Principal strains are defined as normal strains that are either tensile or compressive, not shear strains. They occur on principal planes that are orthogonal to the principal stress planes, which are also normal in nature. The discussion confirms that principal strains are maximum strains at a point and emphasizes their orthogonality, supported by formal derivations found in textbooks. Thus, principal strains cannot be shear strains, as they are inherently linked to normal stress conditions.
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