Mechanics: Nature and Types of Forces

Study Notes

Nature and Types of Forces: Forces can be pushes, pulls, or twists. They are measured in Newtons.
Concurrent Forces: Forces whose lines of action meet at a common point.
Collinear Forces: Forces acting along the same line of action.
Coplanar Forces: Forces contained within the same two-dimensional plane.
Adding Vectors: Magnitude and direction of forces can be determined graphically (draw-to-scale) or using trigonometry. The head-to-tail method is commonly used.
Moments of a Force: Torque is calculated as force multiplied by the perpendicular distance from the pivot point to the line of action of the force (Moment = Force x Distance).
Forces at Angles: Forces acting at angles to a reference axis require analysis of vertical and horizontal components.
Force Couple Systems: A pair of parallel forces with equal magnitude but opposite direction. Moment of a couple = magnitude of one force x perpendicular distance between forces (F x d).

Structure of Materials: Materials are categorized by primary bonding (e.g., ionic, covalent, metallic) and secondary bonding (e.g., intermolecular forces). Crystal structure influences the types of slip planes (bcc, fcc, cph).
Ferrous Metals: Iron-based alloys.
Main Allotropes (Iron Microstructures):
Ferrite: Soft and ductile, low carbon content.
Cementite: Hard and brittle, high carbon content.
Pearlite: Alternating layers of ferrite and cementite.
Austenite: High-temperature phase, same carbon percentage as cementite.
Martensite: Brittle, high carbon content; quenched austenitic structure.
Steel Grades: Different steel grades have varying carbon content and microstructural characteristics affecting their properties (e.g., tough, hard, machinable). For example, low carbon steel is soft, ductile, sustains cold working.
General Rule for Cast Iron: As carbon content increases, pearlite increases, ferrite decreases.

Cast Iron: Allotropes classified by silicon percentage; silicon catalyses graphite formation.
White Cast Iron: Less than 1% silicon; hard, brittle, good castability; dendrites of pearlite in cementite matrix.
Grey Cast Iron: Sufficient silicon to form graphite; soft, machinable, good castability, self-lubricating; graphite flakes in pearlite or ferrite matrix.
Malleable Cast Iron: High carbon and silicon; graphite rosettes in pearlite or ferrite matrix; heat-treated white cast iron.
Ductile/Nodular/Spheroidal Cast Iron: Graphite spheroids; high carbon, silicon, and magnesium (or cerium); soft, malleable, ductile, higher tensile strength and ductility. Graphite spheroids in pearlite or ferrite matrix.
Carbon Content and Hardness: Carbon content influences the hardness of steels. A graph demonstrates the relationship between carbon content and the percentage of ferrite, pearlite, and cementite in the steel. Eutectoid point (0.8%) is the point where ferrite and pearlite combine. Hyper-eutectoid composition suggests cementite precipitates.