Trans RINA, Vol 152, Part B2, Intl J Small Craft Tech, 2010 Jul-Dec


the three different layups were tested. The specimens were numbered A01 to A05 for lay-up 1, B01 to B05 for lay-up 2 and C01 to C05 for lay-up 3, with the sub- notation of ‘AE’ being used to note the specimen which used the acoustic emission set-up during the test. Figures 3.1, 4.1 and 5.1 show the crack initiation and progressive failure at various stages of testing. The load- displacement plots for all the five specimens of each layup are shown in Figures 3.2, 4.2 & 5.2.


In this experimental program the progressive failure


behaviour of the specimens having different composite layups were analysed by comparing four acoustic emission parametric variables along with the load vs. time plot. The results of the parametric correlations are shown in Figures 3.3, 4.3 & 5.3. Each graph contains four plots, (1) Amplitude vs. Time, (2) Event duration vs. Time, (3) Event Rise time vs. Time and (4) Energy release vs. Time. In case of a fracture, elastic energy is released and can be measured by acoustic emission. The history of AE events represents the damage initiation and progression in the material. The non-linear behaviour of the specimens and the AE counts were studied as a function


of time. When the laminated composite


structure begins to generate more AE events, it is an indication of the damage initiation in the material. Layup 2 showed signs of saturation with the accumulation rate being stagnant until final failure. The other two layups showed increase in the number of AE demonstrating


the damage progression.


events, The


accumulation rate of AE increased even though a decrease in load was observed, demonstrating the damage progression as shown in Figures 3.3, 4.3 & 5.3. Load drop at the end of the curves (load-deflection plot) indicate the


extensive material. All the three


damage development different


in the layups displayed


extensive damage due to non-homogenous stress fields between the layers composed of strongly anisotropic plies with different fibre orientations.


4.1(a) Layup 1 (Csm) Experimental Results


For Layup 1 specimens, the initial delamination was observed at 162.92N at a displacement of 66.27mm (Specimen # A05). A crack was developed between the


second and third layers as shown in Figure 3.1. This is shown in the load-displacement plot Figure 3.2 in the form of a small kink. With further application of load, the specimen began to straighten up, creating excessive interlaminar stresses in the bend. Outer layers were subjected to interlaminar compressive stresses. The outer three layers buckled and the fibres sheared off in compression


(Figure 3.1). No delaminations were


observed between other plies. During the final stages of loading, a small delamination occurred between the 6th and 7th layers. This caused further delamination in the specimen and ultimate collapse occurred at 616N and a 85.58mm.


4.1(b) Layup 1 (Csm) Acoustic Emission Correlation


In the layup 1, all layers were CSM; hence there was no difference in the elastic modulus between the individual layers. Four distinguishable damages were analysed by AE as shown in Figure 4.3. The initial scatter up to 375 sec accounts for matrix cracking. The first major damage was observed at 375 sec between second and third layers at 30% of ultimate load. The primary sign of initial delamination is shown as the energy release of 563 eu, amplitude of 78 dB, event rise time of 510μs and event duration of 7139 μs. The small load drop in the load-time plot complements the failure analysis of AE. Further damage up to 438 sec was attributed to matrix cracking and


delamination growth. Next major delamination


occurred at 438 sec between third and fourth layer, with energy release of 521eu, amplitude of 92dB, event rise time of 546 μs and event duration of 4327 μs. Later the crack length propagated between third and fourth layers as is shown in Figure 3.3. The third major delamination occurred between fourth and fifth layer at 499 sec with energy release of 490eu, amplitude of 99dB, event rise time of 726 μs and event duration of 1819 μs. No considerable reduction in load was observed between initial delamination at 375sec and final collapse at 546sec. The final collapse at 546 sec was a combination of matrix cracking, fibre failure and delamination with an energy release of 15609eu, amplitude of 100dB, event rise time of 464 μs and duration of 162365 μs.


Figure 3.1: Progressive failure of Layup 1 specimens


©2010: The Royal Institution of Naval Architects


B-97


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