Roughness-induced crack closure in the threshold region for high strength aircraft structural alloys

Walker, K 2015, Roughness-induced crack closure in the threshold region for high strength aircraft structural alloys, Doctor of Philosophy (PhD), Aerospace, Mechanical and Manufacturing Engineering, RMIT University.

Document type: Thesis
Collection: Theses

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Title Roughness-induced crack closure in the threshold region for high strength aircraft structural alloys
Author(s) Walker, K
Year 2015
Abstract Understanding the significance of crack closure is essential to analyse the behaviour of fatigue cracks in high-strength aircraft metallic alloy materials and structures. Closure is caused by a number of mechanisms including plasticity, roughness and debris build-up. Plastic deformations associated with the crack-tip singularity are the most dominant mechanism. But in certain regimes, such as near the threshold for crack growth, and for some materials, such as β-annealed Ti-6Al-4V ELI and 7050-T7451, other mechanisms such as roughness induced closure become important. The objective of the work presented here was to better understand the contribution of the roughness mechanism for a range of materials, and to develop suitable analytical methods to correlate the behaviour. Fatigue crack growth experiments were conducted for a range of materials under constant amplitude and spectrum loading, and closure levels were measured and/or inferred. Experimental data from the literature were also used where appropriate. Extreme cases of roughness were investigated, from 7249-T76511 which exhibited very smooth and flat crack surface morphology, through to β-annealed Ti-6Al-4V ELI which exhibited very rough and tortuous crack surfaces. Other materials in between those extremes included 7075-T6 and 7050-T7451.

Compression pre-cracking methods were used extensively to ensure that threshold region crack growth rate data would be obtained as accurately as possible, and several methods for performing the load reduction to threshold were also investigated. Compliance based crack closure measurements were also used extensively, both to assist in characterising the baseline crack growth rate characteristics, and to evaluate roughness closure effects and facilitate model development and validation. Optical interferometry methods were used to characterise and quantify crack surface characteristics for the materials considered. Quantitative fractography results were also used to infer crack closure levels in some cases.

An existing geometric asperity based model for roughness-induced crack closure under constant amplitude conditions was evaluated and extended.

The key finding from the work was that roughness induced crack closure effects can be significant under constant amplitude loading, but they are significantly less important during spectrum loading. Previously, very little attention has been directed to roughness-induced crack closure during spectrum loading conditions.

The final result from this work is a new approach using a combination of experimental techniques and analytical models to correlate combined roughness and plasticity induced closure effects for a range of high-strength aircraft alloys and structures. This is essential for maximising aircraft performance and safety while also minimising maintenance costs.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Aerospace, Mechanical and Manufacturing Engineering
Keyword(s) Fatigue
Fatigue crack growth
Crack closure
roughness induced crack closure
aluminium alloy
stress intensity factor
spectrum loading
constant amplitude loading
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Created: Fri, 04 Sep 2015, 10:39:04 EST by Denise Paciocco
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