Showing papers by "Ajit Mal published in 2020"

Modeling Ultrasonic Elastic Waves in Fiber-Metal Laminate Structures in Presence of Sources and Defects

Abstract: This paper presents a modeling and simulation method for studying ultrasonic guided wave propagation in hybrid metal-composites, also known as fiber-metal laminates. The objective is to develop an efficient and versatile modeling tool to aid in the design of cost-effective nondestructive evaluation technologies. The global–local method, which combines finite element discretization and Lamb wave modal expansion is used. An extension to the traditional global–local method is made to couple the source problem with the scattering problem to deal with a surface source generating Lamb waves that interact with defects in multilayered structures. This framework is used to study the sensitivity of different excitation frequencies to ply gap defects of various sizes. The coupled model considers the transducer contact conditions and the ultrasonic system response in the Lamb wave excitation, along with the scattering phenomenon caused by the defects. This combined result is used to define the optimal excitation frequency for the strongest transmission or reflection for a given defect size that can be observed in a physical experiment. Such results can be applied to the design of a damage detection scheme in realistic aerospace structures.

Frequency conversion through nonlinear mixing in acoustic waves

Abstract: Frequency conversion is an essential tool in modern communication devices. Traditionally, frequency conversion is achieved through parametric coupling via nonlinear inductors or capacitors whose reactance is modulated by a carrier wave. In this study, nonlinear acoustic Lamb wave devices are explored for simultaneous signal filtering and frequency conversion. Three sets of interdigitated transducers are fabricated on a piezoelectric aluminum nitride (AlN) thin film to provide a carrier wave, a low-power signal wave, and to receive a frequency converted mixed wave. Two devices are fabricated and tested to demonstrate frequency upconversion and downconversion by utilizing mechanical nonlinearity of AlN, and the results are compared to a nonlinear circuit model. The nonlinear circuit model is used to link experimentally observed phenomenon to the acoustic material's intrinsic nonlinearity. The nonlinearity of AlN reaches a maximum of 2.8% with a carrier wave power at 28 dBm. An analytical model is used to predict device performance along with physical dimensions. These analytical results show that nonlinear acoustic mixers and filters can approach sub-millimeter sizes, which is orders-of-magnitude smaller than conventional structures using nonlinear inductors and capacitors.

An Improved Damage Index for Nondestructive Evaluation of a Disbond in Honeycomb Sandwich Structure Using Guided Waves

Abstract: Honeycomb sandwich structures (HSS) are widely used in the aerospace industry due to their high strength-to-stiffness ratio. However, these materials are susceptible to damage during manufacturing or service that can cause great loss in the load bearing capacity or even failure. Thus, periodic or continuous nondestructive evaluation (NDE) of HSS is essential for safe operation. Development of effective NDE technique is challenging due to the geometric complexity of the honeycomb core. Guided ultrasonic waves are ideal for large-scale testing because of their large propagation range and high sensitivity to defects in their path. In this paper, an improved NDE method for detecting disbonds at the top and bottom interfaces between the core and facesheets is proposed based on experimental studies. By applying excitation signals at different frequencies, the responses at the top and bottom surface of plate-like HSS component are compared and analyzed. The response in a specific frequency range is further studied by introducing disbonds at the top interface. It is shown that some components of the recorded signal in a specific frequency range are more sensitive for detecting the disbond. In addition, an improvement of the conventional damage index based on the damage feature is proposed, and a systematic procedure for detecting damage inside HSS is conducted on an elevator section of an Airbus 330. The results show that the optimized damage index greatly improves the resolution and adaptability of damage detection in the structures.

Uncertainty characterization of guided ultrasonic wave properties in composite materials

Abstract: Guided ultrasonic wave-based methods are promising for structural health monitoring of isotropic and composite materials and structures. 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 guided wave propagation and scattering phenomena as well as damage detection. However, very little research was geared towards the influence of the uncertainty in the material properties for the calculation of the dispersion curves. The lack of knowledge of the exact material properties together with manufacturing tolerances could lead to erroneous conclusions. Hence, in this study, an uncertainty analysis for the material properties of fiber-reinforced composites is conducted to quantify the effect of uncertain material constants on the dispersion curves. A fuzzy arithmetical approach based on the Transformation Method is used to generate the dispersion curves with uncertain parameters in conjunction with a root-finding algorithm. The uncertain parameters are modeled as linear fuzzy numbers. Using triangular membership functions, both the nominal value and the worst-case interval are adequately combined into one fuzzy number. Furthermore, it is shown that the measure of influence for the uncertain material parameters on the group velocities of the considered Lamb waves is not equally weighted. These findings might allow for the development of efficient, nondestructive material characterization techniques in the future.

Numerical and experimental investigation of damage detection in stiffened composite panels using guided ultrasonic waves

Abstract: Fiber-reinforced polymer matrix composites have excellent in-plane stiffness and strength properties, and are therefore ideal for usage in panels of aircraft wings or fuselage as well as launch vehicle case segments. Those thin plates or shell structures are often stiffened with many locally increased thickness regions, or beams of various cross-sectional shapes such as flat or T-shaped. Small defects in any of those stiffened regions would greatly reduce the structural performance as a whole. Locating such defects is time consuming because of the large extend of the panels as well as the number of stiffeners. Guided ultrasonic wave-based techniques could be applied for damage detection in large areas. However, the scattering characteristics of stiffeners are complex. In particular, when multiple stiffeners are present, the incident Lamb wave signal is altered with the passing of each stiffener. Thus, the goal of this work is to efficiently model Lamb wave propagation when multiple stiffeners are present, with and without defects, in an effort to identify useful signal features for damage detection. To this end, the so-called global-local method is used for Lamb wave modeling. The global functions are used to represent the nominal composite region – parameters are obtained by means of waveguide finite element (WFE) method – and the stiffened region is represented by finite element discretization. With a recently developed coupling technique, a source problem, representing a surface-mounted transducer is coupled with multiple stiffener-scattering models to examine the transmission characteristics. The global-local model is validated by laboratory waveform measurements on a stiffened composite plate. The results from global-local method can then be used to efficiently determine the maximum number of stiffeners before the transmitted Lamb waves become too weak to identify defects.