Automated guided wave characterization of Marcelling: In-plane fiber waviness in thermoplastic composites

Thermoplastic composite (TPC) materials often develop in-plane fiber waviness during manufacturing, a defect known as marcelling. Marcelling negatively impacts the strength and overall performance of the composite and are hard to control. While it may appear as a surface imperfection, it can also extend partially through the thickness of the material. Currently, there is no established nondestructive evaluation (NDE) method to reliably detect and quantify marcelling in composites after manufacturing. As marcelling is not a local defect but spans across the surface in in-plane dimensions of the structure, traditional pulse-echo and phased-array ultrasonic NDE are ineffective. Automated multi-directional guided wave NDE, however, offer a promising solution. In this study it is shown that fusing guided wave data with wave vectors along the fiber direction, orthogonal to the fiber direction and along the 45° to the fiber direction would create unique combination to detect and quantify marcelling. The area of marcelling in TPC is called Marcelling Affected Zones (MAZEs). Unknown marcelling was first quantified at each MAZE by measuring in-plane fiber wave lengths and amplitudes of waviness obtained from Scanning Acoustic Microscopy. Statistical distribution of these geometric parameters resulted in three different Marcelling Aspect Ratios (MAR). MAZEs with specific MAR were inspected using a newly built automated guided wave system. The guided wave features such as velocity, amplitude and bi-directional phase were analyzed in relation to the degree of marcelling (defined by MAR). It was found that with increasing MAR the antisymmetric wave velocity and aggregated frequency amplitudes decreases. Effect of the MAZEs on the frequency amplitude is higher when the wave vector direction is orthogonal or at an angle to the fiber direction. This study creates opportunity to use guided wave inspection tool as an enabling technology to quantify marcelling or fiber waviness in TPC after manufacturing.