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ASME PTB-6-2013 pdf download

ASME PTB-6-2013 pdf download.Guidelines for Strain Gaging of Pressure Vessels Subjected to External Pressure Loading in the PVHO-1 Standard.
Manufacturers’ literature present other types of rectangular and delta strain gage rosettes not shown in Figure 2.1. Also given in the literature, both the manufacturers’ and classical texts are the appropriate equations for calculating the maximum and minimum principal strains and stresses from the measured strains. As shown in Figure 2.1, the strain gages are planar and therefore are applied on a surface of the structure. This results in the fact that only the principal strains and stresses in the plane of the applied strain gage rosette can be determined. For a 3-dimensional structure, a third principal strain and stress exists, which is perpendicular to the plane on which the strain gage rosette is applied. The third principal stress is determined by other means. For example, given a cylinder under external pressure, the longitudinal and circumferential strains and stresses (2 of the 3 principal strains and stresses) can be measured using a strain gage rosette since these lie in a plane. The third principal stress is known from the fact that on the outer surface of the cylinder, the radial stress (the third principal stress) is equal in magnitude (in compression) to the external pressure. Likewise, if located on the inside surface of the cylinder, the radial stress (the third principal stress) is equal to zero (0), assuming no internal gage pressure exists. Once the 3 principal stresses are known, other 3D stress quantities such as von Mises (or equivalent) stress can be calculated. It is important to note that when comparing only the planar principal strains and stresses obtained experimentally (from the strain gages) with those obtained analytically, care must be taken that only the corresponding analytical planar values be used in the comparison study. In almost all cases, the purpose of using strain gages during a test is to determine the planar principal strains and stresses. Knowledge about the principal strain/stress directions is required to choose the correct type of strain gage rosette or uniaxial strain gage. For pressure vessels subjected to external pressure loadings, it is expected that most (if not all) strain gages will be biaxial or triaxial strain gage rosettes. 2.2 Use of Uniaxial Strain Gages If the state of strain and stress is uniaxial, then the principal strain/stress direction is known. The use of a uniaxial strain gage at this location would be sufficient to determine the principal strain and stress. A simplistic example of uniaxial strain is a straight bar in tension. An example of such a location in a pressure hull is on the inside surface of the flange of a tee stiffener (circumferential direction, mid-width of flange) for a tee-stiffened cylindrical hull. 2.3 Use of Biaxial (Tee) Strain Gage Rosettes When the directions of the planar principal strains/stresses are known, then the use of a biaxial (tee) strain gage rosette is satisfactory. In order to measure the principal strains, the rosette is oriented such that the gages are in line with the principal directions. The principal stresses are then calculated by closed form equations. It is noted that there will be some strain measurement error if the gages of the rosette are misaligned with these principal directions. For more information on this type of measurement error, the reader is referred to literature regarding this subject available from strain gage manufacturers. An example of a structure where the principal strain directions are known is a cylinder under internal or external pressure. The principal strain directions are the longitudinal and circumferential directions. It is noted that this is true only if no other localized structural features or loads are in close proximity that can affect the principal directions.

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