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Importance of Coefficient of Moisture Expansion (CME) Testing

What is Coefficient of Moisture Expansion (CME)? The CME of a material is determined by measuring the moisture content and strain change between two moisture equilibrium states.  It is expressed as the change in strain per unit mass change between the two moisture states.

Why do you need to know the CME? CME measurements support the proper design and construction of high-stability structural components. PMIC has extensive experience determining the CME in composite plates, tubes, and honeycomb structures with a wide variety of sizes, shapes, temperatures, and humidities. PMIC’s test laboratory is designed to control the necessary environmental variables to achieve the highest level of accuracy possible.

CME can affect building materials, electronics, satellite structures and more. Determining the CME of a material provides insight on expansion and contraction, as well as moisture’s effect on material strength over time.

In a common application, think of siding on houses. Through constant changes in humidity and heat, expansion and contraction can impact how boards fit together.

Image of specialized LVDT based dilatometer.
Image of specialized LVDT based dilatometer.

For electronics, CME measurements provide insight into how materials will react in various humidity environments. It helps people understand if a material will experience buckling or warping when exposed to different environments.

PMIC’s CME procedure is proprietry. However the following ASTM standards apply to the methodology:

ASTM StandardName
C271Standard Test Method for Density of Sandwich Core Materials
C272Standard Test Method for Water Absorption of Core Materials for Sandwich Constructions
D570Standard Test Method for Water Absorption of Plastics
D5229/D5229MStandard Test Method for Moisture Absorption Properties and Equilibrium Conditioning of Polymer Matrix Composite Materials
E104Standard Practice for Maintaining Constant Relative Humidity by Means of Aqueous Solutions

For space flight structures, it is helpful to obtain CME data to see how much expansion or contraction can be anticipated while assembling structures. Without this information, the variance could be the difference between a successful flight, or components detaching and weakening.

Adhesives are another great example of the impact humidity can have on materials. Have you ever noticed how a sticky note in a humid area can detach from the surface it was adhered to? CME provides an understanding of how epoxies will perform in different humidity environments.

For more information on how PMIC can help you with your CME needs, please visit our website or call us at (541) 753-0607.