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

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Section 8 15-RT Specimen decrease slightly with increasing distance, r. A slight decrease is also observed for particles lying at a large angles, θ > 50 °. The particle volume fraction has no simple effect on the σ p max-values. For the different aging conditions and particle volume fractions, the computed average values of the maximum principal stresses in the particles for fractured particles and at the interface for debonded particles show a roughly linear dependency on the composite yield strength. This can be explained, if it is assumed that the void initiation occurs at the onset of gross plastic yielding in the matrix at the location of the first particles in front of the crack tip. A rise of the σ max interf dec-values might be also related with increasing volume fraction of the spinel phase with increasing aging time at the particle/matrix interface, leading to enhanced interfacial bonding. It is shown that the distributions of strengths of ceramic reinforcements in the cast and PM MMCs is not in contradiction to the Weibull distribution. The values of the Weibull modulus, m, determined from the values of the particles strength, are in good accordance with values given in literature. A good correlation is found between the local Ji-values evaluated from the average of the local CODi-values and the global Ji-values determined from the fracture mechanics tests for all investigated materials, excepting the PM MMCs. This is explained when it is assumed that the fracture of one particle might trigger the fracture of the neighboring particles within a certain region so that the area of local crack extension is larger than the resolution of the potential drop technique. The larger particle size (100 µm, compared to 10 µm for the cast MMC) facilitates this effect. The proposed procedure is applied to the MnS-inclusions in the mild steel St37 to study the effect of the inclusion size on the local fracture properties and local conditions for void initiation, as well. It is shown that the CODvi-values strongly decrease with increasing diameter of MnS-inclusions. A clear reduction of the σ interf max-values with increasing diameter of the MnS-inclusions is found, as well. It is shown that equal channel angular pressing (ECAP) leads to an increase of the uniformity of the particle distribution in PM MMCs with small particle sizes and clustered particle distribution. The model proposed by Tan and Zhang [23] about the estimation of the critical value of the particle size which is re quired for a homogeneous particle distribution in the matrix is extended to the case of a combination of extrusion and ECAP. A good accordance between the modified Tan and Zhang model and the quantitative analysis of the particle distribution by the quadrat method is found. 99
Section 8 The effect of the homogeneity of the particle distribution in MMCs on the global and local fracture properties is investigated. An increase of the global fracture properties both the fracture initiation toughness and the slope of the J-∆a-curve, dJ/d(∆a) with increasing homogeneity of the particle distribution is revealed. The scatter of the local fracture properties depends on the local homogeneity of the particle distribution, A high scatter is found in the material with a clustered particle distribution. At the same time, in the material subjected to 7 ECAP passes (with homogeneous particle distribution), the scatter of the CODi-values is significantly less. It is shown that the values of the local fracture toughness at the particle clusters are extremely low, whereas, in the regions with homogeneous particle distribution, higher values of the local fracture toughness appear. 100
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MONTANUNIVERSITÄT MONTANUNIVERSIT
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TABLE OF CONTENTS 1. Introduction 5
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7.4.4. The effect of ECAP on the lo
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Section 1 The main results of this
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Section 2 2.2. The influence of the
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Section 2 high strength, fine reinf
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Section 2 etched to mark the inclus
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Section 2 (4.) The specimen prepara
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Section 3 Fig. 3.1. The digital ele
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height [µm] height [µm] 10 0 -10
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Section 3 Fig. 3.5. The observation
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Section 3 3.3.2. The determination
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Section 3 The Eshelby tensor does n
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equivalent stress Section 3 Fig. 3.
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Assume σ m eq Calculate matrix sec
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Section 4 Table 4.1. Chemical compo
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4.1.1.3. Fracture mechanics tests S
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Section 4 Fig. 4.4. A view of a com
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Section 4 Table 4.3. Mechanical pro
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εfr [%] εfr [%] N 16 12 8 4 0 150
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Section 4 4.2. The effect of the he
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Section 4 It should be noted that n
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CODi [µm] 40 30 20 10 0 Section 4
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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
Page 57 and 58: Section 4 Table 4.6. The values of
Page 59 and 60: σ p max [MPa] σ p max [MPa] σ p
Page 61 and 62: σ p max [MPa] σ p max [MPa] 1200
Page 63 and 64: σ interf max [MPa] σ interf max [
Page 65 and 66: Section 4 4.5. The relation between
Page 67 and 68: Section 4 Table 4.9. Conditions for
Page 69 and 70: Ln(Ln(1/(1-F σ))) Ln(Ln(1/(1-F σ)
Page 71 and 72: Section 5 diameter of 2…8 µm and
Page 73 and 74: MnS-inclusion r θ MnS-inclusion Se
Page 75 and 76: σ interf [MPa] σ interf [MPa] σ
Page 77 and 78: Section 6 where n is the number of
Page 79 and 80: Ji [kN/m] J i [kN/m] 25 20 15 10 5
Page 81 and 82: Section 7 7.2. A model by Tan and Z
Page 83 and 84: Section 7 its initial length, li. O
Page 85 and 86: 1 µm Section 7 5 µm Fig. 7.3. A s
Page 87 and 88: Section 7 a) b) c) Fig. 7.4. Micros
Page 89 and 90: Section 7 It is important that no p
Page 91 and 92: Section 7 between the Rtot and the
Page 93 and 94: Section 7 Fig. 7.8. The fracture su
Page 95 and 96: 10 z [mm] 5 -10 -15 Section 7 0 -20
Page 97 and 98: Section 7 Table 7.3. Data on measur
Page 99: 8. Summary Section 8 This work deal
Page 103 and 104: Table 9.1. Data on stress for Speci
Page 113 and 114: Table 9.11. Data on stress for Spec
Page 119 and 120: Table 9.17. Results of the stereoph
Page 121 and 122: 1 2 1 2 3 3 4 5 6 5 9 119 7 7 8 9 6
Page 123 and 124: 8 9 10 6 10 122 11 12 13 14 11 12 1
Page 125 and 126: 9 10 11 12 13 124 50 µm Fig. 9.6.
Page 127 and 128: 1 2 1 2 3 3 4 4 126 5 5 6 9 7 50 µ
Page 129 and 130: 1 1 2 2 3 4 3 4 5 5 128 6 7 6 7 50
Page 131 and 132: Fig. 9.12. Fracture surface with ma
Page 133 and 134: 1 2 3 1 2 3 4 5 6 7 8 9 10 4 5 132
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Fig. 9.32. Fracture surface with ma
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Fig. 9.34. Fracture surface with ma
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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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