Fatigue Analysis Report
Fatigue Analysis Report2011-1-0610-403-GRO1
Table of ContentsExecutive Summary3Hypothesis5Introduction and Background5Alternate Experiments6Results and Conclusion7Appendix A8Part Measurements& Initial Calculations8Appendix B9Initial S/N Diagram9Completed Post Annealed S/N Diagram11Appendix C13Finite Element Analysis13Appendix D20Fatigue Test20Pre-Annealed Fatigue Testing20Post Annealed Fatigue Testing25Appendix E28Tensile Testing28Tensile Test of the pre-annealed Samples28Tensile Test of the post-annealed Samples30Appendix F32Hardness32Pre Annealed Hardness Testing32Post Annealed Hardness Testing34Appendix G36Combustion-Infrared Absorption36Appendix H38Spark Test38
Appendix I40Metallography40Pre-Annealed Metallography Test40Post-Annealed Metallography Test43Appendix J45Annealing45Appendix K46Cost Analysis46Appendix L47Green Belt Tools47Project Charter47Gantt chart49Stages of Team Development50PDCA51Process Flow Chart52High Level SIPOC53Box Plot Statistics on Testing54Two Sample T-Test: Non Annealed/Annealed56ANOVA58Cause and Effect Fatigue Diagram60Cause and Effect Material Diagram61C & E Checklist62Appendix M63Meeting Notes63References65
Executive SummaryTo accurately understand the failure characteristics of materials, an investigation on 10 specimens of AISI 1018 Cold Rolled steel was undertaken. The test specimens were initially fatigue tested under fully reversed load conditions in a cantilever beam model. To begin to understand the failure characteristics the 10 specimens were stressed in a fatigue tester. Rotating at an average of 3450 rpm and loaded at one end, similar to a cantilever beam. Loads varied on a team to team basis, but were in the range of published data of AISI 1018 CRS steels mean strength (Sm) and Endurance Limit (Se). The loads ranged from 90N to 175N. An S-N Diagram for the material was generated based on the correction factors estimated from the published values. The completed corrected S-N diagram was our starting point. The S-N diagram was referenced to determine load characteristics required for the fatigue test. The 10 specimens were individually loaded into the fatigue tester. Once the machine was fully operational the load was introduced to the free end by rotating the dial knob on the machine. It took about 1000-2000 revolutions before the specimen reached the desired load value for each test run. This variation, in what should have been a constant load was negligible for the overall experiment. The trend with the load and its expected number of cycles to failure showed a much higher mean strength. This translated to a much higher Ultimate Tensile Strength being found from the test values. The results showed a nearly 29% increase compared to the expected value. The expected value was 65.3ksi and experimental value was calculated as 84.2ksi. This created many doubts as to the authenticity of the material or the process of fabrication. As a class, it was established that either the material was not actually 1018 CRS, or the fabrication process induced significant internal stresses by process of work hardening the specimen. To confirm our hypothesis of the material two groups of experiments and testing were done. One test involved the authentication of the material, while the other involved the possibility of internal stresses. In an attempt to verify the quality of output of the fatigue tester, parts of the 10 initial specimens were tensile tested. The Tensile tests results corresponded with the fatigue test giving an average Sut of 100.4ksi. The different between the results of the fatigue test and tensile test were relatively close but still substantially high compared to the critical to quality value of 65.3ksi. The results between the fatigue and tensile test were neglected, due to the bigger issue. These differences may have occurred due to specimen loading conditions, system calibration errors or other random errors. To further verify the results a Rockwell Hardness test was conducted on the specimens. The hardness test results showed an average ultimate tensile strength of 98ksi. After three multiple test results, all verifying an accurate read of 80+ Sut, a spark test was performed to confirm the carbon content. A Combustion test performed by an outsourced laboratory, IMR Test Labs, also confirmed the carbon content of 1 specimen as 0.19% of weight. This test result indicted the specimens were in fact AISI 1018 Cold Rolled Steel. At this point it was clear that the only variance that had not formally been taken into account was manufacturing process. Work hardened materials deviate from most published values, as was observed through test experiments. However, this claim needed to be backed up. A metallurgical grain structure test would enable us to read the grain structure of our specimens and compare it to published data from the American Metallurgical Society. Studying the specimens under a high powered microscope, visually the data corresponded with that of published images. However, photographs were not conclusive enough, or a reliable source of data to draw any conclusion; There can be errors such as human eye errors. At this point it was verified that we know the samples were of AISI 1018 CRS. To bring the Sut down closer to its published values within 3-4% range, a set of new 1018 CRS specimens from which the same batch the original was received. These specimens were put into a furnace to undergo full annealing. Upon the completion of the annealing process, the five specimens were then re-tested for fatigue, tensile, hardness and metallurgy. The results of the post annealed specimens were found to be close to published value of 65.3ksi within 3-4%. Therefore, as hypothesized, the material of the specimens was in fact AISI 1018 Cold Rolled Steel which was induced with internal stresses. To effectively complete this project, Six Sigma tools were utilized to better define, measure, analyze, improve and control (DMAIC) the main objective and customer need which was to determine the fatigue characteristics of given samples of AISI 1018 cold rolled steel. Tools such as project plan using a Gantt chart, process flow chart, diagrams, brainstorming, PDCA, ANOVA, SIPOC, two sample T-test, box plots, and cost analysis were incorporated into the report to be better organized & utilize the DMAIC process.
HypothesisAfter conducting the fatigue analysis experiments, we encountered a problem. The actual fatigue data was significantly higher than both the published and calculated corrected data. We suspect that the cause of this material's increase in fatigue characteristics is a result of strain hardening that occurred during the cold drawing manufacturing process of these specimens. To verify our suspicions, we will conduct a series of tests to verify the SUT of the specimens. This will ensure that there was no major error during the fatigue testing of the material. Spark and metallurgy test will be done to verify that material is actually 1018 CRS. At the completion of these tests, we should be able to conclude that the material is still in fact 1018 CRS with an increase in SUT due to strain hardening during manufacturing process of the specimens.Introduction and BackgroundFor the 2011 fall quarter Failure Mechanics class, a CTQ (Critical to Quality) was given to determine the fatigue characteristics of AISI 1018 CRS within a 99% confidence level and compare them to the published values. In the process of reaching this CTQ, Six Sigma tools were utilized. With these found fatigue characteristics, analysis were to be done to show the general applicable engineering design in terms of safety factors. The failure of materials theoretically occurs at much lower stress levels than the published values of the ultimate tensile strength (SUT). An important part of Failure Mechanics is to understand the conditions of bodies that incur alternating stresses under cyclic loading. With this understanding, predictions of failure can be achieved, and designs can be created at variable FOS (Factors of Safety) for different applications. Using a fatigue tester is a favorable way to test the fatigue characteristics of a material. Usage of this machine requires knowledge of Failure Mechanics and strength of materials in order to utilize the data from this machine. With this knowledge a controlled set of test can be conducted and the data can be interrupted into a SUT. This is achieved with the creation of a SN-Diagram (Stress vs. Number of cycles). To further constrain that the material and verify the material properties, other test can be conducted. These test include, hardness, metallurgy, spark, and tensile test. A collection of this data and published data will provide controlled fatigue characteristics with verified material properties.
Alternate ExperimentsTo qualify the legitimacy of the Fatigue test results, a tensile test and a Hardness test were performed. The specimens used for both these experiments were parts of the broken specimens from the fatigue tests. For all the following experiments parts of the same specimens used for the fatigue test were used. This was to ensure no corruption of the sample set. For the Tensile test the neck of the specimen was used. Tensile test results, performed on 4 specimens were concurrent with those from the fatigue test. The average ultimate tensile strength derived from the experiment was 100ksi. The tensile test was however not enough evidence to dismiss either one of the hypothesis. A hardness test was then performed on 4 samples of the fatigue test. The average HRB number derived from the hardness test was 94.4 HRB. According to the Rockwell Hardness Conversion charts this indicted an ultimate tensile strength of approximately 98ksi. The results of the Tensile and Hardness test were concurrent with those of the Fatigue test. Precise re