Test: Mechanical Properties of Materials - 2 - Mechanical Engineering MCQ

Stress is best defined as the force applied per unit area. It measures how much force is distributed over a specific area and is crucial in understanding material behaviour under load. The correct definition can be summarised as follows:

• Force per unit area indicates how pressure is applied to an object.
• This concept is essential in fields such as engineering and physics.
• Understanding stress helps predict how materials will respond to external forces.

Young's modulus is a measure of the stiffness of a material. In the SI system, the unit of Young's modulus is the Pascal, abbreviated as Pa.

Hooke’s law describes the behaviour of solids under tensile stress within the elastic limit. It states that the strain in a material is proportional to the applied stress, provided the material's elastic limit is not exceeded. This can be summarised as:

• Stress is the force applied per unit area.
• Strain is the deformation experienced by the material.
• In simple terms, as you stretch a solid, it stretches proportionally up to a certain point.

Strain is a measure of deformation representing the displacement between particles in a material body. It is expressed as a ratio of change in length to the original length, making it a dimensionless quantity. Here are key points regarding strain:

• Strain has no units, as it is a ratio.
• It is not measured in N/m²; that is a unit for stress.
• Strain does not equal the applied force; it represents how much a material deforms.

Thus, the correct statement is that strain is a dimensionless quantity.

Bulk modulus is defined as:

• Pressure divided by volume strain. So (a) is the right option.

Poisson’s ratio is defined as the ratio of:

• Lateral strain to longitudinal strain.

This means that when a material is stretched, the amount it contracts in the perpendicular direction (lateral strain) compared to the amount it stretches in the direction of the applied force (longitudinal strain) gives us the Poisson’s ratio.

If a wire is stretched by a force and its length increases by 1%, the strain can be calculated as follows:

• Strain is defined as the change in length divided by the original length.
• In this case, the change in length is 1% of the original length.
• Therefore, strain = (Change in Length) / (Original Length).
• As a decimal, 1% is equal to 0.01.

Thus, the strain is 0.01. The correct answer is B.

The modulus of rigidity is specifically related to:

• Shearing stress

This property measures a material's response to shear forces, which cause deformation. In contrast, tensile and compressive stresses relate to stretching and squeezing, while hydraulic pressure does not directly relate to rigidity.

The point at which permanent deformation begins in a stress-strain curve is called the yield point. This is where the material starts to change shape irreversibly. Understanding this term is crucial for materials science and engineering.

The true stress-strain method provides a more accurate representation of material behaviour because it considers the actual cross-sectional area during deformation, making it sensitive to metallurgical and mechanical changes.

• Statement 1: It offers a precise measure of ductility by accounting for necking effects.
• Statement 2: True stress-strain data can be correlated with results from other tests like torsion or impact due to its fundamental nature.
• Statement 3: The method is applicable to compression tests as it focuses on deformation and load regardless of test type.
• Statement 4: Therefore, all four statements are correct.