ASME STP-PT-081:2017 pdf free download

ASME STP-PT-081:2017 pdf free download.CYCLIC STRESS-STRAIN CURVES.
The cyclic stress range usually changes as a function of a number of cycles (hardeningsoftening) and therefore a “typical” stress range must be chosen. By convention, the stress range at Nd2 is used in almost all cases. Other definitions are occasionally used, but in this report the Nfl approach is used almost exclusively.
K’ fl’S’) and n’ were determined by analysis of literature data for the different groups of materials The results are summarized in Figure S-i. The procedures given are only valid for materials not hardened by cold deformation or yield stresses far outside the ASMF. code specifications. They should only be used for temperatures governed by time-independent properties. For higher temperatures. creep effects tuight impact the cyclic behavior. Within these limitations, it is possible to determine representative cyclic stress-strain curves for several materials presented in the ASME BPVC Section II Tables lID.
It is important to stress that, with this approach. only typical average values could be determined which allow an assessment of the cyclic response of a material (e.g.. for i-integral assessments). Excel worksheets for monotonie and cyclic stress-strain curves were developed.
Raw data created during the project and literature used is also presented and discussed with respect to eventual implementation into the ASME Materials Database. In addition to die literature cited in the document, the References section contains all literature used to establish the results of this report.
Details are summarized in Appendices A-D:
• Appendix A: Description of the Excel Worksheet for determination of stress-strain curves
• Appendix B: Representation of cyclic data for eventual inclusion into the ASME database
• Appendix C: Example for inconsistencies once not well correlated monotonic and cyclic data are used
• Appendix D: Examples for validity of concept
Important Remarks:
In contrast to monotonic stress-strain curves, the procedure for determination of cyclic stress-strain curves is not very well established, and large differences between the results of different investigations exist. For this investigation, results from single-specimen tests were used, and the representative hysteresis loop was the loop at half lifetime.
The cyclic strains leading to fatigue failure are in the 1-2 percent range. Therefore, no discrimination between engineering and true stresses and strains is necessary.
The spreadsheet for evaluation of stress-strain curves is not part of the report.
Disclaimer:
Results gained with the introduced worksheet can only serve as technical information to assess materials properties. At the current stage they may not be used for any safety-relevant calculations or considerations.
The total strain (elastic + plastic strain) can be represented by the elastic portion and a power law function for the plastic part. The coefTiciciit K’ and the exponent n’ are material-dependent consLants.
From the determinaLion of cyclic stress-strain curses, an extremely high scatter can be expected. which is only partly due to the material itself. The main source of scatter is the performance of fatigue tess. This asscssmcnt is based on the experience of the author who lcd a group on LCF and creep at BHC/AHH
Switierland:
• Many data givcn in the literature were created in the l9Nts and 1970s. just when suitable testing cquipmcrn was becoming available
• A stiff and extremely well-aligned testing machine must be available (particularly during the compression phase of the tests)
• Sometimes tests are reported with displacement control (instead of strain control)
• Sample geometry and strain measurements: hourglass-samples with diametric strain measurements (mostly one gage only) are compared with dog-bone type samples and mostly 2 strain gages getting the signals from the movement of the shafl of the samples. Sometimes also scissor gages are (were) in use,
• Only a limited number of Lests are performed. making the determination of the power law coerncien and exponent very uncertain
• Different ways ol determination of the cyclic stress-strain curse (companion sample. incremental step. etc.)
Such experimental deficiencies are a main reason for the wide scatter and differences in experimental results. Large differences can occur particularly when the experimental data were gained from tests performed at relatively low strain ranges. Figure 2-I compares two cyclic stress-strain curves for the same material under comparable conditions.