Trans RINA, Vol 152, Part B2, Intl J Small Craft Tech, 2010 Jul-Dec


defined by the thickness of each layer in the composite laminate. Eight-node, isoparametric, arbitrary hexahedral brick element was used and the stiffness of this element was calculated using eight-point Gaussian integration. Two boundary conditions modelled were the actual representation of the experimental setup. Base nodes of the leg were constrained in all directions. The load was applied in y-direction up to a deflection of 100mm at the loading point as shown in the Figure 2.1. The three different layups used in the experimentation presented in Table 1 and they were analysed using FEA with the lamina mechanical properties presented in Table 2.


Table 2: Material Properties Mechanical Properties CSM 9600 9600 6062 0.347 0.139 0.108 2602 1847 1847 9.5 28


E11 (MPa)b E22 (MPa)c E33 (MPa)c í12 (MPa) c í23 (MPa) c í31 (MPa) c G12 (MPa) c G23 (MPa) c G31 (MPa) c ILTS (MPa)a ILSS (MPa)a


Flexure Modulus (MPa)b Flexure Strength (MPa)b


Fracture Toughness GC (kJ/m2)a


Critical opening displacement (mm)a


1693 214


0.68 DB


7850 7850 6673 0.62


0.146 0.133 7157 2050 2050 10.5 30


1158 76


1.04 0.05 0.05


(CoDA)’ software using resin and fibre properties Delamination


calculations were UD


23500 6560 6560 0.369 0.249 0.087 2265 1847 2265 10.5 28


4196 433


0.84 0.05


a Literature b Experimental testing c Obtained through 'Component Design & Analysis


performed using


cohesive zone elements. These elements combine the aspects of strength based analysis to predict the onset of damage at the interface and fracture mechanics to predict the propagation of a delamination. Interface elements are separate


finite element entities, which are modelled


between the substructures of a composite material as a means of inserting a damageable layer for delamination modelling. These are designed to represent the separation at the zero-thickness interface between the layers of 3D elements


during delamination.


elements are sufficiently stiff in compression to prevent interpenetration of the delaminated surfaces. Each layer was modelled as a separate material and the delamination was defined as the splitting of the mesh between two materials (with composite, they are individual layers).


Damage onset was predicted using a quadratic stress criterion allowing the mesh to split between the materials as shown in Equation (1). The stresses were extrapolated to the nodes and transformed to the material interface.


B-96


4. CORRELATION OF EXPERIMENTAL & ACOUSTIC EMISSION (AE) RESULTS


Also the cohesive


The curved laminates were loaded such that the radius of curvature increased and interlaminar tensile stresses were induced in the inner surface of the curved laminate. This mechanism was described by Li & Kelly [23] that the interlaminar tensile stresses are generally maximum within 20-50% of the thickness while zero at the surfaces. The load was applied effectively to pull the arm of the specimen, i.e.,


to straighten it. Thus peak


interlaminar tensile stress occurs at the mid width of the specimen as the loading point was at the mid-width of the specimen. The tests were terminated when the displacement reached 100mm. Five specimens of each of


©2010: The Royal Institution of Naval Architects


⎜⎟ ⎜ ⎝⎠ ⎝


⎛⎞ ⎛ +


SS σσ


22


nt 1 nt


⎟ = ⎠


⎞ (1)


Where σn was the normal stress, σt was the tangential stress and Sn, St, were the critical values of normal and tangential stresses. Crack propagation was simulated by introducing interface elements where the constitutive behaviour is expressed in terms of tractions versus relative displacements between the top and bottom edge/surface of the elements as shown in Fig. 2.2c.


Figure 2.2c Bilinear cohesive material model [32]


The effective traction is introduced as a function of the effective opening displacement and is characterized by an initial reversible response followed by an irreversible response as soon as a critical effective opening displacement has been reached as shown in Equation (2). The irreversible part is characterized by increasing damage ranging from 0 (onset of delamination) to 1 (full delamination) [32] 2 c


t G v


c = m


Where, tc is the effective traction, Gc is the critical strain energy release rate (Cohesive energy) and vm is the maximum effective opening displacement. When this criterion is satisfied at the crack tip element, the stresses in the element are alleviated and the crack tip moves to the next element, thus initiating crack propagation. The corresponding values of critical strain energy release rate (Gc) and the critical opening displacement (Vc) [33] for CSM DB and UD materials are presented in Table 2.


(2)


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