Category Archives: Materials Science

About Tsai-Wu failure criterion

Predicting material failure is always a challenge, especially when it comes to composites and advanced materials. There are plenty of theories that try to provide a numerical approach to solve this complex problem, such as Maximum Stress/Strain Theories,  Hashin, Tsai-Hill or Tsai-Wu. Although all of them brought something valuable to the table, some of them don’t seem to be that precise when accurate results are needed. In these terms, Tsai-Wu is my least favourite criterion and I’ll explain the reasons for that.


First of all, Tsai-Wu is an interactive failure criterion for composite materials. This means that the theory takes into account the interaction of different stress components in order to predict failure. Basically, the criterion uses equation 1 (subjected to the condition given by equation 2) to calculate an index and, if its value is one, then it means the material is failing. Please note that i,j=1,2,…,6, where subindices 1 to 3 represent normal stress components and 4 to 6 are shear stress components. In the original publication, authors explain how the different coefficient can be determined through experimental tests (e.g. compression, tension, biaxial…). So far, so good.

01
Equation 1
02
Equation 2

Spanish university to collaborate with the development of intelligent materials

The University of Alicante (Spain) is taking part in a project that will develop intelligent materials for aerospace, automotive and transportation industries. The main aim will be to improve the safety of occupants and the durability of the components.


Researchers from the Department of Civil Engineering from the University of Alicante and the tech company Applynano Solutions are carrying out this project known as MASTRO, which stands for Intelligent Bulk Materials for Smart Transport Industries. The project is part of the Horizon 2020 programme, which is the biggest investment system for R&D in Europe.

Their goal is to develop intelligent materials for the transportation sector. In particular, the aerospace and automotive industries will be the main targets. Amongst other innovations,  these materials will be able to monitor their own deformations and they will also be capable of heating and defrosting their surfaces. Besides, thanks to their capability to repair and protect themselves from damage, they will improve their efficiency, their durability and users’ safety. At the same time, manufacturing and maintenance costs will be reduced, as well as emissions.

In order to develop these materials, different matrices will be used, including polymers, concrete and carbon nanomaterials. Their functions will be based on three processes: the variation of electric resistivity when a material is subjected to a mechanical load, the relation between the heat that is generated and the electric flux, and electrostatic discharge.

One the one hand, the Spanish university will work on the development of the function related to perception of strain and damage on structures made of reinforced concrete. In addition, the previously mentioned institution will also focus on the heating of surfaces made of asphalt and concrete in order to avoid the formation of ice.

On the other hand, Applynano Solutions will work on the development of the carbon nanomaterials, the manufacturing of composites and the production of prototypes.

These are exciting news for the European research community, since not only Spain but also institutions from United Kingdom, Portugal, Italy, France, Germany and Sweden will collaborate with the MASTRO project. Hopefully, we’ll see encouraging results in the near future! I’ll keep you updated!

The Secret Science of Superheroes

Do you like science? Are you a comic geek? If your answer to both questions is “yes”, then “The Secret Science of Superheroes” is your book!


Last August I got myself an autographed copy of “The Secret Sicence of Superheroes”, thanks to Dr David Jesson and Dr Mark Whiting (University of Surrey) and I must say I don’t regret it at all! The book is distributed by the Royal Society of Chemistry and it was edited by Mark Lorch and Andy Miah. When I first heard about this book, Dr David Jesson told me that the whole thing was completed in just one weekend during an event in Manchester and that each chapter was written by a different author and it related a specific superheroe topic with the author’s field of expertise. Interesting, right?

I would review every single chapter, I really would, but… then you wouldn’t read the book! So, I’m just going to talk very briefly about the things I enjoyed the most. Basically, the text is written for a general audience, introducing the scientific concepts as the authors try to make their point. Continue reading

Introduction to Impact Dynamics: the ideal collision when playing pool

While I was playing pool the other day, I remembered a lesson that I learned during my time at the University of Seville as well as at Cranfield University. It is related to the quite complicated subject of Impact Dynamics… But don’t be afraid, today I’ll just cover a simple case as an introduction. In particular, I’m going to write about how the ideal collision between the white ball and the other ball that we are aiming to move.


First of all, we need to differentiate between different types of collisions, depending on the loss of translational kinetic energy or, in other words, the conversion of kinetic energy to rotations, vibrations or heat. For this case, let’s consider two bodies: one in motion (impactor) and another in stationary conditions. Hence, we can distinguish between the following categories:

  • Elastic collision: all the energy is transmitted from one body to another, i.e. the impactor stops and the stationary mass starts moving at the same speed as the initial one from the moving mass.
  • Completely inelastic collision: the moving mass stops after hitting the stationary body. The stationary mass remains as it was.
  • Inelastic collision: the impactor suffers a decrease in speed after the collision, whereas the stationary mass starts moving at a certain speed.
  • Superelastic collision: if additional energy is provided to the stationary body during the impact, then it will start moving at a higher speed than the one at which the impactor hit it.

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Graphene-based composite nanomaterial can prevent the overheating of lithium-ion batteries

Although the performance of lithium-ion batteries (LIBs) have been remarkably improved in the past decades, there is a big risk in the use of this type of battery: they can catch fire when they are subjected to abuse. Researchers from Stanford University have developed a nanocomposite material which can be included into the electrodes in order to prevent the explosion of the battery.


To perform in an efficient way, LIBs require operation conditions which are within a specific range of current density, voltage and temperature. Nevertheless, when they are subjected to abuse conditions, exothermic reactions can take place, leading to a fast increase in internal temperature and pressure. What does it mean? Well, our battery is likely to explode!

Current LIBs include external sensors to prevent overheating and overpressure but, unfortunately, temperature and pressure inside the cells can actually increase much faster than they can be detected by those external sensors. Because of that, several alternatives have been developed in order to include internal components to solve the problem. For example, ceramic coating has been proven to be an effective way to shut down the battery and improving the thermal tolerance. However, after the battery is shut down, it cannot be used again. Using solid-state electrolytes can be another option, but the overall performance of the battery is decreased due to their low ionic conductivity. Continue reading

Mechanics of composites

A while ago, I wrote a simple document for undergraduates in order to explain that composite materials can fail in different ways. This was created as a high level document which could be used to find useful references with regards to failure modes, basic failure criteria and damage propagation models. I wanted to share this with you in case you are new in this field or just if you simply want to learn some basics of composites!


A composite can be defined as a material which is composed of two or more constituents of different chemical properties, with resultant properties different to those of the individual components. They usually consist of a continuous phase (matrix) and a distributed phase (reinforcement). These reinforcements can be fibrous, particulate or lamellar and they are usually stiff and strong, so that they are responsible for providing the stiffness and the strength of the composite. On the other hand, the matrix provides shear strength, toughness and resistance to the environment.

Fibre reinforced composites are considered as the strongest and sometimes also the stiffest, due to:

  • Alignment of molecules or structural elements.
  • Very fine structures.
  • Elimination of defects.
  • Unique structures.
  • Statistical factors.

With regards to fibre reinforced composite materials, their main failure modes are:

  • Fibre failure induced by tension in fibre direction.
  • Fibre failure induced by compression in fibre direction.
  • Matrix fracture induced by tension.
  • Matrix fracture induced by compression.
  • Delamination

It is remarkable that fibre failure typically caused composite failure, whereas matrix failure may not cause the same drastic effect. Continue reading

Recycling of Carbon Fibre Reinforced Polymers

The use of carbon fibre reinforced polymers (CFRP) is increasing every day. This type of material have been used in aerospace and automotive industries (amongst others) for years, but now the cost of manufacturing components made of carbon fibre is becoming more accessible for mass production and more companies are introducing CFRP parts in their products because of their good mechanical properties, energy absorption capability and low weight. However, since a large increment in the production is observed, companies need to be aware of the different recycling techniques that are currently available for these materials.



Nowadays there are different ways to recycle composite materials and some of them are more developed than others. However, the use of recycled carbon fibres (rCF) is not that common in industry, mainly because of the lack of confidence in their performance when compared to virgin carbon fibres (vCF). In addition, there is a clear disadvantage: rCFs cannot be used for the same applications as what they were originally designed for. Because of this, I want to introduce some of the recycling techniques which are currently available for composites.

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Classic Laminate Theory: how to derive the reduced stiffness matrix

Those of you who work with composite materials will be familiar with the Classic Laminate Theory. According to this theory, from the properties of the material, the stiffness matrix can be easily obtained. Then, using very tedious expressions, the coefficients of the reduced stiffness matrix can be calculated. The thing is that, based on my own experience, those expressions are introduced without any explanation, so most of the people just use them, ignoring where they come from. So, if you’re curious or you just want to understand a bit more about this theory, keep reading this post!


The first thing that we have to consider is the rotation of our coordinate system . We need to know how to express our new coordinate system in terms of the original one. Following Figure 1 and using basic trigonometry, the relationship can be found.

coordinate_system

Figure 1 Rotation of the original coordinate system

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Carbon Ceramic Brakes

Nowadays, if you are lucky enough to be able to afford a sport car, one decision has to be made with regards to the extras: the brakes. Are the famous carbon ceramic brakes that special? Let´s find out some of their features.


ceramic

Carbon ceramic brake (source: Olivier Delorme)

Carbon ceramic brakes consist of carbon fibre reinforced silicon carbide. In this case, the matrix is made of silicon carbide (SiC) and silicon (Si), whereas the reinforcement is made of popular carbon fibre. The matrix provides the hardness to the composite material and the fibres are responsible for the fracture toughness.

The main advantage of this type of brake is its capability to absorb extremely high temperatures. Why is this important? Well, sport cars can go fast… very fast. Therefore, there is also a need for reducing the speed of the vehicle as fast as possible. Since the brakes use friction in order to slow the cars down, heat is generated and it can decrease the efficiency of those particular components. Hence, having brake disks which contain modifications in order to withstand those high temperatures, is a must.

In addition, this composite material reduces the weight of brake disks up to 50% when compared to conventional ones. Furthermore, carbon ceramic brakes do not suffer corrosion, which is a major problem for iron brake disks. Apart from that, other merits include: longer life, less dust (in metal brakes, the dust have magnetic properties due to static electricity, resulting in particles which remain on metal parts around the disk) and less noise.

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About volume and foams

If you had the opportunity to learn the basics of rational mechanics in high school or impact dynamics during your degree, you may be familiar with one specific condition which was specially useful in order to solve problems: the conservation of volume. But, what if I told you that there are certain cases where that particular assumption can be totally wrong?


Let’s start with the so-called “conservation of volume condition”. This condition assumes that when we have a component (e.g. a beam or a bar) and it goes from one state to another (e.g. it is impacted by another body), no mass will be lost. In other terms, it considers that the component will not break into pieces. However, this doesn’t mean that your particular component cannot change its shape. Thus, it is usually taken for granted that even if the shape changes, the volume should remain constant due to the fact that the density (mass over volume) of a certain material should remain the same. Or at least, that’s what we are usually taught…

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