Natural materials!! As the name suggests it refers to materials which are made by nature like silk, wood, leather etc, But what are they good for? Do they have unique properties which we can learn from? The answer is yes! Natural materials possess many good qualities like strength, toughness, crack resistance, fire resistance and self healing. But all the properties prior mentioned, are ways of measuring and classifying materials for better human understanding. The materials in nature are capable of serving multiple functions at the same time. Doesn’t this sounds obvious? An example would better explain the thought process. Today for the topic of natural materials I will discuss about natural materials and how nature uses the combination of different materials for achieving desired properties, followed by a few more in some weeks.

Strength:

Woodpecker birds of the family Picidae, are known for their strong beaks, they drum/peck tree barks to establish territories, attract mates or in search of food. A woodpecker is known to drum the hard bark of trees at a rate of 18 to 22 times per second which results in deceleration forces of up to 1200 g’s [1]. In spite of this they don’t seem to break or damage the skull, though this drumming of the tree barks happens repeatedly over the day. A simple allometric comparison of G-tolerance between human and woodpecker shows that woodpeckers are capable of withstanding 13 times more G’s compared to humans [5].

A lot of research has been made in order to fully understand how the woodpecker’s head can withstand the forces arising from drumming of trees. But contrary to one might think the beak is not alone responsible for the needed strength for drumming and withstanding the resulting forces due to drumming. The following points explain how the woodpecker performs this feat without smashing the head or damaging the brain.

• The beak of the woodpecker compared to other birds is large and flat, the beak is also extended untill the neck [2] shown in figure 1 (a). The physical characteristics of its beak enable the woodpecker to direct the stresses generated on the tip of the beak to the neck and the study also states that the neck muscles of the woodpecker are strong enough to transfer the stresses.
• The sponge bone shown in circled section in figure 1 (a) is presumed to help disperse the shock wave before it reaches the brain. Having some material with a low Young’s modulus like the spongy bone helps direct the shocks away from the brain [2].
• The hyoid bone figure 1 (b) in the woodpeckers is extended from the jaw to the rear side of the skull and is also connected to the nasal cavity, supposedly for structural reinforcement. The hyoid bone acts like a seatbelt for the skull of the woodpecker [2]. The presence of some cerebral fluid between the brain and the skull also prevents the brain from moving.

Figure 1 : Skull morphology of Wood Pecker(Awokera)

This example of a woodpecker shows that nature uses different materials in combination to achieve a desired effect. The morphology of the woodpeckers’ skull can be used as an inspiration for solving many modern days technical problems, such as shock absorption using light weight materials, load or force dissipation and light weight design.

Micro-architecture:

Nature uses different materials for gaining desired properties; it also uses the design of the materials itself for maximum efficiency. The design of the materials in other words the micro-architecture plays an important role in overall strength of natural systems. The following example of seashells explains in detail how micro-architecture is used by nature for gaining desired properties from a material.

Toughness of Shells:

Shells in nature are subjected to harsh conditions like crashing of waves and danger from predators and in order to survive this, the shells need to have a considerably tough exterior. Strombus gigas also known as queen shell, are of the class gastropoda and order mesogastropoda, they grow up to five pounds and are usually found in sand or coral reef habitats. They have a hard exterior shell, but are soft bodied animals and the toughness is due to the material of the shell, which is mostly calcium carbonate [3]. But the material itself is not responsible for the toughness of the shell, but the micro architecture of the assembly of the material. In conch shells calcium carbonate is arranged in sheaths of proteins, the strong, but brittle calcium carbonate is separated by much weaker layers of proteins, these layers are build up in cris-crossing beams pattern [4]. The conch shell comprises of arognite (one of the two types of calcium carbonate) and the basic block is aragonite wrapped in protein skins and these skins are stacked in sheets called laellae, then these laellae are stacked to form layers [4]. The conch shell has cross-lamellae structure and each layer is rotated 90 degrees from the next element shown in the figure below. This arrangement itself is like brick and mortar, which is quite common in modern day constructions.

Figure 2 : Microscopic pictures of a Conch shell

When a force is first incident on the shell, the soft regions of the proteins develop micro cracks; these cracks are actually advantageous as they add to the toughness of the shell. The second energy dissipation mechanism comes from the cross-lamellar structure [3]. As the cracks have to propagate in a zigzag manner to overcome the adjacent layers it requires more energy to overcome. The first energy dissipating mechanism is exploited to the max, as the load is incident on the shell more micro-cracks are formed, but they propagate along the axis of the lamellae sheets and thus are trapped in between the laminates. The inner layer can take many more micro-cracks as long as the middle layer is twice as strong as the protein layer, but through experiments it was found that the ratio is four time more stronger in conch shells [3]. The cracking of the inner layer alone was found to require 20 percent more work compared to abiogenic aragonite [4]. However, under increased loads the cracks will propagate thought the middle layer at an fortyfive degree interface between the layers. The queen conch shell exhibit excellent energy absorption tactics not just by the use of suitable materials but also incorporating micro architecture.

This it for today, in some weeks we will discuss about a few more natural materials and how they function as one for a better performance.

References:

1. Sang-Hee Yoon1,2,3 and Sungmin Park2; „A mechanical analysis of woodpecker drumming and its application to shock-absorbing systems“.
2. Oda, Juhachi, Jiro Sakamoto, and Kenichi Sakano. „Mechanical evaluation of the skeletal structure and tissue of the woodpecker and its shock absorbing system.“JSME International Journal Series A Solid Mechanics and Material Engineering 3 (2006): 390-396.
3. Ballarini, Roberto, and Arthur H. Heuer. „Secrets in the Shell.“Journal Title: American scientist 5.
4. Barthelat, Francois, Jee E. Rim, and Horacio D. Espinosa. „A review on the structure and mechanical properties of mollusk shells–perspectives on synthetic biomimetic materials.“Applied Scanning Probe Methods XIII. Springer Berlin Heidelberg, 2009. 17-44.
5. Juhachi Oda, Jiro Sakamoto, and Kenichi Sakano. “Mechanical evaluation of the skeletal
   structure and tissue of the woodpecker and its shock absorbing system”. In: JSME
   International Journal Series A 49.3 (2006), pp. 390–396.

Figure 1 : A mechanical analysis of woodpecker drumming and its application to shock-absorbing systems Sang-Hee Yoon1,2,3 and Sungmin Park2

Figure 2 : Barthelat, Francois, Jee E. Rim, and Horacio D. Espinosa. „A review on the structure and mechanical properties of mollusk shells–perspectives on synthetic biomimetic materials.“Applied Scanning Probe Methods XIII. Springer Berlin Heidelberg, 2009. 17-44.