Vinay Dayal

Leaky Lamb waves in an anisotropic plate. II: Nondestructive evaluation of matrix cracks in fiber-reinforced composites

This paper is concerned with the use of leaky Lamb waves for the nondestructive evaluation (NDE) of damage in anisotropic materials such as fiber-reinforced composites. Two fundamental acoustic properties of the material, namely, the wave speed and attenuation have been measured. Stiffness is deduced from the wave speed. The damage mode selected for this study is matrix cracking. As expected, the in-plane stiffness decreases and the attenuation increases with an increase in the linear crack density.

Longitudinal waves in homogeneous anisotropic cylindrical bars immersed in fluid

The propagation of a longitudinal wave in an anisotropic cylindrical bar immersed in water is considered. Energy is leaked into the surrounding fluid in the form of traveling waves, and this leakage determines the amplitude of the signal in the rod. This aspect is important in nondestructive evaluation of composite rods. The governing equation of the longitudinal waves traveling in the rod is obtained and is solved numerically to obtain the dispersion curves and the attenuation, which is due to the energy leaked into the fluid. Results are presented for rods of five different materials.

Physical basis of tap test as a quantitative imaging tool for composite structures on aircraft

Tap test is a simple but effective way for finding flaws in composite and honeycomb sandwich structures; it has been practiced in aircraft inspection for decades. The mechanics of tap test was extensively researched by P. Cawley et al., and several versions of instrumented tap test have emerged in recent years. This paper describes a quantitative study of the impact duration as a function of the mass, radius, velocity, and material property of the impactor. The impact response is compared to the predictions of Hertzian-type contact theory and a simple spring model. The electronically measured impact duration, τ, is used for generating images of the tapped region. Using the spring model, the images are converted into images of a spring constant, k, which is a measure of the local contact stiffness. The images of k, largely independent of tapper mass and impact velocity, reveal the size, shape and severity (cf. Percent stiffness reduction) of defects and damages, as well as the presence of substructures and...

An Automated Simultaneous Measurement of Thickness and Wave Velocity by Ultrasound

The measurement of wave velocity in a sample requires an accurate determination of the sample thickness at the points of interrogation. The theory of an automated technique is presented. In its application, the thickness of the sample is first calculated by the knowledge of the wave velocity in the immersion fluid. This thickness is then used to calculate the wave velocity in the sample. The results of the application of the technique to isotropic and anisotropic materials are presented.

Effects of bend-twist coupling on composite propeller performance

An investigation of a conventional propeller, made from composite materials, was conducted in which propeller characteristics were studied under quasi-static aerodynamic loading. Emphasis was placed on understanding the effects of bend-twist coupling of composite laminates on propeller performance. The classical blade element theory of propellers was used to calculate propeller characteristics and the aerodynamic force distribution acting on the propeller. The finite-element method was used to calculate the resulting deformation of the propeller blades. A simple algorithm was used in which propeller blade deformation and resulting changes in loading conditions were calculated repeatedly until equilibrium between deformation and loading was achieved. Results show that improvements in propeller performance are possible using the bend-twist coupling of composite laminates. Additionally, this study shows that the numerical approach developed by the authors is well suited for analyzing the performance characte...An investigation of a conventional propeller, made from composite materials, was conducted in which propeller characteristics were studied under quasi-static aerodynamic loading. Emphasis was placed on understanding the effects of bend-twist coupling of composite laminates on propeller performance. The classical blade element theory of propellers was used to calculate propeller characteristics and the aerodynamic force distribution acting on the propeller. The finite-element method was used to calculate the resulting deformation of the propeller blades. A simple algorithm was used in which propeller blade deformation and resulting changes in loading conditions were calculated repeatedly until equilibrium between deformation and loading was achieved. Results show that improvements in propeller performance are possible using the bend-twist coupling of composite laminates. Additionally, this study shows that the numerical approach developed by the authors is well suited for analyzing the performance characteristics of composite propellers.

Characterization of waviness in wind turbine blades using air coupled ultrasonics

Waviness in glass fiber reinforced composite is of great interest in composite research, since it results in the loss of stiffness. Several NDE techniques have been used previously to detect waviness. This work is concerned with waves normal to the plies in a composite. Air‐coupled ultrasonics was used to detect waviness in thick composites used in the manufacturing of wind turbine blades. Composite samples with different wave aspect ratios were studied. Different wavy samples were characterized, and a three step process was developed to make sure the technique is field implementable. This gives us a better understanding of the effect of waviness in thick composites, and how it affects the life and performance of the composite.

The interaction of Rayleigh waves with delaminations in composite laminates

In the present work, the interaction of Rayleigh waves with a delamination in a fiber reinforced composite plate was analyzed. Rayleigh waves, upon interacting with delamination mode, convert into Lamb waves in the delamination zone. These guided Lamb modes have the capability to mode convert back into Rayleigh modes when they interact with the edge of the delamination. A unidirectional glass/epoxy laminate with a delamination of known size was fabricated and tested using air-coupled ultrasonics. Finite element models were developed to understand the mode conversions occurring at various sections of the delamination. Particle displacements along with numerical and experimental velocities were considered to identify each mode. Conclusions were drawn based on the velocity analysis.

Interlaminar shear strength measurement of ceramic composites by asymmetric four point bend shear test

Asymmetric four-point bend test is used for the measurement of interlaminar shear strength of ceramic composite. It is shown by finite element analysis that the method provides a reasonably good approximation to a constant shear state in the sample. The use of very small samples makes this method very attractive, especially during the material development stage, when larger samples may be impractical or impossible to fabricate. This technique is also useful when bigger samples would be prohibitively expensive or impractical.

Wave propagation in a composite with a wavy sublamina

The through-thickness waviness in a sublamina in a composite laminate is the subject of study for the purposes of nondestructive evaluation by ultrasound. The model consists of a flat composite laminate with one sublamina which has sinusoidal waviness. The theoretical analysis of a longitudinal wave propagating through such a laminate is presented and the reflection coefficients at various locations on the plate are calculated. The results are validated by experimental ultrasonic C-scan.

Air‐Coupled Ultrasonic Measurements in Composites

Air‐coupled ultrasound is a non‐contact technique and has clear advantages over water‐coupled testing. Research of air‐coupled ultrasonics, especially using capacitance and micromachined transducers, has been extensively reported in the literature. This paper reports our experience of applying piezoceramic air‐coupled transducers for nondestructive evaluation of composites. The beam profiles of air‐coupled piezoceramic transducers, with and without apodization, were mapped out. The transmission of air‐coupled ultrasonic energy through composite plates of different thickness was measured experimentally; model calculation of the transmission coefficient, taking into account the frequency bandwidth of the transducer, agreed with the measurement results. The occurrence of diffraction phenomenon (“Poisson bright spot”) while imaging flaws in composite laminates was investigated. The resolution of scanned images obtained with air‐coupled transducers was investigated for different frequency, focusing, and apodiz...