Shaping machinery elements exposed to cyclic load

Abstract

On the grounds of previous research it can be concluded that any change in shape causes disturbance in the stress state of the strained element and the concentration of stress due to the diversion and densification of stress lines. As a rule, the concentration of stress leads to an increase in the actual stress compared to nominal. Slight changes in shape or cross section; create a slight concentration of stress, often negligible, but the extreme-severe form of change leads to a strong concentration of stress, which must be taken into account. The actual stresses are the greatest in places of concentration sources, where sometimes, depending on the geometric proportions of elements, the big differences between actual and nominal stresses are noticeable. But the difference between the actual and nominal stress becomes less at a relatively small distance from the stress concentration source. Concentration of stress is not caused only by the shapes determined by structural needs, but also by flaws and gaps in the material, or by external damage. They are particularly harmful, even dangerous, when it comes to sharp cuts that create unexpected, but often very high concentration of stress. The risk increases when the cuts are placed on the outer fibers of the elements, usually exposed to the greatest stress due to their position. Machinery elements almost always have geometric shapes of variable cross-section - at least in some places. The shapes of the elements vary and depend on their function; they are often very irregular, or asymmetric, and therefore hard to classtbi. Since the stress concentration depends on the shape, it is logical that the most closely examined shapes are those commonly found in machines. These are the types with stepped transitions (shafts, spindles, bolts, levers...), shapes with outer grooves, shapes with openings... In addition to the stress concentration due to shape there is a stress concentration due to contact ("contact stress concentration'), which occurs in the material under the influence of mutual pressure of the touching elements. It depends on the way in which the contact pressure is transmitted. Simultaneous influence of stress concentration due to shape and due to contact can sometimes have very negative consequences on the stress state of elements. The consequence of stress concentration phenomenon is uneven distribution of stress. Some fibers are " often much stronger than the others. In such a case the material is not evenly strained, and therefore not evenly used. Mass transfer of material from the less strained into areas exposed to greater strain can substantially reduce the unevenness of the stress state and thereby reduce the weight of some construction elements, and the structure as a whole. It ensues that successful resolution of the problem of stress equalization implies both knowing where the greatest stress occurs and what the quantity of stress at each point is.