ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE MECHNICAL DEPARTMENT 5th semester DESIGN OF MACHINE ELEMENT Stress Concentration & fatigue failure PREPARED BY: Nakrani ankit.H(130030119565) Rathod himat.R(130030119588) Vadhel Hitesh j.(130030119612)

Fatigue Failure It has been recognized that a metal subjected to a repetitive or fluctuating stress will fail at a stress much lower than that required to cause failure on a single application of load. Failures occurring under conditions of dynamic loading are called fatigue failures. Fatigue failure is characterized by three stages Crack Initiation Crack Propagation Final Fracture

Jack hammer component, shows no yielding before fracture. Crack initiation site Propagation zone, striation Fracture zone MAE dept., SJSU

VW crank shaft – fatigue failure due to cyclic bending and torsional stresses Propagation zone, striations aCrack initiation site Fracture area

928 Porsche timing pulley Crack started at the fillet

Fracture Surface Characteristics Mode of fracture Typical surface characteristics Ductile Cup and Cone Dimples Dull Surface Inclusion at the bottom of the dimple Brittle Intergranular Shiny Grain Boundary cracking Brittle Transgranular Shiny Cleavage fractures Flat Fatigue Beachmarks Striations (SEM) Initiation sites Propagation zone Final fracture zone

Fatigue Failure – Type of Fluctuating Stresses max min 2 Alternating stress Mean stress m = max min 2 + MAE dept., SJSU

Fatigue Failure, S-N Curve Test specimen geometry for R.R. Moore rotating beam machine. The surface is polished in the axial direction. A constant bending load is applied. Motor Load Rotating beam machine – applies fully reverse bending stress Typical testing apparatus, pure bending

Fatigue Failure, S-N Curve Finite life Infinite life S′e = endurance limit of the specimen Se ′

Concept of Stress Concentration Theoretical stress concentration factor, Kt Maximum stress at the discontinuity Nominal stress, max stress with no discontinuity Kt is used for normal stresses and Kts for shear stresses.

Concept of Stress Concentration Consider an elliptical hole in an infinitely large plate loaded in uniform tension. Using the theory of elasticity the theoretical stress concentration factor is given by the equation below. 2b 2a Elliptical hole For a circular hole Kt = 3 The equation can be applied to a longitudinal crack where b << a

Stress Concentration Factor Stress concentration factor is found using experimental methods. Photoelasticity – a plane polarized light is passed thru a photelastic material (all transparent plastics) resulting in a colorful fringe pattern indicating the intensity of the stress.

Stress Concentration Factor Brittle Coating – a specially prepared lacquers are usually applied by spraying on the actual part. After air drying, the part is subjected to stress. A pattern of small cracks appear on the surface. Data could be used to locate strain gages for precise measurement of the stress. The method is sensitive to temperature and humidity.

Stress Concentration Factor Electrical strain gauges The method is the most popular and widely accepted for strain measurements and stress analysis. The strain gauge consists of a grid of strain-sensitive metal foil bonded to a plastic backing material. When the gauge is subjected to a mechanical deformation, its electrical resistance changes proportionally. The change in voltage is converted to strain and the stress is calculated from the strain. Ken Youssefi

Stress Concentration Factor

Thank you