Carlota Auguet

Modeling of spatiotemporal patterns in bacterial colonies.

A diffusion-reaction model for the growth of bacterial colonies is presented. The often observed cooperative behavior developed by bacteria which increases their motility in adverse growth conditions is here introduced as a nonlinear diffusion term. The presence of this mechanism depends on a response which can present hysteresis. By changing only the concentrations of agar and initial nutrient, numerical integration of the proposed model reproduces the different patterns shown by Bacillus subtilis OG-01.

Damping in civil engineering using SMA. The fatigue behavior and stability of CuAlBe and NiTi alloys

Two types of application in damping of structures by SMA in Civil Engineering are considered. The first one is related to the reduction of the damage produced by earthquakes. The second one is concerned with the increase of the lifetime of the stayed cables in bridges. The analyses of the experimental conditions required for each application are different: Several years or decades without any activity (excepted the summer-winter room temperature parasitic effects) followed by one or two minutes of oscillations under the earthquake affects, or near 100000 oscillations per day with pauses of several hours or days in the damping of stayed cables in bridges. This article analyzes the fatigue behavior of the CuAlBe alloy (appropriate for earthquakes) and of the NiTi alloy. Measurements of the damping of stayed cables indicate that the oscillation amplitude could be reduced up to one-third by using a NiTi wire as a damper device.

Damping in civil engineering using SMA Part 2 – particular properties of NiTi for damping of stayed cables in bridges

Abstract Shape memory alloys (SMAs) show particular properties associated to their martensitic transformation between metastable phases. Their use requires a deep knowledge of the SMA behaviour according to the requirements of the application. In this paper, the following are studied: the required fatigue life of the NiTi SMA that easily overcomes 1 million of oscillations for a complete storm; the suitable thermomechanical treatment to minimise or stabilise the SMA creep; and the temperature effects on the damper and the dynamic phenomena effects modifying the hysteretic energy via the self-heating associated to the release and the absorption of the latent heat were studied. The study includes the effects on SMA of the external summer–winter temperatures. La transformation martensitique entre phases métastables est à l’origine des propriétés particulières des Alliages à Mémoire de Forme (AMF). L’utilisation des AMF nécessite une connaissance approfondie de leur comportement associé aux exigences de l’application. Dans cet article, on étudie: (a) la durée de vie requise de l’AMF NiTi, qui peut facilement avoir à surmonter 1 million d’oscillations lors d’une tempête entière. (b) Le traitement thermomécanique approprié afin de minimiser ou stabiliser le fluage de l’AMF. (c) Les effets de température sur l’amortisseur ainsi que des phénomènes dynamiques modifiant l’hystérésis de l’énergie par l’auto-échauffement associé au dégagement et à l’absorption de la chaleur latente. On étudie également l’effet des températures extérieures de l’été et de l’hiver sur l’AMF.

SMA in Mitigation of Extreme Loads in Civil Engineering: Damping Actions in Stayed Cables

Reliable use of Shape Memory Alloys (SMA) in mitigation of extreme load effects requires a deep study of the SMA behavior according to the necessities of the application. Damping the oscillations induced by one storm of three or four days with a strong wind and/or rain usually requires more than one million of working cycles. A SMA damper is checked in to two realistic cables of ELSA and of IFSTTAR. The measurements establish that the SMA damping device reduces drastically the oscillation amplitude. Technical suggestions for the preparation of the dampers built by several SMA wires of NiTi with 2.46 mm of diameter are included. The choice of appropriate length and the choice of the number of required SMA wires are suggested. Moreover, a suitable simulation by proprietary SMA routine inside ANSYS is included.

SMA (Cu-BASED, NiTi) FOR USE IN DAMPING: THE IMPLICATIONS OF RELIABILITY FOR LONG TIME APPLICATIONS AND AGING BEHAVIOR

Guaranteed behavior of SMA in damping needs deep study of the alloy behavior according to the requirements. Pseudoelastic window, evolution of properties, self-heating and fatigue are the major concerns. The analysis shows that selected alloys can do the work. The application to portico and to cable oscillations in realistic systems shows an excellent damping effect. Useful simulation requires a good phenomenological model. A cubic model is built with improved results at low deformation.

Behavior of NiTi Wires for Dampers and Actuators in Extreme Conditions

Shape memory alloys are considered smart materials because of their singular thermo-mechanical properties, due to a thermoelastic martensitic transformation, enabling possible uses as actuators (because of mechanical recovery induced from temperature changes) and as dampers (because of hysteresis). NiTi wires for dampers in Civil Engineering had been characterized and tested in facilities. Guaranteed performance needs to know behavior during fatigue life and knowledge of effects in the event of extreme conditions, as eventual overstraining. In this work, we check the possibilities to absorb mechanical energy on the fatigue life depending on stress level and explore the consequences of overstraining the material during installation, the possibilities of partial healing by moderate heating, and some effects of over-stressing the wires. The mechanical energy absorbed by the unit weight of damper wire might be very high during its lifetime if maximum stresses remain relatively low allowing high fatigue life. We show also some results on NiTi wire working as an actuator. The lifetime mechanical work performed by an actuator wire can be very high if applied stresses are limited. The overstraining produces relevant “residual” deformation, which can be to some extent reversed by moderate heating at zero stress. The reason for the observed characteristics seems to be that when external high stresses are applied to an NiTi wire, it undergoes some plastic deformation, leaving a distribution of internal stresses that alter the shape and position of the macroscopic stress-strain transformation path.

The SMA: An effective damper in Civil Engineering that smoothes oscillations

The properties of SMA (Shape Memory Alloys, smart materials) are associated to a first order phase transition named martensitic transformation that occurs between metastable phases: austenite and martensite. At higher temperature phase or at lower stress the austenite is the metastable phase. The martensite appears at lower temperature phase or higher stresses. The hysteresis of the transformation permits different levels of applications, i.e., in their use as a damper. Two types of applications can be considered in damping of structures in Civil Engineering. The first one is related to diminishing the damage induced by earthquakes. The second one is a reduction of oscillation amplitude associate to an increase of the lifetime for the stayed cables in bridges. Different fundamental behavior of the SMA needs to be guaranteed in each case.

SMA in mitigation of extreme loads in civil engineering : study of their application in a realistic steel portico

Guaranteeing the use of Shape Memory Alloys (SMA) in mitigation of extreme load effects requires a deep study of the SMA behavior according to the specific requirements of the applications in damping. The damper was defined according the expected requirements (length of SMA and number of SMA wires). It is applied to two types of alloys (CuAlBe and NiTi) in the diagonals of a realistic steel portico (2.47 m x 4.10 m). The measurements establish that the used SMA reduces the oscillation amplitude to a less than a half.

Thermomechanical Fatigue Behavior of NiTi Wires

Applications of the functional, smart materials SMA need a careful evaluation of the working conditions and fatigue life. Structural fatigue failure in NiTi usually comes from a surface defect inducing crack growth, and this means that fatigue has to be studied for concrete applications, with the correct samples, as the state of the material presents size effects. Testing machine experiments on Ni-rich pseudoelastic wires indicate that the main parameter controlling the fatigue life in the traction-traction experiments is the effective stress on the NiTi wire. Long wire life (in the million cycle regime) can be achieved under limited stresses (under 170 MPa) in traction-traction tests. Also, experiments have been done on thermal actuation of NiTi wire under traction at constant load. Long actuator life (larger than 400000 cycles) can be achieved at low stresses (around 100 MPa), coherently with the results from the mechanical cycling.