MatSci Ch 6 Concepts

mechanical properties

• describes how materials respond when forces are applied.

• determine whether a material can safely carry loads, resist deformation, absorb energy, or survive harsh service conditions.

• engineering designs involve balancing properties such as strength, stiffness, toughness, hardness, and ductility

• a good understanding allows engineers to select the right material for a given application.

engineering stress

• applied force divided by the original cross-sectional area of a specimen

• intensity of loading experienced by the material rather than the total force alone

• the same force can have very different effects depending on the area over which it acts. Small areas experience higher stresses than large areas carrying the same load. Engineers use stress because it allows meaningful comparison between specimens of different sizes

engineering strain

• measures deformation by comparing the change in length to the original length of a specimen

• strain has no units because it is a ratio of two lengths

• how much a material deforms under loading. Two materials may experience the same stress but exhibit very different strains. Consequently, strain provides information about a material's flexibility and compliance

difference between stress and strain

• Stress: applied load normalized by area and represents the external demand placed on a material

  • "How hard am I pushing?"

• Strain: measures the material's response to that demand

  • "How much did the material deform?"

tensile loading

• occurs when forces act to pull a material apart

• material experiences elongation and positive strain

• Tensile testing is therefore one of the most important procedures in materials science because it reveals numerous mechanical properties simultaneously

compressive loading

• pushes material together and tends to shorten its length

• Under compression, fractures are less likely to open because applied forces tend to close cracks. Consequently, brittle materials often perform better in compression than in tension.

shear stress

• acts parallel to a surface rather than perpendicular to it

• plastic deformation at the atomic level actually occurs through shear

• dislocation motion and slip require shear stress

torsion

• twisting caused by an applied torque

• torsion generates shear stresses throughout the material

• Many engineering components such as drive shafts, drills, and rotating machinery operate under torsional loading. Failure analysis frequently requires understanding how torsional stresses are distributed within a part

tensile tests

• tensile test provides a complete picture of mechanical behavior using a single experiment

• from resulting stress-strain curve: engineers can determine stiffness, yield strength, tensile strength, ductility, toughness, and resilience

stress-strain curve

• summarizes how a material responds as load increases

• initial slope: reveals stiffness

• yielding: indicates the beginning of permanent deformation

• maximum stress: indicates tensile strength

• total area under the curve: indicates toughness

elastic deformation

• temporary and completely reversible

• atomic bonds stretch slightly but atoms do not permanently change positions

• stored elastic energy causes the atoms to return to their original spacing

• no permanent shape change remains after unloading

plastic deformation

• permanent deformation that remains after removal of the applied load

• plastic deformation occurs primarily through dislocation motion and slip

• atoms occupy new positions after unloading

• no driving force to restore the original shape completely

Young's modulus?

• measures stiffness, which is a material's resistance to elastic deformation

• slope of the elastic region of the stress-strain curve

• high elastic moduli deform very little under load

• low elastic moduli deform more easily

• controlled primarily by interatomic bond strength rather than microstructure

stiffness

• how much a material elastically deforms under loading

• a material may resist elastic deformation exceptionally well but still yield at relatively low stress

Hooke's Law

• stress is proportional to strain within the elastic region

• linear elastic behavior and indicates that deformation increases proportionally with applied load

• constant linking stress and strain is Young's modulus

• valid only until the onset of yielding.

Poisson's ratio

• quantifies this relationship of stretching/contraction in 2 directions

• how deformation is distributed throughout a material and is important in three-dimensional stress analysis

yield strength

• stress at which significant plastic deformation begins

• below YS: deformation is largely recoverable

• above YS: permanent deformation accumulates

• engineers usually want structures to remain below the yielding condition during service

0.2% offset method

• consistent definition of yielding

• practical and reproducible way to determine yield strength

tensile strength

• maximum engineering stress reached during a tensile test

• highest load-carrying capacity based on the original specimen area

• NOTE: fracture generally occurs after the tensile strength has already been reached

necking

• occurs when deformation becomes localized in a small region of the specimen

• cross-sectional area decreases rapidly in that region

• engineering stress decreases even though the actual local stress may continue increasing

• ultimately leads to fracture

fracture strength

• stress corresponding to final failure

• fracture strength may be greater than, equal to, or less than the yield strength

• helps engineers determine the ultimate limits of material performance

resilience

• measures the amount of elastic energy stored per unit volume before yielding occurs

• area under the elastic portion of the stress-strain curve

• Materials with high resilience are particularly useful in springs and energy-storage applications because they can absorb and release energy repeatedly without permanent deformation

toughness

• measures the total energy absorbed before fracture

• entire area under the stress-strain curve

• tough material

• requires both resistance to deformation and the ability to deform significantly before failure

• extremely important in applications where impact loading or sudden overloads may occur

ductility

• measures the amount of plastic deformation that occurs prior to fracture

  • %EL: percentage of plastic strain at fracture

  • %RA: percentage of area reduction at fracture surface

• local stress concentrations to be redistributed through plastic deformation

• Highly ductile materials can undergo substantial shape change before failing

• provides warning before failure and permits manufacturing operations such as forming and drawing

true stress

• uses the instantaneous area

• cross-sectional area continuously decreases during tensile loading

• valid only to the onset of necking

• more realistic description of actual material behavior, especially after yielding

true strain

• measure of deformation that accounts for the changing size of a material as it stretches or compresses

• valid only to the onset of necking

strain hardening

• occurs when a material becomes stronger during plastic deformation

• true stress increases with increasing strain during plastic deformation

• dislocation density increases dramatically as deformation progresses

• growing number of dislocations creates additional obstacles to movement

• further deformation becomes more difficult

• yield strength, tensile strength, and hardness increase, while ductility decreases

hardness

• resistence to permanently indenting the surface

• large hardness:

  • resistance to plastic deformation/cracking in compression

  • better wear properties

• Brinell Hardness Test