This is relatively a recent development whereby two or more different kinds of fibers are used as reinforcements in a ‘hybrid’ composite. The advantage of incorporating hybrid fibers is basically what composites are all about: achieve better overall properties. Consider using carbon fibers with glass fibers as reinforcement inside a single matrix. Carbon fibers are stiffer and stronger and lighter than glass fibers but glass fibers are a lot cheaper and tougher than the carbon fibers. This enables the composite formed with a polymer resin to be cheaper, tougher and lighter than using all carbon fiber reinforcement or stronger and stiffer than using only glass fibers as reinforcements. How would this arrangement work? Naturally carbon fibers will bear the brunt of the load and as they are stiff they will fracture first as the intact glass fibers serve to provide toughness until the fail and the matrix has to come in to play to sustain the load. These better overall properties can be achieved in other ways as well because hybrids offer a chance to manipulate the ‘structure’ of the composite as well. For instance, fibers could be laid out as laminates with each layer composed of a single fiber and these laminates could be alternated to make up the composite. Presently at least, from a commercial stand point probably the greatest advantage of using hybrid fiber is the potential reduction in the cost of the composite other than that, hybrid composites are mainly being used as structural components in transport vehicles and some orthopedic components. Integrating fibers such as capture silk with other fibers have similar applications to those mentioned above, namely to increase toughness and strength of the composite and decrease the overall weight. Because, as of now, some kinds of silk such as dragline silk have strength and toughness much greater than even high-tensile steel. According to research, dragline silk has a strength-to-weight ratio five times greater than high tensile steel. While the arrangement between other fibers to construct hybrid reinforcement is useful, using the amazing silk fibers in a composite is not a feasible proposition and combining them with other fibers to achieve a hybrid may be even harder. Firstly, extracting silk naturally or through artificial methods is extremely expensive. For use, a large scale manufacture will be required but the costs are too damning to make this possible. Second, as the nature of these natural fibers and man-made fibers are so different, finding a material which can work for both these fibers is difficult. Silk may ‘wet’ a particular matrix much better than the man-made fiber reinforcement which will be used in conjunction with it, so while the silk is a good fit with the matrix, the man-made fiber may not be. Wetting broadly refers to how good an interface is formed between the reinforcement and the matrix. Better wetting means a good interface which allows the matrix to transfer load effectively to the reinforcement fibers.
Monday 14 November 2016
Friday 11 November 2016
Hardenability Quenchants and heat dissipation in steels
Hardenability can simply be defined as the susceptibility to hardening by rapid cooling or the ease with which a material can be hardened. On some occasions, hardenablity is described as the depth to which a material can be hardened. This is the case applied to an end quenched bar of metal where quenching media is introduced to one end of a metal bar so the rate heat transfer decreases as distance from the quenched end increases.
One must understand the material science involved. Take for instance carbon steel. As the alloy is rapidly cooled (quenched) it undergoes a diffusion-less transformation known as martensitic transformation. The cooling rate is so high that the carbon atoms do not get enough time to re arrange themselves into a stable structure and hence form a metastable structure which is very hard.This hardness comes at a price. The material becomes brittle due to the presence of residual stresses in the steel. How easily can this alloy form martensite is also hardenability or the ease with which martensitic tranformation takes place is hardenability.
The rapid cooling or 'quenching' has two factors that effects its potency; the ability of the material to dissipate the heat from the interior to the surface and the other being the ability of the quenching medium to remove the heat from the surface. What happens is that in dissipation step the role of thermal diffusivity comes into play, higher the diffusivity lesser the time the material bar retains heat.This factor can not be controlled or always considered because it is material dependent. While the second factor is something we can control so it is mostly considered as the important one. It can briefly be described in three stages, considering the material to be steel:
*Apart from the mentioned media we will highlight other less common quenchants that are used in heat treatment process for hardenability in an another blog.
One must understand the material science involved. Take for instance carbon steel. As the alloy is rapidly cooled (quenched) it undergoes a diffusion-less transformation known as martensitic transformation. The cooling rate is so high that the carbon atoms do not get enough time to re arrange themselves into a stable structure and hence form a metastable structure which is very hard.This hardness comes at a price. The material becomes brittle due to the presence of residual stresses in the steel. How easily can this alloy form martensite is also hardenability or the ease with which martensitic tranformation takes place is hardenability.
The rapid cooling or 'quenching' has two factors that effects its potency; the ability of the material to dissipate the heat from the interior to the surface and the other being the ability of the quenching medium to remove the heat from the surface. What happens is that in dissipation step the role of thermal diffusivity comes into play, higher the diffusivity lesser the time the material bar retains heat.This factor can not be controlled or always considered because it is material dependent. While the second factor is something we can control so it is mostly considered as the important one. It can briefly be described in three stages, considering the material to be steel:
- A layer of vapor/steam forms adjacent to the surface of the steel. The steam insulates the surface and produces a cooling rate.
- The heat transfer makes the continuous production of vapors to break down upon bubble formation thus allowing more of the water or medium to come in contact with the surface, cooling it this way. Rapid cooling takes place here.
- As the medium lowers the temperature of the steel down below the medium's boiling point the vapor formation stops and the surface now is in contact with medium directly as it starts to cool through convection and conduction.
Quenching Media:
We discuss a few of the common quenchants used in industry and laboratories here.Brine:
It is a severe quenchant. The reason for the severity is that brine does not dissolve the atmospheric gases which inherits less bubble formation. As a result the surface is continuously kept wet so the metal piece cools rapidly. Its rapid cooling character comes with a drawback as the severity causes distortion. Because we know the transformation is rapid so the crystal does not get time to relieve itself from the thermal stresses introduced. Also the solution is corrosive to many non ferrous metals hence it is not preferred to use it with them.Water:
It is a very common quenchant and probably the oldest too. It is less severe than brine though still a good quenchant. Its severity does cause distortion and cracking as well. But another trait to it is that it follows an uneven manner in wetting and rewetting the surface of the metal. This non-uniformity causes thermal gradient over the surface and thus promoting distortion that may lead to cracking.Air:
The metal piece should be placed in an open rack where the air can cool it uniformly from all sides. It does not harden the metal to the standards of other quenched media but still a property of mildly hardened and high ductility can be achieved.Oil:
It is the preferred quenchant in industry as it gives an intermediate severity. Which results in achieving less distortion and compromised hardenability.*Apart from the mentioned media we will highlight other less common quenchants that are used in heat treatment process for hardenability in an another blog.
Friday 21 October 2016
Shrimp inspired 'Super-tough' Composites
When you look around, every invention seems to show a glimpse of nature. Be it the head lights of a car inspired from human eyes or a light bulb prompting the working of Sun. There are direct or indirect influences of nature in our advancements. They all seem to make sense considering the powerful bodies that inspired them. However, strange is the case to find how on earth a 'shrimp' would become a root study for developing the next generation of airplanes, body armour and helmets.
The creature is known as Mantis Shrimp also called as stomatopods. The shrimp has an evolutionary technique of killing their prey in two ways. One using a spear that is driven through the prey and the other is a club that smashes the skin of the prey by pulverising it with high speed and force. The 'Dactyl club' can achieve accelerations of 10,000g unleashing great amount of impact with speed equivalent to .22 caliber bullet.
The idea of generating such large force onto another object can be considered suicidal if the right mechanism of absorption is ignored. But the mantis shrimp is assimilate the force onto itself thanks to the herringbone structure it possesses. This structure is the main inspiration for developing composite that can absorb large impact forces without fracturing.
The dactyl club when studied is found to be heavily mineralised, containing chitin fibers in hydroxyapatite matrix, allowing it to withstand repeated impacts without failure. The strength relies on the factor of arrangement of the fibers which are rotated by a small angle with respect to each layer below forming a herringbone structure.
The herringbone not only provides the failure resistance but also gives the shrimp to damage its prey severely by the transfer of momentum. The properties can be further simplified by understanding the structure in more detail the researchers found that the mineralized did not break because the crack is propagated by the structure and restricting it to the fibers and disallowing the crack to propagate to the surface.
A group of researchers imitated the structure of the dactyl club using carbon fiber epoxy composite. Forming helicoidal structures with different low angle rotations and then compared their performance to conventional unidirectional and quasi-directional fiber composites. The latter two when tested with 'drop weight' test were found to be fractured by the impact whereas the helicoidal structure showed a lower dent than the two, on average a 49% shallower dent. Following up with compressional tests also proved durability of the structure as 15-20% increase in residual strength was observed in the composites compared to the conventional structures.
The creature is known as Mantis Shrimp also called as stomatopods. The shrimp has an evolutionary technique of killing their prey in two ways. One using a spear that is driven through the prey and the other is a club that smashes the skin of the prey by pulverising it with high speed and force. The 'Dactyl club' can achieve accelerations of 10,000g unleashing great amount of impact with speed equivalent to .22 caliber bullet.
The idea of generating such large force onto another object can be considered suicidal if the right mechanism of absorption is ignored. But the mantis shrimp is assimilate the force onto itself thanks to the herringbone structure it possesses. This structure is the main inspiration for developing composite that can absorb large impact forces without fracturing.
The dactyl club when studied is found to be heavily mineralised, containing chitin fibers in hydroxyapatite matrix, allowing it to withstand repeated impacts without failure. The strength relies on the factor of arrangement of the fibers which are rotated by a small angle with respect to each layer below forming a herringbone structure.
The herringbone not only provides the failure resistance but also gives the shrimp to damage its prey severely by the transfer of momentum. The properties can be further simplified by understanding the structure in more detail the researchers found that the mineralized did not break because the crack is propagated by the structure and restricting it to the fibers and disallowing the crack to propagate to the surface.
A group of researchers imitated the structure of the dactyl club using carbon fiber epoxy composite. Forming helicoidal structures with different low angle rotations and then compared their performance to conventional unidirectional and quasi-directional fiber composites. The latter two when tested with 'drop weight' test were found to be fractured by the impact whereas the helicoidal structure showed a lower dent than the two, on average a 49% shallower dent. Following up with compressional tests also proved durability of the structure as 15-20% increase in residual strength was observed in the composites compared to the conventional structures.
Wednesday 12 October 2016
Binary Phase Diagrams
A binary phase diagram consists of two variables, temperature and percentage composition. Although, phased diagrams will vary according to different pressures, binary phase diagrams are normally viewed at constant 1 atm pressure. A phase diagram can have more than two components as in the case of ternary phase diagrams but these are a lot more complex.
The binary phase diagrams are all drawn at an equilibrium state meaning that cooling rates during phase transformations are extremely low i.e. the decrease in temperatures which leads to the change in the phase is very slow. This almost never happens in real situations. The cooling rates which coincide with the change in phases is never really at an equilibrium temperature hence, the map or the data gathered from a phase diagram is not accurate. In practical situations cooling rates are usually too fast to be assumed as equilibrium. Take the iron carbide phase diagram, it shows that austenite phase of iron and carbon at a eutectoid composition (0.78% carbon) transforms to pearlite at 723 degrees Celsius, but under normal conditions with faster cooling rates this transformation occurs at lower temperature.
The binary phase diagrams are all drawn at an equilibrium state meaning that cooling rates during phase transformations are extremely low i.e. the decrease in temperatures which leads to the change in the phase is very slow. This almost never happens in real situations. The cooling rates which coincide with the change in phases is never really at an equilibrium temperature hence, the map or the data gathered from a phase diagram is not accurate. In practical situations cooling rates are usually too fast to be assumed as equilibrium. Take the iron carbide phase diagram, it shows that austenite phase of iron and carbon at a eutectoid composition (0.78% carbon) transforms to pearlite at 723 degrees Celsius, but under normal conditions with faster cooling rates this transformation occurs at lower temperature.
So then why do we use a binary phase diagram? These phase diagrams provide a much simpler way to understand the phase transformations. It can also help us predict the microstructures which could be formed under equilibrium or non-equilibrium conditions. One could estimate the approximate solubility limit of alloy addition at various temperatures.
Tuesday 11 October 2016
What are composites?
A composite is a material which consists a reinforcement
phase (the part which provides strength to overall material) and a matrix phase
which completely surrounds/binds the reinforcement phase. These two phases have
to be dissimilar. The dissimilarity is usually very easily identifiable. A
composite can even have more than two distinct phases.
Take the example of the fiber glass. Small glass fibers that can be easily identified provide the strength to the composite whereas a thermosetting polymer forms a matrix around these glass fibers. The glass fibers are embedded into the matrix. In fiber glass typically epoxy, vinyl ester or polyester is used as a matrix material. The matrix by way of surrounding the glass fibers protects them from the environment and is also used to transfer load to the fibers.
Consider a sheet of fiberglass which undergoes tensile force at each ends. Although the force is being applied only at the ends of the sheet, the load will be transferred to all glass fibers in the composite through the matrix.
Composite matrix can be a metal (metal matrix composite), ceramic (ceramic matrix composite) or a plastic/polymer (polymer matrix composite).
In the same way the reinforcement can be of different materials, but as the function of this phase is to provide strength, hence it is stronger than its matrix. The reinforcement can have different geometries. Sometimes reinforcements are employed as particulates or aggregates as those used to make concrete; they can be fibers (glass fibers in fiberglass) or reinforcement can come in the form of a structure such as mats.
Take the example of the fiber glass. Small glass fibers that can be easily identified provide the strength to the composite whereas a thermosetting polymer forms a matrix around these glass fibers. The glass fibers are embedded into the matrix. In fiber glass typically epoxy, vinyl ester or polyester is used as a matrix material. The matrix by way of surrounding the glass fibers protects them from the environment and is also used to transfer load to the fibers.
Consider a sheet of fiberglass which undergoes tensile force at each ends. Although the force is being applied only at the ends of the sheet, the load will be transferred to all glass fibers in the composite through the matrix.
Composite matrix can be a metal (metal matrix composite), ceramic (ceramic matrix composite) or a plastic/polymer (polymer matrix composite).
In the same way the reinforcement can be of different materials, but as the function of this phase is to provide strength, hence it is stronger than its matrix. The reinforcement can have different geometries. Sometimes reinforcements are employed as particulates or aggregates as those used to make concrete; they can be fibers (glass fibers in fiberglass) or reinforcement can come in the form of a structure such as mats.