Microstructure-based material model forthermo-mechanical fatigue of cylinder heads

J.C. Pina, V.G. Kouznetsova, M.G.D. Geers

M2i-Materials Innovation InstituteEindhoven University of Technology, Faculty of Mechanical Engineering

P.O. Box 513, 5600 MB Eindhoven, The NetherlandsPhone: 040-2475169, e-mail: J.C.Pina@tue.nl

IntroductionThermo-mechanical fatigue (TMF) arises as a conse-quence of thermal related stresses that develop dueto thermo-mechanical cycling loading.TMF failure in truck engines can be described as:

• Engine start-stop cycle results in large temperaturevariations.

• The constrained condition found in cylinder heads,produces compressive and tensile stresses.

• Continuous thermal cycling produce valve bridgecracking, resulting in cylinder head TMF failure(Figure 1).

DAF XF Truck engine

DAF XF Truck cylinder head

TMF cracks at cylinderhead valve bridge

Figure 1 : TMF cracks at valve bridges (Courtesy ofDAF Trucks N.V.).

Cast iron heterogonous microstructure (fer-rite/pearlite matrix & graphite inclusions, Figure 2),makes prediction of its response at the macro levelvery complex.

a) Flake graphite b) Compacted graphite c) Spheroidal graphite

Figure 2 : Graphite morphologies found in cast irons [1].

Aim of the project Develop a microstructure-basedmodel for TMF life prediction that considers the differ-ent microstructural phases, their interaction and im-pact on TMF response.

Modelling approachThe basic phenomena to include in the model are:

• 3D Representative Volume Element (RVE) sincethe microstructure is inherently three-dimensional.

• Microcrack initiation & propagation through thegraphite-matrix interface.

• Graphite anisotropy of mechanical & thermal prop-erties.

• Creep & stress relaxation.• Thermo-mechanical cycling.

Example Figure 3 shows a microstructure-basedmaterial model for nodular cast iron [2].

Equivalent von Mises stress

3.80e+08

3.70e+08

3.60e+08

3.50e+08

3.40e+08

3.30e+08

3.20e+08

3.10e+08

3.00e+08

2.90e+08

2.80e+08

Figure 3 : Microstructure-based material model ofnodular cast iron under static tensile loading (verticaldirection) [2].

Future work• Improvement of existing cast iron model [2], by in-

cluding graphite anisotropy and the different matrixphases (pearlite & ferrite).

• Development of cast iron microstructure-basedmodel for thermo-mechanical loading conditions,including creep and stress relaxation.

• Development of cast iron microstructure-basedmodel for TMF life prediction, where the influenceof damage evolution at the micro level is incorpo-rated at the macro level.

References1. Sjogren, T. (2005), PhD Thesis, Jonkoping University, Swe-

den.2. van den Oord, G.A.H. (2009), Master Thesis, Eindhoven Uni-

versity of Technology, The Netherlands.