Fatigue Testing On Metals

Although yield strength is suitable for criterion for designing components which are subjected to be static loads, for cyclic loading the behavior of a material must be evaluated under dynamic conditions. The fatigue strength or endurance limit of a material is therefore used in the design of parts subjected to repeated alternating stresses over an extended period of time.

The fatigue test determines the stresses which a sample of a material of standard dimensions can safely endure for a given number of cycles. This is accomplished using a specimen having a round cross section, loaded at two points as a rotating simple beam, and supported as its ends. The top surface of such a specimen is always in compression and the bottom surface is always in tension. The maximum stress always occurs at surface , halfway along the length of the specimen , where the cross - section is minimum. For each complete rotation of the specimen, a point in the surface originally at the top center goes alternately from a maximum in compression to a maximum in tension and then back to the same maximum in compression.

Specimens are tested to failure using different loads, and the number of cycles before failure is noted for each load. The results of such tests are plotted as graphs of applied stress against the logarithm of the number of cycles to failure. the curve is known as S-N curve. The usual procedure is to make a number of specimens of the same material and test other one under different stress conditions.

Two different curves of S-N curves occur, one showing a definite levelling of the curve , which indicates that the material has a definite fatigue limit. Commonly, ferrous metals shows distinct fatigue limit, while non ferrous metals do not. The fatigue limit is about 40 to 60 percent of the tensile strength.

Comments

ELECTRICAL COMPARATORS AND ITS WORKING

Electrical Comparators are used as a means of detecting and amplifying small movements of a work contacting elements. An electrical comparator consists essentially of a pick-up head or transducer for converting a displacement into a corresponding change in current and a meter or recorder connected in the circuit to indicate the electrical change, calibrated to show in terms of displacement. Generally, an amplifier is needed to provide the requisite sensitivity and to match the characteristics of different parts of the circuit. Electrical comparators can be classified according to the electrical principle used in the pick-up head. Most of the comparators use either a differential transformer, an inductance bridge, a strain gauge or a capacitor as a means of detecting movement of the gauging element. There are different types of electrical comparators. One kind called an electrolimit gauge is used to check or measure the outside diameter of a roll. The objec...

TOP POLLUTION RECORDED IN WORLD

CHERNOBYL NUCLEAR DISASTER (NUCLEAR POLLUTION) : In April 26,1986, the melt down of the Chernobyl nuclear reactor , in Russia, has leaked out the radioactive rays and radioactive materials. Effects : 1. In this Pollution nearly 2000 persons have been killed by the accident. 2. People suffered due to the illness such as, degeneration of the cells, severe bleeding, anemia, skin cancer. 3. Animals, Plants are also affected by nuclear radiation. MINAMATA EPIDEMIC (1953) : MARINE POLLUTION ( CASE STUDY ) Minamata is a small coastal village in Japan. The Chisso Chemical Company , which produces vinyl polymer plastics used organomercurial compound. The industry released its effluent into Minamata sea. The effluent containing mercury ions is converted into methyl mercury, which is highly toxic. The marine organisms like phytoplankton and zoo-planktons easily absorb the toxic methyl mercury compound. These organisms are consumed by small fishes and these are in ...

MELTING FURNACES FOR FERROUS METALS : CUPOLA AND ITS WORKING

Various types of melting furnaces are used in different foundry shops, depending upon the quantity of metal to be melted at a time, and the nature of work that is carried out in the shop. The primary objective in cupola is to produce iron of desired composition, temperature and properties at the required rate in the most economical manner. Besides, this furnace has many distinct advantages over the other types, e.g., simplicity of operation, continuity of production, and increased output coupled with a high degree of efficiency. Description Of a Cupola : The Cupola Furnace consists of a vertical, cylindrical steel sheet, 6 to 12 mm thick, and lined inside with acid refractory bricks or acid tamping clay. The refractory bricks or the tamping clay used consist of silicon oxide acid (SiO2) and alumina (Al2O3). The lining is generally thicker in the lower where the temperatures encountered are higher than in the upper region. The shell is mounted either on a brick work fo...

Mechinopedia For Mechanical Engineers

Search This Blog