Title

Oblique angle pulse-echo ultrasound characterization of barely visible impact damage in polymer matrix composites

Date of Award

1-1-2019

Degree Name

Ph.D. in Materials Engineering

Department

Department of Chemical, Materials and Bioengineering

Advisor/Chair

Advisor: Amanda Criner

Second Advisor

Advisor: Donald Klosterman

Abstract

The motivation for this effort is the desire of the United States Air Force (USAF) to implement slow crack growth damage tolerance lifing (DTL) for its polymer matrix composite (PMC) components. Implementing DTL for PMCs would allow the USAF to life manage its PMC components in a similar manner to its metal components. To realize this goal, the USAF must be able to characterize damage in PMCs with a fidelity that is sufficient to generate a reliable prediction of remaining life or strength with existing damage propagation models.To that end, a statistical meta-analysis of barely visible impact damage (BVID) in composites is conducted using data from the literature to understand the scope of the BVID problem. The analysis performed shows that delamination morphologies that have hidden delaminations (delaminations that are not detectable with normal incidence ultrasonic nondestructive evaluation (NDE), the current sustainment characterization tool) are likely to occur in 1/3 of barely visible impacts. Characterization of hidden delaminations is therefore, a significant challenge to implementing DTL. Statistical meta-analysis of the BVID data from the literature shows that most (92%, 95% CI (0.81, 1.03), n = 25) of the time a cone shaped BVID field results when the PMC has 90░ plies at the center of the laminate and the cross-section view of the BVID morphology was taken along the center plies. This delamination morphology information can be used a priori to inform the NDE inspection direction and optimize the sensitivity of the inspection to defects of interest.In order to detect and characterize hidden delaminations with a single sided, fieldable NDE inspection (USAF constraints on BVID characterization) the technique of oblique angle pulse-echo ultrasound is investigated. An understanding of oblique angle ultrasound wave propagation, reflection and diffraction are presented. Model results suggest oblique angle pulse-echo ultrasound is sensitive to hidden delaminations. However, experimental results do not. Furthermore, unobstructed delamination edges show much larger return signals in models than compared to corresponding experiments. The weak to non-existent delamination edge direct return signal illustrates how difficult the detection of hidden delaminations with oblique angle pulse-echo ultrasound will be. The signal from a hidden delamination is expected to be of much lower amplitude than the direct reflected signal from an unobstructed delamination edge, based on model results, further increasing the difficulty of the inspection. As a result, oblique angle pulse-echo ultrasound is not seen as a viable technique to detect and characterize hidden delaminations at the present time.Lastly, the literature, as well as oblique angle pulse-echo ultrasound measurements conducted in this effort, suggests oblique angle pulse-echo ultrasound scans conducted as a function of specimen rotation (azimuthally scanned oblique angle pulse-echo ultrasound) is sensitive to preferentially oriented defects such as transverse matrix cracks. While delaminations are the primary defect input for DTL models, transverse cracks are also of interest to DTL model developers and DTL model users. A proof of concept is performed demonstrating the ability of azimuthally scanned oblique angle pulse-echo ultrasound to detect and locate preferentially oriented matrix cracks. It is envisioned that coupling normal incidence ultrasound inspection results with azimuthally scanned oblique angle pulse-echo ultrasound inspection results with data fusion schemes could generate a combined damage map that would provide much better characterization capability of BVID (both unobstructed delaminations and transverse cracks) than is currently available with current field inspection tools (unobstructed delaminations only). With additional work, it is believed that the characterization provided by this combined approach would be of high enough fidelity to enable remaining strength or remaining life estimates of sufficient quality to realize DTL for PMCs.

Keywords

Engineering, Materials Science, Aerospace Materials, Aerospace Engineering, nondestructive evaluation, NDE, nondestructive inspection, NDI, nondestructive testing, NDT, barely visible impact damage, BVID, polymer matrix composites, PMC, ultrasound, statistical meta-analysis, delamination, carbon fiber reinforced polymer, CFRP

Rights Statement

Copyright 2019, author

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