Damage detection with genetic algorithms taking into account a crack contact model

This paper deals with the crack detection in structural elements by means of a genetic algorithm optimization method. The crack model takes into account the existence of contact between the interfaces of the crack. Many of the methods to detect a crack in beam-like structures are based on linear one dimensional models and are not straightforwardly applicable to structures such as beams or arcs with a breathing crack with or without contact. The present study addresses bi- and three-dimensional models to handle the dynamics of a structural element with a transverse breathing crack. The methodology is not restricted to beam-like structures since it can be applied to any arbitrary shaped 3D element. The crack is simulated as a notch or a wedge with a unilateral Signorini contact model. The contact can be partial or total. All the simulations are carried out using the general purpose partial differential solver FlexPDE, a finite element (FE) code. A genetic algorithm (GA) optimization method is successfully employed for the crack detection. The dynamic response at some points of the damaged structures are compared with the solution of the computational (FE) model using least squares for each proposed crack depth and location. An objective function arises which is then optimized to obtain an estimate of both parameters. Physical experiments were performed with a cantilever damaged beam and the resulting data used as input in the detection algorithm.     

Charge transfer in blends of P3HT and colloidally prepared CuInS2 nanocrystals

CuInS₂ (CIS) is a suitable semiconductor for thin film photovoltaics and also a promising candidate to replace CdSe in hybrid polymer/nanoparticle solar cells, advantages being the lower toxicity and the better absorption properties. In this work, crystalline nanoparticles of CuInS₂ were successfully synthesized with uniform size and shape by means of colloidal chemistry. Photo-induced absorption spectroscopy was used to study charge transfer between poly(3-hexylthiophene) (P3HT) and the CIS nanoparticles. The appearance of signals corresponding to positive polarons provides evidence that charge transfer is possible in the P3HT/CIS system.     

An electrochemical impedance spectroscopy study of polymer electrolyte membrane fuel cells electrocatalyst single wall carbon nanohorns-supported

Electrochemical impedance spectroscopy (EIS) was used to study the polymer electrolyte membrane fuel cells (PEMFC) performance when using single wall carbon nanohorns (SWNH) to support Pt nanoparticles. Additionally, as-prepared and oxidized SWNH Pt-supports were compared with conventional carbon black. Two different oxidizing treatments were considered: oxygen flow at 500 degrees C and reflux in an acid solution at 85 degrees C. Both oxidizing treatments increased SWNH surface area; oxygen treatment increased surface area 4 times while acid treatment increased 2.6 times. The increase in surface area should be related to the opening access to the inner tube of SWNH. Acid treatment of SWNH increased chemical fragility and decreased electrocatalyst load in comparison with as-prepared SWNH. On the other hand, the oxygen treated SWNH sample allowed to obtain the highest electrocatalyst load. The use of as-prepared and oxygen treated SWNH showed in both cases catalytic activities 60% higher than using conventional carbon black as electrocatalyst support in PEMFC. Moreover, EIS analysis indicated that the major improvement in performance is related to the cathode kinetics in the as-prepared SWNH sample, while concerning the oxidized SWNH sample, the improvements are related to the electrokinetics in both anode and cathode electrodes. These improvements should be related with differences in the hydrophobic character between SWNH and carbon black.     

Local bifurcation and instability theory applied to formability analysis

The design and optimization of both sheet metal formed parts and processes are nowadays carried out virtually making use of numerical tools by finite element analysis. Such virtual try-out approach contributes with significant savings in terms of money, time and effort in the design, production and process set-up of deep drawn parts. The analysis of either forming success operation or surface defects, in each of the development phases, is generally performed by means of the material’s forming limit diagram (FLD), since it allows to define a safe region that reduces the probability of: (i) necking; (ii) wrinkling and (iii) large deformation occurrence. However, the FLD represented in the strain space is known to present some disadvantages. To overcome this problem, Ito and Goya proposed a local bifurcation criterion that defines the critical state for a local bifurcation to set in as a function of the stress level to work-hardening rate ratio, leading to a FLD represented in the stress space. This suggests that the FLD obtained is completely objective in the sense that it is completely independent of the strain or stress history paths (Ito et al. 2000). In this work the Ito and Goya model is used to evaluate formability, as well as fracture mode and direction on the deep drawing of a square cup. Since the analysis is performed based on the stress state, it is also possible to determine an instability factor that “measures” the degree of acceleration by current stress for the local bifurcation mode towards fracture. The selected example highlights the potential use of the criterion which, once combined with the finite element analysis, can undeniably improve the mechanical design of forming processes.     

Isolation,structural features and rheological properties of water‐extractable β‐glucans from different Greek barley cultivars

β‐Glucans were isolated from six Greek barley cultivars (Persefoni, Kos, Thessaloniki, Athinaida, Dimitra and Triptolemos) by water extraction at 47 °C, enzymatic removal of starch and protein and subsequent precipitation of the water‐soluble β‐glucans with 37% (w/v) ammonium sulfate saturation. The purity of barley β‐glucans was high (>93% dry basis) with some small contamination by protein (<3.84%). The molecular size of the β‐glucan isolates was determined by high‐performance size‐exclusion chromatography (HPSEC); the weight‐average molecular weights and the intrinsic viscosities ranged between 0.45 × 10⁶ and 1.32 × 10⁶ and 2.77 and 4.11 dl g^?1, respectively. Structural features of barley β‐glucans were revealed by ¹³C NMR spectroscopy and high‐performance anion‐exchange chromatography (HPAEC) of the oligomers released by the hydrolytic action of lichenase. Lichenase degradation showed that β‐glucans from all barley cultivars consisted of blocks of cellotriosyl and cellotetraosyl units, accounting for 90.6–92.3% of the total oligomers released, with a molar proportion of these units between 2.31 and 2.77. Rheological measurements of aqueous solutions/dispersions of β‐glucans showed the behaviour of non‐interacting polysaccharides and a transition from the typical viscoelastic response to gel‐like properties after a time period that depended on the molecular size of the polysaccharide. The lowest molecular size β‐glucans from the Triptolemos cultivar showed shorter gelation times than their higher molecular weight counterparts. The effect of sugar incorporation (glucose, fructose, sucrose, xylose and ribose), at a concentration of 30% (w/v), to the β‐glucans gels (6% w/v) on compression parameters seemed to be related to the type of sugar used; the pentose sugars substantially reduced gel firming. Copyright © 2004 Society of Chemical Industry     

Functionality of enterococci in meat products

The presence of enterococci in meat fermentation is a constant as reported in the literature. Despite the concern about pathogenicity of enterococci, recent studies point out that food and meat enterococci, especially Enterococcus faecium have a much lower pathogenicity potential than clinical strains. Enterococci possess a competitive advantage over other microbiota in meat fermentations, and many enterococci isolated from sausages have the ability to produce enterocins harbouring antimicrobial activity against pathogens and spoilage microorganisms of meat concern. The application of enterocins producing enterococci or their purified metabolites, as extra hurdles for preservation in sausage fermentation and in sliced-vacuum packed cooked meat products can be beneficial, preventing the outgrowth of Listeria monocytogenes and slime-producing lactic acid bacteria. Enterocins and bacteriocinogenic enterococci hold considerable promise as alternatives to traditional chemical preservatives and they could be exploited for the control of emergent pathogens in meat products. Their inhibitory effect can be increased when used in conjunction with particular physical and chemical processes, but current regulation is hampering the application of purified bacteriocins.     

Serotonin‐transporter promoter polymorphism modulates the ability to control food intake: Effect on total weight loss

Scope

The biggest challenge for losing weight is the ability to control the amount of food eaten; the tendency to overeat is called disinhibition. Our aims were to determine whether (a) the SLC6A 4‐promoter variant (5‐HTTLPR) relates to disinhibition; (b) this association could affect total weight‐loss during a behavioral/dietary treatment for obesity.

Methods and results

A total of 2961 subjects attended voluntarily five weight‐loss clinics; a subsample (n = 624) was recruited for SLC6A4 genotyping. Total weight‐loss, emotional‐eating‐score and disinhibition‐score were examined. We observed that: (a) the reduced ability to control food intake (disinhibition) is implicated in the impairment to lose weight; (b) SLC6A 4‐promoter variant is implicated in disinhibition. S carriers (low‐expressing) of the SLC6A4‐promoter variant had a lower inhibition capacity and showed more failure (1.6 times) to control the amount of food eaten than LL (p < 0.05); other factors such as eating while bored, overeating after work at night, or craving for specific foods were associated to the SLC6A4 genotype (p < 0.05); (c) The combination of disinhibition (high disinhibition) and genetics (S carrier) had a higher impact on total weight loss than each factor separately.

Conclusions

SLC6A 4‐promoter variant is associated with the ability to control food intake and interacts with emotional eating to modulate total weight loss.

     

Application of nisin,CO2 and a permeabilizing agent in the preservation of refrigerated blue whiting (Micromesistius poutassou)

Gram‐negative bacteria are mainly responsible for spoilage of refrigerated fish. Nevertheless, to preserve refrigerated fresh fish no additives are permitted and only packaging as in a modified atmosphere can be used. Despite the fact that the present‐day application of nisin has been extended to dairy and meat products, its ineffectiveness against Gram‐negative bacteria complicates application to fresh fish unless combined with other additives such as chelators. With this aim, agents that increase the permeability of the membrane and facilitate the diffusion of many hydrophobic antimicrobial agents were screened for their combined activity with nisin against different genera of Gram‐negative bacteria. Quantification of the synergistic effects has shown sodium hexamethaphosphate (SMP) and ethylenediaminetetraacetic acid (EDTA) were the most effective. Further studies were done on the combined effects of nisin, SMP and CO₂ in the preservation of fresh fish. A factorial experimental design was used to determine synergism between these variables. Logarithmic values of total viable counts and total volatile bases after 12 days of storage were described by empirical equations. Analysis of the results showed a marked effect of CO₂ in decreasing both variables. Whereas CO₂ interacted positively with nisin and SMP, a non‐significant interaction was observed between nisin and SMP. It is concluded that CO₂ interacts readily with SMP, and may offer an additional advantage for CO₂‐rich atmospheres in food preservation. Copyright © 2005 Society of Chemical Industry