Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Mechanical Engineering

Committee Chair/Advisor

Dr. Gang Li

Committee Member

Dr. Srikanth Pilla

Committee Member

Dr. Huijuan Zhao

Committee Member

Dr. Paul Joseph

Committee Member

Dr. Hongseok Choi


The ever-growing pressure of reducing the adverse impact of transportation systems on environment has pushed industries towards fuel-efficient and sustainable solutions. While several approaches have been used to improve fuel efficiency, the light-weighting of structural components has proven broadly effective. In this regard, reinforced thermoplastic composites (RTPC), owing to their high recyclability, higher impact strength and fast cycle times, have become competitive candidates at an industrial scale. However, to implement RTPC toward large scale structural applications several challenges pertaining to material design and manufacturing effects need to be addressed. To this end, a computational study is carried out to address three key challenges that limit the design and development of RTPC structures: (a) enhancing filler/matrix bonding strength at the microscale level, (b) understanding the effect of interfacial microstructure on the macroscale material behavior and (c) understanding the effect of manufacturing process on the mechanical performance at the structural level.

First, the interface bonding strength between the non-polar/non-reactive polymer matrix and filler surface is generally poor and hence, may fail locally at these interfaces upon external loading. Many interfacial strengthening studies in the past lack precision and control in obtaining higher interfacial strength. The concept of controlled mechanical interlocking between fiber and matrix interface is explored computationally on an E-glass/polypropylene (PP) composite material system. To understand the micromechanics under dynamic and impact loads, the interfacial behavior at various strain rates is studied. A finite element parametric model is setup where different surface morphologies of E-glass/PP are investigated. The strength calculations consider both material failure and detachment of the matrix material from the anchoring sites. Second, to incorporate the effect of interfacial microstructure into macroscale material behavior, a continuum constitutive model is developed. The bulk material response at macroscale is evaluated by using a unit cell homogenization method. The mechanical behavior of the composite material at macroscale is modeled using a rheological three network viscoplastic (TNV) model. Numerical biaxial and shear tests are conducted for several geometric configurations of the fiber/matrix interface. The numerical data generated is used further to develop the TNV material model. The developed material model is validated for a laminated beam structure. The study elucidates the impact of various interface design variables on the material model parameters by establishing analytical relationships.

Third, to investigate the effect of manufacturing process on the mechanical performance a manufacturing to response (MTR) pathway is established. This pathway was developed collabora-tively and consists of both computational methods to simulate and integrate all relevant manufac-turing process steps as well as experimental methods to validate each step from coupon to structural level. The composite material system selected for this study is AS4/Nylon-6 (PA6) with woven layup. The thermoforming process simulations are carried out using anistropic hyperelastic material model and the thickness variation, fiber orientations and residual stresses are captured from the analysis. Residual stresses developed in the formed structure during quench cooling from the elevated temperature are predicted by implementation of classical laminate theory (CLT). The static and dynamic performance for the thermoformed structure is evaluated and the effects of thermo-forming process are compared numerically, for the cases with and without inclusion of process effects.

Lastly, the MTR pathway is implemented to perform a design optimization study of an ultra-lightweight fiber RTPC door of a vehicle. The objective of the optimization was to reduce the weight of an existing door of an OEM’s mid-size SUV by 42.5% while also satisfying various static and dynamic structural load requirements. Several static and dynamic studies are carried out at structural length scale. The door stiffness optimization is first carried out for various linear static load cases. This is followed by evaluation of crashworthiness performance under three non-linear load cases: (a) quasi-static pole test (FMVSS 214S) (b) full pole test (FMVSS 214) and (c) moving deformable barrier test (IIHS SI MDB).

Author ORCID Identifier


Available for download on Wednesday, November 30, 2022