Scattering Revelations

Fiberglass is a composite material consisting of small-diameter glass fibers supported by a matrix material, typically a thermosetting plastic. One of the qualities of fiberglass composites that makes them attractive for modern applications is their ability to tolerate damage; a relatively large amount of work needs to be done to a fiberglass part to break it.

Damage in a fiberglass part occurs in stages. The first damage that occurs is matrix cracking; however, the composite still performs nearly as well as it did before the damage was introduced. It is when strain increases to the point at which fibers break that the performance of the part becomes significantly compromised.

For various reasons, it is usually desirable to detect the presence of cracked matrix material. Few nondestructive evaluation (NDE) methods currently detect damage in fiberglass. Ultrasonic and computed tomography methods have been investigated, but complexity, equipment cost, safety, and training issues limit their usefulness.

We developed an IR NDE method for fiberglass damage testing. The mechanism is simple. Light scatters at the new surfaces introduced when matrix cracking occurs. By illuminating the fiberglass and monitoring the transmitted light, we can detect cracking episodes.

To test our theory, we fitted an IR LED on one side of a fiberglass sample and placed a phototransistor directly adjacent to the IR LED on the opposite side of the sample. The system had an effective field of view of roughly 0.025 in. The transistor voltage varied relative to the transmitted light; during the test, we collected transistor voltage response and mechanical performance data (load and deflection). We loaded samples until fiber failure occurred.

As matrix crack density increased, the transmittance decreased almost linearly until fiber failure began, demon-strating that monitoring transmission during tensile loading is a good way to detect damage (see figure 1).

Figure 1. A plot of load and IR phototransistor response versus strain during tensile test to failure shows a distinct elbow at the onset of matrix cracking at 0.3 to 0.5% strain.

In most applications, however, it is necessary to detect damage that has occurred after an event, not during it. We conducted tests to investigate the transmission properties of fiberglass during and after successive damage events by loading a sample to a level at which matrix cracking typically occurs and then unloading it (see figure 2). The transmittance decreased as the sample was loaded, consistent with previous tests, and increased when the load was removed. Note, however, that it did not increase to its previous unloaded value. Successive load cycles showed additional decreases, but to a lesser degree, suggesting we can use transmission methods to detect levels of progressive damage.

Figure 2. Phototransistor response during and after successive damaging 414 MPa load events shows that transmission does not rebound completely when the sample is unloaded, revealing residual damage levels.

The IR NDE method has several potential applications. Since the components are relatively small, they can be laminated into large structures and queried periodically for health monitoring. Some manufacturing challenges arise when laminating the components into a structure, but those should be overcome easily. Several configurations of hand-held devices have been constructed for "spot-checking." Since the mechanism is simple light scattering, this same approach can be used in a manufacturing environment for porosity and other part-quality measurements. oe