chapter 2 (part 3)
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Chapter 2 (Part 3): Failure Analysis and Prevention
& Material Selection for Gas Systems
BKG3493 GAS SYSTEM MATERIALS &
COMPONENTS
CHAPTER OUTCOMES
At the end of this part, you should be able to:
i) Explain the other material failure properties such impact energy, fracture toughness, and fatique.
ii) Explain the common techniques of nondestructive testing, instruments being used, and the other nondestructive tests.
iii) Give the example of failure modes typically occur in engineering design
iv) Highlight general criteria in selecting material for gas systems and components.
FAILURE OF MATERIALS
At room temperature, metal alloys and polymers stressed beyond their elastic limit eventually fracture following a period of plastic deformation. Brittle ceramics and glasses typically break following elastic deformation, without plastic deformation. The inherent brittleness of ceramics and glasses combined with their common use at high temperatures make thermal shock a major concern. With continuously service at relatively high temperatures, any engineering material can fracture when creep deformation reaches its limit.
We will look at additional ways in which materials fail!!
IMPACT ENERGY
Corresponds to the measurement of toughness, or area under the stress-versus-strain curve. The energy necessary to fracture a standard test piece under an impact load, is a similar analog of toughness. The most common laboratory measurement of impact energy is the Charpy test. For polymers, the impact energy is typically measured by the Izod test. In general, we expect alloys with large values of both strength (Y.S and T.S) and ductility to have large-impact facture energies.
Impact test
The Charpy impact test, also known as the Charpy v-notch test, is a standardized high strain-rate test which determines the amount of energy absorbed by a material during fracture.
This absorbed energy is a measure of a given material's toughness and acts as a tool to study temperature-dependent brittle-ductile transition.
If the material breaks on a flat plane, the fracture was brittle, and if the material breaks with jagged edges or shear lips, then the fracture was ductile.
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final height initial height
Adapted from Fig. 8.12(b), Callister 7e. (Fig. 8.12(b) is adapted from
H.W. Hayden, W.G. Moffatt, and J. Wulff, The Structure and Properties
of Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc.
(1965) p. 13.)
(Charpy) Impact testing
CHARPY TEST
IMPACT TEST (CHARPY) DATA FOR SOME ALLOYS
Alloy Impact energy (J)
1040 carbon steel 180
8630 low alloy steel 55
Monel 400 (nickel alloy) 298
IMPACT TEST (IZOD) DATA FOR SOME POLYMERS
Polymer Impact Energy (J)
PE (high density) 1.4-16
PE (low density) 22
Polyamides (nylon 66) 1.4
Very Ductile
Moderately Ductile Brittle Fracture
behavior:
Large Moderate
%EL
Small
• Ductile fracture is
usually more desirable
than brittle fracture!
Ductile:
Warning before
fracture
Brittle:
No
warning
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• Ductile failure:
-- one piece
-- large deformation
Figures from V.J. Colangelo and F.A.
Heiser, Analysis of Metallurgical Failures
(2nd ed.), Fig. 4.1(a) and (b), p. 66 John
Wiley and Sons, Inc., 1987. Used with
permission.
Example: Pipe Failures
• Brittle failure:
-- many pieces
-- small deformations
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• Evolution to failure:
• Resulting
fracture
surfaces
(steel)
50 mm
particles
serve as void
nucleation
sites.
50 mm
From V.J. Colangelo and F.A. Heiser,
Analysis of Metallurgical Failures (2nd
ed.), Fig. 11.28, p. 294, John Wiley and
Sons, Inc., 1987. (Orig. source: P.
Thornton, J. Mater. Sci., Vol. 6, 1971, pp.
347-56.)
100 mm
Fracture surface of tire cord wire
loaded in tension. Courtesy of F.
Roehrig, CC Technologies, Dublin,
OH. Used with permission.
Moderately Ductile Failure
necking
s
void nucleation
void growth and linkage
shearing at surface
fracture
FRACTURE TOUGHNESS
The term fracture mechanics has come to mean the general analysis of failure of structural materials with preexisting flaws- has brought the focus of much active research.
One of the parameter of fracture mechanics – fracture toughness.
Fracture toughness, KIC, is the critical value of the stress intensity factor at a crack tip necessary to produce catastrophic failure under simple uniaxial loading
A simple example- blow a balloon containing a small pinhole. When the internal pressure of the balloon reaches a critical value, catastrophic failure originates at the pinhole (the balloon pops)
FATIGUE
So far, we have characterized the mechanical behavior of metals under a single load application either slowly (tensile test) or rapidly (impact test). Many structural applications involve cyclic rather than static loading, and a special problem arises. Fatigue – is the general phenomenon of material failure after several cycles of loading to a stress level below the ultimate tensile stress.
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Fatigue
• Fatigue = failure under cyclic stress.
• Stress varies with time.
-- key parameters are S, sm, and
frequency
s max
s min
s
time
s m S
• Key points: Fatigue...
-- can cause part failure, even though smax < sc.
-- causes ~ 90% of mechanical engineering failures.
Adapted from Fig. 9.24,
Callister & Rethwisch 3e.
(Fig. 9.24 is from Materials
Science in Engineering, 4/E
by Carl. A. Keyser, Pearson
Education, Inc., Upper
Saddle River, NJ.) tension on bottom
compression on top
counter motor
flex coupling
specimen
bearing bearing
NONDESTRUCTIVE TESTING (NDT)
Method applied for the detection of surface breaking defects.
Is the evaluation of engineering materials without impairing their usefulness.
A central focus of many of the nondestructive testing is the identification of potentially critical flaws, such as surface and internal cracks.
For fracture mechanics, nondestructive testing can serve to analyze an existing failure or it can be used to prevent future failures.
The dominant techniques- x-radiography and ultrasonics
NONDESTRUCTIVE TESTING (NDT)
The types of defect / flaw and degradation that can be detected using NDT are summarized as: Planar defects - these include flaws such as fatigue cracks, lack of side-wall fusion in welds, environmental assisted cracking such as hydrogen cracking and stress corrosion cracks; cold shuts in castings etc; Laminations - these include flaws such as rolling and forging laminations, laminar inclusions and de-laminations in composites; Voids and inclusions - these include flaws such as voids, slag and porosity in welds and voids in castings and forgings; Wall thinning - through life wall loss due to corrosion and erosion
Testing of Quality of Weld Joints.
NONDESTRUCTIVE TESTING (NDT)
X-RADIOGRAPHY
Radiography is the detection of material loss by the variation in applied radiation, g or x-ray, passing through a component and impinging on a film.
Sensitive to material loss is better suited to the detection of volumetric defects such as slag or porosity
Defects are identified by abrupt changes in the density of the developed film: the film density is related to the exposure it has received from the radiation.
The quality and sensitivity of a radiograph are measured by the density of the film and the use of an IQI (Image quality indicators)
X-RADIOGRAPHY
ULTRASONIC TESTING
Use of high frequency sound waves in a similar manner to sonar or radar: sound pulses are reflected from interfaces or discontinuities.
In thickness checking the reflections from the wall surfaces are measured.
In defect detection reflections from cracks, voids and inclusions are detected and assessed.
The transfer of sound from the ultrasonic probe to the component requires a coupling medium, which is usually water or gel.
The condition of the interface determines how much sound is transferred into the component, how much is scattered and how much noise is produced.
A typical ultrasonic source involving a piezoelectric transducer
ULTRASONIC TESTING
OTHER NDT
Eddy-current testing Magnetic-particle testing Liquid-penetrant testing A coustic-emission testing
FAILURE ANALYSIS & PREVENTION
The related issue of failure prevention is equally important for avoiding future disasters.
Ethical and legal issues are moving the field of material science and engineering into a central role in the broader topic of engineering design.
A wide spectrum of failure modes has been identified: 1) Ductile fracture 2) Brittle fracture 3) Fatigue fracture 4) Corrosion-fatigue failure 5) Stress-corrosion cracking (SCC) 6) Liquid-erosion failure 7)Liquid-metal embrittlement 8) Hydrogen embrittlement 9) Creep and stress-rapture failures, and etc
EXAMPLE OF FAILURE MODES
Ductile fracture- Is observed in a large number of the failures occurring in metals due to overload (i.e., taking a material beyond the elastic limit and subsequently to fracture)
Brittle fracture – is characterized by rapid crack propagation without significant plastic deformation on macroscopic scale
Fatigue failure- by a mechanism of slow crack growth gives the distinctive “clamshell” fatigue-fracture surface
Corrosion-fatigue failure – is due to the combined actions of a cyclic stress and a corrosive environment. In general, the fatigue strength of the metal will be decreased in the presence of an aggressive, chemical environment.
Stress-corrosion cracking (SCC)- is another combined mechanical and chemical failure mechanism in which a non-cyclic tensile stress (below the Y.S) leads to the initiation and propagation of fracture in a relatively mild chemical environment. Can be intergranular, transgranular, or a combination of the two.
EXAMPLE OF FAILURE MODES Liquid-erosion failure – a special form of wear damage in which a liquid is responsible for the removal of material. Its damage typically results in a pitted or honeycomb-like surface region.
Liquid-metal embrittlement – involves the material losing some degree of ductility or fracturing below its yield stress in conjunction with its surface being wetted by a lower-melting-point liquid metal.
Hydrogen embrittlement - the notorious form of catastrophic failure in high-strength steels. A few parts per million of hydrogen dissolved in these materials can produce substantial internal pressure, leading to fine, hairline cracks and loss of ductility.
Creep and stress-rapture failures - occur near room temperature for many polymers and certain low-melting-point metals, such as lead, but may occur above 1000oC in many ceramics and certain high-melting-point metals.
MATERIAL SELECTION
Design
Service conditions
Function
Cost
Processing
Equipment selection
Influence on properties
Cost
Materials
Properties
Availibility
Cost
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MATERIAL SELECTION FOR GAS SYSTEMS AND COMPONENTS
Factors to be considered: Ability to accommodate/withstand
• Chemical reaction and soil type • Animal activity •Microorganism
Engineering aspect
• Corrosion control • Lifetime/lifespan
Cost involve
• Material cost/pipe cost – pipe cost depend on pipe thickness and size • Installation cost – labor cost, welder and their assistance, etc