t
However, production of double cantilever beam samples (DCB) was more
involved than this and required extensive technique development before
the commencement of testing.
The object was to produce a flat sheet 5mm thick, with an internal
fold of aluminium to form a pre-crack through the centre of the sheet
thickness, up to 3 5 mm from one end of the sheet.
It is important that the orientation of the aluminium is central and
parallel with respect to thickness. This permits the crack to propa
gate down the centre of the sample, so both halves of the cracked sample
are of equal thickness, giving true cantilever beam conditions. In
the case of continuous fibre reinforced samples, this presents few
problems, as the prepreg naturally orientates the foil correctly in a
being used as base material here presented many problems and numerous
systems were attempted to establish a successful moulding technique
as follows:
(a) A single moulding operation was carried out in which 100g of
Mono Munched material was added to the mould and levelled. The fold
of aluminium was placed on top of this and the final 100g placed on top of this and moulding carried out in the previously described manner.
A cross-section through the sheet revealed an unsatisfactory pre-crack
which was neither straight nor central.
(b) In an attempt to prevent excessive wandering, 70g of Mono Munched
material was given a standard moulding cycle.. A further 30g was then
added and levelled before the aluminium fold was positioned and
covered with 30g of material before a second moulding operation. Finally, a further 70g of material was added and a final moulding
operation carried out. Obviously this was very time consuming and
showed very little improvement in the orientation of the pre-crack.
(c) To utilise the stiffening effect of continuous fibres, method (b)
was repeated using 0° orientated fibres to construct the top and bottom layers of 70g. Such a moulding was sampled and placed under
test, but the crack migrated to the continuous layer interface and
thus any such testing would be unrepresentative of the composite under
evaluation. •
(d) 100g of Mono Munched material was moulded, the aluminium fold
placed directly on top of this and the final 100g of material added before a second and final moulding. It was thought that the flat
moulded surface would act as a former, preventing the foil from move
ment, so holding it true and central, and this was-achieved to some
degree. Two samples were taken from such a moulding and tested.
t
The first showed different degrees of bending in each half and thus
a loss of the required DCB conditions. The second sample was reinforced
by glass fibre epoxy strips glued to the outside in an attempt to
minimise bending, but this was unsuccessful because the adhesion
interface failed after very little displacement. It was thought
that a contributory factor to the problem could be that some parts
of the moulding were receiving two or more cycles while others
received one. consequently, a final moulding technique was attempted
in order to eliminate this, as described in (e).
(e) Finally, 100g of Mono Munched material was moulded and removed
from the mould. Another 100g was moulded. The adjoining faces
were degreased and the first sheet moulded onto the second with the
/
aluminium fold inserted between the two prior to final moulding.
The resultant sheet contained a very straight pre-crack and this was
the moulding technique employed for subsequent DCB specimen prepara
tion .
The moulding procedure was thus defined, and with this a range of seven
particle sizes varying from 6.35 - 25.4 mm were taken and samples pro
duced destined for the mechanical testing procedures outlined later.
The particle size was increased by equal increments between samples,
and thus a complete range of the particle size effect could be accumu
lated .
When injection moulding APC-2 offcuts, dilution of the fibre volume
fraction was necessary, in compression mouldings, this presented a
problem with the distribution of the additional resin.
The dilution was attempted using PEEK granules without any success
because the PEEK granules migrated to the bottom of the mould, leaving
APC-2 in the upper regions, thus producing a distinct two part
moulding.
(
Eventually, dilutions of 55, 60 and 65% PEEK by weight were achieved
particles within the mould to achieve the predetermined dilution.
Another alternative which it was thought may offer useful theoretical
evidence was to produce an ordered lay up of particles within the
moulding. This effect was created in mouldings with 25.4mm square
particles stacked to produce lay ups with fibre orientations of
(0)t6? ^ ^ 1 6 and (0*9 0)0. The system of particle arrangement was a regular pattern as presented in Figure 17 and thus approaching the
theoretical situation.
Other particle effects could prove interesting such as:
(a) The effect of combining two particle sizes in one moulding. The
two sizes chosen initially were 25.4mm and 1 2.7mm square.
(b) The effect in a woven product. A mould was made of woven 12.7mm
wide strips in eight mats.
(c) The effect of elevated temperature. Flexural testing at 200°C
was used to ascertain this.
(d) The effect upon final properties if the base material for the
moulding was preconsolidated sheet stock. Pieces were cut from 8 ply preconsolidated material and moulded as previously described to
establish this.
(e) The effect of extended time at moulding temperature upon the
material properties, because if mouldings were made from offcuts•and
scrap parts, then APC-2 would be subjected to longer times at the
moulding temperature. Mouldings were made, holding at temperature
for up to eight hours to establish this.