JEC COMPOSITES MAGAZINE - Issue #101 - November/December 2015 - 36

ageing

Fig. 1: Weight change during hygrothermal ageing

Fig. 2: Fatigue S-N curve evolution during hygrothermal ageing

Focus
The intrinsic hydrophobicity of pDCPD imparts resistance to
hygrothermal ageing, and the ultra-low viscosity is beneficial
to infusion of thick and large parts.

The long-term (one year) effects of water immersion ageing (60°C,
deionized) on unidirectional (UD) glass/pDCPD composites was
investigated using a traditional glass/epoxy composite as a benchmark material for comparison. The pDCPD composites absorbed
less water than epoxy composites and also exhibited superior
fatigue strength retention. The thermal oxidation ageing (100°C,
air) behaviour of pDCPD composites was also analyzed. Stable
mechanical properties were observed after half a year of oxidation,
indicating their superior resistance to thermal ageing.
Figure 1 shows the effects of water immersion ageing on pDCPD
and epoxy composites. The pDCPD composites showed less water
uptake than epoxy composites, and this behaviour was attributed
to their intrinsic hydrophobicity. At the molecular level, pDCPD
polymer chains lack hydrophilic groups, while epoxy polymer
chains contain hydrophilic hydroxyl groups that foster water
uptake. Dynamic mechanical analysis (DMA) also revealed that
after ageing of neat epoxy resin, plasticization occurred and the
Tg decreased from 100°C to 85°C. In comparison, plasticization
was not observed in pDCPD, and the Tg remained stable at 130°C
during ageing.
Figure 2 shows the tension-tension fatigue behaviour of unidirectional 0° composites after hygrothermal ageing. On the plot, S
indicates the maximum cyclic stress, while N indicates cycles to
failure. A stress ratio of R = 0.1 and a loading frequency of 5 Hz
were used. The two composites showed similar S-N curves for the
pre-aged condition, indicating comparable resistance to fatigue
loading. However, after one month of ageing, the fatigue strength
of pDCPD composites increased slightly. This was attributed to
post-curing and/or physical ageing of the matrix. In contrast, the
fatigue strength of epoxy composites decreased slightly, a be-

Fig. 3: Fibre push-out test of a glass/pDCPD composite

haviour due to fibre strength decrease, matrix plasticization, and
interface degradation, as evidenced by decreases in tensile strength
and interlaminar shear strength (not shown in this paper). After
three months, the fatigue strength of both composites decreased,
although pDCPD composites showed superior strength retention
relative to epoxy composites. The fatigue life cycles at S = 450 MPa
of pDCPD composites decreased by 88% after three months, while
the fatigue life of epoxy composites decreased by 99% under the
same load.
In many cases, ageing strongly influences the fibre-matrix interface strength of composites. In this work, a single fibre push-out
technique was used to evaluate such changes. Figure 3 shows the
push-out device at the moment of interface debonding during a
test. A diamond tip indenter was driven by a stepper motor and the
load curve was recorded during the process. The apparent interface
strength (τapp) was calculated from the debonding load and fibre
dimensions.
Figure 4 shows the dependence of interface strength (τapp) on hygrothermal ageing time for the two composites. The τapp decreases

36 jec composites magazine / No101 November - December 2015

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #101 - November/December 2015