Showing papers by "Ajit Mal published in 2019"

Effects of Homogenization and Quasi-Isotropy Assumptions on Guided Wave-Based Nondestructive Testing Methods

Abstract: Past research on guided ultrasonic wave-based methods for nondestructive testing of composite materials often assumed quasi-isotropy of composite laminates. Addition- ally, the quasi-isotropic laminate is homogenized and effective material properties for the laminate, modeled as a single layer material, are employed in subsequent analytical, numerical or experimental studies. However, deviations from the intended layup cause a directional dependency of the propagation velocities, thus requiring the modeling of the entire laminate. This not only alters wave fronts but it may also lead to significant errors in time-of-flight analyses for longer propagation ranges. Hence, in this paper, a comprehensive numerical-experimental approach is taken to investigate the quasi-isotropy of composite laminates in the context of guided ultrasonic wave propagation. A combination of semi-analytical and analytical tools are used to determine the dispersion curves and the propagation characteristics for several composite plate structures. The methods are applied for quasi-isotropic laminate that can be homogenized as well as a general composite laminate. The results are validated through dispersion experiments, and implications for damage detection are highlighted.

Feasibility Analysis of Various Sensing Methods for Nondestructive Testing of Composites

Abstract: Guided ultrasonic wave-based methods are promising for detecting defects over long ranges in isotropic and composite materials. The technology has seen a lot of attention in the research community over the past decades, and many analytical and numerical methods have been developed to describe different aspects of wave propagation and scattering phenomena. However, very little research was geared towards the physical implementation in practical applications. Hence, in this study, the detectability of defects in composite materials (delaminations and core-skin disbonds) with different sensor technologies is investigated. To induce and record scattered guided ultrasonic waves, broadband contact transducers, air-coupled transducers and a laser Doppler vibrometer are used. It is shown that the thickness of typical sandwich panels used in aerospace industry and the non-homogeneous nature of the honeycomb core material make precise localization and identification of defects challenging, in particular in non-contact scenarios.

An improved damage index for non-destructive testing of a honeycomb sandwich panel using guided waves

Abstract: Honeycomb sandwich panels (HSP) are widely used in the aerospace industry due to their high strength to stiffness ratio. Conducting non-destructive evaluation (NDE) of HSP is a topic of great current interest. However, the geometric complexity of honeycomb core makes NDE of HSP extremely difficult. Guided ultrasonic waves are ideal for large scale testing due to its large range and high sensitivity to defects in their path. Previous research has been successful in detecting disbonds at the core-skin interface using guided waves, but few of them have focused on the detection of disbonds at the bottom interface. An improved NDE method for detecting disbonds at the top and bottom interfaces is proposed based on experimental results. By applying excitation signals for different frequencies, the responses at the top and bottom skins are compared and analyzed. The response in a specific frequency range is further studied by introducing disbond at the bottom interface. It is shown that some components of the recorded signal in specific frequency range are more sensitive to the disbond and can be related to the size of the disbond. Finally, an improvement of the conventional damage index based on propagation velocity of guided waves is provided. The results show that the optimized damage index greatly improves the resolution and flexibility of NDE on HSP.