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APPLIED SCIENCE 279 LABORATORY #3

FATIGUE AND BRITTLE FRACTURE

Keywords Fatigue, striations, beach marks, stress concentration, fatigue limit, S-N diagram, notchtoughness, fracture surface, shelf energy, impact test.

PURPOSE

This laboratory demonstrates fatigue failure resulting from dynamic fluctuating stresses and the phenomenon of ductile-brittle transition. The materials used are steel and aluinum. The effect of a stressconcentration (in this case a machined notch) on fatigue life is also shown. Since flaws such as cracks candramatically alter the fracture characteristics of any material, seeral common nondestructie tests (N!T)will be used to locate flaws within steel test specimens.

PROCEDURE

• Fatigue test the smooth and notched  specimens (cold rolled mild steel) and record the number of stress

cycles to failure. Note" the minimum daimeter (#.$% mm) is the same for both.

• !etermine the &harpy impact energy ('oules) for the .*& steel specimens at the specified

temperatures. +amine the fracture surface for each test and estmate the * shear (ductile) failure.

• se the ballistic pendulum to determine the angular deflection θ for the .* & steel specimens (/&&

structure) oer the temperature range -0#°& to 12°&. 3erform the same test on aluminum specimens

(F&& structure) at -0#°&, 12°& and one intermediate temperature.

• se ultrasonic, magnetic and die penetrant techni4ues to locate cracks within each of the steel test

specimens. 5ecord the specimen number, the type of flaw detected (surface or internal) and the position (distance from the numbered end in mm).

• +amine the fracture characteristics of the arious failed industrial components on display.

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T!E REPORT

6 clean copy of the data sheets with all data and calculations must be included. The report should alsoinclude responses to the following "

. se the data gien in the handout to plot an S-N (stress s log cycles) diagram for the mild steel.!etermine the fatigue limit  in 73a and compare this to it8s TS which is 9 73a. /e sure to alsoshow your data on the plot.

3lotting the data of Stress :s. ;og cycles we get"

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T-e 01,"e 45, o6sered s 16o, 3&&8P1 w-/- s 4ess ,-1. ,-e UTS 7&&8P1'*

2. n designing a structure sub?ect to fluctuating stresses, how you would utili@e the information deriedfrom an S-N diagram= >f made from aluminum can this structure hae an infinite serice life=

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To des". 1 s,r/,re s6- 0or 145.5 we /1. see ,-1, ,-e "r1>- 6e4ow

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  As we /1. see ,-ere s .o edr1./e 45, 0or 145.5 so 145.5 w44 .o, -1e .0.,e 40e*

. ;ist at least four factors which affect the in-service fatigue strength of metals.

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%. For the ballistic pendulum test, plot θ ersus T for both the steel and aluminum. &alculate the !/TT

for each. 6boe the transition to brittle fracture, θ  increases as temperature decreases. +plain why

including a sketch of the stress-strain cure at arious temperatures to to support your answer.

http://www.interfaceforce.com/technical-library/load-cell-fatigue.phphttp://www.interfaceforce.com/technical-library/load-cell-fatigue.php

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- ,-e DBTT 0or s,ee4 s 6e,wee. ?$&2 1.d ?$$% de"ree /e4/s $&9 de"ree /e4/s'

- T-ere s .o DBTT 0or 145.5

- A6oe ,-e ,r1.s,o. ,o 6r,,4e 0r1/,re ,-e d/,4e re"o.' we see ./re1se 1s ,e5>er1,re

de/re1ses* ./re1ses 5e1. 5ore e.er"y s 16sor6ed 6y ,-e 51,er14* As ,e5>er1,re

de/re1ses ,-e ye4d." s,re.",- o0 ,-e 51,er14 ./re1ses ,-e 1re1 .der ,-e s,ress?s,r1./res e41s,/ re"o. ./re1ses so ,-e 51,er14 /1. 16sor6 5ore e.er"y* !oweer w-e. ,-eye4d." s,re.",- e/eed /er,1. >o., ,-e 1re1 .der ,-e s,ress?s,r1. /res e41s,/ re"o.s,1r, ,o de/re1se ,-1, 5e1. ,-e s4o>e 1."4e s -"-er ,-1. )% de"ree' 1.d ,-1, s w-e. ,-e,r1.s,o. 6e,wee. ,-e d/,4e 1.d 6r,,4e o//r*

#. /ased on the * shear alues estimated during the lab, sketch the fracture surfaces for the &harpyspecimens.

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T-e s-e1r ./re1se ,-e e.er"y 16sor6ed . ,-e 0r1/,re ./re1ses 6e/1se dr." s-e1r ,-ee.er"y s 16sor6ed ,o /1se >41s,/ de0or51,o.* I0 we 4oo= 1, ,-e "r1>- 6e4ow we w44 see ,-1,,-e 1re1 .der ,-e s,ress s,r1. /re s "re1,er w-/- 5e1. ,-1, ,-e 51,er14 s 5ore ,o"- 1.d

5ore ,o"- 5e1.s 6e." 164e ,o 16sor6 5ore e.er"y*

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NON?DESTRUCTIE TESTING

>t is clearly useful to detect flaws and fatigue cracks in parts before sudden failure occurs. For this purposea number of non-destructive techni4ues can be used, including dye penetrants, magnetic particles, -raysand ultrasonic testing.

Dye Pe.e,r1.,s Sr01/e F41ws'

>n this techni4ue, a coloured or fluorescent dye, dissoled in carrier li4uid of low surface tension, is appliedto the surface. The dye then penetrates surface cracks. The part is then washed to remoe ecess dye and

a Ablotting8 agent (deeloper) is applied to draw the dye from within the cracks to the surface giing aisible indication of location.

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U4,r1so./ I.s>e/,o. I.,er.14 1.d Sr01/e F41ws'

This method is based on the fact that dense substances (such as metals) propagate sound waes or high-fre4uency ibrations more readily than low-density substances such as air, oil, or water. ltrasonic ( to 2million C@) waes generated by a pie@oelectric crystal are passed through the part to be inspected. 6sshown in the figure below, the waes are reflected at cracks and flaws (as well as the surface of thematerial). 6 li4uid Acouplant8 such as oil or water is used aid transmission of the waes into the part. The position of defects, which appear on the oscilloscope out as shown in the figure, indicated by the timere4uired for the signal to reach the defect relatie to the time for the waes to penetrate the specimen.

Oscilloscpope

readout

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