Effect of stress intensity factor on aging of short glass fiber reinforced thermoplastics

Short fiber reinforced thermoplastics use in different engineering applications is on the rise due to their ease of manufacturing, as they can be injected molded into complex shapes needed in industrial machines. It is important to know the mechanical behavior as well as aging mechanisms of these components, as a degradation of their properties can cause severe damage to the structures they form part of. This thesis studies aging due to differences in the stress intensity factor of three short fiber reinforced thermoplastics, polypropylene and polyamide-12, with a fiber reinforcement of 50%, and polyetheretherketone with a fiber reinforcement of 14%. The materials were processed by injection molding and specimens were tested considering the effect of the fiber orientation. Creep tests have been conducted in Single Edged Notched Tension (SENT) specimens at different stress intensity factors to study their static fracture mechanical behaviour, with failure times of up to 2*106 s. Their infrared spectra have been studied to determine the type and level of aging they suffer along the crack path. Additionally, fatigue tests were performed in polyamide and polypropylene single edge notched (SENT) specimens and compared to Compact Tension (CT) specimens of the same material and fiber content, but these results are not fully conclusive. Results for the creep tests indicate that all three materials are more sensitive in the fiber direction than in the perpendicular one, most probably due to poor adhesion between fiber and matrix. Infrared spectra show water absorption, carbonyl intake and crystallization as factors of induced aging in the specimens. However, these last results should be interpreted with caution due to different aspects in the material which cannot be captured with the used measurement configuration.