(688e) Cure Monitoring of Glass-Fiber Reinforced Composite (GFRP) Laminates By in-Situ Strain Measurement

CURE-MONITORING
OF GLASS FIBER REINFORCED (GFRP) COMPOSITE LAMINATES BY IN-SITU STRAIN
MEASUREMENT

Santoshi
Mohanta ¹, Swati Neogi ²

^1,
2 Department
of Chemical Engineering, Indian Institute of Technology,Kharagpur,
India

E.Mail:
santoshimohanta@iitkgp.ac.in,
swati@che.iitkgp.ernet.in

Abstract

The
widespread application of composite materials is limited by the instability of
product quality due to variances in the manufacturing parameters.  One such important
parameter is the degree of cure which depends on the chemistry, kinetic and
rheological properties of the resin and the curing strategy specific to the resin.
Optimized curing strategy includes the optimization of temperature and time
cycle. Optimization of curing time and temperature will allow one to shorten
the manufacturing time. It will also provide the information about the
progression of cure reaction, extent of cure, building up of residual stresses
and ultimately minimize the cost [1]. However, for most of the
manufacturing processes, the curing time is not optimized and excess time is allocated
than the actual time required. This is because the time for complete cure
during manufacturing cannot be estimated without monitoring the cure cycle. 
The time can be reduced significantly if the curing process can be monitored
and the cure completion time can be detected.

Resins,
consisting of low molecular weight polymeric chains react to form a three-dimensional
network by crosslinking when mixed with proper curing agent [2]. It results in significant
volume shrinkage which results in development of internal strain.  In-situ
strain measurement using embedded-strain gauges during manufacture can be a
viable option for monitoring the cure and optimize the curing strategy which
has been explored in the present study.

In the
current study, a glass fiber reinforced composite laminate and an epoxy plaque
have been manufactured by embedding strain gauges as shown in Figure 1a.  Amine
cured epoxy resin has been used here. Curing parameters such as onset, peak,
and final temperatures are determined by DSC as shown in Figure 1b. The
laminate was cured at 85 °C which is above the onset temperature.

Figure 2a shows the strain and temperature variation with time as
measured by the embedded strain gauge and thermocouple. Percentage of cure is
determined from the DSC curve given in Figure 2 b. During
the heating up phase, strain continuously increases as temperature
is raised up to 85°C as shown in Figure 2 a. During this period, the
resin viscosity decreases with application of heat. The
indication of gelation is observed by the remarkable change of strain which is
induced by the increase in density as cross-linking of polymer chains occur at
around 1.43 h. Here the percentage of cure is 56% as obtained from DSC. After
this, when the temperature reaches 85°C, a minimal change in strain is observed.The remaining cure occurs in the next 2-3hrs
till cure percentage reaches 99.99%. The strain developed for epoxy(EP) resin
plaque (cured resin without reinforcement) is compared with the EP-FRP laminate to obtain a clear
understanding of the curing cycle and to predict the cure completion time which
can help in shortening the manufacturing time without affecting the quality of
product.  The strain sensors used are able to record mechanical
deformations which arise inside the laminates during the manufacturing.

Key words:
Glass fiber reinforced composites, Cure monitoring, Strain sensors, Percentage
conversion

Figure
1 a. Glass fiber reinforcement and strain sensor placement

Figure 1 b. Heat flow
curve of amine cured epoxy resin at different temperatures as obtained from DSC

Figure 2 a.
Strain-gauge strain and temperature measurement along with conversion percentage
in the EP-plaque and EP-FRPs during curing.

Figure 2 b. Heat flow
curve of epoxy resin (amine cured) when cured in DSC as per the cycle followed
during manufacturing.

References:

[1]      D.
Mulligan, Cure Monitoring for Composites and Adhesives, vol. 14.
iSmithere Rapra Publishing, 2003.

[2]      M. Younes, S. Wartewig, D.
Lellinger, B. Strehmel, and V. Strehmel, “The curing of epoxy resins as studied
by various methods,” Polymer (Guildf)., vol. 35, no. 24, pp. 5269–5278,
1994.