However the laminate stack method using compressive and tensile strength and first ply to failure would show a very


significant difference in moment


depending on which outer ply (i.e. glass or aramid, or carbon or aramid) was in compression since the ratio of compressive to tensile strengths is 70% (glass), 62% (carbon) and 36% (aramid) according to Annex C [1].


Comparing the ratio of first ply to failure to moment capability after the loss of the first outer ply to fail, the stack method using the strains from Table 1 gave 0.78*, 1.17 and 1.15 for the three alternate ply lay-ups (glass- carbon, glass-aramid, aramid-carbon). The ratios using the current Annex H method (obtained


using the


HullScant software, developed by the Wolfson Unit of the University of Southampton) were 0.79, 0.84 and 1.06. In addition, [1] formally recognizes the first ply to failure moment only.


* ratio less than 1.0 means first ply to failure moment is synonymous with ultimate failure, which was the case for all but panels I and L .


The calculated failure moment was calculated using laminate stack analysis with the modulus taken from Annex C [1] plus the direct measurement value of fibre content by mass (Table 2) and the ply failure strains from Table 1. The calculated flexural rigidity (EI) was obtained as part of the stack analysis and was virtually identical to HullScant calculations. Calculated thickness was used in the stack analysis.


The results of the comparison between calculations and test values are shown in Table 6 (see back of paper), which also includes calculations for the failure moment derived using the current format of Annex H


4. a)


CONCLUSIONS


The level of under estimation calculated (using the current ISO-12215-5 method i.e. in-plane strength based) versus test values was 60% on average.


This may be regarded as two


Whatever view is taken, it must be desirable to bring the


ISO single-skin


requirements into closer agreement than is currently the case.


The exploration of simple


methods to facilitate this is the main concern of this paper.


b)


Ideally, the new apparent ply failure strain- based calculated values should lie a little below the de-rated test values as these were based on laboratory produced samples.


This is generally the case: The calculated tensile strength and stiffness under predicted test values


too conservative. panel


capacity


by margins of between 1 and 26% (see Table 4). Calculated flexural stiffness values were under predicted for E-glass and carbon dominated lay- ups, but not


for aramid dominated lay-ups.


Flexural strengths as given in Table 6 were nearly always under predicted by about 15% on average, although there were significant variations within that average, especially for aramid dominated lay-ups.


c) At present there is no intention to propose a revision to the standard.


under development since 1989.


ISO-12215 has been It has been a


massive labour thus far and the working group members are fully aware of the issue addressed in this paper. Only the desire to ‘get the standard out there’ has prevented the closure of this issue.


It should be borne in mind that the majority of boats designed using equation 2a (single skin) or equation 1 (sandwich) will experience no conflict. It is only for that select group of single skin laminates (possible using advanced fibres) where laminate stack analysis is contemplated that the ‘discontinuity’ of the two ISO methods becomes an issue.


For the present, this paper


represents an attempt to indicate the ‘state of play’ and perhaps to offer a possible solution for more general discussion, in the event of any future revision. More experimental work will be required before any final


‘thin apparent ply


failure strain’ values can be proposed and at least the level of conservatism of the current methodology [1] has been quantified.


d) Two final comments are offered:


It should be understood by all users of the ‘first ply to failure simplified laminate stack method’ (i.e. Annex H [1]) that this generally works well only for 0/90 lay-ups (with respect to the panel boundaries) and for plies which have broadly similar modulus and apparent failure strains. For other lay-ups, the use of classical lamination theory is permitted in [1].


Resin choice: The test values are based on epoxy resin and the ISO default mechanical properties are based


on ‘compatible In this paper, it is resin’


systems. For glass, this means the vast majority of the data applies to polyester/vinyl ester based samples; for carbon/aramid both polyester and epoxy are well represented.


assumed that resin failure strains comfortably exceed fibre failure strains and hence that the ISO default properties give a reasonable lower bound indication of properties. Table 4 would seem to bear that out, at least as far as static


© 2008: Royal Institution of Naval Architects


B-37


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