AME 4 Q1

Correct Answer: C) Connecting Rods

• Explanation: The connecting rods in a 4-stroke diesel engine are subjected to constant cyclic stress due to the repetitive forces they experience during the engine's operation. During each engine cycle, the connecting rods undergo alternating tensile and compressive stresses as they transfer the forces between the piston and the crankshaft. Over time, this cyclic loading can lead to fatigue, which is why the connecting rods typically have a maximum recommended service life to ensure they don’t fail unexpectedly due to fatigue.

Incorrect Options:

A) Crankshaft

• Explanation: While the crankshaft does experience significant stress and cyclic loading, it is typically designed to have a longer service life compared to components like the connecting rods. The crankshaft is engineered to withstand the forces generated during engine operation, and it is less likely to be the component with the shortest recommended service life due to cyclic stress. Additionally, crankshafts are often inspected and maintained rather than replaced on a regular schedule.

B) Camshaft

• Explanation: The camshaft experiences cyclic loading as it opens and closes the engine valves, but the stress levels are generally lower compared to those in the connecting rods. Camshafts are designed for durability and usually have a long service life. They are less likely to fail due to cyclic stress compared to components like the connecting rods.

D) Cylinder Head Gasket

• Explanation: The cylinder head gasket does experience pressure and temperature fluctuations, but it is not subjected to the same kind of cyclic mechanical stress as the connecting rods. The gasket’s primary role is to maintain a seal between the cylinder head and the engine block, and while it can fail over time due to thermal cycling or pressure, it is not considered to be a component with a maximum service life specifically due to cyclic stress.

Correct Answer: A) The internal resistance of a material to forces that pull it apart, calculated as the force applied per unit area.

• Explanation: Tensile stress refers to the stress experienced by a material when it is subjected to forces that tend to stretch or elongate it. This stress is calculated as the force applied along the length of the material divided by the cross-sectional area over which the force is distributed. Tensile stress measures how much resistance the material offers to being pulled apart, which is essential in understanding the material’s behavior under tensile loads.

Incorrect Options:

B) The internal resistance of a material to forces that push or compress it together, calculated as the force applied per unit area.

• Explanation: This describes compressive stress, not tensile stress. Compressive stress occurs when forces push a material together, causing it to shorten or compress. While both tensile and compressive stresses involve force per unit area, they act in opposite directions—tensile stress pulls the material, while compressive stress pushes it.

C) The internal resistance of a material to forces that twist or slide parallel to the surface, calculated as the force applied parallel to the area divided by that area.

• Explanation: This describes shear stress, not tensile stress. Shear stress occurs when forces are applied parallel to the surface of a material, causing different layers of the material to slide against each other. Tensile stress, on the other hand, is related to forces that pull the material apart in a straight line.

D) The internal resistance of a material to forces that bend it, calculated as the force applied perpendicular to its length divided by the cross-sectional area.

• Explanation: This describes bending stress, not tensile stress. Bending stress results from forces that cause a material to bend, creating tension on one side and compression on the other. Tensile stress specifically refers to the stress due to forces pulling the material apart, rather than bending it.

Correct Answers:

B) Connecting Rods

• Explanation: The connecting rods in a diesel engine experience tensile stress during the power stroke when the combustion force pushes the piston downward. This force creates tension in the connecting rod as it pulls on the crankshaft, making it a key component subject to tensile stress.

C) Cylinder Head Bolts

• Explanation: Cylinder head bolts are under tensile stress because they are used to hold the cylinder head securely against the engine block. The force exerted by the combustion pressure and thermal expansion during engine operation pulls on these bolts, creating tensile stress as they work to maintain the seal between the head and block.

Incorrect Options:

A) Pistons

• Explanation: Pistons are primarily subject to compressive stress rather than tensile stress. During the compression and power strokes, the pistons are pushed by the force of combustion, causing compression rather than tension within the piston itself.

D) Crankshaft Bearings

• Explanation: Crankshaft bearings experience compressive and shear stresses but not tensile stress. These bearings support the crankshaft and absorb the forces from the rotating crankshaft, primarily dealing with compressive forces and shear forces due to the movement of the crankshaft.

E) Camshaft

• Explanation: The camshaft is mainly subjected to shear stress as it rotates and drives the cam followers that open and close the engine valves. It is not typically subjected to significant tensile stress, making it incorrect in this context.

Correct Answer: C) The internal resistance of a material to forces that push or compress it together, calculated as the force applied per unit area.

• Explanation: Compressive stress refers to the stress experienced by a material when it is subjected to forces that push or compress it together. This stress is calculated as the compressive force divided by the cross-sectional area over which the force is applied. Compressive stress measures how much resistance the material offers to being compressed or shortened, which is essential in understanding how materials behave under compression.

Incorrect Options:

A) The internal resistance of a material to forces that pull it apart, calculated as the force applied per unit area.

• Explanation: This describes tensile stress, not compressive stress. Tensile stress occurs when forces are applied to pull a material apart, causing it to elongate. While both tensile and compressive stresses involve force per unit area, they act in opposite directions—tensile stress pulls the material apart, whereas compressive stress pushes it together.

B) The internal resistance of a material to forces that twist or slide parallel to the surface, calculated as the force applied parallel to the area divided by that area.

• Explanation: This describes shear stress, not compressive stress. Shear stress occurs when forces are applied parallel to the surface of a material, causing different layers to slide against each other. Compressive stress, in contrast, involves forces that push the material together along its axis.

D) The internal resistance of a material to forces that bend it, calculated as the force applied perpendicular to its length divided by the cross-sectional area.

• Explanation: This describes bending stress, not compressive stress. Bending stress results from forces that cause a material to bend, creating tension on one side and compression on the other. Compressive stress specifically refers to forces that push or compress the material along its axis, rather than bending it.