: I presume that the samples with two layers of cloth were layed-up
: with the yarns parallel ? Do you think there would be any benefit
: in having the yarns at 45 degrees to each other ? That might be
: difficult to achieve on a boat, depending on the width of the cloth,
: but having the length of the cloth aligned 22.5 degrees to the boat
: centre line, one layer having that angle to the left, the other to
: the right might achieve a more uniformly strong composite, even if
: it wasn't actually any stronger in its strongest direction. Want to
: do more tests, or is the single layer 6 oz s-2 glass so much superior
: that it isn't worth knowing about anything else ?

: Andy

Andy,

Good question , I’m sure it’s well formed based on knowledge of the directional properties of composites but there are a few things to consider. For one, those directional properties are measured using uniaxial tests – tension, compression, or bending about a single axis. Also recall that the stiffness varies in much the same way as the strength, so in the weak direction(s) the composite will deform more prior to failure. When something like a rod impacts a composite plate, it imparts a strain field as the sample tries to conform to the end of the rod. Since the shape of the rod is uniform, the local strain field is uniform. In the lab, this is referred to as strain control – we impart a strain, and measure the resulting stress. As a result, failure will occur first along the orientations that have the lowest strain to failure. In woven composites that orientation is also the strongest orientation, so failure will occur along lines perpendicular to the fibers. Because the plane of the sample is symmetric about two axes, separated by 90 degrees, a 0-90 orientation will provide essentially the same resistance to this type of impact as any other combination of 0-90 layers regardless of the relative orientations between the layers.

Based on the above reasoning (and my own observations of smashed samples), I would not expect any real benefit to having fiber orientations other than 0 and 90 for the rods, and only minor differences for the screwdriver. For the screwdriver, one might expect small differences related to the relative angle between the blade and the fibers because the shape of the blade imparts a very local, non-uniform strain field. But for each orientation that might show less damage, there would be another orientation that would show more damage. The damage might have a different shape, but the energy dissipated in damage would be very nearly the same.

Note that we haven’t mentioned delamination, which is one way to dissipate impact energy through damage. But the role of delamination in thin composites having only a few lamina is small.

Thanks,
Sam