Abstract
Bonded composite repairs of metallic aircraft structures were evaluated experimentally. Metal surface treatments for structural bonding were tested to assess their performance in aluminium, titanium, and stainless-steel epoxy bonding. Cracked aluminium plates were repaired with composite reinforcements to evaluate the effects of repair configuration parameters. The studied surface treatment methods included currently used grit blast silane and AC-130 Sol-Gel treatments and new nanoscale diamond-like carbon vacuum plasma coating DIARC. The DIARC-treated surface does not require any additional primers containing hazardous CrVI chromium. Static and fatigue double lap shear (DLS) specimens were tested as dry and wet at room temperature to study performance of surface treatment methods. Wedge tests were performed under hot and wet exposure, in hot fresh water and salt water immersions and in a salt fog exposure. Elastic-plastic stainless-steel wedge test specimens were analysed using the Virtual Crack Closure Technique (VCCT). Acceptable silane-based field-level methods were found for unclad and clad 7075 aluminium bonding without primers. With titanium and stainless-steel, the use of a BR 6747-1 primer was found to be necessary. In a case study, AC-130 Sol-Gel without a primer on naval grade 5083 aluminium provided good results also when immersed in Baltic Sea water.A five-year-old DIARC coating was tested and noted to perform equally to the freshly bonded DIARC specimens. The DIARC results were comparable with the best results achieved with grit-blast silane and grit-blast Sol-Gel primer methods with aluminium and titanium.In fatigue testing, the DIARC coated DLS specimens were cycled with con-stant amplitude loading until failure or until 10 million cycles. The fatigue performance of the DIARC coating was found to be acceptable. Aircraft repair conditions were simulated with pre-cracked plate specimens by using a steel bonding rig that constrained the effective thermal expansion to the level measured from locally heated aircraft structures. Residual thermal stresses of repaired plates were varied using the rig and by varying the curing and testing temperatures. The repairs were tested using a variable amplitude loading (R=-0.14) with anti-buckling edge supports. In plate specimen repair testing the residual thermal stresses had a rectilinear and distinct effect on the crack growth rate. The structural support against bending had a significant effect on the fatigue life of the repairs. The fatigue life improvement factor with the centre-cracked single-sided repairs was highest with wet-laminated carbon/epoxy repairs, with a stiffness ratio close to one.
Translated title of the contribution | Lentokoneiden metallirakenteiden liimatut korjaukset : Pintakäsittelymenetelmien ja väsymissäröjen komposiittikorjausten kokeellinen tutkiminen |
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Original language | English |
Qualification | Doctor's degree |
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Print ISBNs | 978-952-60-7776-5 |
Electronic ISBNs | 978-952-60-7777-2 |
Publication status | Published - 2017 |
MoE publication type | G5 Doctoral dissertation (article) |
Keywords
- metal surface treatment for bonding
- Sol-Gel
- DIARC
- composite repair
- aircraft structures