Microscale and ultrafast high cycle fatigue testing of titanium
<p>The fatigue properties and crack growth behaviour in Grade 2 commercially pure titanium (CP Ti) was investigated using a miniature ultrafast testing method. Using finite element modelling (FEM) a dogbone type mesoscale cantilever specimen was designed, that facilitated the shift of stress c...
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| Format: | Thesis |
| Language: | English |
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2019
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| _version_ | 1797079614138351616 |
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| author | Arutyunyan, A |
| author2 | Wilkinson, A |
| author_facet | Wilkinson, A Arutyunyan, A |
| author_sort | Arutyunyan, A |
| collection | OXFORD |
| description | <p>The fatigue properties and crack growth behaviour in Grade 2 commercially pure titanium (CP Ti) was investigated using a miniature ultrafast testing method. Using finite element modelling (FEM) a dogbone type mesoscale cantilever specimen was designed, that facilitated the shift of stress concentrations from sharp corners in a simple initial cantilever design to the centre of a beam. This ensured that failure was more dependent on microstructure than sample geometry.</p>
<p>Monotonic single crystal microscale cantilever tests were performed that isolated <a> prism, <a> basal and <c+a> first order pyramidal planes. The bulk critically resolved shear stress (CRSS) for these slip systems were 130 MPa, 298 MPa and 795 MPa respectively. These experiments showed that slip is most likely to occur on <a> prism planes. In addition, using von Mises cri- terion to determine the yield stress from the CRSS of <a> prism (225 MPa) was comparable to the mean bulk 0.2 yield stress measured by monotonic tensile tests (243 MPa). The mono- tonic bulk tensile tests on typical dogbone samples also demonstrated that the elastic modulus along the length axis of a microcantilever as measured from the microscale tests of <a> prism had good agreement with bulk data (~105 GPa). It has also been shown that in ultrafast mesoscale tests there is good agreement with both microscale and macroscale tests as it has been shown that failure is extremely unlikely to occur below ~240 MPa.</p>
<p>By comparing both single-step and multi-step testing approaches to ultrasonic fatigue test- ing it has been shown that there is little/no coaxing effect in Grade 2 CP Ti. By fitting a Basquin power law fit to the fatigue data generated, a fatigue coefficient (S) of 675 MPa and fatigue expo- nent (b) of -0.048 was achieved. In-situ ultrasonic mesoscale testing has also shown the ability to track crack growth and demonstrate the importance of fatigue crack initiation resistance when designing for high cycle fatigue resistance. In addition, the testing method has been able to distin- guish between sample surface conditions and calculate the fatigue performance deficit as a result. Fractography data has also informed that fatigue cracks in these specimens initiate primarily from the surface and the edges of the beam. Furthermore, fractography has allowed for the observation of the steady-state nature of crack growth as the crack extends into the centre of the beam.</p>
<p>A significant correlation was established between increased local GND density as revealed by HR-EBSD, and local surface slip features imaged in SEM, with grain boundaries at which the slip systems available in the two grains were misaligned. Increased plastic work and poor stress transfer between the misaligned shear planes lead to slowing of the fatigue crack and/or a change in crack path generally increasing resistance to short fatigue crack growth at these features.</p> |
| first_indexed | 2024-03-07T00:48:24Z |
| format | Thesis |
| id | oxford-uuid:857ceea6-56d3-4726-abff-216a7fd2837e |
| institution | University of Oxford |
| language | English |
| last_indexed | 2024-03-07T00:48:24Z |
| publishDate | 2019 |
| record_format | dspace |
| spelling | oxford-uuid:857ceea6-56d3-4726-abff-216a7fd2837e2022-03-26T21:57:59ZMicroscale and ultrafast high cycle fatigue testing of titaniumThesishttp://purl.org/coar/resource_type/c_db06uuid:857ceea6-56d3-4726-abff-216a7fd2837eMaterials scienceEnglishHyrax Deposit2019Arutyunyan, AWilkinson, AGong, J<p>The fatigue properties and crack growth behaviour in Grade 2 commercially pure titanium (CP Ti) was investigated using a miniature ultrafast testing method. Using finite element modelling (FEM) a dogbone type mesoscale cantilever specimen was designed, that facilitated the shift of stress concentrations from sharp corners in a simple initial cantilever design to the centre of a beam. This ensured that failure was more dependent on microstructure than sample geometry.</p> <p>Monotonic single crystal microscale cantilever tests were performed that isolated <a> prism, <a> basal and <c+a> first order pyramidal planes. The bulk critically resolved shear stress (CRSS) for these slip systems were 130 MPa, 298 MPa and 795 MPa respectively. These experiments showed that slip is most likely to occur on <a> prism planes. In addition, using von Mises cri- terion to determine the yield stress from the CRSS of <a> prism (225 MPa) was comparable to the mean bulk 0.2 yield stress measured by monotonic tensile tests (243 MPa). The mono- tonic bulk tensile tests on typical dogbone samples also demonstrated that the elastic modulus along the length axis of a microcantilever as measured from the microscale tests of <a> prism had good agreement with bulk data (~105 GPa). It has also been shown that in ultrafast mesoscale tests there is good agreement with both microscale and macroscale tests as it has been shown that failure is extremely unlikely to occur below ~240 MPa.</p> <p>By comparing both single-step and multi-step testing approaches to ultrasonic fatigue test- ing it has been shown that there is little/no coaxing effect in Grade 2 CP Ti. By fitting a Basquin power law fit to the fatigue data generated, a fatigue coefficient (S) of 675 MPa and fatigue expo- nent (b) of -0.048 was achieved. In-situ ultrasonic mesoscale testing has also shown the ability to track crack growth and demonstrate the importance of fatigue crack initiation resistance when designing for high cycle fatigue resistance. In addition, the testing method has been able to distin- guish between sample surface conditions and calculate the fatigue performance deficit as a result. Fractography data has also informed that fatigue cracks in these specimens initiate primarily from the surface and the edges of the beam. Furthermore, fractography has allowed for the observation of the steady-state nature of crack growth as the crack extends into the centre of the beam.</p> <p>A significant correlation was established between increased local GND density as revealed by HR-EBSD, and local surface slip features imaged in SEM, with grain boundaries at which the slip systems available in the two grains were misaligned. Increased plastic work and poor stress transfer between the misaligned shear planes lead to slowing of the fatigue crack and/or a change in crack path generally increasing resistance to short fatigue crack growth at these features.</p> |
| spellingShingle | Materials science Arutyunyan, A Microscale and ultrafast high cycle fatigue testing of titanium |
| title | Microscale and ultrafast high cycle fatigue testing of titanium |
| title_full | Microscale and ultrafast high cycle fatigue testing of titanium |
| title_fullStr | Microscale and ultrafast high cycle fatigue testing of titanium |
| title_full_unstemmed | Microscale and ultrafast high cycle fatigue testing of titanium |
| title_short | Microscale and ultrafast high cycle fatigue testing of titanium |
| title_sort | microscale and ultrafast high cycle fatigue testing of titanium |
| topic | Materials science |
| work_keys_str_mv | AT arutyunyana microscaleandultrafasthighcyclefatiguetestingoftitanium |