International Journal of Small Craft Technology
AN INDEPENDENT ASSESSMENT OF SIMPLIFIED METHODS FOR DESIGN OF COMPOSITE SINGLE SKIN PANELS R Loscombe, Southampton Solent University, UK SUMMARY
The simplified laminate stack method provides a useful tool for designers to evaluate different material and stacking options. This method is described in Annex H ‘laminate stack analysis’ of the ISO/FDIS-12215-5 scantling draft standard [1], where bending strength calculations are based on the tensile and compressive strengths for each ply, with the minimum moment to failure obtained by examining each ply. This approach can be rather conservative when compared with a flexural property based method, a method which is also included in [1] as an alternative to Annex H using the same design loads and stress factors.
An additional area of interest is how the ‘lower-bound’ default
mechanical property data [1] behave. As ISO-12215-5 approaches the final stage of harmonisation, it was felt useful to review the current position and to examine how well the Annex H method stands up to independent assessment against new empirical data/analyses not used in the original development work. Given the current status of the standard, the assessment is submitted for wider discussion rather than as an internal working group document.
NOMENCLATURE
E in-plane elastic modulus of ply ‘i’ (N/mm2) E general symbol for elastic modulus EINA
K aspect ratio coefficient (0.308 – 0.5) (-) MFPF
p design pressure (N/mm2) Pfail
s
Smean Sk
flexural rigidity of laminate stack (Nmm2/mm)
bending moment to cause first ply to fail per unit width (Nmm/mm)
ultimate tensile load per unit width (N/mm) short panel side (mm)
average of strength measurements
characteristic strength (95% chance of S > Sk) where S is the strength concerned (e.g. flexural, tensile)
t thickness (mm) Vf Vt zi
coupon volume stressed in flexure coupon volume stressed in tension
distance from neutral axis to top or bottom of the i’th ply (mm) Weibull shape factor
stress factor = design stress/ultimate strength (-) ui
ultimate strength of ply ‘i’ (N/mm2)
strength (subscript ‘t’ tension, ‘f’ flexure) CSM chopped strand mat WR
woven roving
gsm grams per square metre (fibre mass) 1.
INTRODUCTION The performance of composite single skin panels,
sandwich panels and stiffeners depends on the choice of materials and the stacking sequence. This is particularly so for single skin panels. Although classical lamination theory is widely used, structural designers of small craft also tend to use a simplified laminate stack procedure that involves finding the flexural rigidity per unit width
© 2008: Royal Institution of Naval Architects
(EINA) and examining each ply to determine the bending moment which will cause a ply to fail.
The minimum value is the first ply to failure moment capability, i.e.:
M FPF min{ EI
zE
NA ui ii
This method ignores any coupling between in-plane and out of plane and is strictly only correct for specially- orthotropic layups. Nevertheless, it is widely used [1, 2]. Most laminate stack methods employ in-plane properties for each ply, i.e. ui is the ultimate tensile or compressive strength of ply i. The logic behind this is that each ply is thin compared to its distance from the stack neutral axis and hence experiences compression.
almost pure tension or
An alternative design method for single skin involves use of the measured apparent
flexural strength for the laminate stack [1, 3]. The flexural strength for the stack
(f-stack) is used to calculate the required thickness directly from: .
. fstack ts kp
(2a)
k is an aspect ratio factor, p the design pressure, is the stress factor (typically 0.3 – 0.5 for composites) and s is the short panel dimension.
Alternatively the moment to cause failure may be obtained from:
M FPF
fstack .t 6
2 (2b) } (1)
B-31
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