Tag Archives: Engineering

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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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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How could grid-tied Photovoltaic Plants become more competitive when it comes to National Electricity Markets participation? (Part I)

If someone mentions the term Photovoltaic System (PV Systems), the first thing that one would probably think about is a solar panel installed on top of the roof of a house, like the one shown below. Well, even though this equipment is undeniably the most popular worldwide, this article deals with the current and short-term situation of those huge PV Power Plants which are connected to a National Electric Grid and, therefore, participate in a National Electricity Market.


First of all, I would like to start by providing some background and a number of previous concepts that might be needed later. A simple diagram is shown below, which represents a normal grid-tied PV Plant:

esq1

Source: Tersa

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What’s the best way to go over a speed bump?

I’ve noticed that a lot of people try to avoid using the four wheels of the car when they go over a speed bump. Out of curiosity, I asked some of those drivers and all of them gave me the exact same answer: “Because if you only hit the obstacle  with the wheels of one side of the car, you will cause less damage to the vehicle and, besides, it is less uncomfortable for occupants”. Is that true? Let’s find out.


Let´s start from the beginning. The first thing we need to know is that cars have two axles (i.e.front and rear) and each of them has two wheels (i.e. right and left). On the other hand, speed bumpers are road obstacles which are designed to make drivers reduce the speed in certain areas and they are usually as wide as the lane. Why is that? Well, basically bumps are thought to be encountered by the two wheels of each axle simultaneously, creating a scenario known as “vertical symmetric load case”. This situation causes results in a bending moment which is applied to the structure of the car.

However, sometimes we can find some bumps which present a smaller width or even gaps. These are the situations where some drivers decide to vary the direction of the car so that the wheels of one of the sides avoid the contact with the obstacle. Therefore, only one wheel goes over the bump. Hence, the vehicle will suffer an “asymmetric vertical load case”. In other words, the automotive structure will be subjected to a torsional load, which is a worse scenario than the one introduced above since it can cause one of the wheels to lift off. I will show you how a relatively simple approach can be used to prove this statement.

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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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About Hashin´s failure criteria in FEA

When using Finite Element Analysis (FEA) for studying composite materials, one of the most used failure criterion is the one which was proposed by Hashin in 1980. This theory is included in all the main FEA packages and, probably, you are more than familiar with this particular model. However, what you might not know is that the failure criteria that you are defining is not exactly the Hashin’s one. If you want to know why, this is your place.


Since the available failure criteria at that point presented some inconsistencies, in 1980 Hashin developed a new criteria which differentiated between failure modes. His theory considered four different ways in which the material could fail:

Hasin_original

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