Is Spider Silk Stronger than Steel?

This article was discussed on the very popular Skeptic’ s Guide to the Universe podcast: https://www.theskepticsguide.org/podcast/sgu/670

Nearly a year later they briefly discussed the article as being very impactful in correcting some common knowledge they had always believed: https://www.theskepticsguide.org/podcast/sgu/714

Thanks to Richard Patterson and Bruno Flior for becoming Knife Steel Nerds Patreon supporters. We have almost reached our second goal of funding a toughness study! Just need a couple more supporters.

I’ve heard many times through random internet articles, tv documentaries, and trivia questions that a spider web is stronger than steel. I always accepted this as fact. It is exciting to analyze naturally-occurring materials to see if we can learn to make our current materials better. Here is an example of an article making such claims about spider silk: Spider Silk Is Stronger Than Steel – And Now It Can Be Made In A Lab.

Spider silk is not stronger than steel. In a review of studies on spider silk properties the strongest reported value was 1652 MPa ultimate tensile strength [1]. If you have a block of knives in your kitchen you own steel that is stronger than the strongest spider silk ever reported. In fact, the strongest steel that I am aware of was reported as 6350 MPa [2]. The study that reported that high strength value for steel was not even an attempt to set a record, but a study on the effect of different annealing treatments on high strength steel wire. They got the steel from a commercial supplier. However, articles for the public about steel being super strong are not as sexy as talking about spider silk.

Strength of Materials

The many claims of being “stronger than steel” by many popular articles are not as clear cut as they appear. There are many measures of strength, such as yield strength, ultimate tensile strength, compressive strength, bending fracture strength, specific strength, stiffness, etc. It would take too much room in this article to explain them all. And the comparison to the strength of steel is often to “mild steel” which is in fact quite soft, less than 300 MPa ultimate tensile strength. Therefore, these articles claiming new materials that are stronger than steel only need to find one measure of strength with which to compare with steel, and then they compare it with the lowest strength steel available, and we have a new material that is “stronger than steel.”

Sometimes scientists are more honest than to make such frivolous comparisons, but this message is often lost. In the NPR article I linked to at the top, the follow exchange occurred:

SIMON: And how do spiders do it? What makes it stronger than steel?

RISING: Well, spider silk is made up of proteins, and these proteins are then assembled into a fiber, and it is the binding between the proteins that makes the silk so tough.

Toughness of Materials

Notice the interviewer asked “What makes it stronger than steel?” but Rising, the scientist, responded with “makes the silk so tough.” While listening to an interview that distinction may be lost, but to a materials scientist, toughness and strength are not the same. Just like strength there are many measures of toughness, but the one Rising is referring to is the energy absorbed by a material during a tensile test, where the material is pulled until it breaks. This generates a stress-strain curve, such as this one comparing spider silk to kevlar [3]:

Tensile strength (y-axis) is clearly higher for kevlar than spider silk, contrary to claims by some articles [4]. However, the area under the tensile curves is greater for spider silk, which is a measure of toughness. Because spider silk has high ductility (strain in the curve above) along with relatively high strength means that the area under the curve is large and therefore the toughness is high. So Rising gently corrected the interviewer by restating the question and answering that spider silk is “tough.” In the same literature review I referenced above, the average value of toughness for the best spider silk recorded was 354 MJ/m^3. Is that better than steel? Well, again, it depends on what steel you mean. It is certainly an impressive number, but there are steels that are better.

TWIP steel

With a cursory search I came up with one reported value of 500 MJ/m^3 for a type of steel referred to as TWIP steel, or twinning-induced plasticity steel [5]. There are likely higher values reported for TWIP steels but this is the value I found on page one of a search, and I’m feeling lazy. They made no claims of this being the highest value ever recorded; the toughness value was incidental to the study. You can see a comparison between the tensile properties of spider silk and TWIP steel below [5][6]:

This is just one value from spider silk and TWIP steel, but it is enough to get the point. The strength of spider silk in the review ranged from only 20 MPa up to the aforementioned 1652 MPa, and it has also been reported that spider silk has high variability in properties, even from the same spider [1]. So I used a reported stress-strain curve from a good value of spider silk, along with the TWIP steel study that gave a value of 500 MJ/m^3 [5]. You can see that the TWIP steel has both higher strength and ductility than the comparison spider silk. TWIP steels can have a range of strength and ductility values depending on the design. And that is the beauty of steel: it is produced in a variety of shapes, sizes, strength levels, costs, and made for a variety of applications including corrosive environments, high temperature environments, and more. TWIP steels have been around for decades, and are currently commercially available [7]. And they are available in sheets, not just thin strands, but they are not as exciting as spider silk, apparently.

How do TWIP steels gain their high combination of strength and ductility? As is described by their name, it occurs through a process called twinning. First, the steels are designed to have high amounts of carbon and manganese so that a phase called austenite is stable at room temperature. Normally steel transforms to austenite at high temperature, such as above 912°C in pure iron. This phase is nonmagnetic and has different properties than the normal room temperature phase called ferrite. The composition of the steel is also carefully controlled to have a low “stacking fault energy” which causes it to twin during deformation. Mechanical twinning is a shift of a portion of atoms within the crystal structure of a material, as is illustrated here:

These twins form in the material continuously as it is deformed, which modifies the microstructure, as can be seen in these micrographs of TWIP steel at different levels of deformation [8]:

The strength of crystalline materials (metals, for example) is controlled by the movement of dislocations, which are line defects that naturally occur in the atomic structure. The material permanently deforms when the dislocations move. In a normal material, the grain size is a strong controlling factor for strength as they act as barriers to the movement of dislocations. An introduction to dislocations and grain boundaries can be read here: How Does Grain Refinement Lead to Improved Properties? In typical materials the larger the grain size, the less stress it requires for dislocations, or line defects, to overcome grain boundaries so that the material deforms. When the dislocations overcome their barriers, then the “yield stress” is exceed as indicated on the following stress-strain curve:

Prior to the yield point is the elastic region where the material always returns to its original shape if the load is removed. You can think of it as flexing but not staying bent. What causes the strength to continue to increase past the yield stress? As the material is deformed, more and more dislocations are generated which continuously strengthens the material because dislocations also impede the movement of other dislocations. The stress on the material affects “dislocation sources” which generate the new dislocations. A video of a dislocation source can be seen here, where new dislocations are shooting out of the source and are stopped when they run into other dislocations:

It works in a similar manner with TWIP steel; the stress increases and the dislocations pileup at grain boundaries, but then twins form, as shown in this schematic [9]:

Twins also act as boundaries to dislocation movement. So as the twins continuously form during deformation, the effective grain size is also going down. In a normal material, the dislocation density gradually increases during deformation until the material can no longer accommodate any strain and it fractures. But with a TWIP steel, there is a combination of typical hardening through dislocation generation and also through twin formation. Once the effective grain size is reduced then the stress must be further increased for dislocations to overcome barriers. It’s almost like every time a twin forms it is a new material in a new tensile test. So this behavior leads to very high combinations of strength and ductility.

Artificial Spider Silk

So I have thoroughly covered the erroneous claim by the NPR interview headline that spider silk is stronger than steel, but not the claim that “now it can be made in the lab.” Rising and coworkers are not the first to attempt to make artificial spider silk, there have been many others [10]. The reported breakthrough from Rising and coworkers was that they could produce artificial spider silk of a reasonable length, rather than the very short silk reported by others. However, the reported strength from their experiments was only 162 MPa. Not exactly impressive. That isn’t even as strong as mild steel. So if the strength they measured is so much lower than the “super strong” spider silk that is reported so often, than what exactly have they created? I’m not saying that working on artificial spider silk is a fruitless endeavor, but no one is currently making artificial spider silk that is nearly as strong as the very good silk reported in many articles. Contrary to the article’s title, they did not make spider silk stronger than steel in the lab. Obviously, making silk on a commercial scale has another set of challenges even if they had successfully made high strength silk in the lab, which they didn’t.

The steel that is 20x stronger than steel

Previously in this article I referenced the strongest recorded steel at 6350 MPa ultimate tensile strength. My subheading for this section helps to illustrate the ridiculous headlines of “stronger than steel” since comparing steel to itself is, of course, impossible without stating what two steels you are referring to. The study in question is on a type of steel wire sometimes called piano wire, which has been around for decades. It is produced through heavy cold-drawing of steel wire. The wire is brought through progressively smaller dies, elongating the wire, decreasing its diameter, and increasing its strength [11]:

The high strength of the wire is obtained through interesting modifications to the microstructure of the steel. The wire starts out as a pearlitic microstructure, which is made up of alternating bands of cementite and ferrite. Pearlite forms in steel through slow cooling from high temperature. Ferrite is the typical room temperature phase of steel and is generally soft and low in carbon. Cementite is a hard phase made up of carbon and iron. However, the composite strength of cementite and ferrite is still typically low. An image of pearlite with increasing deformation can be seen below, with alternating white cementite bands and grey ferrite bands. Deformation is in the x-axis, so the pearlite becomes elongated horizontally [2]:

Once the pearlite is highly strained the cementite begins to decompose, so that carbon is diffusing into the surrounding ferrite. Carbon greatly increases the strength of ferrite, but ferrite has a low solubility for carbon, so generally it isn’t possible to add very much. Typically to produce the highest strength steel a phase called martensite is formed, as it can have a much higher carbon content, as described in this article: What Makes Quenched Steel so Hard? The effect of carbon on steel strength can be seen here [12]:

The decomposition of pearlite and the resulting high carbon content of the ferrite gives the heavily drawn wire a very high tensile strength, much higher than spider silk, and even, in fact, higher than many materials known for their high tensile strength like kevlar. Ultra high tensile strength material is difficult to develop, in part, because strength and ductility are opposed to each other. Many very strong materials like ceramics have low measured tensile strength because they are brittle and therefore fracture before their peak strength can be reached. Those types of materials are often tested in compression instead since that test is not as affected by low ductility. So the very high tensile strength possible with steel wire is impressive.

Conclusion

I started the article with a spider silk hook, and then tricked you into learning about steel. Studying natural materials like spider silk is fascinating, to be sure, but I wish the articles written for popular audiences would dial it back a bit and attempt to present reality instead. Steel continues to be a fascinating material with many different applications and sets of properties, and is actually produced for purchase. Unfortunately, articles from the popular press about amazing properties of new types of steel don’t see as much attention because you can’t have a sexy headline about being “20x stronger than steel.” We metallurgists will keep studying it anyway.

 

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[1] Agnarsson, Ingi, Matjaž Kuntner, and Todd A. Blackledge. “Bioprospecting finds the toughest biological material: extraordinary silk from a giant riverine orb spider.” PloS one 5, no. 9 (2010): e11234.

[2] Li, Y. J., P. Choi, S. Goto, C. Borchers, D. Raabe, and R. Kirchheim. “Evolution of strength and microstructure during annealing of heavily cold-drawn 6.3 GPa hypereutectoid pearlitic steel wire.” Acta Materialia 60, no. 9 (2012): 4005-4016.

[3] https://en.wikipedia.org/wiki/Spider_silk#/media/File:Wikipedia_Kevlar_Silk_Comparison.jpg

[4] https://www.seeker.com/body-armor-made-from-spider-silk-1765595936.html

[5] Frommeyer, Georg, Udo Brüx, and Peter Neumann. “Supra-ductile and high-strength manganese-TRIP/TWIP steels for high energy absorption purposes.” ISIJ international 43, no. 3 (2003): 438-446.

[6] Zhang, Lei. “Study on structure and mechanical properties of spider dragline silk fibers.” (2013).

[7] http://www.posco.co.kr/homepage/docs/eng5/dn/company/product/k_car_pdf_2014.pdf

[8] Pierce, Donald T., Jose Antonio Jiménez, James Bentley, Dierk Raabe, and James E. Wittig. “The influence of stacking fault energy on the microstructural and strain-hardening evolution of Fe–Mn–Al–Si steels during tensile deformation.” Acta Materialia 100 (2015): 178-190.

[9] De Cooman, B. C., Ohjoon Kwon, and Kwang-Geun Chin. “State-of-the-knowledge on TWIP steel.” Materials Science and Technology 28, no. 5 (2012): 513-527.

[10] Koeppel, Andreas, and Chris Holland. “Progress and trends in artificial silk spinning: a systematic review.” ACS Biomaterials Science & Engineering 3, no. 3 (2017): 226-237.

[11] https://www.precisionkidd.com/technology.htm

[12] Pickering, F. B. “Microalloying 75.” Union Carbide Corp., New York, NY 9 (1977).

Featured image: Euler, Manfred. “Hooke’s law and material science projects: exploring energy and entropy springs.” Physics Education 43, no. 1 (2008): 57.