Environmental Geology - - GEOL 406/506

Rock Strength and Behavior

As you might imagine, natural substances behave according to their internal properties. They are either brittle or ductile, depending on the amount of stress applied. The stress/strain graphs below illustrate the bahavior of materials under stress.



Elastic Deformation

A brittle substance fails (ruptures) after elastic deformation. An example is a piece of window glass which will bend slightly, then break if enough stress is applied. If stress is released before the glass breaks, it will return to its original shape.

A = substance with high yield strength, very little deformation prior to failure.

B = substance with intermediate strength, some deformation prior to failure.

C = substance with low strength, bends easily before failure.




Ductile Deformation

Ductile deformation occurs when a substance is stressed to a point where it begins to behave like a plastic. At this point the proportional elastic limit has been exceeded and the substance begins to deform with less and less applied stress. The ultimate strength of the material is the highest point on the curve in the ductile region. The rupture strength is usually lower than the ultimate strength and depends on many factors, including the confining pressure and the amount of strain that has occurred.

Most rocks have a very small region of elastic behavior and behave more like a plastic with increased stress (pressure).

Permanent Strain

Permanent strain occurs when the proportional elastic limit is exceeded and the substance does not return to its original shape when the stress is released. The small region in green illustrates the amount of strain that cannot be recovered once the stress/strain curve has reached point "A". A piece of wire is a good example of a substance that, when subjected to slight stress, will return to its original shape, but, when the elastic limit is exceeded the wire is bent.

A piece of wire is not a good example of a natural substance that has a finite rupture strength. Why?