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Stress Intensity Factor Fracture Mechanics Linear Elastic Fracture Mechanics II Linear Elastic Fracture Mechanics II Presented by Calvin M. Stewart, PhD MECH 5390-6390 Fall 2020 Outline • Definition • Fracture Modes • Useful Expressions • Finite Width • Superposition • Determination of SIF Definition Stress Intensity Factor and Fracture Toughness • Definition: The Stress Intensity Factor (SIF) is a fracture mechanics parameter used to characterize the strength of the singularity at the crack tip, denoted by K. • The subscripts I,II, or III correspond to the opening, sliding, and tearing modes of fracture, respectively. • Definition: The Critical Fracture Toughness is a cracked material’s ability to resist fracture, denoted by Kc. It is considered a material property and commonly used in design. • Units are ksi in or MPA m ksi in=1.099 MPA m LEFM Inequality • We can apply these two parameters to make decision concerning the fracture resistance of a structure. • If this inequality is true, Fracture has occurred Stress Intensity Factor (SIF) Equation Critical Fracture Toughness Property KKIc Equation. Function of Material Property Function of • Loading Conditions Environment • Body Geometry Surface Roughness • Crack Geometry Microstructural Mechanisms Fracture Modes Stress Intensity Factor • All stresses in the vicinity of the crack tip can be divided into three basic types, each associated with a local mode of crack surface displacement. • KI (Opening Mode) • KII (In-plane shear) • KIII (Out-of-plane shear • / tearing) • Mode 1 – Opening Fracture MOdes • Mode 2 – In-Plane Shear • Mode 3 – Out-of-Plane Shear / Tearing Useful Expressions Cartesian and Principal Stresses; Elastic Displacement; Crack Flank Displacement; Mode II; Mode III Useful Expressions • Let us remembers Westergaards solution for an infinite width plate under equi-biaxial loading. • Cartesian Notation (x,y,z) KI 3 x =−cos 1 sin sin 2 r 2 2 2 KI 3 y =+cos 1 sin sin 2 r 2 2 2 KI 3 xy = sin cos cos 2 r 2 2 2 Principal Stresses • The stress field equations can be expressed in principal stresses by apply the transformation equation from Mohr’s circle. • Principal Stresses Principal Directions For Plane Stress 3 = 0 For Plane Strain Elastic Displacement • The elastic displacement field around a crack can be derived from the elastic stress field using the two-dimensional form of Hooke’s law, valid for either a plane stress or a plane strain condition For Plane Stress For Plane Strain • Where u and v are the displacement in the x and y directions respectively. Crack Flank Displacement • Another useful expression is the Crack Flank Displacement, v which is the displacement at any position along the crack flank. For Plane Stress For Plane Strain Stress Field Equations for Modes II and III • The stress field equations for mode II and mode lII loading may be obtained in a similar manner to Westergaard’s solution for mode I. Finite Width Finite Width Crack Bodies • Closed form solutions (like those found for infinite boundary problems) are either • Difficulty or impossible to achieve for finite boundary problems • As the crack length, a increase or width, W decreases the outer boundaries have an increased influence on the crack tip Finite Width Crack Bodies • Free boundary affects the crack tip stress distribution • Most Solutions to finite boundary problems are expressed in the general form K= C a f a ( W ) • C=fitting constant • f=dimensionless geometric fractor Center Cracked Plate in Tension (CCT) • The earliest attempt to obtain a solution to a finite-body problem is attributed to Irwin (1957) for the infinite plate in tension with co- linear cracks of equal length and spacing Center Cracked Plate in Tension (CCT) • The Westergaard stress function is proposed as Zz( ) = sin2 ( a ) 1− W sin2 z ( W ) • noting, d22 Z d = =ReZ + y Im Z rr dx22 dx Center Cracked Plate in Tension (CCT) • At the crack tip A, the stress intensity factor can be expressed as x sin 2W K=− 2 ( x a) sin22xa− sin ( 22WW) ( ) • After manipulation Irwin solution, 2Wa A Ka= tan aW2 • As a/W approaches 1 • K approaches infinite Finite Width Centre Crack Specimen • The solution for a Finite Width Centre Cracked specimen can be distilled to Wa Ka= tan aW • where note, the width is now described as W instead of 2W. Center Cracked Plate in Tension (CCT) • The formulation was later improved with a polynomial expression Brown Solution 23 a a a Ka=1 + 0.256 − 1.152 + 12.200 WWW • Or by Fedderson Solution, a Ka= sec W • Or by Isidia Solution, 24 a a a Ka= sec 1 − 0.025 + 0.06 WWW Edge Notched Specimen The single and double edge notched specimens. Edge Notched Problems • The slight increase in the S.I.F. of both cases can be compared to the CCT case. • The Closing effect is absent in the edge crack case, More Restraint • The Edge crack opens more because it is less restrained. • A through crack must be longer (a increased) to achieve the same opening displacement Less Restraint • Solutions to S.I.F.’s for more complex geometries are determined experimentally, numerically, and analytically or with mixed methods. Edge Notched Specimen • Approximate Solution • For longer cracks, the finite geometry results in stress enhancement. Correction factors have taken both the free edge effect and finite geometry effect into account. Edge Notched Specimen • Double edge notched specimen Superposition Superposition • The total stress field due to two or more different mode I loading systems can be obtained by an algebraic summation of the respective stress intensity factors. This is called the superposition principle. • Only valid for combinations of the same mode of loading, i.e. all mode I, all mode II or all mode III. • By using the superposition principle the stress intensity factor for a number of seemingly complicated problems can be readily obtained. Through Crack under Internal Pressure Subsisting, P=-σ in case D gives the required result, P(πa)^1/2 −a Semi-Elliptical Surface Crack in Cylindrical Pressure Vessel • Hoop Stress • Stress due to internal pressure on crack surfaces. • Superposition Determination of SIF Determination of SIF • Often, when applying fracture mechanics you will find that there is no standard solution for your problem of interest. • For example, a cracked turbine blade. • Unique geometry, Load conditions, and location of the crack results in the stress fields being difficult to derive. https://amses-journal.springeropen.com/articles/10.1186/s40323-016-0083-7 Determination of SIF • Recommended Approach 1. Literature Review - Check the Literature for a direct solution (i.e. analytical solution). 2. Analytical Solution - If no solution is directly available, assess the effort needed to solve the problem analytically. This effort depends on the seriousness of the problem, the desired accuracy, and how many times the solution will be useful. If worthwhile, derive the analytical solution. A strong mathematical background is required. 3. Computational Solution – If deriving the analytical solution is not worthwhile and/or the problem is too complex to obtain a closed-form solution, applying computational tools to approximate the for your problem. 4. Experimental Solution – Alternatively, a more straightforward approach is to perform a series of fracture/fatigue experiments with different initial crack lengths. Analytical Methods • Books and Various Design Codes and Standards • Sih, G.C. (Editor), Methods of Analysis and Solutions of Crack Problems, Noordhoff International Publishing (1973): Leiden. • Paris, P.C., McMeeking, R.M. and Tada, H., The Weight Function Method for Determining Stress • Intensity Factors, Cracks and Fracture, ASTM STP 601, American Society for Testing and Materials, pp. 471 489 (1976): Philadelphia. • Rooke, D.P. and Cartwright, D.J., Compendium of Stress Intensity Factors, Her Majesty’s Stationery Office (1976): London. • Tada, H., Paris, P.C. and Irwin, G.R., The Stress Analysis of Cracks Handbook, Paris Productions Incorporated (1985): St. Louis, Missouri. • Murakami, Y., Stress Intensity Factors Handbook, Vols. 1, 2 and 3, Pergamon Press (1987 vols. 1 and 2, 1992 vol. 3): Oxford. Computational Methods ➢ Finite Element Method (FEM) ➢ Boundary Element Method (BEM) ➢ Extended Finite Element Method (XFEM) Experimental Methods • Def: Standard – an accepted guide that governs concepts, procedure, definitions, etc. • Ex: ASTM E399 - Standard Test Method for Plane- Strain Fracture Toughness of Metallic Materials • Ex: ISO 12135:2002- Metallic materials -- Unified method of test for the determination of quasistatic fracture toughness Compendium of SIF Solutions Each reference has a library of direction solution. In our textbook by Janssen see pages 53 -59 And many More Summary ➢ The Stress field equations can be expressed in different notations, Cylindrical, Cartesian, Principal, etc. ➢ The analytical solution to Finite Width Specimen are more difficult to obtain but solution do exist. ➢ Superposition can be applied to combine the effect of multiple loading conditions. Note, these loading conditions must be all of the same type. All mode 1 ; all mode 2; or all mode 3. ➢ The determination of SIF should generally proceed as follows: literature review, analytical solution, computational solution, experimental solution. ➢ Cost/Value should be considered in the determination of SIF. Homework 5 • Recreate figure 2.8 from the textbook which compares the correction factors for a single edge notch specimen. Read
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