Question

Q-1. You are testing a circular beam of two metals, 1045 steel and 2014-T6 aluminum, in a rotating-cantilever beam fatigue test. If you need to choose one of these metals so that you can essentially design a part made of that metal, and subject the part to a repetitive stress- amplitude without ever failing due to fatigue, which one would you choose (See the attached figure) 600 T-51-25 tanium alloy 500 00 Maximum 8 MP 300 Ductile Castro 200 700 30 2016-16 Ally 100 ERLA 16 Mg alloy 10 10 10 10 Cycles to Q-2. What is the fatigue strength for the 1045 steel alloy, if you expect to survive 100,000 cycles of repetitive stress? Answer: Q-3. What is the stress below which you could essentially claim that a part made of 4340 steel alloy would not fail due to fatigue? Answer: Q-4. What is the stress below which you could essentially claim that a part made of 2014-T6 AI alloy would not fail due to fatigue? Answer: Q-5. Plastic deformation of a metal at a low (-room) temperature is (a) A mechanism for strengthening a metal () A manufacturing method for producing metal parts (e) Both a manufacturing method and a strengthening mechanism Q-6. A large strain-hardening coefficient, n, in the true stress-true strain equation, say n=0.5, (a) A given amount of plastic strain produces only a small increase in strength (6) Strain hardening (cold working) is virtually impossible to achieve (c) A given amount of plastic strain produces a large increase in strength means that 2

Answer 1

Fatigue is the behavior of materials under complex repetitive loads.

Fatigue is a complex process (causing a localized permanent change in the material) in which repetitive stresses/strains involving at one or more points, which leads to the crack initiation.

And further repetitions(stresses/strains) by loads induce the crack propagation at some points and leads to a complete fracture.

Roughly fatigue shall happen when the stress ranges are above the (0.5-0.4)*ultimate strength.

There is a limit called endurance limit where any number of repetitions of stress cycles shall not induce any damage to the material.

(Note: Endurance limit is the place where curve flattened)

The material in the image 4340 steel, 1045 steel, Ductile cast iron, 70cr Brass, Titanium alloy, and EQ21A-T6 Mg alloy have an endurance limit.

600 T15A12.55 titanium alloy 500 Maximum SP 1045 300 Ductile Castro 700 30 2012- 16 A 100 EQ2LAT6 Mg alloy 104 10 10 10 Cycles of

In question number 1, the selected materials are 1045 Steel, and 2014-T6 aluminum is chosen.

1045 Steel has an endurance limit, and 2014-T6 aluminum doesn't have an endurance limit.

The endurance limit of 1045 steel is around 300MPa

which means it can sustain any infinite cycles whose cycle ranges lesser than 300MPa.

while the endurance limit of is not defined for 2014 T-6 al alloy and for the same cycle range of stress 2014 T-6 ally will undergo more fatigue than steel alloy.

Ex at the cycle ranges of 300Mpa 1045 steel have infinite life but 2014 T-6 al alloy will fail after (10^4 to 10^5) cycles.

Now which one we have to select

It depends on the number of cycles of stress and the corresponding cycle range that shall act in the design life of the structure (and also on the cost of material/partially).

If the cycles of range 300Mpa in the design life occurs for 10*4-10^5 times then material failure shall happen due to fatigue in  2014 T-6 al alloy.

while steel there wont be any damage.

So it is wise to select the 1045 steel

700 600 T1542 58 titanium alloy 500 400 Maximum se, 8 MP3 1045 1 300 Dutcast iron 200 700 SOZ 2012-T6 Alloy 100 E021A T6 My alloy 10+ 10° 10 10 10 Cycles to failure

  

2) 100,000 cycles = 10⁵ cycles. Now we shall see the S-N curve in the below image.

600 11.542.5 titanium alloy 400+ Maximum SMP 1045 300 Ductile cast iron 200 700 2012-16 A 100 21A T6 Mg alloy 104 10° 10 10 10 10 Cycles to file -

For 10^5 cycle repetitions, it can sustain a stress of 400MPa. (Observe in the graph above)..

3) What is the stress value at which 4340 steel shall not fail?

The flattened part of the curve describes the endurance limit at the endurance limit for the given stress material that will never fail for any number of cycles.

The curve flattened at the stress 500Mpa.

So, the stress level below the 500Mpa will never induce damage to the material.

700 600 115412.55 titanium 500 00 Maximum , 8 MP 1045 300 Quctie castro 700302 2014-16 AM 100 E021A 16 My 10 100 10 10 10 Cycles to

4) There is no flattened portion of the material 2014-T6 Al alloy. So it needs to be tested at lower stress levels less than 100MPa and observe whether the curve flattens or not.

So as of now, with this data, it is not possible to answer the endurance limit.

I hope you have understood.

All the Best.!
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