Carbon fiber composite laminate containing symmetric internal ply-drop simulating thickness variation was fabricated and tested to determine fatigue life under a standard mini-FALSTAFF spectrum load sequence. The fatigue life of ply-drop composite was significantly lower than that of plain composite due mainly to initiation and growth of de-lamination near the ply-drop location. The spectrum fatigue life was also predicted by empirical method. For this purpose, static and constant amplitude fatigue data was generated to construct constant life diagrams. The spectrum fatigue life predicted using CLDs was in good agreement with experimental results for both plain and ply-drop composites.
Fiber reinforced polymer (FRP) composites are extensively used in construction of primary aircraft structural components. Recent airframes are built with as high as 50% of FRPs. The high specific strength, stiffness and tailorability of composites make them very attractive materials for airframe applications. The laminated structural FRP composites generally consist of continuous carbon fibers reinforced in a thermosetting epoxy matrix. Each of the lamina is laid up in a specific orientation to produce multidirectional laminates of required strength and stiffness. Due to design requirements, the thickness of composite needs to be varied from place to place within a single component such as in the wing of an airframe. Such thickness variations in composites are incorporated by use of ply-drops. There are several basic types of thickness variations induced in laminated composites through ply-drop construction and can be broadly classified as external, internal and mid-plane ply-drop constructions.
The structural components experience several types of static and fatigue loads in service. Being a stress concentration location, presence of ply-drop in the component could be detrimental to the strength and durability of the structure. Several studies have shown that stress concentration due to ply-drop reduce the static strength of composites. They also brought out the susceptibility of ply-drop zones to the onset and growth of de-lamination. Similarly, Steeves and Fleck observed that de-lamination initiate near the ply-drop leading to failure and result in reduction of compressive strength of a tapered laminates.
Many studies on FRP laminates have shown that the presence of ply-drop reduces the fatigue strength of composite significantly. Fatigue damage initiation and growth according to Cairns et al is strongly sensitive to ply-drop location and manufacturing details. They observed that it is difficult to completely suppress damage and delamination initiation at ply-drop. Helmy and Hoa observed that fatigue crack initiates at the resin pocket near ply-drop and propagates along the interface under mode two conditions. They showed that addition of nano-clay in the epoxy enhance the fatigue life by reducing speed of growth of de-lamination. Giannis used fracture mechanics parameters to estimate the fatigue performance of FRP tapered laminates. He observed that onset of de-lamination and growth at ply-drop could be accurately evaluated using stain energy release rate.
Compressive fatigue failure of composites containing ply-drop was investigated by Wang et al who observed that specimens with longitudinal discontinuous plies did not fail in compression but rather failed by initiating at ply-drop. Also, they observed similar failure mechanisms under both static and fatigue loads.
Finite element modeling and stress analysis of ply-drop in composites has been carried out by several investigators. Her used a combination of analytical and numerical method to analyze the ply drop-off problem. Dhurvey and Mittal recently reviewed the analysis efforts made on tapered laminates with respect to static and dynamics analysis, buckling analysis, vibration analysis, de-lamination and inter-laminar stress analysis.
Most of the fatigue studies on laminates with ply-drop or tapered laminates are limited to constant amplitude fatigue loads. Very few studies have been made on the ply-drop laminates under spectrum fatigue simulating service loads. Meirinhos et al showed that the stress amplitude of the civil-aircraft fatigue spectrum did not create any damage in the tapered section and has no contribution to the propagation of de-lamination when maximum 20° tapered specimens are used. The main aim of this work was to study the effect of symmetric ply-drop on the fatigue life of composite under a standard fighter aircraft spectrum load sequence. Also, an attempt was made to predict the fatigue life under the spectrum load sequence using empirical method and compare with experimental results.