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Fig. 5: Local strength reduction factors for face/core debonds 4 00.10.20.30.40.50.60.70.80.9100.10.20.30.40.50.60.70.80.91Position in short direction d /LPosition in long direction Position in short direction 22 2 2 3 3 334 44 4 1 < Sp < 2 00.20.40.60.811.20 100 200 300Debond diameter (mm)Local strength reduction factor RlAnalysisExperimentLocal strength reduction factor Rl Larger debonds and cases with more complex loading have to be considered as Level 2 damage. In these cases the above-mentioned methods can be applied for esti-mating the strength reduction, but the analysis has to be applied directly to the panel with specified damage, and Rp is found directly. Panel Strength Reduction: Wrinkle Defects A wrinkle is a type of production defect that is caused by a slight excess of reinforcement in the skin laminate in relation to the surface area available or by uneven-ness arising from the production or transportation of the fabric. Thus one or more reinforcement layers is unable to lie completely flat and forms a small, outward buckle or wrinkle. The defect often affects only the topmost layer or layers of fabric. The region below the wrinkle is usually filled with resin, but the presence of a void can-not be ruled out. Such wrinkles usually occur over a significant part of the panel width or length. They reduce the compressive strength of a laminate for in-plane loading applied per-pendicularly to the line of the wrinkle, but the strength for loading applied parallel to the wrinkle is not be-lieved to be significantly reduced. Fig 6 shows the panel strength reduction factor Rp for sandwich CFRP face laminates with compressive load-ing perpendicular to the axis of the wrinkle, plotted as a function of the proportion of the plies involved in the wrinkle. The curve is based on the results of four-point bending tests performed on sandwich beams having wrinkles across the compressive face laminate in the mid-span region. The lay-ups were quadri-axial, quasi-isotropic CFRP in which the outermost plies were in the loading direction. Fig. 6: Panel strength reduction factor for CFRP sandwich panels with wrinkle defects. Note that the results presented in Fig. 6 are provisional and are to be verified by further testing and modelling studies now in progress. In particular, the face lami-nates of the panels used in these tests had somewhat low compressive strengths in the regions without wrinkles.
For laminates with higher initial strength values, the reductions due to wrinkles may well be greater. Wrinkle defects may be considered as Level 2 damage. They are not small in the sense of being confined to a small area over which the stresses can be considered constant; however, they do not normally affect the panel stiffness significantly. Panel Strength Reduction: Core Shear Cracks and Debonds Core shear cracking is usually accompanied by debond-ing of the face from the core. This may be fairly local-ised, e.g. along one edge of the panel, or may cover a considerable part of the panel area. This type of dam-age affects the stiffness of the panel for some types of loading so it must be considered as Level 3 damage. Models for strength and stiffness reduction for panels with core cracks and associated debonds are currently under development. Global Strength Reduction The ship strength reduction factor Rs is the proportion of the global strength that remains when the damage is taken into account. It can be defined in a way that is analogous to the panel strength reduction factor Rp. The method of estimating Rs depends on the type and size of damage, as described below. Estimating Rs for Level 1 or Level 2 damage For cases of small and medium local damage (Level 1 and Level 2 damage) the change in panel stiffness due to the damage can be neglected. A global panel location and load case factor Ss, can be defined that is analogous to the local location factor Sl. The global load reduction factor Rs is then given by s p s S R R = or 0 . 1 = s R (3) whichever gives the lower value. Here Rp must be found for the loading condition on the panel that is in-volved in the global strength in question. However, in some cases it may be relevant to consider in addition the maximum allowable global load on the ship when the damaged panel is completely removed, and calculate the corresponding strength reduction Ro. This may be used as an alternative to Rs in assessing the acceptability of the strength reduction. In some cases it may give a more favourable value than Rs. Estimating Rs for Level 3 or Level 4 damage For cases of large local damage and extensive damage (Level 3 and Level 4 damage) the change in panel stiff-ness due to the damage cannot be neglected. In cases of Level 4 damage it is normally necessary to perform a fully detailed analysis of the ship with the damaged panels removed or having appropriately re-duced stiffnesses. This is also the case for Level 3 dam-age, especially if the structure is a complex, three-dimensional one and the influence of the damage on global behaviour is unclear. However, for many cases of Level 3 damage it is possi-ble to estimate the residual strength using approaches based on pre-calculation of standard cases and simpli-0.00.20.40.60.81.00 0.2 0.4 0.6 0.8 1Fraction of plies in wrinkleStrength reduction factor RpTestsAverage fied treatment of stress redistribution. Allowable Strength Reduction There is normally no need to consider an allowable strength reduction if the panel strength reduction factor Rp = 1. However, if Rp < 1 the possibility of accepting a reduction of panel strength must be considered unless the damage can be fully repaired immediately. The global strength reduction Rs must also be evaluated. In some cases it is not possible to accept any reduction in the global or local strength of the structure. How-ever, not all parts of the structure of a ship are highly stressed. In many cases a given panel may be exposed to a maximum loading that is lower than the allowable value because the design gave more than the minimum required reserve of strength, i.e. there is a lower utilisa-tion of the panel than the maximum that is allowed. In such cases it will normally be acceptable to reduce the panel strength by an amount that reflects this extra re-serve of strength in the intact structure.