One of the unique capabilities of Diagnostic Acoustic Emission technology is identification of failure mechanisms and flaw assessment in composite structures. Main failure mechanisms of fiber reinforced plastic structures include:
- Epoxy matrix cracking.
- Fiber breakage.
- Fiber bundle fracture.
- Delamination growth.
- Intralaminar cracking.
Identification, monitoring and assessment of fracture development in fiber reinforced composites are based on two principal types of analyses: analysis of possible failure modes in the inspected structure and the analysis of acoustic emission activity itself. For example, when pultrusion structures are subjected to bending, we can expect epoxy matrix cracking due to shear stresses and then initiation and development of delaminations that later may be followed by intralaminar cracking. In this scenario, analysis of AE activity will target detection and identification of delamination initiation and growth while distinguishing it from AE related to epoxy matrix cracking.
How we can do that? Identification of delamination is based on the fact that delamination growth in pultrusion are long duration events that take place for several hundred miliseconds while matrix cracking in pultrusion takes 100-200 microseconds only. Another factor that helps to distinguish matrix cracking and delamination is the fact that amount of energy released at the event of delamination growth can be by an order higher compared to AE energy released during individual matrix cracking.
In the case of the same pultrusion structure but which is loaded by tension, fracture development is different and so accompanied acoustic emission. In this loading mode, we expect the following stages of fracture development:
- Matrix cracking.
- Fracture of individual fibers.
- Fiber bundle fracture and acceleration of damage development to failure.
Acoustic emission can reliably distinguish between the above stages by detecting and identification AE events suspected to individual fiber and fiber bundle breakage from matrix cracking. Such identification is based on several AE analysis methods like frequency separation (matrix cracking tend to lower frequencies compared with brittle fracture of fiber which tend to high frequencies) and amplitude/energy separation (maximum energy of matrix cracking is considerably lower of fiber breakage energy).
In our experience, it is possible very effectively אם detect, identify and monitor different stages of fracture development in glass, Kevlar, carbon fiber over-wraps, pultrusion, wet layups, prepreg, honeycomb, Rohacell, Microballons structures subjected to tension, bending, compression and shear loading.