ÜYD Properties of Material

1. ÜYD Properties of Material ANADOLU U N I V E R S I T Y Industrial Engineering Department 2 – Properties of Materials Spring 2005 Saleh AMAITIK

2. Manufacturing Processes Properties of Materials Spring 2005

3. Manufacturing Processes Properties of Materials Mechanical Properties. Mechanical properties are useful to estimate how parts will behave when they are subjected to mechanical loads (stresses) Properties include: Strength, ductility, hardness, elasticity, toughness, creep, fatigue …etc. Physical Properties. Physical properties define the behavior of materials in response to physical forces other than mechanical Properties include: density, specific heat, melting point, thermal expansion, conductivity, magnetic properties. The manufacturing engineer should appreciate the design viewpoint and the designer should be aware of the manufacturing viewpoint Spring 2005

4. Manufacturing Processes Mechanical Properties Stress - Strain Relationships. Elasticity Strength Ductility Fatigue. Creep. Toughness Hardness These mechanical properties are usually determined by subjecting prepared specimens to standard laboratory tests Spring 2005

5. Manufacturing Processes Stress – Strain Relationships Three types of static stresses to which materials can be subjected: • Tensile Stress - tend to stretch the material • Compressive Stress - tend to squeeze it • Shear Stress - tend to cause adjacent portions of material to slide against each other Spring 2005

6. Manufacturing Processes Tensile Test Most common test for studying stress‑strain relationship, especially metals In the test, a force pulls the material, elongating it and reducing its diameter Spring 2005

7. Manufacturing Processes Tensile Test ASTM (American Society for Testing and Materials) specifies preparation of test specimen Spring 2005

8. Manufacturing Processes Tensile Test Machine Spring 2005

9. Manufacturing Processes Tensile Test Typical tensile test progress • beginning of test, no load; • uniform elongation and reduction of cross‑sectional area; • continued elongation, maximum load reached; • necking begins, load begins to decrease; and • fracture. • If pieces are put back together as in (6), final length can be measured Spring 2005

10. Manufacturing Processes Tensile Test Video Clip Spring 2005

11. Manufacturing Processes Tensile Test Video Clip Spring 2005

12. Manufacturing Processes Engineering Stress Defined as force divided by original area: where e = engineering stress; F = applied force; and Ao = original area of test specimen. Spring 2005

13. Manufacturing Processes Engineering Strain Defined at any point in the test as where e = engineering strain; L = length at any point during elongation (instantaneous length); and Lo = original gage length. Spring 2005

14. Manufacturing Processes It shows the basic relationship between stress and strain Engineering Stress – Strain Curve in Tensile Test Spring 2005

15. Manufacturing Processes Two Regions of Stress‑Strain Curve Engineering Stress – Strain Curve • The two regions indicate two distinct forms of behavior: • Elastic region – prior to yielding of the material • Plastic region – after yielding of the material Spring 2005

16. Manufacturing Processes Elastic Region in Stress – Strain Curve • Relationship between stress and strain is linear • Material returns to its original length when stress is removed • The material obeys Hooke's Law: e = E e where E = modulus of elasticity (Young's modulus ) Spring 2005

17. Manufacturing Processes Young’s Modulus Young's modulus measures the resistance of a material to elastic (recoverable) deformation under load. Its value differs for different materials Astiffmaterial has a high Young's modulus and changes its shape only slightly under elastic loads (e.g. diamond). Astiffmaterial requires high loads toelasticallydeform Aflexiblematerial has a low Young's modulus and changes its shape considerably (e.g. rubbers). The stiffness of a component means how much it deflects under a given load. Spring 2005

18. Manufacturing Processes Yield Strength in Stress – Strain Curve As stress increases, a point in the linear relationship is finally reached when the material begins to yield. • Yield StrengthY can be identified by the change in slope at the upper end of the linear region • Y = a strength property • Other names for yield strength = yield point, yield stress, and elastic limit Spring 2005

19. Manufacturing Processes Plastic Region in Stress – Strain Curve Yield point marks the beginning of plastic deformation. The stress-strain relationship is no longer guided by Hooke's Law . As load is increased beyond Y, elongation proceeds at a much faster rate than before, causing the slope of the curve to change dramatically. Spring 2005

20. Manufacturing Processes Tensile Strength in Stress – Strain Curve As the load is further increased, the engineering stress reaches a maximum and then begins to decrease. TS = The maximum engineering stress is called the Tensile Strength - TS (or Ultimate tensile Strength – UTS) of the material. Spring 2005

21. Manufacturing Processes Ductility in Tensile Test There are two common measures of Ductility Ability of a material to plastically deform without fracture % Elongation lf = specimen length at fracture; and lo = original specimen length lf is measured as the distance between gage marks after two pieces of specimen are put back together % Reduction of Area lf = final (fracture) cross-sectional area of the specimen; and lo = original cross-sectional area of the specimen Brittleness is simply the lack of significant ductility Spring 2005

22. Manufacturing Processes Mechanical Properties of Various Materials Spring 2005

23. Manufacturing Processes True Stress True Stress is defined as the ratio of the applied load F to the actual (instantaneous) cross-sectional area A of the specimen. where  = true stress; F = applied force; and A = actual (instantaneous) area resisting the load Spring 2005

24. Manufacturing Processes True Stress True Strain (natural or logarithmic strain) is calculated as where  = engineering strain; L = length at any point during elongation (instantaneous length); and Lo = original gage length. Spring 2005

25. Manufacturing Processes True Stress – Strain Curve If previous engineering stress‑strain curve were plotted using true stress and strain values Spring 2005

26. Manufacturing Processes Compression Test Applies a load that squeezes the ends of a cylindrical specimen between two platens (1) compression force applied to test piece; and (2) resulting change in height Spring 2005

27. Manufacturing Processes Compression Test Machine Video Spring 2005

28. Manufacturing Processes Engineering Stress in Compression As the specimen is compressed, its height is reduced and cross‑sectional area is increased where F = applied force Ao = original area of the specimen Spring 2005

29. Manufacturing Processes Engineering Stress in Compression As the specimen is compressed, its height is reduced and cross‑sectional area is increased where F = applied force Ao = original area of the specimen Spring 2005

30. Manufacturing Processes Engineering Strain in Compression Engineering strain is defined where h = height at any point during elongation (instantaneous); and ho = original height of the specimen Since height is reduced during compression, value of e is negative (the negative sign is usually ignored when expressing compression strain) Spring 2005

31. Manufacturing Processes Engineering Stress – Strain Curve in Compression Test Shape of plastic region is different from tensile test because cross‑section increases Spring 2005

32. Manufacturing Processes Shear Stress and Shear Strain Application of stresses in opposite directions on either side of a thin element Shear Stress Shear Strain Shear Strain is defined as Shear stress is defined as where F = applied force; and A = area over which deflection occurs. Where  = deflection element; and b = distance over which deflection occurs Spring 2005

33. Manufacturing Processes Shear Stress – Strain Curve Typical shear stress‑strain curve from a torsion test Spring 2005

34. Manufacturing Processes Shear Elastic Stress – Strain Relationship In the elastic region, the relationship is defined as where G = shear modulus, or shear modulus of elasticity For most materials, G 0.4E whereE = elastic modulus Spring 2005

35. Manufacturing Processes Shear Plastic Stress – Strain Relationship shear strengthS = Shear stress at fracture • Shear strength can be estimated from tensile strength: S  0.7(TS) Since cross‑sectional area of test specimen in torsion test does not change as in tensile and compression, engineering stress‑strain curve for shear  true stress‑strain curve Spring 2005

36. Manufacturing Processes Hardness The ability of a material to resist scratching, wear and indentation. • Good hardness generally means material is resistant to scratching and wear • Most tooling used in manufacturing must be hard for scratch and wear resistance • Commonly used for assessing material properties because they are quick and convenient. • Several methods have been developed to measure the hardness of materials. • Most well‑known hardness tests are Brinell and Rockwell and Vickers. Spring 2005

37. Manufacturing Processes Hardness Tests Spring 2005

38. Manufacturing Processes Brinell Hardness Test Widely used for testing metals and nonmetals of low to medium hardness A hard ball is pressed into specimen surface with a load of 500, 1500, or 3000 kg Spring 2005

39. Manufacturing Processes Brinell Hardness Test Brinell Hardness Number (BHN) = Load divided into indentation area where BHN = Brinell Hardness Number; F = indentation load, kg; Db = diameter of ball, mm; and Di = diameter of indentation, mm Spring 2005

40. Manufacturing Processes Temperature Effect General effect of temperature on strength and ductility Spring 2005

41. Manufacturing Processes Toughness Toughness is an estimate of how much energy is consumed before the material fractures. Energy consumed = work done = force x distance which you can easily see, is related to the stress and strain. So: Toughness = the strain energy = area under the stress-strain curve To compute toughness, True stress and True strain are used, which measure the instantaneous stress. The tensile test can provide a measure of this property. Spring 2005

42. Manufacturing Processes Creep If a material is kept under a constant load over a long period of time, it undergoes permanent deformation. This phenomenon is seen in many metals and several non-metals. For most materials, creep rate increases with increase in temperature. The phenomenon does not have much direct implication in manufacturing, but has significant use in design of parts that, for example, carry a load permanently during their use. Spring 2005

43. Manufacturing Processes Fatigue Fatigue is the fracture/failure of a material that is subjected to repeating cyclical loading, or cyclic stresses. There are two factors: the magnitude of the loading and the number of cycles before the material fails. The behavior is different for different materials Spring 2005

44. Manufacturing Processes Machinability, Formability and Weldability Machinability: depends not only on worked material but on applied machining process (range of meanings). Formability (malleability, workability): materials suitability for plastic deformation (depends on process conditions). Weldability: depends on particular welding (joining) technique. Spring 2005

45. Manufacturing Processes Physical Properties Defined • Properties that define the behavior of materials in response to physical forces other than mechanical • Includes: volumetric, thermal, electrical, and electrochemical properties • Components in a product must do more than simply withstand mechanical stresses • They must conduct electricity (or prevent conduction), allow heat to transfer (or allow its escape), transmit light (or block transmission), and satisfy many other functions Spring 2005

46. Manufacturing Processes Physical Properties in Manufacturing • Important in manufacturing because they often influence process performance • Example: • In machining, thermal properties of the work material determine the cutting temperature, which affects how long tool can be used before failure Spring 2005

47. Manufacturing Processes Thermal Expansion • Thermal expansion is the name for this effect of temperature on manufactured part. • Measured by coefficient of thermal expansion  Coefficient of thermal expansion is defined as change in length per degree of temperature, such as mm/mm/C. • Change in length for a given temperature change is: • L2 ‑ L1 = L1 (T2 ‑ T1) • where • = coefficient of thermal expansion; L1 and L2 are lengths corresponding respectively to temperatures T1 and T2 Spring 2005

48. Manufacturing Processes Thermal Expansion in Manufacturing • Thermal expansion is used in shrink fit and expansion fit assemblies • Part is heated to increase size or cooled to decrease size to permit insertion into another part • When part returns to ambient temperature, a tightly‑fitted assembly is obtained • Thermal expansion can be a problem in heat treatment and welding due to thermal stresses that develop in material during these processes Spring 2005

49. Manufacturing Processes Melting Properties of Metals • Melting pointTm of a pure element = temperature at which it transforms from solid to liquid state • The reverse transformation occurs at the same temperature and is called the freezing point • Heat of fusion = heat energy required at Tm to accomplish transformation from solid to liquid Spring 2005

50. Manufacturing Processes Importance of Melting in Manufacturing • Metal casting - the metal is melted and then poured into a mold cavity • Metals with lower melting points are generally easier to cast • Plastic molding - melting characteristics of polymers are important in nearly all polymer shaping processes • Sintering of powdered metals- sintering does not melt the material, but temperatures must approach the melting point in order to achieve the required bonding of powders Spring 2005