Sunil Kishore Chakrapani

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.

Detection and Characterization of Waviness in Unidirectional GFRP Using Rayleigh Wave Air Coupled Ultrasonic Testing (RAC-UT)

In this article, we propose a method of detection and quantification of fiber waviness or “marcels” in unidirectional glass fiber reinforced composite plates using air coupled ultrasonics. Severity of waviness was defined with the help of aspect ratio, which is the geometric characteristic of the marcel. Several wavy samples with different aspect ratios were fabricated and tested with the defined method. Waviness was detected by performing C-Scans and characterized with the help of B-Scans to determine the change in time of flight while traversing along the length of the wave. This method can be effectively implemented as a field technique for marcel characterization in thick composites.

Influence of fiber orientation on the inherent acoustic nonlinearity in carbon fiber reinforced composites

This paper presents the study of non-classical nonlinear response of fiber-reinforced composites. Nonlinear elastic wave methods such as nonlinear resonant ultrasound spectroscopy (NRUS) and nonlinear wave modulation spectroscopy have been used earlier to detect damages in several materials. It was observed that applying these techniques to composites materials becomes difficult due to the significant inherent baseline nonlinearity. Understanding the non-classical nonlinear nature of the composites plays a vital role in implementing nonlinear acoustic techniques for material characterization as well as qualitative nondestructive testing of composites. Since fiber reinforced composites are orthotropic in nature, the baseline response variation with fiber orientation is very important. This work explores the nature of the inherent nonlinearity by performing nonlinear resonant spectroscopy (NRS) in intact unidirectional carbon/epoxy samples with different fiber orientations with respect to major axis of the sample. Factors such as frequency shifts, modal damping ratio, and higher harmonics were analyzed to explore the non-classical nonlinear nature of these materials. Conclusions were drawn based on the experimental observations.

Ultrasonic Rayleigh wave inspection of waviness in wind turbine blades: Experimental and finite element method

This paper presents the investigation of discrete, out-of-plane waviness in thick composite plates with applications to wind turbine blades. The investigation was carried out with the help of air coupled ultrasonics and a two-step procedure was framed to assist production line implementation. The first step involved detection of marcels, and the second step involved the characterization of these marcels with the help of an index called aspect ratio. A set of standardized samples with known aspect ratios were fabricated and used for this study. Finite element models were created to understand the wave propagation in wavy composite plates. All the experimental data was correlated with numerical B-Scans and conclusions concerning the method were made.

Measurement of nonlinearity parameter (β) of water using commercial immersion transducers

The determination of absolute nonlinearity parameter (β) for water using a finite-amplitude method is described here. Measurements were carried out using commercial immersion transducers, thus making this technique field adaptable. Calibration of immersion transducers is based on a simplified version of the calibration technique proposed by Dace et al. With corrections for diffraction and attenuation, the nonlinearity parameter calculated for water is in good agreement with literature, providing a validation for the methods developed. The current study is aimed at progressing towards measuring the nonlinearity parameter of immersed solids using commercially available ultrasonic equipment. This work is supported by the Industry/University Cooperative Research Center members of the Center for Nondestructive Evaluation at Iowa State University.

Nondestructive evaluation of helicopter rotor blades using guided Lamb modes

This paper presents an application for turning and direct modes in a complex composite laminate structure. The propagation and interaction of turning modes and fundamental Lamb modes are investigated in the skin, spar and web sections of a helicopter rotor blade. Finite element models were used to understand the various mode conversions at geometric discontinuities such as web-spar joints. Experimental investigation was carried out with the help of air coupled ultrasonic transducers. The turning and direct modes were confirmed with the help of particle displacements and velocities. Experimental B-Scans were performed on damaged and undamaged samples for qualitative and quantitative assessment of the structure. A strong correlation between the numerical and experimental results was observed and reported.

Detection of in-plane fiber waviness in composite laminates using guided Lamb modes

This paper presents the study of Lamb wave interaction with in-plane fiber waviness in thin composite laminates. Previous studies show that a reduction in stiffness of the laminate occurs due to in-plane waviness. Aspect ratio and shear angles were used to define severity of waviness. Air coupled ultrasonic transducers were used to generate the fundamental anti-symmetric mode (A0), which was used in this study. A reduction in velocity of the A0 mode was observed when it interacts with in-plane waviness. To confirm the findings, dispersion curves were plotted for straight fiber zone and waviness zone. Experimental B-Scans of wavy laminate were used to evaluate the velocity change. Conclusions were made based on the correlation between experimentally and analytically observed velocities.

Inspection of helicopter rotor blades with the help of guided waves and "turning modes": Experimental and finite element analysis

Modern helicopter rotor blades constructed of composite materials offer significant inspection challenges, particularly at inner structures, where geometry and differing material properties and anisotropy make placement of the probing energy difficult. This paper presents an application of Lamb waves to these structures, where mode conversion occurs at internal geometric discontinuities. These additional modes were found to successfully propagate to the targeted regions inside the rotor and back out, allowing evaluation of the structure. A finite element model was developed to simulate wave propagation and mode conversion in the structure and aid in identifying the signals received in the laboratory experiment. A good correlation between numerical and experimental results was observed.

Ultrasonic testing of adhesive bonds of thick composites with applications to wind turbine blades

This paper discusses the use of pulse echo based ultrasonic testing for the inspection of adhesive bonds between very thick composite plates (thickness greater than 30 mm). Large wind turbine blades use very thick composite plates for its main structural members, and the inspection of adhesive bond-line is very vital. A wide gamut of samples was created by changing the thickness of plate and the adhesive. The influence of experimental parameters such as frequency on measurement is studied in this paper. Two different frequencies are chosen, and the measurement error bars are determined experimentally. T-Ray measurements were used to verify and correct results, and conclusions are made based on the combined results.