{"id":248661,"date":"2023-02-05T19:06:53","date_gmt":"2023-02-05T19:06:53","guid":{"rendered":"\/forum\/forums\/topic\/sn-curves-for-different-levels-of-mean-stress\/"},"modified":"2024-02-29T15:12:39","modified_gmt":"2024-02-29T15:12:39","slug":"sn-curves-for-different-levels-of-mean-stress","status":"closed","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/sn-curves-for-different-levels-of-mean-stress\/","title":{"rendered":"SN-Curves for different levels of mean stress"},"content":{"rendered":"<p>Hi there<\/p>\n<p>I have a question concerning the fatigue tool of Ansys Workbench:<\/p>\n<p>I already figured out how to define S-N-curves for different levels of mean stress in the technical worksheet of Ansys Workbench. So, to make sure I understand what the fatigue tool is doing, I defined three S-N-curves: One for a fully reversed loading, one for a zero based loading and one where the maximum load is zero. As an example, lets say the endurance limits (the amplitudes) are as follows:<\/p>\n<ul>\n<li>Mean stress = 0 MPa&nbsp; &nbsp; &nbsp; &nbsp;==&gt; endurance limit = 200 MPa<\/li>\n<li>Mean stress = 180 MPa&nbsp; &nbsp;==&gt; endurance limit = 180 MPa<\/li>\n<li>Mean stress = -220&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ==&gt; endurance limit = 220 MPa<\/li>\n<\/ul>\n<p>I then simulated a test geometry and tuned the load so the maximum stress is 200 MPa. I made a fully reversed fatigue assessment (10e9 cycles) and got a safety factor of 1 as expected.<\/p>\n<p>I then tuned the load so the maximum tensile stress is 360 MPa (mean stress of 180 MPa plus the amplitude of 180 MPa) and made a zero based fatigue assessment (10e9 cycles). After playing with the mean stress theory used, I finally got a safety factor of 1 as expected. So far so good.<\/p>\n<p>But when I tune the load so the maximum compressive stress is -440 MPa (mean stress of -220 MPa minus the amplitude of 220 MPa) and make a zero based fatigue assessment (10e9 cycles) I do not get a safety factor of 1 (but lower) no matter what mean stress theory I choose!<\/p>\n<p>How does Ansys interpret or uses the specified S-N curves? It seems like Ansys is using some type of mean stress correction curves (see image) but not the values I deliberately specified for different mean stresses.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" title=\"Mean stress curves of fatigue tool\" src=\"\/forum\/wp-content\/uploads\/sites\/2\/2023\/02\/05-02-2023-1675623315-Mittelspanungskurven.png\" alt=\"Mean stress curves of fatigue tool\" width=\"1210\" height=\"290\"><\/p>\n<p>Obviously, I expect Ansys to figure out by itself, which areas of the part have positive, negative (or no) mean stresses, even if the analysis is defined as <em>zero based<\/em>, even though the compressively loaded area of the part only sees negative stresses \/ compressive stresses.<\/p>\n<p>Thanks in advance,<\/p>\n<p>Roy<\/p>\n","protected":false},"template":"","class_list":["post-248661","topic","type-topic","status-closed","hentry","topic-tag-fatigue-analysis"],"aioseo_notices":[],"acf":[],"custom_fields":[{"0":{"_bbp_subscription":["274279","490"],"_bbp_author_ip":["23.192.164.18"]," _bbp_last_reply_id":["0"]," _bbp_likes_count":["0"],"_btv_view_count":["4804"],"_bbp_topic_status":["answered"],"_bbp_topic_id":["248661"],"_bbp_forum_id":["27791"],"_bbp_engagement":["490","274279"],"_bbp_voice_count":["2"],"_bbp_reply_count":["6"],"_bbp_last_reply_id":["249344"],"_bbp_last_active_id":["249344"],"_bbp_last_active_time":["2023-02-10 07:30:05"]},"test":"roy-roffleresoro-ch"}],"_links":{"self":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/248661","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics"}],"about":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/types\/topic"}],"version-history":[{"count":1,"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/248661\/revisions"}],"predecessor-version":[{"id":354584,"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/topics\/248661\/revisions\/354584"}],"wp:attachment":[{"href":"https:\/\/innovationspace.ansys.com\/forum\/wp-json\/wp\/v2\/media?parent=248661"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}