Multi-scale Study On Interfacial Performance And Toughening Of Carbon Fiber Sandwich Structures

Carbon fiber sandwich structures, with higher specific stiffness, specific strength, energy-absorbed ability, fatigue resistance and thermal functions, have been widely used in aerospace, automotive, marine and civil construction. However, there is stress concentrationat at interface zone between face sheet and core, due to the significant property mismatch between the face sheet and core. The stress concentrationat and relative weak properties of the interface lead to frequent damage and fracture at the interface during service, which could impair the structural integrity and mechanical performance of sandwich structures. Therefore, macroscopic experimental verifications of toughening and reinforcing effects of short-aramid-fiber toughening method on carbon fiber sandwich structures were firstly studied in this thesis. Then, the microscopic underlying toughening mechanisms of the short-aramid-fiber toughening method were investigated. Finally, analysis and prediction were conducted for toughening effect of the short-aramid-fiber toughening method on critical energy release rate (Gc) and mechanical behavior of sandwich beam with aramid-fiber interleaf, based on the aforementioned experimental studies. This study is part of the project of National Basic Research Program of China (973Project)"Study on multi-functional innovation construction of extra-waighted and porous materials&structures’"(No.2006CB601205,2010-2011), National Science Foundation in China (No.11102032,2012-2014) and special funding of State Key Laboratory of Structural Analysis for Industrial Equipment (S S13205,2013-2014). The specifci research topics of this study are as follows:1. Investigation on manufacturing of sandwich structures with low-density short aramid fiber toughened interfaceDebonding and fracture failures frequently occurs at the interface between face and core of carbon fiber aluminum foam sandwich during service, which could impair the properties of sandwich and lead to structural failure. Aramid fibers, which obtained high strength, high stiffness, good flexibility and low density, were selected to manufacture low-density tissue of short aramid fibers by two methods. The low-density tissue of short aramid fibers were then placed between the carbon fiber face sheet and the aluminum foam core during the manufacturing process of sandwich beams, to form a interface toughened by short aramid fibers, and therefore to improved the interfacial properties and structural properties. 2. Experimental study on mechanical peoperties and behavior of sandwich structures with short aramid fiber tougheningThe toughening and reinforcing effects of short-aramid-fiber toughening method on interfacial and structural properties of carbon fiber sandwich were macroscopic experimentally verified and studied by three-point bending, in-plane compression and asymmetric double cantilever beam (ADCB) tests. The results of three-point bending and in-plane compression test indicated that the short-aramid-fiber interleaf could effectively reduce the debonding area at interface under loading, keep structural integrity and thus increase the peak load, critical displacement and energy absorption ability of sandwich structures. ADCB test showed that the Gc of the face/core interface was significantly improved by short aramid fibers. Parameter studies demonstrated that the Gc of the toughened face/core interface could be further improved by reducing the length of aramid fibers or increasing the surface roughness of core.3. Microscopic behavior and toughening mechanism of short-aramid-fiber toughened interfaceMicroscopic behavior and toughening mechanism of short-aramid-fiber toughened interface were observed by scanning electron microscopy (SEM), photography and optical microscopy to explain the toughening effect of short aramid fibers on interface and entire structure. SEM observation and photography indicated that the short aramid fibers firstly formed micro "bridge" structure, which improved the critical load for crack extension, between face and core after interfacial debonding. The micro aramid-fiber "bridge" could also increase the Gc by absorbing energy through pulling-out and tensile failure of aramid fibers. In addition, it is evidenced that the short-aramid-fiber toughened interface formed "fillet" reinforcement between foam core and face sheet, based on fractography of sandwich samples. Such "fillet" reinforcement increased contect area between core and face sheet, and enhanced bonding properties of interface. These observations explained the toughening effect and mechanism of short aramid fibers4. Prediction of Gc of short-aramid-fiber toughened interface and analysi of mechanical behavior of sandwich structure with aramid-fiber interleafThe short-aramid-fiber toughening method provided effective reinforcing effect on face/core interface of sandwich structures with promising prospects. To analysis the toughening effect of short-aramid-fiber toughened interface and to predict the Gc, a microscopic analytical model was proposed based on the peeling-off, pulling-out and tensile failure behavior of aramid fibers during crack extension, and considering the competition effect and random distribution of aramid fibers. The accuracy of the microscopic analytical model was verified by experimental Gc measurements of interface between carbon fiber face sheet and aluminum substrate. Moreover, an extended Finite Element Model (XFEM), which considered the complex stress field at the crack tip of interface, was proposed for analysis of interfacial fracture of sandwich structures.