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.
Have you ever wondered why Formula 1 cars have those extremely complex front wings? Some people may think that these structures are only there for producing downforce, but in reality their function goes beyond that. Do you want to find out more? Well, here’s your chance!
A few years ago I had the opportunity to meet Craig Scarborough during one of his pesentations about Formula 1 at Cranfield University (United Kingdom). For those who are not familiar with that name, Mr Scarborough is a well known expert in motorsport and, just so you know, he’s quite a celebrity on social media (Twitter, LinkedIn…), where he usually shares top quality information about racing and the engineering behind it.
Yesterday, I contacted him after watching his latest video for motorsport.com in which he discusses the function of a front wing with Willem Toet, one of the best aerdynamicist in the world. They use a 3D airflow animation in order to illustrate how the wing of the McLaren MCL-32 works. After asking for his approval, Mr Scarborough was kind enough to give me permission to share the video with the audience of Engineering Breakdown, so here it is! I hope you enjoy it!
(Please note that in order to watch the videos, you need to reproduce them on Youtube, following the instructions).
Let me introduce you to Dr Nicholas Brown, one of the Composite Design Engineers at McLaren Racing and former EngD at the University of Surrey. It was a real pleasure having a conversation with him at the McLaren Technology Centre (MTC) in Woking, UK. We covered different topics about what is like to work at the top level of automotive engineering, including some tips for getting where he is now! Enjoy it!
First of all, I’d like to say how grateful I am to have you here, since I know you are extremely busy at the moment. Thank you for your time and your kindness. And now, let’s get started. Can you tell us a bit about your background?
So I did my masters first in engineering at Loughborough University; that was Aeronautical Engineering. I spent five years up there and did a placement year as well. So my placement year was with an aerospace electronic sort of warfare defense company, but I was doing more of the support work reliability team and things like that, writing general reports… Didn’t really do anything fancy, so I came out of there not wanting to do that and not really wanting to go on a graduate scheme. Then I had a year just between jobs and then the EngD came up, so I chose to do the EngD that as you know is a great opportunity. And then towards the end of it I was looking for more job roles and one came up at McLaren Racing as a Design Engineer, which implied using my composites knowledge for a more applied role. There are research aspects as well, but it’s mainly applying my knowledge. That was about a year and a half ago and now I’m still here! It’s quite fun! It’s good to apply all the things you know. As I said we do research up there but it is completely different to the research I did as an EngD.
Basically, when we want to determine the forces and displacements in a certain structure using Finite Element Analysis (FEA), what we are doing is creating a system of equations that relates the stiffness of the elements to the displacements and forces in each node. When we run a simulation, we do not see all the calculations. For that reason, today I want to illustrate a simple case that can be easily solved by hand applying that methodology.
Before getting started, just think of a spring. Everyone has come across the Hooke’s law at a certain point during school. It states that the force in the spring is proportional to a constant “k” multiplied by the variation in length of the spring. FEA follows the same principle, but in this case the “k” constant is the stiffness matrix and the variation in length is a vector of displacements and rotations, depending on the case.
Let’s study a simple static case. Our structure consists of two bar elements connected at a common node, where a load “P” is applied. The other two nodes have both horizontal and vertical displacements constrained (see the boundary conditions). For this particular case, the reactions in nodes 1 and 3 and the displacements of node 2 are requested. I have solved the problem by hand following a few steps that, based on my experience, can be generalised for more complex problems. Pretty much, the summary of the methodology is:Continue reading →
Last February I participated in the Young Persons’ Lecture Competition, organised by IOM3. In particular, the local heat took place at the University of Surrey. I want to share with you the transcript of my presentation. I have to say that I tried to present a quite complex topic in a very simple way so that anybody without an engineering background could follow it. Hope you enjoy it!
Abstract: What would happen if you removed the roof of your car? First of all, you would have a convertble vehicle to enjoy that one sunny day we have in England. Second and most importantly, you would probably be the bravest person on earth. Driving on a bad road or even going over a speed bump could have dramatic results. Using simple engineering concepts, logic and a shoe box you will be able to understand why that could happen ad how automotive companies overcome this issue.
Let’s start from the beginning. What is Strength of Materials? It is the science that studies the behaviour of solid objects when they are subjected to stresses and strains. So, first question: what kind of objects? Basically we can have 1D, 2D and… Exactly! 3D elements! Some examples could be a bar (1D), a shell or a plate (2D) and a hexaedron (3D). For this particular topic, I’m going to focus on 2D elements, since the body panels of a car can be considered as very thin shells assembled together.
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.
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.
It is remarkable that fibre failure typically caused composite failure, whereas matrix failure may not cause the same drastic effect.Continue reading →
Last month I travelled to Geneva (Switzerland) in order to attend the most important motor show of the year and here you will find some of the best pictures that I took during the event.
For those of you who are not familiar with the automotive world, I should start this post by stating that during the year there are several events known as “motor shows”, where Original Equipment Manufacturers (OEMs) exhibit their new vehicles and technologies. Since not all OEMs go to every event, there are some motor shows which are flagged in every calendar due to their relevance in terms of the companies and public that will be attending. In these terms, the Geneva International Motor Show is considered as the most important event of the year, followed by Frankfurt.
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.
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.
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.
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…