Preparation of Glass-reinforced plastics duct rods
Apparatus and materials
Mould, in the form of a rigid hollow cylinder having a minimum length of 400 mm and an internal diameter of, preferably, 4 mm ± 0.3 mm or 6 mm ± 0.3 mm. If other diameters are chosen, these shall be selected by agreement between the interested parties in the range from 4 mm to 10 mm. However, the diameter used shall in all cases be reported, and only test data obtained with moulds of the same diameter shall be used for comparative purposes.
The mould may be constructed of glass or polytetrafluoroethylene (PTFE). A release agent may be used in the mould if the shrinkage of the moulding is very low (typical of epoxy resins). However, an internal release agent (a release agent that is mixed into the resin) will affect the test results. Adding a release agent is not recommended.
Resin, suitable for use with the reinforcement to be tested (the manufacturer of the reinforcement roving normally gives general recommendations).
Metal wire or yarn, for pulling impregnated rovings into the mould.
Impregnation equipment (see Figure 1), comprising a long, shallow impregnation bath to impregnate a hank of rovings with resin. It is recommended that the bath be made of metal to facilitate heating of the resin to lower its viscosity if required. This is common practice for those resins, such as epoxy, whose room temperature viscosity may be too high to adequately impregnate the hanks of roving.
It is recommended that the rovings be pre-soaked in the bath of resin to facilitate wet-out and air removal. When a large number of rods are to be made, having two or more baths and switching the pulling of hanks from one bath to another while rovings soak in the other baths has been found an effective technique.
Lining the impregnation bath with thin cellophane foil keeps the bath from becoming coated with resin and makes cleaning faster with less need for cleaning solvents. The mould support shall be fitted with a device to prevent the mould moving as rovings are pulled into it. The rovings are pulled into the mould by means of a metal wire or yarn attached to the hank of roving. The wire or yarn can be pulled by a winding device. The winder can be hand cranked or motor-driven. Care shall be taken that the hanks of roving are pulled into the mould at a slow enough speed for the minimum amount of air to be trapped in the rovings.
Figure 1 — Example drawing of equipment set-up for impregnation of rovings
Circulating-air oven, for curing and/or post-curing the resin at the recommended temperature.
Diamond-tipped saw, for cutting the cured rods to required length.
Heating device and suitable glassware, if boiling-water pretreatment of the glass-reinforced plastics rods is required.
Former (see Figure 2), with a circumference of 1 m, for winding a hank of rovings.
Figure 2 — Winding a hank of rovings
Preparation of hanks of rovings
First, calculate the mass of roving needed to give a glass content of (65 ± 1) % by mass in the specimens produced. Then calculate the number of turns N of roving which will have to be wound on the former to give this mass. To calculate N, it is necessary to know the mass per unit length of the rovings.
Preparation of rods
Prepare an amount of resin sufficient to prepare the required number of rods. Use the resin either at ambient temperature in a conditioned room or at an elevated temperature if it is necessary to lower the resin viscosity. The elevated temperature can be specified by the manufacturer or be determined by preliminary trials. When the rovings have been conditioned, pour the resin into the impregnating bath(s).
Impregnate the roving by immersion in a bath containing the resin. In order to facilitate immersion and to enable air bubbles to escape, push the rovings gently (avoiding damage) to the bottom of the bath with a plastic spoon or wooden stirrer. The wire or yarn connected to the hank shall be kept out of the bath.
Because deficiencies in impregnation may cause considerable scatter in test results, care is necessary in this operation. The hank of rovings shall be completely impregnated before it is pulled into the mould. Good-quality impregnation is characterized by the absence of trapped air bubbles and usually the apparent disappearance of the fibres in the resin as wet-out occurs. The impregnation time shall be at least 10 min in all cases. With rovings that are hard to impregnate, use longer times and push the rovings around the bath with the stirrer or spoon to allow the air to escape. A bar near the entrance to the mould may be used to assist in removing air.
After impregnation, pull the hank of rovings into the mould. Adjust the pulling speed so that excess resin and air bubbles are expelled steadily and continuously. When the hank is in the mould, it is recommended that the mould be stoppered with a cork or other suitable stopper to prevent resin flowing out and air getting in.
The number of rods made will depend on their length and the number of tests to be performed. For each of the three types of test (flexural, compressive and interlaminar shear), a minimum of eight specimens shall be tested. Furthermore, besides testing the rods "as-moulded", testing can also be performed after boiling specimens in water or another medium. Different immersion times can be used. For each type of pretreatment or set of test conditions, another set of specimens will be necessary. The number of specimens in each set shall be at least eight. If specific statistical limits are agreed between interested parties, additional specimens may be tested to permit statistical analysis of the results.
Curing the rod
Cure the rod in the mould. Support the mould in the horizontal position in the curing oven. The curing and post-curing conditions shall be as specified for the resin system used. The curing conditions shall be included in the test report.
Cutting the FRP duct rod
If necessary, prior to demoulding remove the cork or stopper and cut off any impregnated roving protruding from the lower end of the mould. Remove the rod from the mould. After demoulding, trim square both ends of the rod, cutting off the loop at the upper end (which will not be cylindrical) as well as about 40 mm at the other end (where the cork or stopper was) because of possible variations in glass content in that section of the rod.
When rods pretreated with boiling water are to be tested, immerse the specimens in boiling distilled or deionized water (or another medium agreed between the interested parties). The immersion time for polyester-resin-based rods is typically 16 h or 40 h, while for epoxy-resin-based rods the immersion time may be 72 h, 144 h or 288 h.
After pretreatment, transfer the specimens to water (or the other medium, if used) at room temperature and allow them to cool to room temperature. Specimens shall be tested within 24 h of being pretreated. Take each specimen out of the water (or other medium) immediately before testing and wipe it dry.
Determination of mechanical properties on FRP duct rods made of roving-reinforced resin
- Determination of flexural strength
A duct rod is laid horizontally on two supports and bent at constant speed by applying a force at midspan until the specimen breaks.
The flexural strength of the FRP duct rod, expressed in megapascals, is the calculated maximum bending stress in the rod at break.
- Determination of compressive strength
A FRP duct rod is compressed longitudinally by applying forces at the specimen ends at a constant speed until the rod breaks or until the deformation has reached a predetermined level.
The compressive strength of the FRP duct rod, expressed in megapascals, is the calculated compressive stress at the highest load applied.
- Determination of apparent interlaminar shear strength
A flexural test is carried out. However, the span used is significantly shorter in order to induce interlaminar shear failure as a result of the relatively high shear stresses in the midplane of the glass-reinforced plastics rods.
The apparent interlaminar shear strength of the FRP rod, expressed in megapascals, is the calculated shear stress in the midplane of the glass-reinforced plastics rod at the highest load applied.
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