Twintex®, a relatively new thermoplastic composite material system made of comingled polypropylene (PP) and glass fibers, has shown superior toughness in comparison with traditional glass-fiber epoxy composites. Along this improvement, this thesis aims at enhancing impact characterization of Twintex composites and developing a systematic approach for weave pattern selection and lay-up optimization of their laminates under impact. The research was conducted in experimental and numerical parts with a case study on potential application of Twintex in a highway guardrail.

In the first part, a set of 180 drop-weight-impact and four-point-flexural experiments were performed for mechanical characterization of PP/glass laminates with different fiber architectures (balanced plain, balanced will, unbalanced plain, and unidirectional tape) before and after impact. Another set of the experiments was designed based on a Taguchi design of experiment (DOE) method and showed that this method has a good accuracy in predicting impact response of hybrid fiber reinforced plastic (FRP) composites such as Twintex. The X-ray microtomography and visual inspection techniques were also employed to investigate the interior and exterior damages induced to the specimens due to impact. These nondestructive evaluation techniques revealed that the impact damage mechanisms are highly dependent on the selected architecture of fibers. Also it is shown that, contrary to some previously published reports, impact resistance of FRP composites cannot be evaluated solely based on the extent of visible damages, and that inner damage along with associated damage modes must also be taken into account. Next, a case study was conducted to extract the criteria that may be used by engineers to assess impact-resistance of different Twintex FRP laminates for a potential guardrail application. Namely, a decision matrix with nine criteria was formed and three weighting techniques were developed under a multicriteria decision making environment to select the most appropriate fabric weave pattern for this specific application. The proposed multi-criteria approach is general and can assist designers to select optimum composite materials in other similar applications. The last part of this thesis deals with a simplified approach for numerical characterization (finite element analysis) of Twintex laminates under impact without a need for additional subroutine codes.