Research Issues in the Repair of Composite Structures

Abstract

Abstract The composites repair in the past was mostly for nonstructural applications. For the present emerging systems and to extend service life of structural members, repairs need to be made at the structural level. Several issues that are imuortant to bonded composite repair and its performance in realistic environmental conditions are discussed, Some of the recent work performed at the Air Force Materials Directorate is presented. The effects of surface preparation and laminate stacking sequence on strength of double-lap shear specimens are described. Introduction The Air Force maintains a number of composite aircraft, and these numbers will increase in the future. Maintaining these aircraft will require extensive repair of damaged composite structures during their operational lifetimes. AS a result there is an urgent need for developing durable and cost effective repair methodology. The existing repair techniques usually use monolithic or composite materials as patches. doublers. etc., which are applicable for the repair of secondary structures and for small strain levels. But the present advanced aircraft, e.g., F-22, B-2, use a significant amount of composite structural parts and operate at higher strain levels. In order to maintain these advanced aircraft, there is a need to develop repair methodology for significantly higher strain levels. The bolted and bonded repairs are both feasible repair techniques. But, the bolted joints in general weigh more than the bonded repair, and the holes created for the bolts also cause undesirable stress concentrations in the parent structures. Due to the availability of high performance and damage tolerant structural adhesives, the bonded repair is preferred over bolted. Thus issues related to the bonded repair will be emphasized and addressed in this project. Research Issues The structural performance of a bonded joint repair greatly depend on the performance of the adhesive that is used in the joint. The durability, damage tolerance, environmental effects, thermo-mechanical fatigue performance, etc. of a bonded joints are not yet well investigated. Of particular concern is the combined effects of repeated moisture exposure and drying cycles, in conjunction with thermal spiking and mechanical loading. Quite often repair (patching) requires to be performed in the field and on aircraft. The equipment availability in field repair is different from that performed in depot. But large and structurally reliable repair in the field must be performed to reduce down time of aircraft. In the past repair used to on nonstructural parts, often small and easy to do. For the emerging aircraft systems and to extend life of existing aircrafts, repairs need to be performed on structural members and the repaired part should be able to reliably perform around a strain level of 5000 microstrain. As a result, there is a need for new repair materials, processes, analyses, and nondestructive inspection (NDI) techniques