Fail Safe Design With Crack Stoppers And Warners
Free (open access)
71 - 77
C. Mattheck, Ch. Wissner, K. Bethge & O. Kraft
Trees attach thicker tree rings at overloaded places. This principle is mimicked here in order to delay crack initiation and propagation. Bulges are attached beside notches to reduce stress concentrations. The bulges have to be shape optimized so as to not cause notch stresses themselves. Brittle coating can act as a warner near notches. The crack stoppers are described by FEM and the result is verified by comparative fatigue testing of prototypes leading to up to 21 times longer fatigue life. Keywords: crack stoppers, crack warners. 1 Introduction The resistance of a mechanical stressed structure against crack initiation is at the one hand a material attribute but on the other hand a matter of the mechanical effective tensions and so directly depending on the shape of the structure. Former studies showed the great effect of a shape optimization to reduce the maximum stresses and to get a more homogeneous tension distribution. The Computer Aided Optimization (CAO) was inspired by the growth of trees, who attach more material, thicker tree rings, at highly stressed regions. As a complex an iterative computer algorithm, this method achieves very good results, need otherwise special computer programmes and advanced users. A few years ago with the Method of Tensile Triangles a method was created by Mattheck , which everybody can use. Without computers, only with pencil and setsquare. The simple way of construction is shown at figure 1. The basic idea of the Method of Tensile Triangles is to find a shape that fits to the flow of forces. This means, that no abrupt changes in the flow of forces
crack stoppers, crack warners.