Design of Pseudo-ductile Composites by Combining Fiber-hybridization and Discontinuous Reinforcements

Conventional fiber-reinforced polymer composites are either stiff but suffer from sudden brittle failure, or they are compliant but possess high ductility. Seldom conventional fiber-reinforced composites are both stiff and ductile. Fiber-hybridization is a promising way to break through the stiffness-ductility dilemma. Fiber-hybrids normally demonstrate a unique combination of stiffness and ductility in combination with low density. Moreover, with careful structural design, the hybrid composites can exhibit a pseudo-ductile failure behavior. The advantages of pseudo-ductile behavior are to avoid catastrophic failure while providing a wide safety margin before final failure and to maintain the residual load-carrying capacity. These advantages are vital for extending the application of high-performance fiber-reinforced composites, such as carbon fiber composites, which normally suffer from catastrophic failure without warning and poor residual load-bearing capacity. However, fiber-hybridization alone is not necessarily sufficient to achieve pseudo-ductility for a given material combination. Alternatively, natural composites, such as nacre and bone, also demonstrate a pseudo-ductile behavior under tensile loading, which is attributed to their sophisticated structures. The common structural features of these biological composites include discontinuous reinforcements, structural hierarchy, aligned and staggered arrangement of the reinforcements. Among these common features, the most prominent one is discontinuity of the reinforcement. This thesis aims to achieve pseudo-ductile behavior by combining fiber-hybridization and the structural features of biological composites. The resultant discontinuous fiber-hybrids should demonstrate a pseudo-ductile behavior along with a balanced mechanical performance. To achieve the goal, it is of great importance to understand the role of discontinuities in affecting the tensile behavior of interlayer hybrid composites. Therefore, discontinuities at different scales were introduced into the hybrid structure. The meso-scale, micro-scale, and nano-scale are defined as millimeter-, micrometer-, and nanometer-level, respectively. At the meso-scale, the discontinuities were introduced into the carbon layer of hybrid carbon fiber (CF)/self-reinforced polypropylene (SRPP) composites by laser cutting the carbon fibers. The cutting pattern was initially simple, such as a single large cut or multiple small cuts in a row. Different failure modes were obtained by changing the cut length as well as the number of cuts. This validated the concept that the discontinuities can be used as a way to influence the tensile behavior of hybrid composites. Later, the laser cuts were arranged in a staggered manner. Three parameters were used to define the staggered pattern and the effects of the pattern parameters on the tensile behavior of hybrid CF/SRPP composites were studied. Guidelines on how to optimize the tensile performance using the staggered discontinuities were proposed. An analytical model was also developed to predict the stress-strain response of the hybrid CF/SRPP composites with staggered discontinuities. Finally, more complex cutting patterns, such as hierarchical and polygonal patterns, were investigated. The optimal tensile behavior was achieved with dispersed polygonal patterns. With 4% carbon fiber volume fraction, the resultant discontinuous CF/SRPP hybrid composites exhibited a stiffness of 10 GPa and an ultimate failure strain up to around 16%. These tensile properties outperform many of the engineering plastics and random glass fiber polymer composites. More importantly, such hybrids revealed the potential for pseudo-ductile failure, which was promoted by progressive fiber bundle pull-out while avoiding the fiber bundle fracture. At the micro-scale, aligned discontinuous carbon fibers were hybridized with SRPP. Different layups of the interlayer hybrids were manufactured to study the effects of structural parameters, such as carbon layer thickness and carbon layer volume fraction, and interfacial properties. In this micro-scale discontinuities case, the damage was initiated at the fiber level, but the tensile stress-strain response was determined by the ply level failure mechanisms. With the best laminate design, the hybrid composite demonstrated a graceful deformation up to failure in combination with a stiffness of around 10 GPa and a failure strain up to 14%. At the nano-scale, the aligned carbon nanotubes (CNTs) were hybridized with thin-ply glass fiber (GF) prepregs in a layer-by-layer configuration. A novel hybrid layup method was developed to correctly test the unidirectional fiber composites by avoiding failure at the grips. Preliminary results indicated that the CNT volume fraction is critical in affecting the tensile behavior of the CNT/GF hybrid composites. Only marginal improvements in mechanical performance were found due to the low CNT volume fraction, even though long CNT pull-out was clearly observed on the fracture surface. Although the work focuses on a specific combination of materials, the concept of utilizing discontinuities and fiber-hybridization can be extended to influence the tensile behavior of interlayer hybrid composites with other fiber types. The proposed guidelines can be applied to tailor the tensile behavior of hybrid composites, hence achieving pseudo-ductile behavior. In comparison to continuous fiber-hybrids, introducing discontinuities into fiber-hybrids also enriches the failure mechanisms, which extends the design space for tailoring the tensile behavior of hybrid composites.

Publication year:2019

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