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

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Section 5 5. The relation between the inclusion size and the local fracture properties in a mild steel The knowledge of the maximum particle stresses or the maximum decohesion stresses at the moment of void initiation would be interesting for other materials, too. To check whether our procedure can be used to more generally used engineering materials, it shall be applied in this section for MnS-inclusions in a mild steel St37. 5.1. Material and mechanical properties 5.1.1 Materials characterization The chemical composition of the St37 is given in Table 5.1. Test pieces were cut transverse to the rolling direction. They were austenized at 870°C for 45 minutes and quenched in salt water to achieve a uniform distribution of the carbon content. Then they were tempered at 700°C for 4 hours and furnace cooled down to 500°C at a rate of 2° per min. to spheroidize the iron carbides. The resulting microstructure after heat treatment consist of large MnS-inclusions with a Table 5.1. Chemical composition of the mild steel St37. C Mn Si Ni Mo P S Cr Cu As 0.17 0.54 0.01 0.04 0.01 0.019 0.018 0.01 0.01 0.002 00 µm Fig. 5.1. The microstructure of the mild steel St37. 69 50 µm
Section 5 diameter of 2…8 µm and a length of 4…100 µm (Fig. 5.1). The MnS-inclusions are elongated in the rolling direction. Very small Fe3C particles embedded in the ferrite matrix are observed, as well. 5.1.2. Tensile tests A standard tensile specimen with a gage length of 30 mm and a diameter of 6 mm is used. The mechanical test is conducted using a “ZWICK” machine at a the loading rate of 6.9×10 -4 s -1 . The experimental true stress - strain and theoretical power-law curves are given in Figure 5.2. The results of mechanical tensile tests are listed in Table 5.2. True stress [MPa] 600 500 400 300 200 100 0 5.1.3. Fracture mechanics tests 0 4 8 12 16 Strain [%] Fig. 5.2. The true stress-strain curve for the mild steel St37. 70 experiment power-law Standard CT specimens were machined, with a thickness of 25 mm and a short-transverse crack plane orientation. Elongated MnS inclusions lie parallel to the crack front. The specimens are pre-fatigued and fracture mechanics tests are conducted using a “ZWICK” test machine [78]. The value of the fracture toughness is given. Large elongated dimples formed by debonded MnS inclusions are observed on the fracture surface (Fig. 5.3). These dimples are surrounded by very small equiaxed cavities generated by iron carbide inclusions Fe3C. The width of the primary dimples is larger than the diameter of
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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
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
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: Ln(Ln(1/(1-F σ))) Ln(Ln(1/(1-F σ)
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 and 100: 8. Summary Section 8 This work deal
Page 101 and 102: Section 8 The effect of the homogen
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
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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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