scholarly journals Thermo-Mechanical Fatigue Lifetime Assessment of Spheroidal Cast Iron at Different Thermal Constraint Levels

Metals ◽  
2019 ◽  
Vol 9 (10) ◽  
pp. 1068 ◽  
Author(s):  
Ghodrat ◽  
Kalra ◽  
Kestens ◽  
Riemslag
Keyword(s):  
Cast Iron ◽  
Crack Growth ◽  
Crack Tip ◽  
Local Strain ◽  
Compacted Graphite ◽  
Cyclic Plastic ◽  
Paris Law ◽  
Mechanical Fatigue ◽  
Strain Model ◽  
Constraint Level

In 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.

Microstructural Evolution of Compacted Graphite Iron under Thermo-Mechanical Fatigue Conditions

2011 ◽  
Vol 409 ◽  
pp. 757-762 ◽  
Author(s):  
S. Ghodrat ◽  
M. Janssen ◽  
Roumen H. Petrov ◽  
Leo Kestens ◽  
Jilt Sietsma
Keyword(s):  
Mechanical Properties ◽  
Cast Iron ◽  
Elevated Temperatures ◽  
Easy Access ◽  
Compacted Graphite Iron ◽  
Microstructural Changes ◽  
Oxide Layers ◽  
Cooling Cycle ◽  
Compacted Graphite ◽  
Mechanical Fatigue

Cast iron components in combustion engines, such as cylinder blocks and heads, are exposed for long periods of time to elevated temperatures and subjected to large numbers of heating and cooling cycles. In complex components, these cycles can lead to localized cracking due to stresses that develop as a result of thermal gradients and thermal mismatch. This phenomenon is known as Thermo-Mechanical Fatigue (TMF). Compacted Graphite Iron (CGI) provides a suitable combination of thermal and mechanical properties to satisfy the performance of engine components. However, TMF conditions cause microstructural changes, accompanied by the formation of oxides at and close to the surface, which together lead to a growth in size of the cast iron. These microstructural changes affect the mechanical properties and accordingly the thermo-mechanical fatigue properties. The aim of this research is to provide insight into the microstructure evolution of CGI, with its complex morphology, under TMF conditions. For this, optical and scanning electron microscopy observations are made after cyclic exposure to air at high temperature, both without and with mechanical loading. It was found that the oxide layers, which develop at elevated temperatures, crack during the cooling cycle of TMF. The cracking results from tensile stresses developing during the cooling cycle. Therefore, paths for easy access of oxygen into the material are formed. Fatigue cracks that develop also show oxidation at their flanks. In order to quantify the oxide layers surrounding the graphite particles, Energy Dispersive X-Ray Analysis (SEM-EDX) and Electron Probe Micro Analysis (EPMA) are used.

EFFECT OF HYDROGEN ON UNIAXIAL TENSILE BEHAVIORS OF A DUCTILE CAST IRON

2012 ◽  
Vol 06 ◽  
pp. 407-412
Author(s):  
K. Matsuno ◽  
H. Matsunaga ◽  
M. Endo ◽  
K. Yanase
Keyword(s):  
Cast Iron ◽  
Strain Rate ◽  
Crack Growth ◽  
Crack Tip ◽  
Thermal Desorption Spectroscopy ◽  
Ductile Cast Iron ◽  
Uniaxial Tensile ◽  
Matrix Interface ◽  
Hydrogen Charging ◽  
Graphite Matrix

Effect of the hydrogen-charging on the uniaxial tensile behaviors of a ductile cast iron was investigated. It was found that the hydrogen-charging accelerated the process of crack growth from graphite in the uniaxial tensile loading condition. Further, the accelerated crack growth had a marked influence on the reduction of area at the final fracture (RA) of specimens. For instance, for the uncharged specimens, the RA was nearly constant irrespective of the strain rate. In contrast, for the hydrogen-charged specimens, the RA gradually decreased as the strain rate decreased. Thermal desorption spectroscopy and hydrogen microprint technique revealed that, in the hydrogen-charged specimen, most of solute hydrogen was diffusive one, which was mainly segregated at graphite, graphite/matrix interface zone and pearlite. Based on these experimental observations, we consider that the hydrogen-induced degradation behavior was caused mainly by a combination of the following three mechanisms: (i) supplement of hydrogen to the crack tip from graphite and graphite–matrix interface, (ii) hydrogen-enhanced pearlite cracking and, (iii) successive hydrogen-emission from graphite and additional hydrogen-supplement to the crack tip.

A Model of Fatigue Crack Growth Based on Damage Accumulation

2004 ◽  
Author(s):  
Satish Chand ◽  
K. N. Pandey
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Plastic Zone ◽  
Crack Growth ◽  
Damage Accumulation ◽  
Process Zone ◽  
Crack Tip ◽  
Cyclic Plastic ◽  
Cyclic Plastic Zone ◽  
Material Element

A fatigue crack growth model based on cumulative damage is presented, when a material element ahead of the crack tip, is approached by the tip of the crack. The cyclic plastic zone and process zone ahead of the crack tip are taken as the area where damage accumulation takes place when the material element, first, enters into the cyclic plastic zone and then into the process zone. During this period, the Coffin-Manson damage law in conjunction with Miner’s linear damage accumulation is used to determine the damage in the material element. A constant strain gradient was assumed along the process zone ahead of the crack tip and the size of process zone was taken to be variable and dependent on the range of stress intensity factor. For a cyclic loading, the effective crack driving force takes into consideration the crack tip blunting process. The model results are in good agreement with experimental data available in literature for a number of materials.

Assessment of Fatigue Crack Growth Data Available for the P355NL1 Steel Using a Local Strain-Based Approach

2011 ◽  
Author(s):  
Abi´lio M. P. de Jesus ◽  
Jose´ A. F. O. Correia
Keyword(s):  
Residual Stress ◽  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Propagation ◽  
Crack Growth ◽  
Fatigue Crack Propagation ◽  
Crack Tip ◽  
Local Strain ◽  
Element Analysis ◽  
R Ratio

Fatigue crack growth models based on elastic–plastic stress–strain histories at the crack tip region and strain-life damage models have been proposed in literature. The UniGrow model fits this particular class of fatigue crack propagation models. The residual stresses developed at the crack tip play a central role in these models, since they are used to assess the actual crack driving force, taking into account mean stress and loading sequence effects. The performance of the UniGrow model is assessed based on available experimental constant amplitude crack propagation data, derived for P355NL1 steel. Key issues in fatigue crack growth prediction using the UniGrow model are discussed; in particular, the assessment of the elementary material block size, the elastoplastic analysis used to estimate the residual stress distribution ahead of the crack tip and the adopted strain-life relation. The use of finite element analysis to estimate the residual stress field, in lieu of a simplified analysis based on the analytical multiaxial Neuber approach, and the use of the Morrow strain-life equation, resulted in fatigue crack propagation rates consistent with the experimental results available for P355NL1 steel, for several stress R-ratios. The use of the SWT parameter for the local strain-life relation, which has often been proposed in the literature, leads to overprediction of stress R-ratio effects.

scholarly journals Study on the Influence of the Gurson–Tvergaard–Needleman Damage Model on the Fatigue Crack Growth Rate

Metals ◽  
2021 ◽  
Vol 11 (8) ◽  
pp. 1183
Author(s):  
Edmundo R. Sérgio ◽  
Fernando V. Antunes ◽  
Diogo M. Neto ◽  
Micael F. Borges
Keyword(s):  
Growth Rate ◽  
Fatigue Crack ◽  
Crack Growth Rate ◽  
Fatigue Crack Growth ◽  
Plastic Strain ◽  
Crack Growth ◽  
Damage Model ◽  
Crack Tip ◽  
Strength Parameter ◽  
Stress Intensity Factor Range

The fatigue crack growth (FCG) process is usually accessed through the stress intensity factor range, ΔK, which has some limitations. The cumulative plastic strain at the crack tip has provided results in good agreement with the experimental observations. Also, it allows understanding the crack tip phenomena leading to FCG. Plastic deformation inevitably leads to micro-porosity occurrence and damage accumulation, which can be evaluated with a damage model, such as Gurson–Tvergaard–Needleman (GTN). This study aims to access the influence of the GTN parameters, related to growth and nucleation of micro-voids, on the predicted crack growth rate. The results show the connection between the porosity values and the crack closure level. Although the effect of the porosity on the plastic strain, the predicted effect of the initial porosity on the predicted crack growth rate is small. The sensitivity analysis identified the nucleation amplitude and Tvergaard’s loss of strength parameter as the main factors, whose variation leads to larger changes in the crack growth rate.

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