Service Attributes of Manufactured Parts.pptx

Service attributes of the manufactured parts for manufacturing engineers

1. Service Attributes of Manufactured Parts Mechanical properties and their determination Ductile and Brittle behaviour of materials The harmful effects of internal defects, inclusions and minute surface defects Tribological physical and chemical properties

2. Service Attributes of Manufactured Parts The properties that make the manufactured products valuable are called service attributes . They are important because, service properties often dictate the choice of materials in the design stage. The acceptability of the finished product is judged on the basis of tests in which conformance to specifications is checked.

3. Service Attributes of Manufactured Parts Materials of construction can be briefly classified as metallics, ceramics, plastics and composites. Different tests may be applicable to different materials and the results are affected by the method itself; therefore tests must be conducted in conformance to standards. A most obvious property of a manufactured product is that they are able to support loads. Static loading Dynamic loading Loading at elevated temperatures

4. Service Attributes of Manufactured Parts The Tension Test For components under tensile tests, properties are routinely checked in the tension test subject to ASTM standard E8. There are standard specimen geometries for round and flat specimens.On imposing a load, the weaker portion of the uniform cross section (gage length) deforms. Tensile specimens made from an aluminum alloy. The left two specimens have a round cross-section and threaded shoulders . The right two are flat specimen designed to be used with serrated grips The magnitude of P is measured by a dynamometer. Extension of the specimen is measured by attaching an extensometer at the gage length. Transducers or non-contacting optical methods may also be used to get elongation Δ l . The SI unit of stress is N/m 2 (called Pa)this represents very small sterss, therefore, MN/m 2 or MPa is often used.

5. The Tension Test In the course of testing, both load and extension are seen to change continuously. Transducer outputs can be used to drive x-y recorders so that a force vs extension recording is obtained.Outputs can be digitized using a data acquisition system The specimen is held in self aligning heads of the machine, to ensure only pure tensile loads will be imposed. The test machine is a hydraulic or mechanical presss in which the moving cross head is displaced in a controlled manner. The movement of the cross head develops a load P, which is balanced by a reaction foce.

6. Tensile test specimen nomenclature

7. Stress- Strain Curve The pattern of force displacement diagram (stress-strain curve) for a ductile material, tested at room temperature is shown. If specimens of a larger cross section were tested, different curves would be obtained, as it takes a greater force to deform a larger specimen. 1. Ultimate strength, 2. Yield strength, 3. Proportional limit stress, 4. Fracture , 5.Offset strain(typically 0.2%) The experimental results can be normalized by dividing the force P by the area A over which it acts, to get the stress, σ = P/A. Elongation too can be normalized by taking the change in length and dividing by the original length; this is termed as engineering strain (e t ) = Δ l/l .

8. Stress vs. strain curve typical of structural steel 1. Ultimate strength, 2. Yield strength, 3. Fracture, 4. Strain hardening region, 5. Necking regionA : Engineering stressB : True stress

9. Strength in Tension Inspection of the stress-strain curve shows a number of critical points that can be used to characterize the material. 1)Elastic Modulus: At the beginning if the test, the force is proportional to the strain: the stress- strain curve obeys the Hook’s law; σ = E e t The proportionality constant (slope of the curve) is called the elastic modulus or Young’s modulus. E = σ / e t If the specimen is unloaded in this region it will return to its original length. The elastic modulus reflects the basic structure and bond strength of the materials.

10. Strength in Tension 2)Yield strength When ductile materials are tested, at some critical stress, the slope of the curve changes and this stress is termed proportionality limit. It is customery to choose a point at which the specimen deforms permanently, the corresponding engineering stress is called the yield strength. Typical yield behavior for non-ferrous alloys. 1: True elastic limit, 2: Proportionality limit, 3: Elastic limit, 4: Offset yield strength

11. Strength in Tension Round bar tensile specimen after testing   The engineering stress at the maximum load is calledthe tensile strength or Ultimate Tensile Strength, TS = P max /A

12. Ductility in Tension The engineering stress-strain curve also provides information of the ductilityof the material. Prior to necking the cross section reduces roughly uniformly along the gage length. So, the engineering strain sustained at the point of maximum load is called the uniform elongation. More frequently, the total elongation to fracture is measured most by placing the broken parts of the specimen together and measuring the distance between the gage marks, e f = (l f – l o )/l o . The gage length must always be stated. The most sensitive measure of ductility is the reduction in area measured at fracture. During uniform elongation, the material obeys the constancy of volume to compensate for the increase in length; the cross section reduces, A 1 l 1 = A l =Al . The stress state is that of uniaxial tension. All this changes at the onset of necking.The material in the neck cannot deform freely because of the adjacent nondeforming material. This stes up radial tensile stress; thus within the neck, there is triaxial tension. Triaxial tension state literally tears apart the material. The minimum cross sectional area of the fractured test specimen can be measured and the reduction in area calculated.

13. Strength in Tension 3)Tensile strength On further loading and elongation, the gage section of the specimen elongates uniformly along its length, yet force gradually increases. At some critical deformation level, typical of the material, strain hardening cannot counterbalance the loss of strength and a neck forms at the weakest point while the force P declines. Finally, fracture occurs . 4)Toughness The area under the stress strain curve has the dimension of force times distance., i.e., work. It can be regarded as the measure of toughness,i.e., the energy absorbed by the material prior to fracture. Ductile materials such as low carbon steel, copper, aluminium alloys have a much greater toughness than brittle materials.

14. Notch Effects Internal defects or inclusions may reduce the ductility of the material. Even more harmful can be the surface defects(notches). Notches cause sterss concentration- a local increase in stress . When the maximum stress or strain reaches a critical value, a crack develops and propogates at high speed through the part. The fracture stress can be shown to depend on the crack radius, r c , and crack depth, a, as σ fr = [C . (r c /a) ] 1/2 where C is a material constant. For truly brittle materials r c is of the order of atomic radii. An edge crack (flaw) of length a in a material.

15. Schematic and dimensions of a compact tension specimen

16. Brittle fracture A completely brittle material deforms only elastically. At some critical stress, fracture occurs suddenly, usually in a plane perpendicularto the load application. Fracture originates from minute cracks that locally raise the stress. Brittle behaviour, identified by zero elongation is typical of a few metals, all ceramics, thermosetting polymers. Some materials (ex. GCI) gradually yield from the beginning and Hook’s law does not hold.

17. Graph comparing stress-strain curves for brittle and ductile materials Brittle fracture in cast iron tensile testpieces

18. Bending Tests The specimen is supported at two points(ASTMF417). In the three point test a force P is applied at the center. The specimen bends and the outer half is put into tension whereas the inner half is put into compression. Failure (fracture) occurs when the maximum tensile stress, at the outer surface & midway between the supports, reaches a critical value- rupture strength For rectangular beams, σ B = 3/2 ( P l / b h 2 ) For a round specimenm, σ B = 8 P l / π d 3

19. Bending Tests 1940s flexural test machinery working on a sample of concrete The four point test generates uniform tensile stress betweenthe loading points. If a= l/3, the modulus of rupture for a rectangular specimen is σ B = P l /b h 2

20. References Introduction to Manufacturing Processes, John A Schey , 3 rd Edition, Mc Graw Hill International Edition, Singapore, 2000 ( www.en.wikipedia.org )