Debonded area Notch tip Matrix crack branching Debonded/delaminated fibers Matrix zone (a) (b) Figure 2: Microscopic picture showing (a) debonded area and matrix crack with branching in MSEN specimen having Vf = 2:4 % and 0 Æ notch(b)delaminatedanddebonded bersasbrightspotsin MSEN specimens having Vf =33:0 %and 0 Æ notch Table 1: Damage propagation velocity and rate of growth of damage in glass/polyester composites Volume fraction Notch orientation Velocity (m=s) Rate of growth (m2 =sec) % deg Initial Final Initial Final Woven berclothcomposite 2.4 0 460 260 3.49 { 15 400 210 3.35 { 30 420 320 3.21 { 45 470 150 3.42 { 5.3 0 490 0 2.50 0.45 30 380 0 2.22 0.13 45 450 0 2.92 0.98 Chopped strand mat composite 10.0 0 500 0 { { 15 250 0 { { Anotherimportantfeatureobservedinspecimenswithlow bervolumefractionsisthedamagezone splitting,analogoustothecrackbranchinginhomogeneousmaterials.Forlow bervolumefractions, themechanismsinvolvedintheformationofdamagezoneare,thematrixcracking, ber-matrixinterface debonding and branching in matrix region (Figure 2a). On the other hand, the mechanisms involvedinhigh bervolumefractionarematrixcracking, ber-matrixinterfacedebondinganddelamination (Figure 2b). Table1givesthedamagevelocityfordi erentspecimensstudied.Itisobservedthattheinitialvelocity is approximately same for both Vf =2:4%and 5:3 %. But during the end of the observation period, the damage slows down in single layer composites (Vf = 2:4 %), while it gets arrested in two layer composites (Vf =5:3 %). Further investigations revealed that the damage traveled to the entire width of the specimen in single layer composite which is not seen in other specimens. This lowering and arrest may be due to the unloading of the damage zone as the trailing part of the stress wavepassesoverandduetotheincreasedresistanceduetorisein bervolumefraction.
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