Mechanical Properties of Metals. Goals of this unit

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1 Mechanical Properties of Metals Instructor: Joshua U. Otaigbe Iowa State University Goals of this unit Quick survey of important metal systems Detailed coverage of basic mechanical properties, especially as they apply to metals

2 Survey of Important Metal Systems Ferrous Alloys Aluminum Alloys Magnesium Alloys Titanium Alloys Copper Alloys Nickel Alloys Ferrous Alloys (based on Fe) Carbon and low alloy steels wt% C, <5 wt% non-c additives High-alloy steels >5 wt% non-c additives stainless steels (Cr and Ni important) tool steels superalloys (for high-t use) Cast irons (>2 wt% C, Si also important) most forms are quite brittle

3 Aluminum Alloys Low density and corrosion resistant Commonly alloyed with Si, Mg, Cu pure Al is soft but can be precipitation hardened through alloying and heat treatment Magnesium Alloys Very low density with high strength-toweight ratio however, is of low ductility (hcp)

4 Titanium Alloys Fairly low density and very good strength give high strength-to-weight ratio (important in aerospace) Quite corrosion resistant and retains strength to fairly high temperatures Difficult to fabricate because of low ductility (hcp) Quite expensive Copper alloys Used in pure form for electrical conductor (but very soft) Often alloyed to give better strength with Zn to give brasses with Sn, Si, Al, Be, etc to give bronzes

5 Nickel Alloys Ni-based alloys are usually quite resistant to oxidation and temperature Alloyed with Cu to form monels Important ingredient in superalloys such as Inconel and Hastelloy Important ingredient in stainless steels Expensive Mechanical Properties (Materials utilized in load-bearing applications)

6 Goals Describe the concepts of stress and strain Differentiate between elastic and plastic deformation Quantify elastic properties of materials Describe measures of hardness, ductility, toughness and strength Understand fatigue and creep failures Mechanical Properties Most materials are subject to some force or load in use, e.g., airplane wing car axle brick in a wall automobile windshield deck on a bridge Think about the kinds of loads in each case

7 What do we need to know? What are the mechanical properties of various classes of materials Reliability (or variability of properties) Fracture characteristics How microstructure affects mechanical properties How we can influence the development of microstructure to tailor mechanical properties What are mechanical properties? Strength (what does that really mean?) compressive tensile shear Hardness Ductility Stiffness values depend on T, and type of loading, and always on microstructure

8 Stress and Strain - Review Engineering Stress - Force/original cross sectional area Length Force Area Displacement (U) Stress Strain σ = F/A 0 (N/m ) ε = U/L 2 For tensile or compressive (axial) stresses There are also shear and torsional stresses Shear Stress and Shear Strain P s A s P s t = P s / A s g = D y / z o

9 Stress - Strain Behavior Stress - σ = P/A o (where A o is the original cross sectional area) Strain - ε = l/l o (where l o is the original length) Stress is in units of: psi (pounds force per square inch) MPa (megapascals = 10 6 N/m 2 ) Strain is unitless sometimes expressed as a percentage Tensile testing One of the most common stress-strain tests performed is tensile testing There are standards for the shape and size and finish of test specimens Tensile testing equipment elongates a specimen at a constant rate and measures: Load (load cell) Elongation (extensometer)

10 Standard Tensile Test UTS s engr = P / A O e engr = D l / l O Tensile Testing Elongation Necking and Failure

11 Elastic Deformation Definition when stress and strain are proportional Non-permanent deformation When stress is removed, strain disappears i.e. the sample returns to it s original shape What is happening? small changes in interatomic spacing bonds are stretching but not breaking Elastic deformation and atomic displacement (reversible strain)

12 Modulus of Elasticity Slope of stress-strain curve in elastic region σ = (E)(ε) (Hooke s Law) σ E - modulus of elasticity (Young s modulus) Slope = E Material E (MPa) Steel 20.7 x 10 4 Aluminum 6.9 x 10 4 Al2O3 37 x 10 4 SiC 47 x 10 4 ε The greater the modulus the stiffer the material Poisson s ratio Q. When a specimen is elongated in one direction - what happens in the other two directions? A. They contract. The ratio of lateral to axial strains is called Poisson s ratio υ = ε x ε z = ε y ε z The - sign assures ν will be positive

13 Poisson s ratio, n ν = - ε x / ε z Poisson s ratio, cont. Q. What is Poisson s ratio for an isotropic material? A. If the properties are the same in all directions, then ν = 0.25 Most metals have a ν = 0.25 to 0.35 The maximum value ν = 0.5 (for no volume change)

14 Plastic Deformation There is a limit to how much a metal can be deformed before it will not return to its original shape when the stress is removed Beyond this point, stress and strain not proportional (Hooke s law is not valid) Plastic deformation occurs (the elastic limit has been exceeded) Plastic Deformation Elastic Stress Plastic Strain Curvature in stress-strain curve indiates the onset of plastic deformation. Plastic deformation corresponds to the breaking of bonds with atom neighbors and reforming bonds with new neighbors (slip).

15 Slip produces plastic deformation During plastic deformation, shearing stresses cause dislocation movement resulting in slip. This deformation is permanent (not recovered when stress is removed.) Necking during extensive Plastic Deformation necking

16 Yielding and Yield Strength Most structures are designed such that only elastic deformation occurs when a stress is applied The point at which plastic deformation occurs must be known (what stress level will bend the metal permanently?) Phenomenon is called yielding For metals that experience a gradual transition, the point is called the elastic limit Elastic Limit and Yield Strength σ y Stress Strain How do you know where the elastic limit is? By convention, a specified strain offset of is used to identify the yield strength, σ y.

17 Elastic recovery after plastic deformation Shackelford 7-6 Work Hardening (Strain Hardening) Process of plastic deformation (slip) multiplies the number of dislocations As each increment of plastic deformation occurs, dislocations find it harder and harder to move because of entanglement with ever increasing number of dislocations Result is that yield strength increases after plastic deformation ( strain hardening )

18 Work Hardening (Strain Hardening) Yield Characteristic of low-c Steels

19 Tensile Strength After yielding, stress increases to a maximum, then decreases, and eventually the material fractures Tensile strength is the stress at the maximum of the stress vs strain curve. Deformation up to this point is uniform throughout the sample After maximum stress, necking occurs Specimen Geometry Changes with Plastic Deformation

20 True vs. Engineering Stress and Strain Does material actually get weaker after TS has been exceeded? No, that is an artifact of using engineering stress instead of true stress in the plot. X-sectional area is decreasing, and especially after necking starts. Tensile Strength Tensile strength may vary from 7,000 psi to 450,000 psi (1 psi = 6895 Pa) When strength of a metal is cited, for design puposes, the yield strength is used. The fracture strength is the stress at fracture

21 Ductility Measure of degree of plastic deformation that has been sustained at fracture If there is little plastic deformation before fracture --- called brittle Ductility = percent elongation %EL = (l f l o ) l o x 100 Ductility

22 Ductility Why is ductility important? Specifies how much a structure will deform before fracture Specifies how much deformation is allowable during fabrication Ductility is strongly temperature dependent i.e., ductile-to-brittle transitions Toughness Describes the combination of strength and ductility Total area under the stress-strain curve Seldom have complete stess-strain curve, so an impact test is usually used to measure toughness

23 Charpy Impact Test of Toughness Summary of Mechanical Characteristics 1 Elastic Modulus 2 Yield strength (YS) 3 UTS 4 Ductility (100 * e F ) 5 Toughness

24 Summary of Mechanical Characteristics Comparison of Mechanical Characteristics

25 Hardness Measure of resistance to localized deformation (a dent or scratch) Early tests were based on minerals (which mineral could scratch another) Mohs scale; 1 (talc) to 10 (diamond) Qualitative method Qualititative means use a standarized small indenter forced into the surface Hardness Testing

26 Hardness and Strength There is a correlation between tensile strength and hardness Hardness tests are simple and inexpensive Hardness tests are nondestructive (you still have a usable sample when you are done) Other properties can be estimated from hardness information. Tensile Strength often scales with Hardness

27 Hardness Tests Although the scales are quantitative, the numbers are only relative (rather than absolute values), so you should only compare hardness values obtained using the same method Methods of testing Rockwell Hardness Brinell Hardness Knoop and Vickers Microhardness Rockwell Hardness Most common method Indentors are hardened steel balls or cones of various diameters The hardness is determined by the difference in depth of the indentation with two different loads Modern instruments are automated

28 Brinell Hardness Hard, spherical indenter is forced into the surface (like for Rockwell) The indentor is steel or WC (tungsten carbide) Standard loads are used The load is maintained for a specified amount of time The diameter of the indentation is measured with a microscope Knoop and Vickers Very small diamond indenter with a pyramid geometry is forced into the specimen. The resulting impression is measured microscopically Knoop is frequently used for ceramics Show Table 6.4, Callister 2000 (summary of hardness tests)

29 Summary of Standard Hardness Tests Failure modes Failure by excessive deformation Failure by fracture ductile fracture brittle fracture (covered in ceramic section) Fatique failure (cyclic loading) Creep failure (high temperature)

30 Fracture Fracture can occur in any mode of loading tensile compressive shear torsion Modes of fracture (based on amount of local deformation) ductile brittle Fracture Proceeds in two stages crack formation and crack growth Ductile fracture extensive plastic deformation slow crack growth - called stable Brittle fracture almost no plastic deformation very rapid crack growth - called unstable

31 Ductile Fracture Ductile fracture is usually preferred Presence of plastic deformation gives warning of imminent failure Large amount of strain energy is required to induce ductile fracture Ductile vs. brittle fracture Configuration of ductile fracture Highly ductile Moderately Ductile Brittle

32 Appearance of Fracture Zone Cup and cone fracture in aluminum Brittle fracture in mild steel Ductile vs Brittle Fracture* Fractured Specimens Cold - Worked Specimens

33 Ductile to Brittle Transition on cooling Fatigue failure Fatigue failure is fracture that occurs under cyclic loading well below the static strength value of the material Fatigue occurs in both ductile and brittle materials Most metallic fractures in use are fatigue failures bridges, aircraft, all types of machinery Often characterized by cycles to failure

34 Failure under Cyclic Loading Fatigue failure (S-N curve)

35 Improving Fatigue Resistance Creep (High-temperature deformation) Continuous plastic deformation of materials subjected to a constant stress at T>0.4T m Occurs at stresses well below room temperature yield strength Both temperature and applied stress influence creep behavior Alloys resistant to creep have high E and high melting T

36 Stages of Creep creep rupture Creep depends on s and T Creep rate affected by changes in applied stress Creep rate affected by changes in temperature

37 End of Unit 2 1 & Review Describe the concepts of Stress and Strain Differentiate between Elastic and Plastic Deformation Quantify Elastic Properties of Materials Describe measures of hardness Describe different modes of failure Explain fatigue and creep Read Class Notes and Relevant portions of Callister 2000 OR Shackelford 2000