Reconstruction of fatigue crack growth in aluminum lithium and aluminum copper fastened lap joints under marker band loading

The application of polyvinylidene di-fluoride (PVDF) film as an acoustic emission (AE) sensor appears to be practical to detect the fatigue crack growth in composite materials and structures. A commercially available PVDF film sensor was employed to detect AE due to fatigue crack growth in the single-lap joints of graphite/epoxy laminates. Although the signal-to-noise ratio is not as high as those measured by PZT sensors, the result showed that the location of crack front could be predicted by the linear location of AE signals detected by the PVDF sensors. Since the composite materials usually produce very energetic AE signals, the extremely cost-effective form of PVDF sensors can be permanently mounted on composite structures for structural health monitoring as disposable ones. Piezoelectric polymer sensors are expected to be eventually embedded in composite structures provided the current limit of use temperature being increased by introducing co-polymerization with some heat resistant constituents.

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A Modified Compliance Method for Automatically Measuring Fatigue Crack Growth In Situ during Spectrum Fatigue Testing

Advanced Materials Research ◽

10.4028/www.scientific.net/amr.891-892.732 ◽

2014 ◽

Vol 891-892 ◽

pp. 732-738

Author(s):

Wyman Zhuang ◽

Qian Chu Liu

Keyword(s):

Fatigue Crack ◽

Fatigue Crack Growth ◽

Crack Growth ◽

Operating Conditions ◽

Loading Conditions ◽

Variable Amplitude Loading ◽

Variable Amplitude ◽

Marker Band ◽

Spectrum Loading

The fatigue critical structures of military aircraft are generally subjected to variable amplitude flight spectrum loading. Maintaining aircraft structural integrity to ensure safe operation of the fleet is critically dependent on accurate analysis and reliable prediction of fatigue crack growth in those structures under service operating conditions. To achieve this goal, laboratory experimental methods that can accurately measure and monitor fatigue crack growth under variable amplitude loading are required. This can be challenging as no test standard exists to guide the process of fatigue crack growth measurement under variable amplitude loading conditions to ensure the accuracy of the test results. This challenge was addressed by developing a modified compliance method as described in this paper. The results presented employed a modified compliance method complemented with a travelling microscope technique and marker band loads. The modified compliance method developed is able to measure in-situ, fatigue crack growth of standard compact-tension specimens under a fighter flight spectrum loading. The marker band loads and microscope readings were used to assist the post-test validation using quantitative fractography. The results from this study have demonstrated that the modified compliance method can produce consistent and accurate fatigue crack growth data under variable amplitude loading conditions.

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