Basic Marine Engineering – Unit 1 – Strength of materials
In the study of strength of materials, it is important to understand key concepts such as mass, weight, force, and stress. This article provides an overview of these concepts and their relevance in the field of strength of materials.
Mass (M) and Weight (W):
Mass refers to the amount of matter in an object and remains constant regardless of its position on the Earth’s surface. It is measured using a lever balance or physical balance by comparing it with a standard mass. On the other hand, weight is the force exerted by the Earth on an object due to gravitational pull. Weight varies with the distance of the object from the Earth’s center and its position on the Earth’s surface. It is measured using a spring balance or spring gauge. For example, moving a wooden object and a solid steel object of the same volume in space would require different forces to initiate motion due to the difference in their masses.
Force refers to an action applied to an object that can cause it to move, change its current motion, or alter its shape. It can be a push (compressive force) or a pull (tensile force). Forces can be exerted on objects in various scenarios, such as pushing or pulling a desk or applying force to prevent a football from entering a goal. The unit of force is the newton (N), and it is calculated by multiplying the mass (M) by the acceleration due to gravity (A).
Load and Force Vector:
A load represents the collective forces acting on an object. A force vector is a graphical representation of a force, typically depicted as an arrow indicating the direction, magnitude, and point of application of the force. It is essential in determining the object’s motion or the counter-force acting on a stationary object. There are five fundamental forces that can act on an object: tensile force, compressive force, bending force, shear force, and torsional force. Each force type exerts a specific effect on the material.
Stress and Strain:
Stress refers to the force applied to a specific cross-sectional area of an object. In the case of a bar subjected to axial loading or tension, stress is calculated by dividing the applied force by the cross-sectional area. The stress-strain curve depicts the relationship between stress and strain in a material. The linear-elastic region of the curve represents the range where stress and strain increase linearly. Hook’s Law applies in this region, where the ratio of stress to strain is constant. The slope of the linear-elastic region is known as the modulus of elasticity or Young’s modulus, measured in pascals (Pa).
Yield Strength and Ultimate Tensile Strength:
The yield strength is the point on the stress-strain curve where the relationship deviates from the linear-elastic region, indicating some permanent (plastic) deformation in the material. It represents the stress required to produce a small amount of plastic deformation. The ultimate tensile strength is the maximum stress a material can withstand without breaking. It is the highest point on the stress-strain curve, where the line momentarily flattens.
Breaking strength refers to the stress at which fracture or failure occurs in a material. It represents the maximum stress applied during a tension test, where the material reaches its breaking point.
Understanding these concepts is vital in analyzing the behavior of materials under different forces and loads. By studying the strength of materials, engineers and designers can ensure the structural integrity and reliability of various structures and components.