Nondestructive evaluation of the integrity of thick composite structures
Creators: name: IBRAHIM, Matthew
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400102
subject: Aerospace structures

description: Nondestructive testing and evaluation (NDT&amp;E) is concerned with the detection, characterisation, and dimensional measurement of internal flaws in structural components. While the techniques in use today were developed largely for quality assurance in metal and alloy production, the continual drive to improve mechanical properties while reducing costs and hence weight has led to the gradual replacement of metallic alloys in high-end applications with fibre-reinforced polymer matrix composites (PMCs). Structures in which these feature prominently are: aircraft, wind turbines, petrochemical storage tanks, and even entire marine vessels. Techniques applicable to the evaluation of flaws in the thin quasi-isotropic carbon-fibre/epoxy skins used in aircraft have been well documented in the technical literature. However, the increasing use of thick PMC materials for primary load-bearing structures makes them impervious to many NDT techniques, and hampers the ability to detect and size internal defects.

This Thesis addresses key gaps in the quantitative evaluation of `thick' composites - those for which the thickness is sufficient to offer variation in mechanical properties and which must be interrogated nondestructively in order to be qualified for service. It generates new understanding in the quantitative detection, sizing and characterisation of major production and in-service defects occurring in thick composite structures. The objectives of the Thesis are:
i. to define the set of NDT techniques that can be employed in applied inspection scenarios;
ii. to determine the relationship between defect detectability and depth in thick composite laminates using acoustic methods, generating or verifying where possible simulation models that can be used to further explore thick composite inspection problems;
iii. to quantify defect sizing limitations in composite structures under load, and explain changes to the elastic wave propagation occurring when altering the stress state of a thick laminate;
iv. to assess the ability of passive acoustic methods to predict failure in thick composite structures under cyclical loads, and
v. explore the capability of non-acoustic methods to overcome acoustic shadowing and to discriminate between air- and water-based defects within a glass PMC.

The suitability of a wide range of techniques to quantitatively inspect such materials is evaluated, and the effects of thickness on defect detectability and size measurements is determined.

Significant new knowledge has been generated from this research project in a number of areas. These include:
i. A comprehensive review of nondestructive evaluation technology specifically applicable to thick composites, including a reference guide for inspectors;
ii. A model to correct for the effect of thick composites on ultrasonic beam shape, and that significantly improves the ability to detect and size small defects at depth, as demonstrated against the best published sizing data in thick PMC laminates;
iii. Novel scientific analysis of the effect of hybridised carbon- and glass-fibre composite laminates on ultrasound propagation and defect detectability;
iv. The first demonstration of the ultrasonic crack closure effect in composite materials undergoing compressive loads;
v. The demonstration and comprehensive analysis of a material attenuation effect in glass-fibre composites undergoing bending loads;
vi. The first application of acoustic emission to the prediction of fatigue failure in a solid structural hybrid marine composite;
vii. The first demonstration of terahertz radiation to detect moisture ingress in solid glass-fibre composite materials.

While this Thesis identifies various avenues of inquiry requiring further investigation, it records substantial progress in the understanding of the effect of thick PMC materials on NDT energies and how these may be accounted for when inspecting such structures.