PSI - Issue 5

Martin Kadlec et al. / Procedia Structural Integrity 5 (2017) 1342–1348 Petr Homola / Structural Integrity Procedia 00 (2017) 000 – 000

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1. Introduction

Thermoplastic-matrix composites (TPMC) reinforced by continuous carbon fibres are used due to their high strength and stiffness to weight ratio. The thermoplastic matrix has several advantages over epoxy resins. High fracture toughness is the most significant advantage for aerospace applications where the impact damage cause severe material degradation and the brittle epoxy resins has significant problems with impact damage ( Kadlec and Růžek (2012) ). On the other hand, the TPMCs have very good impact resistance (Vieille et al., (2014)), they are recyclable (Boria et al. (2016)) and there is possibility to thermoform them to manufacture complex structures more easily (Okereke (2016)). The constant thickness material is not very efficient for integral structures where the load is transferred via various paths. Changing of the thickness enables to decrease weight and maintain reliability of the construction which is needed especially for aerospace industry. To change the thickness, terminating of plies (ply drop) can be embedded between plies or at the surface. The technology needed to achieve the tapering is called tailored blanks. This technology enables to vary not only composite thickness, but also fibre lay-up, composition, and shape based on the need. Considering fatigue behaviour, ply drop is crucial point where stress and strain concentrations appears. Helmy and Hoa (2014) studied tensile fatigue behaviour for internal ply drop and observed that fatigue crack initiates the near ply-drop and propagates along the interface to the thicker section under mode II. Similarly, Thawre et al. (2016) stated that the fatigue life of ply-drop composite was significantly lower than that of plain composite due mainly to initiation and growth of delamination near the ply-drop location. Among the aerospace industry, an extensive review by He et al. (2000) mentioned applications of TPMCs with tapering for structures such as helicopter yoke, composite aircraft-wing skins, helicopter flexbeams, fly-wheels, etc. The application for this technique is convenient also for airframe parts such as skin, ribs, stiffeners, and beams. This paper describes experimental investigation of an asymmetrically tapered carbon/PPS laminate with internal ply drop loaded in tension-tension fatigue. The aim of the work was to obtain data for specific design and fatigue strength of a rib demonstrator. The effects of specimen edge quality, and two ply drop configurations were considered.

2. Materials and methods

2.1. Material properties

Test specimens were manufactured from carbon fibre fabric prepreg according to AIMS 05-09-002 (Airbus (1998)) with areal weight of 285 ±15 g/m 2 . The fabric had 5-harness satin weave which is more flexible for curved surface than a plain weave. The matrix was polyphenylene sulfide (PPS) with weight ratio of 43%. Nominal thickness of a lamina was 0.31 mm. Two alternatives were chosen for the thickness transition. The alternative A has two transitions from 16 to 13 and from 13 to 11 layers (Fig. 1a ). The layup can be coded as follows for A: (0/90, ±45)∫(0/90)| ±45| (0/90, ±45, 0/90)∫ ±45| (±45, 0/90, ±45, 0/90)∫ ±45| (0/90)| (±45, 0/90)∫, where ∫ mea ns continuous plies and | means ply drop (cut lamina end). The tapered angles for alternative A were 3.5°. Nominal thicknesses of the sections were 4.96 mm for 16 layers, 4.03 mm for 13 layers and 3.41 mm for 11 layers.

Fig. 1. (a) Lay-up detail of the tapered section of alternative A; (b) alternative B.