Basics of Material selection for mechanical design engineers

Discover how design engineers use material properties to select suitable materials for an application. Assess weight, cost, strength, toughness, deformation, hardness, creep, corrosion, and thermal, environmental, and sustainability factors.

Density explains how tightly atoms pack into a material, linking mass, volume, and weight, and higher density increases mass for same geometry, guiding lightweight design choices like aluminium over steel.

Assess how material cost depends on availability, market conditions, supply chain, and regional manufacturing infrastructure, then weigh processing and economies of scale when comparing mild steel, aluminum, and stainless steel.

Explore how ductility and malleability measure a metal's plasticity under tensile and compressive stresses. Discover how wire drawing and sheet metal fabrication reveal these properties in gold and copper.

Compare brittleness and ductility through glass, ceramics, and metals, noting brittle vs semi-ductile vs ductile fracture, and how heat treatment and carbon content affect brittleness.

Understand elastic modulus and its role in material stiffness under different loading modes. Compare Young's, shear, and bulk moduli to see how geometry and material choices influence deformation.

Learn how strength defines a material’s resistance to plastic deformation and fracture, comparing ductile and brittle behavior via the stress–strain diagram, including elastic region, yield, necking, and ultimate tensile strength.

compare tensile and compressive strengths: metals show similar values, while ceramics like glass have much higher compressive strength than tension; cracks propagate in tension but close under compression.

Understand hardness as a material’s resistance to scratching, abrasion, and localized deformation. The Brinell hardness test measures this by indentation, linking microstructure and strong bonds to performance in crushing machinery.

Examine wear in mechanical components, highlighting adhesive, abrasive, and corrosive wear as sliding contacts generate debris, surface degradation, and increased oxidation in gears, belts, and pulleys.

Quantify wear using a dimensionless constant and compare debris volumes from sliding under a normal load; PIDF shows a much lower wear constant than steel, indicating superior wear resistance.

Assess hardness and surface condition to guide wear-appropriate material selection for mechanical design engineers. Consider temperature resistance, friction heat, and lubrication viscosity and compatibility to maintain material properties under operation.

Explore resilience as the maximum elastic energy a material can absorb, illustrated by a stress-strain curve, yield point, and ultimate strength, and relate it to designing energy-storing springs.

Toughness measures a material's ability to absorb energy before fracture, visualized as the area under the stress–strain curve, with metals typically tougher than ceramics and polymers.

Explore fracture toughness, the material's resistance to crack propagation under internal flaws, cyclic and impact loading, and how a ductile crack-tip plastic zone delays failure.

Explore fatigue under cyclic loading and how S-N curves show fatigue life and endurance limits, noting steel may have infinite life below a threshold and aluminum often does not.

Factors influencing fatigue include part size and geometry, surface condition, corrosion, temperature effects, residual stresses, and stress concentrations that affect crack initiation and propagation.

Creep is the permanent deformation of a material under static load, even below the elastic limit, with stages primary, secondary, tertiary, and varying by materials like metals, elastomers, and wood.

Explore energy loss in loading-unloading cycles and hysteresis in viscoelastic materials. See how the area difference between loading and unloading curves quantifies damping and loss coefficient in vibration.

Derive the loss coefficient from energy dissipated in loading–unloading cycles by comparing the enclosed area to elastic energy, highlighting tan delta and internal damping in elastomers.

Understand thermal conductivity and heat flux, where Q = -K ΔT, comparing copper and aluminum conductors with ceramics, plastics, and fiberglass insulators in heat exchangers and engine cooling.

Explore thermal expansion and the coefficient of thermal expansion and their effects on geometry. Examine how geometry changes trigger residual stresses and gaps, with rail tracks as an example.

Learn heat capacity and specific heat capacity, defined by mass and temperature change, with the unit joule per kilogram Celsius, and see examples like utensils, heaters, heat sinks, and irons.

Explore the basics of material selection by examining temperature resistance, highlighting aluminum, steel, and ceramics, and how corrosion resistance may worsen at higher temperatures.

Corrosion resistance measures how well a material withstands oxidation and chemical reactions, protecting its surface properties. Stainless steel and aluminum offer high corrosion resistance, often enhanced by surface treatments.

explores plastics and polymers as key engineering materials, classifying them into thermosets and thermoplastics, and into amorphous and semi-crystalline varieties with distinct melting behavior and dimensional stability.

Understand how glass transition temperature governs polymer behavior, with amorphous plastics softening above Tg and semi-crystalline materials behaving differently across temperature ranges for plastics design.

Explore elastomers as amorphous polymers with rubber-like elasticity, showing viscoelastic behavior and energy loss during loading and unloading. Recognize they are often thermosetting, making processing harder; examples include natural rubber.

Iron plus carbon alloys form ferrous materials; cast iron and steel show carbon increases strength and hardness but also brittleness, with heat treatment and alloying producing over 3,500 steel grades.

Explore microstructure, the tiny crystal lattice of metals, and see how heat treatment, temperature, and cooling shape ferrite and cementite in steel, determining hardness, strength, and ductility.

Understand heat treatment parameters by analyzing crystallisation temperature and processes like annealing, hardening, tempering, and quenching, and how cooling rate affects ductility, strength, and brittleness.

Alloying elements such as carbon, chromium, nickel, and molybdenum shape steel properties, boosting tensile strength, hardness, wear and corrosion resistance, and toughness.

Shows steel's ubiquity in mechanical design through cost, toughness, and creep resistance. Illustrates carbon steel grades for cutting tools, gears, and structures, and stainless steel for chemical, food, and implants.

Explore aluminum alloys, the second most used metal after steel, covering wrought and cast varieties, annealing and hardening, and forming processes like forging, rolling, extrusion, and casting for corrosion-resistant designs.

Compare steel and aluminum to weigh cost, corrosion resistance, fatigue, wear, and weight, highlighting aluminum for lightweight automotive body and panels and design geometry to replace steel.

Explore engineering ceramics such as silicon carbide and zirconia, noting their high hardness and heat resistance, brittle failure in tension, and uses in cutting tools, bearings, and space shuttle tiles.

Explore hybrids and composites, including matrix and reinforcement, with examples like concrete, reinforced plastics, plywood, and sandwich structures, and note their application-specific design and anisotropy.

Wood shows anisotropy: it is strong parallel to the grain but weak perpendicular to it, influencing its use in furniture, houses, and bending structures.

Examine material selection using two-property plots, focusing on strength-to-weight ratios to compare materials via ashby charts and identify the best performing option for lightweight, strong designs.

Analyze the asby chart of Young's modulus versus density on a log scale, comparing ceramics, steel, aluminum alloys, elastomers, polymers, composites, and wood orientation.

Explore the strength–density trend on a logarithmic scale, noting wood's anisotropy and steel alloys' wide strength ranges, with diamond as the highest yet costly.

Compare plastics and metals by cost versus strength, from expensive plastics like Piqué and Peterffy to stainless steel, carbon steels, aluminum, titanium, CFR Okapi, and GFR for aerospace relevance.

Translate design requirements into material properties and prioritize mechanical, thermal, and chemical attributes. Match the property profile to the design and consider constraints and free variables to optimize material selection.

Learn how material indices quantify performance as a function of functional requirements, geometry, and material properties, and apply axial, bending, and torsional loading to optimize for minimum wage.

Maximize material index E over rho to design a light tie rod, using m equals rho A L and k equals A E divided by L to relate density, modulus.

Explore applying the material index on Ashby charts of modulus versus density to compare stiffness-to-weight ratios and choose materials from wood and leather to CFRP and ceramics.

Select materials for the tie rod to maximize strength and minimize weight, using strength = force/area and mass = density × volume; the Zimm index links density to strength.

Explore a material index on a log-log chart of strength versus density to rank materials by strength-to-density, showing plastics, wood, aluminum, steels, and carbon fiber reinforced plastics as options.

Select materials for a beam to minimize mass while maximizing stiffness, using the material index to optimize stiffness-to-density ratio in a center-loaded, square cross-section beam.

This lecture demonstrates using a modulus versus density chart to select materials by plotting straight lines with slope two, comparing steels, aluminum, wood, CFRP, and bamboo for beam stiffness.

derive a beam model with minimum mass and maximum strength by linking mass, cross-section geometry (height, width, aspect ratio alpha), bending stress, and moment of inertia to optimize strength-to-weight.

Apply material index on Ashby charts to guide material selection using lines with slope three by two in log strength–density, highlighting steels, aluminum alloys, titanium, magnesium, plastics, nylon, and wood.

Apply the Madrid index to design a shaft for minimum mass with optimized torsional stiffness, deriving the angle of twist from torque, length, and polar moment of inertia.

This lecture drives the material index for optimization of strength in a solid circular shaft, deriving and simplifying the relation between shear strength, outer radius, and polar moment of inertia.

This bonus lecture thanks you for taking the course and outlines key topics in mechanical design, including material selection, product development, manufacturing process selection, and design for manufacturing.