Characterization of Fatigue Behavior of Bonded Composite Repairs

Abstract

Composite patches are bonded to a cracked metallic surface either symmetrically (double sided) or unsymmetrically (single sided) to extend service life. The stresses in the metallic panel are greatly affected by the repair symmetry. Unsymmetric repairs present the greatest challenge because of the presence of out-of-plane bending. Thermal residual stresses are present because of the thermal coef cient mismatch of the patch and the aluminum plate. Debonding along an adhesive-adherend interface can reduce the patch effectiveness. A simple analysis with Mindlin plate theory is investigated to model the host and the repair plate. The two plates are connected by an adhesive layer modeled by effective springs. Large de ection theory is used in the case of unsymmetric repairs. The springs are ineffective in the debond zone and are removed. Both the aluminum and the debond cracks are characterized by fracture mechanics by use of the stress intensity factor and strain-energy release rate, respectively. Experiments on aluminum 2024-T3 plate, AS4/3501-6 carbon/epoxy composite patch and FM73 adhesive include determining the thermal residual stresses in the aluminum plate and observation of debond development by use of an ultrasonic C-scan. Tests are conducted to examine the metallic and debond crack growth interaction on unsymmetric repairs.

Comparison of double and single bonded repairs to symmetric composite structures: a numerical analysis

“…It is firstly noted that the asymptotic behaviour is visible for the two cases and there is a reduction of the asymptotic value about 10% by the use of double symmetric patch, it means that the fatigue life of the structure can be improved considerably. This tendency is in agreement with the analysis of Klug et al [14] whom showed that the use of a double patch increase twice as much the fatigue life of the structure compared with the single patch. It can also be see in Fig.…”

Bonded Composite Patch Repairs on Cracked Aluminum Plates: Theory, Modeling and Experiments

“…As a result, the crack growth rate does not depend on of crack length according to the Paris law. Klug (Klug et al, 1999) investigated the fatigue behaviour of pre-cracked 2024-T3 aluminum plates repaired with a bonded carbon/epoxy patch. Single sided repairs were found to provide about a 4-5 times improvement in the fatigue life.…”

Debonding monitoring of a composite repair patch using small-diameter FBG sensors

“…They indicated that the bonded repair patches are effective for repairing the fatigue and/or corrosion cracks in an airframe. Residual strength and stress concentration in the repaired part were discussed by many researchers because they depended on various details such as surface treatment, geometry of patch, material properties and adhesive thickness [3][4][5][6][7][8][9][10][11][12]. They reported that the composite patch is debonded under cyclic loading condition and additionally the debonding cause the strength deterioration of the substrates.…”