97.5 sec 114.0 sec 127.5 sec 205.5 sec Figure 1: Damage growth pattern in glass/polyester composite MSEN (Vf =5:3 %and 0 Æ notch) EXPERIMENTAL PROCEDURE Material Optically transparent glass/polyester composite plates have been fabricated in the laboratory from sheetgradepolyesterresin(ParikhChemicals,Kanpur208016,INDIA)andglass bersintheform of cloth (balanced weave) and chopped strand mat (Harsh-Deep Industries, Ahmedabad 380023, INDIA).Therefractiveindexofthepolyesterresinismodi edtomatchwiththatoftheglass bers by adding 5% di-butyl pthalate and 2% di-vinyl benzene. The resin mixture has been cured using 1% MEKP and 0:03% cobalt octate at room temperature for 24h and post cured at 80 Æ C for 8h. Theplatethusfabricatedwith33% bervolumefractionshowsatransmissionratioof69:5%. Testing Dynamicdamagegrowthin bercompositeshavebeenstudiedusingmodi edsingleedgenotched (MSEN) specimens containing an initial notch length of 0.25 times the width. The specimen is mounted on the optical bench of Cranz-Schardin high speed camera. Dynamic loads are produced by the simultaneous detonation of two explosive charges on both side-shoulders of the specimen. For the present study, 50 mg and 25 mg of PETN explosive charge with lead azide as explosion initiator have been used respectively for cloth and mat composites. Detonation generates compressive waves whichgetre ectedastensilewavesfromthefreeend. TheendshapeoftheMSENhasbeenselectedinsuchawaythatthere ectedtensilewavespropagateasaplanarwavefront.Ithasbeen established[4]thataplanefrontedloadingtensilewavecanbeachievedwiththespecimencon guration considered for the present analysis. The planar wave loads the notch and initiates the damage. Damage Zone Measurements The detonation simultaneously sends signal to the high speed camera to start recording twenty pictures of damage initiation and propagation history at 5 sec interval. Image processing tool, IDRISI has been used to obtain the useful informations, like the damage area etc., from the recorded images. RESULT AND DISCUSSION Damage in Woven Fabric Composites The damage pattern and the propagation history in woven fabric (cloth) composite specimen having Vf =5:3%with =0 Æ are shown in Figure 1. The dark area ahead of the machined notch are the 3 dimensional damage occurred as the notch is loaded with the stress waves. Similar images have been obtained for all the cases considered. From these images, it is observed that the damage propagates perpendicular to the loading direction and parallel to the notch plane. In homogeneous materials, single crack propagates with possible crack branching under appropriate loading conditions, while in composites, a damage zone is formed ahead of the notch and this damage zone propagates. Further, it isobservedthatforagivenenergy,thedamagezonesizereducesasthe bervolumefractionincreases.
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