Infinite progress through finite elements

Infinite progress through finite elements

Computer-aided engineering tools (CAE) are more pervasive nowadays, and finite element analysis is having more impact than at any other time. In the past, CAE abilities have been used in specific fields with highly trained engineer teams and large computing facilities. For example, in the aeronautical industry the objective is, among others, to design more efficient airliners and the automotive industry must produce safer cars in case of accident.

Currently there are not field of science or engineering that has not been affected, and in some cases transformed, by computer simulation. Almost most manufacturing companies, regardless of the industry, can take advance of CAE abilities to simulate their process and improve their performance.

Sport industry show off this fact, for example, SPEEDO produced swimsuits including compressing effects for changing, in certain way, the shape of the swimmer’s body. Using this idea, SPEDOO designed suit able to achieve drag reductions in more than 15 per cent. In the JJ.OO. of Beijing, 94% of the medals were won by swimmers dressed with SPEDOO swimsuits (Michael Phelps, Mireia Belmonte …) and 23 out of 25 Olympic  records where beaten using this technology (according to data from FINA).

For an airliner or for an Olympic swimmer the engineering problem is essentially the same. There is a shape moving through a fluid and the drag must be minimized. That is, advanced engineering aerodynamic concepts also works in the textile industry. This example clearly defines the current situation of CAE abilities, where high technology is used to solve what we could define as trifling problems.

According to Lesa Roe, NASA Langley Research Center director, “Modeling and simulation is older than NASA”. Since the first models of digital calculators, computing machines evolved step by step and around year 2000 some experts believed that engineering simulation programmes had reached its peak due to the big improvement in abilities for the limited supply of high-level engineers.

However, the more powerful computers and the friendly integrated analysis environments have allowed companies to take advantage of the enormous potential of simulation programmes to make accurate predictions about natural phenomena, providing compelling evidence that we are really gaining in our understanding of how the products, processes and services can be optimized. Therefore, the almost endless engineering simulation techniques provide big growth opportunities, based on the current needs and the challenges that this poses.

As the saying goes, “Necessity is the mother of invention” CARTIF believes and works in endless possibilities to help customers develop better products and processes. I would like to stress in a particular application: the estimation of the static and buckling behaviour of very thin walled containers for food packaging.

Through simulation programmes we are able to detect weak points and design failures prior to manufacture, with consequent savings in time, material and money. Note that the containers are manufactured by plastic injection machines using expensive cast moulds.  During the analysis are taken into account parameters such as constitutive material properties (PET, HDPS, aluminium …), thickness, type of liquid or granular product to be content, etc. that define the containers and allows us to predict its performance, resulting in deformation curves under load, loads of collapse, tensions and stretching under certain loads which it may be subjected to circumstances of the production process, storage and transport conditions, including temperature, pressure and impact effects among others.

Beside theses services, being aware that “data is the new currency”, CARTIF is also working on structural health monitoring in civil structures. The aim of this work is predict when maintenance will be needed or what the expected behaviour of structure should be if the real system begins to deviate from the digital models’ behaviour. This idea can be reviewed in my previous post ‘When structures age’.

When structures age

When structures age

How to reduce structural conservation task expenses by implementing monitoring systems?

The structures are not everlasting. They are projected to play a role for a certain number of years. Thus, a wind turbine mast lifespan is about 20 years while in the case of a bridge it depends on the type and the material used. According to Guy Grattesat, metal bridge will have a lifespan of 40 years, 100 years for a reinforced concrete, between 15 and 20 years for those made of wood and about 200 years for masonry. Nevertheless, exceeding the life expectancy does not necessarily mean dismantling the structure. Generally, what we do is a more comprehensive monitoring effort and implementing conservation works if necessary.

Wire and accelerations sensor installation process inside the handrail

The European bridge stock is catching its lifespan and it is showing signs of fatigue. According to Eva Lantsoght, Professor of engineering at the University of San Francisco de Quito, “European bridges are old, but their replacement involves a great investment. Only in the Netherlands, there are about 3,000 bridges that could cause problems, being their refurbishment cost around one million euros each”. To mention another example, the first wind turbine parks (1984), whose technological emergence came in 2002 (according to the GWEC, Global Wind Statistics), have great needs for maintenance and the increase of these needs will have the same exponential curve, I can imagine, that its development has had in the past.

18 triaxial MENS accelerometers (sensors developed by CARTIF) installed equidistant along the span inside the handrail.

Having this background in mind, I would like to highlight the importance of implementing new maintenance strategies to reduce structural conservation task expenses. State-of-the-art monitoring systems, developed in recent years at increasingly low costs can be the solution. Usually in this type of structures, the procedure consists on the installation of a network of accelerometers to record the ambient vibration response of the structure. Using identification techniques, the modal characteristics can be estimated and be used to evaluate the structural integrity.

The idea is to know the structural behavior in operation conditions and to determine the valid range by controlling only a few parameters (frequency, mode and damping), being these parameters easy to evaluate and to record their trends along the time. Although the meaning of the word ‘monitoring’ refers to the capacity of the acquisition system to record certain values, it is also important to add the ability to process the data and to generate alarm codes, if it would be necessary.

In this aim, researchers require a deep understanding of the technical matter involved as well as a big experience in experimental techniques and data processing. OMA (Operational Modal Analysis) identification techniques are based just on the acceleration records but if some loading signal applied on the structure could be also record, additional information can be obtained using EMA (Experimental Modal Analysis “EMA”). Both approaches should gradually replace, or at least complement, the traditional visual inspections and static loading tests. Monitoring systems could be an emergent business for technology-based companies in cooperation with new maintenance standards or strategies by the infrastructure’s authorities.

In this regard, I can mention the success of low-cost monitoring system of Pedro Gómez Bosque footbridge (built in Valladolid) operated by CARTIF team since 2012. This stress-ribbon structure undergoes no linear effects. The collected data has been useful so far to understand the behavior under dense pedestrian loading, at different met conditions (summer or winter, strong wind…), etc. Analyzing this information, we know what it is normal and what is not, so we can establish an optimal operational range and determine when anomalies appear and why. In these terms, the structure is an ideal benchmark not only for structural monitoring but also for understanding mechanical problems, pedestrian loading and other engineering issues.