Donald Robert Howard

Quantitative evaluation of discrete failure events in composites using infrared imaging and acoustic emission

We describe the successful synchronization of thermal imaging with acoustic emission to observe and characterize for the first time the thermal responses of discrete fiber breaks and matrix cracks in composite panels under load in real time. These events can be accurately described by ideal buried point and line thermal sources.

Thermal Imaging at General Electric

We will describe advances in and current applications of thermal imaging at General Electric. Time‐of‐Flight methods are in use at several GE facilities. This approach is sufficiently robust to be used in production. Examples presented include evaluation of metal airfoils and ceramic materials.

Lateral heat flow method for thickness independent determination of thermal diffusivity

A pulsed transient thermography method is described where a high-intensity light pulse is used to heat a long, uniform stripe on the surface of a plate. A high spatial resolution, high frame rate focal plane array infrared camera is used to monitor surface temperature. We explain the theoretical model and data analysis framework used to experimentally determine all three thermal diffusivity components from the temperature measurements. The analysis does not require any fitting to the temperature profile and is based on the creation of thermal time-of-flight (tof) images from the temperature data and the relationship between tof and the distance from the stripe edge. The in-plane components of thermal diffusivity are obtained without the need for thickness information. Experimental validation of this procedure was carried out using anisotropic carbon fiber reinforced polymer composites.

Advances in Thermal Time‐of‐Flight Imaging with Flash Quenching

Flash quenching, that is, cutting off the tail of the exponentially decaying light flash, permits great strides in realizing the ideal analysis of high resolution IR data. We will describe some current applications of Quenched Thermal Time‐of‐Flight Imaging to high‐resolution evaluation of metals and fast materials.

QUANTITATIVE INFRARED FULL‐AREA IMAGING OF COATING THICKNESS AND INTEGRITY

Infrared Imaging of coatings is not new. In the past it has been accomplished either comparatively, for example using additional thickness standards, as a point‐wise inspection or as a simple non‐quantitative image showing debonds at most. In the present paper we describe the first quantitative infrared image of both coating thickness and integrity. No reference standard is necessary. This coating thickness imaging can be applied over 100% area of complex‐shape components such as airfoils. It is fast and a by‐product of this inspection is an image of coating integrity which may be graded from complete disbond to poorly‐coupled, for example, indications of porosity at the bondline. We will present a first quantitative coating thickness IR image.

INFRARED IMAGING OF CARBON AND CERAMIC COMPOSITES: DATA REPRODUCIBILITY

Infrared NDE techniques have proven to be superior for imaging of flaws in ceramic matrix composites (CMC) and carbon silicon carbide composites (C/SiC). Not only can one obtain accurate depth gauging of flaws such as delaminations and layered porosity in complex‐shaped components such as airfoils and other aeronautical components, but also excellent reproducibility of image data is obtainable using the STTOF (Synthetic Thermal Time‐of‐Flight) methodology. The imaging of large complex shapes is fast and reliable. This methodology as applied to large C/SiC flight components at the NASA Dryden Flight Research Center will be described.

Synchronization of Infrared Imaging with Acoustic Emission for Depth Measurement of Discrete Failure Events in Composites

Infrared (IR) imaging together with acoustic emission (AE) sensing are employed to follow the progression to failure of graphite epoxy test panels under dynamic loading. Acoustic emission generated by fiber breaks and matrix cracks define the “zero” time for surface observation by an IR camera of the deep heat source from that event. The AE clock and IR clock must be synchronized. The timing of the surface event relative to “zero” permits the calculation of its depth. The position and depth of the IR events can then be related to crack evolution and modeling for failure prediction.