High cycle compressive fatigue of unidirectional glass/polyester performed at high frequency

This thesis is part of current materials research aimed at studying the behavior of polymer matrix composites at high cycles. The need stems from wind turbines which experience roughly 10^8 to 10^9 fatigue cycles (application and relaxation of load) during their 20 to 30 year lifetimes. Conventional fatigue testing is limited to 20 Hz (cycles per second) because of hysteresis heating problems caused by poor heat transfer capabilities of polymer matrix composites. However, at 20 Hz it would take 58 days to almost 1.5 years to reach the desired 10^8-10^9 cycle range. Previous work by Creed [3] demonstrated that high frequency (75 to 100 Hz) tensile fatigue testing can be accomplished using a specimen that is sufficiently thin that hysteretic heating is no longer a problem. The purposes of this research were to develop a high frequency compressive fatigue test method and obtain base line fatigue data for unidirectional glass/ polyester composites at R values (minimum stress/ maximum stress) of ten and two. Two-ply specimens were cut from composite plates manufactured by resin transfer molding (RTM). Specimen geometry measured 2.2 inches long by 0.25 inches wide by approximately 0.035 inches thick. Fiber volume fraction was found to vary between the composite plate and individual specimens depending on the number of fiber bundles present in an individual specimen; however, no significant effects. of fiber content were observed in the S/N (stress versus number of cycles to failure) curves. Specimen porosity ranged from 2% to 4%. S/N data and specimen failure modes for R=10 were in good agreement with literature data and with data from standard sized coupons manufactured from the same material as the high frequency specimens but tested at lower frequencies (10-20 Hz). The data approximately followed an 8% per decade decay line. For R=2 the data approximately followed a least squares 5.5% per decade decay line. Above 10^5 cycles the S/N curves for R=10 and R=2 flattened, and least square curve fits of these points (including runouts) showed that projected failure stresses at 10^9 cycles rose for both curves compared to curve fits using the entire data set. Frequency effects were examined using high frequency specimens tested at various frequencies but the same stress level. Data at 30 and 50 Hz correlated well, but some specimens at 10 Hz had longer lifetimes. HIGH CYCLE COMPRESSIVE FATIGUE OF UNIDIRECTIONAL GLASS/POLYESTER PERFORMED AT HIGH FREQUENCY by Andrew Jay Belinky A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Chemical Engineering MONTANA STATE UNIVERSITY Bozeman, Montana