Journal of AE, Volume 23, 2005 (ca. 43 MB) - AEWG

detected, at higher strain values then Felicity effect is sensed. Finally, load takes the maternal tofracture and fiber breakage is perceived by AE signal. For these materials, however, it is verydifficult to assure when the significant cracks initiate.Of particular interest in this work is the relationship of the complex fiber architecture of theknitted fabric composite to the damage accumulation under load. For this reason AE techniquewas used to find out the strain value where micro-damage initiates in the composites materials.Obtaining the data, investigation was done at a macroscopic level by cutting and polishing severalpieces in different planes in order to relate the damage initiation and propagation with thegeometry of the textile. The two principal material directions, wale and course, were investigatedsince they represent the two extremes in the orientation of the knitted fabric. Finally, analysis ofa course sample loaded to a strain of 0.7%, which is just below the onset of matrix cracking inthese specimens (which occurs at 0.8%) provided further evidence of early damage initiation inthe form of cracking at the loop crossover points (Fig. 15). This represent fiber/matrix debondinginterface detected by AE signal at low strain values (Fig. 5), which some researchers [4, 5] haveidentified as the first stage in the composite fracture mechanisms: interface debonding, matrixcracking and finally fiber breakage.4. ConclusionsFig. 15 Evidence of crack initiation sites on crossover points.The advantage of acoustic emission (AE) technique is shown as applied to composite materialsin support of the analysis of damage development at lower strain to understand the crackingprocess by identifying initiation sites, cracking progression and final fracture. By AE, variousfailure mechanisms such as resin matrix cracking, fiber bundle debonding and tensile fracture offiber bundles were distinguished. Initial AE signal (event count rate) was attribute to the fiberbundle debonding at the crossover initiation sites, followed by visual matrix cracking on thesample. Micro-cracks originating at the debonded fiber bundles propagated into the resin-richregions of the laminate and cracks coalesced until catastrophic fracture. Similar mechanism offracture were observed in both the model and the RTM materials that suggest the intimate effectof the knitted fabric architecture in the final mechanical behavior of the composite. The modelsandwich laminates enabled the damage behavior and failure mechanisms to be related to theorientation of the fabric. The observed pre-damage in the knitted fabric layer was identified for35