MSE 321 MECHANICAL BEHAVIOUR OF MATERIALS

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1 MSE 321 MECHANICAL BEHAVIOUR OF MATERIALS Yahya K. Tür GYTE öğrencileri tarafından çoğaltılmasında

2 Things to Know Course hours: Friday 9:00-12:00 Office hours: Wednesday 16:00-17:00 Room : MLZ 221, Phone: Sources : A comprehensive list will be given at the end of the lecture In Turkish: Malzemelerin yapısı ve mekanik davranışları, Kayalı ve Çimenoğlu, İTÜ Kimya Metalurji Fakültesi Grading : Midterm (35 %) + Final (55 %) + Attendance (10 %) One A4 sheet hand written notes is permitted in the exams. Second sheet and photocopied sheets are to be collected.

3 Mechanical Behaviour of Materials Objective: Macro and micro mechanical behaviour of materials under the influence of external forces Part I Mechanical fundamentals Stress and strain relationships of elastic behaviour Plastic deformation Part II Metallurgical fundamentals Plastic deformation of single crystals Dislocation theory Strengthening mechanisms Fracture and Fatigue Part III Mechanical behaviour of ceramics Mechanical behaviour of polymers.

4 Common States of Stress Simple tension: cable F A o = cross sectional area (when unloaded) s = F A o s F s Torsion (a form of shear): drive shaft A c M 2R M F s A o t = F s A o Note: t = M r/j o here. Ski lift (photo courtesy P.M. Anderson)

5 OTHER COMMON STRESS STATES (1) Simple compression: A o Canyon Bridge, Los Alamos, NM (photo courtesy P.M. Anderson) Balanced Rock, Arches National Park (photo courtesy P.M. Anderson) s = F A o Note: compressive structure member (s < 0 here).

6 OTHER COMMON STRESS STATES (2) Bi-axial tension: Hydrostatic compression: Pressurized tank (photo courtesy P.M. Anderson) s q > 0 Fish under water (photo courtesy P.M. Anderson) s z > 0 s < 0 h

7 Why failure in materials Seven of the Liberty Ships built during the world war II has broken completely in two as a result of brittle fractures. Over 1000 of approximately 5000 merchant ships built during World War II had developed cracks of considerable size by Failure of Liberty Ships during services in World War II.

8 Why failure in materials Collapse of Point Pleasant suspension bridge, West Virginia, on December 15, The bridge building industry did not pay particular attention to the possibility of brittle failure until the failure of Point Pleasant bridge in The bridge collapsed without warning, costing 46 lives.

9 Why failure in materials The aircraft was used for interisland transportation for 19 years before failed. Failure has been attributed to multiple-site-damage. Failed fuselage of the Aloha 737 aircraft in 1988.

10 Material property assessments Hardness : Micro/Macro hardness tests Strength, Ductility (elongation, reduction of area): Tension tests Torsion: Torsion tests Toughness (resistance to failure) : Impact tests; Fracture toughness tests Fatigue: S-N fatigue tests;fatigue crack growth tests Creep (elevated temperature strength): Creep tests

11 Hardness tests Hardness is a property which is a measure of a resistance to permanent or plastic deformation. Large hardness means: --resistance to plastic deformation or cracking in compression. --better wear properties. most plastics brasses Al alloys easy to machine steels file hard cutting tools nitrided steels diamond increasing hardness

12 Stress-Strain Testing (Tensile Test) Typical tensile test machine Typical tensile specimen extensometer specimen Adapted from Fig. 6.2, Callister 7e. gauge length Adapted from Fig. 6.3, Callister 7e. (Fig. 6.3 is taken from H.W. Hayden, W.G. Moffatt, and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, p. 2, John Wiley GYTE and öğrencileri Sons, New tarafından York, 1965.) oğaltılmasında

13 Yield Strength, s y Stress at which noticeable plastic deformation has occurred. when e p = s y tensile stress, s s y = yield strength Note: for 50 mm sample e = = z/z z = 0.1 mm e p = engineering strain, e Adapted from Fig (a), Callister 7e.

14 engineering stress Tensile Strength, TS Maximum stress on engineering stress-strain curve. TS s y Adapted from Fig. 6.11, Callister 7e. F = fracture or ultimate strength Typical response of a metal strain engineering strain Neck acts as stress concentrator Metals: occurs when noticeable necking starts. Polymers: occurs when polymer backbone chains are aligned and about to break.

15 Impact Tets Measure toughness of materials in terms of energy absorption. Specimen is impacted by a hammer and the energy absorbed during fracture is measured in Joule.

16 Fracture mechanics Resistance of materials to crack propagation (to failure). Crack propagation can be predicted before failure. Material will fail when the stress intensity factor K reaches the critical value K C.

17 Fatigue tests Material is subjected to a repetitive or fluctuating stress (cyclic loading) and will fail at a stress level much lower than that causes failure in statistic loading. Parameters: Fracture life (fatigue strength) Fatigue crack growth resistance Paris exponent (m) Fatigue threshold (K th ) Stresses in fatigue loading

18 To Improve Material Properties

19 Main references Hibbeler, R.C. Mechanics of materials, 2005, SI second edition, Person Prentice Hall, ISBN Callister, W. D. Jr., Materials Science and Engineering: An Introduction, 2007, John Wiley & Sons, Inc., ISBN-13: Dieter, G.E., Mechanical metallurgy, 1988, SI metric edition, McGraw-Hill, ISBN Meyers, M. A. and Chawla, K. K., Mechanical Behavior of Materials, 2009, Cambridge University Press, ISBN Rösler, J., Harders, H., Baeker M., Mechanical Behaviour of Engineering Materials 2007, Springer, ISBN Hertzberg, R. W., Deformation and Fracture Mechanics of Engineering Materials, 1995, Wiley; 4 edition, ISBN-13: Udomphol, T., Mechanical Metallurgy lecture notes,