John C. Duke

Thermographic investigation of high-power ultrasonic heating in materials

Abstract Results are reported on the use of an infrared thermovision imaging system to observe the surface temperature distribution caused by the application of high-power 20 kHz ultrasound to a variety of metal specimens and one fluoroplastic. Temperature increases of the order of 200°C were found to occur 20 to 30 s after initiation of insonation in resonant specimens of fine-grained polycrystalline brass, copper, and steel. Observation of extremely rapid temperature increases localized to the points of attachment of thermocouples to the test specimens casts strong doubt on all previous thermocouple temperature measurements made during similar experiments by earlier investigators. Fatigue cracks, artificially induced defects, and grain boundaries were also found to be sites of rapid localized heating during insonation.

DAMAGE MECHANICS AND NDE OF COMPOSITE LAMINATES

ABSTRACT The mechanics of the response of composite laminates with damage is discussed in the context of damage initiation, damage growth, stress redistribution, fracture, and nondestructive testing.

An integrated ultrasonic sensor for monitoring gradual wear on-line during turning operations

Abstract The condition of the tool and the cutting process are essential inputs to any productivity improvements through process optimization in conventional and unmanned machining. Tool replacement and tool wear compensation strategies, which are based on prior experience and/or tool history are, in general, under performing. Currently, the methods of tool condition monitoring are either time consuming, as in the case of off-line direct measurements of the tool, or are modestly successful, as in the case of the on-line indirect measurements, such as forces or acoustic emissions. This in part is due to the lack of suitable sensors and/or exact dynamic model, which relate the indirect measurements to the actual tool condition. This paper describes a promising ultrasonic method for on-line direct measurement of gradual wear in turning operations. An integrated (transmit and receive) single ultrasonic transducer operating at a frequency of 10 MHz is placed in contact with the tool. The change in the amount of the reflected energy from the nose and the flanks of the tool can be related to the level of gradual wear and the mechanical integrity of the tool. The experimental results show that under laboratory conditions, a correlation exists between the ultrasonic measurement and gradual wear and that it is tool dependent.

Health monitoring of FRP bridge decks

Statistics released in the fall 1989 show that 238,357 (41%) of the nations 577,710 bridges are either structurally deficient or functionally obsolete. New materials are being explored for use in bridge systems to solve this problem. These materials are less affected by corrosive environmental conditions than conventional civil engineering materials and thus, require less maintenance and potentially provide a longer life span. A material being considered for these applications is glass fiber reinforced vinyl ester matrix composites. Fiber reinforced plastic (FRP) composite deck systems made of this material are favorable potential replacements for deteriorating conventional bridge decks. The decreased specific weight of the FRP greatly reduces the dead load of the superstructure helping avoid load posting of bridges. However there is a lack of long-term durability data concerning this material system in typical bridge environments. Thus, an efficient and effective method must be devised to monitor the health of an FRP structure in-situ. This paper will discuss the use of Infrared Thermography as a means of detecting structural imperfections -- delaminations, disbonds, voids -- caused by conditions encountered both in fabrication and in the field. As forced convection hot air is circulated through the bridge deck, delaminations and disbonds in the top of the deck appear cold while defects in the bottom of the deck give rise to areas with higher temperatures. The discontinuities in thermal propagation patterns are detected with a thermal imaging system and indicate present and possible future structural deficiencies. Laboratory results revealing fabrication/installation problems and those from field tests will be presented.

Fiber optic acoustic emission sensors for harsh environment health monitoring

Optical fiber sensors are rapidly emerging as viable alternatives to piezoelectric devices as effective means of detecting and quantifying acoustic emission (AE). Compared to traditional piezoelectric-based sensors, optical fiber sensors offer much smaller size, reduced weight, ability to operate at temperatures up to 2000 degree(s)C, immunity to electromagnetic interference, resistance to corrosive environments, inherent safety within flammable environments, and the ability to multiplex multiple sensors on a single fiber. The authors have investigated low-profile fiber optic-based AE sensors for non-destructive evaluation (NDE) systems. In particular, broadband and resonant type optical fiber sensors were developed for monitoring acoustic emission for NDE of pressurized composite vessels and commercial airframe structures. The authors developed an in-plane, broadband sensor design based on optical strain gage technology. In addition, an out-of-plane, resonant sensor was developed using micromachining techniques. The sensors have been evaluated for performance using swept frequency and impulse excitation techniques and compared to conventional piezoelectric transducers. Further, application experiments were conducted using these sensors on both aluminum lap-joints and composite fracture coupons, with collocated piezoelectric transducers. The results indicate that optical fiber AE sensors can be used as transducers sensitive to acoustic events and the indication of imminent failure of a structure, making these sensors useful in many applications where conventional piezoelectric transducers are not well suited.

Acousto-ultrasonic (AU) Technique for Assuring Adhesive Bond Quality

Abstract This paper examines the feasibility of using the Acousto-Ultrasonics (AU), nondestructive technique, for assuring the quality of adhesively bonded sheet-metal used for automobiles. Kissing bonds or regions lacking adhesive were easily identified by this technique. A bond quality (BQ) model is introduced that takes into account the mixed mode failure. Destructive testing results showing fairly consistent correlation of BQ values with the breaking strength of the adhesive joint failing in mixed mode failure are presented.

Characterization of composite materials by means of the ultrasonic stress wave factor

The usual approach to nondestructively evaluating a composite structure involves inspection and mechanical analysis of the inspection results. Such an approach has met with only limited success. On the other hand, the ultrasonic stress wave factor technique directly evaluates the material. Despite requiring access to only one surface of the material, the technique interrogates the material in the directions of applied load. Using the stress wave factor technique it is possible to determine the failure location in the material. The correlation of the stress wave factor with stiffness is shown. In addition, the use of the technique for determining the strength or life of composite material structures is discussed.

Fiber Optic Acoustic Emission Sensors and Detection

Optical fiber sensors are rapidly emerging as viable alternatives to piezoelectric devices as effective means of detecting and quantifying acoustic emission (AE). Compared to traditional piezoelectric-based sensors, optical fiber sensors offer much smaller size, reduced weight, ability to operate at temperatures up to 2000 degrees Celsius, immunity to electromagnetic interference, resistance to corrosive environments, inherent safety within flammable environments, and the ability to multiplex multiple sensors on a single fiber. The authors have investigated low-profile fiber optic- based AE sensors for non-destructive evaluation (NDE) systems. In particular, broadband optical fiber sensors were developed for monitoring acoustic emission for NDE of pressurized composite vessels. The authors conducted experiments by surface attaching sensors to aluminum compact tension specimens using a piezoelectric transducer as a reference sensor. Both the fiber optic and piezoelectric sensors accurately measured a representative acoustic event. The response of the fiber optic AE sensors were also compared to existing piezoelectric sensors during pencil lead break tests on an aluminum panel. The results indicate that optical fiber AE sensors can be used as highly sensitive transducers in many applications where conventional piezoelectric transducers are not suited.

Characterizing the Damage State of Composite Laminates VIA the Acousto-Ultrasonic Technique

The acousto-ultrasonic, or stress wave factor, technique was developed by Vary and co-workers. This ultrasonic technique provides a quantitative ultrasonic parameter which has been found to correlate with several different mechanical properties in both homogeneous and composite materials. We have found the technique to be quite sensitive to both in-situ damage in a virgin specimen and to damage developing during mechanical loading. The technique differs from standard transmit-receive ultrasonic techniques only in that the two transducers are not in a direct line of sight (including reflection by Snell’s Law). That is, one cannot follow the ray path from the sender to the receiver, even when reflections off boundary surfaces are considered. The receiver signal is composed of surface waves, body waves, and plate waves, established through scattering and diffraction processes. In this work we will present the basic technique, examples of experimental results relating the ultrasonic signals to mechanical properties of composites, and experimental results from studies of the basic nature of the waves involved in this technique.

Material Fatigue Prognosis with Higher-Order Spectral Analysis of Nonlinear Ultrasonic Characteristics

Nonlinear aspects of ultrasonic wave propagation in a specimen that is subjected to increasing intervals of cyclical loadings are characterized. The ultimate goal is to develop an approach that would facilitate monitoring the extent of the deterioration occurring during the precursor to crack formation. The efiectiveness of using higher-order spectral analysis tools, and more particularly the bispectrum, in characterizing certain nonlinear aspects of the ultrasonic wave propagation are presented. The results show that the bispectrum is capable of detecting couplings between the fundamental frequency of the exciting burst, its subharmonic, its flrst harmonic and its sidebands. Difierences in coupling levels between these modes are observed as the number of cycles is increased. This suggests that such levels could be used to derive parameters that can be directly related to the fatigue damage state of the material. The results also present a discussion on the shortcomings of using the amplitudes of generated harmonics to determine the fatigue damage state in a specimen.