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Discontinuities are even more damaging to fatigue strength than to tensile properties.


Design of Castings In his Hoyt Lecture of


1961, J.B. Caine said ‘‘the advantage of castings is de- rived from the ability of liquid metal to assume any shape, shapes that cannot be formed effi ciently by any other form- ing process. T is fl exibility enables the design of shapes as castings that will uniformly distribute the load … no part of the casting is overloaded … and stress concentration is at a minimum.’’ T e means to compete


with wrought metals of higher strength and internal integrity comes from careful analysis of the design of a part. Modifi - cations of the design usually can result in the use of a lower strength cast metal and arrive at a cast part with equal or higher strength. Castings can be designed with the most favorable geometries to reduce stress concen- tration by adding stock at high stress areas, increasing radii, using curved surfaces and adding metal where needed to reduce stress. Often, metal can be removed at locations where it is not needed to maintain or even reduce the gross weight of the part.


Types of Discontinuities in Castings


Discontinuities in iron castings


include microshrinkage porosity, gas porosity, oxide films, slag inclusions, coarse graphite particles, nodule clustering, degenerate graphite, flake graphite at the cast surface, and other issues. T e Al–Si casting alloys are subject


to gas porosity, microshrinkage porosity, coarse eutectic silicon phase and several secondary phases resulting from elevated levels of iron and copper in the alloys. In ductile iron, discontinuities consist of degenerate graphite particles, dross inclusions, and microshrinkage porosity. In steel castings, the discontinuities con-


continuity, the stress increases upon approaching the edge of the discontinuity. Eventually, the local stress exceeds the yield strength of the metal and yielding occurs. T us, in softer metals, the theoreti- cal peak stresses predicted at a notch of discontinuity are never attained because yield- ing occurs; however, in higher strength metals the full eff ect of the stress concentration may be realized. When the shape of the


discontinuity is severe and its size large enough, the metal surrounding the tip of the dis- continuity plastically deforms. Plastic deformation will lead to crack formation. T e stress concentration


Fig. 1. Graph shows the stress concentration factor as a function of the notch geometry. (a) Plate with hole in tension, (b) Notched bar in tension.


sist of microshrinkage porosity, slag and oxide inclusions, unfavorable distribu- tions of oxysulfi des and grain boundary aluminum nitride inclusions.


Stress Concentration at Discontinuities


It is well known that geometric


notches and fi llets produce stress con- centration. Internal discontinuities also produce stress concentration (Figure 1). Locally around the notch or dis-


at the very tip of the discon- tinuity or crack is described by the stress intensity factor K—a term used in fracture mechanics. Fracture mechan- ics is a study of the stress state associated with cracks in metal. Fracture mechan- ics allows one to predict


the fracture strength and even the fatigue life in components with a fl aw. Fracture strength is predicted from the fl aw size and location; the geometry of the part; externally applied loads and the material properties of the component. When the stress intensity at the perimeter of a fl aw reaches the critical stress intensity factor Kc, the crack advances and crack growth can become unstable, leading to rapid crack propagation and failure. Crack growth can occur by dif-


ferent modes—tensile and shear. Probably the most common mode encountered in structural compo- nents is the tensile mode, or ‘Mode I’ loading. Hence, the most commonly determined and reported critical stress intensity factor for metals is the KIc, which is also a material property called the plane strain fracture toughness.


Embrittlement in Castings


Fig. 2. This photo shows the fracture face of a ductile iron test specimen.


Embrittlement could be defi ned as any mechanism that causes both tensile


Nov/Dec 2016 | METAL CASTING DESIGN & PURCHASING | 35


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