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12 September 2015 Tablets & Capsules well as other factors related to the safe use of the tooling or any part or assembly associated with it. With FEA, design engineers can analyze complex cup geometry and see where stress concentrations develop and use that informa- tion to improve tablet and tool design. FEA works just as its name implies, by defining discrete elements within a larger model and analyzing them piece- by-piece. To begin, the model is broken down into a finite number of small pieces, the size of which the user controls. This collection of elements is known as a mesh (Figure 2). Next a material, with all the necessary properties defined, is assigned to the model, and the desired force vectors are applied to all surfaces of interest. The model is then constrained to simulate how it would act in a real-world installation. Last, a failure criterion is selected, which is a set of values at which the material and part combination would likely fail. With these parameters in place, the analysis begins, and the FEA software exam- ines each element of the mesh to analyze the stress states of the model. The result: a detailed map showing where maxi- mum stresses and resulting failures are likely to occur, enabling engineers to modify the design. For ductile materials, such as metals, von Mises stress— also called equivalent tensile stress—is generally accepted as the best failure criterion. It states that when an elastically deformable body is subjected to three-dimensional loading, it will develop a complex network of three-dimensional stresses. Von Mises stress can also be formulated in terms of von Mises yield criterion, which states that, even when none of the three principal stresses exceeds the yield strength, the yield may still be reached as a result of the combination of stresses. This criterion works particularly well when FEA uses a mesh model because it narrows the almost infinite number of degrees of freedom. That is, the calculated von Mises stress combines all stresses into an equivalent tensor value that engineers can compare to a material's known yield strength. Figure 3 shows the von Mises stress distribu- tions of a flat-face, bevel-edge tablet tool and a flat-face, radius-edge tool. Note the stress concentrations (red areas) around the bevel edge of the punch face. Figure 2 Mesh created by FEA modeling software Figure 3 Von Mises stress distribution plots: Flat-face bevel edge versus flat-face radius edge Section view of bevel edge Flat-face bevel edge Flat-face radius edge Section view of radius edge 5,140.7 23,426.3 78,427.1 60,105.5 115,070.3 170,035.2 206,678.4 151,713.6 188,356.8 225,000.0 Von Mises (psi) yield strength 275,000.0