The Autonomous Crack Monitoring project measures not total crack width but change in crack width. We will define crack displacement as this change in crack width. The crack displacement measured by the sensors may be driven by any combination of the factors listed below. to as displacement.

Differential thermal expansion Structural overloading Chemical changes in mortar, bricks, plaster, and stucco Shrinkage and swelling of wood with temperature and humidity changes Fatigue and aging of wall coverings Differential foundation settlement

The measured change in crack width is very small. The maximum crack displacement measured in our Evanston test house over the 1999-2000 heating season amounts to slightly more than three times the width of a human hair. Average daily crack displacement is less than 1/10 the diameter of a human hair.