Mechanical properties of plastics 5

[elogesh]

Plastics,

For evaluation of the strength of the plastics,I have referred the mechanical properties of plastics.I have following doubts,

1) For some plastics(usually without additives),only tensile strength is specified and compressive strength is not

specified.Can, I consider tensile strength is equivalent to compressive strength.

2) For some plastics tensile strength at break or ultimate tensile strength is specfied without referring to
yield strength. Whether these plastics has brittle fracture...

3) For some plastics ultimate Tensile strength values mentioned are usually less compared to yield strength. Can someone explain
how this is possible...

In metals Tensile strength is higher than yield strength.

4) For some plastics elongation at yield is specified as 3%.Does this means that strain greater than 0.03
will result in permeant set.For the same material elongation at break is specified as 1.9-150%.The lower limit
of elongation at break less than corresponding value for break!!!

5) How flexural modulus is different from tensile modulus and compressive modulus?

Can someone help in this regard.....

Currently I am working with following plastics,

GE volex,Delrin,PBT,NYlon66,Nylon6,etc......

Regards,
Logesh.E

 

[patprimmer]

The test results for plastics can be quite dependant on details in test conditions.

For example unfilled nylon 6 gains about 70% in flex mod when conditioned to equilibrium moisture content vs dry as moulded.

Also speed of head travel when testing elongation and tensile can have a significant effect.

Are the figures you are comparing from the same source, for the exact same grade, and prepared and tested under identical conditions.

CAMPUS standards go a long way toward reducing these variables.

In my experience, Bayer and DuPont produce reliable and complete data for their materials.

Valox is PBT.

Regards
pat pprimmer@acay.com.au
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[patprimmer]

For semi crystalline materials, and in particular, polyesters (PET & PBT) are susceptible to post moulding crystallisation. This can be having a measurable effect on properties for quite a few days after moulding.

Regards
pat pprimmer@acay.com.au
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[CoryPad]

1) You should not assume tension and compression strength are equal.

2) It is possible that some plastics (for example, high glass fiber content) have low fracture strain, thus no appreciable yielding behavior. Or, it could be that the data source did not test for yield or did not report it.

3) For some plastics, after yielding the microstructure changes and results in strain softening. This results in fracture stress less than yield stress.

4) You are correct about the 0.03 strain as the recoverable deformation limit. The 1.9% to 150% values may be due to different test conditions as pat mentioned, or it could be a result of different material suppliers with different additives, etc.

5) Flexural modulus is determined by bending tests, while tension and compression are determined by uniaxial tension and compression, respectively.

Regards,

Cory

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[RichGeoffroy]

Elogesh:

Let’s take your points one at a time.

1. For some plastics (usually without additives), only tensile strength is specified and compressive strength is not specified. Can, I consider tensile strength is equivalent to compressive strength?

Compressive strength is not an often-used property; therefore it is not usually measured nor reported as a typical property for a material. Do not consider it to be the same as the tensile strength. If you need that specific property for the material, have it measured by a competent laboratory.

2. For some plastics tensile strength at break or ultimate tensile strength is specified without referring to yield strength. Whether these plastics has brittle fracture...

Tensile strength is defined as the highest tensile stress achieved --- sometimes it is the tensile yield strength, for other materials it is the tensile strength at break. If tensile yield strength is not reported, DO NOT ASSUME that the material exhibits brittle fracture (i.e., rupture prior to yield). The material may just exhibit a higher stress at break than at yield.

3. For some plastics ultimate tensile strength values mentioned are usually less compared to yield strength. Can someone explain how this is possible? In metals tensile strength is higher than yield strength.

I am not sure if it’s always the case with metals that the tensile yield strength is higher than the strength at break. However, for some polymers, orientation during tensile testing significantly alters the material’s mechanical behavior as the test proceeds. In metals, I believe this is referred to as “work hardening”. In any event, as the material elongates the polymer molecules orient, in some cases enhancing crystallization, and the polymer becomes stronger and more rigid. The tensile stress will begin to rise with further extension and may exceed the yield stress.

Also, keep in mind that while most ductile polymers “yield”, they may not exhibit the classical “yield point” (the point at which there is a zero slope). Many ductile polymers exhibit “pseudo-yield”, that is they deviate significantly from linear behavior but their stress continues to rise with increased strain.

4. For some plastics, elongation at yield is specified as 3%. Does this means that strain greater than 0.03 will result in permanent set? For the same material elongation at break is specified as 1.9-150%. The lower limit of elongation at break less than corresponding value for break!!!

Actually, permanent set can occur well before the yield strain. Viscoelastic flow occurs as soon as the elastic limit is exceeded, i.e., anywhere after the linear portion of the stress-strain curve. Most of that viscoelastic flow that occurs prior to yield is generally recoverable with time --- but not all of it. That which is not recoverable is permanent set.

The second part of this question I don’t understand. Rather than guess at what you might mean, I’d rather not answer it without further clarification.

5. How flexural modulus is different from tensile modulus and compressive modulus?

Ideally, they should all be the same. The fact of the matter is that we can’t measure the properties of materials in ideal states. Because of the way we have to test materials, not everything can be held constant --- things change --- and these changes, many of which occur during the test, alter the outcome of the test.

A case in point, flexural modulus is generally measured in a 3-point bend test. A rectangular specimen is supported horizontally by two steel pins and the plastic bar is loaded at the midpoint of the two supports.

One limitation with flex test is that it is not a “pure” stressed state. The stress is calculated as the maximum “fiber” stress that occurs directly under the load on the underside surface of the bar --- this is the only point at which that maximum fiber stress exists. Actually, the stress distribution through the bar varies from tensile on the underside surface of the specimen to compressive stress on the topside surface. The compressive stress tends to inhibit the deflection of the specimen, artificially raising the apparent modulus of the material.

Also, as the specimen deflects, the bar must move along the supports to accommodate the deflection. If the calculated fiber strain on the underside surface exceeds about 5%, a significant portion of the load is consumed as the driving force to “push” the specimen through the supports --- rather than merely bend the specimen.

In addition, at some strain level, probably around 5%, the actual strain on the bar at the point of loading stops increasing --- no further curvature occurs in the central area of the specimen --- but the specimen continues bend toward the ends of the bar as it slides through the supports. Inasmuch as the apparent fiber strain is calculated based on the amount of deflection from the original horizontal position, the method begins to yield erroneous data.

The latter two points most likely would not affect the reported modulus, but should be considered in any other data reported for the test.



Rich Geoffroy
Polymer Services Group
POLYSERV@aol.com

 

[elogesh]

Hai,
Thanks for all the replies.

Betterway to show the thanks in eng-tips is putting a star for valuable post.

Particularly the replies from Rich geoffrey is very useful.
____________________________________________________
Pat:

I wrongly mentioned valox and PBT. Thanks for pointing it out.
_____________________________________________________
Rich Geoffrey:

I have corrected the third question

3) For some plastics, elongation at yield is specified as 3%. Does this means that strain greater than 0.03 will result in permanent set? For the same material elongation at break is specified as 1.9-150%. The lower limit of elongation at break(1.9%) less than corresponding value for yield(3%)!.
_________________________________________________________


I am working in automotive company involved in design and manufacture of braking systems.Gradually,we are converting few metallic components into plastic components.

For the past few months,we have seen few failure of plasic components during accelerated life testing.Hence we are reviewing the material properties of plastics,design of plastics,wear resistance,influence of temperature on the properties(young's modulus,etc) and fatigue strength,economy,etc.For referring the fatigue strength we have "book on fatigue and tribological properties of plastics and elastomers" ,plastics design library" and fewmore text books.
The web-sites and catalogues of dow,DSM,GE-plastics also provides useful material properties.

Regarding the design of plastics,I have a doubt.

1)This is in regard to new component under developement...

Usually in our industry it was guidelined to use factor of safety of greater than 3 to 5 for plastics.The loading conditions used to be repeated loading for morethan a million cycles for this new component.Hence fatigue strength is accounted for allowable strength.
we bought the book on fatigue and tribolgical properties of plastics and elastomers, we started referring the fatigue properties from this hand book.


we used a plastic material with minimum tensile strength of 165 N/mm2.The fatigue strength is 40 N/mm2.Currently we are limiting the maximum stress on the component to 13 N/mm2,keeping a factor of safety of 3 with respect to fatigue strength.

My confusion is, whether designing like this, is an over design.That is keeping a factor of safety of 3 based on fatigue strength is an overdesign.

Regards,
Logesh.E

 

[rnd2]

elogesh
Always take into account that even moderate temperature rise can have a profound effect on the strength of a plastic part.
If you are testing at 20 deg C and the part may reach working temperature at say 50 deg C you need to be certain it will continue to function perfectly at the higher temp.

 

[RichGeoffroy]

Elogesh:

In the situation that you propose, I suspect that you’re looking at a database that is providing a range of values that corresponds to many different grades or compounds of the same base resin. In the instance, of the typical tensile break at 1.9% strain (in a material that typically exhibits 3% strain to yield), you can probably expect a fairly brittle fracture. In all likelihood this particular compound is a highly-filled compound and not neat resin. The other option is that the resin may be a very hi-flow, low-molecular weight grade, which could also account for the low strain to failure.

The high-strain-to-failure grade could be an unfilled resin with good ductility, or possibly a plasticized or rubber-modified compound of a generally rigid base resin. And those compounds in between, they can be any of a multitude of different formulations using the same base resin. It’s hard to tell with the little information provided.

You should have a general understanding of the base resin you’re contemplating using. Try to interpret the data with your own personal knowledge. Try to make some sense of the information being provided --- then test it.

DON’T DESIGN FROM A BOOK!! Data bases are there for your help --- but don’t count on them to do your work for you. The typical properties presented on a data sheet can assist you in getting a fair understanding of the material's properties, but you still need to test the material, and eventually the product, to make a reasonable judgement as to whether the material and/or the design will satisfy the needs for your application.

As for your question regarding designing a part for cyclic loading, plastic is similar to most materials in that it can fail at stresses lower than the strength of the material if a repeating stress application occurs at a sufficiently high frequency and magnitude on a continuous basis over a period of time. Your general guideline of using a design factor of between 3 and 5 (i.e., minimum tensile strength multiplied by a factor of 0.2-0.33) is the general rule of thumb for designing plastic parts under long-term static load. In applications where cyclic loading is anticipated, 80% of the long-term static design stress is a reasonable guideline. Based on these rules-of-thumb, it's possible that the your design stress is fairly conservative. However, if you're not testing, you're better off being very conservative.

Now, I need to go back to something I said at the beginning of this post, DON’T DESIGN FROM A BOOK!! The numbers that you’re quoting may be absolutely perfect for designing your application, or they may be terribly misleading. Go back and test the material to ensure that the data fits your needs. Design with your data and then retest the product.




Rich Geoffroy
Polymer Services Group
POLYSERV@aol.com

 

[patprimmer]

I must agree with Rich Geoffroy.

Don't design from a data sheet.

Data sheets give results for a perfect moulding in the shape of a test bar, and details are often chosen that show the material in to good a light.

For instance data for nylon is mostly quoted dry, where as some are dramatically different when moisture conditioned (By the way I quoted it wrong in my earlier post. It should have said looses, not gains 70% of flex mod on conditioning)

Also colours and moulding conditions can cause a significant deviation from data sheet properties, as can part and mould design.

An example of this is glass fibre orientation. Fibre orientation can have a dramatic effect on properties. Glass fibre orientation is dependant on a number of factors, including direction of flow, injection speed, back pressure, section thickness and hold pressures and times.

Other factors that significantly effect part life in applications like brake parts are chemical environment and temperature. The chemicals the part is exposed to might not be identical to the specified chemicals.

I know that DSM, GE and DuPont have all supplied material for parts like brake booster canisters and power assist valves. They should have considerable data derived from projects dating as far back as about 1977.

Regards
pat pprimmer@acay.com.au
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[elogesh]

Hai,

Thanks to Rich Geoffroy and pat and a star.

We decide to test the material. We are exploring the feasibility of using in house testing facility for gathering the data.

If I have any issues in future, I will get back to these forum.

With Thanks and Regards,
Logesh.E