Bonded Repair of Aircraft StructuresA. Baker, Jones The conventional approach to through-life-support for aircraft structures can be divided into the following phases: (i) detection of defects, (ii) diagnosis of their nature and significance, (iii) forecasting future behaviour-prognosis, and (iv) pre scription and implementation of remedial measures including repairs. Considerable scientific effort has been devoted to developing the science and technology base for the first three phases. Of particular note is the development of fracture mechanics as a major analytical tool for metals, for predicting residual strength in the presence of cracks ( damage tolerance) and rate of crack propagation under service loading. Intensive effort is currently being devoted to developing similar approaches for fibre composite structures, particularly to assess damage tolerance and durability in the presence of delamination damage. Until recently there has been no major attempt to develop a science and tech nology base for the last phase, particularly with respect to the development of repairs. Approaches are required which will allow assessment of the type and magnitude of defects amenable to repair and the influence of the repair on the stress intensity factor (or some related parameter). Approaches are also required for the development and design of optimum repairs and for assessment of their durability. |
Contents
Introductory chapter | 1 |
11 Bonded vs bolted repairs | 3 |
12 Combined bondedbolted repairs | 8 |
13 Adhesives | 9 |
14 Adhesive testing | 11 |
15 Surface Preparation | 13 |
16 Environmental Behaviour | 14 |
17 Summary | 17 |
Appendix A | 76 |
Theoretical analysis of crack patching | 77 |
52 Formulation and notation | 79 |
53 Load transfer to bonded reinforcements | 81 |
54 Twostage analytical solution | 83 |
55 Residual thermal stress due to adhesive curing | 90 |
56 Bending effects | 92 |
57 Partial reinforcement | 99 |
Surface treatments for bonded repair of metals | 19 |
22 Background | 21 |
23 Structural aluminum alloys | 22 |
24 Phosphoric acid anodizing | 23 |
25 Chromic acid anodizing | 28 |
Design and analysis of bonded repairs for metal aircraft structures | 31 |
32 Design of adhesivebonded repairs in thin sheet metal contruction | 32 |
33 Residual strength of flawed or damaged adhesive bonded joints | 34 |
34 Acceptance criteria for bond flaws and damage | 39 |
35 The pitfalls of life prediction for adhesivebonded joints | 41 |
36 Surface preparation for adhesivebonded repair of metal structure | 45 |
37 Conclusions | 46 |
Crack patching design aspects | 49 |
42 The finite element formulation | 50 |
43 Repair of cracks in Mirage III lower wing skin a design study | 53 |
44 Neutral axis offset effects | 57 |
46 Comparison with experimental and 3D results | 59 |
47 Repair of semielliptical surface flaws | 63 |
48 Repair of cracked holes | 65 |
49 Repair of cracked fastener holes | 74 |
58 Conclusion | 105 |
Crack patching experimental studies practical applications | 107 |
62 Adhesive system and process selection | 109 |
63 Thermal and residualstress problems | 122 |
64 Design correlations and materials allowables | 131 |
65 A preliminary design approach | 146 |
66 Crack propagation behaviour in patched specimen | 153 |
67 Applications of crackpatching | 162 |
Repair of composite aircraft | 175 |
72 Composite fabrication | 178 |
73 Defects | 180 |
74 Repair materials | 183 |
75 Composite repair concepts | 187 |
76 The effect of moisture on bonded repair of composites | 196 |
77 Design of bonded repairs | 199 |
78 Composite service damage experience | 202 |
79 Specific component repair | 203 |
710 Future requirements | 209 |
213 | |
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Common terms and phrases
adherends adhesive AF126 adhesive bonding adhesive cure adhesive layer adhesive-bonded aircraft structure aluminium alloy analysis analytical anodise anodize application approach b/ep bolted bond flaws bonded joints bonded repairs bonded structure boron Composite Materials composite patch configuration crack growth crack length crack patching crack tip cycles damage defects delaminations developed disbond displacement durability effective epoxy equation experimental failure fasteners fatigue crack fibre film adhesives finite element fracture mechanics gr/ep graphite/epoxy honeycomb kN/m laminate load transfer maximum metal structure metallic components Mirage III moisture overlap joint panel parameters peel phosphoric acid plate plies precured prepreg pressure primer ratio reduced reinforcing patch residual stress resin rivet scarf shear modulus shear strain shear strength shear stress shown in Figure silane specimens stiffness strength stress intensity factor surface preparation surface-treatment Table techniques temperature tensile thermal thickness treatment unpatched wing skin