Tag Archives: Materials

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

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

Carbures is back on track

After a relatively long period of instabilities, the Spanish composite manufacturer Carbures is raising again. This is really good news for the Spanish industry and for all those engineers who are interested in this kind of advanced material.


It’s just been made public that in 2016 Carbures reached their historic record in terms of the production of aircraft components made of composite materials. As a matter of fact, their production has increased 16.2% with respect to 2015, manufacturing a total of 45,695 aircraft parts. Therefore, we can say that Carbures have returned to the place where they belong: being one of the top composite manufacturers for the European aerospace and defence sectors.

For those who don’t know the company, they produce structures for quite a few members of the Airbus fleet. For instance, some of the civil airplanes which use their components are: A320, A320NEO, A330, A340, A350 or even the impressive A380. In addition, they also contribute to the military sector (e.g. A400M). The parts which are manufactured by Carbures include from lids of the oil tanks of the engine to parts of the fuselage. 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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Modelling foam cores in LS-DYNA

It´s been a while since the last time I wrote about Finite Element Analysis. For that reason, this week I would like to express some of my concerns about two material models which are available in LS-DYNA for crushable foams.


Crushable foams are widely used in the aerospace and automotive industries due to their energy absorption capabilities and their low weight. This means that companies can take advantage of those properties in order to produce lightweight vehicles, improving the efficiency in terms of fuel consumption while making the structures safer for the occupants.

In these terms, original equipment manufacturers (OEMs) normally use foams as the core of sandwich structures, in order to combine the properties of different materials. Nevertheless, both the manufacturing process and the experimental tests are usually expensive and time consuming, and this can lead to non-profitable results. Because of that, FEA has become an extremely powerful tool for analysing and predicting the behaviour of structures. The fact that the set up of the FE models usually requires simple tests reduces the cost of the process, even more if we take into account that once the models are validated, they can be used for predicting other type of scenarios which would be extremely expensive to test in reality.

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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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Material Science in surfboards

During my recent visit to the American West Coast, I had the opportunity to have a first contact with surfing. Although I didn’t have a totally satisfactory experience, I must say I found this sport quite interesting and, for that reason, I started asking some questions about the materials which are used to manufacture surfboards to my surfing expert brother. In this post I will introduce very briefly the main materials and a bit of comparison between their performance.


To begin with, the surfboards can be classified in two main categories with regards to the materials used in the outer part. For people who, like me, are not familiar with this sport, the way for identifying which of the two types of surfboards is in front of us is simply to look at the external appearance of the board itself. Trust me, anyone can spot the difference!

The first type corresponds to the surfboards made of glass fibre. This boards have been around for over half a century and this fact makes them be considered as the traditional board. The inner part is made of polyurethane (PU), a polymer which stands out for being lightweight and providing a good buoyancy to the final structure. Usually a wooden element is embedded in the core in order to increase the stiffness of the product. Then, the whole thing is surrounded by some layers of glass fibre cloth which provides the strength and resistance to the board. The amount of fibres and their orientation can vary from one surfboard to another, resulting in different strengths and stiffness without changing the weight of the structure too much.

Then, we have the epoxy boards. The first thing we notice about this category is its plastic exterior appearance. In this case, the core is made of polystyrene (PS) foam, and it is worth saying that depending on its type, the foam will absorb a different quantity of water. This material is also the responsible for the buoyancy of the surfboard. Then this core is coated with an epoxy resin, being this the reason behind its name. Another option within this category is the use of an expanded polysterene (EPS) foam core, which is an even lighter material that we can find, for instance, in bicycle helmets. It should be said that the use of this kind surfboards was not very extended until they started appearing in main events where professional surfers had a great performance.

surf6

Surfer in Malibu, California (extracted from my personal Flickr account)

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Split Hopkinson Pressure Bar Test

One of the experimental techniques for characterising materials under the effect of high strain rates is the split Hopkinson pressure bar (SHPB) test. This week a brief introduction to the basics of this technique is covered.


If you have an engineering background, it is likely that you have come across one of the most famous testing methods for characterising materials: the tensile/compression test. In that case, a sample is usually subjected to a controlled displacement (usually in mm/min). The result is the force-displacement curve and from those results and the geometry of the sample, the determination of the stress-strain curve for the material is pretty straight forward. Despite the fact that this test can be performed for a different range of speeds, these velocities are normally quite low. In these terms, there are many cases where engineers are interested in the stress-strain curve for dynamic cases, since some materials can exhibit different behaviour depending on the strain rate (e.g. crushing of an automotive component). For that reason, in order to obtain the desired characteristic curve, other methods have to be used, such as the split Hopkinson pressure bar test.

One of the most interesting things about the SHPB test is that there is no official standard to follow. However, there are some common features with regards to the necessary equipment. In these terms, every Hopkinson bar test should have:

  • Two cylindrical long bars. Their length has to be big enough in order to obtain one-dimensional wave propagation. They are called incident and transmitted bar, respectively.
  • Fixtures to ensure that the bars are perfectly aligned and that they can freely move after an impact occurs.
  • Gas gun. This device is the launches a striker bar which impacts the incident bar. Hence, a controlled compressive pulse is achieved.
  • Two sets of strain gages. Each set should be placed in the middle of both the incident and transmitted bars.
  • Equipment for data acquisition (e.g. amplifiers, oscilloscopes…).

Capture

Figure 1 Schematic of the split Hopkinson pressure bar test [1]

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