Lecture 17 introducing FATIGUE FAILURE Atta ul Haq GIK Institute-Fall 20131

Just to have an overview where we are now Atta ul Haq GIK Institute-Fall 20132

Major Issues to be addressed; 1 What is FATIGUE 2 STRESS CYCLES 4 Fatigue Testing 3S-NCURVES Atta ul Haq GIK Institute-Fall 20133

What IS a FATIGUE ? 1 Atta ul Haq GIK Institute-Fall 20134

Object --> Repeated Motion--> Weakness/tiredness --> Fatigue Example: Your legs/ muscles  decrease in output due to constant running Example: Your legs/ muscles  decrease in output due to constant running Fatigue in materials Metals --> Weak/fracture due to repeated loading 90% service failure  Fatigue Examples: materials in continuous loading-unloading applications like bolts/ shafts (Brittle type fracture—why?) “The PREMATURE failure/lowering of SRENGTH of a material due to repetitive stresses which may be below or above its yield strength ” Getting Started- Fatigue basics Atta ul Haq GIK Institute-Fall Automobile Turbine blades, bridges… Aircraft

Requirements for Fatigue Failure This type of failure occurs in Metals and Polymers, While Ceramics do not undergo this type of failure!! Atta ul Haq GIK Institute-Fall Cyclic loading Tensile stresses Plastic strain

FATIGUE FAILURE Fracture Surface in a Fatigue failure: STAGES OF FATIGUE FAILURE STAGES OF FATIGUE FAILURE Two Main fracture ZONES  Smooth Surface  Crystalline Surface 1)CRACK initiation 2)Crack Propagation 3)Catastrophic rupture Atta ul Haq GIK Institute-Fall 20137

Striation High magnification SEM Image Fatigue in Engine Crankshaft Atta ul Haq GIK Institute-Fall Fracture Surface in a Fatigue failure:

Atta ul Haq GIK Institute-Fall Fracture Surface in a Fatigue failure: Macroscopic Examination Microscopic Examination Clamshell/beach markings Finer markings-Striations Intrusions and Extrusions SEM high magnification SEM image start progress End Dislocation 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 Extrusions

CRACK Initiation and Propagation !  Crack formation and propagation is a Micromechanism in fatigue  Fatigue crack initiation is a surface phenomenon  SLIP BANDS Formation (Persistent Slip Bands)  Crack initiates Atta ul Haq GIK Institute-Fall

CRACK Propagation Mechanism ! Atta ul Haq GIK Institute-Fall Cyclic LoadCrack Initiation (microcrack) Crack Propagation Stage ICrack Propagation Stage IIFracture We will study Crack propagation Stages again once we get familiarize with the loading cycles

2 STRESS CYCLES Atta ul Haq GIK Institute-Fall

Atta ul Haq GIK Institute-Fall Types Of Fluctuating Stresses Reversed Stress cycleRepeated Stress cycle Random/irregular stress cycles

Stress Cycles in Fatigue Atta ul Haq GIK Institute-Fall Mean Stress: Amplitude Stress: Range: Radius Ratio: Amplitude Ratio:

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CRACK Propagation Mechanism ! Atta ul Haq GIK Institute-Fall Cyclic LoadCrack Initiation (microcrack) Crack Propagation Stage ICrack Propagation Stage IIFracture

Atta ul Haq GIK Institute-Fall Let us consider three samples with same initial crack length subjected to different repeated cycles. Please note that material is also same; Higher Crack Growth rate at higher magnitude of applied stresses and vice versa Materials life to Fracture is now highly dependent on magnitude of applied stress and final crack length (which is related to materials structural property (resistant to deformation etc..)

Fracture Mechanics for Fatigue Atta ul Haq GIK Institute-Fall Fatigue Life

3 S-N Curves Wöhler curve/SN curves - A graph showing stress as a function of number of cycles in fatigue. Atta ul Haq GIK Institute-Fall

Presenting Fatigue Data Atta ul Haq GIK Institute-Fall

Fatigue Limit/Endurance Limit: “The stress below which a material will not fail by fatigue” Non-ferrous ferrous Atta ul Haq GIK Institute-Fall

Fatigue Life: The number of cycles permitted at a particular stress before a material fails by fatigue. The stress required to cause failure by fatigue in a given number of cycles. Fatigue Strength: Atta ul Haq GIK Institute-Fall

4 Fatigue Testing Atta ul Haq GIK Institute-Fall

Atta ul Haq GIK Institute-Fall Four Point Bending Set up Cantilever loading

Modified Goodman Relation a = e (1 – m / u ) Soderberg Relation Gerber Relation Failure Criteria can also be established by plotting fatigue test data Predicts safer regions for fatigue a = e (1 – m / u ) 2 a = e (1 – m / y ) Atta ul Haq GIK Institute-Fall

Miner’s Law and Cumulative Damage Remaining cycles permissible at S 2 N 1 = lifetime at S 1 n 1 = Number of cycles applied till S 1 Fatigue life consumed at S 1 Atta ul Haq GIK Institute-Fall Fatigue Failure will occur when

Factors Effecting Fatigue  Loading and Loading Cycles  Surface Conditions  Design Consideration  Nature of Environment Atta ul Haq GIK Institute-Fall The nature of Surface strongly influence fatigue strength: -Surface Hardness -Surface Roughness -Residual Stresses

--Method 1: shot peening put surface into compression shot --Method 2: carburizing C-rich gas 2. Remove stress concentrators. bad better 1. Surface Treatments, induce compressive stressess. How to Improve Fatigue Performance Atta ul Haq GIK Institute-Fall

Atta ul Haq GIK Institute-Fall Fatigue in Polymers Fatigue behaviour and SN curves similar to that of metals except the crack initiation: There is no Concept of Persistent Slip Bands (PSBs) in Polymers, rather crack is initiated with CRAZING phenomenon.

QUIZ-5/6 ON THURSDAY, 8 TH MLH Atta ul Haq GIK Institute-Fall