ME4255 NUS Failure Analysis
-
Upload
oliverqueen -
Category
Documents
-
view
41 -
download
0
description
Transcript of ME4255 NUS Failure Analysis
-
1
GENERAL FAILURE ANALYSIS
http://en.wikipedia.org/wiki/Failure_analysis#See_also
-
2
Non-destructive methods
General failure analysis procedures
Fracture surface detection and analysis
Mechanical Testing
-
3
NONDESTRUCTIVE TESTING
Nondestructive testing (NDT) or Nondestructive evaluation (NDE) means to perform some evaluation on a piece of material or
structure to determine on a piece of material or structure to
determine if it contains any flaw that could affect serviceability.
The evaluation process must not destroy or alter the material or structure that is being assessed.
It is used on metals, plastics, ceramics, composites, cermets, and coatings.
It is used on standard shapes of materials as they come from their manufacture (rods, billets, flats, sheets, bars, and others)
It is used to detect cracks, internal voids, surface cavities, delamination, incomplete or defective welds any type of flaw that could lead to premature failure.
-
4
COMMONLY USED NDT
TECHNIQUES
Technique Capability Limitations
Visual inspection Macroscopic
surface flaws
Small flaws are difficult to detect,
no subsurface flaws
Microscopy
(optical/electron)
Small surface
flaws
Not applicable to large structures;
no subsurface flaws
Radiography (x-
ray gamma rays)
Subsurface flaws Smallest defect detectable is 2%
of the thickness; radiation
protection needed
Dye Penetration Surface flaws No subsurface flaws, not for
porous materials
Ultrasonics Subsurface flaws Material must be good conductor
of sound
Magnetic particle Surface and near-
surface flaws
Limited subsurface capability;
only for ferromagnetic materials
-
5
COMMONLY USED NDT
TECHNIQUES
Technique Capability Limitations
Eddy current Surface and near-
surface flaws
Difficult to interpret in some
application; only for metals.
Acoustic
emission
Can analyze entire
structures
Difficult to interpret; expensive
equipment
-
6
-
7
FAILURE ANALYSIS
PROCEDURES
Collecting history and information related to the failure
Visual examination: low-magnification examination of the two fracture surfaces.
Stress analysis involved on the operations for which the components or devices were designed: some
experimental work may need to be done.
Metallographic evaluation of the material used and its heat treatment: to examine the microstructure of the
material used to manufacture the wire cutters.
Detailed fractographic examination: SEM fractography to examine the fracture surface by using Scanning
Electron Microscope (SEM).
-
8
FAILURE ANALYSIS
PROCEDURES
Chemical analysis to determine whether any corrosion failure involved.
Characterize the properties, especially, mechanical properties of the materials used, sometime, it is need a
simple experiment in which cannot destroy the samples.
Hardness test is the most common used method for such
purpose.
Simulation of failure: sometimes looking for a similar component (made of the same batch, same material, same
process etc), let it run in the real conditions as the failed
sample, however, this may very expensive.
Conclusions to be made.
-
9
HANDLING THE FRACTURE
PIECES
-
10
HANDLING THE FRACTURE
PIECES
Fracture surface contains a wealth of information, it is important to understand the types of damage that can obscure or obliterate
fracture features and obstruct interpretation.
There are two types of damage, i.e., chemical and mechanical damage. These damage can occur during or after the fracture
event.
If damage occurs during the fracture event, very little can usually be done to done to minimize it. However, proper handling and
care of fractures can minimize damage that can occur after the
fracture.
Education in the proper handling of specimens prior to any fractography examination is strongly recommended for anyone
dealing in fractures either in the field or in the laboratory.
-
11
VISUAL EXAMINATION
Schematic representation of
the information conveyed by
crack branching with regard
to the location of the crack
origin
Schematic representation of the
T-junction method of
determining which fracture
surface to search to locate the
crack origin. Because B does not
cross A but meets it at about
90, B occurred later and cannot contain the crack origin.
-
12
Intergranular cracking
-
13
IDENTIFY THE FRACTURE
ORIGIN
Locating the origin in an impact
fracture, produced by two hammer
blows, in a notched bar of 12% Cr
steel. Fracture origin can be found in
three ways: by tracing the radial marks
in the lower portion of the fracture to
their point of convergence (the arrows
on the curved lines indicate the
direction of crack propagation); by
drawing normals to the crack-arrest
fronts labeled A and B; and by
projecting the tangents to the final
radial marks at C and D toward the
bottom. The crack came to a full stop
at B with the first hammer blow and
resumed motion at the second hammer
blow. Light fractograph. 3
-
14
EXAMINE FRACTURE BY SEM
The principal categories of fracture features are:
Cleavage features
Quasicleavage features
Dimples from microvoid coalescence
Tear ridges
Fatigue striations
Separated-grain facets (intergranular fracture)
Mixed fracture features, include mixture of above features
Features of fractures resulting from chemical and thermal
environments.
-
15
EXAMINE FRACTURE BY SEM
Cleavage fracture in a notched impact specimen of hot-rolled 1040 steel broken at
-196 C (-321 F), shown at three magnifications. The specimen was tilted in the scanning electron microscope at an angle of 40 to the electron beam. The cleavage planes followed by the crack show various alignments, as influenced by
the orientations of the individual grains. Grain A, at the center in fractograph (a),
shows two sets of tongues (see arrowheads in fractograph b) as the result of local
cleavage along the {112} planes of microtwins created by plastic deformation at
the tip of the main crack on {100} planes. Grain B and many other facets show
the cleavage steps of river patterns. The junctions of the steps point in the
direction of crack propagation from grain A through grain B, at an angle of about
22 to the horizontal plane. The details of these forks are clear in fractograph (c).
-
16
EXAMINE FRACTURE BY SEM
Dimples and cleavage facets exhibited in three aspects of a Charpy impact fracture at
room temperature in a specimen of hot-rolled 1040 steel, tilted in the scanning
electron microscope at an angle of 30 to the electron beam. The machined notch of the specimen was below the region shown in (a). The overall direction of crack
propagation was upward. Although equiaxed dimples pre-dominate, certain grain
orientations near the top of (a) were unfavorable for ductile fracture by microvoid
coalescence and local cleavage occurred, as shown in detail in (b), which is a higher-
magnification view of the outlined area in (a). Fractograph (c), a higher-magnification
view of the region at the center of (a), shows a deep dimple, which initiated the local
ductile fracture immediately surrounding it. The smooth surface at A shows no river
patterns and should not be identified as a cleavage facet; it could be a grain-boundary
surface, or perhaps a region of stretching.
-
17
EXAMINE FRACTURE BY SEM
Intergranular brittle fractures in tungsten, iridium, and a tungsten-3 wt% rhenium
alloy. (a) Sintered tungsten rod drawn to 1.5 mm (0.060 in.) diam, recrystallized for
100 h at 10-6 torr and 2600 C (4712 F), and fractured in tension. (b) Iridium sheet annealed for 50 h in purified helium at 1700 C (3092 F) and broken by bending. (c) Tungsten-3 wt% rhenium alloy that was prepared in the same manner as the
sintered tungsten rod in fractograph (a). Microvoids ("bubbles") at grain boundaries
resulted from segregation of potassium (an impurity).
-
18
Materials Failure Analysis
Mechanical Physical Chemical
Static, dynamic,
strength of the
Materials, etc
Physical property
Thermo-condition
Fluid exchange
Chemistry, fluid
mechanics,
thermodynamics
English
Report Social Knowledge Engineering Ethics
-
19
-
20
-
21
Tensile test
Bending test Shear test Compressive test
-
22
Mechanical Testing
Fatigue
Creep
Friction and wear
Fracture toughness
Corrosion
etc.
-
23