Sentence:  The concept of the strain energy as a Gibbs function difference * * f and exerting a force normal to the shearing face is compatible with the information obtained from optical birefringence studies of fluids undergoing <WSD>shear</WSD> . 
Meaning of the word marked with <WSD> tags:  (physics) a deformation of an object in which parallel planes remain parallel but are shifted in a direction parallel to themselves