How to Eliminate Mechanical, Vibrational Loosening, Clampload & Corrosion Failures In Your Products
Mechanical Failures
-
Upload
pinchi877012 -
Category
Documents
-
view
231 -
download
0
Transcript of Mechanical Failures
-
8/13/2019 Mechanical Failures
1/65
Mechanical Behavior,
Properties and Testing ofMaterials
Umair Bin Asim
-
8/13/2019 Mechanical Failures
2/65
Overview
Mechanical application of material is to keep
the things that are supposed to be stationary,
stand their ground; Static loading, and thethings that are supposed to be moving to
move only in the intended fashion, i.e. not
more not less; Dynamic loading
-
8/13/2019 Mechanical Failures
3/65
Overview
What does standing there ground implies
Bearing the intended (sometime even more)
static loads Keep on bearing loads when its value is
constantly changing; Fatigue loading
Enduring extreme conditions, loads, elevated
temperature and extended time periods;Creep
Bearing the load even when it was suddenly
applied; Impact loading
-
8/13/2019 Mechanical Failures
4/65
Overview
Even when the material is subjected tobattering in the form of key holes, holes forfasteners, sudden changes in cross sections;Stress concentration
And also to face some adverse effects likecorrosion
-
8/13/2019 Mechanical Failures
5/65
Static Loading
Tension Test
Strength
Ductility
Toughness
Elastic Modulus
Test Specimen
Dog-bone shape
ASTM Standard E8
Original Gauge length lo Cross-sectional area Ao
-
8/13/2019 Mechanical Failures
6/65
Tension
Stress-strain curves
Linear elastic: elongation in the specimen that
is proportional to the applied load. Engineering stress: the ratio of the applied
load P, to the original cross-sectional area, Ao,
of the specimen.
Engineering stress equation: = P/Ao
Engineering strain equation: e = (l-lo)/lo
-
8/13/2019 Mechanical Failures
7/65
Tension
Yield Stress: the stress at which
permanent (plastic) deformation occurs.
Permanent (plastic) deformation: stressand strain are no longer proportional.
Ultimate tensile strength (UTS): the
maximum engineering stress.
-
8/13/2019 Mechanical Failures
8/65
Tension
If the specimen is
loaded beyond its
UTS it begins to
neck.
Fracture stress: the
engineering stress at
fracture.
-
8/13/2019 Mechanical Failures
9/65
Tension
Modulus of elasticity: ratio of stress to
strain in the elastic region.
Modulus of elasticity equation: E = /e This linear relationship is known as
Hookes Law.
Poisons Ratio: the ratio of the lateral strainto the longitudinal strain.
-
8/13/2019 Mechanical Failures
10/65
Ductility
Ductility: extent of plastic deformation that
the material undergoes before fracture.
Two measures of ductility: Total elongation: (lf-lo)/lox 100%
Reduction of Area: (Ao-Af)/Aox 100%
-
8/13/2019 Mechanical Failures
11/65
True-Stress and True-
Strain
True-stress: ratio of the load, P, to the
instantaneous cross-sectional area, A, of
the specimen. True-strain: the sum of all the
instantaneous engineering strains.
True-stress equation: = P/A True-strain equation: e = ln(l/lo)
-
8/13/2019 Mechanical Failures
12/65
Construction of Stress-
Strain Curves
-
8/13/2019 Mechanical Failures
13/65
Temperature Effects
As temperatureincreases: Ductility and toughness
increase.
Yield stress and themodulus of elasticitydecrease.
-
8/13/2019 Mechanical Failures
14/65
Other Stress-Strain
Characteristics
Compression, Torsion, and Bending
Hardness, Toughness, and Strength
-
8/13/2019 Mechanical Failures
15/65
Compression Test
Simplistically, compression
testing is the opposite of
tensile testing
Load tends to squeeze orcompact the specimen
Metals and many plastics, are
more efficient at resisting
tensile loads. Therefore, theyare more commonly tested
using tensile loading
-
8/13/2019 Mechanical Failures
16/65
Disk Test
The cylindrical specimenssurface begins to bulge, knownas barreling.
Compression test developed for
brittle materials such asceramics and glass.
Tensile stress from this test canbe calculated with the followingequation: = 2P/dt P is load
at fracture, d is diameter of disk,t is thickness.
-
8/13/2019 Mechanical Failures
17/65
Torsion Test
In addition to tension and compression, a work-piecemay be subjected to shear strains.
Punching holes in sheet metal. Metal cutting.
Torsion test used for determination of properties inshear.Usually performed on a thin tubular specimen.
Shear stress can be calculated with formula: T/2r2t T is torque, r is average radius of tube, t is thickness of tube.
Shear strain is calculated with formula: r/l r is radius of tube, is angle of twist in radians, and l is length of
tube.
-
8/13/2019 Mechanical Failures
18/65
Torsion Test
The ratio of the shear stress to the shear
strain in the elastic range is known as the
shear modulusor modulus of rigidity. The angle of twist, , to fracture in the
torsion of solid round bars and elevated
temp can help estimate forge-ability of
metals.
-
8/13/2019 Mechanical Failures
19/65
Bending
Preparing specimens from brittle materials,
such as ceramics and carbides, is difficult
because of problems in shaping andmachining them to certain dimensions.
The most common test for brittle materials
is the bendor flexure test.
-
8/13/2019 Mechanical Failures
20/65
Bend / Flexure Test
Rectangular specimen
supported at its ends.
Load is applied vertically at 1 or
2 pts. The stress at fracture in
bending is known as the
modulus of rupture, flexural
strength, or transverserupture strength.
-
8/13/2019 Mechanical Failures
21/65
Hardness
Commonly used property which gives indication
of resistance to scratch and wear of a
material/specimen.
Widely used
Relatively inexpensive and simple
Non-destructive
Other mechanical properties can be estimated Hardness is not a fundamental property because
indentation depends on shape of indenter and
load applied.
-
8/13/2019 Mechanical Failures
22/65
Brinell Test
J. A. Brinell 1900
Involves pressing a steelor carbide ball of 10mmagainst a surface withvarious loads. 500, 1500, or 3000 kg
Measures diameter ofindentation.
Harder surfaces havesmall indentation whilesofter surfaces havelarger indentation.
-
8/13/2019 Mechanical Failures
23/65
Rockwell Test
S. P. Rockwell 1922
Test measures depth
rather than diameter of
indentation. Diamond indenter
presses against surface
with minor load and then
major load. The difference in depths ofpenetration is a measure of
the hardness of material.
-
8/13/2019 Mechanical Failures
24/65
Vickers Test
Developed in 1922.
Comparable to Brinell
Test except using a
pyramid shaped
diamond to make
indentation.
Lighter loads thanBrinell Test
From 1 to 120 kg
-
8/13/2019 Mechanical Failures
25/65
Knoop Test
Developed in 1939.
Comparable to Brinell and Vickers test.
Uses an elongated pyramid shaped diamond to make
indentations. Uses very light loads. From 25 g to 5 kg.
Known as a micro-hardnesstest because of the lightsloads.
Suitable for very small or very thin specimens.
Test also used for measuring the hardness of individualgrains and components in a metal alloy.
-
8/13/2019 Mechanical Failures
26/65
Fatigue
FatigueComponents in manufacturing
equipment are subjected to fluctuating
cyclic (periodic) loads and static loads.
Cyclic Stresson wings, landing gears
Thermal Stress -- exhaust nozzle
Both stresses may cause part failure at stresslevels below normal static stress loading
-
8/13/2019 Mechanical Failures
27/65
Fatigue
Fatigue Failure -- Failure
associated with every
stress cycle, propagated
through the material until
critical crack is reached
and material fractures.
Fatigue Testing --
Various stresses, tension
then bending to a
maximum load limit (total
failure.)
-
8/13/2019 Mechanical Failures
28/65
Fatigue
S-N Curves
Stress Amplitude (S) --
Maximum stress
specimen is subjected Number of Cycles (N)
Level of stress a
material tolerates
decreases with an
increase in cycles.
-
8/13/2019 Mechanical Failures
29/65
Fatigue
Endurance (Fatigue Limit) -- Maximumstress material may be subjected withoutfatigue failure.
Aluminum Alloys and similar materials exhibitan indefinite endurance limit.
Fatigue strength is specified at a certainnumber of cycles (10^7.)
Carbon Steels have a proportional endurancelimit and tensile strength, usually 0.4 to 0.5.
-
8/13/2019 Mechanical Failures
30/65
Fatigue
Improving Fatigue Strength -- Fatigue life is
influenced greatly by the method of preparation
to the surfaces of the part or specimen.
Fatigue Strength of manufactured products may beimproved overall by
Inducing compressive residual stresses on
surfaces
Shot Peening or Roller Burnishing
Case hardening (surface hardening) by various
means
-
8/13/2019 Mechanical Failures
31/65
Fatigue
Fatigue Strength of manufactured products
may be improved overall by
Providing a fine surface finish and thereby reducing
the effects of notches and other surfaceimperfections.
Selecting appropriate materials and ensuring that
they are free from significant amounts of inclusions,
voids, and impurities.
-
8/13/2019 Mechanical Failures
32/65
Creep
Permanent elongation of a component under a
static load maintained for a period of time.
Grain-Boundary Sliding -- Mechanism of creep
at an elevated temperature in metals.
In high-temperature applications, gas-turbine blades,
jet engines, and rocket motors.
-
8/13/2019 Mechanical Failures
33/65
Creep
Creep Testing --
Subjecting a specimen to
a constant tensile load
(engineering stress) at a
certain temperature,
measuring the length
changes at various time
increments.
Primary, secondary, andtertiary stages
-
8/13/2019 Mechanical Failures
34/65
Creep
Rupture (Creep Rupture) -- Failure bynecking and fractures
Creep rate increases with specimen
temperature and the applied load. Secondary Linear ranges and slopes aid to
determine reliable design.
A higher melting point generally is related toan increase in creep resistance. Stainless Steels, Super-alloys and Refractory
metals and alloys
-
8/13/2019 Mechanical Failures
35/65
Impact
Testing consists of
placing a notched
specimen in an impact
tester and breaking it with
a swinging pendulum.
Impact or Dynamic
Loading
CharpyTest -- Specimen
supported at both ends.
Izod Test -- Specimen
supported at one end.
-
8/13/2019 Mechanical Failures
36/65
Impact
Impact Toughness -- The energy
dissipated in breaking the specimen may
be obtained from the amount of swing in
the pendulum.
Useful in determining the ductile-brittle
transition temperature of materials.
High Impact ResistanceHigh StrengthHighDuctilityHigh Toughness
-
8/13/2019 Mechanical Failures
37/65
Impact
Notch Sensitivity --
Sensitivities to surface
defects, lowers impact
toughness.
-
8/13/2019 Mechanical Failures
38/65
Mechanical & Aerospace Engr., SJSU
Discontinuity in
Cross Section
Stepped shafts
Discontinuity
Discontinuity
-
8/13/2019 Mechanical Failures
39/65
Bracket with screw holes
High
stress
Discontinuity in
Cross Section
-
8/13/2019 Mechanical Failures
40/65
Infrared analysis reveals the points of
greatest stress on a Renault parts surface
caused by weld points and helps estimate
fatigue limits for those critical areas
(bottom).
Discontinuity in
Cross Section
-
8/13/2019 Mechanical Failures
41/65
Bolt failure at the junction of
the thread and solid section.
Location of higher stresses.
Discontinuity
Discontinuity in
Cross Section
-
8/13/2019 Mechanical Failures
42/65
Maximum stress at the discontinuity
Nominal stress, max stress
with no discontinuity
Ktis used for normal
stresses and Ktsfor
shear stresses.
Discontinuity in
Cross Section
-
8/13/2019 Mechanical Failures
43/65
Stress concentration factor is found using experimental methods.
Photoelasticitya plane polarized light is passed thru aphotelastic material (all transparent plastics) resulting in a
colorful fringe pattern indicating the intensity of the stress.
Discontinuity in
Cross Section
-
8/13/2019 Mechanical Failures
44/65
Brittle Coatinga speciallyprepared lacquers are usually
applied by spraying on the actualpart. After air drying, the part is
subjected to stress. A pattern of
small cracks appear on the surface.
Data could be used to locate strain
gages for precise measurement of
the stress. The method is sensitiveto temperature and humidity.
Discontinuity in
Cross Section
-
8/13/2019 Mechanical Failures
45/65
The CAD model is subdivided into many small pieces of simple
shapes called elements.
FEA program writes the equations governing the behavior of each
element taking into consideration its connectivity to other elements.
These equations relate the unknowns, for example displacements instress analysis, to known material properties, restraints and loads.
The program assembles the
equations into a large set of
simultaneous algebraic
equations (thousands or evenmillions).
These equations are then
solved by the program to
obtain the stress distribution
for the entire model.
Discontinuity in
Cross Section
-
8/13/2019 Mechanical Failures
46/65
Stress Concentration
FactorRound shafts in axial Tension
-
8/13/2019 Mechanical Failures
47/65
Round section
with a fillet
Bending
Torsion
Discontinuity in
Cross Section
-
8/13/2019 Mechanical Failures
48/65
Ken Youssefi Mechanical & Aerospace Engr., SJSU
Flat plate in Tension
with a hole
Flat plate in Bending
with a hole
Discontinuity in
Cross Section
-
8/13/2019 Mechanical Failures
49/65
Failure and Fracture of
Material
Failure -- One of the most important
aspects of material behavior. It directly
influences the selection of a material for a
particular application, the methods of
manufacturing, and the service life of the
component.
-
8/13/2019 Mechanical Failures
50/65
Failure and Fracture of
Material
In selecting and processing
materials
Fracture -- Either internal or
external. Sub-classified intoDuctile or Brittle.
Ductile materials - extensive
plastic deformation and energy
absorption (toughness) beforefracture
Brittle materials - little plastic
deformation and low energy
absorption before fracture
-
8/13/2019 Mechanical Failures
51/65
Failure and Fracture of
Materials
Ductile Fracture -- Plastic deformation
proceeds failure.
Highly ductile materials neck down to a point
before failing.
Most metals and alloys will neck down to a
finite area and then fail.
Generally ductile fractures take place alongplanes which shear stress is a maximum.
-
8/13/2019 Mechanical Failures
52/65
Failure and Fracture in
Materials
Ductile Fracture -- Plastic deformationproceeds failure.
Close examination of ductile fracture surface
shows a fibrous pattern with dimples. Failure is initiated with formation of tiny voids which
grow and coalesce, developing micro-cracksleading to fracture.
In tension-test, fracture begins at the center ofthe necked region as a result of the growthand coalescences of cavities.
-
8/13/2019 Mechanical Failures
53/65
Failure and Fracture in
Material
Cup-and-Cone Fracture -- Due to appearance,the fracture surface of a tension-test specimen.
-
8/13/2019 Mechanical Failures
54/65
Failure and Fracture
in Material
Crack propagation is fast
Propagates nearly
perpendicular to direction ofapplied stress
Often propagates by
cleavage - breaking of
atomic bonds along specificcrystallographic planes
No appreciable plastic
deformation
Brittle fracture in a mild steel
-
8/13/2019 Mechanical Failures
55/65
Failure and Fracture in
Material
Defects -- Scratches, flaws, and pre-existing
external or internal cracks.
The high tensile stresses subject the tip of the crack to
propagate the crack rapidly due to the materialsinability to dissipate energy.
Catastrophic failure occurs under tensile stresses
when compared to their strength in compression.
-
8/13/2019 Mechanical Failures
56/65
Failure and Fracture in
Material
Fatigue Failure -- Minute external or internal
cracks develop at pre-existing flaws or defects in
the material. The cracks propagate over a
period of time and leads to total and sudden
failure of the part.
-
8/13/2019 Mechanical Failures
57/65
Failure and Fatigue in
Material
Beach Marks -- Term given the fracture surface
in fatigue.
Striations -- On the fracture surface, several
appearing on each beach mark.
-
8/13/2019 Mechanical Failures
58/65
Corrosion Resistance
Corrosion
Typically used to
describe metal or
ceramic deterioration Similar phenomena
occur in plastics
Often referred to as
degradation
-
8/13/2019 Mechanical Failures
59/65
Corrosion Resistance
Types of corrosion
Pitting
Intergranular
Crevice
Galvanic cell
Stress-corrosion cracking
Selective Leaching
Oxidation
Passivation
-
8/13/2019 Mechanical Failures
60/65
Corrosion Resistance
Pitting
Can occur over the
entire surface or be
localized
Intergranular
Occurs along grain
boundaries
-
8/13/2019 Mechanical Failures
61/65
Corrosion Resistance
Crevice Occurs at the interface of
bolted or riveted joints
Galvanic cell Occurs between dissimilar
metals when an electrolyte
is present Not as common in pure
metals or single-phasealloys
-
8/13/2019 Mechanical Failures
62/65
Corrosion Resistance
Stress-corrosioncracking Cold worked metals
are most susceptible
Selective leaching Occurs when
metalworking fluidattacks specificelements in tool anddie materials
-
8/13/2019 Mechanical Failures
63/65
Corrosion Resistance
Oxidation
A chemical reaction which leaves a small layerof oxidized material on the surface
Resists further corrosion Aluminum & Titanium
Passivation
The development of a protective film bychemical reaction Stainless Steel
-
8/13/2019 Mechanical Failures
64/65
http://www.hardwaresquare.com/category/hardware/nails/
http://www.boeing.com/companyoffices/gallery/images/space/delta_i
v/d4_1st_heavy_24.html
http://en.wikipedia.org/wiki/Density
http://www.nyu.edu/pages/mathmol/textbook/density.html TEXTBOOK
http://www.american-carbide.com/EndMills/DHEM.aspx
http://www.roymech.co.uk/Useful_Tables/Rivets.html
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/conins.html http://en.wikipedia.org/wiki/Oxidation
http://www.hardwaresquare.com/category/hardware/nails/http://www.boeing.com/companyoffices/gallery/images/space/delta_iv/d4_1st_heavy_24.htmlhttp://www.boeing.com/companyoffices/gallery/images/space/delta_iv/d4_1st_heavy_24.htmlhttp://en.wikipedia.org/wiki/Densityhttp://www.nyu.edu/pages/mathmol/textbook/density.htmlhttp://www.american-carbide.com/EndMills/DHEM.aspxhttp://www.roymech.co.uk/Useful_Tables/Rivets.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/electric/conins.htmlhttp://en.wikipedia.org/wiki/Oxidationhttp://en.wikipedia.org/wiki/Oxidationhttp://hyperphysics.phy-astr.gsu.edu/hbase/electric/conins.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/electric/conins.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/electric/conins.htmlhttp://www.roymech.co.uk/Useful_Tables/Rivets.htmlhttp://www.american-carbide.com/EndMills/DHEM.aspxhttp://www.american-carbide.com/EndMills/DHEM.aspxhttp://www.american-carbide.com/EndMills/DHEM.aspxhttp://www.nyu.edu/pages/mathmol/textbook/density.htmlhttp://en.wikipedia.org/wiki/Densityhttp://www.boeing.com/companyoffices/gallery/images/space/delta_iv/d4_1st_heavy_24.htmlhttp://www.boeing.com/companyoffices/gallery/images/space/delta_iv/d4_1st_heavy_24.htmlhttp://www.hardwaresquare.com/category/hardware/nails/ -
8/13/2019 Mechanical Failures
65/65
http://www.materialsengineer.com/G-Pitting-Corrosion.htm
http://www.corrosion-doctors.org/Forms/intergranular.htm
http://www.textronfasteningsystems.com/appsol/galv_corr.html
http://www.materialsengineer.com/G-Pitting-Corrosion.htmhttp://www.corrosion-doctors.org/Forms/intergranular.htmhttp://www.textronfasteningsystems.com/appsol/galv_corr.htmlhttp://www.textronfasteningsystems.com/appsol/galv_corr.htmlhttp://www.corrosion-doctors.org/Forms/intergranular.htmhttp://www.corrosion-doctors.org/Forms/intergranular.htmhttp://www.corrosion-doctors.org/Forms/intergranular.htmhttp://www.materialsengineer.com/G-Pitting-Corrosion.htmhttp://www.materialsengineer.com/G-Pitting-Corrosion.htmhttp://www.materialsengineer.com/G-Pitting-Corrosion.htmhttp://www.materialsengineer.com/G-Pitting-Corrosion.htmhttp://www.materialsengineer.com/G-Pitting-Corrosion.htm