PSI - Issue 57

Giovanni M. Teixeira et al. / Procedia Structural Integrity 57 (2024) 670–691 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

683

14

= 1 1 2 ⋅ − 4 1 ⋅ ( ′ ⋅ TMF ) − 2 1 The integration of equation 53 results in: =( 1 2 − 1 0 1 2 2 ) 1 14 ( ′ TMF ) − 1 2

(54)

(55)

Note the remarkable similarity of equations 50 and 55. The total damage caused at every reversal ( the n subscripts in equation 55 below ) is calculated from both equations 50 and 55 as per Palmgren-Miner: 1 =( 1 fc + 1 ox ) 1 +( 1 fc + 1 ox ) 2 +⋯( 1 fc + 1 ox ) (56) Since the DTMF approach accounts for creep by means of a modifying factor (F cr ), the best way to evaluate the creep contribution to damage is by evaluating equation 49 having the creep factor (F cr ) set to its real value and having the creep factor (F cr ) set to 1. The difference is the creep contribution. 3. The Exhaust Manifold Exhaust manifolds are the automotive components responsible for collecting the gases burned in the multiple combustion chambers into just one exhaust pipe that drives the gases to the catalytic converter and the muffler, filtering the toxic components and reducing noise. The exhaust manifolds are generally made of cast iron or cast stainless steel, depending on the application and operating conditions ( maximum gas temperature ). The SiMo (Silicon-Molybdenum) family of materials is recommended for Diesel engines where the exhaust gas reaches temperatures about 750 o C [19]. It is basically a ferritic ductile cast iron that presents excellent castability. The austenitic ductile cast iron (e.g., Ni-resist D5S) and ferritic cast stainless steel (e.g., Star Cast DCR3) are used for applications where the temperature is in the range of 750 o C to 950 o C. The austenitic cast stainless steel (e.g., HK30, EN 1.4980) is used for the most critical operating conditions where the gas temperatures can reach values higher than 950 o C. It is highly corrosion resistant, durable, formable and weldable. Figure 11 below shows a finite element representation of an exhaust manifold bolted to the cylinder head.

Fig. 11. Finite Element Model of a Typical Exhaust Manifold

The thermomechanical cycle consists in stresses and strains that are generated from the moment the model starts