Understanding the influence of laminate stacking sequence on strain/stress concentrations in thin laminates at repair holes with large scarf angles

Abstract

Scarf repair is widely used in the restoration of structural performance of damaged aircraft secondary structures. Such repairs result in reduced thickness sections which are significantly larger than those associated with typical fastener holes. Significant literature exists on the distribution of strain/stress concentration in fastener hole geometries, both straight sided and countersunk, but is lacking for the geometries associated with shallow scarf angles and thin laminates. Hence, herein three-dimensional finite element models are developed to understand the influence of stacking sequence and scarf angle on strain/stress concentrations. The results demonstrate and quantify for the first time that strain concentrations are not only dependant on the laminate membrane stiffness but also on laminate bending stiffness, due to the anisotropy created as a result of scarfing angle, hole geometry and laminate thickness. Scarfing is demonstrated, for typical repair geometry associated with foreign object damage (hole diameter 20 mm, scarf angles 3° to 7°), to elevate strains by up to 2.5 times when compared to equivalent diameter straight-sided holes in laminates of thickness ≈1 mm.

Bonded flush repairs for aerospace composite structures: A review on modelling strategies and application to repairs optimization, reliability and durability

“…Main design parameters of flush repairs are repair angle or ratio, adhesive thickness, plies thickness, laminates stacking sequence and overplies configuration. The sensitivity of stress distribution in the bondline to those parameters was addressed by numerical parametric studies [107][108][109][110]. Shallower repair angles, thicker plies and thinner adhesive layer tend to reduce the peak shear and peel stresses along the bondline, but addition of overplies, i.e.…”

Potential energy as a new approach for detection and monitoring of fatigue damage in scarf adhesive joints modified with nanoparticles

“…The extensometer has a gauge length of 50 mm and a travel of ±5 mm. The clamping position of the extensometer in Figure 2 was selected by many researchers 4,16,21,23,25,26 . Hence, the measured local strain includes the strains of the adhesive layer and the adherends.…”

“…In comparison with single lapjoints (SLJs), the smooth surface of the SAJs minimize the aerodynamic contour changes, which makes it more suitable for repairs of the external structures. 4 The insignificant 1 weight added to the bonded/repaired scarfed structure increase their application in aerospace industries compared to the over lapped joints. Javaid et al 5 pointed out that the joint performance should be characterize under fatigue loading, which is the main reason for catastrophic failure of about 50% to 90% of mechanical structures.…”

Influence of modelling hypotheses on strength assessment of CFRP stepped repairs

“…Hence, this paper attempts to fill this gap. It focuses on stepped repairs rather than scarf repairs, because an 4 industrial wet-layup repair is generally likely to be closer to a stepped configuration than a scarf configuration due to plies drop-off [15]. This paper investigates the influence of modelling hypotheses on the predicted behaviour of a stepped repair in terms of bondline stress and failure strength.…”