@phdthesis{oai:sucra.repo.nii.ac.jp:00010290,
 author = {AKM, Asif Iqbal},
 month = {},
 note = {xiv, 110 p., The development of metal matrix composites (MMCs) has set the stage for a new revolution in materials. The cast hybrid MMC reinforced with 21 vol% of SiC particles and 9 vol% of Al2O3 whiskers is developed to use as a brake disc of high speed rail way coach. The fatigue properties of this material are of critical interest as MMC materials suffer cyclic plastic deformation in the structural applications. When a component is subjected to relatively high cyclic loads, high densities of microcracks form simultaneously. From one crack, a larger crack grows that eventually propagates and causes failure. Elucidating the mechanisms of the initiation and growth of this main crack are essential to understanding and predicting the fatigue life of a material. Therefore, the evaluation of fatigue life and the fracture mechanism of cast hybrid MMC have been proposed in this research. The initiation mechanism of fatigue microcracks and stage by stage growth till to final failure as well as fatigue crack growth in the near-threshold and stable-crack-growth regions were examined in three different types of materials: cast hybrid MMC (SiCp+ Al2O3 whisker), cast MMC with Al2O3 whisker and monolithic cast Al alloy. The effect of hybrid reinforcement on the fatigue crack initiation and propagation mechanisms were explained through the comparison of the observed mechanism in the three materials. Conventional three point bending fatigue tests were performed in a rectangular bar smooth specimen. The plastic replica technique was used to observe the initiation and early propagation of microcracks at various times during the fatigue life. The tensile and fracture surfaces were comprehensively examined using scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS) to characterize the crack initiation site. In Al alloy, microcracks were observed to initiate in the Al grain, but when the matrix was reinforced, the initiation location changed to the whisker-matrix and particle-matrix interfaces and the hybridization effect reduced the resistance to crack initiation. Moreover, the two MMC materials exhibited similar interface debonding in fracture, which created additional secondary microcracks due to continued fatigue cycling. Numerous voids were formed ahead of the crack tip, and the microcracks intersected with other nearby microcracks. However, in the Al alloy, the microcracks propagated through the boundaries between Si particle clusters and the Al grain through void nucleation and coalescence or through striation formation in the Al grain. To observe the fatigue crack growth (FCG) in the near-threshold and stable-crack growth regions in hybrid MMC during high cycle fatigue, experiments were carried out on rectangular bar single edge notched specimen in accordance with the guidelines in ASTM E647. The hybrid MMC showed a higher threshold stress intensity factor range, ΔKth, than the MMC with Al2O3 whisker and Al alloy, indicating better resistance to crack growth in a lower stress intensity factor range, ΔK. This effect occurred due to the higher roughness induced crack closure in hybrid MMC. Moreover, in the near-threshold region with decreasing ΔK, the two composite materials exhibited similar debonding of the reinforcement–matrix interface due to the modulus mismatch followed by void nucleation and coalescence in the Al matrix. At higher ΔK in the stable- or mid-crack-growth region, in addition to the debonding of the particle-matrix and whisker-matrix interface caused by cycle-by-cycle crack growth on the interface, the FCG occurred predominantly by striation formation in the Al matrix. Moreover, void nucleation and coalescence in the Al matrix and transgranular fracture of SiC particles and Al2O3 whiskers at high ΔK were also observed as the local unstable fracture mechanisms. However, the FCG of the monolithic Al alloy was dominated by void nucleation and coalescence at lower ΔK, whereas the FCG at higher ΔK was controlled mainly by striation formation in the Al grains followed by void nucleation and coalescence in the Si clusters. In order to validate the experimental results, numerical analysis was conducted by using finite element method (FEM) to observe the characteristics of the elastic-plastic stress field in the clustering and non-clustering regions of cast hybrid MMC. The numerical analysis confirmed that the high stress was developed in the reinforcements located in the clustering region and stress concentration occurred on the particle-matrix interfaces. Moreover, the high volume fraction reinforced hybrid clustering region is found to be highly vulnerable to initiate crack in cast hybrid MMC during low cycle fatigue., ABSTRACT ......................................................................................................... i
ACKNOWLEDGEMENTS ............................................................................... iv
LIST OF FIGURES ............................................................................................ vii
LIST OF TABLES .............................................................................................. xiv
CHAPTER 1: INTRODUCTION ...................................................................... 1
1.1 Background ............................................................................................... 1
1.2 Application of MMCs ............................................................................... 5
1.3 Literature review ....................................................................................... 6
1.3.1 Fatigue damage and crack initiation in MMC .................................. 7
1.3.2 Preferential sites for crack initiation in MMC .................................. 8
1.3.3 Fatigue crack growth (FCG) in MMC .............................................. 10
1.3.4 Deformation and fracture of MMC .................................................. 12
1.3.5 Research on hybrid MMC ................................................................ 14
1.4 Scope and objectives ................................................................................. 16
1.5 Outline of the thesis .................................................................................. 17
CHAPTER 2: MATERIALS AND EXPERIMENTAL PROCEDURES ..... 18
2.1 Introduction ............................................................................................... 18
2.2 Materials fabrication ................................................................................. 18
2.3 Specimen preparation ................................................................................ 20
2.4 Microstructural features ............................................................................ 23
2.5 Experimental set up and procedures ......................................................... 25
2.5.1 Microcrack initiation test in low cycle fatigue ................................. 25
2.5.2 Crack propagation test in high cycle fatigue .................................... 26
CHAPTER 3: EXPERIMENTAL RESULTS .................................................. 29
3.1 Introduction ............................................................................................... 29
3.2 Monotonic test ........................................................................................... 29
3.3 Low cycle fatigue test ............................................................................... 32
3.4 Microcrack initiation and propagation in low cycle fatigue ..................... 32
3.4.1 Hybrid MMC .................................................................................... 32
3.4.2 MMC with Al2O3 whisker ................................................................ 40
3.4.3 Al alloy ............................................................................................. 45
3.4.4 Discussion ......................................................................................... 50
3.5 Fatigue crack growth in high cycle fatigue ............................................... 52
3.6 Fatigue crack growth behaviour in high cycle fatigue .............................. 54
3.6.1 Hybrid MMC .................................................................................... 54
3.6.2 MMC with Al2O3 whisker ................................................................ 63
3.6.3 Al alloy ............................................................................................. 69
3.6.4 Discussion ......................................................................................... 74
CHAPTER 4: NUMERICAL ANALYSIS ....................................................... 77
4.1 Introduction ............................................................................................... 77
4.2 Numerical model ....................................................................................... 84
4.3 Numerical results and discussion .............................................................. 88
4.4 Summary............................................................................................... 92
CHAPTER 5: CONCLUSIONS AND RECOMMENDATIONS ................... 93
5.1 Conclusions ......................................................................................... 93
5.2 Recommendations for further studies .................................................. 96
REFERENCES .................................................................................................... 98, 主指導教員 : 荒居善雄, text, application/pdf},
 school = {埼玉大学},
 title = {Study on fatigue crack initiation and propagation mechanisms in Al based cast hybrid metal matrix composite reinforced with SiC particles and Al2O3 whiskers},
 year = {2013},
 yomi = {エーケーエム, アシフ イクバル}
}