1. Materials Characterization and the Selection Process Metallurgy
METL 1301
Introduction to Metallurgy Lecture2
Materials Characterization and the Selection Process Metallurgy

2. Materials Requirements
The requirements placed on materials are derived from a study of the desired performance of the product or system being developed as well as the environment in which it will operate.

3. Engineering Design
Engineering design of material components is a complex task requiring consideration of many interrelated factors, not all of which are necessarily compatible.
Compromises and trade-offs among various design factors are routinely made.

4. Engineering Design
A designer must know the relative importance of various design factors and how they interact before intelligent choices between conflicting requirements can be made.

5. Factors in Material Selection
Selecting the most suitable material for a given application is by no means a simple matter, for there are many factors to be considered.
There is frequently no single answer since several materials, each with its particular advantages and disadvantages, may be about equally suitable.

6. Factors in Material Selection
The engineer must use his best judgment based on his own experience and study, and that of other engineers, scientists, and technicians.
In general, the material most suitable for a given use will be that material which most nearly supplies the necessary properties and durability with a satisfactory appearance at the lowest cost.

7. Engineering Materials
Although the focus in this lesson is on metals, it is helpful to define other materials which are used in engineering applications, such as polymers, ceramics, and composites.
Polymers are always composed of atoms of carbon in combination with other elements.

8. Polymers
A polymer (meaning many parts) is a large molecule, or macromolecule, composed of many repeated sub-units.
Polymers are “built” from chemical units called monomers.
A monomer (meaning one part) is a small molecule that may bind chemically to other molecules to form a polymer.

9. Monomers & Polymers

10. Polymers
Plastics (one type of polymer) are solid in their finished state, but at some stage in manufacture, approached a liquid condition and were capable of being formed into various shapes.
Usually through the application of heat and/or pressure.

11. Polymers
Polymer chemists utilize only eight of the more than 100 known elements to create thousands of different plastics.
These eight elements are: hydrogen, carbon, nitrogen, oxygen, fluorine, silicon, sulfur, and chlorine.

12. Polymers
The main distinguishing factor between the two classes of polymers is whether the polymer chains remain linear and separate after molding (thermo plastic).
Or whether they undergo three-dimensional chain combination by cross linking (thermo set).

13. Polymers
Linear plastics are chemically unchanged during molding (except for possible degradation) and can be remolded again and again.
Cross linked plastics start with linear chains that are joined irreversibly during molding into an interconnected, molecular network, and cannot be remolded.

14. Linear Vs Cross Linked
Knowing whether a plastic is linear or cross linked can be very helpful.
It not only characterizes the method of molding but also describes many inherent properties of the material.

15. Linear Vs Cross Linked

16. Polymer Chains

17. Linear Polymers

18. Polymerization

19. Linear Vs Cross Linked
A cross linked plastic, compared to the same plastic in linear form, has improved resistance to heat, chemical attack, stress-cracking, and creep.
There are trade-offs, of course: The greater the cross linking, the less ductile the molded part and the more involved the processing.

20. Linear Vs Cross Linked
Designers should be given understandable classification terminology which is directly useful for designing the best part at the lowest price.

21. Linear Vs Cross Linked
Critics of the present terminology suggest that the plastics industry could take the first step by eliminating the restrictive and contradictory terms of the earlier years.
The terms “thermoplastic” and “thermoset” can easily be replaced by the more logically based terms, “linear” and “cross linked”.

22. Ceramics
Ceramics, in the broadest sense, is a term that describes materials ranging from window glass to furnace brick.
Ceramics that are considered structural engineering materials include only a handful of types.

23. Ceramics
Structural engineering type ceramics are variously called high-performance, structural, technical, advanced, or engineering ceramics.
They are selected for load-bearing applications under combinations of high temperature, severe corrosion, abrasion, and thermal or electrical insulating requirements.

24. Ceramics
A ceramic material may be generally characterized as being brittle, with a high melting temperature.
Most ceramics are poor conductors of electricity and are nonmagnetic.
The strength property used most frequently to characterize ceramics is not the tensile test (used for metals), but the modulus of rupture or MOR.

25. Ceramics
MOR also called flexural strength, is usually used to measure the strength of ceramics for critical, high-strength applications.
In the MOR test, the sample — usually a rectangular plate — is supported near the ends, and a bending load is applied at its center.

26. Ceramics
The load is increased until the sample ruptures, at which point the applied load is recorded.
Two loading conditions are commonly used: In a three-point test, the load is applied at one point midway between the two supports.
A more uniform four-point version calls for a load applied at two points equidistant from the supports.

27. Composites
The term advanced composite came into being in the mid-1960’s to designate certain composite materials having properties considerably superior to those of earlier composites.
The term is ambiguous, however, because it does not identify specific material combinations, nor does it indicate their arrangement or configuration in the composite.

28. Composites
The term can describe materials having reinforcing fibers that are either continuous, chopped, or both.
The reinforcing fibers may be oriented or randomly distributed in the matrix.
A composite can include metal, wood, foam, or other material layers, in addition to the fiber/resin components.

29. Composites
Nevertheless, an advanced composite has come to denote, to most engineers, a resin-matrix material reinforced with high-strength, high-modulus fibers of glass, carbon, aramid, or even boron, and usually laid up in layers to form an engineered component.

30. Composites
Early uses of these materials in sports and recreational equipment provided excellent test products.
The aircraft industry was quick to see the advantages of using carbon/epoxy composites which offered light weight, high strength, elastic modulus, and-most importantly- excellent fatigue performance.

31. Composites
Most of the engineering and manufacturing experience with composites has been gained in developing components for military aircraft.

32. Materials Properties and Characterization
Let’s examine several commonly measured material properties, and significant materials characteristics.

33. Mechanical Properties
One question usually asked in selecting materials is whether strength is adequate to withstand the stresses imposed by service loading.

34. Mechanical Properties
Although the primary selection criterion is often strength.
It may also be toughness, corrosion resistance, electrical conductivity, magnetic characteristics, thermal conductivity, specific gravity, strength-to-weight ratio or some other property.

35. Mechanical Properties
In general, the usual criterion for selection is not just one property, such as strength, but some combination of properties, manufacturing characteristics and cost.

36. Tensile Strength
Tensile strength is a measurement of the force required to pull something such as rope, wire, or a structural beam to the point where it breaks.

37. Ultimate Tensile Strength
Ultimate tensile strength (UTS), is the capacity of a material or structure to withstand loads tending to elongate, as opposed to compressive strength, which withstands loads tending to reduce size.
In other words, tensile strength resists tension (being pulled apart), whereas compressive strength resists compression (being pushed together).

38. Tensile Strength
There are three typical definitions of tensile strength.
Yield strength - Indicates the lowest stress at which measurable permanent deformation occurs, or the stress which will cause a permanent deformation of 0.2% of the original dimension.
Ultimate strength - The maximum stress a material can withstand.
Breaking strength - The stress coordinate on the stress-strain curve at the point of rupture.

39. Tensile Strength
A commonly measured and widely reported indication of the ability of a material to withstand loads.
The direct application of tensile strength data to design problems is extremely difficult.

40. Tensile Strength
Finally, there seems to be only a rough correlation between tensile strength and material properties such as hardness and fatigue strength at a specified number of cycles.
No correlation whatsoever, exist between tensile strength and properties such as resistance to crack propagation, impact resistance or proportional limit.

41. Yield Strength
Indicates the lowest stress at which measurable permanent deformation occurs.
This information is necessary to estimate the forces required for forming operations.
Yield strength is also useful in considering the effects of a single application overload; most structures must be designed so that a foreseeable overload will not exceed the yield strength.

42. Hardness
Another widely measured material property useful for estimating the wear resistance of materials and estimating approximate strength of steels.
The most widespread application for hardness testing, is for quality assurance in heat treating.
However, only rough correlation can be made between hardness and other mechanical properties or between hardness and behavior of materials in service.

43. Ductility
Is usually measured as the percent reduction in area or elongation that occurs during a tensile test, and is often considered an important factor in material selection.
It is assumed that, if a metal has a certain minimum elongation in tensile testing, it will not fail in service through brittle fracture.

44. Ductility
It is assumed that if a little ductility is good, a lot is better.
Neither assumptions is accurate.
How much ductility is actually usable under service conditions, how to measure it, and its relationship to fabrication and formability are not well-established.

45. Other Factors
Durability, If a material is to be satisfactory for a given use, it must be durable.
That is, it must continue to function properly during the design life of the structure of which it is a part.
To be durable, a material must resist all forms of destruction to such a degree that the system or device will not be rendered unsafe or inefficient at any time during its prescribed life.

46. Other Factors
Fabricability, In principle, the fabricability of a material is a measure of its ability to be worked or shaped into a finished and useful form or part.
The two main considerations in the fabrication of any engineering component are forming and joining.

47. Other Factors
Forming, includes casting, drawing and extruding, forging, machining, and sintering.
Joining, includes both welding, brazing, and soldering, on one hand, and the use of fasteners such as screws and rivets on the other.

48. Other Factors Castability, the ease with which a material can be cast.
Machinability, the ease with which a material can be machined.

49. Other Factors
It is important to remember that processing operations will almost always have some effect on a material’s functional or service performance.
The effect may either improve or reduce performance.