dissertation global and local fracture properties of metal matrix ...

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1. Introduction Section 1 In spite of the attractiveness of metal matrix composites (MMCs) because of their advantages, such as increased strength, stiffness, and modulus, their low fracture toughness in comparison with the fracture toughness of the matrix material is a primary obstacle against their widespread application in materials engineering. Many investigations have been already devoted to the mechanical behavior of MMCs and its relation to the microstructure, but both the microstructure and the properties have been considered at the global level: the variation of strength and fracture toughness, measured by conventional tensile and fracture mechanics tests, have been studied as a function of parameters, such as the particle volume fraction, the average particle size, the particle shape, the mean particle orientation with respect to the crack plane, etc. [1-5]. In comparison to previous investigations, the idea of this work is to study experimentally the local fracture properties: the conditions for void and fracture initiation. The motivation to study it is explained in Section 3. A method to estimate the local conditions for void initiation near the crack tip and the position of the particle location with respect to the crack tip is developed. The main tool of this method is a quantitative analysis of the fracture surfaces via automated stereophotogrammetry. By this tool, the local crack tip opening displacement at the moment of fracture initiation, CODi, (a measure of the local fracture initiation toughness) and the crack tip opening displacement at the moment of the void initiation, CODvi, (which can be considered as a local void initiation toughness) are determined. From the local void initiation toughness, the maximum normal stresses in the particles and at the interface at the moment of void initiation are evaluated by means of the HRR-field theory, a Mori-Tanaka type mean field analysis, and according to the model by Argon et al. This method is described in details in Section 4. The method is applied to Al6061 based MMCs. The results of investigation are presented and discussed in Sections 5 and 6. The procedure is also applied to MnS-inclusions in the mild steel St37 (Section 6). Another topic of our investigations is an improvement of the homogeneity of particle distribution in MMCs reinforced by small (less than 10 µm) particles. As known, these materials are prone to cause an inhomogeneous particle distribution. A method for equal channel angular pressing (ECAP) is proposed to solve this problem. The effect of ECAP on the homogeneity of the particle distribution in an Al6061-20%Al2O3 MMC processed by powder metallurgy (PM) technology on the global and local fracture properties near the crack tip is investigated by automated stereophotogrammetry (Section 7). 5
Section 1 The main results of this investigation are following: the local fracture properties are influenced by the particle location with respect to the crack tip, the particle size, the material strength. A correlation between the local and global fracture properties is found. It should be noted that this work a part of the research devoted to the local deformation and fracture properties of MMCs performed in frames of the FWF-project P14333-PHY. 6
Page 1 and 2: MONTANUNIVERSITÄT MONTANUNIVERSIT
Page 3 and 4: TABLE OF CONTENTS 1. Introduction 5
Page 5: 7.4.4. The effect of ECAP on the lo
Page 9 and 10: Section 2 2.2. The influence of the
Page 11 and 12: Section 2 high strength, fine reinf
Page 13 and 14: Section 2 etched to mark the inclus
Page 15 and 16: Section 2 (4.) The specimen prepara
Page 17 and 18: Section 3 Fig. 3.1. The digital ele
Page 19 and 20: height [µm] height [µm] 10 0 -10
Page 21 and 22: Section 3 Fig. 3.5. The observation
Page 23 and 24: Section 3 3.3.2. The determination
Page 25 and 26: Section 3 The Eshelby tensor does n
Page 27 and 28: equivalent stress Section 3 Fig. 3.
Page 29 and 30: Assume σ m eq Calculate matrix sec
Page 31 and 32: Section 4 Table 4.1. Chemical compo
Page 33 and 34: 4.1.1.3. Fracture mechanics tests S
Page 35 and 36: Section 4 Fig. 4.4. A view of a com
Page 37 and 38: Section 4 Table 4.3. Mechanical pro
Page 39 and 40: εfr [%] εfr [%] N 16 12 8 4 0 150
Page 41 and 42: Section 4 4.2. The effect of the he
Page 43 and 44: Section 4 It should be noted that n
Page 45 and 46: CODi [µm] 40 30 20 10 0 Section 4
Page 47 and 48: Section 4 Table 4.4. Results of the
Page 49 and 50: CODi [µm] COD i [µm] CODi [µm] 2
Page 51 and 52: Cast MMCs 10%-RT 10%-8h 10%-24h 10%
Page 53 and 54: COD vi [µm] COD vi [µm] CODvi [µ
Page 55 and 56: CODvi [µm] CODvi [µm] 80 60 40 20
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Section 4 Table 4.6. The values of
Page 59 and 60:
σ p max [MPa] σ p max [MPa] σ p
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σ p max [MPa] σ p max [MPa] 1200
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σ interf max [MPa] σ interf max [
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Section 4 4.5. The relation between
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Section 4 Table 4.9. Conditions for
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Ln(Ln(1/(1-F σ))) Ln(Ln(1/(1-F σ)
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Section 5 diameter of 2…8 µm and
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MnS-inclusion r θ MnS-inclusion Se
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σ interf [MPa] σ interf [MPa] σ
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Section 6 where n is the number of
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Ji [kN/m] J i [kN/m] 25 20 15 10 5
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Section 7 7.2. A model by Tan and Z
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Section 7 its initial length, li. O
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1 µm Section 7 5 µm Fig. 7.3. A s
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Section 7 a) b) c) Fig. 7.4. Micros
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Section 7 It is important that no p
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Section 7 between the Rtot and the
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Section 7 Fig. 7.8. The fracture su
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10 z [mm] 5 -10 -15 Section 7 0 -20
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Section 7 Table 7.3. Data on measur
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8. Summary Section 8 This work deal
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Section 8 The effect of the homogen
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Table 9.1. Data on stress for Speci
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Table 9.11. Data on stress for Spec
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Table 9.17. Results of the stereoph
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1 2 1 2 3 3 4 5 6 5 9 119 7 7 8 9 6
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8 9 10 6 10 122 11 12 13 14 11 12 1
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9 10 11 12 13 124 50 µm Fig. 9.6.
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1 2 1 2 3 3 4 4 126 5 5 6 9 7 50 µ
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1 1 2 2 3 4 3 4 5 5 128 6 7 6 7 50
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Fig. 9.12. Fracture surface with ma
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1 2 3 1 2 3 4 5 6 7 8 9 10 4 5 132
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Fig. 12.36. Fracture surface of the
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[14] Lewandowski JJ, Liu C, Hunt WH
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[42] Rice JR, Rosengren GF. Plane s
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[70] ASTM E1820-01, “Standard Tes
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