Gary F. Hawkins

Machine-augmented composites

We have recently demonstrated that composites with unique properties can be manufactured by embedding many small simple machines in a matrix instead of fibers. We have been referring to these as Machine Augmented Composites (MAC). The simple machines modify the forces inside the material in a manner chosen by the material designer. When these machines are densely packed, the MAC takes on the properties of the machines as a fiber-reinforced composite takes on the properties of the fibers. In this paper we describe the Machine Augmented Composite concept and give the results of both theoretical and experimental studies. Applications for the material in clamping mechanisms, fasteners, gaskets and seals are presented. In addition, manufacturing issues are discussed showing how the material can be produced inexpensively.

NDE of Thick Composites in the Aerospace Industry — An Overview

Designers are turning to thick fiber reinforced composites, with increasing success, in order to meet the unique structural requirements which arise within the aerospace industry. These composites offer many superior properties, especially in the design of solid rocket motor cases, where there is a tremendous potential for advantage in strength-to-weight and stiffness-to-weight ratios over conventional materials [1,2]. The increased use of thick fiber reinforced composites presents quite a challenge to the NonDestructive Evaluation (NDE) community. To inspect these materials, conventional NDE techniques must be modified and/or new techniques must be developed to permit interrogation of the full material thickness and adjacent bondlines. Thick fiber reinforced composites exhibit a degree of anisotropy that is orders of magnitude above that of previously employed structural materials (i.e., metals). This anisotropy stems not only from the basic construction of the composite (oriented fibers imbedded within a matrix), but also from what are currently considered “acceptable flaws” within the material (varying degrees of delamination, matrix cracking, porosity, etc.). Successful NDE requires that one be able to distinguish the signals from these “acceptable flaws” from those deemed unacceptable. For the detection of some types of flaws, conventional techniques can be applied to composites with only slight modification, whereas for others, new techniques must be developed. For instance, recently, a large (expensive) solid rocket motor segment sustained an accidental impact. Standard ultrasonic inspection techniques successfully revealed delaminations between a number of layers in the composite case beneath the point of contact. Structural analysis, however, indicated that additional information regarding the degree of fiber breakage was needed. Unfortunately, since no NDE technique was available to assess the degree of fiber breakage, the contractor had to assume the worst and, consequently, scrap the motor.

Temperature Measurements with Micrometer Spatial Resolution

A novel technique has been developed to measure temperatures with a spatial resolution less than 10 μm. This method uses the temperature‐sensitive time decay fluorescence of a phosphor as a surface sensor. The spatial resolution is obtained by using an electron beam to excite individual phosphor grains deposited on the surface of interest. The phosphor selected for its dynamic range and chemical stability is the inorganic compound magnesium fluorogermanate activated with manganese. The time decay constant of its fluorescence decreases monotonically with temperature between −200 °C to 450 °C. This method was verified on a cross‐sectioned Zener diode. A description of the technique and a temperature map of the sectioned area of an operating diode are presented.

NDE of composite seismic retrofits to bridges

Composite materials are being used for bridge column seismic retrofits and to rehabilitate other concrete structures. There are three different manufacturing methods for applying composites to concrete columns which are outlined in this paper. Each method has the potential for creating debonds at the composite-concrete interface and within the composite itself. Thermography is a non-destructive evaluation technique which can be used to image debonds below the composite surface. Data from thermographic tests of a variety of retrofit applications, which include examples for each of the three aforementioned manufacturing processes, are presented.

Method and apparatus for detecting acoustic emissions from metal matrix wire

Apparatus for detecting acoustic emissions from metal matrix wire allows on-line measurement of the transverse strength of the wire as it is moving through its manufacturing process. A series of end and middle rollers guide the wire and form a bend in it. The location of the bend in the wire is maintained in contact with a liquid bath which, in turn, is contacted by the sensing surface of an acoustic transducer. Acoustic emissions from breaking of the fiber-matrix interface of the wire are transmitted by the bath and detected by the transducer.

Machine-augmented composites displaying control of unusual spin for a bouncing ball

One unique property of a Machine Augmented Composite (MAC) is its ability to convert a compressive force into a shear force, and vice versa, simply by the geometry of its angled sidewalls. We have discovered that a non-spinning ball dropped at a normal angle onto the MAC’s surface rebounds from that surface at an angle different from the normal and develops a significant rotational velocity. The MAC can be designed so that the spin imparted on the ball is either clockwise or counterclockwise and tailored so that the ball’s oblique angle rebound is either positive or negative from its normal angle. Through finite-element analyses and experiments, the magnitude and direction of the spin can be precisely controlled by tailoring the stiffness of the MAC through the properties and dimensions of its constituent materials.

Thermographic Detection of Buried Debonds

The purpose of this study was to test the sensitivity of thermographic imaging for detection of deeply buried flaws in a structure. The flaws of interest are adhesive debonds underneath 3/8″ of steel and 1/4″ of rubber insulation. This study was stimulated by the necessity of increasing the reliability of solid rocket motors. Consequently, the specimen and techniques described were chosen with this task in mind.

Detection of manufacturing flaws in composite retrofits

Composite materials are being used for bridge column seismic retrofits and to rehabilitate other concrete structures. There are three different manufacturing methods for applying composites to concrete columns which are outlined in this paper. Each method has the potential for creating debonds at the composite-concrete interface and within the composite itself. Thermography is a non-destructive evaluation technique which can be used to image debonds below the composite surface. Background fundamentals of the thermographic technique are discussed. Data from thermographic tests of a variety of retrofit applications, which include examples for each of the three aforementioned manufacturing processes, are then presented. The paper concludes with a list of issues which need to be addressed when performing a thermographic inspection in the field.

Detection of Magnetic Oxide Filled Cracks in Steel

The magnetic leakage field produced by a discontinuity draws and holds the ferromagnetic particles used in magnetic particle inspection (MPI). The particles held by the leakage field then provide the visible evidence to the location of the discontinuity. A change in the leakage field could, potentially, change the detectability of the crack. [1] Typically, the study of magnetic leakage fields has been limited to those emanating from air-filled discontinuities. However, for this investigation, the leakage fields from surface cracks filled with a magnetic iron oxide scale were observed. The goal was to determine if cracks filled with common magnetic oxides could essentially “bridge the magnetic gap” and, therefore, be masked during MPI.