Typical top-hat-stiffener configuration 

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... In structural composites, the initiation of a crack/fracture does not indicate ultimate failure. Generally, a stable crack propagation stage, associated with a steady increase in the external load, precedes catastrophic failure. This is frequently seen in the structural application of composites. In some structures, reserve strength (due to the alignment of the fibres to the load path) is seen even after the composite laminate has suffered significant damage and this reserve strength steadily increases during crack propagation. In this paper, experiments on composite top- hat-stiffeners are carried out and further analysis using FEA are presented. This top-hat-stiffener demonstrates secondary stiffening particularly when layers of glass fibre are aligned with the load direction. The design and assessment of such structures require adequate considera- tion of strength and fracture. Top-hat-stiffeners are generally used in the fabrication of stiffened panels, such as decks, bulkheads or the hull shell, joining the keel to the boat hull (Dodkins et al. 1994). The keel is normally attached using a series of bolts that penetrate the hull. These bolts load the hull through washer plates, which bear down upon the flanges of top-hat-stiffeners called keel floors. The key benefits of using top-hat-stiffeners are high bending stiffness and torsional resistance. As the name suggests, the stiffener has a hat shape with a flange, web and a crown, as shown in Figure 1. The top bend between the crown and the web is stiffened by adding more composite layers (preferably unidirectional). Due to the absence of through-thickness rein- forcements and the lack of bond ductility, out-of-plane joints suffer from relatively low strength and fatigue resistance against out-of-plane tensile, bending and shear loads (Smith. 1990, Shenoi and Hawkins. 1995) In addition, they are susceptible to failure by peel or delamination well before the ultimate in-plane laminate material stress is reached (Dodkins et al. 1994, Raju et al. 2010a and 2010b). Curved composites are inevitable in the design of many structural applications. The primary mode of failure in curved composites is delamination, which occurs when the bends are opened or closed owing to an external load or pressure. The stress distribution is complex and delamination is caused by through-thickness tension and a lack of through-thickness strength. The delaminations are generally embedded between the layers and frequently are undetected. The interlaminar tensile strength (ILTS) and interlaminar shear strength (ILSS) are the key parameters in defining the delamination strength of the composite. Many methods have been suggested to arrest the interlaminar failure such as using tougher matrix polymers, interleaf layers or through-thickness rein- forcements, improving the fibre – matrix strength and op- timising fabrication (Mouritz et al. 1997a and 1997b). The study of the interlaminar stress distribution at the bend provides a tool for understanding and predicting delaminations in these structures. Prevention of delamination may be possible with techniques such as fabric stitching and z pinning, but practically they may not be used owing to constraints such as the cost, weight, manufacturing process and analysis complexity. MANUFACTURING METHODS Experimental specimens were manufactured using both the hand layup and vacuum infusion process. The detailed description of the manufacturing methods and the specimen configuration are discussed in this section. Four specimens were tested in tension until failure. The specimen testing was conducted in the solids laboratory on an Instron 8805 universal testing machine. The experiments were conducted using 250 kN load cell. The load-deflection plots were monitored using Bluehill software compatible with the Instron. Special care was exer- cised when installing specimens within the grips to ensure proper alignment. The specimens are 100 mm wide and 200 mm high, with detailed dimensions shown in Figure 2. The basic materials used in manufacturing the specimens are are given in Table 1. The layup configuration for the specimen tested is presented in Table 2. with the detailed geometry shown in Figure 3. Two 10 mm steel backing plates below and one 10 mm steel backing plate above the crown were placed and fastened to replicate the actual keel fixture (Fig. 3). The flange was secured using clamps and the displacement load was applied at a rate of 2 mm/min using the centre bolt. The loading mechanism is similar to that of the load applied by a keel bolt to the hull transverse floor. Layup 3 consists of three DB layers (611 gsm) and three UD layers (461 gsm), each sandwiched between six CSM layers (225 gsm). The UD layers were placed at the lower part of the laminate and DB layers on the upper end. Four specimens (C01, C02, C03) were tested until failure. The experimental load-deflection plot for all specimens using Layup 3 is shown in Figure 4 and the fail- ure/stiffness values are presented in Table 3. A mismatch between the elastic properties of the adjacent CSM and UD layers might have created excessive interlaminar tensile stress, causing the structure to suffer premature failure between the second and third layers. Simi- lar to earlier layups, the crown bend failed prior to the flange bend. Continuing application of the load after the initial failure caused other bends to fail without a significant increase in the load up to a deflection of 22 mm. Thirteen visible load drops can be identified to assess the structural damage. The progressive damage can be seen in Figure 5. At 20.20 mm deflection and 21.24 kN load, all the four bends of the specimen began to straighten and all the layers suffered delamination. The structure resembled a trapezium with relatively sharp corners. The secondary strength mechanism is triggered at this stage due to the presence of UD fibres. The secondary slope (4.399 kN/mm) in Figure 4 shows the reserve strength — it continued to handle the load beyond 45 kN. During this stage, slippage at the grips was observed and the test was terminated. The average initial specimen stiffness was 2.625 kN/mm. Two behavioural changes were observed during the test. At 20.2 mm deflection, all the bends completely disappeared and the structure was shaped like a trapezium. The secondary reserve strength (secondary stiffness shown in Figure 3) carried the load up to 45 kN, ...