Optimisation and Improvement of the Design of Scarf Repairs to Aircraft
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Optimisation and Improvement of the Design of Scarf Repairs to Aircraft Alex Harman BE (Aero. Hons.), GradIEAust A thesis submitted in fulfilment of the degree of Doctor of Philosophy School of Mechanical and Manufacturing Engineering The University of New South Wales August 2006 The work presented in this thesis was conducted as part of a combined research program with the Cooperative Research Centre for Advanced Composite Structures (CRC-ACS) and the Defence Science and Technology Organisation (DSTO). PLEASE TYPE THE UNIVERSITY OF NEW SOUTH WALES Thesis/Dissertation Sheet Surname or Family name: Harman First name: Alex Other name/s: Bruce Abbreviation for degree as given in the University calendar: Phd. School: Mechanical and Manufacturing Engineering Faculty: Engineering Title: Optimisation and improvement of the design of scarf repairs to aircraft Abstract 350 words maximum: (PLEASE TYPE) Flush repairs to military aircraft are expected to become more prevalent as more thick skin composites are used, particularly on the surface of the fuselage, wings and other external surfaces. The use of these repairs, whilst difficult to manufacture provide an aerodynamic, “stealthy” finish that is also more structurally efficient than overlap repairs. This research was undertaken to improve the design methodology of scarf repairs with reduced material removal and to investigate the damage tolerance of scarf repair to low velocity impact damage. Scarf repairs involve shallow bevel angles to ensure the shear stress in the adhesive does not exceed allowable strength. This is important when repairing structures that need to withstand hot and humid conditions, when the adhesive properties degrade. Therefore, considerable amounts of parent material must be machined away prior to repair. The tips of the repair patch and the parent laminate are very sharp, thus a scarf repair is susceptible to accidental damage. The original contributions include: x Developed analytic means of predicting the stresses within optimised scarf joints with dissimilar materials. New equations were developed and solved using numerical algorithms. x Verified using finite element modelling that a scarfed insert with dissimilar modulus subjected to uniaxial loading attracted the same amount of load as an insert without a scarf. As such, the simple analytic formula used to predict load attraction/diversion through a plate with an insert may be used to predict the load attraction/diversion into a scarf repair that contains a dissimilar adherend patch. x Developed a more efficient flush joint with a doubler insert placed near the mid line of the parent structure material. This joint configuration has a lower load eccentricity than external doubler joint. x Investigated the damage tolerance of scarf joints, with and without the external doubler. The results showed that scarf joints without external doublers exhibited a considerable strength reduction following low velocity impact. Based on the observations, the major damage mechanics in the scarf joint region following impact have been identified. These results demonstrated that it is important to incorporate damage tolerance in the design of scarf repairs. Declaration relating to disposition of project thesis/dissertation I hereby grant to the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or in part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all property rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstracts International (this is applicable to doctoral theses only). …………………………………………………………… ……………………………………..……………… ……….……………………...…….… Signature Witness Date The University recognises that there may be exceptional circumstances requiring restrictions on copying or conditions on use. Requests for restriction for a period of up to 2 years must be made in writing. Requests for a longer period of restriction may be considered in exceptional circumstances and require the approval of the Dean of Graduate Research. FOR OFFICE USE ONLY Date of completion of requirements for Award: THIS SHEET IS TO BE GLUED TO THE INSIDE FRONT COVER OF THE THESIS Abstract Flush repairs to military aircraft are expected to become more prevalent as more thick skin composite structures are used, particularly on the surface of the fuselage, wings and other control surfaces. The use of these repairs, whilst difficult to manufacture provide a more aerodynamic, “stealthy” finish that is also more structurally efficient than standard overlap repairs. A lot of design work has been conducted to improve the state of the art in scarf repair design, but some significant shortcomings were identified that led to this research project. This research was undertaken to improve the design methodology of scarf repairs with reduced material removal and to investigate the damage tolerance of scarf repair to low velocity impact damage. Scarf repairs often involve very shallow bevel angles to ensure the shear stress in the adhesive does not exceed allowable strength. This is particularly important when repairing structures that need to withstand hot and humid environmental conditions, when the adhesive properties can degrade to 50% of its room temperature values. As such, a large amount of “good” parent material must be machined away prior to the application of flush repairs. With the tips of the bevelled repair patch and the parent laminate being very sharp, a scarf repair is susceptible to accidental damage. The original contributions of this thesis include x Developed a validated analytic means of predicting the adhesive stresses within optimised scarf joints with dissimilar materials. New governing equations were developed and solved using finite difference algorithms. These equations were validated with finite element methods. x Verified using finite element modelling that a scarfed insert with dissimilar modulus subjected to uniaxial loading attracted the same amount of load as an insert without a scarf. As such, the simple analytic formula used to predict load attraction/diversion through a plate with an insert may be used to predict the load attraction/diversion into a scarf repair that contains a dissimilar adherend patch. x Developed a more efficient flush joint with a doubler insert placed near the mid-line of the parent structure material. This joint configuration has a lower load eccentricity than external doubler joint, thus providing a stronger joint. Experiments and computational analyses have been carried out to assess suitable failure criteria predicting the strength of flush joints. x Investigated the damage tolerance of scarf joints, with and without external doubler. The results showed that scarf joints without external doubler exhibited a considerable strength reduction following low velocity impact. Based on the experimental observations, the major damage mechanics in the scarf joint region following impact have been identified. These results demonstrated that it is important to incorporate damage tolerance in the design of scarf repairs. i List of papers and Reports 1. Harman, A., (2004), Review of literature pertaining to the design and experimentation of aircraft scarf repairs. CRC-ACS TM-05086. 2. Harman, A., (2004), Testing of thick adherend bonded scarf joint 2D test coupons. CRC-ACS TM 04x25. 3. Harman, A. and C. Wang, (2005), Analytic and finite element stress predictions in two-dimensional scarf joints (Paper 364), Proceedings of 11th Australian International Aerospace Congress. 2005. 4. Harman, A.B., Wang C.H., and D. Kelly, (2006), Improved flush repair design for aerospace structures using modified biscuit joints, Proceedings of Structural Integrity and Failure, Sydney. 5. Wang, C.H. and A.B. Harman, (2006), Optimal Scarf Joint between unmatched adherends, Proceedings of SAMPE 2006, Long Beach, CA, USA. 6. Harman, A.B. and Wang, C.H., (2006), Improved design methods for scarf repairs to highly strained composite aircraft structure. Composite Structures, Vol. 75, 132- 144. 7. Harman, A.B. and Wang, C.H., (2007), Damage tolerance of composite scarf joints, Abstract submitted to ICCM-16. ii Originality Statement I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or substantial proportions of material which have been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project’s design and conception or in style, presentation and linguistic expression is acknowledged. Alex Harman iii Copyright Statement I hereby grant the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain