Stress concentration associated with circular holes in hybrid loading (i.e., tension-compression ratios of 0.25, 0.50, and 0.75) achieved maximum reduction of 31.7%. This reduction is obtained by introducing elliptical defense holes along the principal stress direction. Finite element analysis is used to optimize the size and location for defense hole system. The effect of the stacking sequence, the fiber orientation, and the stiffness of both the fiber and the matrix are investigated.
TopIntroduction
Composite materials offer many advantages over metals such as: high strength and high stiffness-to-weight ratio, good fatigue strength, corrosion resistance and low thermal expansion. Composite materials such as glass fiber, aramide fiber, boron fiber and carbon-fiber-reinforced plastics have been used for a few decades, especially in the aircraft industry. Aircraft structures also include a large number of open holes and cut-outs e.g. holes for electric wires and hydraulic pipes or holes required for assembly or maintenance where a laminate containing open holes is subjected to shear loading i.e. biaxial loading case. Joining by mechanical fasteners is one of the common practices in the assembly of structural components. Among the most important elements in aircraft structures in general and in composite structures in particular are mechanically fastened joints. Improper design of the joints may lead to structural problems. Also, conservative design may lead indirectly to overweight structures and high life-cycle cost of the aircraft. Typical examples of mechanically fastened joints in composite aircraft structures are: the skin-to-spar/rib connections in e.g. the wing structure, the wing-to-fuselage connection etc. Since the failure of the joints can lead to the catastrophic failure of the structures, an accurate design methodology is essential for an adequate design of the joints.
Introducing auxiliary holes (defense holes) in the neighborhood of a main hole to reduce the stress concentration is called defense hole (DH) theory which has been known since the early years of last century. Most of the work that has been done so far deals with defense hole system (DHS) under uniaxial loading on sheet metals (isotropic material). Some efforts are done for shear loading.
Numerical and experimental studies for reducing stress levels in structures by introducing other geometric discontinuities are very few. Erickson and Riley (1978) investigated the effect of the defense holes on the stress concentration around the original hole using two- dimensional photoelasticity. Jindal (1983) examined the reduction of stress concentration around circular and oblong holes using the finite element (FE) analysis and photo-elasticity analysis. Meguid (1986 & 1989) studied the reduction of stress concentration in a uniaxialy-loaded plate with two co-axial holes using the FE analysis. Rajaih and Naik (1986) investigated hole – shape optimization in a finite plate in the presence of auxiliary holes using the two dimensional photoelastic methods. Ulrich and Moslehy (1995) used boundary element methods to reduce stress concentration in plates by introducing optimal auxiliary holes. Durelli (1978) investigated the optimization geometric discontinuities in stress field under uniaxial loading. Dhir (1981) studied the hole shape optimization in plate structure under tension and shear loading.
Akour et. al (2003) studied the design of a DHS for a pure shear-loaded plate. Mittal and Jain (2008) investigated the effect of fiber orientation on stress concentration factor in a laminated composite plate with central hole under in-plane static loading.
Most of the previous work in DHS has been done for sheet metal plates (isotropic material). Some attempts are made to investigate the stress distribution in the vicinity of a circular hole for composite plate under uniaxial loading (William, 1985, Yoshiaki and Kiyoshi, 1987). In the current research, design and optimization of stress relief system for composite laminate is investigated and unveiled the optimum design parameters of the DHS.
The research has investigated the optimum design of the DHS by utilizing univariate and pattern search optimization technique (Chapra and Canal, 2006). This technique is multi-dimensional numerical optimization technique. DHS under hybrid in-plane loading is investigated. The optimum size, shape and position of the defense holes, the effect of laminate thickness and the stacking sequence such as cross-ply and angle ply laminate are revealed. The Experimental verification for selected cases is conducted using RGB photoelasticity techniques.
TopPhysical Model
This section presents the physical model of the composite plate, which includes geometry, boundary conditions as well as the materials used in the investigation.