Final answer:
The maximum in-plane shear stress with respect to an element occurs at a 45-degree orientation to the principal planes where normal stresses are present.
Step-by-step explanation:
The orientation of the maximum in-plane shear stress with respect to an element is 45 degrees. This is a fundamental concept in the study of materials and mechanics of solids, particularly when evaluating stress on various planes within a material subjected to external forces. When normal stresses are present on the principal planes, the maximum shear stresses occur on planes that are at a 45-degree angle to the principal planes. This is because the shear stress is zero on the principal planes themselves and reaches its maximum on planes that are oriented at 45 degrees to these principal planes.