Лако-красочные материалы — производство

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Andrea Bernasconi and Md Kharshiduzzaman

Politecnico di Milano, Dipartimento di Meccanica, Milano, Italy

Abstract

In modern engineering applications, use of bonded composite joints is becoming increasingly widespread due to the numerous advantages over conventional mechanical fastening. For load bearing applications, fatigue cracks, either induced by defects or by applied stresses, may appear and propagate, thus becoming potentially harmful for the structural integrity of a part or a whole structure. Therefore, in-situ structural health monitoring (SHM) of bonded joints is essential to maintain reliable and safe operaional life of these structures. This chapter presents various in-situ SHM techniques which are used to monitor bonded composite joints with a focus on fatigue crack monitoring based on the backface strain (BFS) technique. Case studies are presented and discussed for adhesively bonded single lap joint (SLJ). Sensors associated with this technique are also explained, with emphasis on Fiber Bragg Grating (FBG) optical sensors.

Andrea Bernasconi, Associate Professor, Department of Mechanical Engineering, Politecnico di Milano, Via La Masa 1, 20156 Milano, Italy Email: andrea. bernasconi@polimi. it; Tel.: +39-02-2399-8222 ; fax: +39-02-2399-8263.

Keywords: Structural Health Monitoring (SHM), Composite material, Bonded joint, Back Face Strain (BFS) technique, Fiber Bragg Grating (FBG) sensor, Single Lap Joint (SLJ)

Introduction

Composites have emerged as important materials in the last few decades because of their high specific stiffness and high specific strength, allowing for lighweight design solutions; moreover, they possess excellent fatigue resistance and outstanding corrosion resistance compared to most common metallic alloys, such as steel and aluminum alloys. Besides, composites include the ability to fabricate directional mechanical properties, low thermal expansion properties and high dimensional stability. It is the combination of outstanding physical, thermal and mechanical properties that makes composites attractive to use in place of metals in many applications, particularly when weight-saving is required [1, 2].

Bonded joints are extensively employed in the construction of composite structures in aerospace applications, maritime structures, lifting equipment, wind mills as well as automotive industries [3, 4, 5]. Unlike the bolt hole in mechanical fastening that causes a stress concentration in the composite joint plates, adhesively bonded joints minimize the potential for stress concentration within the joint. Besides, applications where lower structural weight, improved damage tolerance design philosophy are required, adhesively bonded joints provides a potential solution. Bonded joints are an efficent fastening solution also for hybrid structures, i. e., structures where composite parts are connected to metal parts.

In the case of high strength joints, difficulties arise because manufacturing defects and operational loads (static, cyclic fatigue loads or impacts) can cause local debonding. Therefore, to maintain damage tolerance design philosophy in cases of composite structures as well as increasing safety and decreasing cost service and maintenance purposes, developing structural health monitoring (SHM) technique for evaluating and monitoring critical bonded composite joints is extremely important.

In this chapter, various SHM techniques applicable to monitoring the structural integrity are presented and discussed. Special attention is given to the SHM technique to monitor fatigue crack propagation based on strain field measurement. Basic principle of strain based SHM method is explained and some case studies, referring to this technique applied to bonded joints using both electrical resistance strain gauge and Fiber Bragg Grating (FBG) optical sensor, are predented.