All ETDs from UAB

Advisory Committee Chair

Gregg M Janowski

Advisory Committee Members

R Michael Banish

Derrick R Dean

Nasim Uddin

Uday K Vaidya

Document Type


Date of Award


Degree Name by School

Doctor of Philosophy (PhD) School of Engineering


Long fiber thermoplastic (LFT) composites are used primarily in various automotive and military applications. LFT mechanical properties are dictated by the polymer matrix and fiber reinforcements, and are affected by processing. As LFTs deform under moderate loads and moderate temperatures, flexural creep raises concerns about longevity. The present study is an analysis of creep as a function of constituents and variables, and the development of predictive estimation methods. Compressionmolded high-density polyethylene, nylon 6/6, and polypropylene and their 10 and 40 wt. % LFT composites were studied. The LFTs showed non-linear viscoelasticity, quantitatively described using an empirical model formerly used for short fiber thermoplastic composites. The greatest creep resistance was observed in nylon 6/6 LFTs, followed by polypropylene and highdensity polyethylene. Long-term creep resistance estimated by time−temperature−stress superpositioning was consistent with the corresponding static flexural strength, decreasing with increasing temperature. The short-term and long-term creep resistance for neat polymer and its 10 wt. % and 40 wt. % LFTs increased with increasing fiber volume fraction. Irrespective of initial fiber length, the fiber length distribution in the extrusion/compression molded parts highlighted the reduction in fiber length by about 30% during extrusion and compression iv molding. Creep resistance increased with longitudinally aligned fibers and was favored by a near-random alignment. The TTSSP data was validated with long-term (~ 168 h) tests, and creep strain rates were analyzed at various applied stresses with constant temperatures to determine excess stress using metal matrix composite theories. With some exceptions, the excess stress was found to be a fraction of the applied stress, and was strongly dependent on the load-transfer ratio and weakly dependent on temperature. Moisture absorption suggested reversible compositional changes in nylon 6/6 LFTs, as it showed a creep resistance comparable to unexposed material. Ultraviolet radiation caused yellowing and surface cracking in polypropylene LFTs at short exposure time (~ 400 h) and in nylon 6/6 LFTs with prolonged exposure (~ 3600 h). The creep resistance of the polypropylene LFTs showed a decrease in creep resistance with an increase in ultraviolet exposure time, whereas the nylon 6/6 LFTs showed an incubation period where a slight increase in creep resistance with ultraviolet exposure was observed.

Included in

Engineering Commons



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