kn is a sample size dependent factor corresponding to the 95% exceedance point [11].


Characteristic stiffness values are taken as the mean value for reasons explained in 2.3.


The characteristic strength is taken to be the failure moment per unit width, not the apparent flexural strength (for three-point bending) and the failure force per unit width, not the ultimate tensile strength (tensile tests). Likewise the flexural rigidity per unit width is used in preference to the apparent flexural modulus (three-point bending) and the axial stiffness per unit width in preference to the tensile modulus.


The reason for this approach is that it generally accepted that thickness calculations in composites are less meaningful than those associated with metal or wood. Composite thickness calculations are merely a means to an end. It is often better to work with the measured failure moment or the flexural rigidity rather than the flexural stress or modulus. consider


that moments and rigidity should only be


converted to stress and modulus using calculated and not measured thickness. Given the presence of voids and variations in


fibre mass (one manufacturer quotes


variations of up to 5% from the nominal grams per square metre), it is generally considered satisfactory if calculated thickness’ are within 15% of measured values [3]. In this


study, on average, measured thickness


exceeded calculated values by about 7%. Given that the calculated thickness assumes a zero void content (typically 2%); the agreement is considered to be reasonable.


3.3 TENSILE TESTS


The ISO default modulus values are used as input to the calculated flexural rigidity and failure moments (see 3.4) and the level


of difference between calculated and


measured would directly impact on the comparison between three-point bending tests and laminate stack analysis.


Three panels (all glass, all carbon, all aramid) were cut into 14 x ISO 527-4, type 2 specimens. were conducted following reference Southampton Solent University’s Lloyd


Instruments


LR30k with 50mm gauge length extensometer. Modulus values were obtained by software supplied by Lloyd Instruments.


These results may be translated into the apparent tensile failure strain. See Tables 4 and 5 (see end of paper).


The most significant difference is between ISO and measured tensile stiffness for carbon. On the basis of these tests, the laminate stack analysis method (assuming the strains in Table 1 are correct) should give good


Tensile tests 13


using


agreement with glass, but will under predict carbon by about 20%.


3.4 THREE-POINT BENDING TESTS


Three-point bending Tests were conducted following references


14 and 15, using Southampton Solent


University’s Lloyd Instruments LR30k and 5kN three- point bending jig. Modulus values were obtained by software supplied by Lloyd Instruments.


Twelve single skin panels were made up as follows:


 Panels A-C: All glass, all carbon, all aramid  Panels D-F: Three combinations of glass and carbon  Panels G-I: Three combinations of glass and aramid  Panels J-L: Three combinations of carbon


Some authorities [1, 12]


aramid and


Each panel consisted of 8 plies of 295gsm (E-glass), 280gsm (carbon) and/or 290gsm (aramid). Each three hybrid set consisted of 2 x material A + 4 x material B + 2 x material A, 2 x material B + 4 x material A + 2 x material B and alternate plies of material A and B.


Hence, the stacks were mostly mid-plane symmetrical and


used 0-90 plies in order to avoid the added


complication of coupling. As the coupons consisted of only 8 plies, first ply failure and ultimate failure is calculated to be synonymous for all but two of the twelve panels, the exceptions


aramid-carbon alternate plies lay-up. Each panel was cut


being the aramid-glass and The ratio of the


ultimate moment to first ply to failure moment was calculated to be about 1.15 for both these lay-ups.


into 20 coupons meaning 240


coupons were tested in total. This is way beyond the normal requirements of five or so samples laid out in test standards [14, 15]


but it was felt important coefficients of variation (CoV) should be reliable.


As a check on the ‘serendipity factor’ which the author has come to associate with composite testing, coupons 1 to 10 were tested with the mould tool surface (first ply to be laid) in compression and coupons 11-20 with the last ply to be laid in compression. Of course for the nine symmetrical lay-ups the differences should be small. The average


difference between the two that


sets of


measurement where found to be 4% (stiffness) and 11% (strength). Of course, the calculation would show no difference, but respective CoV’s for the nine panels were 6% and 10%, suggesting reasonable symmetry.


The three alternate plies lay-ups are slightly


unsymmetrical. The average difference between the two sets of measurement where found to be 7% (stiffness) and 12% (strength), with respective CoV’s of 7% and 8%.


B-36


© 2008: Royal Institution of Naval Architects


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