failure prediction static
TRANSCRIPT
7/23/2019 Failure Prediction Static
http://slidepdf.com/reader/full/failure-prediction-static 1/16
Failure Prediction for Static Loading
Image: The Liberty Bell,a classic case of brittle fracture.
crack
7/23/2019 Failure Prediction Static
http://slidepdf.com/reader/full/failure-prediction-static 2/16
Bow portion of the Titanic.(Painting by Ken Marschall, based on photographs
taken aboard Alvin, Angus, and Argo research vessels. (Courtesey Dr. Robert D.
Ballard, The Discovery of the Titanic.)
FAILURE PREDICTION OF MACHINE COMPONENT BASED ON TYPE OF LOADING :
1. STATIC LOADING
2. CYCLIC LOADING
3. IMPACT LOADING
BASED ON SCOPE OF ITS ANALYSIS :
1. MACROMECHANIC (STRESS, STRAIN, STRAIN RATE, STRESS RATE)
2. MICROMECHANIC (FRACTURE SURFACE ANALYSIS, DISLOCATION,).
3. MESOMECHANICS
4. NANOMECHANICS
BASED ON FAILURE CRITERIA :
1. CONVENTIONAL METHOD (STATIC LOADING)– ( Stress < Strength )
2. FATIGUE FRACTURE CONCEPT(S – N diagram )
3. DAMAGE TOLERANCE APPROACH ( KIC )
( FRACTURE MECHANICS CONCEPT )
7/23/2019 Failure Prediction Static
http://slidepdf.com/reader/full/failure-prediction-static 3/16
DEFINISION OF FAILURE:Failure is a general term for a condition in which a member is
subjected to plastic deformation; in other words, whereirreversible traces are observed in a member.
CLASIFICATION OF FAILURE (BROADLY) :
1. Damage
2. Fracture
3. Break
4. Rupture.
85 – 90 % of failure are generally caused directly by fatigue.
Losses of failure are classified as :1. Direct losses.
2. Indirect losses.
7/23/2019 Failure Prediction Static
http://slidepdf.com/reader/full/failure-prediction-static 4/16
FAILURE AND LOSSES
CLASSIFICATION OF FAILURE ACCORDING TO FAILED
MEMBERS.
7/23/2019 Failure Prediction Static
http://slidepdf.com/reader/full/failure-prediction-static 5/16
CLASSIFICATION OF FAILURES ACCORDING TO CAUSE
CLASSIFICATION OF FAILURES ACCORDING TO FACTOR
7/23/2019 Failure Prediction Static
http://slidepdf.com/reader/full/failure-prediction-static 6/16
COMMONLY OBSERVED
MODES OFMECHANICAL FAILYREIN MACHINECOMPONENT
Axial Load onPlate with Hole
Rectangular plate with hole subjected to axial load. (a) Plate with
cross-sectional plane. (b) Half of plate with stress distribution.
Stress concentration is theregion in which stress raisersare present.
Kc is stress concentration factor
Stress Concentration
Stress Average
Stress Maximum ActualK c
7/23/2019 Failure Prediction Static
http://slidepdf.com/reader/full/failure-prediction-static 7/16
Stress Concentrations for Plate with Hole
Stress concentration factor for rectangular plate with central hole. (a)Axial Load. [Adapted from Collins (1981).]
Stress Concentrations for Plate with Hole (cont.)
Stress concentration factor for rectangular plate with central hole. (b)Bending. [Adapted from Collins (1981).]
7/23/2019 Failure Prediction Static
http://slidepdf.com/reader/full/failure-prediction-static 8/16
Stress Concentrations for Plate with Fillet
Stress concentration factor for rectangular plate with fillet. (a) AxialLoad. [Adapted from Collins (1981).]
Stress concentration factor for rectangular plate with fillet. (b)Bending Load. [Adapted from Collins (1981).]
Stress Concentrations for Plate with Fillet
(cont.)
7/23/2019 Failure Prediction Static
http://slidepdf.com/reader/full/failure-prediction-static 9/16
Stress Concentrations for Plate with Groove
Stress concentration factor for rectangular plate with groove. (a) AxialLoad. [Adapted from Collins (1981).]
Stress Concentrations for Plate with Groove (cont.)
Stress concentration factor for rectangular plate with groove. (b)Bending. [Adapted from Collins (1981).]
7/23/2019 Failure Prediction Static
http://slidepdf.com/reader/full/failure-prediction-static 10/16
Stress Concentrations for Bar with Fillet
Stress concentration factor for round bar with fillet. (a) Axial load. [Adapted from
Collins (1981).]
Stress concentration factor for round bar with fillet. (b) Bending. [Adapted from
Collins (1981).]
Stress Concentrations for Bar with Fillet (cont.)
7/23/2019 Failure Prediction Static
http://slidepdf.com/reader/full/failure-prediction-static 11/16
Stress Concentrations for Bar with Fillet (cont.)
Stress concentration factor for round bar with fillet. (c) Torsion. [Adapted from
Collins (1981).]
Stress Concentrations for Bar with Groove
Stress concentration factor for round bar with groove. (a) Axial load. [Adapted from
Collins (1981).]
7/23/2019 Failure Prediction Static
http://slidepdf.com/reader/full/failure-prediction-static 12/16
Stress Concentrations for Bar with Groove (cont.)
Stress concentration factor for round bar with groove. (b) Bending. [Adapted from
Collins (1981).]
Stress Concentrations for Bar with Groove (cont.)
Stress concentration factor for round bar with groove. (c) Torsion. [Adapted from
Collins (1981).]
7/23/2019 Failure Prediction Static
http://slidepdf.com/reader/full/failure-prediction-static 13/16
FLOW ANALOGY
-To reduce stress concentration as much as possible.-To observe similarity understanding the flowvelocity of fluid in a channel and the stressdistribution of an axially loaded plate.
- The analogy is accurate, since equation of flowpotential in fluid mech. and stress potential in solidmechanics are of the same form.
the volume flow is :
From solid mechanics, the force must be constant atany point :
q
udAq
dAP
Stress Contours in Bar
Bar with fillet axially loaded showing stress contours through a flatplate for (a) square corners, (b) rounded corners (c) small groove, and(d) small holes.
7/23/2019 Failure Prediction Static
http://slidepdf.com/reader/full/failure-prediction-static 14/16
7/23/2019 Failure Prediction Static
http://slidepdf.com/reader/full/failure-prediction-static 15/16
Modes of Crack Displacement
aY K nomci
Three modes of crack displacement. (a) Mode I, opening; (b) mode II,sliding; (c) mode III, tearing.
KcI = stress intensity factor
Critical stress intensity = fracture toughness (fracture resistance)
FRACTURE MECHANICS
Y = dimensionless correction factor that accounts for geometry of partcontaining crack
Nominal stress at fracture, MPaa = Half of crack length, m
nom
7/23/2019 Failure Prediction Static
http://slidepdf.com/reader/full/failure-prediction-static 16/16
FRACTURE MECHANICS
Structural studies that consider crack extension as a function ofapplied load are performed in fracture mechanics.
Fracture failure occur when stress intensity is greater that criticalstress intensity factor, as shown in equation below :
KI ≥ KIC or KII ≥ KIIC or KIII ≥ KIIIC
or combination of each mode.
The broad field of fracture mechanics
Yield Stress and Fracture Toughness Data
Yield stress and fracture toughness data for selected engineeringmaterials at room temperature [From ASM International (1989)].
Material Yield Stress , S y Fracture Toughness , K ciks i Mpa ksi in1/2 Mpa m1/2
MetalsAluminum alloy
2024-T35147 325 33 36
Aluminum alloy7075-T651
73 505 26 29
Alloy steel 4340tempered at 260°C
238 1640 45.8 50.0
Alloy steel 4340tempered at 425°C
206 1420 80.0 87.4
Titanium alloyTi-6Al-4V
130 910 40-60 44-66
CeramicsAluminum oxide 2.7-4.8 3.0-5.3Soda-lime glass 0.64-0.73 0.7-0.8Concrete 0.18-1.27 0.2-1.4
Polymers
Polymethyl methacrylate 0.9 1.0Polystyrene 0.73-1.0 0.8-1.1