1. Thermo-mechanical behavior and strain rate sensitivity of 3D-printed polylactic acid (PLA) below glass transition temperature (Tg)Vukašin Slavković, Blaž Hanželič, Vasja Plesec, Strahinja Milenković, Gregor Harih, 2024, original scientific article Abstract: This study investigated the thermomechanical behavior of 4D-printed polylactic acid (PLA), focusing on its response to varying temperatures and strain rates in a wide range below the glass transition temperature (Tg). The material was characterized using tension, compression, and dynamic mechanical thermal analysis (DMTA), confirming PLA’s strong dependency on strain rate and temperature. The glass transition temperature of 4D-printed PLA was determined to be 65 °C using a thermal analysis (DMTA). The elastic modulus changed from 1045.7 MPa in the glassy phase to 1.2 MPa in the rubber phase, showing the great shape memory potential of 4D-printed PLA. The filament tension tests revealed that the material’s yield stress strongly depended on the strain rate at room temperature, with values ranging from 56 MPa to 43 MPA as the strain rate decreased. Using a commercial FDM Ultimaker printer, cylindrical compression samples were 3D-printed and then characterized under thermo-mechanical conditions. Thermo-mechanical compression tests were conducted at strain rates ranging from 0.0001 s−1 to 0.1 s−1 and at temperatures below the glass transition temperature (Tg) at 25, 37, and 50 °C. The conducted experimental tests showed that the material had distinct yield stress, strain softening, and strain hardening at very large deformations. Clear strain rate dependence was observed, particularly at quasi-static rates, with the temperature and strain rate significantly influencing PLA’s mechanical properties, including yield stress. Yield stress values varied from 110 MPa at room temperature with a strain rate of 0.1 s−1 to 42 MPa at 50 °C with a strain rate of 0.0001 s−1. This study also included thermo-mechanical adiabatic tests, which revealed that higher strain rates of 0.01 s−1 and 0.1 s−1 led to self-heating due to non-dissipated generated heat. This internal heating caused additional softening at higher strain rates and lower stress values. Thermal imaging revealed temperature increases of 15 °C and 18 °C for strain rates of 0.01 s−1 and 0.1 s−1, respectively.
Keywords: smart materials, shape memory polymer, 3D printing, 4D printing, thermo-mechanical experiments
Published in DKUM: 29.05.2024; Views: 248; Downloads: 14
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2. Ephemerality in personal memory travel : encountering the changing environmentAndreja Trdina, Maruša Pušnik, 2023, original scientific article Abstract: This article approaches personal memory travels with the aim of investigating people’s encounters with the changing environment in relation to time. It centers on the intra-actions with perceptual ephemera as immaterial non-solid entities to argue that our relational reality is a cyclical, dis-continuous process of past and present encounters. The study is grounded in the post-humanist performativ-ity approach and based on personal life histories of memory trips gathered in Slovenia with the help of in-depth interviews and photo-elicitation. Findings suggest that travelers’ engagements with ephemera at site illustrate complex entanglements between human and other (immaterial) entities as co-constituted in time and from within time, provoking the production of situated and temporally sensitive knowledge about the more-than-human world. The article endorses the significance of memory work and human engagements with absences and resonances, both allusions to other (past) contexts, as fundamental for cultivating post-anthropocentric attitudes and advancing the habit of timefulness.
Keywords: personal memory travel, encounters, environment, absence, anthropocene, ephemera, memory, time, trave
Published in DKUM: 16.02.2024; Views: 255; Downloads: 13
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3. Layered battleship game changer password systemBoštjan Brumen, Darko Crepulja, Leon Bošnjak, 2022, original scientific article Abstract: The paper presents a secure and usable variant of the Game Changer Password System, first proposed by McLennan, Manning, and Tuft. Unlike the initial proposal based on inadequately secure Monopoly and Chess, we propose an improved version based on a layered “Battleship” game resilient against brute force and dictionary attacks. Since the initially proposed scheme did not check for the memorability and usability of a layered version, we conducted an experiment on the usability and memorability aspects. Surprisingly, layered passwords are just as memorable as single ones and, with an 80% recall rate, comparable to other graphical password systems. The claim that memorability is the most vital aspect of game-based password systems cannot be disproved. However, the experiment revealed that the usability decreased to such a low level that users felt less inclined to use such a system daily or recommend it to others.
Our study has once again shown that optimizing the password security–memorability–usability triangle is hard to achieve without compromising one of its cornerstones. However, the layered Game Changer Password System can be used in specific applications where usability is of secondary importance, while security and memorability augmented by its graphical interface are at the forefront.
Keywords: security, authentication, passwords, graphical passwords, cryptanalysis, games, memory, memorability, usability
Published in DKUM: 22.08.2023; Views: 334; Downloads: 26
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4. The role of brain oscillations in working memoryAnja Pahor, 2017, doctoral dissertation Abstract: Working memory is important for a number of higher cognitive functions such as problem solving, reasoning, reading and language comprehension. Moreover, working memory measures are significantly correlated with measures of intellectual abilities. Investigating the neural basis of working memory provides the opportunity to gain a deeper understanding of individual differences in general cognitive ability. This thesis aimed to elucidate the roles of brain oscillations in working memory, with a particular focus on theta and gamma frequency bands. Two techniques were employed that are best suited for the non-invasive study of brain oscillations: scalp recorded EEG and transcranial alternating current stimulation (tACS). In the first step, correlational studies were conducted followed by neuromodulatory studies in the next step. The results showed that individuals with high working memory capacity, contrasted against individuals with low working memory capacity, display stronger alpha and gamma band desynchronisation and increased coherence in the theta frequency band between fronto- parietal areas during maintenance and between frontal brain areas during retrieval. The results further demonstrated that interactions between theta and gamma frequency bands are related to individual differences in working memory capacity. The neuromodulatory studies showed that theta tACS applied over parietal brain areas can be used to enhance performance on working memory tasks, thereby providing support for the causal role of theta band oscillations in working memory.
Keywords: working memory, brain oscillations, capacity, electroencephalograpy, tACS, theta, gamma
Published in DKUM: 28.07.2017; Views: 2099; Downloads: 208
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5. Long memory in the Croatian and Hungarian stock market returnsMejra Festić, Alenka Kavkler, Silvo Dajčman, 2012, original scientific article Abstract: The objective of this paper is to analyze and compare the fractal structure of the Croatian and Hungarian stock market returns. The presence of long memory components in asset returns provides evidence against the weak-form of stock market effi ciency. The starting working hypothesis that there is no long memory in the Croatian and Hungarian stock market returns is tested by applying the Kwiatkowski-Phillips-Schmidt-Shin (KPSS) (1992) test, Loʼs (1991) modified rescaled range (R/S) test, and the wavelet ordinary least squares (WOLS) estimator of Jensen (1999). The research showed that the WOLS estimator may lead to different conclusions regarding long memory presence in the stock returns from the KPSS and unit root tests or Loʼs R/S test. Furthermore, it proved that the fractal structure of individual stock returns may be masked in aggregated stock market returns (i.e. in returns of stock index). The main finding of the paper is that both the Croatian stock index Crobex and individual stocks in this index exhibit long memory. Long memory is identified for some stocks in the Hungarian stock market as well, but not for the stock market index BUX. Based on the results of the long memory tests, it can be concluded that while the Hungarian stock market is weak form efficient, the Croatian stock market is not.
Keywords: stock market, long memory, efficient-market hypothesis, Croatia, Hungary
Published in DKUM: 18.07.2017; Views: 1163; Downloads: 100
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6. Microstructure of NiTi orthodontic wires observations using transmission electron microscopyJanko Ferčec, Darja Feizpour, Borut Buchmeister, Franc Rojko, Bojan Budič, Borut Kosec, Rebeka Rudolf, 2014, original scientific article Abstract: This work presents the results of the microstructure observation of six different types of NiTi orthodontic wires by using Transmission Electron Microscopy (TEM). Within these analyses the chemical compositions of each wire were observed in different places by applying the EDS detector. Namely, the chemical composition in the orthodontic wires is very important because it shows the dependence between the phase temperatures and mechanical properties. Micro- structure observations showed that orthodontic wires consist of nano-sized grains containing precipitates of Ti2Ni and/or TiC. The first precipitated Ti2Ni are rich in Ti, while the precipitated TiC is rich in C. Further investigation showed that there was a difference in average grain size in the NiTi matrix. The sizes of grains in orthodontic wires are in the range from approximately 50 to 160 nm and the sizes of precipitate are in the range from 0,3 μm to 5 μm.
Keywords: orthodontic wires, nickel-titanium orthodontic wire, NiTi wire, shape memory alloys, SMA wires, microstructure, transmission electron microscopy, TEM, average grain size
Published in DKUM: 03.07.2017; Views: 1524; Downloads: 127
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7. Microstructural and phase analysis of CuAlNi shape-memory alloy after continuous castingMirko Gojić, Stjepan Kožuh, Ivan Anžel, Gorazd Lojen, Ivana Ivanić, Borut Kosec, 2013, original scientific article Abstract: The results of the characterization of a CuAlNi shape-memory alloy after continuous casting technology are shown. Using this procedure a bar with a diameter of 8 mm was manufactured. After solidification of the alloy the microstructure characterization was carried out using optic microscopy (OM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) methods. Our results showed that the as-cast alloy consisted of the parent β1 and β1’ martensite phases. The martensite phase primary as the needle-like inside grains was observed. Martensite laths have different orientations inside particular grains. It was found that the average grains size is 98.78 µm. The grain diameter near to the external surface is higher than in the center. The average hardness of the alloy was 275 HV1.
Keywords: shape memory alloys, martensite, continuous casting, grain size
Published in DKUM: 16.03.2017; Views: 1237; Downloads: 124
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8. Suggestopedia - a theoretical approach to understanding its principles and reasons for its use in practiceAndrej Karner, 2016, undergraduate thesis Abstract: Suggestopedia is a teaching method, originally developed by psychiatrist Dr. Georgi Lozanov, the foundations of which are built upon the knowledge of psychology, neurology and linguistics.
By combining the knowledge of suggestology, medical facts and well-established psychological effects, Lozanov attempted to reconcile various aspects of this knowledge by their systematic and strategic integration. This has been achieved by using suggestion as a common denominator, and creating the carefully coordinated “set-up” (preparation of the classroom and the students) which would then make tapping into the “hidden reserves” of our brain possible.
Lozanov argues that these reserve capacities, although present, are normally more or less inhibited by parasitic conscious and subconscious thought processes, which he calls barriers, and that it is critical to discard or override, i.e. de-suggest them, and then suggest new standards prior to any language teaching. Hence, this involves the process of suggesting as well as the process of de-suggesting, i.e. erasing negative ideas by setting and anchoring positive ones.
In every-day life, language (mother tongue or foreign language) is by far more dependent on subconscious processes (automaticity) than on conscious ones. In fact, it is believed that about 95% of the language acquisition happens by employing these subconscious mechanisms. And this is believed to give us fluency and grammatical accuracy at the same time.
Since it makes sense that the acquisition of knowledge should follow the principle of economy and efficiency, and, at the same time, should have a solid scientific support, the fact that suggestopedia, in addition to employing various types of suggestions, also makes use of almost all known beneficial psychological effects found in humans makes this method rather scientifically plausible.
In Slovenian elementary schools suggestopedia did not establish itself either as the principal or alternative teaching method. One definitive reason for this situation is that the method is almost unknown. The other reason lies in the lack of appropriate (suggestopedic) teacher training which is mainly due to the first reason.
In terms of suggestopedia being applied as an alternative teaching method in Slovenian elementary schools, although a fair amount of mistrust towards the effectiveness of the method has been revealed, the general attitude of the teachers toward suggestopedia, nevertheless, is not negative or reluctive, but rather positive and embracive, and there is a general consensus that suggestopedia is an acceptable alternative to the standard way of teaching.
Keywords: Suggestopedia, Superlearning, memory enhancement, language acquisition, neurons, episodic memory, semantic memory
Published in DKUM: 25.11.2016; Views: 2522; Downloads: 123
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9. Testing pronunciation varieties of English in primary school through pictorial and textual inputMejade Tomažič, 2016, undergraduate thesis Abstract: We are surrounded with different varieties of English language. We listen to English music or radio shows, watch English movies and shows and watch English videos on the internet. The most common varieties that we encounter are British English and American English, which have distinctive differences in stress and pronunciation. Because we encounter both varieties daily, we have found it interesting to see which one prevails in our primary schools. The British English variety is primarily taught in school and the American variety is all around us and is thus more likely to be acquired. Children acquire a foreign language very easily if they are in contact with it on a regular basis. This is as nowadays, as we all hear English lyrics on the radio or hear English conversations on TV.
The thesis focuses on the factors that might influence the choice of the English variety. It explores if the input, whether it is pictorial with pictures or textual with words, influences the choice of variety. It also presents connections between the pronunciation of more frequent and less frequent words with choice of variety, the influence of the pupils’ backgrounds, and way of learning English.
Keywords: word recognition, working and long-term memory, second language, language acquisition, language learning, British and American English pronunciation.
Published in DKUM: 15.11.2016; Views: 1886; Downloads: 74
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10. Comparison of NiTi orthodontic archwires and a determination of the charasteristic propertiesJanko Ferčec, Matija Kos, Mihael Brunčko, Ivan Anžel, Branislav Glišić, Evgenija Marković, Rebeka Rudolf, 2014, original scientific article Abstract: The aim of this paper was to analyse the characteristic properties of six different, commercially available nickel-titaniumorthodontic wires with a diameter of 0.305 mm (0.014"). The characteristic properties were determined by usingsemi-quantitative EDX analyses, DSC analyses for a determination of the phase temperatures, and a tensile test to obtain themechanical properties of the wires. The investigation of the chemical composition showed an equiatomic NiTi alloy. Analysesof phase temperatures showed that the nickel-titanium orthodontic wires were, in an austenitic microstructure, exhibiting asuperelastic effect in the oral environment. The uniaxial tensile stressstrain curves showed different values for the beginningand the end of the transformation range during the loading.
Keywords: shape-memory alloy, phase transformation, thermal analysis, tensile test
Published in DKUM: 21.12.2015; Views: 1548; Downloads: 113
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