Finite Element Thermomechanical Analysisof an Exhaust Manifold System of anEngine for Automotive ApplicationsMatteo Giacopini, Roberto Rosi, Simone Sissa
Outline• Aim and scope of the activity• Modeling strategy, based on thermal and structural decoupled simulations• Thermal analysis: gas/solid heat transfer• Mechanical analysis: thermostructural loading FEM analysis• Fatigue analysis: energy based LCF criterion• Conclusions
Aim and scopeThe present activity take its bases from some crack propagations that actually havebeen observed during engine bench tests. In particular, cracks have been detected inthe region of the manifold bellows:This activity aims at evaluating the mechanical behaviour of the exhaust manifoldsystem and at investigating if low cycles fatigue phenomena could be identified as thereason behind these crack propagations.
Modeling strategyNon-linear Finite Element models have been employed to mimic the manifold systembehaviour when subjected to the exhaust gases thermal loading.Engine Head InterfacesThe modeling strategy developed in this study consists in separated thermal andmechanical simulations, performed using the commercial Finite Element softwareAltair HyperMesh and RADIOSS (Bulk Data Format) solver v12.0.Engine Block InterfacesTurbineFlangeExhaust ManifoldBellows
Modeling strategyA model of the bellows alone is then employed to better investigate the mechanicalbehaviour of the bellows in the crack regions.Double layer bellows
Modeling strategyParticular care has been devoted to the mesh generation process. A uniformthickness boundary layer has been created on the manifold surface.Advantage:1. Better contact detection2. Optimal surface resolution in stress and strain calculation
Thermal analysisThe thermal analysis is based on a previous CFD 1D-3D coupled simulation of awhole engine cycle.Due to high thermal inertia of metals, the instantaneous gas temperatures and heattransfer coefficients have been cycle-averaged and they have been mapped on theexhaust system FEM model with an ad-hoc developed fortran routine.HTC [W/mm2/K]
External surfacesh = 5∙10-5 [W/mm2/K]T = 60 [°C]Engine HeadT = 170 [°C]Thermal analysisExhaust gasesh = mapped from previous CFD simulationsT = 820 [°C]Thermal boundary conditions:• Heat transfer coefficient and reference temperatures on external surfaces andinternal surfaces touched by exhaust gases.• Fixed temperature on the engine head interfaces.
Thermal analysisThermal contacts are modeled with CGAP elements and the contact thermalresistance for closed gap imposed in PGAPHT card is estimated with the Yovanovicexpression for thermal resistance of conforming rough contacts as follow:where:𝑘 𝑠: thermal conductivity [W/m/K]𝑚: mean absolute surface slope𝜎: rms surface roughness [μm]𝑃: pressure [Pa]𝐻 𝑚𝑖𝑐: microhardness of the softer material in contact [GPa]𝐴: contact area [m]𝑛_𝐶𝐺𝐴𝑃 : number of CGAP element involved in contact surfaceℎ 𝑠 = 1.25𝑘 𝑠𝑚𝜎𝑃𝐻 𝑚𝑖𝑐0.95𝐾𝐴𝐻𝑇 =ℎ 𝑠 𝐴𝑛_𝐶𝐺𝐴𝑃
Mechanical analysisThe mechanical model consists in a subsequent applying and removal of thetemperature distribution. The aim is to evaluate if some point eventually undergoes aplastic hysteresis cycle.Bolt Tightening1 I Warming2I Cooling3II Warming4II Cooling5
Mechanical analysisBolt tighteningF = 22000 [N]Engine head/block interfacesdof1 = 0dof2 = 0dof3 = 0Engine head/block interfacesdof1 ≠ 0dof2 ≠ 0dof3 ≠ 0Mechanical boundary conditions:• Fixed zero displacements are applied on the engine head and engine blockinterfaces during bolt tightening.• Displacements evaluated with a previous engine head/block thermomechanicalFEM analysis are superimposed to the model during warming/cooling loadings.
Mechanical analysisThe non-linear temperature-dependent mechanical behaviour of all the materialsinvolved in the analysis has been considered to carry out plastic strain by simulations.
Mechanical analysisDue to the high temperatures and to the particular bellows geometry, thermal lowcycle fatigue phenomena could be considered as a possible reason of crack initiation.The employed approach consists in the computation of the plastic energy dissipatedper cycle at a certain location, whose value can be directly correlated to the low cyclefatigue life of the component.Thermal Load HistoryPlastic Strain/StressHysteresis CycleLow Cycle Fatigue-300-200-1000100200300-0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08stress[MPa]plastic strainstrass - plastic strain curves20 °C250 °C
Mechanical analysis resultsResults obtained by the previous whole model analysis show two critical areaslocated at the bellows extremities. Therefore, a model of the bellows alone has beencreated to better evaluate stress and strain in these regions.
Mechanical analysis resultsDisplacements previously evaluated on the welded joints at the both sides of thebellows have been applied to the corresponding nodes of the bellows alone model.REB2 used to imposeglobal displacements
Conclusions• A thermomechanical analysis has been performed in order to evaluate thethermomechanical behaviour of an exhaust system;• First, decoupled thermal and mechanical FEM models have been developed of thewhole exhaust system. Secondly, a mechanical model of the bellows alone has beenintroduced to better compute stress and strain in this critical region;• An energetic LCF criterion based on the plastic dissipated energy per cycle hasbeen employed in order to rationalise some crack propagations observed in thebellows during engine bench tests;• The methodology has been shown to be able to correctly locate the most criticalareas in terms of Low Cycle Fatigue life and it constitutes a valid instrument for futuredesign optimizations of the bellows geometry.