This page contains a Flash digital edition of a book.
Applying Fracture Mechanics to the Design of


Low-Alloy Steel Castings


A crawler shoe case study illustrates how designers can use fracture mechanics to avoid over-designing and over-specifying a casting, which will reduce part cost. PATRICK SEVERSON, UNIVERSITY OF WISCONSIN-MILWAUKEE (MILWAUKEE); HATHIBELAGAL ROSHAN, MAYNARD STEEL CASTING CO. (MILWAUKEE); R. EL-HAJJAR AND PRADEEP ROHATGI, UNIVERSITY OF WISCONSIN-MILWAUKEE (MILWAUKEE)


M


any times, castings are rejected because of a flaw that may not have an influ-


ence on the performance or life of the part. Tey are rejected, or repaired, as a conservative way to make sure the part won’t fail when in service. By analyzing fracture mechanics designers can not only optimize the geometry of a casting, they may also be able to determine whether a flaw is truly detrimental, possibly saving thousands of dollars on scrapping or repair. Many fabrication techniques are used in steel structures for heavy mobile equipment, including forgings, castings, and weldments. Low-alloy steel castings are common among these techniques and can be used as a stand-alone component or incorpo- rated into part of a weldment. When designing steel components for differ- ent types of heavy mobile equipment, different failure modes considered include static strength, fatigue (stress life and strain life), buckling, and connection failures (pins, bolts, etc.). However, current methods of analysis do not take advantage of linear elastic or elastic-plastic fracture mechanics to optimize structural design of low-alloy steel cast components. Current analysis of cast compo- nents consists of generalizing the


Table 1. Mechanical Properties of AISI 8630 Alloy


0.2% Proof Stress (MPa)


985


Ultimate Tensile Strength (MPa)


1,144


component as an elastic continuum without any defects or inclusions while assuming isotropic stiffness behav- ior. Multiple static or dynamic load cases that represent a full duty cycle are applied to determine the mean stress, stress range, and von Mises stress throughout the component. Modified Goodman and Miner’s Rule calculations are applied to determine adequate fatigue life while von Mises stresses are compared to yield stresses to determine adequate safety factors. Fracture toughness is not consid-


Fracture Toughness at -45oC (MPa√m)


106


Modulus of Elasticity (MPa)


207,000


ered in the analysis of low-alloy steel castings because high safety factors are used to ensure fatigue crack propaga- tion is not an issue. Tis type of design methodology can lead to over designed components typically resulting in expensive steel structures. Tensile properties and impact


Fig 1. The geometry of a crawler shoe was studied using fracture mechanics.


strength are insufficient to charac- terize the behavior of materials and the corresponding structures they are included in. Modern design approaches require fracture mechanics characterization of materials. Not only would a fracture mechanics approach help optimize structural design, but unnecessary weld repair operations could be avoided if it is determined that stress intensity factors of an existing crack would never get to their critical values. In turn, this methodology could be applied to other components with different fabrication techniques, leading to optimization of struc- tures throughout an entire machine. Inspection guidelines from ASTM A903 (Magnetic Particle Inspection) and ASTM A609 (Ultrasonic Inspec- tion) can be used to determine mini-


January 2018 MODERN CASTING | 35


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60