Innovative test methodology for shelf life extension of carbon fibre prepregs

Abstract

The aerospace industry makes extensive use of composite materials in the form of fibre fabrics pre-impregnated with thermosetting resin, called prepregs. In order to minimize the resin polymerization before curing, prepregs must be stored at -18°C (0°F). There are therefore expiration dates for prepregs before use. Although manufacturers try to minimize storage time, offcuts and time out of the freezer, it is estimated that 30% to 40% of the prepregs are not used [1]. Today, recertification of expired materials is still complex and expensive, therefore it is generally chosen to send expired prepregs to landfill. The purpose of this work is to correlate physicochemical measurements with the loss of mechanical performance in order to point out and measure the real aging effects during excessive storage time. Processability, physicochemical and mechanical tests were performed in order to understand which tests are truly representative of ageing. This study was illustrated by testing on unidirectional Hexcel carbon/epoxy prepreg. Different expiry dates of this material were studied and the properties were compared. It was shown that the main observed degradation was the processability of the prepreg while mechanical performance was minimally degraded after the expiry date. This study could lead to a simpler measurement of the actual expiry rate of prepregs, which could be useful to speed up recertification procedures or to propose new scenarios to extend the shelf-life of expired prepregs [2].

Simplification of requalification procedure of outdated carbon/epoxy prepregs and scenarios of reuse

“…This section provides an overview of the results obtained from all the tests carried out, separated into two parts, those showing evolution of properties and those that do not. Some of the results have already been detailed in the former paper [4].…”

“…Many physical-chemical, mechanical and processability tests have already been performed and published in a previous study [4]. New tests will be detailed here and compared to the previous results to complete the discussion.…”

“…As a matter of fact, the requalification procedure corresponds to a full qualification procedure. Many unnecessary tests are then carried out which explains that the cost may be higher than the cost of the virgin material [4].…”

“…The shelf life indicated by the manufacturer is 1 year. Three different expired material were compared: i) outdated shelf life of 1 year ii) outdated shelf life of 5 years iii) outdated shelf life of 9 years Previous analyses [4] determined the properties of the compliant material (unexpired). These materials were analysed uncured for physical-chemical testing.…”

“…The metal substrate is then placed vertically and a visual control of the adhesion of the prepregs is performed after 30 minutes according to NCAMP standards [7]. The grades are defined from 1 (dry and brittle) to 6 (wet with resin transfer) [4].…”