IS:800 Section 13 FATIGUE

Introduction Mechanism of Fatigue Fracture Factors Affecting Fatigue Strength Design Strength & Cumulative Fatigue Damage IS:800 Design Provisions Prudent Design Practices Summary FATIGUE DESIGN OF STEEL OFF-SHORE PLATFORMS

INTRODUCTION Modes of Failure of Steel Structures Yielding or excessive plastic deformation. Elastic or inelastic buckling or instability. Fatigue fracture due to repeated cyclic loading. Brittle fracture.

Definition of Fatigue Fracture Process of progressive localised permanent structural change occurring in a material subjected to conditions, which produce fluctuating stresses and strains at some point or points and which may culminate in cracks or complete fracture after a sufficient number of fluctuations - ASTM E206-62T INTRODUCTION

MECHANISM OF FATIGUE FRACTURE ‘a’ Crack Size ‘N’ Number of Cycles (3) (2) (1) Stress Range=  1 Stress Range =  2 a cr, plastic III IV Fig. 1 Fatigue Crack Growth II I a cr, elasto-plastic a cr, elastic (1) Sub-critical range (2) Stable crack propagation (3) Unstable Crack Propagation Effect of: I Stress Range II Quality Control III Toughness Elastic – Elastoplastic IV Toughness Elastoplastic - Plastic

FACTORS INFLEUNCING FATIGE STRENGTH Material Toughness Stress Range  max -  min Number of Cycles Life Stress Concentration Increases stress range Residual Stress Compressive Corrosive Environment D ecreases life Frequency of Loading Negligible effect Member Size Only in small spec.

FATIGUE BEHAVIOUR OF STRUCTURAL DETAILS Structural Member –Tension, welding, cover plate, splices, stiffeers Riveted Connections –Stress concentration, steel strength, details, rivet tension Bolted Connections –Bearing/ friction, prying effect, eccentricity Welded Connections –In the weld metal –In the line of fusion –In the heat affected zone –At the toe edge of the weld.

DESIGN STRENGTH Minimum Stress in Cycle Maximum Stress in cycle 0 + _ R = 1 R = 0 R = -1 n1n1 n2n2 Fig. 3 Modified Goodman Diagram Log S Log. N Fig.2 S-N Curve Endurance limit

9 SECTION 13 FATIGUE - Contents 13.1 General 13.2 Definitions 13.3 Design Reference Design Conditions Design Spectrum Stress Evaluation Design Stress Spectrum Low Fatigue Partial Safety Factors Partial Safety Factor for Actions and their effects(  mft =1.0 ) Partial Safety Factor for Fatigue Strength 13.4 Detail Category 13.5 Fatigue Strength 13.6 Fatigue Assessment 13.7 Necessity for Fatigue Assessment

IS:800 Section 13 FATIGUE V. Kalyanaraman Indian Institute of Technology Madras 10 SECTION 13 FATIGUE S-N Curve for normal Stress (Continued)

11 SECTION 13 FATIGUE S-N CURVE FOR SHEAR STRESS (Continued)

IS:800 Section 13 FATIGUE V. Kalyanaraman Indian Institute of Technology Madras 12 DETAIL CATOGORY CLASSIFICATION (2) (3) (3)(3) (2)(2) (1)(1) (1) (7) (9)(9) (8)(8) (8) (9)

13 DETAIL CATOGORY CLASSIFICATION (10) (11) (12) (16) (17) (18) 83 (20) (21) (19) 66 (22) 1:4 < Taper  1:2.5 (14) 59

14 Assumptions Redundant load path Stress range evaluated by conventional method Load cycles not highly irregular Detail amenable for inspection Welded plate thickness < 25 mm

15 Normal Stresses n<5x10**6 n>5X10**6 Shear Stresses FATIGUE STRENGTH

16 DESIGN STRENGTH

17 Inspection and Access Consequence of failure Fail-safe Non-fail- safe Periodic inspection and maintenance, accessibility to detail is good Periodic inspection and maintenance, poor accessibility for detail Partial Safety Fators

18 MODIFICATION FACTORS Weld in plates thicker than 25 mm  t = (25/t p ) 0.25  1.0

CUMULATIVE FATIGUE DAMAGE Variable Repeated Loading Minors Hypothesis time Load

20 VARIABLE STRESS RANGE Normal Stress Range Shear Stress Range

Prudent Design Practices in Fatigue Avoid details that produce severe stress concentrations or poor stress distribution. Provide gradual transitions in sections and avoid reentrant notch like corners. Avoid abrupt changes of section or stiffness of members or components. Align points so as to eliminate eccentricities or reduce them to a minimum.

Avoid making attachments on parts subjected to severe fatigue loadings. If attachments in such locations are unavoidable, the weld profile should merge smoothly into the parent metal. Use continuous rather than intermittent welds. Avoid details that introduce localized constraint. Prudent Design Practices in Fatigue

During fabrication provide suitable inspection to guarantee proper riveting, adequate clamping of high-strength bolts, and proper deposition of welds. Provide for suitable inspection during the fabrication and erection of structures. Intersection of welds should be avoided. Prudent Design Practices in Fatigue

Edge preparation for butt-welding should be designed with a view of using minimum weld metal so as to minimize warping and residual stress build up. Ask for pre and post heating, if necessary to relieve the build-up of residual stresses. Fillet welds carrying longitudinal shear should not be larger in size than necessary from design considerations. Prudent Design Practices in Fatigue

Deep penetration fillet welds should be used in preference to normal fillet welds. Structures subjected to fatigue loading, especially critical locations in such structures, should be regularly inspected for the presence of fatigue cracks and when such cracks are discovered, immediate steps to prevent them further propagation into the structure should be taken. Prudent Design Practices in Fatigue

Any repair measures taken should be designed to avoid introduction of more severe fatigue condition. Provide multiple load path and/or structural redundancy in the structure to avoid overall collapse of the structure due to failure of one element in the structure in fatigue. Provide crack arresting features in the design at critical locations to avoid propagation of cracks into the entire member. Prudent Design Practices in Fatigue

FITNESS FOR SERVICE Weld Defect Acceptance Criteria Effect of defect on stress concentration FEA of weld defects & Fracture Tests Critical Defects Under cut, centre line notch, overlap, convexity, concavity Lack of weld penetration

SUMMARY Mechanism of Fatigue Fracture Design Under Fatigue loading Inspection, Rehabilitation, Maintenance Acceptance Criteria

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