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# สมบัติเชิงกลของวัสดุ PowerPoint PPT Presentation

บทที่ 5. สมบัติเชิงกลของวัสดุ. 1. ความเค้น. ความเค้น ( stress) คือ  แรงกระทำในเนื้อวัสดุต่อหนึ่งหน่วยพื้นที่ที่ตั้งฉากหรือขนานกับแรงนั้น . มีหน่วยเป็น N/m 2 หรือ  Pa หรือ lb/in 2 1 N/m 2 = 1 Pa 1 lb/in 2 = 6.89 10 3 Pa.

สมบัติเชิงกลของวัสดุ

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5

1.

(stress)

N/m2 Pa lb/in2

1 N/m2 = 1 Pa

1 lb/in2 = 6.89103 Pa

Ex = 0.500 2500 lb

= 2500/ (/4)(0.5)2

= 12700 lb/m2

2. (Strain)

() = (l - lo) (lo)

Ex : 0.5 0.04 8 2.00 2.65

3. Poisson Ratio

http://training.ce.washington.edu/WSDOT/Modules/03_materials/Images/Main%20Page%20Pictures/poisson.jpg

Typical Values of Poissons Ratio

4. Shear

http://www.eng.uc.edu/~gbeaucag/RyanBreese/Rheolog/rheology_files/image001.jpg

Tensile Testing

http://www.mtec.or.th/labs/mech/tips7.html

http://www.twi.co.uk/twiimages/jk69f4.gif

Stress vs. Strain curve typical of structural steel

1. Ultimate Strength2. Yield Strength3. Rupture4. Strain hardeningregion5. Necking region

A: Apparent stress (F/A0)B: Actual stress (F/A)

1. Elastic modulus

• Elastic modulus : Hooks law

• = E

• E = /

• E = Elastic modulus = Youngs modulus

• : psi / Pa

• 2. Yield strength

• 0.2% offset (proof strength/ offset yield strength)

• (plastic deformation)

3. Ultimate strength

• -

4. % Elongation

5. % Reduction Area

http://www.auf.asn.au/const_images/stress_strain4.jpg

The Bend Test for Brittle Materials

Yield strength, Tensile strength, Breaking strength : are the same

Flexural strength = modulus of rupture

L = distance between the two outer point

w = width of the specimen

h = height of the specimen

http://www.emeraldinsight.com/fig/0180570504001.png

www.aws.org/itrends/11-03/feature1.html

www.thefabricator.com/WeldingInspection/Weldi...

(Hardness Test)

Brinell Rockwell

Shore , Vickers, Rockwell superficial Herbert

(microhardness tester) Knoop

### Brinell

• 10 mm.

• 3000 kg 1500 kg 500 kg

www.twi.co.uk/content/jk74.html

### Rockwell

• Rockwell

• Brinell Rockwell

• Brinell Rockwell ASTM E 18

• 10 kg

• 60 100 kg 150 kg 1/16 1/8 1/4 1/2

### Rockwell

(kg)

A 60

B 1/16 100

C 150

D 100

E 1/8 100

F 1/16 60

G 1/16 150

Principal of the Rockwell Test

1. The indenter moves down into position on the part surface

2. A minor load is applied and a zero reference position is established

3. The major load is applied for a specified time period (dwell time) beyond zero

The resulting Rockwell number represents the difference in depth from the zero reference position as a result of the application of the major load.

home.iitk.ac.in/~kamalkk/ME-371.htm

The Hardness Test : Rockwell Hardness Test

• Rockwell hardness values are expressed as a combination of a hardness number and a scale symbol representing the indenter and the minor and major loads.

• The hardness number is expressed by the symbol HR and the scale designation. There are 30 different scales. The majority of applications are covered by the Rockwell C and B scales for testing steel, brass, and other metals. However, the increasing use of materials other than steel and brass as well as thin materials necessitates a basic knowledge of the factors that must be considered in choosing the correct scale to ensure an accurate Rockwell test.

• The choice is not only between the regular hardness test and superficial hardness test, with three different major loads for each, but also between the diamond indenter and the 1/16, 1/8, 1/4 and 1/2 in. diameter steel ball indenters.

• If no specification exists or there is doubt about the suitability of the specified scale, an analysis should be made of the following factors that control scale selection:

• Type of material

• Specimen thickness

• Test location

• Scale limitations

HR = E e

F0 = preliminary minor load in kgf

e = permanent increase in depth of penetration due to major load F1 measured in units of 0.002 mm

E= a constant depending on form of indenter: 100 units for diamond indenter, 130 units for steel ball indenter

HR = Rockwell hardness number

D = diameter of steel ball.

home.iitk.ac.in/~kamalkk/ME-371.htm

Advantages of the Rockwell hardness method include the direct Rockwell hardness number readout and rapid testing time. Disadvantages include many arbitrary non-related scales and possible effects from the specimen support anvil (try putting a cigarette paper under a test block and take note of the effect on the hardness reading! Vickers and Brinell methods don't suffer from this effect).

www.ami.ac.uk/courses/topics/0123_mpm/index.html

Rockwell 100

0.002 mm 4 53 HRB 56 HRB 3 x 0.002 mm 0.006 mm

B 0 100 HRB 100 HRB

C 100 HRB 70 HRC C 20 HRC 20 HRC

### Vickers

136

5-120 5 Brinell

F= Load in kgfd = Arithmetic mean of the two diagonals, d1 and d2 in mmHV = Vickers hardness

www.twi.co.uk/content/jk74.html

www.bestar-steel.com/cn/hardnessTesting.php

http://www.nhml.com/i/resources_NHML_hardness_testing_fig2.gif

Hardness of plastics and elastomers

www.engineeringpolymers.com/epbb/ch09.html

http://www.iun.edu/~geos/Zoran%20IUN/G%20101/Photos/Mohs%20%20scale.jpg

Impact Test

www.wmtr.com/Content/impact_testing.htm

www.castingmanufacturers.com/facilities.asp

Charpy impact test

http://www.twi.co.uk/twiimages/jk71f2.gif

:

www.steinwall.com/ART-impact-testing.html

http://www.matweb.com/reference/images/izod_apparatus.gif

Creep Test

http://www.pktooling.co.uk/pk-images/gm-aerosol-creep-test-fixture.jpg

www.twi.co.uk/content/jk81.html

www.twi.co.uk/content/jk81.html