研究的主要内容,并展望了其发展趋势.     

An Affordable Approach for Robust Design of Thick Laminated Composite Structure

A systematic and affordable approach is proposed for the robust design of thick laminated composite structures. Our approach integrates the principles of the Robust Concept Exploration Method (RCEM) for designing complex engineering systems and the hierarchical multi-level optimization procedure for managing the complexity of composite structure optimization. Foundational to the proposed approach is the use of Design of Experiments (DOE) techniques and the Response Surface Methodology (RSM) for improving computational efficiency in using high fidelity design simulations; and the use of the robust design method for improving the quality of a product that is insensitive to potential variations of design parameters. Our approach is illustrated through the design of a laminated composite femoral component for hip joint arthroplasty. The solution yields the robust design of a composite hip implant, which is applicable for a range of bone stiffness, thereby eliminating the need to design specifically for an individual.     

Optimization of Embedded Optical Sensor Location in Composite Repairs

Optical fibers were embedded in a bonded composite patch in order to detect the strain field variations of a load bearing structure. The study concentrated on a classical cracked metallic structure repaired with this smart patch and using finite element analysis. Six different laminates constituted the model of the composite patch, a layered structure with three-dimensional elements. Each laminate is assumed to have different mechanical properties, according to the case under any specific study, in order to simulate different stacking sequence or material used. A resin rich eye pocket has also been modeled in order to simulate the exact form of the resin area produced during the manufacturing process. The patch is bonded over a cracked aluminum sheet through a small adhesive layer placed in between. External loads were applied only on the metal structure, as in a real repair case. The primary loading axis of the metal was assumed to be parallel to the direction of the optical fibers. The different nature of the materials that form the composite patch generated complex mechanical interactions between the fibers and the surrounding material, resulting in a complicated stress field along the optical fiber sensor, which affects the structural integrity of both the patch and the repair. Different optical fiber positions were considered, moving towards the horizontal and vertical dimensions of the patch, as well as different patch architectures (single and double patch configurations), with the hope of studying their effect on the structural integrity of the patch.     

Repair Substantiation for a Bonded Composite Repair to F111 Lower Wing Skin

This paper presents the results obtained to date as part of a comprehensive repair-substantiation program currently in progress for a safety-critical repair to an F-111 lower wing skin. From the viewpoint of demonstrating compliance with certification requirements, this F-111 repair represents the most technically challenging bonded repair undertaken to date by the RAAF. It is particularly significant that the repair was designed and implemented entirely by RAAF personnel, on the basis of design principles and application procedures developed by DSTO. This constitutes an important milestone in technology transfer from DSTO to the RAAF.The repair substantiation involves both detailed finite element (FE) stress analyses and structural testing at three levels, ranging from coupon-size specimens (representative bonded joints) to quasi- full-scale specimens representing a spar-stiffened wing-box structure. The close coupling of analysis and testing is shown to lead to a time- and cost-efficient certification package, with a confidence level comparable with full-scale structural testing.     

Three-Dimensional Static and Dynamic Analysis of a Composite Cruciform Structure Subjected to Biaxial Loading: A Discontinuum Approach

A three-dimensional structural integrity analysis using the eXtended Finite Element Method (XFEM) is considered for simulating the crack behaviour of a chopped fibre-glass-reinforced polyester (CGRP) cruciform specimen subjected to a quasi-static tensile biaxial loading. This is the first time this problem is accomplished for computing the stress intensity factors (SIFs) produced in the biaxially loaded area of the cruciform specimen. A static crack analysis for the calculation of the mixed-mode SIFs is carried out. SIFs are calculated for infinite plates under biaxial loading as well as for the CGRP cruciform specimens in order to review the possible edge effects. A ratio relating the side of the central zone of the cruciform and the crack length is proposed. Additionally, the initiation and evolution of a three-dimensional crack are successfully simulated. Specific challenges such as the 3D crack initiation, based on a principal stress criterion, and its front propagation, in perpendicular to the principal stress direction, are conveniently addressed. No initial crack location is pre-defined and an unique crack is developed. Finally, computational outputs are compared with theoretical and experimental results validating the analysis.     

复合钕铁硼/铁氧体黏结磁体电泳工艺研究

复合钕铁硼磁体在高温潮湿环境下易被腐蚀,因而需要涂层保护.研究了复合永磁体基体性能和阴极电泳环氧树脂工艺参数对复合钕铁硼/铁氧体黏结磁体的电泳涂漆质量的影响,通过QJ57直流电阻电桥测试了磁体的电阻,采用盐雾试验和湿热试验测试了涂层耐腐蚀性能.结果表明,复合黏结磁体中锶铁氧体磁粉含量质量分数超过30%时,电阻率急剧升高,难以获得优良的涂层.采用EED-060M环氧树脂电泳漆,在漆液温度28～32 ℃,电压120～180 V,固化温度150～170 ℃的电泳工艺条件下,在锶铁氧体含量质量分数少于30%的磁体表面可包覆上一层致密、光滑的,且具有优异耐蚀性能的涂层.     

塑料粘结钕铁硼─铁氧体材料的复合效应研究

本文研究了ＮｄＦｅＢ－铁氧体－塑料３者复合成的永磁材料的复合效应。结果表明塑料粘结ＮｄＦｅＢ－铁氧体复合材料的＿ｂＨｃ、＿ｊＨｃ、（ＢＨ）＿ｍ随ＮｄＦｅＢ含量增加而升高，呈中间偏凹的趋势，压缩强度和硬度下降；同时随着塑料含量增加，磁性能Ｂ＿ｒ，＿ｂＨｃ，（ＢＨ）＿ｍ降低，而内禀矫顽力＿ｊＨｃ却上升。     

Biodegradable Materials for Bone Repairs:A Review

With attractive research and development of biomaterials,more and more opportunities have been brought to the treatments of human tissue repairs.The implant is usually no need to exist in the body accompanied with the recovery or regeneration of the tissue lesions,and the long-term effect of exotic substance to human body should be reduced as lower as possible.For this purpose,biodegradable materials,including polymers, magnesium alloys and ceramics,have attracted much attention for medical applications due to their biodegradable characters in body environment.This paper in turn introduces these three different types of widely studied biodegradable materials as well as their advantages as implants in applications for bone repairs.Relevant history and research progresses are summarized.     

A Cohesive Zone Approach for Fatigue-Driven Delamination Analysis in Composite Materials

A new model for prediction of fatigue-driven delamination in laminated composites is proposed using cohesive interface elements. The presented model provides a link between cohesive elements damage evolution rate and crack growth rate of Paris law. This is beneficial since no additional material parameters are required and the well-known Paris law constants are used. The link between the cohesive zone method and fracture mechanics is achieved without use of effective length which has led to more accurate results. The problem of unknown failure path in calculation of the energy release rate is solved by imposing a condition on the damage model which leads to completely vertical failure path. A global measure of energy release rate is used for the whole cohesive zone which is computationally more efficient compared to previous similar models. The performance of the proposed model is investigated by simulation of well-known delamination tests and comparison against experimental data of the literature.     

Damage Analyses of Adhesively Bonded Single Lap Joints Due to Delaminated FRP Composite Adherends

Three-dimensional non-linear Finite Element Analyses (FEA) due to an in-plane loading have been performed to evaluate the out-of-plane normal and shear stresses over the overlap region of a Single Lap Joint (SLJ) on different surfaces. These surfaces have been considered as; (i) two interfacial surfaces between the adherends and the adhesive layer, (ii) the mid-surface of the adhesive layer and (iii) two surfaces beneath the surface ply of both the adherends adjacent to the adhesive layer. The critical locations of onset of adhesion, cohesion and delamination failures on the above mentioned surfaces of the SLJ have been determined using suitable damage criteria. A comparative study due to adhesion, cohesion and delamination failures in the SLJ with Fiber Reinforced Polymeric (FRP) composite adherends have been presented. The effects of simultaneous variations of the delamination positions on the out-of-plane peel and shear stress components have been studied by pre-embedding the delamination damages at the critical locations in both the adherends. It has been observed that the possibilities of onset of cohesion failures in the adhesive layer are higher compared to the adhesion and delamination failures. The detailed analyses showed that secondary peaks of out-of-plane stress components (σ _z , τ _yz and τ _xz ) on the mid surface of the adhesive layer appeared at the locations closer to the delamination fronts due to pre-embedded delamination damages. The highest stress magnitudes on the overlap edge of the SLJ have been reduced significantly when the centers of the delamination damages are exactly aligned with the overlap ends of the joint. No significant variations of stress magnitudes have been noticed either when the delaminations are pre-embedded outside the overlap regions or when the delamination damages are completely entrapped within the overlap region.     

New Approach for the Application of Functional Ceramic Material in Carbon Bonded Doloma Refractories to Reduce Emissions

Doloma carbon bricks with graphite contents of approximately 2 wt% are widely used in the production of stainless steels in argon oxygen decarburisation (AOD) or in vacuum oxygen decarburisation (VOD) vessels as lining material. The application of doloma refractories is connected with metallurgical benefits such as high oxidic stability of its oxides, and the ability to bond sulphur from the hot metal. The production and application of carbon bonded refractories is linked with environmental harmful emissions in the broadest sense. Amongst the aspect of environmental friendly refractory systems this work has observed and shown the interaction of functional ceramic material TiO₂ with the organic binder system. In the centre of this work is the aspect of increased residual carbon content of the binder resin due to TiO₂ addition. The increased residual carbon content of the binder resin connected with improved mechanical, physical and thermomechanical properties due to sub‐micro scaled TiO₂ addition offers the feasibility to reduce the total carbon content without downgrading the brick properties. This aspect has not been observed yet and is of high interest with respect to reduced emissions and environmental friendly refractories. Previous works have investigated the influence of TiO₂ on other carbon bonded refractory systems such as alumina carbon and magnesia carbon. As illustrated in this work and previous work, TiO₂ is working completely different in the Doloma Carbon system from other systems.     

复合材料胶接修理层合板拉伸性能及影响参数

胶接修理是效率较高、应用较广的复合材料结构修补技术。对采用不同参数进行挖补和贴补修理的复合材料层合板的拉伸性能进行实验研究。结果表明:挖补修理实验件的强度恢复率约为66%~91%,贴补修理实验件的强度恢复率约为44%~61%。在挖补修理实验件中,减小挖补斜度、采用双面挖补、使用热压罐固化,在贴补修理实验件中,采用双面贴补、增大补片尺寸,均可得到更高的强度恢复率。在实验基础上建立的有限元模型,能够有效预测实验件的失效载荷、破坏模式,并可分析实验件的应力分布和渐进损伤过程,为设计修理方案提供参考。     

民机复合材料结构适航审定现状

随着民用飞机上复合材料结构所占比例的不断增加,在适航审定领域,复合材料航空件占有了越来越大的比重.阐述了复合材料在民用飞机结构中的应用情况,重点评述了国内外民机复合材料结构适航审定技术的现状和发展趋势,在此基础上指出了我国民机复合材料结构适航审定中存在的问题.