Understanding Stress Development in Constrained Heated Parts

When a material is heated, its temperature increases, which typically causes it to expand in size. This phenomenon is known as thermal expansion. The extent of expansion depends on the material properties, specifically its coefficient of thermal expansion, and the temperature change.

Effect of Constraint on Thermal Expansion

If a part is free to expand when heated, it will do so without developing internal stress. However, the question states that the part is "constrained to move". This means its ability to expand freely is restricted or completely prevented by external constraints.

When the material tries to expand due to heating, but is prevented from doing so by constraints, internal forces are generated within the material. These internal forces resist the attempted expansion. This resistance to deformation (the inhibited thermal expansion) leads to the development of stress within the material.

Identifying the Type of Stress

Let's consider the intended action of the material when heated versus what the constraint forces it to do:

• Heating causes the material to want to increase its volume and length (expansion).
• Constraints prevent this increase in volume and length.

Because the constraints are preventing the material from expanding to its natural size at the elevated temperature, they are effectively forcing the material into a smaller volume than it would occupy if unconstrained. This condition, where a material is forced into a smaller space than its unrestrained state, results in the development of internal forces that are pushing inward, which is characteristic of compressive stress.

Imagine trying to fit an object into a box that is slightly too small for its expanded size. You would have to compress the object to make it fit. Similarly, the constraints compress the heated material, preventing its full thermal expansion.

Analyzing the Options

Based on the above explanation, let's look at the given options:

• Principal stress: Principal stresses are the normal stresses acting on planes where the shear stress is zero. While compressive stress can be a principal stress, stating "principal stress" is a general term and not specific to the type of stress developed in this scenario.
• Tensile stress: Tensile stress occurs when a material is pulled or stretched. In this case, the material wants to expand but is prevented, leading to a pushing or compressing effect, not a pulling one.
• Compressive stress: Compressive stress occurs when a material is pushed or compressed. This aligns with the scenario where inhibited thermal expansion leads to the material being effectively compressed by the constraints.
• Shear stress: Shear stress occurs when forces act parallel to a surface, causing layers to slide past each other. While shear stresses can exist in complex constraint scenarios, the primary stress directly resulting from prevented thermal expansion under simple constraints is normal stress, specifically compressive.

Therefore, when a part is constrained from movement and heated, the primary stress developed due to the inhibited thermal expansion is compressive stress.