tailieunhanh - Volume 08 - Mechanical Testing and Evaluation Part 3

Tham khảo tài liệu 'volume 08 - mechanical testing and evaluation part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Engineering stress s The force at any time during the test divided by the initial area of the test piece s F A 0 where F is the force and A0 is the initial cross section of a test piece. True stress o The force at any time divided by the instantaneous area of the test piece o F Ai where F is the force and Ai is the instantaneous cross section of a test piece. Because an increasing force stretches a test piece thus decreasing its cross-sectional area the value of true stress will always be greater than the nominal or engineering stress. These two definitions of stress are further related to one another in terms of the strain that occurs when the deformation is assumed to occur at a constant volume as it frequently is . As previously noted strain can be expressed as either engineering strain e or true strain where the two expressions of strain are related as ln 1 e . When the test-piece volume is constant during deformation . AL A0L0 then the instantaneous cross section Ai is related to the initial cross section A0 where A A0 exp - A0 1 e If these expressions for instantaneous and initial cross sections are divided into the applied force to obtain values of true stress at the instantaneous cross section Ai and engineering stress at the initial cross section A0 then o s exp s 1 e Typically engineering stress is more commonly considered during uniaxial tension tests. All discussions in this article are based on nominal engineering stress and strain unless otherwise noted. More detailed discussions on true stress and true strain are in the article Mechanical Behavior under Tensile and Compressive Loads in this Volume. Uniaxial Tension Testing John M. Tim Holt Alpha Consultants and Engineering Stress-Strain Behavior During a tension test the force applied to the test piece and the amount of elongation of the test piece are measured simultaneously. The applied force is measured by the test machine or by accessory force-measuring devices. The amount of stretching or .