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How strong is plastic really?

“Modern polymers are stronger than steel”. This is a frequent article claim. Is it true?

Let’s make a comparison: The strongest plastic that could be found on the Campus database was PPA Grivory GV XE 16102 (GVL-6H HP) by EMS. According to the datasheet, its Tensile Strength in room temperature and dry conditions is a staggering 315 MPa. "Steel" is a broad term. As examples I have selected high strength steel ASTM A514 and structural steel S275. The given Yield Strength values for them are 690 and 275 MPa. In this case, Tensile Strength corresponds to Yield Strength since the selected plastic does not yield.

 

Conclusion: Clearly, according to the datasheet values, plastic is not consistently stronger than all steel grades but it can be stronger than a particular steel grade.

 

How does this relate to reality then?

The presented polymer is an uncommon plastic grade that product designers rarely use for their products because of its cost. It is primarily used in highly demanding applications like airplane engines. When compared with more common plastics the chart looks like this:

 

These examples roughly represent typical strength values for different types of plastics (commodity, filled and unfilled).

  • PP: Unfilled commodity polyolefin, Yield Strength 25-35 MPa
  • ABS: Most unfilled plastic grades, amorphous and semi-crystalline: Yield Strength 35-60 MPa
  • PBT+15%G: Common engineering plastic with moderate glass rate, Tensile Strength 80-120 MPa
  • PA+40%GF: Advanced engineering plastic with high glass rate, Tensile Strength 130-180 MPa

These are the relevant values in normal plastic product design and demonstrate the strenght of plastic better the earlier example.

How strong is plastic compared with metal?

If we want to compare plastics to metals, instead of steel, we should rather see what their properties look like next to the common die cast metals that often compete with plastics:

 

At first glance this chart would seem to indicate that heavily filled engineering plastics are stronger than die cast metals. However, the metal values represent Yield Strength, the stress level that leads to permanent deformation. The stress required for reaching their Ultimate Tensile Strength might be substantially higher. Still, I prefer to use Yield Strength value because it is what usually matters. More important, the strength of plastic is less predictable than that of metals. In all plastic product design temperature, continuous load, moisture absorption (especially in case of PA), and fatigue must be taken into account. Plastics are far more sensitive to them than metals. Also weld lines and fiber orientation might weaken injection molded plastic parts more than pressure die cast metal components.

Does this mean that metal is always better option than plastic?

No, comparing the strength values of plastics and metals is very theoretical. What matters is the final strength and durability of the end-product. Product strength is a combination of material properties and product geometry. The guidelines and principles for metal and plastic components are different and this should be seen in the geometry of the product. When properly designed, plastic products can be strong and durable, but they also have other benefits when it comes to performance, weight, cost, manufacturability and in many cases even environmental impact. I favor plastics because of my background but sometimes die cast metals might offer better alternatives. It depends on case.

Plastic vs. organic materials

When writing this article I ran into interesting strength values among organic materials. Here are some of them next to the earlier plastic types (still on theoretical bases):

 

It seems like engineers still have work to do if they want to exceed nature in material science.      

Material Strength [MPa)
Structural Steel S275 (YS) 275
Steel, high strength alloy ASTM A514 (YS) 690
Cast iron (TS) 170
Aluminium alloy 6061-T6 (YS) 241
Aluminium ALSi12, Pressure die casting (YS) 170
Magnesium AZ91HP, Pressure die casting (YS) 160
Zinc ZnA14Cu1, Pressure die casting (YS) 230
PP, Borealis HD003WG (YS) 33
ABS, CYCOLAC Resin MG47, unreinforced (YS) 43
PBT+15GF, Crastin® SK602 NC010 (TS) 109
PA+40GF,Lanxess Durethan® BKV 140, PA+40GF, dry (TS) 180
PPA+60%GF, Grivory GV XE 16102 (GVL-6H HP), dry (TS) 315
Human skin (YS) 15
Bone (TS) 130
Human Hair (TS) 380
Spider silk (TS) 1000

References:

http://www.azom.com/article.aspx?ArticleID=6022

http://en.wikipedia.org/wiki/Bone http://en.wikipedia.org/wiki/Ultimate_tensile_strength www.campusplastics.com

http://www.ssab.com/Global/SSAB/SSAB_Americas/en/015_ASTM%20A514%20Gr%20...

http://www.campusplastics.com/campus/en/datasheet/Grivory+GV+XE+16102+%2...  

Comments

avinash egala
Could you advice on something that is significantly light with MPa >300 and easily castable probably possible even in your back yard
Markus
Avinash, engineering plastics like PBT or PA with high glass-fibre content you can reach level of 130-180 MPa. This is in room temperature even then theoretical since orientation of the fibres, weld lines etc. affect the strength. To me it sounds like you need to go with thermosets. Those you can laminate in your "backyard" and utilize continuous glass fibres (or carbon fibres) with optimal orientation.
Markus
Mark, Does it really need to have the strength of structural steel or does it just replace steel? Typically constructions that are made of steel are much stronger than they actually should be... aimply because high strength is an inherent property of the material. Anyway, if you do you frame using some heavily glass-inforced plastic (PP, PA, PBT) you might theoretically reach a strength level of app 100 MPa. In correspondence to steel that is app. 1/4. This means that only to compete with straight pull, the cross-section of your frame should be 4 times larger. But if you need to compete with bending as well, you need to look at the tensile modulus values, which are on the level of 8-12 GPa vs. 200 GPa. This means that your construction should be significantly higher (in the direction of bending) in order to be equally stiff. Furthermore, unlike plastics, steel is not very sensitive to elevated temperature or long term static load. If you have to take these in account, you should have at least safety factor of 2-3 on data sheet values. I must say that this case does not sound very promising to me. Good luck, Markus

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