Letter from Daniel D. Barnard to George Sibley, June 4, 1844

Transcript of Letter from Daniel D. Barnard to George Sibley, June 4, 1844. Barnard thanks Sibley for nominating him; nullifyers

Measurements of nonlinear harmonic waves at cracked interfaces

Nonlinear harmonic waves generated at cracked interfaces are investigated both experimentally and theoretically. A compact tension specimen is fabricated and the amplitude of transmitted wave is analyzed as a function of position along the fatigued crack surface. In order to measure as many nonlinear harmonic components as possible a broadband Lithium Niobate (LiNbO3) transducers are employed together with a calibration technique for making absolute amplitude measurements with fluid‐coupled receiving transducers. Cracked interfaces are shown to generate high acoustic nonlinearities which are manifested as harmonics in the power spectrum of the received signal. The first subharmonic (f∕2) and the second harmonic (2f) waves are found to be dominant nonlinear components for an incident toneburst signal of frequency f. To explain the observed nonlinear behavior a partially closed crack is modeled by planar half interfaces that can account for crack parameters such as crack opening displacement and crack surface conditions. The simulation results show reasonable agreements with the experimental results

Development of a portable mechanical hysteresis measurement and imaging system for impact characterization in honeycomb sandwich structures

Honeycomb sandwich materials are commonly used for aero‐structures, but because the outer skins are typically thin, 2–10 plys, the structures are susceptible to impact damage. NDI methods such as tap tests, bond testers and TTU ultrasound are successfully deployed to find impact damage, but identifying the type∕degree of damage is troublesome. As the type∕degree of impact damage guides decisions by the maintenance, repair and overhaul (MRO) community regarding repair, the ability to characterize impacts is of interest. Previous work demonstrated that additional impact characterization may be gleaned from hysteresis loop area, as determined from an out‐of‐plane load‐vs‐displacement plot, where this parameter shows a correlation with impact energy. This presentation reports on current work involving the development of a portable hysteresis measurement and imaging system based on an instrumented tapper. Data processing and analysis methods that allow production of the load∕displacement data from a single accelerometer are discussed, with additional reporting of tests of software to automatically vary pixel size during scanning to decrease C‐scans inspection time

Field and Beta-Site Testing of the Dripless Bubbler Ultrasonic Scanner

The Dripless Bubbler technique [1–4] has demonstrated in both laboratory and field trials the ability to identify adhesive disbonds and quantify the metal loss due to corrosion in aircraft fuselage structures. In the latest round of field trials, this technique was successfully applied to aid in characterizing exfoliation corrosion around fasteners in thick wing skins (0.190 – 0.500 inches). In two Beta-site tests, the technique was used to identify delaminations, verify ply drop-offs and evaluate repairs in aircraft composite structures such as rudders, spoilers and flaps

Modeling the Effects of Beam Size and Flaw Morphology on Ultrasonic Pulse/Echo Sizing of Delaminations in Carbon Composites

The size and shape of a delamination in a multi-layered structure can be estimated in various ways from an ultrasonic pulse/echo image. For example the -6dB contours of measured response provide one simple estimate of the boundary. More sophisticated approaches can be imagined where one adjusts the proposed boundary to bring measured and predicted UT images into optimal agreement. Such approaches require suitable models of the inspection process. In this paper we explore issues pertaining to model-based size estimation for delaminations in carbon fiber reinforced laminates. In particular we consider the influence on sizing when the delamination is non-planar or partially transmitting in certain regions. Two models for predicting broadband sonic time-domain responses are considered: (1) a fast "simple" model using paraxial beam expansions and Kirchhoff and phase-screen approximations; and (2) the more exact (but computationally intensive) 3D elastodynamic finite integration technique (EFIT). Model-to-model and model-to experiment comparisons are made for delaminations in uniaxial composite plates, and the simple model is then used to critique the -6dB rule for delamination sizing

Damage characterization of carbon/epoxy laminates using compression-after-impact (CAI) and ultrasonic NDE

A study of impact damage morphology in unidirectional carbon/epoxy laminates was performed. A “load drop” method was investigated for prediction of the delamination threshold energy (EDT) for impact. The impacted samples were subjected to uniaxial, in-plane compression to observe the growth of damage, failure modes, and residual strength. Samples were scanned before and after CAI using air-coupled through-transmission ultrasound and amplitude C-scans were collected for visual inspection of damage

Inspection and evaluation of dry fibers in thick composites

This paper presents the investigation of dry fibers in thick composites. Dry fibers can be defined as lack of epoxy in localized regions. These local regions act as potential defects in thick composites under compressive loads and act as an initiation point for defects such as matrix cracking and delamination. Detection and characterization of dryness in glass/epoxy composites, with thickness greater than one inch, is presented in this paper. One inch samples with dry fibers were fabricated and tested. Detection is carried out with the help of air coupled ultrasonics in both through transmission and singled sided inspection. To characterize the amount of epoxy, time of flight data was correlated with density and fiber volume fraction. Based on the observations a technique was developed for characterization of dryness

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

Review of ultrasonic testing for NDE of composite wind turbine blades

This paper will present a review of the use of Air coupled ultrasonics for nondestructive evaluation of wind turbine blades carried out at the CNDE, Iowa State University. Results will show the use of air coupled ultrasonics to scan thick composite sections of 40m wind turbine blades including the spar cap, the leading and trailing edges. Detection of defects such as slots, holes, fiber waviness (both in-plane and out-of-plane waviness), delaminations and dry fibers will be presented. Other features like adhesive thickness and length estimation in spar cap/shear web section and the trailing edge section will also be presented. The use of air coupled UT is not limited to just detection, but also characterization of the detected defects. Results highlighting the use of ultrasonic velocity to characterize defects will be presented. This includes defining damage index in terms of velocity, as well as development of stiffness maps to show reduction in structural stiffness based on the defect. Additionally, the challenges of NDT of wind turbine blades using conventional ultrasonics will also be presented

Nonlinear Ultrasounic Resonance Spectroscopy of Intact Carbon Fiber/Epoxy Composites

The present study reports the influence of fiber orientation, laminate sequence and fabric type on the inherent nonlinearity of intact carbon fiber/epoxy composites using nonlinear resonant ultrasonic spectroscopy (NRUS) [1-2]. Fiber orientation of the CFRP samples were changed relative to the length of the sample. Since the fundamental flexural mode is excited, the properties of the composite along the length of the sample would influence the nonlinear response [3]. Two hypothesis were developed using the orientation results and theory of interlaminar stress. Additionally four different laminate sequences were also tested to study the influence of laminate sequence on the inherent nonlinear response. Finally the effect of fabric type was investigated by comparing the nonlinearity of woven fabric and continuous fabric [4]. For each case comparisons were made by capturing the frequency shifts, modal damping ratios, and higher harmonics. The hypotheses were tested using the NRUS results for the laminate sequence and fabric study. Conjunctures for physical interpretation were developed using these results