5 research outputs found
Thermo-mechanical fatigue lifetime assessment of spheroidal cast I\iron at different thermal constraint levels
No full textIn previous work on the thermo-mechanical fatigue (TMF) of compacted graphite iron (CGI), lifetimes measured under total constraint were confirmed analytically by numerical integration of Paris’ crack-growth law. In current work, the results for CGI are further validated for spheroidal cast iron (SGI), while TMF tests at different constraint levels were additionally performed. The Paris crack-growth law is found to require a different CParis parameter value per distinct constraint level, indicating that Paris’ law does not capture all physical backgrounds of TMF crack growth, such as the effect of constraint level. An adapted version of Paris’ law is developed, designated as the local strain model. The new model considers cyclic plastic strains at the crack tip to control crack growth and is found to predict TMF lifetimes of SGI very well for all constraint levels with a single set of parameters. This includes not only full constraint but also over and partial constraint conditions, as encountered in diesel engine service conditions. The local strain model considers the crack tip to experience a distinct sharpening and blunting stage during each TMF cycle, with separate contributions to crack-tip plasticity, originating from cyclic bulk stresses in the sharpening stage and cyclic plastic bulk strains in the blunting stage.Emerging Materials(OLD) MSE-3(OLD) MSE-
A Comparative Study of Thermo-Mechanical Fatigue Performance of Different Grades of SiMo Nodular Cast Iron
No full textThis thesis is a comparative study of the Thermo-Mechanical Fatigue (TMF) performance of different grades of SiMo nodular cast iron for heavy-duty diesel engine exhaust gas manifold applications. The TMF performance of the current SiMo variant used to manufacture exhaust manifolds - SiMo 5.10 (C-3.25Si-4.45Mo-0.76), is compared with that of the variants SiMo 4.05 (C-3.22Si-4.66Mo-0.56) and SiMoNi (C-3.3Si-4.5Mo-1Ni-1.3) by performing three out-of-phase (OP) TMF test series under partial constraint conditions. A benchmark TMF test series in the temperature range: 50 ˚C to 550 ˚C with a hold time of 30 s at 550 ˚C showed that SiMo 5.10 had relatively better performance due to development of lower mechanical crack driving forces compared to other variants. However, a long holding time of 600 s at 550 ˚C saw a larger decrease of average TMF lifetimes for SiMo 5.10 than that of SiMo 4.05 despite similar crack driving forces. An investigation of the stress relaxation during TMF of the two variants showed that the SiMo 4.05 performs better during long hold time due to better stress relaxation properties. The SiMoNi variant which is very brittle at low temperatures was found to fail by a fracture by overloading mechanism taking over quite early in the fatigue cycle; which is confirmed by examination of the fracture surfaces and numerical estimations. This also explained the low lifetimes and scatter in previously performed TMF tests under total constraint conditions. The TMF test series performed in the temperature range: 150 ˚C to 550 ˚C with a hold time of 30 s at 550 ˚C found that a heat-treatment seemed to reduce the TMF performance of the SiMo 5.10 variant. Metallographic investigations and hardness measurements of as-cast and heat-treated materials revealed that the distribution of the Mo-rich phase from the grain boundary regions into the matrix due to an annealing heat-treatment seemed to affect the TMF performance.Materials Science and Engineerin
Thermo-mechanical fatigue lifetime assessment of spheroidal cast iron at different thermal constraint levels
No full textIn previous work on the thermo-mechanical fatigue (TMF) of compacted graphite iron (CGI), lifetimes measured under total constraint were confirmed analytically by numerical integration of Paris' crack-growth law. In current work, the results for CGI are further validated for spheroidal cast iron (SGI), while TMF tests at different constraint levels were additionally performed. The Paris crack-growth law is found to require a different C-Paris parameter value per distinct constraint level, indicating that Paris' law does not capture all physical backgrounds of TMF crack growth, such as the effect of constraint level. An adapted version of Paris' law is developed, designated as the local strain model. The new model considers cyclic plastic strains at the crack tip to control crack growth and is found to predict TMF lifetimes of SGI very well for all constraint levels with a single set of parameters. This includes not only full constraint but also over and partial constraint conditions, as encountered in diesel engine service conditions. The local strain model considers the crack tip to experience a distinct sharpening and blunting stage during each TMF cycle, with separate contributions to crack-tip plasticity, originating from cyclic bulk stresses in the sharpening stage and cyclic plastic bulk strains in the blunting stage
Weibull analysis of the effect of modified aging treatments on fatigue life of cast aluminium alloy 354
No full textCast aluminium alloy 354 has extensive applications in the automobile industry. Due to its attractive combination of mechanical properties and excellent castability, it is being used in production of automobile components like the compressor wheel for turbochargers. Performance of this component under fatigue loading conditions is a critical issue. The present study explores the possibility of improving the fatigue life of the component by bringing in process changes - (i) adopting a two-step aging treatment in place of the normally used single step aging treatment (ii) adopting a lower artificial aging temperature (171°C) instead of the temperature normally used for artificial aging(188°C) while performing T61 treatment. In all cases Weibull analysis of fatigue test results was carried out. Weibull analysis of Ultimate Tensile Strength (UTS) values obtained after artificial aging at 171°C and 188°C was also carried out. Among the four variants of two-step aging treatment carried out, the one consisting of 100°C for 5 hours followed by 170°C for 5 hours was found to have the best characteristic fatigue life for the components. The modified T61 treatment where aging was carried out at 171°C instead of the normally used 188°C yielded better characteristic fatigue life as well as better Ultimate Tensile Strength (UTS). </p
Weibull analysis of the effect of interrupted aging treatments on the fatigue life of components made of cast Aluminium alloy 354
No full textCast aluminum alloy 354 is widely used in the automobile industry due to its attractive set of mechanical properties and excellent castability. The compressor wheel in turbochargers, for example, is used for the production of this alloy. Apart from mechanical properties like fracture toughness and tensile strength, the fatigue life of the component is also a critical issue while considering the performance. This study makes an attempt to improve the fatigue life of a component made out of this alloy by subjecting it to interrupted aging cycles similar to T6I4 and T6I6 (discussed in the published literature) instead of the normally used T61 standard aging treatment. Results show that subjecting the material to these interrupted aging treatments gives lower fatigue life than that obtained after subjecting it to standard T61 conditions.Also, T6I4 treatment yields better fatigue life as compared to T6I6. </p
