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reduce stress concentrations by a special curved contour

IP.com Disclosure Number: IPCOM000228039D
Publication Date: 2013-Jun-04
Document File: 3 page(s) / 252K

Publishing Venue

The IP.com Prior Art Database

Abstract

The curves published by Mr. Baud are describing a way how to reduce stress concentrations by a special curved contour, which gives a constant stress distribution along the contour of the hole for the shown loadcase. As hole is the summation of 4 radii, i.e. a original circular hole can be replaced by four segments of a Baud-contour (see Fig. 1).

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1.     Abstract

The curves published by Mr. Baud are describing a way how to reduce stress concentrations by a special curved contour, which gives a constant stress distribution along the contour of the hole for the shown loadcase. As hole is the summation of 4 radii, i.e. a original circular hole can be replaced by four segments of a Baud-contour (see Fig. 1).

2.     Disclosure

Each hole in a structure creates fatigue relevant stress concentrations. The level of stress concentration is determined by the radius of the hole. Thus, the design driver for the thickness and width of a flange is mostly driven by the allowable given by the hole diameter of the attachment point.

Reducing the stress concentration of the hole will reduce the thickness of the flange/skin into which the proposed fastener element is applied. Therefore, the weight of the flange/skin can be reduced

A circular radius or a hole in any structural part is still a fatigue critical area caused by the stress concentration factor. Notches with circular radii are a commonly accepted standard in engineering to reduce notch stresses compared to sharp corners. However, in the transition area of the contour between straight part and curved radius the rapid change of curvature still creates notch stresses. These are lower than for a sharp corner, but in practice still 2-4 times higher than for the undisturbed net section of the structural part.

It possible to reduce notch stresses by shaping the notch in a way that stress concentration are decreased significantly and in the ideal case completely disappear:

In 1934 R. V. Baud showed by photo elasticity experiments that a special shape of a notch contour does not create such kind of stress concentrations.

The experimental results of Baud were confirmed by an analytical proof by Neuber in 1971.

In the 1990s C. Mattheck did extent this knowledge to biologic structures. He discovered that trees, bones, claws and thorns have the same type of contour which eliminates stress concentrations.

One characteristic of the contours is that in the transition zone between the straight part and the curved notch its curvature is continuous, which means without rapid change. This is not the case for the transition from a straight contour to a circular arc. In this case the curvature changes suddenly in one step from 0 to the finite curvature of the radius. Although the straight and curved contour have the same tangent at their connection, the curvature of the contour is discontinuous. As a result this discontinuity in curvature generates notch stresses.

So far th...