Development of design techniques for the avoidance of acoustic fatigue in aircraft box-type structures

Abstract

This thesis describes a programme of work carried out on the acoustic fatigue of aluminium alloy, carbon fibre reinforced plastic (CFRP) and GLARE box-type structures representative of aircraft flaps. GLARE is a hybrid material made of aluminium alloy and glass reinforced plastic. 'Tee-coupon" specimens were also used to obtain fatigue data for the composite materials. A brief introduction is given concerning CFRP and GLARE composites used to build the test coupons and box structures. A review of the state-of-the-art in the research on the acoustic fatigue of aircraft structures is also presented.

The characteristics of the sound pressure field at the test section of the progressive wave tube (PWT) used in the acoustic fatigue testing were investigated. The highest overall sound pressure level measured was 162dB. Non-Gaussian distribution behaviour in measured sound pressure signals was observed. Results showed that sound pressure field was uniformly distributed in amplitude around the test section but spatial phase change occurred in the direction along the axis of the PWT.

Damping measurements for coupon specimens revealed that the CFRP coupons had higher loss factors than the GLARE coupons. Fatigue tests were carried out to generate fatigue data for CFRP and GLARE coupons. It has been found that the fatigue damage patterns for the CFRP coupons were cracks in the joint region of the skin and stiffener and delamination of the skin plate. For the GLARE coupons, the 'fibre bridging effect' was not as effective as expected.

Mode shapes, resonance frequencies and modal damping ratios of the box structures were obtained by the means of forced vibration tests. Results showed that stiffeners behaved differently at low and high frequencies. The CFRP box had the highest damping compared with the GLARE and aluminium alloy boxes. The acoustic excitation tests of three box-type structures showed that the strain responses of the two skin panels were coupled at high excitation levels. Non-linear behaviour in the forms of resonance peak broadening, peak frequency shifting and strain energy redistribution as the excitation level increased were observed. Fatigue damage in the form of cracks in the metallic and hybrid structures was induced and propagation rates noted. Damage to rivets occurred in the CFRP box, but this was the most acoustic fatigue resistant structure followed by the GLARE and aluminium alloy constructions. Formulae based on the fatigue data of coupon tests and Miner's accumulation theory were derived for the fatigue life predication of the CFRP and GLARE structures. Estimated fatigue life gave good indication of the fatigue resistance of the composite structures.

FE analysis was carried out for both coupon specimens and box structures. A good agreement was achieved for RMS strain response and spectral densities at various locations on the test boxes and coupon specimens.

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