Overview of Mechanical Testing

The Role of Testing

Material test data typically used for: Design/construction of new mechanical or structural

elements Control of established processes Material development Scientific knowledge (i.e., understanding how certain

factors affect materials) Others?????

Engineers must have a general understanding of: Common test methods for material properties What constitutes a valid test Applicability of data / limitations of tests

“Types” of material testing Commercial testing

Concerned mostly with checking acceptability of materials under purchase specs

Standard procedures are used Objective to determine if properties of material or part fall

within required limits Materials research testing

To obtain a new understanding of known materials Characterize properties of new materials Develop new or refine existing test standards or material

quality standards Scientific testing

Provide data to support development/verification of models, analyses, etc.

“Types” of Tests Field tests

Testing done on location (such as flight line, construction site, etc)

Typically have lower precision than laboratory tests

May better represent actual use environment

Laboratory tests Tests done in laboratory using

load frames and other specialized equipment under controlled conditions

Typically more expensive and complicated than field tests

Usually have greater precision than field tests.

Destructive Involve breaking or damaging

the sample Not applicable for finished

parts Examples, tensile testing,

hardness testing, fatigue testing

Non destructive Do not damage the sample Often used for quality control Examples, proof testing,

radiography, microhardness testing

Structural – tests done on components or structures

Coupon – tests done on small samples of material

Significance of Tests Concepts of properties/ testing of materials is oversimplified

(many assumptions) Properties measured by tests affected by test conditions, method

of testing, quality of test samples, quality of test equipment, etc. Uncertainties in data

Those associated with material properties due to manufacturing, processing

Those associated with level and type of loading and actual service/environmental conditions

Significance of test measured by precision Reliability - within lab variability Reproducibility – between lab variability

Accuracy of test = closeness to true value** a test can be precise, but not accurate **

Materials Properties Mechanical

Microstructure Sensitive Describe how a material responds

to an applied force

Physical Microstructure Insensitive Describes a materials response to

an applied field or chemical

Stress conditions

Fundamental stress conditions describe mechanical behavior features of components and assemblies: Axial tension or compression Bending, shear or torsion Internal/external pressure Stress concentrations and localized contact loads.

Tensile Loading Axial tensile loading = F/A Design such that app < failure

Where failurecan be: u (UTS) if fracture is criterion for failure

Ductile material: UTS = stress where necking occurs Brittle material: UTS = stress where material breaks

o (yield strength) if permanent deformation is criterion for failure

AF

Stiffness in Tension Elastic deformation governed by stiffness

L = L = strain L = length of bar

= t/l = Poisson’s ratio In the elastic range of deformation

= E E = elastic modulus

Can be considered a physical property because it is fundamentally related to bond strength, not affected much by microstructure

Can vary with direction if material has anisotropic structure Design of stiffness critical applications

L = FL/AE < = design limit change in length

l

l

F

Load cell Strain gages mounted on precision

machined alloy steel elements Load cell mounted such that specimen

is in direct contact with load cell or indirectly loaded through the machine crosshead, table, columns of load frame

Calibrated to provide specific voltage output signal when a certain force is detected

Can be used in tension or compression and available with variety of temperature compensation capabilities

Source: www.inston.com

Clip on extensometer Attached to test sample Measures elongation or strain as

load is applied Typically have fixed gage lengths (0.

in – 2 in, etc.) Used to measure axial strain,

available to measure transverse strain to determine reduction in width or diameter

Source: ASM Mechanical Testing and Eval Handbook

Tensile Testing - Engineering Stress Strain Curve

Tensile Testing - Engineering Stress vs. Strain Curve

Yield Point yield strength: stress at which slip becomes noticeable and significant - transition between elastic

and plastic deformation, y (lb/in2);

yield strain: strain at which slip becomes noticeable and significant, e y (in/in or %) Offset yield strength (0.2% or 0.002 common): stress at which material changes from elastic to

plastic is not always well defined, therefore define an offset yield strength Ultimate Tensile Strength (UTS): Stress at highest applied force Breaking Strength: stress at which fracture occurs Modulus: Slope of elastic portion of stress vs strain curve, E (lb/in2) Resilience:

area under stress strain curve in elastic region; indicates amount of energy a material can absorb in elastic range

Toughness: area under stress strain curve usually associated with shock or impact loadings

% Elongation = ((lf-lo)/lo) x 100%, lf = gage length at failure, lo = initial gage length

% Reduction In Area = ((Ao-Af)/Ao) x 100%, Ao= original area; Af=final area

Askeland, Phule The Science and Engineering of Materials

Tensile Testing - Failure Modes

•Ductile Failure–Cup cone fracture–Dimpled failure surface–Significant plastic deformation–Has “necked” or localized deformation region

•Brittle Failure–Flat fracture–Cleavage(radial lines) failure surface–Little to no plastic deformation–Does not have “necked” region

Compression loading

Isotropic materials uc equal to uT

Anisotropic materials uc not equal to uT

Buckling may preceed other forms of failure b = ( 2 E I)/(L2 A)

I = moment of inertia of cross section of bar

Dynamic Properties Impact Loading/Impact loading occurs if time duration is less than the

natural period of vibration of part or structure Depends on material parameters and geometric factors Design stress, s = V (Em/Al)o.5

V = velocity of mass, m A, l = cross-sectional area and length of bar E = elastic modulus

Impact tests Charpy, Izod, Hopkinson bar, Others Factors that affect data:

loading rate: faster => less energy, higher transition temperature slower => more energy, lower transition temperature

specimen size and configuration smaller energies might be required to break thicker samples notch configuration

Impact data should be used comparatively materials screening not appropriate for design data

Temperature

Transition temperature Temp at which a

material changes from ductile to brittle

BCC metals have distinct transition temp

FCC metals do not have distinct transition temp.

Abs

orbe

d E

nerg

y, f

t-lb

Test Temp, F

brittle

ductile

Impact Tests

Impact Resistance: mat’ls ability to withstand a sudden intense, blow

Toughness: provides a measure of impact resistance; ability of mat’l to absorb energy prior to failure area under true stress - true strain curve low toughness

clean break brittle material little to no plastic deformation

high toughness significant plastic deformation ductile material

Tru

e st

ress

, psi

True strain, in/in

Brittle Material

Tru

e st

ress

, psi

True strain, in/in

Ductile Material

Impact Tests -Charpy and Izod

Heavy pendulum of mass, m, is dropped from a height, ho. Pendulum swings through arc, strikes and breaks specimen, rebounds to

height of hf. Energy dissipated via elastic, plastic deformation and fracture Potential Energy difference (ft-lb or Joules) read from impact tester. Charpy:

Izod

Pendulum

Notch

Specimen(10 x 10 x 55 mm)

PendulumSpecimen

(10 x 10 x 75 mm)

Impact Tests - Izod and Charpy

Potential Energy Difference ‘ U = U1 -U2 = mg (ho-hf)

Where: ‘ U = potential energy difference m = mass of pendulum g = gravity h0 = drop height

hf = rebound height 1 ft -lb = 1.356 joules

ho

hf

1

2

Hardness Testing

Not a fundamental property Provides quick, easy, cheap indication and comparative information

regarding material's strength Used as quality control technique Widely used for steels Many different types of hardness tests

Macrohardness Brinell, Rockwell, etc. “Destructive” test

Microhardness Vickers, Knoop, etc. “Non destructive” test

Steel or tungsten carbide ball (10 mm dia) pressed against material Load of 500, 1500 or 3000 kg applied for 5 - 10 seconds Diameter of indention measured using microscope BHN = P/ (( D/2) x (D - (D2 - d2)1/2))

Where: P = applied load [kg] D = diameter of ball [mm] d = diameter of resultant penetration [mm] BHN = Brinell Hardness Number (Pa)

Advantages: measure hardness over large area, indifferent to small scale variations in

structure; simple and easy to conduct used a lot for steels and irons.

Hardness Strength Relationship (for steels using 3000 kg load): UTS (MPa) = 3.5 HB UTS (psi) = 500 HB

Macrohardness Testing - Brinell

P

d

D

Indenter pressed on surface of material with a minor load, then major load Difference in depth or penetration automatically measured => HR

Indenter Types and Scales

Superficial Hardness Rockwell test conducted using light loads Produces shallow indentions Useful for evaluating surface treatments and thin materials

Macrohardness Testing - Rockwell

P = 0 P = minor load P = major load

Rockwell TestingDisadvantages and Advantages

Limitations: not useful for mat’ls < 1/16 in thick; not useful for mat’ls with rough surfaces; not useful for non-homogeneous materials (e.g. gray cast iron) composition and structure can greatly influence results;

Advantages: provides direct hardness reading in a single step; quick, easy to use; provides for relatively small indentions that can be easily concealed

or removed via finishing.

Hardness Testing - Miscellaneous Methods

Vickers

Knoop

Sceleroscope: diamond tipped indentor or hammer enclosed in a glass tube hardness related to rebound of indentor

Mohs: scratch resistance

Durometer: measures hardness of rubbers, plastics and similar soft and elastic

materials.

Hardness Testing Precautions

Location should be at least two indenter diameters from specimen edge

Thickness: should be at least ten times the depth of penetration

Successive indentions: should be far enough apart to not allow indentions to interact

Resultant Penetration Size: should be large enough to give a representative hardness value for the bulk material

Surface Prep not critical for Brinell somewhat important for Rockwell very important for tests having small indenter size (smaller the indenter size the more

surface prep is required) polishing surface provides more accurate results.

Bending

Normal stress distribution in bending = Mb Z/I = stress in bending Mb = bending moment Z measured from neutral

axis I = moment of inertia

Bend Test For Brittle Materials

Translational mode of loading Shear stress acting on shear plane

= F / As

As = total area of shear planes F = transmitted load

Can extend shear strength of material from tension test via: o = s0 / (3)0.5

Linear shear (translational shear) affected significantly by microstructural anisotropy and can require specialized tests

Shear Loading

Stress Concentrations Irregular geometries => stress concentrations

Fillet Radii Notches Holes

Simple relation for stress concentrations max = kt a = (1 + 2a/b) a

kt = stress concentration factor a = dimensions of geometric irregularity perpendicular to hole b = dimensions of geometric irregularity parallel to hole

Small cracks perpendicular to load, a >> b Variety of kt developed through extensive

experimentation and analysis

Fracture Toughness

All materials contain flaws or defects material defects (pores, cracks inclusions) manufacturing defects (machining tool marks, arc strikes, contact damage) design defects (abrupt section changes, excessively small fillet radii, holes)

Fractures initiate at defects Defects have sharp geometries (a >> b) => high localized stresses =>

catastrophic failure Unsteady crack growth occurs when elastic energy released by growth of

defect exceeds energy required to form crack surfaces. Design equation for stable crack growth:

K = Y ( a ) 0.5 < Kc K = stress intensity factor Y = factor depending on geometry of crack relative to geometry of part = applied stress A = crack length (defect size) Kc = critical value = fracture toughness of material

Fatigue Materials under cyclic stress undergo progressive damage which

lowers resistance to fracture Fatigue failures count for 90% of all mechanical failures Fatigue caused by simultaneous action of cyclic stress, tensile

stress, plastic strain. Plastic strain resulting from cyclic stress initiates crack, tensile stress

promotes crack growth Fatigue cracks typically initiate near or at “defects” that lie on or near

the surface.

Fatigue testing Stressed based fatigue testing

Fatigue endurance limit = e: lower stress limit of S-N curve for which fracture does not occur ~ 10 7 cycles Does not exist for all materials Greatly affected by

Presence of stress risers: (small surface cracks, machining cracks, surface gouges) Operating temperature (increase in temp => drop in fatigue resistance) Environment (humidity, atmosphere, interaction with cycle frequency)

slow frequency - environment has more time to react higher frequency - environment has less time to react

Residual Stresses (compressive residual stresses => increase fatigue life) Strain based fatigue testing

Cycles to failure measured and plotted versus strain Very useful in determining conditions for initiation fatigue Used in designs where a major portion of the total life is exhausted in crack initiation phase of

fatigue. Fatigue crack growth rates are measured under conditions of cyclic stress intensity (K)

at subcritical levels (K < Kc)

Fatigue FailureFatigue Failure

Beach or clamshell markings: Formed when load is changed during service or when loading is intermittent

Striations: finer marks associated with position of crack tip after each cycle.

Creep Higher temp + app < ys => cavitation, creep elongation and

rupture of material Tensile test subjected to constant load within high temp

environment and measure elongation with time => creep curve Creep rate: rate of elongation in second stage, de/dt Time to rupture: total elapsed time

For design f is the creep rupture strength

Str

ain,

in/i

n

Time, hours

de/dt

I IIIII Fracture

eo

Creep - Microstructural Mechanism Dislocation Climb:

Movement of dislocation perpendicular to its slip plane by diffusion of atoms to or from the dislocation line

Dislocations escape from lattice imperfections, continue to slip and causes additional deformation of specimen even at low applied stress

Diffusion controlled phenomenon (therefore occurs more quickly at higher temperatures)

Arrhenius Relationship - Creep Rate creep rate = K n exp (Q c / R T)

Where: R = gas constant T = temp, K c, K, n= material constants Q = Activation energy related to

self diffusion when dislocation climb is important

Stress Corrosion Cracking (SCC)

Combination of applied stress plus corrosive environment => corrosion of part that would normally be resistant to corrosion

Stress may be result of residual stresses

Occurs for select metal environment pairs only such as High strength Al alloys in NaCl,

seawater, water vapor Cu alloys (including brass) in

ammonia, mercury salt solutions, amines, water

Low carbon steel in NaOH, Nitrate solutions, acidic Hydrogen sulfide, seawater

400 and 300 series stainless steels in various environments

etc,.

Physical Properties

Thermal Properties heat capacity thermal conductivity thermal expansion

Electrical conductivity Magnetic Response Weight Density Melting/Boiling Point Optical Properties