There are some shelves in my garden shed that provide a clear and rather extreme example of the phenomenon known as ‘creep’. I hasten to add that I did not make this particular shelf!
Wood is a visco-elastic material (last discussed in WBPI 22(3):48) which means that it exhibits both elastic behaviour and plastic behaviour, depending on the test conditions and the level of applied stress.
Elastic behaviour is observed if relatively low loads are applied for short periods of time and plastic behaviour is seen when the load is applied for a long time. Time is the most crucial factor as creep can be observed in wooden objects even when subject to low loads but over long periods of time. The ability to observe the deformation is dependent on the accuracy of the measurement system.
The rate of deformation changes with time; initially the object, a shelf for example, will deform rapidly and then at an ever-decreasing rate. Creep occurs whatever types of forces are applied, eg tension, compression, bending, torsion, etc.
Greater deformations are observed at a given load at high wood moisture contents, which is often explained by the ‘lubrication’ of movement between wood polymer chains by water. This is relatively easy to comprehend, but what is less easy to understand is that wood creeps even faster when its moisture content changes. So a shelf which is subjected to a series of high and low humidities (cold rainy days followed by hot dry days) will deform more than a shelf in a constant high humidity! This phenomenon is known as ‘mechanosorption’.
My poor shelf is therefore an excellent example of mechanosorptive creep which I will fondly keep until it breaks.