TY - THES

T1 - Characterization of short fiber reinforced polymers in the very high cycle fatigue regime

AU - Puregger, Franz

N1 - no embargo

PY - 2024

Y1 - 2024

N2 - The present work deals with the development of a test setup for investigating the fatigue behavior of short fiber-reinforced thermoplastics in the VHCF range (> 107 cycles). The main objective was to reduce the test duration for high cycle numbers by increasing the frequency and to assess the feasibility of high-frequency testing on the examined materials. A test concept involving the application of test force by an electrodynamic shaker was developed based on a test setup for metals. The mean stress is 0 MPa, and the stress ratio R = -1. The testing involves alternating tensile/compressive loading. The test concept utilizes the resonance principle. Three versions of the test setup were designed and implemented based on this concept, with each new version representing an improvement. Each version was systematically examined using various sensors and methods to identify deviations from the test concept. The final setup closely adheres to the test concept and allows testing at 230 Hz. With this testing frequency, 107 cycles can be achieved in half a day, while conventional testing methods require 115 days for the same test. In an initial test series, carbon- and glass-fiber-reinforced PEEK with 30% fiber volume content was investigated. To rule out unacceptable temperature increases due to hysteretic heating, the temperature was monitored during the test. The temperature increase was not critical across all tests. The generated S/N curves show a clear increase in fatigue strength with decreasing stress amplitude, as observed in conventional S/N tests for this material. This confirms the fundamental testability of this material at high frequencies. However, the position and slope of the S/N curves do not match conventional tests, requiring further investigation to explain this discrepancy.

AB - The present work deals with the development of a test setup for investigating the fatigue behavior of short fiber-reinforced thermoplastics in the VHCF range (> 107 cycles). The main objective was to reduce the test duration for high cycle numbers by increasing the frequency and to assess the feasibility of high-frequency testing on the examined materials. A test concept involving the application of test force by an electrodynamic shaker was developed based on a test setup for metals. The mean stress is 0 MPa, and the stress ratio R = -1. The testing involves alternating tensile/compressive loading. The test concept utilizes the resonance principle. Three versions of the test setup were designed and implemented based on this concept, with each new version representing an improvement. Each version was systematically examined using various sensors and methods to identify deviations from the test concept. The final setup closely adheres to the test concept and allows testing at 230 Hz. With this testing frequency, 107 cycles can be achieved in half a day, while conventional testing methods require 115 days for the same test. In an initial test series, carbon- and glass-fiber-reinforced PEEK with 30% fiber volume content was investigated. To rule out unacceptable temperature increases due to hysteretic heating, the temperature was monitored during the test. The temperature increase was not critical across all tests. The generated S/N curves show a clear increase in fatigue strength with decreasing stress amplitude, as observed in conventional S/N tests for this material. This confirms the fundamental testability of this material at high frequencies. However, the position and slope of the S/N curves do not match conventional tests, requiring further investigation to explain this discrepancy.

KW - Ermüdung

KW - Wöhlerlinie

KW - VHCF

KW - kurzfaserverstärkte Thermoplaste

KW - Shaker

KW - Fatigue

KW - SN-curve

KW - VHCF

KW - short fiber reinforced thermoplasitcs

KW - Shaker

M3 - Master's Thesis

ER -