Strain-Rate Effects on Unidirectional Carbon-Fiber Composites

Strain-rate effects on mode I fracture of unidirectional carbon-e ber tow composites corresponding to crack propagation parallel to the e ber tow direction was investigated. Precracked unidirectional stitched carbon-e ber specimensweresubjected to a staticandlow-velocity-impactthree-pointbend test.Thecrack position asa function oftime and hencethecrack-propagation velocity were measured with thehelp ofspecial crack-propagation gauges and a high-resolution digital camera. Load vs load point displacement was measured for every test. The effect of strain rate on fracture energy was characterized. The Iosipescu shear test under static and low-velocity-impact loading conditions wasused to characterizetherate-dependent shearresponse ofthematerial. In addition, tension and compression responses were characterized using American Society for Testing and Materials standard test cone gurations. It is found that the mode I fracture energy decreases with an increase in the rate of loading. with the failure event. The resin used for the tubes is slightly rate sensitive; however,thisindicatesthat theresin isstiffer and stronger under dynamic conditions. Yet, the dynamically crushed tubes con- sistently absorblessenergythan thestatically crushedtubes,always atalowermeanplateauload.Thus,itappearsthattheratesensitivity of the fracture events warrant a careful examination. The basic building block of braided composite plaques are e ber tows that are braided into different microstructural architectures prior to being infused with resin. Different types of braided archi- tectures are summarized in the text. 1 Prior to studying the fracture propertiesofthebraidedplaques (thebraidedplaquescontainacom- plex internal microstructure ), it is prudent to understand the various fracturemechanisms andfractureproperties of thetows themselves. Fundamental issues related to mode I, mode II, and mixed mode fracture of stitched tow-reinforced composites need investigation. The mode I, mode II, and mixed mode fracture energies are fun- damental properties of a e ber-reinforced composite. Consequently, measurement of these fracture energies is necessary for properly characterizing the response and failure of structures made of these composites. In this paper we present the results of an experimental study that examined the mode I fracture of unidirectional carbon- e ber tow composites with crack propagation along the e ber tow direction. The present investigation of crack growth is limited to low ve- locity impact (LVI) conditions. Under these conditions the dy- namic stress e eld produced by the impact loading subsides, and this transient e eld occurs at the very early stages of loading. In the present experiments the maximum impactor velocity is 4.6 m/s, re- sulting in maximum crack-propagation velocities on the order of 350 m/s. These velocities are a small fraction of the shear wave speed (1540 m/s) and Rayleigh wave speed of the material. Con- sequently, dynamic effects can be neglected. Researchers 7i11 have conducted an extensive experimental and numerical investigation of dynamic crack propagation in unidirectional continuous e ber (prepreg)-laminated composites. The present study examines con- tinuous e ber (e ber tows) unidirectional composites under LVI con- ditions.