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Thus values have to be given for specific damage types and loading cases. The local location factor Sp is defined as follows: aiacpPPS = (1) where Pac is the value of load that causes the critical stress or strain component at the damage location to reach its maximum allowable value ignoring the dam-age, and Pai is the maximum allowable value of load on the intact panel. For face sheet damage, the critical stress or strain component is usually assumed to be the in- plane compressive stress or strain at the point in question. The value of Sp should be taken as 1.0 unless a higher value can be demonstrated; Sp can never be smaller than unity. As an example, suppose a panel under uniform, lateral pressure is designed so that it reaches the allowable limit for core shear stresses before the allowable stresses for face laminate failure. (This will often be the case for panels designed to classification rules, for example, if the minimum thickness requirements result in an in-crease of laminate thickness relative to the basic strength requirements.) Then Pai is the pressure load at which the allowable core shear stress is reached. Suppose that there is a local impact damage that has reduced the local in-plane compressive strength of the face laminate so this has to be checked at the damage location. Then Pac is the pressure applied to the panel at which the allowable compressive stress (or strain) in the face laminate at that location reaches its maximum al-lowable value, also calculated for the intact panel. Note that all these quantities are calculated for the intact panel. This means that maps of Sp values can be drawn for panels with given dimensions and lay-ups for simple load configurations like uniform lateral pressure. Such a map for the specific case of a simply supported 3 m x 2 m panel with 1.8 mm quasi-isotropic CFRP face laminates and 60 mm H80 core under uniform lateral pressure loading is shown in Fig. 4. The safety factors incorporated in the allowable stress levels are 3.33 for compressive failure of the face laminate and 2.5 for core shear failure. Fig. 4: Local location factor Sp for simply supported sandwich panel (contour plot) Such figures can be developed for a panel with any lay-up and dimensions, for either local face sheet damage or local core damage. They are dependent only on the undamaged properties of the panel. In the case shown, the cross-shaped area within which Sp < 2 is the area most sensitive to face sheet damage. Plotting the same figure with finer resolution shows that the corner re-gions are in fact more sensitive (having lower Sp values) than the centre.
Estimation of panel strength reduction factor, Rp The panel strength reduction factor Rp is a measure of the extent to which the panel strength is reduced by the given damage at the given location on the panel. For cases of Level 1 damage, the panel strength reduc-tion factor is given by p l p S R R = or 0 . 1 = p R (2) whichever gives the lower value. Note that Rp = 1 implies no reduction in panel strength caused by the damage. Note also that Rp calculated in accordance with the first equation above may be greater than 1.0. If Rp = 1 the damage has no immediate consequences for the strength of the panel and immediate repair is not normally required unless either watertightness or other important serviceability characteristics are affected. However, the possibility of damage growth under re-peated loading may also need to be evaluated. Panel Strength Reduction: Face/core Debonds As with face sheet damage, the reduction in in-plane compressive strength caused by face/core debonds is found to be insensitive to the panel size provided the debonds are below a certain size. Furthermore, their influence on panel stiffness is very small. This means that small face/core debonds can be treated as Level 1 defects. The local strength reduction Rl may be established, for example, using the methods of Berggreen (2004) or established software such as DEBUGS (Nilsson et al, 1997, 1999). Fig. 5 shows an example established by Berggreen, including corresponding experimental re-sults. This applies to a sandwich lay-up with quasi-isotropic GRP face laminates and H80 PVC foam core. For small debonds the local location factor Sp and panel strength reduction factor Rp can then be calculated by a similar method to that described above for local impact damage. However, for panels with lateral loading it is necessary to take account of the out-of-plane shear stress as well as in-plane compressive stress as this influences the possible kinking of the crack from the face/core interface into the core.