Failure sequence determination in sandwich structures using concurrent acoustic emission monitoring and postmortem thermography

This study provides a novel approach for damage classification and failure sequence evaluation in sandwich panel composite materials under flexural loading condition merely by using acoustic emission monitoring in interrupted mechanical tests and postmortem lock in thermography analysis. The studied sandwich panels consist of glass fiber reinforced phenolic skins and Nomex honeycomb core which are used extensively in the aerospace industry. While one of the standard flexural samples is mechanically tested up to global failure, the other specimens are loaded till a certain load level and the tests are interrupted. Acoustic emission hits are recorded throughout the mechanical tests by using piezoelectric sensors, and are classified using a well-established clustering algorithm. The damaged samples are then investigated via lock in thermography technique to identify the hidden failure progress inside the sandwich structure. Acoustic emission monitoring is successfully used to mark the change of mechanical response under flexural loading condition. The results of acoustic emission analysis indicated four different clusters associated with four major failure modes occurring due to mechanical loading. Correlating failure progress observed in thermography results with the fraction of acoustic emission clusters registered during mechanical tests helped to attribute each class of acoustic emission hits to a specific failure type. Upon using the interrupted test methodology together with the lock-in thermography technique, one can reliably identify the damage types and their sequence of manifestation during flexural tests. The results show that successful analysis of visually hidden failure progress in sandwich panel composites under out of plane loading condition can be attained by using structural health monitoring techniques and quasi static tests.