The development of lightweight automotive parts is an important issue for improving the efficiency of vehicles. Polymer composites have been widely applied to reduce weight and improve mechanical properties by mixing polymers with carbon fibers, glass fibers, and carbon nanotubes. Polypropylene (PP) has been added to carbon fiber-reinforced nylon-6 (CF/PA6) composite to achieve further weight reduction and water resistance. However, the mechanical properties were reduced by the addition of PP. In this research, multiwalled carbon nanotubes (CNTs) were added to compensate for the reduced mechanical properties experienced when adding PP. Tensile testing and bending tests were carried out to evaluate the mechanical properties. A small amount of CNTs improved the mechanical properties of carbon fiber-reinforced PA6/PP composites. For example, the density of CF/PA6 was reduced from 1.214 to 1.131 g/cm³ (6.8%) by adding 30 wt % PP, and the tensile strength of 30 wt % PP composite was improved from 168 to 173 MPa (3.0%) by adding 0.5 wt % CNTs with small increase of density (1.135 g/cm³). The developed composite will be widely used for lightweight automotive parts with improved mechanical properties.

This study was systematically conducted to investigate the dependence of mechanical behaviors of double-lap bi-material joints on various environmental conditions, including room temperature dry (RTD), cold temperature dry (CTD), elevated temperature dry (ETD), elevated temperature wet (ETW), salty spray (SS), and thermal shock cycles (TS) conditions. Compared to the RTD condition, the shear strength of the joint under the CTD and TS conditions shows increases of approximately 50.0% and 9.09%, respectively, but lower values are obtained under ETD (18.18%), ETW (72.73%), and SS (27.27%) conditions. Failure modes of lap-joint are analyzed on fractured surfaces and confirmed by finite element simulation in which the cohesive failure mode mainly occurs with the highest distributed stresses. However, an experimental adhesive failure mode partially appears under the ETD, ETW, and SS conditions. Furthermore, as the less prevalent conditions, the dependencies of the shear strength and failure mode on the exposure time under SS and TS conditions are more thoroughly evaluated based on the scanning electron microscope (SEM) and the energy-dispersive X-ray spectroscopy (EDS) to show microstructures and chemical compositions of the fracture surface, respectively. The effects of environmental conditions, particularly under SS and TS conditions, could provide new insight into designing specific bonded joints.