Trans RINA, Vol 152, Part B2, Intl J Small Craft Tech, 2010 Jul-Dec
polymer blends, particle-filled and FRP composites [20, 21, 22]. AE measurement is based on the detection of the surface movements caused by stress waves of the fracture process in a microscopic scale
[23]. Two
approaches of AE study are generally performed: Parametric AE analysis and Transient AE analysis. In the current study, parametric AE analysis is used based on the extraction of a number of parameters (such as arrival time, peak amplitude, rise-time, signal duration, counts, elastic energy released during fracture (measured in atto Joules, 1atto=10-18, referred to as eu), RMS, etc) from individual AE signals [24]. One of the initial studies [25] showed only two levels of failure, matrix cracking and fibre fracture. Over the years, due to sophistication in the data acquisition technology, the detection of other failure modes in the
laminate are possible. A number of
investigators have attempted to use the AE technique for detection and evaluation of damage mechanisms in composites [23, 26, 27, 28, 29].
with unidirectional and cross ply mats. Most of loading parameters used were either pure bending or pure edge loading, based on Lekhnitskii’s solutions. The L-bends laminates were fixed at the base and a pure moment was applied on the top arm until the final collapse and failure modes were studied.
1.2 CURRENT WORK
The structures in general experience various types of loading, not just the pure bending or pure edge loading. Hence an attempt is made in this paper to study the failure mechanism of L-bend specimens subjected to flexural load. Boat builders tend to use uni-directional fibres to increase the strength. An attempt to check the effectiveness of addition uni-directional fibres is focused in this paper.
The key aims of this paper are three-fold, to study the: • through thickness stress distribution of
composite laminate with three layups subjected to bending load.
the curved different material
• effect of inclusion of uni-directional (UD) layer in the layup configuration
• failure characterisation of curved composite using Acoustic Emission (AE).
2. Figure 1: A typical AE Signal [24] Huget et al [28] studied the damage mode and the
corresponding amplitude output of GFRP composites. The authors proposed that the amplitudes for matrix cracking, debonding, fibre pull out and fibre fracture had 40-55dB, 60-65dB, 65-85dB and 85-95dB respectively. The study of Ageorges et al. [30] showed that the interface debonding had amplitude of 40-60dB whereas fibre breakage at 75-90dB. Failure mechanism based on the event duration can be classified as Debonding ~ 10ms, Fibre breakage ~15ms and Delamination > 25ms [31].
Various studies have been conducted to assess the curved composite delamination due to interlaminar
stresses. Martin et al. [2] studied failure of curved composites Table 1- Specimen Layup
Layer 1 Layer 2 Layer 3 Layer 4 Layer 5 Layer 6 Layer 7
Normalised fibre weight B-94
Layup 1 – 4.62mm Layup2 – 4.62mm Layup3 – 4.62 mm CSM 450gsm CSM 450gsm DB 611gsm CSM 450gsm DB 611gsm
CSM 450gsm
CSM 450gsm CSM 450gsm UD 451gsm CSM 450gsm DB 611gsm
DB 611gsm
CSM 450gsm CSM 450gsm CSM 450gsm CSM 450gsm DB 611gsm
UD451gsm
CSM 450gsm CSM 450gsm DB 611gsm 1.00
1.15 1.10 ©2010: The Royal Institution of Naval Architects
In the experimental investigation, composite laminates were manufactured using E-glass fibre and Vinylester resin (FGI – SPV 6036) by hand layup and cured at room temperature for 24hrs. The specimens were manufactured with three different layups (Table 1) with a uniform thickness of 4.62mm, each lamina being 0.66mm thick. Layup 1 had seven CSM layers, Layup 2 had four CSM and three DB layers and Layup 3 consisted of two CSM, three DB and two UD layers. The ply layup sequence is 1 to 7 from the inner surface to outer surface is shown in Figure 2.1 and numbered as shown in Table 1. The specimens had an arm of 115mm,
leg of 100mm and
were cut to 25mm width. The radius of the bend was 19mm as shown in Figure 2.1. Five specimens were tested until failure for each layup.
EXPERIMENTAL INVESTIGATION 2.1 EXPERIMENTAL SETUP
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