CHAPTER 4

CHAPTER 4 METAL FATIGUE INTRODUCTION It has long been known that a component subjected to fluctuating stresses may fail at stress levels much lower than its monotonic fracture strength, due to a process called Fatigue. A form of failure that occur in structure subjected to dynamic and cyclic stresses. Possible failure at a stress level lower than the tensile or yield strength for a static load and after a lengthy period of repeated stress or strain cycling. It is believed that more than 95 % of all mechanical failures can be attributed to fatigue. Many engineering materials such as those used in cars, planes, turbine engines, machinery, shoes, etc are subjected constantly to repetitive stresses in the form of tension, compression, bending, vibration, thermal expansion and contraction or other stresses.

2TYPES OF FATIGUE LOADING Three types of stress cycle are :

1.Completely reversed cycle of stress

2. repeated stress cycles

3. irregular or random stress cycle

Illustrates the type of fatigue loading where a member is subjected to opposite loads alternately with a means of zero.

For example bending of steel wire continuously in either direction leads to alternate tensile and compressive stresses on its surface layers and failure fatigue.

If the applied load changes from any magnitude in one direction to the same magnitude in the opposite direction, the loading is termed completely reversed,

Reversed stress cycle

Type of fatigue loading where a member is subjected to only tension but to various degrees.

A spring subjected to repeated tension as in a toy would lead to fatigue failure.

Repeated stress cycle

This type of fatigue loading where a member could be subjected to irregular loads just as in the case of an aircraft wing subjected to wind loadsRandom stress cycleConsider a ductile material which is subjected to simple alternating tensile and compressive stressesThere are typically three stages to fatigue failure

First, a small crack is initiated or nucleates at the surface and can include scratches, pits, sharp corners due to poor design or manufacture, inclusions, grain boundaries or dislocation concentrations.

Second, the crack gradually propagates as the load continues to cycle.

Third, a sudden fracture of the material occurs when the remaining cross-section of the material is too small to support the applied load.

MECHANISM OF FATIGUE FAILURE 8 The Process of Fatigue

The Materials Science Perspective:

Cyclic slip,

Fatigue crack initiation,

Stage I fatigue crack growth,

Stage II fatigue crack growth,

Brittle fracture or ductile rupture8

Crack nucleation: During the first few cycles of loading, localized changes take place in the structure at various places within the material. These changes lead to the formation of submicroscopic cracks.Low Cycle FatigueBased on the LCF local strain philosophy, fatigue cracks initiate as a result of repeated plastic strain cycling at the locations of maximum strain concentration.

These cracks are usually formed at the surface of the specimen.There are several theories like orowans theory, cottell & hull theory etc, which explain the mechanism of crack nucleation.

10Crack growth:

The submicroscopic cracks formed grow as the cycles of loading continue and become microscopic cracks.

Crack propagation:

If a crack exits in the component before it goes unto service for example due to weld fabrication or from some other cause, the 'initiation stage is by-passed and the fatigue failure process is taken up entirely with incremental growth and final fracture.

11Mechanisms of fatigue failure

Some of the theories which explain the mechanism of crank nucleation leading to fatigue fracture are mentioned below,

Woods theory Orowans theoryCottrell and Hull theory

Fatigue failures are often easy to identify.The fracture surface near the origin is usually smooth. The surface becomes rougher as the crack increases in size.Microscopic and macroscopic examination reveal a beach mark pattern and striations. Beach mark patterns indicate that the load is changed during service or the load is intermittent. Macroscopic dimension may be observed with an unaided eye. Striations are on a much finer scale and show the position of the crack tip after each cycle. Microscopic and subject to observation with SEM/TEM

13The region of a fracture surface that formed during the crack propagation step may be characterized by two types of markings termed beachmarks and striations. Atta ul Haq GIK Institute-Fall 201314Fracture Surface in a Fatigue failure:

Macroscopic ExaminationMicroscopic Examination

Clamshell/beach markingsFiner markings-StriationsIntrusions and ExtrusionsSEM image @ high magnificationSEM image startprogressEndDislocation motion leads to PERSISTENT SLIP BANDS (PSBs)These SLIP BANDS involve INTRUSIONS and EXTRUSIONS (slip on different set of planes depending on loading cycle) Tiny Steps Stress Raisers Microcrack Initiation (along planes of high shear stress )

Schematic diagram showing Intrusions and ExtrusionsCRACK Propagation Mechanism !Atta ul Haq GIK Institute-Fall 201315

We will study Crack propagation Stages again once we get familiarize with the loading cycles16

Appearance of Failure Surfaces Caused by Various Modes of Loading (SAE Handbook)16Fatigue FailuresTypes of stresses for fatigue tests include,axial (tension compression)flexural (bending)torsional (twisting)From these tests the following data is generated.By convention, tensile stresses are positive and compression stresses are negative.

1718Constant and Variable Amplitude Stress Histories; Definition of a Stress Cycle & Stress ReversalOne cyclemmaxminStressTime0Constant amplitude stress historya)StressTime0Variable amplitude stress historyOne reversal b)

In the case of the peak stress history the important parameters are:Range stress :Stress amplitude: :Mean stress :Stress ratio : 18The most important fatigue data for engineering designs are the S-N curves, which is the Stress-Number of Cycles curves.In a fatigue test, a specimen is subjected to a cyclic stress of a certain form and amplitude and the number of cycles to failure is determined.The number of cycles, N, to failure is a function of the stress amplitude, S.A plot of S versus N is called the S-N curve.

WOHLER or S-N DIAGRAM,

S-N curve for a material that display a fatigue limit.S-N curve for a material that does not display a fatigue limit.

The S-N curves for a tool steel and an aluminum alloy showing the number of cycles to failureFigure 1

22Endurance Limit For some materials such as BCC steels and Ti alloys, the S-N curves become horizontal when the stress amplitude is decreased to a certain level.

This stress level is called the Fatigue Limit, or Endurance Limit, which is typically ~35-60% of the tensile strength for steels.

In some materials, including steels, the endurance limit is approximately half (50%) the tensile strength given by:

Fatigue life

Tells us how long a component survives at particular stress.

For example if the tool steel (Figure 1) is cyclically subjected to an applied stress at 90,000 psi the fatigue life will be 100,000 cycles.

Fatigue Strength:

For materials, which do not show a fatigue limit, i.e., the S-N curves do not become horizontal such as Al, Cu, and Mg (non-ferrous alloys), and some steels with a FCC structure,

fatigue strength is specified as the stress level at which failure will occur for a specified number of cycles, where 107 cycles is often used.

Fatigue strength is necessary for designing with aluminum and polymers which have no endurance limit.

Fatigue strenght is the stress level at which failure will occur for some specified number if cycle 24Fatigue Life Crack Growth RateTo estimate whether a crack will grow, the stress intensity factor (K), which characterizes the crack geometry and the stress amplitude can be used.Below a threshold K a crack doesnt grow.For somewhat higher stress intensities, the cracks grow slowly.For still higher stress-intensities a crack grows at a rate given by: Where C and n are empirical constants that depend on the material.When K is high, the cracks grow in a rapid and unstable manner until fracture occurs.

25The calculation of the rate of propagation of crack enable us to estimate when failure might occur when a crack is present. Fracture Mechanics for FatigueAtta ul Haq GIK Institute-Fall 201326

Fatigue LifeAtta ul Haq GIK Institute-Fall 201327

Four Point Bending Set up Cantilever loadingFatigue testing Factors affecting fatigue 1. Residual stress The mean stress level leads to a decrease in fatigue life

2. Surface finish Most cracks leading to fatigue originate at surface position, specially at stress amplification sites.

3. Environment/ corrosion If a corrosive environment is present during the cyclic stress of a metal, the chemical attack greatly accelerates the rate at crack fatigue propagates. The combination of corrosive attack and cyclic stresses on a metal is known as corrosive fatigue.

4. Design Any notch or geometrical discontinuity can act as a stress raiser and fatigue crack initiaon site,

5. Temperature

The nature of surface strongly influences fatigue strength ; surface hardness , surface roughness , residual stress. 28Stress-Corrosion FailureStress corrosion happens when a material reacts with corrosive chemicals in its environment. Two good examples, salt on the roads reacting with the steel in cars causing reduced lifetime of the cars components such as its frame and suspension system. salt in the ocean reacting with boats and their moorings where the corrosion reduces the life of the engine, which is cooled by the salt water, and the structural integrity of the boat is jeopardized if salt water sits in the hull or around the drive shaft.29Stress-Corrosion FailureStress-corrosion will cause failure of materials below their yield strength because the corrosion will cause cracks to form, usually along grain boundaries.

Usually if there is a corrosion product on the surface where a crack is inside the material.

The surface flaws themselves can be nucleation sites for crack growth.

Usually materials are coated to reduce or prevent corrosion. The automotive industry has shown excellent results by applying metal coatings (Sn) and polymer coatings on the sheet steel used on the body of cars. 30

Intergranular cracks near a stress-corrosion fracture in a metal. Note the many branches where the corrosion has eaten into the grain boundaries of the metal.If you are offered materials to be used for structural purposes that have etch pits on the surface at a reduced price, think again, as the pits are surface cracks that could extend far into the material. As a professional engineer, your career may be jeopardized by poor judgment that saved a little money in material costs but resulted in catastrophic failure during the service life of the material.31Fatigue control Consider actual stresses, including stress concentrations, rather than to nominal average stresses.

2. Visualize load transfer from one part or section to another and the distortions that occur during loading to locate points of high stress

3. Avoid adding secondary brackets, fittings, handles, steps, bosses, grooves, and openings at locations of high stress

4. Use gradual changes in section and symmetry of design to reduce secondary flexure

5. Consider location and types of joints (frequent cause of fatigue problems)

6. Use double shear joints when possible

7. Do not use rivets for carrying repeated tensile loads (bolts superior)

8. Avoid open and loosely filled holes

9. Consider fabrication methods, specify strict requirements when needed

10. Choose proper surface finishes, but not overly severe (rivet holes,welds, openings etc. may be larger drivers)

11. Provide suitable protection against corrosion

12. Avoid metallic plating with widely different properties thanunderlying material

13. Consider prestressing when feasible, to include shot peening and cold working14. Consider maintenance, to include inspections, and protection againstcorrosion, wear, abuse, overheating, and repeated overloading

15. 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)

16. Consider temperature effects.

Goodman and Soderberg Diagram Goodman Diagram

If the point representing the stress amplitude and mean stress for any given set of condition lies within the area bounded by the axes and the Goodman line the shaded area, then according to the Goodman relationship the material should not be fail by fatigue.

Fatigue Failure, when there is no Complete Stress Reversalthese diagrams are used to determine the operational parameters for a material if failure by fatigue is to be avoided.Fatigue Failure, when there is no Complete Stress Reversal

34Soderberg Diagram

Goodman and Soderberg Diagram

Again the point representing the stress amplitude and mean stress for the material must lie within the shaded area bounded by the axes and the Soderberg line if failure by fatigue is to be avoided.Surface treatment Surface polishing Polish the surface to remove stress amplification sites

Shot peening Imposing residual compressive stress within a thin outer surface layer

Case hardening By a carburizing or nitriding process whereby a component is exposed to a carbonaceous or nitrogenous atmosphere at an elevated temperature

ANY QUESTION????????