Downloaded from https://www.everyspec.com STRUC URES creep effects mnay yield important design criteria. Creep data are generally presented in the C STRAIN form of st:ain as a function of time as shown ~-EL . in Figure 2-4. Specimens are subjected to a I____________ conslant load, which cozresponds to a con. - stant stress level based on the original cross- JPRIMAR\ sectionl area of the specimen. Thie instan- S RGE __ taneous value of the strain, t. corresponds to STAGV the elastic deformation calculable from the load. original cross-sectional area and Young's Modulus.
With the passage of tLime,J the strain increasts; the strain rate is high at first and gradually decreases to a constant value. The portion of this creep curve inI which the creep rate is changing is called the I pi inzary stage or primary e 'eep. The portion corresponding to a constant ate of strain is called the seeondary stage, or secondary SEODR creep. In the tertiary stage, the strain rate STNAE suddenly increases until the specimenSAG ruptures. Remova-l of the load during the secondaryj stage results in the recovery of the elastic strain r, and sometimes also of that portion of the creep strain it, obtained by extrap~iat- ing the straight line of the secondary creep stage to t = 0.
Some solid state physicists attribute this to a release of internal stresses which occurred during the loading and creep processes. it should b,.. noted that the true physical nature of cr esp has not yet been esZ abhshed with any &gree of finality. Elongat-s i rn the specimen that occur - - - contraction of the specimen in the lateral TERTIARY STAGE direction. This decreases the cros~s-sectional area and increases the -value of the true stress whici' is the loadO divided by the true RtP7URE cross-sectional area. This process acceleratesTIE rapidly a.2d culmipates in rupture of thet-TM sppcinei when the greatly reduced cross- secticty e,, area can no longer 3upport the ap- plied load.
The portion of the creep curve Figure 2-4. Typical Presentation of Creep Ocita corresponding to increasing rates of creep is the tert-ary stage. it should be mentioned that -met,,ilographic- changes in the material (for instance those caused by annealing dur- ing the test) can also lead to increasing rates of creep