FREQUENCY EFFECT ON FATIGUE CRACK PROPAGATION IN PMMA PANELS
Abstract
The effect of loading frequency on fatigue crack propagation has been found to be quite significant in some alloys and high polymers. A significant decrease in fatigue crack propagation rate with increasing cyclic frequency has been observed. The equation which has been used to describe this phenomenon has the same expression as Paris' power law, in which some parameters are frequency dependent. The determination of these parameters in such power law needs a large number of tests conducted under different loading frequencies. This research is an attempt to find a new fatigue crack propagation law for viscoelastic media under constant amplitude tension-tension cyclic loads. The strain intensity factor is used in developing the new propagation law in lieu of the widely used stress intensity factor. The magnitude of the strain intensity factor is independent of frequency in elastic materials but is a frequency dependent parameter in viscoelastic media. By employing the strain intensity ratio and strain intensity factor, a new fatigue crack propagation law is proposed without the explicit use of loading frequency as a variable. The effect of loading frequency on fatigue crack propagation is through its effect on the strain intensity which is one of the main variables in the crack growth law. The complex compliance and creep compliance of the viscoelastic material are measured first and then used to compute the strain intensity factor range and strain intensity ratio for propagating fatigue crack. If the proposed law is valid, then a master da/dN curve can be established for all loading frequencies, and, as a result, the fatigue test needs only to be conducted under a specific frequency. The experimental work was carried out with cast acrylic panels in order to evalute the new propagation law. A new device called Krak-Gage was employed in the measurement of the crack length. The advantages and disadvantages are discussed. Two test programs were conducted, namely the constant stress ratio test and the constant strain intensity ratio test. Both were conducted under tension-tension sinusoidal loading with three frequencies, 0.1 Hz, 1.0 Hz and 10 Hz. The results for the crack growth rate are plotted against the stress intensity factor range and a modified strain intensity factor range. For the constant strain intensity ratio test, the crack growth rate can be expressed as a single equation in terms of the modified strain intensity factor for all loading frequencies considered.
Degree
Ph.D.
Subject Area
Aerospace materials
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