JEC COMPOSITES MAGAZINE - Issue #68 Special JEC Asia - October 2011 - 82

natural fibre

problems encountered when applying these materials to engineering practice, and how fibre morphology affects the mechanical behaviour of the resulting composites.

Materials and methods
The experimental characterization was carried out on two different natural-fibre-reinforced composites. These are referred to as Case 1 and 2. Case 1 Laminates used in Case 1 consisted of hybrid composites reinforced with glass and jute fibres manufactured by resin transfer moulding. Plain-woven jute fabric (300 g/m2) and E-glass (Vetrotex VR38, 290 g/ m2) were used as reinforcement. The resin was an unsaturated polyester (1629 NT from Lonza). Two different configurations (labelled as Q and T) were obtained, for which stacking sequences and characteristics are summarized in Table 1. From the plates, four-point bend specimens were obtained with 150-mm length, 30-mm width and 5(±0.2)-mm thickness. Specimens from T and Q laminates were impacted and then subjected to post-impact four-point bending tests. Impact tests were performed using an instrumented impact tower fitted with an anti-rebound device. Four different impact energies were considered: 5, 10, 12.5 and 15 J. The flexural tests were carried out in accordance with the ASTM D-790 standard, using a universal testing machine (Zwick Roell Z010) with a support span length of 140 mm and a crosshead speed of 5 mm/min. Post-impact flexural tests were monitored by AE until final fracture occurred using an AMSY-5 AE system from Vallen Systeme GmbH. The AE acquisition settings used throughout were as follows: threshold = 35 dB, RT (rearm time) = 0.4 ms, DDT (duration discrimination time) = 0.2 ms and total gain = 34 dB. Four PZT AE sensors resonant at 150 kHz were used: two sensors were placed on the surface of the specimens at both ends to allow linear localization, while the other two sensors were used as guard sensors in order to discriminate between AE signals and noise.

Case 2 In Case 2, untreated phormium tenax-reinforced epoxy composites were tested. The matrix was an epoxy resin

jec composites magazine / No68 October 2011
Sample name Q T Stacking sequence (G = Glass; J = Jute) Number of jute layers 4 4G/1J/2G/1J/2G/1J/2G/1J/4G 7G/4J/7G 14 4 14

Table 1: Summary of laminate configurations (Case 1)

Number of glass layers

Case 1: jute/glass hybrid laminates In Case 1, the idea was to demonstrate which one of the hybrid configurations – T (sandwich hybrid) or Q (intercalated hybrid) – both with the same amount of plant fibres and therefore with a similar reduction in weight, would prove more suitable to sustain the application of load. Flexural tests demonstrated that T hybrids are superior to Q hybrids at all impact energies by more than 20% both in Table 2: Post-impact flexural strength and AE strength and cumulative-counts "knee" stress for Q and T modulus [7]. glass/jute hybrids impacted at different energies AE analysis was Flexural strength σc (MPa) Specimens focused on the (MPa) measurement of Q0J 209.91±0.37 93.66±3.12 the cumulativeQ5J 172.89±7.18 84.92±9.95 counts curve “knee”: the stress Q10J 160.42±8.98 75.40±7.45 value associated Q12.5J 157.08±10.42 71.07±0.99 with the change Q15J 154.57±3.31 72.50±10.34 in slope of the T0J 269.34±15.47 90.33±6.43 above curve was referred to as σc T5J 244.13±0.69 82.64±4.73 and was T10J 227.43±14.87 79.85±3.11 supposed to be T12.5J 212.76±14.20 80.61±2.56 an acceptable indication of T15J 195.04±18.49 54.50±10.13 the upper limit for materials service. Post-impact flexural and AE test results (σc) are summarized in Table 2. Values of σc were comparable for both the T and Q configurations, representing in general about 30-40% of the maximum strength. However, in the specific Overall fibre case of flexural tests after 15 J impact, the T content [vol %] configuration showed a much lower value of σc than the Q configuration (54.5±10.1 MPa vs. 72.5±10.3 50±2 MPa). Intercalated layers of glass-fibre composite in Q hybrids offer more gradual degradation to the 50±2 material with increasing impact energy, whilst in T hybrids the breakage of jute-fibre core by tearing off of the fibres is the dominating failure mechanism [8]. AE localization plots for Q and T laminates impacted at 15 J are shown in Figures 1 and 2. The AE events are more concentrated

(Ampreg 26) by SP systems. The fibres were cut to a length of approximately 2 cm and randomly arranged in the final composite. The specimens were manufactured using the hand lay-up process in a closed aluminium mould. The final fibre content was 20 wt%. Five specimens (200 mm x 30 mm x 4.5±0.2 mm) were tested in three-point bending on an Instron 5584 at a constant crosshead speed of 2.5 mm/min and with a span-to-thickness ratio of 20:1. These tests were monitored by AE according to what was reported for Case 1.

Results and discussion

JEC COMPOSITES MAGAZINE - Issue #68 Special JEC Asia - October 2011

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