Tensile, Creep, and Fatigue Behaviors of Thermoplastics Including Thickness, Mold Flow Direction, Mean Stress, Temperature, and Loading Rate Effects

The overall objective of this study was to conduct a systematic and comprehensive experimental investigation to evaluate and quantify the tensile, creep, and fatigue behaviors of two thermoplastic polymers. The effects considered in this experimental study include mold flow direction, thickness, mean stress, temperature, and loading rate. Specimens were subjected to monotonic tension, creep, and load-controlled fatigue tests. The two materials considered were a neat impact polypropylene and a mineral and elastomer reinforced polyolefin. A unique test specimen geometry was developed for this study along with a procedure for fatigue testing. Tensile tests were performed at various strain rates at room, -40¿¿C, and 85¿¿C temperatures with specimens cut parallel and perpendicular to the mold flow direction. Tensile properties including elastic modulus, ultimate tensile strength, yield strength, and strain at the ultimate tensile strength were determined from these tests. The Ramberg-Osgood model for metals was used to model the tensile deformation behavior, in addition to other models specific to polymeric materials. Creep tests were conducted at room temperature, 85¿¿C, and 125¿¿C at various stress levels, and data from these tests were used to model the creepdeformation behavior by a power-law model for both materials. The effect of creep damage in fatigue tests with mean stress was also investigated with a linear damage model. Fatigue tests were performed with test specimens of two different thicknesses and cutting directions with respect to the mold flow. These tests were performed at room temperature, -40¿¿C and 85¿¿C in tension-compression and tension-tension. Several frequencies were used for the same stress levels to examine the effect of frequency on fatigue life. Prediction of stress-life fatigue properties from tensile properties was also examined and compared to existing predictions from tensile properties for steels. The effect of mean stress was modeled using various parameters developed for metals and with models specific to polymeric materials obtained from a literature review. In terms of effectiveness, the Walker equation and a simple model with a mean stress sensitivity factor (¿¿) proved to be the most effective. Incremental step cyclic deformation tests were performed to generate cyclic stress-strain curves to enable the determination of strain-life fatigue properties from stress-controlled tests. The effect of self-heating under cyclic loading was modeled with tests that measured the specimen surface temperature as a function of the applied stress amplitude and cycling frequency.

Name thesis:degree_name

Master of Science in Mechanical Engineering

Level thesis:degree_level

masters

Discipline thesis:degree_discipline

College of Engineering

Grantor dc:publisher

University of Toledo

Year dc:date

2012

Author dc:creator

• Mellott, Stephen Richard

Contributors dc:contributor

• Fatemi, Dr. Ali

• Materials Science
• tensile
• creep
• fatigue
• thermoplastic

Statement dc:rights

• unrestricted
• This thesis or dissertation is protected by copyright: all rights reserved. It may not be copied or redistributed beyond the terms of applicable copyright laws.

Language dc:language

English

Repository record dc:identifier

http://rave.ohiolink.edu/etdc/view?acc_num=toledo1344541288

OAI identifier oai:identifier

oai:etd.ohiolink.edu:toledo1344541288