Mechanical Properties of Materials

Mechanical Properties Intro

• Mechanical properties rely on a material's composition and microstructure.
• Composition, bonding, crystal structure, and defects influence the strength and ductility of metallic materials.
• Lower temperatures can cause brittleness in metals and plastics, as seen in the 1986 Challenger accident due to O-ring failure.
• The 2003 space shuttle Columbia was lost due to debris impact on ceramic tiles and failure of carbon-carbon composites.
• The special chemistry of the steel used on the Titanic contributed to the failure of the ship's hull due to low temperatures and stress.
• Weak rivets and design flaws were other contributing factors in the Titanic disaster.
• The chapter introduces basic concepts like hardness, stress, strain, elastic and plastic deformation, viscoelasticity, and strain rate.
• Testing procedures used by engineers to evaluate mechanical properties are reviewed. Real-world applications are used to illustrate these concepts.

Technological Significance

• Today's technologies strongly depend on the mechanical properties of the materials used.
• Aluminum alloys or carbon-reinforced composites in aircrafts must be lightweight, strong, and able to withstand cyclic mechanical loading.
• Steels used for buildings and bridges require enough strength for safety.
• Plastics used for pipes, valves, and flooring must possess adequate mechanical strength.
• Materials for prosthetic heart valves, like pyrolytic graphite or cobalt chromium tungsten alloys, have to be durable.
• Sports equipment performance depends on strength, weight, and the ability to withstand impact loading.
• Mechanical properties are critical in load-bearing applications.
• Material properties play an important role even when the primary function of the material is electrical, magnetic, optical, or biological.
• Optical fibers need strength to withstand stress, while biocompatible titanium alloys for bone implants must endure in the human body.
• Scratch-resistant coatings on lenses must withstand abrasion. Aluminum alloys or glass-ceramic substrates for hard drives need mechanical strength.
• Electronic packages must endure stress and heat during operation. Nanotechnology devices need mechanical robustness. Float glass needs shatter resistance.
• Components made from plastics, metals, and ceramics need adequate toughness and strength at various temperatures.
• Material selection requires matching mechanical properties to design specifications and service conditions, including the need for strength, stiffness, or ductility.
• Materials need to withstand high stress, sudden force, high temperatures, cyclic stresses, and corrosive conditions.
• Preliminary material selection is possible with databases. It's important to understand how properties are obtained and their meanings.
• Idealized tests may not exactly apply to real applications, so properties may vary based on microstructure.
• Temperature changes, stress cycles, chemical changes (oxidation, corrosion, erosion), and defects have a large effect.
• The mechanical properties of materials enable processing into useful shapes.

Terminology for Mechanical Properties

• Materials processing requires understanding mechanical properties at different temperatures and loading conditions.
• The term stress refers to the force acting per unit area.
• Tensile, compressive, and shear stresses are different types of stresses.
• Strain is the change in dimension per unit length.
• Stress is typically measured in psi (pounds per square inch) or Pa (Pascals).
• Strain is dimensionless, expressed as in./in. or cm/cm.
• Tensile and compressive stresses are normal stresses that arise when the applied force acts perpendicular to the area of interest.
• Tension causes elongation, while compression causes shortening.
• Shear stress arises when the applied force acts parallel to the area of interest.
• Many load-bearing applications deal with tensile or compressive stresses.
• Shear stresses are often encountered in polymer extrusion.
• Elastic strain is fully recoverable strain resulting from an applied stress.
• "Elastic" strain develops instantaneously, remains while stress is applied, and recovers when force is withdrawn.
• A material under elastic strain returns to its original shape after stress is removed.
• Elastic stress and strain are linearly related in many materials.
• Young's modulus (E) or modulus of elasticity is the slope of a tensile stress-strain curve in the linear regime.
• E is measured in pounds per square inch (psi) or Pascals (Pa).
• Large elastic deformations occur in elastomers, where the elastic strain and stress relationship is nonlinear.
• Shear modulus (G) is defined as the slope of the linear part of the shear stress-shear strain curve.
• Plastic strain is permanent deformation in a material i.e. it does not go back to its original shape when stress is removed.
• Strain rate is the rate at which strain develops in a material and measured in s⁻¹.
• The rate at which a material is deformed is important from a mechanical properties perspective.
• Many materials considered ductile behave as brittle solids when strain rates are high.
• Impact loading refers to loading materials at high strain rates.
• Viscous Material : Strain develops over time, doesn't return to its original shape after stress removal. The strain is plastic.
• Viscoelastic Material : Behavior between viscous and elastic material, a qualitative description is shown in Figure 6-2.
• Stress relaxation refers to the level of stress decreases over time in viscoelastic solids held under constant strain.
• Understanding of the mechanical properties is required for molten materials, liquids, & dispersions, such as paints or gels.
• Newtonian material: where the relationship between the applied shear stress and the shear strain rate (y) is linear.
• Viscosity is the slope of the shear stress versus the steady-state shear strain rate curve
• Viscosity is measured in Pa.s (SI), Poise (P) or g/cm•s (cgs)
• centipoise (cP) is sometimes used, 1 cP = 10-2 P
• The following relationship defines viscosity: τ = ηγ
• Kinematic viscosity (v) is defined as v = η/ρ, measured in Stokes (St) or equivalently cm²/s.
• centiStokes (cSt) is sometimes used i.e. 1 cSt = 10-2 St.
• Non-Newtonian Materials: the relationship between shear stress and shear strain rate is nonlinear.
• The relationships between the shear stress and shear strain rate are described as τ = ηγm
• Shear thinning materials: (or pseudoplastic) where the apparent viscosity decreases with increasing shear strain rate.
• Example of shear thinning material is paint sitting in storage which is very viscous.
• When brushed, the paint subject to a high shear strain rate is quite the less viscous
• Bingham plastics can be modeled as follows: τ = G·y (when tis less than ty.s) or τ = Ty.s + ηj (when τ ≥ ty.s)
• Thixotropic materials: contain a network that breaks under shear, and reforms slowly when shearing stops.
• They show time-dependent viscosity/deformation.
• Rheopectic Materials: Materials that increase viscosity over time.
• The rheological properties of materials are determined using instruments known as a viscometer or a rheometer.

The Tensile Test: Use of the Stress-Strain Diagram

• The tensile test assesses a material's resistance to a static or slowly applied force. The strain rates in a tensile test are small.
• The general test setup is a specimen has a diameter of 0.505 in. and a gage length of 2 in
• The specimen is placed in the testing machine and a force F, called the load, is applied.
• A strain gage or extensometer measures how much the specimen stretches between gage marks with an applied force.
• the change in length of the specimen (Al) is measured with respect to the original length (lo).
• Information on strength, Young's modulus, and ductility of a material can be found.
• Tensile tests are conducted on metals, alloys, and plastics but also on ceramics
• however, these are often fragile for tensile
• Figures 6-6 contains typical stress-strain curves
• Metal : Metallic and thermoplastic materials show an initial elastic region followed by a non-linear plastic region.
• Thermoplastic : The temperature of the plastic material is assumed to be above its glass-transition temperature (Tg).
• Elastomers: a large portion of the deformation is elastic and nonlinear since the behavior of these materials is different from other polymeric materials.
• Ceramics : show only a linear elastic region and almost no plastic deformation at room temperature.
• recorded data includes load or force as a function of change in length
• Data is converted to stress and strain that are then subsequently converted into stress and strain. This enables material extraction
• Engineering stress and engineering strain of defined by the following equations:
  • Engineering stress = S = F/Ao
  • Engineering strain = e = ΔI/lo

Properties Obtained from Tensile Test

• Yield Strength: Critical stress needed to initiate plastic deformation, usually for dislocation motion in metals, or polymer disentanglement.
• The transition from elastic deformation to plastic flow is abrupt in some materials, known as the yield point phenomenon.
• Stress value drops from upper yield point (S2) to an average value that is called the lower yield point (S1).
• Tensile strength (SUTS) obtained at the highest applied force. In ductile materials, deformation doesn't remain uniform rather local decrease in the cross-sectional area called necking.

Properties

• The slope of the stress-strain curve in the elastic region gives the Young's modulus (E) or modulus of elasticity.
• The stiffness of an object is proportional to its Young's modulus.
• Steel deforms elastically 0.001 in./in. while elastically deforms aluminium three time more.

Poisson's Ratio

• Poisson's ratio (v) relates longitudinal to lateral elastic deformation. It Is typically about 0.3.
• The modulus of resilience (E₁), the area under the elastic portion of a stress-strain curve, is the elastic energy that a material absorbs during loading.

Tensile Toughness

• the energy absorbed by a material prior to fracture that is measured as the area under the true stress-strain curve (work of fracture).

Ductility

• Ductility is the ability of a material to be permanently deformed without breaking.
• Percent elongation quantifies plastic deformation at failure, while percent reduction in area describes thinning by the specimen.
• Yield strength, tensile strength, and modulus of elasticity decrease as temperature increases, while ductility commonly increases.

True Stress and True Strain

• True Stress

  • σ=F/A (where A is the instantaneous area over which the force Fis applied)

• True Strain

  • ε = ln (lo/A) ( l is the instantaneous

• Comparison of engineering and true stress values can be compared

Bend Test For Brittle Materials

• In ductile metallic materials, the engineering stress-strain curve goes through a maximum. Failure occurs at a lower engineering stress.
• Brittle materials: In many brittle materials, a normal tensile test cannot be peformed.
• This is since the testing machine causes cracking often
• Bend Test [Figure 6.16 in the study notes) By loading a a 3 point set-up leads to brittle fracture.

Hardness of Materials

• Hardness test measures the resistance to penetration of surface of material by a hard object
• Brinell Hardness Test: HB= kg/mm^2

##Nanoindentation:

• Hardness testing performed at the nanometer length scale.
• Nanoindenter tips come in varying shapes. A common shape is known as the Berkovich indenter, which is a three-sided pyramid.