Diacetilenos: propiedades, polimerización y aplicaciones

3.5.1. Preparation of Specimens for Testing

3.5.1.1. Polishing of Specimen Edges

In order to obtain edges that are polished to a finish of one micron, an edge-polishing jig for long tensile specimens was designed and manufactured specifically for this purpose.

The whole length of the specimen edge could then be polished, using metallurgical grinding and polishing techniques. This polishing process was conducted prior to end tagging.

Six specimens were placed into the jig (three on either side) with aluminium strips on either side in order to prevent damage to the surface of the specimens by the jig’s tightening screws. The edges to be polished were levelled with respect to each other in the jig, by first finger-tightening the screws and then laying the specimens on a flat glass surface. This is to ensure that all the specimens lie flat whilst grinding and polishing in order that the complete length of all specimens are polished to the same degree. The specimens were then finally tightened in the jig and were ready for metallurgical grinding and polishing, using a Planopol-2 metallurgical polisher, with a Pedemax-2 attachment.

The following polishing sequence was used, which differs from that used for preparing mounted edge specimens (see section 3.6).

The first step was to abrade the specimen edge through a series of grit papers. They were initially abraded using the following papers sequentially: grades 500, 1200, 2400 and then 4000. The lubricant used was water, with an arbitrary machine pressure setting of 3 and no independent movement of the specimen jig with respect to the polishing wheel. The times for the abrasion stage were as follows; 60s, 45s, 45s and 45s for each grade of paper, respectively. Between each grinding cycle, the specimen jig was removed from the Planopol machine for cleaning in an ultrasonic bath for around five minutes. The specimens were then rinsed with water and left to stand for a few minutes, in order to

remove any excess water trapped in between individual specimens. The specimen jig was then placed back onto the Planopol for the next grinding cycle. After the first grinding cycle, the specimen edges were checked under a microscope to ensure that the specimen’s edges remained planar.

Following this, the specimens were then polished using the following sequence. First, the specimens were polished on the 6 micron DUR cloth and 1 micron DUR cloth, using the 6 micron and 1 micron diamond sprays, respectively. A lubricant was used with an arbitrary machine pressure setting of 3 and no independent movement of the specimen jig with respect to the polishing wheel. The time for each polishing cycle was two minutes.

Between each polishing cycle, the specimen jig was removed from the Planopol for cleaning as for the abrasion stage outlined in the previous paragraph, although in this case the specimens were also rinsed with methanol. After each polishing cycle, the specimen edges were checked under a microscope to ensure that there was sufficient removal of scratches at each stage, which was done by repeat polishing cycles until the polishing was satisfactory. Once the polishing process had been completed, the specimens were removed and cleaned with methanol. The specimens were then turned around in the polishing jig so that the other edges could be polished using the same method.

3.5.I.2. End-tagging

Grit paper was used to abrade a 50 mm length at either end of the test specimens in preparation for bonding the end tabs. The abraded surfaces were then cleaned lightly with a water-based alkaline surface cleaner. Strips of aluminium alloy for the end tabs were cut, using a guillotine to dimensions of 20 mm x 50 mm. These were then etched for 30 minutes at 65°C in a concentrated sulphuric acid/sodium dichromate solution in order to promote adhesion. After drying, the tabs were bonded to the specimen ends using Permabond F245 two-part adhesive. The adhesive was applied to the specimen surface and the initiator was lightly applied to the end tab surface and the two were then brought together, ensuring a good mix of the two parts of the adhesive. The end tabs were left overnight with a weight of 500 g on each end to ensure the formation of a strong bond.

This procedure left the specimens with a gauge length of 130 mm. Figure 3.3 illustrates the specimen configuration.

3 5.1.3. Preparation for Strain Measurement

A 50 mm gauge length extensometer was used to measure longitudinal strain. In order to keep the extensometer in place during testing, two grooved seatings were made using two-part rapid-fix Araldite adhesive bonded onto one surface of the specimens. The grooves on the extensometer would sit in these grooves and would also be held in place on the specimen by rubber bands. The grooves were spaced 50 mm apart centred about the mid-length of the specimen. The locations on the specimen where the grooves would lie were abraded using a glass fibre abrasive pen. The two-part rapid-fix Araldite adhesive was mixed just prior to use, then spread evenly across the width of the specimen where the grooves would sit. The grooves were made by placing a V-shaped aluminium

‘groove maker’ on top of the uncured adhesive, and held in place for at least 10 minutes, or until the adhesive had set (see Figure 3.4). Prior to testing, the extensometer would be placed with the knife-edges in the V-shaped groove and held in place using the rubber bands.

3.5.2. Quasi-static Tensile Test Types

Quasi-static tensile tests were conducted to examine the accumulation of damage within the coupons and the effect of the damage on the mechanical properties of the coupons.

The tensile testing was performed on an Instron 1196 machine at a displacement rate of 1 mm/min. A 100 kN load cell was used. Strain measurements were taken using a 50 mm gauge length extensometer and the stress-strain response was recorded using a computer data-logger.

There were three types of quasi-static tensile tests performed. The first two types of test were conducted in order to obtain data for damage accumulation and quantification,

whilst the third type of test was conducted in order to obtain data for the degradation in stiffness as a result of damage accumulation.

In the first type of test, the specimen was loaded continuously to failure, recording the stress as a function of applied strain. The undamaged Young's modulus (E0) was obtained along with the damage initiation and laminate failure strains (sLi, sLf), as well as the ultimate tensile strength (<tts). The measurement of the undamaged Young’s modulus was taken over a range of 0.1 % to 0.4 % strain, which is below the strain corresponding to the onset of matrix cracking. In order to quantify damage (transverse ply crack density, l/2s, where 2s is the average spacing between cracks) as a function of applied stress, in- situ plan view photographs were taken. Photomicrographs of edge sections from failed specimens were also taken in order to show the morphology of damage corresponding to maximum crack density. In the second type of test, the specimen was loaded continuously up to the onset of damage. Plan view photographs were taken in order to show the damage initiation sites and the morphology of cracking. In the third type of test, the specimen was loaded incrementally. Specimens were loaded initially to 0.4 % strain (which is below the onset of damage) and then the load was ramped back down to zero.

After this, repeated loadings were carried out up to progressively higher maximum strains increasing from 0.4 % strain up to imminent laminate failure in 0.1 % strain increments.

The change in Young’s modulus as a function of applied strain and crack density was recorded (see next section). The measurement of the undamaged Young’s modulus was taken over a range of 0.1 % to 0.4 % strain, which is below the strain corresponding to the onset of matrix cracking. Note that the specimen remained in the machine grips at all times. The normalised modulus (E/E0) as a function of applied maximum strain was then calculated. In order to quantify the stiffness reduction resulting from the damage observed, the normalised modulus was correlated with the transverse ply crack density obtained from in-situ plan view photographs. As in the first type of test, in order to quantify damage (crack density, l/2s) as a function of applied maximum strain, in-situ plan view photographs were taken at the point corresponding to the maximum strain for each loading. Photomicrographic edge sections of the damage corresponding to maximum strain were also taken in order to obtain measurements of laminate parameters

required for theoretical modelling (see chapter 5), i.e. the longitudinal ply thickness (b), the transverse ply semi-thickness (d) and the outer resin-rich region thickness (m).

For tests conducted under incremental loading (to obtain data on the degradation in stiffness as a result of damage accumulation), each modulus value relates to the previous maximum strain (and also damage density) that the specimen had experienced. For instance, referring to Figure 3.5a, if a particular specimen was loaded to a maximum strain of s3 % strain (after a previous maximum strain of s2 %), then the modulus value taken (E2) would relate to the previous strain, s2 %, and the damage density, D2 corresponding to the strain s2 %. Tests conducted under continuous loading would have the damage density relating directly to the strain at which the photograph was taken (see Figure 3.5b).