Stronger than Steel, more Elastic than Rubber

As we enter spring, spiders are becoming a nuisance as they make their silky homes on our patios and porches. However, not all byproducts of spiders are considered a nuisance. Not only does Spider-Man boast the super strength of spider silk, but the textile Kevlar® was developed with properties that compare to the fiber. Now, scientists have uncovered fascinating new facts about this material that offer intriguing possibilities for the future. Learn more about these discoveries and the growing repertoire of spider silk uses:

Scientists and engineers have studied and measured the amazing mechanical properties of spider silk for decades. Spider silks are stronger than steel and more elastic than rubber. They’re tough; they can absorb a large amount of energy without breaking, such as when a bee collides with the web. They’re sticky; some silks are spun with glue droplets along their lengths to keep that bee secure. These studies have led to developments in textiles, notably Kevlar® from the DuPont Company, with properties that mimic some of those of spider silk.

One would think that given all the research that has taken place on the mechanical properties of spider silk that there was little more to discover. But scientists at the University of Akron have recently uncovered a fascinating new fact about this material that offers intriguing possibilities for the future.

Spider silk is a fiber comprising complex protein molecules. Spiders manufacture silk from their spinnerets; organs located on the spider’s abdomen. Depending on the species, spiders can have anything from two to eight spinnerets, usually in pairs.

A single spider can produce several different kinds of silk from different pairs of spinnerets, each with varying properties for specific functions. For example, dragline silk, used for a spider web’s outer frame and spokes, is strong and very tough, but not sticky. Capture silk, used to trap and hold the prey, is sticky and elastic.

Silk is not exuded under pressure from the spinneret. The spider touches the spinneret to a surface, adhering the silk, and then its own motion away from that surface draws out the silk. This offers the spider even further control over the properties of the silk as the stiffness varies with the rate, and thus the force, of silk withdrawal1.

Recently Dr. Ali Dhinojwala, the H.A. Morton Professor in the Department of Polymer Science at the University of Akron became interested in an observation made by a colleague, Dr. Todd Blackledge who commented on the fact that spider silk contracted and relaxed under varying humidity conditions.

Drs. Dhinojwala and Blackledge began to research this new behavior by hanging weights on lengths of spider silk and exposing the silk to varying humidity conditions2. They found that the silk contracted as humidity decreased and subsequently relaxed as the humidity increased. The contraction was sufficient to lift the weight. Moreover, the contraction of the silk was directly proportional to the humidity and was repeatable and fast – within three seconds of the humidity change. They calculate that the work generated is up to 50 times greater than that of the equivalent mass of human muscle fiber. However, the overall contraction of the silk is very small at around 2.5 per cent of the overall length.

A further limitation is the difficulty of mass production of the silk. Spiders don’t tend to live too peacefully together; they have a habit of eating one other. Thankfully, the same, albeit somewhat weaker, property of contraction is found in silkworm silk where mass production is more feasible.

Artificial, or biomimetic muscle, in the form of electroactive polymers, has existed for some time. Electroactive polymers undergo changes in size or shape when activated by electricity.

Using humidity control and biological material as a source of power offers a range of fascinating possibilities in industry and the life sciences. Actuators and valves operating environmental control systems, ligaments and tendons in prostheses, all operated by exposure to wet or dry air.

The more we look, the more we learn about the incredible properties of spider silk and that master materials scientist – the humble spider.

1 J. Pérez-Rigueiro, M. Elices, G. Plaza, J. I. Real and G. V. Guinea (2005). The effect of spinning forces on spider silk properties. J. Exp. Biol. 208, 2633-2639

2 Ingi Agnarsson, Ali Dhinojwala, Vasav Sahni and Todd A. Blackledge (2009). Spider silk as a novel high performance biomimetic muscle driven by humidity. J. Exp. Biol. 212, 1990-1994

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Bamboo Bikes - A "Growth" Industry

Bamboo is a natural composite material comprising cellulose fibers embedded in a lignin matrix. The cellulose fibers are aligned along the length of the bamboo providing maximum tensile strength and rigidity in that direction. Moreover, the fibers are most abundant on the outermost part of the bamboo stem while the inner part of the stem is hollow, meaning that bamboo is strong, stiff, and light.

These outstanding properties of bamboo have been used for centuries for many purposes; bamboo buildings, bridges, furniture, and more. But there is one piece of equipment for which stronger, stiffer, and lighter are the ultimate watchwords – the bicycle.


Bamboo: Image on a Creative Commons License

Bike: Image courtesy of Nagooyen Photography and Stalk Bicycles

Bamboo is an ideal material for bike frame construction where stiffness and strength to weight ratios are important. It is light; bamboo stems, known as culms, are hollow. It is stiff; the strong fibers are distributed most densely in the outer surface, the area of greatest longitudinal stress. It is strong; some bamboo fibers can have tensile strengths of up to 40 kN/cm2. Solid transverse nodes divide bamboo culms into segments. Each segment acts as a short strong column, resisting buckling. The nodes also act as crack arresters.

The composite nature of bamboo gives it one more major advantage over steel, aluminum, and carbon fiber. Bamboo frames are excellent for absorbing road shocks and vibrations, as well as their resistance to crash damage. The vibration damping is a performance advantage on longer rides, reducing fatigue and shock associated with carbon frames. The rider experiences a rare combination - a performance frame that absorbs road shock yet remains stiff.

The first bamboo bicycles were shown at the London Stanley Show of 1894. In fact, an English patent was granted that year on the use of bamboo as a bicycle frame. However, it was not until the late 1990s that more extensive interest in bamboo bicycle production took off.

Much of the renewed interest in bamboo for use in bicycles is due to the increased emphasis in environmentalism and sustainability in engineering and design. Bamboo is one of the fastest growing plants in the world; therefore, it is a natural and highly renewable resource. Some species have been recorded as growing up to 100 cm (39 in) in a day.

Bamboo use also has benefits in the new era of carbon-offsetting. The production of 1 ton of steel releases over 2 tons of carbon dioxide into the atmosphere whereas the production of 1 ton of bamboo extracts over 1 ton of carbon dioxide from the atmosphere.*

In recent years, many companies have formed to take advantage of the benefits of bamboo bicycles. A very brief internet search identified a few immediately: Biomega, Denmark; Calfee Design, La Selva Beach, CA; Renovo Bicycles, Portland, OR; Panda Bicycles, Fort Collins, CO; Stalk Bicycles, Oakland, CA; Erba Cycles, Boston, MA; and Grass Frames, Vancouver, Canada.

Manufacturing a bike frame is not just a matter of selecting some bamboo poles of the right length and diameter. While the mechanical properties are impressive, there are many different species of bamboo, and the mechanical properties vary between these species. Being a biological material, the properties of bamboo, even within a single species, are not consistent. Bamboo is an orthotropic material; it has independent mechanical properties in the three main axes: longitudinal, radial, and tangential. The properties vary with the height, age, and water content of the bamboo culm. Also, without treatment, bamboo can be vulnerable to insect and fungus attack.

The individual bamboo sections are selectively chosen for each part of the frame. Different manufacturers use different processes, but in general, the culms of bamboo are heat treated and smoked to prevent cracking. They are coated to protect them from moisture. Some companies join the frame components to one another using hemp wrapping in an epoxy matrix; others use carbon fiber jointing.

Despite the resurgence of interest in bamboo as a material for bike manufacture, the industry must be considered as still in its infancy. The bikes are labor intensive and thus costly to make, although rising prices for steel and aluminum are reducing the differential. To become more than a niche product, the industry must achieve a wider recognition of the enhanced performance attributes. However, the growing number of bamboo bike builders entering the market shows promise in the industry’s future.

*Scientfic Design of Bamboo Structures, Dr. Suresh Bhalla, Department of Civil Engineering, Indian Institute of Technology Delhi. Read more

Strain Measurement When Testing Composites

When testing composite specimens, effective strain measurement often requires the direct bonding of strain gauges to the specimen. It is generally not possible to connect individual strain gauges directly to materials testing systems since the electronics are typically designed for a "full Wheatstone bridge" whereas a single strain gauge is only a "quarter bridge". 

Typically, one would need external signal conditioning and bridge completion electronics such as Vishay or HBM, which bypasses the signal conditioning in the strain channel. We recommend a simpler, less costly approach: a “bridge completion” adapter and cable that connects directly to the signal conditioning in the strain channel, making the strain gauge look like an extensometer; you will no longer need the external conditioning electronics. Calibration in materials testing software is as simple as when using a clip-on extensometer. Read more

Composites Question from a Customer

Q: What is the most effective gripping technique for a composite specimen with a hard, smooth surface?

A: Generally, we recommend a careful choice of grips, grip face finishes, and specimen preparation.

Grips: To avoid specimen slippage, we recommend hydraulic grips since composite materials are typically tabbed and demand high gripping forces. Hydraulic grips offer the additional advantaage of being highly repeatable - a critical element when pursuing consistent test results.

Grip Faces: Use a high friction grip face finish; either serrated diamond-point or Surfally^® coated.

Specimen Preparation: To avoid damaging the surface of the specimen and raising stress points at the grip jaws, particularly f you're using grip faces, we recommen to bond composite tabs to both sides and both ends of the specimen. Read more

Testing Uncovers Titanic Mystery

In light of the 100 year anniversary of the Titanic this weekend, we're sharing a story found on ABCNews.com.

The Unsinkable Ship sank in less than 3 hours back in 1912. Did the Titanic sink simply due to the impact of an iceberg and the speed of the ship or was it a malfunction in the mechanical property of a key material holding the ship together?

A recent study, conducted by Tim Foecke of the National Institute of Standards and Technology and his colleagues, tested the rivets of the ship's hull; rivets that were made of wrought iron, not steel like the rest of the ship's rivets. The one big difference: wrought iron tends to soften at lower temperatures. 

Using a static hydraulic universal testing system, Foecke and colleagues simulated the ship's design with 2 pieces of 1-inch thick steel plates held together with wrought-iron rivets. Through a compression test, they were able to simulate the force on the rivets and found that the rivet heads broke off, proving their substandard quality. As the rivet heads popped, the steel plates separated, allowing water to pour into the ship's hull at a very fast rate.

"If the wrought iron rivets were up to standards, they would have been fine," says Foecke. "But since there was no method for quality checking, the rivets used on the Titanic were not up to standards, which caused them to fail prematurely."


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Doing Their Part to Ensure Safety on the Playing Field

We were approached by several visitors at NPE whose companies were interested in learning more about impact testing of helmets as it relates to increased safety. Many of us that are sports enthusiasts are reading more and more about concussions and the potentially devastating effects they can have on athletes as they occur on the field – and their long-lasting effects later on in life. There is a growing urgency to enhance the safety of athletes – especially in football and hockey.

Not only are our customers focusing on safety, I found this interesting article about two high school althetes who, after witnessing a sports-related injury, decided to enter the Rhode Island Science & Engineering Fair with their findings on impact testing of head gear. I won't spoil their results, but I will mention that they went on to receive blue ribbons for their project! Read more

Fatigue Testing of Stents and Stent Materials

It seems we've been focusing a lot on the medical and biomaterials markets lately, but it's an industry that is growing with new techniques, standards, and more.

Traditional fatigue testing of complete stent devices is addressed by ASTM F2477 “Standard Test Methods for in vitro Pulsatile Durability Testing of Vascular Stents”, which specifies methods for fatigue of complete devices through hydrodynamic pulsation. The method involves placing complete devices into mock arteries and subjecting them to 400 million cycles of internal pressure pulsation (10 years of human heartbeats), forcing them to radially expand and contract in each cycle. The test can either be performed between pressure limits, simulating diastolic and systolic pressures; or displacement controlled, reproducing the minimum and maximum diameters that a stent would see in vivo under worse case conditions. Tests are typically performed at frequencies of up to 50 cycles per second, resulting in typical test durations in the three to six month range.

The acceptance criterion of devices is a simple pass/fail one, in that no fracture of the stent can occur during these in vitro tests for success. Many devices from varying manufacturers have undergone Pre-Market Approval (PMA) by Food Drug Administration (FDA) and have gone into clinical use. Although this traditional “Test to Success” approach of fatigue testing has not resulted in failures, the reality is that many of
these devices are fracturing in vivo.

In early 2006, the FDA and ASTM started looking at ways that could eventually improve the current durability assessment of cardiovascular devices. Initially, two working groups under the ASTM F04.30.06 Endovascular Devices Task Group were established; the first group concentrates on better understanding of the physiological conditions devices undergo in vivo and transferring this knowledge into boundary conditions for use in testing, evaluation and modelling. The second group, entitled “Fatigue to Fracture” (FtF) group, was charged with developing alternative and improved test methods for fatigue testing of cardiovascular devices.

An alternative method that is being rapidly adopted is a “Fatigue to Fracture” approach. A rudimentary technique that is more akin to aerospace testing, this methodology involves a combination of FEA modelling and in vitro testing to assess the durability of stents through established fracture mechanics techniques. These testing guidelines and standards are still under development. Several testing techniques have been developed recently that provide testing results that provide support as manufacturers submit products for regulatory approval.

To enable a representative sample of specimens to be evaluated and to reduce overall test time, multiple samples must be tested. The multi-specimen fixtures assist cardiovascular implant manufacturers to assess these long-term fatigue characteristics of nickel-titanium (Nitinol), CoCr, stainless-steel, and other stent materials and structures. It is important that each specimen station feature a fatigue-rated load cell, precision alignment adjustment, and applicable grips for the material or structure undergoing test. The specimens should be tested in vitro at body temperatures and results should include trend monitoring of forces to determine each specimen fracture. Read more

What's the Buzz from Orlando?

With NPE well underway in sunny Orlando, FL, we've heard from our colleagues in the booth about the excitement and energy that's filling the 920,000 square feet of exhibiting space. Also, one note of surprise is that a lot of the attendees who stop by the booth are not aware that CEAST products are a valuable part of the Instron line.

"It's been interesting for me to see that there are still some customers in the materials testing industry who do not know that Instron offers a full range of plastics solutions," says Marco Bronzoni, CEAST Marketing Manager, Italy. "This is a great chance for all of us in the booth to talk about the integration of CEAST with Instron and how we offer systems for rheology, melt flow, thermal-mechanical, and  specimen preparation."

With that being said, it seems the systems we're exhibiting in the booth meet the expectations of the attendees at the show.

If you're in the Orlando area - or already at NPE - stop by the Instron booth (#2403) to meet Marco and take a look at the systems in action! Read more