Fatigue, creep and corrosion

Last Updated:*
© LMS SEGi education group
Chapter Overview
Chapter 3 exposed you with the different types of metal failures.
You learned about what are the factors that cause the failures.
You also learned how to control these failures.

Last Updated:*
© LMS SEGi education group
To expose the students to different types of metal failure such as fatigue, creep and corrosion, what are the factors that cause these failures and how to control them.
Learning Objectives

Last Updated:*
© LMS SEGi education group
Learning Outcomes
At the end of the lesson, students should be able to identify and explain the types of metal failures, the contributing factors and controlling method.

Last Updated:*
© LMS SEGi education group
INTRODUCTION TO FATIGUE
Definition: The effect on metal of repeated cycles of _________.
The insidious feature of fatigue failure is that there is ______ obvious warning, a crack forms without appreciable deformation of structure making it difficult to detect the presence of growing cracks.
Fracture usually start from small nicks or scratches or fillets which cause a localised concentration of stress.
Failure can be influenced by a number of factors including size, shape and design of the component, condition of the surface or operating environment.

Last Updated:*
© LMS SEGi education group
Fatigue results in a bittle appearing fracture, with no gross deformation at the fracture.
A fatigue failure can usually be recognized from the appearance of the fracture surface which shows a smooth region, due to the rubbing action as the crack propagated through the section, and a rough region, where the member has failed in a ductile manner when the cross section was no longer able to carry the load.

Last Updated:*
© LMS SEGi education group
Three basic factors are necessary to cause fatigue failure. These are:
maximum tensile stress of sufficiently high value,
large enough variation or flucti in the applied stress, and
sufficiently large number of ryeles of the applied stress.

Last Updated:*
© LMS SEGi education group
In addition, there are a host of other variables, such as:
stress concentration,
corosion
temperature
overload,
metallurgical structure,
residual stresses, and
combined stresses, which tend to alter the conditions for fatigue.

Last Updated:*
© LMS SEGi education group
METALLURGY
The science that deals with procedures used in extracting metals from their ores, purifying and alloying metals, and creating useful objects from metals.
The study of metals and their properties in bulk and at the atomic level.

Last Updated:*
© LMS SEGi education group
Residual Stresses
Definition: Stresses that remain in material or body without application of an external load (applied force, displacement of thermal gradient).
Origin: Usually originates during manufacturing and processing of materials due to *heterogeneous plastic deformations, thermal contractions and phase transformations
*Heterogeneous: Consisting of dissimilar elements or parts; not homogeneous.

Last Updated:*
© LMS SEGi education group
Fatigue Design Guideline
(minimize stress concentrations)
Consider all types stresses, including stress concentrations, rather than to nominal average stresses.
Visualize load distribution from one part or section to another and the distortions that occur during loading to locate points of high stress
Avoid adding or attaching more brackets, fittings, handles, steps, bosses, grooves, and openings at locations of high stress
Use gradual changes changes in section and symmetry of design to reduce secondary flexure

Last Updated:*
© LMS SEGi education group
Consider location and types of joints (frequent cause of fatigue problems)
Use double shear joints when possible
Do not use rivets for carrying repeated tensile loads (bolts superior)
Avoid open and loosely filled holes
Consider standard methods,
specify strict requirements when needed
Choose proper surface arushes but not overly severe (rivet holes, welds, openings etc. may be larger drivers)

Last Updated:*
© LMS SEGi education group
Provide suitable protection against corrosion
Avoid metal piahrg plating with widely different properties than underlying material
Consider pre-stressery when feasible
Consider maintenance, to include inspection and protection against corrosion, wear, abuse, overheating, and repeated overloading
Avoid use of structures at critical or fundamental frequency of individual parts or of the structure as a whole (induces many cycles of relatively high stress)
Consider temperature effects

Last Updated:*
© LMS SEGi education group
INTRODUCTION TO CORROSION
Corrosion is chemically induced damage to a material that results in deterioration of the material and its properties.

Last Updated:*
© LMS SEGi education group
WHY METALS CORRODE?
Metals corrode because we use them in environments where they are chemically unstable .Only copper and the precious metals (gold, silver, platinum, etc.) are found in nature in their metallic state. All other metals, to include iron-the metal most commonly used-are processed from minerals or ores into metals which are inherently unstable in their environments.

Last Updated:*
© LMS SEGi education group
Design considerations
Factors that can influence corrosion:
Environment
Chemical
Natural
Storage, transit
Stress
Residual stress from fabrication
Static, variable and alternating operating stresses
Shape
Joints and flanges
Crevices and deposits
Trapped and contained liquid
Compatibility
Metals with metals
Metals with other materials
Quality control of materials
Movement
Flowing fluids
Parts moving in fluids
Two- and three-phase flow
Entrained solids
Vibration and pulsing
Temperature
Oxidation, scales and tarnishes
Heat-transfer effects
Molten deposits
Condensation and dewpoints
Control
Surface cleaning and preparation
Coatings
Cathodic protection
Inhibitors
Inspection
Planned maintenance

Last Updated:*
© LMS SEGi education group
Several factors should be considered during a failure analysis to determine the affect corrosion played in a failure. Examples are listed below:
Type of corrosion
Corrosion rate
The extent of the corrosion
Interaction between corrosion and other failure mechanisms

Last Updated:*
© LMS SEGi education group
COMMON TYPES OF CORROSION
Uniform or General Corrosion
The metal loss is uniform from the surface.
The reaction starts at the surface and proceeds uniformly.
Often combined with high-velocity fluid erosion, with or without abrasives.
Pitting Corrosion
The metal loss is randomly located on the metal surface.
The basis metal is eaten away and perforated in places in the manner of holes, the rest of the surface being affected only slightly or not at all.
Often combined with stagnant fluid or in areas with low fluid velocity.
Galvanic Corrosion
Increased corrosion in crevices or cracks or at contact surfaces between two metal articles.
Occurs when two metals with different electrode potential is connected in a corrosive electrolytic environment.
The anodic metal develops deep pits and groves in the surface.

Last Updated:*
© LMS SEGi education group
*Crevice Corrosion
Occurs at places with gaskets, bolts and lap joints where
crevice exists.
Crevice corrosion creates pits similar to pitting corrosion.
Concentration Cell Corrosion
Occurs where the surface is exposed to an electrolytic environment where the concentration of the corrosive fluid or the dissolved oxygen varies.
Often combined with stagnant fluid or in areas with low fluid velocity.
Graphitic Corrosion
Cast iron loosing iron in salt water or acids.
Leaves the graphite in place, resulting in a soft weak metal.
*Crevice: a long narrow opening [syn: crack*Crevice: a long narrow opening [syn: crack, cleft*Crevice: a long narrow opening [syn: crack, cleft, fissure*Crevice: a long narrow opening [syn: crack, cleft, fissure, scissure]

Last Updated:*
© LMS SEGi education group
Electrochemical corrosion
Four conditions must exist before electrochemical corrosion can proceed:
(1) there must be something that corrodes , the metal ___________, where the oxydation reaction takes place
(2) there must be a __________, where the reduction reaction takes place
(3) there must be continuous conductive liquid ______________ (electrolyte, usually condensate and salt or other contaminations), for example: water, seawater, condensing water, humidity..
(4) there must be a _______________ to carry the flow of electrons from the anode to the cathode.

Last Updated:*
© LMS SEGi education group
E.g. Electrochemical corrosion of iron due to contact with water

Last Updated:*
© LMS SEGi education group
This conductor is usually in the form of metal-to-metal contact such as in bolted or riveted joints.
The elimination of any one of the four conditions will remote the conditions that causes corrosion.
An unbroken (perfect) _____________ on the surface of the metal will prevent the electrolyte from connecting the cathode and anode so the current cannot flow.
Therefore, no corrosion will occur as long as the coating is unbroken.

Last Updated:*
© LMS SEGi education group
Corrosion is a normal, natural process. Corrosion can seldom be totally prevented, but it can be ____________ or controlled by proper choice of material, design, coatings, and occasionally by changing the environment.
Various types of metallic and nonmetallic coatings are regularly used to protect metal parts from corrosion.

Last Updated:*
© LMS SEGi education group
CORROSION CONTROL
There are a number of means of controlling corrosion. The choice of a means of corrosion control depends on
economics,
safety requirements, and
a number of technical considerations.
Failure to control corrosion can lead to:
Increase costs
reduced safety
Negative environmental impact

Last Updated:*
© LMS SEGi education group
CORROSION CONTROL
Protective _____________,
Can be metallic, such as the galvanized steel or they can be applied as a liquid “__________."
____________ protection,
Cathodic protection is an electrical means of corrosion control. It can be applied using sacrificial (galvanic) anodes.
Corrosion inhibitors
Corrosion inhibitors are ____________ that are added to controlled environments to reduce the corrosivity of these environments.

Last Updated:*
© LMS SEGi education group
INTRODUCTION TO CREEP
When a material is subjected to a stress that is greater than or equal to its yield stress, the material deforms plastically. When the stress is below this level, then in principle it should only deform elastically.
However, provided the _________________ is relatively _________, plastic deformation can occur even when the stress is lower than the yield stress. This deformation is time-dependent and is known as creep.

Last Updated:*
© LMS SEGi education group
CREEP OF METALS
High temperature progressive deformation of a material at constant stress is called creep. High temperature is a relative term that is dependent on the materials being evaluated.
Creep occurs under load at high temperature. Boilers, gas turbine engines, and ovens are some of the systems that have components that experience creep.

Last Updated:*
© LMS SEGi education group
For a low melting point metal like lead, creep becomes significant at about 270C, i.e. on a hot day. This may cause drainage piping made from lead to sag between its supports or lead plates to creep in a battery.
For a high melting point metal like tungsten, temperatures above 15000C would be needed to produce creep.

Last Updated:*
© LMS SEGi education group
CREEP CURVE
A typical creep curve is shown below:

Last Updated:*
© LMS SEGi education group
Effect of High Temperature on Metals:
___________ strength.
Greater atomic and dislocation mobility, assisting dislocation climb and diffusion.
New _______________ mechanisms, such as new slip systems or grain boundary sliding.
Recrystallisation and grain growth.
Age hardened alloys will ______________ by particle coarsening and lose strength.
Oxidation and intergranular penetration.