@phdthesis{8678012,
  abstract     = {{One of the engine components subjected to the most challenging thermal and mechanical loads is the cylinder head. From the block side, it is directly exposed to the ignition process, with the corresponding temperature and pressure peaks. Furthermore, it is structurally weakened by means of the intake and exhaust bores that are necessary to transport the atmospheric air and combustion residue flows to and from the combustion cylinder. Inside the cylinder head, cooling channels induce a temperature gradient towards the block. Furthermore, from the top side, structural constraints caused by the fixture mechanisms to the cylinder block, induce mechanical restrictions. Such constraints and thermal gradients in conjunction with engine start-up/shut-down (use related) loops, lead to sequential compression (high temperature) and tension (room temperature) cycles. The phenomenon known as Thermo-Mechanical Fatigue (TMF) may affect some areas of the cylinder head limiting the lifetime of the entire engine. In particular, it is known that the thin region in between the exhaust valve bores on the block side of the cylinder head (called the Valve Bridge (VB)), is especially sensitive to this phenomena. Consequently, a material with both competitive mechanical resistance and thermal conductivity is required for this application. Compacted Graphite Iron (CGI) is the material of choice for engine cylinder heads of heavy-duty trucks. CGI provides the best possible compromise between optimum mechanical properties, compared to flake graphite iron, and optimum thermal conductivity, compared to spheroidal graphite iron. The vermicular-shaped graphite particles, however, act as stress concentrators, and, as a result of delamination from the metal matrix are responsible for crack initiation during the engine TMF cycles. TMF laboratory tests are intended to apply a mechanical constraint to the thermal strain cycles in a tested specimen. The statistical analysis of the relationship between microstructural features on the one hand (such as graphite size, density of graphite particles, morphological aspects, crystal orientation) and macroscopic mechanical features (such as crack growth rate, bulk plastic strain or lifetime) on the other hand, will supply reliable data to computational models, which reduces experimental test time and can be used to reduce the cost of engine design and possibly to reduce emissions and fuel consumption. Consequently, various test setups were implemented to reproduce the TMF behavior of the VB areas, which are specifically prone to TMF. In these laboratory tests, various mechanical boundary conditions were applied including single and double constraints at low and high temperatures. Such designed boundary conditions were created considering two criteria. First, literature well-documented constraint conditions led to single constraint setups that were used for comparison purposes. Second, the output of Finite Elements (FE) computations
considering combustion thermodynamics, fluid dynamics, and the mechanical properties of CGI showed a double constraint behavior of the VB in the cylinder head. Finished the tests, hysteresis loops, hardening and plastic strain calculations were used to evaluate and compare single and double constraint setups. It was found a reduction of the lifetime with increase test severity. Furthermore, the TMF lifetime was satisfactorily modeled implementing the Paris Crack-Growth Law. It was proven that the law description is valid under the wide range of tested boundary conditions. Besides, data post-processing allowed to define an equivalent constraint value based on the Paris Law Cp coefficient that is a single constraint test that yields an identical lifetime as the experiment with a double constraint test. TMF tests were performed using smooth samples, but also notch specimens in order to force crack initiation. However, some notched samples exhibited crack initiation out of the grove. Furthermore, the crack path was found to be meandering and not related to the boundary conditions. This milestone motivated a detailed inquiry of the microstructural influence of matrix and graphite on both, crack initiation and propagation. When it comes to crack initiation studies, the in-situ test is a strong candidate. The technology for the In-situ TMF test is highly specialized but also restricted for testing facilities, sample dimensions, and boundary conditions among others. In return, it was developed a semi in-situ technique that uses existing testing and characterization equipment with a modified dog bone sample (extending in-situ scope). The technique integrates Scanning Electron Microscopy (SEM) and TMF boundary conditions to identify damage and the crack initiation event in CGI during the first TMF cycles of total constraint. In the first cycle, at the bulk graphite particles, it was found delamination perpendicularly oriented to the load direction, whereas at the sample surface cracks linking graphite particles were detected. Based on the observations and some calculations, it is proposed that the latter phenomena are a combination of graphite length distribution, clustering and size of the cyclic plastic zone ahead of the crack and graphite particles. The proposed crack initiation mechanism was used to correct the surface condition of smooth specimens finding good data correlation with literature. Regarding crack propagation, the fracture volume of two failed dog bone samples tested under double constraint TMF were scanned and reconstructed. It was captured enough number of graphite and matrix particles to perform a statistically representative study. The volumes reconstruction showed that there was no influence of the matrix cleavage plane on the crack propagation path. Instead, it was found that the clustered distribution of graphite particles guided the crack propagation path since it was found a possible interaction between the cyclic plastic zones of neighbor delaminated graphite particles and the crack itself. Based on the crack initiation and propagation finds it is clear that the CGI TMF lifetime could be improved if delamination is delayed and therefore crack initiation. In consequence, scratch tests driven over the matrix and into the graphite particles were performed in order to understand and characterize the strength of the metal–graphite interface. Samples extracted from a cylinder head in As Cast condition were compared to samples subjected to a heat-treatment at 700˚C for 60 h. The former samples were composed of primarily pearlitic matrix and graphite particles, whereas, after annealing, a certain pearlite fraction decomposed into Fe and C, producing a microstructure with graphite–ferrite interfaces, exhibiting a partially graphite spiky superficial morphology. The scratch test revealed that the ferrite–graphite interfaces with spiky nature exhibited a stronger resistance to delamination compared to the ferrite–graphite interfaces with smooth morphology. In brief: There were designed TMF setups closer to real engine conditions. The mechanical response was understood and compared with the literature. Furthermore, it was made a link between microstructure and mechanical properties leading to a complete understanding and description of TMF mechanical and microstructural behavior during crack initiation, propagation, and failure.}},
  author       = {{Lopez Covaleda, Edwin Alexis}},
  isbn         = {{9789463553803}},
  keywords     = {{Thermomechanical fatigue,EBSD,Cast Iron}},
  language     = {{eng}},
  pages        = {{xxi, 143}},
  publisher    = {{Universiteit Gent. Faculteit Ingenieurswetenschappen en Architectuur}},
  school       = {{Ghent University}},
  title        = {{Thermomechanical fatigue in cast iron used in truck engines cylinder heads}},
  year         = {{2020}},
}