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Advanced Fabrication and Engineering For when your build is going off the charts. Wildly modified vehicles and straight up buggies welcome.

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  #1  
Old May 3rd, 2006, 09:51
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Yield strength test for the ME's (and good info for the rest of us).

OK, I'm beginning to think about my rear suspension for next winter's project. I won't bore you with details, but there will be shaped links involved. As we have seen from recent experience, 2" x .25" tube is not stout enough to withstand the kind punishment we are dishing out when used in a rear LCA application. It's fine for the front, as rocks tend to slide off, but in the rear, under articulation, you are actually driving into rocks under power, and they bend.

So, I want to construct a lower wishbone out of mild steel that will have 50% greater yield strength than 2" x .25" DOM tube. I want to know what thickness of material will achieve this goal given the following design parameters.

Length of member = 38"
Maximum allowable width = 2"
Maximum allowable height = 3"

Assume the stress to be induced perpendicular to the wishbone for simplicity. What material thickness is needed to achieve 50% greater yield strength than 2" x .25" mild steel DOM round tube. Show your work!

CRASH
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Old May 3rd, 2006, 09:55
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Re: Yield strength test for the ME's (and good info for the rest of us).

2x2 .250 square

or better yet, since your parameters allow, 2x3 .250 rect. tube.

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Originally Posted by CRASH
Show your work!
:flipoff2:



Or screw mild steel. 1.75" .188wall heat treated 4130 is 30% stronger than 2" .250wall DOM (source: PolyPerformance), so I'd imagine 1.75" .250wall heat treated 4130 would be about 50% stronger, and 2" .188 or especially 2" .250 would be way more than 50% stronger.

Last edited by BrettM; May 3rd, 2006 at 10:00.
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Old May 3rd, 2006, 10:03
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Re: Yield strength test for the ME's (and good info for the rest of us).

Oh, and some basic engineering info for the rest of us to ponder (source: Wikepedia):

There are three typical definitions of tensile strength:

* Yield Strength - The stress a material can withstand without permanent deformation.

* Ultimate Strength - The maximum stress a material can withstand.

* Breaking Strength - The stress coordinate on the Stress-strain curve at the point of rupture.


Yield strength, or the yield point, is defined in engineering and materials science as the stress at which a material begins to plastically deform. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed some fraction of the deformation will be permenent and non-reversible. Knowledge of the yield point is vital when designing a component since it generally represents an upper limit to the load that can be applied. It is also important for the control of many materials production techniques such as forging, rolling, or pressing

In structural engineering, yield is the permanent plastic deformation of a structural member under stress. This is a soft failure mode which does not normally cause catastrophic failure unless it accelerates buckling.
Contents

Definition

It is often difficult to precisely define yield due to the wide variety of stress-strain behaviours exhibited by real materials. In addition there are several possible ways to define the yield point in a given material:

* The point at which dislocations first begin to move. Given that dislocations begin to move at very low stresses, and the difficulty in detecting such movement,this definition is rarely used.
* Elastic Limit - The lowest stress at which permenent deformation can be measured. This requires a complex iterative load-unload procedure and is critically dependent on the accuracy of the equipment and the skill of the operator.
* Proportional Limit - The point at which the stress-strain curve becomes non-linear. In most metallic materials the elastic limit and proportional limit are essentially the same.
* Offset Yield Point (proof stress) - Due to the lack of a clear border between the elastic and plastic regions in many materials, the yield point is often defined as the stress at some arbitrary plastic strain (typically 0.2%). This is determined by the intersection of a line offset from the linear region by the required strain. In some materials there is essentially no linear region and so a certain value of plastic strain is defined instead. Although somewhat arbitrary this method does allow for a consistent comparison of materials and is the most common.



Steel has a very linear stress-strain relationship up to a sharply defined yield point, as shown in the figure. For stresses below this yield strength all deformation is recoverable, and the material will relax into its initial shape when the load is removed. For stresses above the yield point, a portion of the deformation is not recoverable, and the material will not relax into its initial shape. This unrecoverable deformation is known as plastic deformation. For many applications plastic deformation is unacceptable, and the yield strength is used as the design limitation.

After the yield point, steel and many other ductile metals will undergo a period of strain hardening, in which the stress increases again with increasing strain up to the ultimate strength. If the material is unloaded at this point, the stress-strain curve will be parallel to that portion of the curve between the origin and the yield point. If it is re-loaded it will follow the unloading curve up again to the ultimate strength, which has become the new yield strength.

After steel has been loaded to its ultimate strength it begins to "neck" as the cross-sectional area of the specimen decreases due to plastic flow. Necking is accompanied by a region of decreasing stress with increasing strain on the stress-strain curve. After a period of necking, the material will rupture and the stored elastic energy is released as noise and heat. The stress on the material at the time of rupture is known as the breaking stress. Note that if the graph is plotted in terms of true stress and true strain necking will not be observed on the curve as true stress is corrected for the decrease in cross-sectional area. Necking is also not observed for materials loaded in compression.

Ductile metals other than steel typically do not have a well defined yield point. For these materials the yield strength is typically defined by the "0.2% offset strain". The yield strength at 0.2% offset is determined by finding the intersection of the stress-strain curve with a line parallel to the initial slope of the curve and which intercepts the abscissa at 0.002. A stress-strain curve typical of aluminum along with the 0.2% offset line is shown in the figure below.

Yielded structures have a lower and less constant modulus of elasticity, so deflections increase and buckling strength decreases, and both become more difficult to predict. When load is removed, the structure will remain permanently bent, and may have residual pre-stress. If buckling is avoided, structures have a tendency to adapt a more efficient shape that will be better able to sustain (or avoid) the loads that bent it. Because of this, highly engineered structures rely on yielding as a graceful failure mode which allows fail-safe operation. In aerospace engineering, for example, no safety factor is needed when comparing limit loads (the highest loads expected during normal operation) to yield criteria. Safety factors are only required when comparing limit loads to ultimate failure criteria, (buckling or rupture.) In other words, a plane which undergoes extraordinary loading beyond its operational envelope may bend a wing slightly, but this is considered to be a fail-safe failure mode which will not prevent it from making an emergency landing.

Last edited by CRASH; May 3rd, 2006 at 10:37.
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  #4  
Old May 3rd, 2006, 10:07
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Re: Yield strength test for the ME's (and good info for the rest of us).

More (source: Knowledge Article from www.Key-to-Steel.com):

The engineering tension test is widely used to provide basic design information on images/the strength of materials and as an acceptance test for the specification of materials. In the tension test a specimen is subjected to a continually increasing uniaxial tensile force while simultaneous observations are made of the elongation of the specimen. An engineering stress-strain curve is constructed from the load elongation measurements (Fig. 1).

Figure 1. The engineering stress-strain curve

It is obtained by dividing the load by the original area of the cross section of the specimen.
(1)

The strain used for the engineering stress-strain curve is the average linear strain, which is obtained by dividing the elongation of the gage length of the specimen, d, by its original length.
(2)

Since both the stress and the strain are obtained by dividing the load and elongation by constant factors, the load-elongation curve will have the same shape as the engineering stress-strain curve. The two curves are frequently used interchangeably.

The shape and magnitude of the stress-strain curve of a metal will depend on its composition, heat treatment, prior history of plastic deformation, and the strain rate, temperature, and state of stress imposed during the testing. The parameters, which are used to describe the stress-strain curve of a metal, are the tensile strength, yield strength or yield point, percent elongation, and reduction of area. The first two are strength parameters; the last two indicate ductility.

The general shape of the engineering stress-strain curve (Fig. 1) requires further explanation. In the elastic region stress is linearly proportional to strain. When the load exceeds a value corresponding to the yield strength, the specimen undergoes gross plastic deformation. It is permanently deformed if the load is released to zero. The stress to produce continued plastic deformation increases with increasing plastic strain, i.e., the metal strain-hardens. The volume of the specimen remains constant during plastic deformation, A·L = A0·L0 and as the specimen elongates, it decreases uniformly along the gage length in cross-sectional area.

Initially the strain hardening more than compensates for this decrease in area and the engineering stress (proportional to load P) continues to rise with increasing strain. Eventually a point is reached where the decrease in specimen cross-sectional area is greater than the increase in deformation load arising from strain hardening. This condition will be reached first at some point in the specimen that is slightly weaker than the rest. All further plastic deformation is concentrated in this region, and the specimen begins to neck or thin down locally. Because the cross-sectional area now is decreasing far more rapidly than strain hardening increases the deformation load, the actual load required to deform the specimen falls off and the engineering stress likewise continues to decrease until fracture occurs.
Tensile Strength

The tensile strength, or ultimate tensile strength (UTS), is the maximum load divided by the original cross-sectional area of the specimen.
(3)

The tensile strength is the value most often quoted from the results of a tension test; yet in reality it is a value of little fundamental significance with regard to the strength of a metal. For ductile metals the tensile strength should be regarded as a measure of the maximum load, which a metal can withstand under the very restrictive conditions of uniaxial loading. It will be shown that this value bears little relation to the useful strength of the metal under the more complex conditions of stress, which are usually encountered.

For many years it was customary to base the strength of members on the tensile strength, suitably reduced by a factor of safety. The current trend is to the more rational approach of basing the static design of ductile metals on the yield strength.

However, because of the long practice of using the tensile strength to determine the strength of materials, it has become a very familiar property, and as such it is a very useful identification of a material in the same sense that the chemical composition serves to identify a metal or alloy.

Further, because the tensile strength is easy to determine and is a quite reproducible property, it is useful for the purposes of specifications and for quality control of a product. Extensive empirical correlations between tensile strength and properties such as hardness and fatigue strength are often quite useful. For brittle materials, the tensile strength is a valid criterion for design.
Measures of Yielding

The stress at which plastic deformation or yielding is observed to begin depends on the sensitivity of the strain measurements. With most materials there is a gradual transition from elastic to plastic behavior, and the point at which plastic deformation begins is hard to define with precision. Various criteria for the initiation of yielding are used depending on the sensitivity of the strain measurements and the intended use of the data.

1. True elastic limit based on micro strain measurements at strains on order of 2 x 10-6 in | in. This elastic limit is a very low value and is related to the motion of a few hundred dislocations.
2. Proportional limit is the highest stress at which stress is directly proportional to strain. It is obtained by observing the deviation from the straight-line portion of the stress-strain curve.
3. Elastic limit is the greatest stress the material can withstand without any measurable permanent strain remaining on the complete release of load. With increasing sensitivity of strain measurement, the value of the elastic limit is decreased until at the limit it equals the true elastic limit determined from micro strain measurements. With the sensitivity of strain usually employed in engineering studies (10-4in | in), the elastic limit is greater than the proportional limit. Determination of the elastic limit requires a tedious incremental loading-unloading test procedure.
4. The yield strength is the stress required to produce a small-specified amount of plastic deformation. The usual definition of this property is the offset yield strength determined by the stress corresponding to the intersection of the stress-strain curve and a line parallel to the elastic part of the curve offset by a specified strain (Fig. 1). In the United States the offset is usually specified as a strain of 0.2 or 0.1 percent (e = 0.002 or 0.001).
(4)

A good way of looking at offset yield strength is that after a specimen has been loaded to its 0.2 percent offset yield strength and then unloaded it will be 0.2 percent longer than before the test. The offset yield strength is often referred to in Great Britain as the proof stress, where offset values are either 0.1 or 0.5 percent. The yield strength obtained by an offset method is commonly used for design and specification purposes because it avoids the practical difficulties of measuring the elastic limit or proportional limit.

Some materials have essentially no linear portion to their stress-strain curve, for example, soft copper or gray cast iron. For these materials the offset method cannot be used and the usual practice is to define the yield strength as the stress to produce some total strain, for example, e = 0.005.
Measures of Ductility

At our present degree of understanding, ductility is a qualitative, subjective property of a material. In general, measurements of ductility are of interest in three ways:

1. To indicate the extent to which a metal can be deformed without fracture in metalworking operations such as rolling and extrusion.
2. To indicate to the designer, in a general way, the ability of the metal to flow plastically before fracture. A high ductility indicates that the material is "forgiving" and likely to deform locally without fracture should the designer err in the stress calculation or the prediction of severe loads.
3. To serve as an indicator of changes in impurity level or processing conditions. Ductility measurements may be specified to assess material quality even though no direct relationship exists between the ductility measurement and performance in service.

The conventional measures of ductility that are obtained from the tension test are the engineering strain at fracture ef (usually called the elongation) and the reduction of area at fracture q. Both of these properties are obtained after fracture by putting the specimen back together and taking measurements of Lf and Af .
(5)
(6)

Because an appreciable fraction of the plastic deformation will be concentrated in the necked region of the tension specimen, the value of ef will depend on the gage length L0 over which the measurement was taken. The smaller the gage length the greater will be the contribution to the overall elongation from the necked region and the higher will be the value of ef. Therefore, when reporting values of percentage elongation, the gage length L0 always should be given.

The reduction of area does not suffer from this difficulty. Reduction of area values can be converted into an equivalent zero-gage-length elongation e0. From the constancy of volume relationship for plastic deformation A*L = A0*L0, we obtain
(7)

This represents the elongation based on a very short gage length near the fracture.

Another way to avoid the complication from necking is to base the percentage elongation on the uniform strain out to the point at which necking begins. The uniform elongation eu correlates well with stretch-forming operations. Since the engineering stress-strain curve often is quite flat in the vicinity of necking, it may be difficult to establish the strain at maximum load without ambiguity. In this case the method suggested by Nelson and Winlock is useful.

Last edited by CRASH; May 3rd, 2006 at 10:40.
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  #5  
Old May 3rd, 2006, 10:13
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Re: Yield strength test for the ME's (and good info for the rest of us).

Since this will be a constructed member, I am not limited to pre-formed shapes. The main trailing portions of the wishbone will see the deflective loads, so they are the only thing I am worried about. Round tube does not fit my design parameters.

In other words, I can use any combination of material thickness commonly availabe in sheet form, and it can be cut into any combination of height and width within, the design parameters.

Oh, an Brett if you were really an engineer, you would have suggested a much more expensive option, requiring months of time to source, and utilizing new tooling for you to play with.

CRASH


Quote:
Originally Posted by BrettM
2x2 .250 square

or better yet, since your parameters allow, 2x3 .250 rect. tube.



:flipoff2:



Or screw mild steel. 1.75" .188wall heat treated 4130 is 30% stronger than 2" .250wall DOM (source: PolyPerformance), so I'd imagine 1.75" .250wall heat treated 4130 would be about 50% stronger, and 2" .188 or especially 2" .250 would be way more than 50% stronger.

Last edited by CRASH; May 3rd, 2006 at 10:42.
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Old May 3rd, 2006, 10:27
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Re: Yield strength test for the ME's (and good info for the rest of us).

Now I know why I don't got into AdFab that much.

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Old May 3rd, 2006, 10:33
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Re: Yield strength test for the ME's (and good info for the rest of us).

Wow a whole semester of materials science in one sitting.....wanna buy the book? I'm don't need mine now that I have seen this.
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Old May 3rd, 2006, 10:43
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Talking Re: Yield strength test for the ME's (and good info for the rest of us).

Quote:
Originally Posted by goodburbon
Wow a whole semester of materials science in one sitting.....wanna buy the book? I'm don't need mine now that I have seen this.
just finished my final - anyone want to buy mine? the book store wont take it back.... ****ing $150 book!

Andy - Is heat trating aloud?
what amount of strain do you want?

Increasing the yeild stress by 50% but making the material too brittle wont do you a damn thing...

take your metal file that you use to de-burr stuff - the yeild strength of that material is really high, but throw it at the ground and it shatters - its much too brittle for what you are wanting to do, but meets your yeild requirement...
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As if the program is running with some high level of integrity and only the best of the best win.
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Old May 3rd, 2006, 10:52
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Re: Yield strength test for the ME's (and good info for the rest of us).

Yeah I have my final next week for MAt Sci. So anyone else need a useless book? Its got a golf ball on the cover. Did you guys have a Lab with it?
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Old May 3rd, 2006, 11:54
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Re: Yield strength test for the ME's (and good info for the rest of us).

Quote:
Originally Posted by XJ_ranger
just finished my final - anyone want to buy mine? the book store wont take it back.... ****ing $150 book!

Andy - Is heat trating aloud?
what amount of strain do you want?

Increasing the yeild stress by 50% but making the material too brittle wont do you a damn thing...

take your metal file that you use to de-burr stuff - the yeild strength of that material is really high, but throw it at the ground and it shatters - its much too brittle for what you are wanting to do, but meets your yeild requirement...
I think for the sake of cost savings, we should stick to mild steel, no heat treatment. This will definately lower yield strength potential, but it probably puts you in the ballpark on ductility. The graph in my second post is mild steel, the file in your example looks much different, right?

Also, on the strain question, I guess that depends on the strain it takes to deform the 2" tube. I guess we need to find a yield value for 2" mild steel tube, and work up from there........aren't common shapes like tube, channel, and I-beams already worked out for various types of steel?
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Old May 3rd, 2006, 12:30
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Talking Re: Yield strength test for the ME's (and good info for the rest of us).

Quote:
Originally Posted by CRASH
The graph in my second post is mild steel, the file in your example looks much different, right?
the numbers are different, but the graph has the same shape... steeper angle = higher modulus of elasticity....
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Old May 3rd, 2006, 12:33
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Re: Yield strength test for the ME's (and good info for the rest of us).

wait... this is a lower wishbone you're making? If it's a wishbone, I don't understand the design parameters of 2"x3". Do you have a basic/initial sketch of this suspension?

(Is there a Hi9 on the way? only reason I can see for a lower wishbone is driveshaft protection)
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Old May 3rd, 2006, 12:39
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Re: Yield strength test for the ME's (and good info for the rest of us).

also need net loaded vehicle weight, tire size, and actual yielded lift
to insure it will hold up to a good smack.
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Old May 3rd, 2006, 12:57
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Re: Yield strength test for the ME's (and good info for the rest of us).

Quote:
Originally Posted by goodburbon
also need net loaded vehicle weight, tire size, and actual yielded lift
to insure it will hold up to a good smack.

See, I don't think you need that.

We know 2"x.25" mild is not quite strong enough for LCA use. So use that as your basis and add 50% to the yield number to get a target to build toward.

The stuff you mention is a constant for our purposes.
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Old May 3rd, 2006, 13:04
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Re: Yield strength test for the ME's (and good info for the rest of us).

Quote:
Originally Posted by BrettM
wait... this is a lower wishbone you're making? If it's a wishbone, I don't understand the design parameters of 2"x3". Do you have a basic/initial sketch of this suspension?

(Is there a Hi9 on the way? only reason I can see for a lower wishbone is driveshaft protection)

The reason for a lower wishbone is two-fold.

A: It will have a shape to it, a down ward sweep, specifically, to help grond clearance while articulated. If you do this with individual links, they flop over when using flex-type joints. Unless you do a set of mini-links or somthing to keep the joints oriented correctly,

B: With a wishbone, you get away with only needing one flex joint at the crossmember, and you can mount sway bar links or anything else you like (air bags, for instance) to the wishbone without worrying about deflection.

I'll be doing a double triangulated system, with an upper wishbone as well. That member is a lot less troublesome than the lower one, however.
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