PSI - Issue 12

A. Castriota et al. / Procedia Structural Integrity 12 (2018) 71–81

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Castriota et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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introduced new problems that were usually addressed with the use of classical materials. In particular, fatigue effects can be very relevant on structural integrity of aero structures (Megson, 1999). Although fatigue behavior is a widely investigated topic for metallic materials, the same cannot be said for composite materials. While in metal materials this phenomenon is already evident for low loads, for the composites it is necessary to apply high loads in order to reach such importance to be taken into consideration (Vassilopoulos et al., 2011). Moreover, the composite materials are inhomogeneous, have an orthotropic behavior and show breakage modes that are completely dissimilar to the classic breaking modes in metallic materials (Mallick, 1993; Jones, 1975). In literature, there are several studies on the fatigue behavior of composite materials but, often, they are only experimental nature, without any comparison with numerical models. For example, Post et al. (2008) have performed a fatigue study in composite materials with the application of damaged models and calculation of residual life. However, their study is based on empirical and semi empirical models. Gathercole et al. (1994) have studied, from the experimental point of view, the constant amplitude fatigue behavior in laminates for various levels of load ratio. In this work, the mechanical behavior of a CFRP aeronautical spar has been studied both from a numerical and experimental point of view. The spar presents a repair by scarfing and hot bond process and has been subjected to a random fatigue load, followed by a final static test. The numerical model has been realized using a FEM code, which reproduces the geometry and loads existing on the real component, the staking sequence and implements the material characteristics. The numerical model allowed to verify the correct positioning during test and to obtain a reference for comparison of numerical and experimental strain data. Strain data were obtained using strain gauge techniques.

Nomenclature CFRP Carbon Fiber Reinforced Polymer DUL Design Ultimate Load ECLF Environmental Compensation Load Factor LVDT Linear Variable Displacement Transducer

2. Material and geometry

The test article is essentially a double T-section in CFRP, having upper and lower skins co-cured on the cap and a hole of about 100 mm in diameter on the core. Around hole, it has been supposed the presence of a damage, which has been repaired by scarfing and hot bond process to restore the structural integrity of the part. This technique is based on the application of a certain number of sub-laminates (called patches) that are polymerized and subsequently superimposed on the part to be repaired, interposing layers of adhesive. In this way, it is possible to make the sub laminates adhere perfectly to the component profile without giving rise to high internal stresses. In the case of composite materials, unlike metals, they are applied with bevel joining techniques, minimizing the increase in thickness and allowing a uniform distribution of shear stress in the adhesive. The spar is constituted of two parts, the spar cap and the spar web. The cap part is realized with 10 plies, while the web part is built with 20 plies. Each ply has a nominal thickness of 0.186 mm. On spar cap a skin with a variable number from 50 (skin L in dark green) to 28 (skin T in light green) additional plies were co-cured. Details about the number of plies and the lamination sequence of the various constituent parts of the spar are summarized in Table 1. This structural component is equipped at the two ends with aluminum supports fixed by rivets in Ti6Al4V to the spar that are used to constraint and load the spar during test.

3. Experimental set-up and method

The test configuration provides that the spar plane of symmetry is placed horizontally to facilitate the application of the load by the actuator. The constraints and the applied load reflect the real load condition of the spar: in particular, the constraint at the base (all DOF blocked) was realized fixing the aluminum structure mounted at the root of the spar to a test bench by threaded connections. The application of the load was carried out with a hydraulic actuator connected