COMPARISON OF THEORIES WITH ЕXРЕRIMENTAL RESULTS

(I)

In general, there is little experimental information on com­bined stress behaviour of materials. Some experiments have been conducted for biaxial tension-tension and tension-compression stress combinations which give good support to the distortion energy theory for predicting the yield strength. A limited amount of experimental data is available on yielding under tri­axial stresses. These results support the distortion energy theory but are restricted to the case where two of the princi­pal stresses are compressive and equal. Few experiments have been conducted to determine ultimate and fracture strengths of ductile and brittle materials under combined states of stress. For biaxial tension-tension, tension-compression, and certain triaxial stress combinations, the internal friction theory ap­pears to be in as good agreement with the test results as any of the theories. It is on the basis of the foregoing observa­tions that the distortion energy theory is recommended for pre­dicting biaxial yield strength while the more conservative in­ternal friction theory is recommended for predicting triaxial yield strengths and biaxial and triaxial ultimate and fracture strength.

A limited number of investigations. Including biaxial ten­sion-tension and biaxial tension-compression, indicate that the simple deformation theory is a good approximation for predict­ing plastic stress-strain relations under combined stresses. There are no sufficient results on triaxial states of stress to conclude which theory agrees best with the experimental re­sults.

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COMPARISON OF THEORIES WITH EXPERIMENTAL RESULTS

(II)

Perhaps the main reason why there is little experimental information available on properties of materials subjected to combined stresses is that special, complicated testing equip­ment is needed for the most of these tests. Another reason is that for certain stress combinations a suitable method of stress­ing the specimens has not been devised. Some of the earlier in­vestigations were unsatisfactory since they were made on speci­mens in which the state of stress throughout the specimen was not uniform. One of these was the solid round specimen subject­ed to torsion and axial tension. Although this loading pro­duces a combined state of stress, the torsional shear stress varies from the inside to the outside of the specimen. This means that the lower stressed inner fibers introduce a streng­thening effect on the outer fibers, thereby increasing the re­sistance to yielding of these fibers. Other specimens in which a strengthening effect influences the results include solid round specimens subjected to torsion and bending, thick-walled cylinders subjected to axial tension and internal pressure, and notched specimens subjected to tension and bending.

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FATIGUE ROPERTIES (I)

Machine and structural members are often subjected to loads and stresses that do not remain constant but vary with time. Fоr example, the aerodynamic loads on an aircraft do not remain fixed in value, and the stresses produced by these loads fluc­tuate in value. Stresses that vary with time are called fluc­tuating, alternating, or fatigue stresses. If the loads are suddenly applied at a high rate of speed, the loads are called impact loads.

The study of fatigue properties of materials was started about 100 years ago, primarily as a result of fatigue failures in railroad equipment. Today the development of modern high-­speed transportation and power equipment of various types has increased the importance of the fatigue properties of engineer­ing materials. The nature of fatigue failure is now reasonably well understood, but the complexity of the problem is such that rational methods of design for fatigue are difficult to develop. Some of the reasons for this difficulty are that fatigue strengths of parts are affected not only by the material but also by de­sign features, fabrication methods, and service conditions. In addition, fatigue strengths of materials are influenced by small cracks and other flaws in the material and, for this rea­son, considerable variation in test results occurs. The great variation in properties makes it necessary to apply statistical procedures in the evaluation of fatigue strength.

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Date: 2015-01-02; view: 939