Using light to detect the internal damage of materials

An image of the polymer composite and a schematic it the composite being stretched and its photoluminescence properties. (X. Jin et al. 2012)

An image of the polymer composite and a schematic it the composite being stretched and its photoluminescence properties. (X. Jin et al. 2012)

You can usually tell when a material is about to fail, you might see sagging, cracks, dents, holes, etc. But sometimes materials can fail suddenly—without warning— this known as catastrophic failures.

Researchers in Germany report for the first time in a recent issue of Advanced Materials, a method of detecting the amount of deformation a material undergoes based on its colour intensity (light emission). The researchers made a polymer composite material containing micrometer sized zinc oxide particles that shows photoluminescence properties when shone with ultraviolet light.

Photoluminescence occurs when an external energy source (i.e. electromagnetic radiation like ultraviolet in this case) is absorbed for an atom/molecule and produces photon emissions (i.e. light). This is a composite material is made of a polymer known as poly(dimethylsiloxane) that’s dispersed with the zinc oxide particles. The zinc oxide particles are filler materials but have the added bonus of strengthening the polymer. They were also chosen to be the filler material because they often show blue-green light emissions at room temperature.

The polymer composite was pulled at constant speed until it broke, and UV light was continuous shone at the centre of composite during the pulling. The amount of light emitted by the composite was recorded relative to the amount of UV light emitted. The researchers report that the amount of light emission from the composite relative to UV emission (input energy source) decreases as the material is stretched. They explain that the volume of material being excited by the UV light is reduced as the material stretched, increasing the amount of UV radiation reaching the surface.

They mention that the light emission of zinc oxide is related to the amount of surface defects. But don’t go into further details about the actual density of surface defects and/or how the defect density in the sample changes as the composite is stretched (although the defect density typically increases).

According to the results, the composite material has a maximum tensile strength of about 1.3 MPa. But the researchers also report some of the fillers were partially separated from polymer matrix, broken into individual whiskers, suggesting that delamination influenced the material strength.

Regardless of the strength of the composite material, the researchers believe that this material has potential to be used in future engineering applications because the internal damage can be detected by photoluminescence. While sensing the stress of a material is a novel concept, the tensile strength of the polymer composite is only about 1.3 MPa—
which is pretty low considering cellophane film ranges from 50-120 MPa [2]. So I think the material’s strength needs to be addressed before engineering applications can truly be considered.

[1] Jin, X., Götz, M., Wille, S., Mishra, Y., Adelung, R., & Zollfrank, C. (2012). A Novel Concept for Self-Reporting Materials: Stress Sensitive Photoluminescence in ZnO Tetrapod Filled Elastomers Advanced Materials DOI: 10.1002/adma.201203849

[2] “Polymer Chemistry” 21 July, 2000. Retrieved from http://faculty.uscupstate.edu/llever/Polymer%20Resources/Mechanical.htm

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