A Novel Ultrasonic Technique for the Detection of Distributed Precursor Damages in Composites

Abstract

Detection of the initial stage of the distributed damage which we call the damage precursor in the composites namely matrix micro cracking and fiber breakage, are extremely important for diagnostic and prognostic of the material. However, existing nondestructive evaluation (NDE) and/or structural health monitoring (SHM) system are not efficient to detect such damages and often demand transformative approach over the existing techniques. Here, we present a novel ultrasonic method for identification of the distribution of the probable damage sites as an indication of the percentage of degradation of the material properties on a typical representative volume element (RVE). An American Society of Testing and Materials (ASTM) standard unidirectional (UD) carbon-fiber-reinforced-plastic (CFRP) specimen was tested under 250,000 cycles mechanical loading and 500 cycles thermal loading. Scanning Acoustic Microcopy (SAM) was used to perform Z-scan of the specimen on a particular area (6mm×6mm) of the specimen. Surface skimming ultrasonic wave velocity (SAW) profile on a representative volume element (RVE) was calculated before and after loading the specimen. A 2D map of decreased SAW regions were calculated after the loading. Statistical analysis of the SAW profile was performed to study the distributed damage evolution in the material. It is found that the damaged regions often coalesce as loading cycle increases and such sites can be predicted from the SAW profile. In addition, we also presented a nonlocal mechanics based ultrasonic technique to study damage evolution with thermomechanical loading. Nonlocal parameters were also calculated on the selected RVE by using Quasi-longitudinal wave velocity along the thickness direction. Using micro-morphic wave dispersion curve, for a selected frequency at 50 MHz. Damage Entropy (DE) was calculated from the distribution of the nonlocal parameter. We also performed optical microcopy (OM) of the selected area to investigate the development of the damages and to validate the SAW results. This work has potential to estimate the remaining useful life of the structure and provide useful information about the distributed damages states, which in turn will help Condition Based Maintenance (CBM+) of the critical locations of the structure while maintaining the aircrafts and the defense equipment.