Abstract
A novel methodology is introduced for quantifying the severity of damage created during testing in composite components. The method uses digital image correlation combined with image processing techniques to monitor the rate at which the strain field changes during mechanical tests. The methodology is demonstrated using two distinct experimental datasets, a ceramic matrix composite specimen loaded in tension at high temperature and nine polymer matrix composite specimens containing fibre-waviness defects loaded in bending. The changes in the strain field owing to damage creation are shown to be a more effective indicator that the specimen has reached its proportional limit than using load-extension diagrams. The technique also introduces a new approach to using experimental data for creating maps indicating the spatio-temporal distribution of damage in a component. These maps indicate where damage occurs in a component, and provide information about its morphology and its time of occurrence. This presentation format is both easier and faster to interpret than the raw data which, for some tests, can consist of tens of thousands of images. This methodology has the potential to reduce the time taken to interpret large material test datasets while increasing the amount of knowledge that can be extracted from each test.
Highlights
Composite materials have complicated heterogeneous microstructures that typically contain many microscale defects, this results in complicated damage mechanics that are difficult royalsocietypublishing.org/journal/rsos R
Several techniques for capturing data during mechanical testing are available within two broad categories: contact methods, such as strain gauges or monitoring failure based on the propagation of sound waves; and noncontact methods, which typically use visible light-based measurements or computed tomography
Non-contact methods such as computed tomography are capable of determining the morphology of damage as it forms in specimens [4,5] but these techniques require expensive infrastructure restricting the number of specimens and the rate of testing
Summary
Composite materials have complicated heterogeneous microstructures that typically contain many microscale defects, this results in complicated damage mechanics that are difficult royalsocietypublishing.org/journal/rsos R. To monitor rapid damage events such as impact, high-speed imaging must be used, resulting in thousands of strain fields showing specimen behaviour [12] This greatly increases the amount of time required to analyse experimental data. It can be difficult to automatically identify significant events when the data contains substantial amounts of measurement noise This is because noise causes the rate of change, s_(t) to occasionally peak, but these peaks only occur for a single measurement; whereas, when damage forms, the strain field at that location is permanently changed. The threshold was determined by calculating the 99.9th percentile of the strain-difference field represented by the feature vector, f(h) − f(0) The changes in this field occurred at the start of the test and were only caused by measurement noise and elastic deformation.
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