Molecular dynamics simulations of sheared brush-like systems

Mit Hilfe von Molekulardynamik-Simulationen untersuchen wir bürstenartige Systeme unter guten Lösungsmittelbedingungen. Diese Systeme sind, dank ihren vielfältigen Beschaffenheiten, die von Molekularparametern und äußeren Bedingungen abhängig sind, wichtig für viele industrielle Anwendungen. Man vermutet, dass die Polymerbürsten eine entscheidende Rolle in der Natur wegen ihrer einzigartigen Gleiteigenschaften spielen. Ein vergröbertes Modell wird verwendet, um die strukturellen und dynamischen Eigenschaften zweier hochkomprimierter Polymerbürsten, die eine niedrige Reibung aufweisen, zu untersuchen. Allerdings sind die Lubrikationseigenschaften dieser Systeme, die in vielen biologischen Systemen vorhanden sind, beeinflußt. Wir untersuchen so-genannte "weiche Kolloide", die zwischen den beiden Polymerbürsten eingebettet sind, und wie diese Makroobjekte auf die Polymerbürsten wirken.rnrnNicht-Gleichgewichts-Molekulardynamik-Simulationen werden durchgeführt, in denen die hydrodynamischen Wechselwirkungen durch die Anwendung des DPD-Thermostaten mit expliziten Lösungsmittelmolekülen berücksichtigt werden. Wir zeigen, dass die Kenntnis der Gleichgewichtseigenschaften des Systems erlaubt, dynamische Nichtgleichgewichtsigenschaften der Doppelschicht vorherzusagen.rnrnWir untersuchen, wie die effektive Wechselwirkung zwischen kolloidalen Einschlüßen durch die Anwesenheit der Bürsten (in Abhängigkeit der Weichheit der Kolloide und der Pfropfdichte der Bürsten) beeinflußt wird. Als nächsten Schritt untersuchen wir die rheologische Antwort von solchen komplexen Doppelschichten auf Scherung. Wir entwickeln eine Skalen-Theorie, die die Abhängigkeit der makroskopischen Transporteigenschaften und der lateralen Ausdehnung der verankerten Ketten von der Weissenberg Zahl oberhalb des Bereichs, in dem die lineare Antwort-Theorie gilt, voraussagt. Die Vorhersagen der Theorie stimmen gut mit unseren und früheren numerischen Ergebnissen und neuen Experimenten überein. Unsere Theorie bietet die Möglichkeit, die Relaxationszeit der Doppelschicht zu berechnen. Wenn diese Zeit mit einer charakteristischen Längenskala kombiniert wird, kann auch das ''transiente'' (nicht-stationäre) Verhalten beschrieben werden.rnrnrnWir untersuchen die Antwort des Drucktensors und die Deformation der Bürsten während der Scherinvertierung für grosse Weissenberg Zahlen. Wir entwickeln eine Vorhersage für die charakteristische Zeit, nach der das System wieder den stationären Zustand erreicht.rnrnrnElektrostatik spielt eine bedeutende Rolle in vielen biologischen Prozessen. Die Lubrikationseigenschaften der Polymerbürsten werden durch die Anwesenheit langreichweitiger Wechselwirkungen stark beeinflusst. Für unterschiedliche Stärken der elektrostatischen Wechselwirkungen untersuchen wir rheologische Eigenschaften der Doppelschicht und vergleichen mit neutralen Systemen. Wir studieren den kontinuierlichen Übergang der Systemeigenschaften von neutralen zu stark geladenen Bürsten durch Variation der Bjerrumlänge und der Ladungsdichte.

Mechanical behavior of flat and curved laminates interleaved by electrospun nanofibers

A major weakness of composite materials is that low-velocity impact, introduced accidentally during manufacture, operation or maintenance of the aircraft, may result in delaminations between the plies. Therefore, the first part of this study is focused on mechanics of curved laminates under impact. For this aim, the effect of preloading on impact response of curved composite laminates is considered. By applying the preload, the stress through the thickness and curvature of the laminates increased. The results showed that all impact parameters are varied significantly. For understanding the contribution rate of preloading and pre-stress on the obtained results another test is designed. The interesting phenomenon is that the preloading can decrease the damaged area when the curvature of the both specimens is the same. Finally the effect of curvature type, concave and convex, is investigated under impact loading. In the second part, a new composition of nanofibrous mats are developed to improve the efficiency of curved laminates under impact loading. Therefore, at first some fracture tests are conducted to consider the effect of Nylon 6,6, PCL, and their mixture on mode I and mode II fracture toughness. For this goal, nanofibers are electrospun and interleaved between mid-plane of laminate composite to conduct mode I and mode II tests. The results shows that efficiency of Nylon 6,6 is better than PCL in mode II, while the effect of PCL on fracture toughness of mode I is more. By mixing these nanofibers the shortage of the individual nanofibers is compensated and so the Nylon 6,6/PCL nanofibers could increased mode I and II fracture toughness. Then all these nanofibers are used between all layers of composite layers to investigate their effect on damaged area. The results showed that PCL could decrease the damaged area about 25% and Nylon 6,6 and mixed nanofibers about 50%.

Laser shock peening treatment to control and moderate fatigue crack growth in aircraft structure based on residual stress engineering approach

Laser Shock Peening (LSP) is a surface enhancement treatment which induces a significant layer of beneficial compressive residual stresses of up to several mm underneath the surface of metal components in order to improve the detrimental effects of the crack growth behavior rate in it. The aim of this thesis is to predict the crack growth behavior in metallic specimens with one or more stripes which define the compressive residual stress area induced by the Laser Shock Peening treatment. The process was applied as crack retardation stripes perpendicular to the crack propagation direction with the object of slowing down the crack when approaching the peened stripes. The finite element method has been applied to simulate the redistribution of stresses in a cracked model when it is subjected to a tension load and to a compressive residual stress field, and to evaluate the Stress Intensity Factor (SIF) in this condition. Finally, the Afgrow software is used to predict the crack growth behavior of the component following the Laser Shock Peening treatment and to detect the improvement in the fatigue life comparing it to the baseline specimen. An educational internship at the “Research & Technologies Germany – Hamburg” department of AIRBUS helped to achieve knowledge and experience to write this thesis. The main tasks of the thesis are the following: •To up to date Literature Survey related to “Laser Shock Peening in Metallic Structures” •To validate the FE model developed against experimental measurements at coupon level •To develop design of crack growth slowdown in Centered Cracked Tension specimens based on residual stress engineering approach using laser peened strip transversal to the crack path •To evaluate the Stress Intensity Factor values for Centered Cracked Tension specimens after the Laser Shock Peening treatment via Finite Element Analysis •To predict the crack growth behavior in Centered Cracked Tension specimens using as input the SIF values evaluated with the FE simulations •To validate the results by means of experimental tests

Cannabinoid CB1 receptor in the regulation of sociability, stress coping, and its interaction with the serotonergic system

Cannabinerge Substanzen können das Verhalten in einer dosisabhängigen, aber biphasischen Weise beeinflussen. Eine Erklärung für diese Art der Effekte könnte die Verteilung des CB1 Rezeptors auf verschiedenen Neuronentypen sein. CB1 Rezeptoren in glutamatergen und GABAergen Neuronen sind hier besonders wichtig, da die entsprechenden Neurotransmitter als Gegenspieler die neuronale Erregung kontrollieren. Spezifische Deletion des CB1 Rezeptor-Gens von einer der beiden Populationen führte zu gegensätzlichen Phenotypen, genauer gesagt, einem erniedrigten, bzw. einem gesteigerten Interaktiondrang. Tiere, bei denen der CB1 Rezeptor ausschließlich in striatalen, GABAergen „Medium Spiny“ Neuronen deletiert wurde, zeigten keinen veränderten Phänotyp. Dies legt nahe, dass der CB1 Rezeptor in kortikalen glutamatergen und GABAergen Neuronen für einen ausgeglichenen Interaktionsdrang entscheidend ist (siehe Kapitel 3).rnDiese dosisabhängigen, biphasischen Effekte auf das Verhalten können auch im „Forced Swim Test“ (FST) beobachtet werden. Ein möglicher Mechanismus, durch den Cannabinoide das Stressverhalten beeinflussen können, wäre die Regulierung der Monoaminausschüttung. Um die Abhängigkeit der Cannabinoideffekte von der Serotonintransmission zu untersuchen, wurden Dosen von CB1 Rezeptoragonisten und –antagonisten mit antidepressiv-induzierenden Eigenschaften bei gleichzeitiger Inhibition der Serotonintransmission im FST getestet. Die Ergebnisse zeigten, dass lediglich der Agonisteffekt durch die Inhibition der Serotoninauschüttung beeinflusst wird. Zusätzlich konnte die Abhängigkeit des Antagonisteneffekts von funktionsfähigen GABAergen CB1 Rezeptoren nachweisen werden. Interessanter Weise konnte der durch die Deletion von glutamatergen CB1 Rezeptoren induzierte Phänotyp durch Inhibition der Serotoninausschüttung blockiert werden (siehe Kapitel 4).rnEin indirekter Einfluss auf Serotoninausschüttung scheint also wahrscheinlich zu sein. Bis jetzt blieb jedoch unklar, inwieweit cannabinerge Substanzen direkt auf serotonerge Neuronen wirken können. Im Jahr 2007 konnte unsere Gruppe die Expression des CB1 Rezeptors in serotonergen Neuronen auf mRNA- und Proteinebene nachweisen. Die Züchtung und Analyse einer mutanten Mauslinie, in welcher der CB1-Rezeptor spezifisch in serotonergen Neuronen ausgeschaltet wurde, zeigte bei männlichen Tieren eine schwache, aber signifikante Verhaltensänderungen, die durch soziale Stimuli und lebensbedrohlichen Situationen ausgelöst wurde. So ist es erstmals gelungen nachzuweisen, dass serotonerge CB1-Rezeptoren eine physiologische Relevanz besitzen (siehe Kapitel 5).rn

Problem behavior of dementia patients predicts low-grade hypercoagulability in spousal caregivers

Low-grade hypercoagulability might be one pathway to explain how the chronic stress of dementia caregiving increases cardiovascular disease risk, but the specific aspects of caregiver stress that elicit hypercoagulability are elusive. We hypothesized that dementia patients' problem behaviors and negative reactions of caregivers to these behaviors would relate to hypercoagulability in caregivers.

Work-related behavior and experience patterns and predictors of mental health in German physicians in medical practice

Background and Objectives: Work-related stress and burnout among physicians are of increasing relevance. The aim of this study was to investigate work-related behavior and experience patterns and predictors of mental health of physicians working in medical practice in Germany. Methods: We surveyed a stratified, random sample of 900 physicians from different specialties. The questionnaire included the standardized instruments Work-related Behavior and Experience Pattern (AVEM) and the Short Form-12 Health Survey (SF-12). Results: Only one third of physicians reported high or very high general satisfaction with their job, but 64% would choose to study medicine again. Only 18% of physicians presented a healthy behavior and experience pattern. Almost 40% presented a pattern of reduced motivation to work, 21% were at risk of overexertion, and 22% at risk for burnout. Willingness to study medicine again, fulfilled job expectations, professional years, marital status, and behavior patterns were significant predictors of mental health and accounted for 35.6% of the variance in mental health scores. Job-related perceptions also had a significant effect on burnout. Conclusions: The strong influence of work-related perceptions suggests a need for realistic expectation management in medical education, as well as support in stress management and coping strategies during medical training.

Role Stress and Job Outcomes in Public Accounting: Have the Gender Experiences Converged?

Using data collected from professionals in a large U.S. national public accounting firm, we explored gender differences in perceived levels of role stress and job outcomes as well as the effects of a healthy lifestyle as a coping mechanism for role stress, burnout and related job outcomes. Our large sample size (1,681) and equal participation by women (49.7%) and men (50.3%) allowed us to analyze the causal relationships of these variables using a previously tested multi-disciplinary research model (Jones, Norman, & Wier, 2010). We found that women and men perceive similar levels of role stress as defined by role ambiguity and role overload, and that women perceive less role conflict. Men and women perceive similar levels of job satisfaction and job performance. Contrary to earlier studies, women do not report higher levels of turnover intentions. Results show that efforts of the public accounting firms over the past decade may be somewhat successful in reducing the levels of role stress and turnover intentions among women. Another plausible explanation could be that an expansionist theory of gender, work and family (Barnett & Hyde, 2001) may now be responsible for improved well-being of females to the point where the genders have converged in their experience of role stress and job outcomes in public accounting.

Beta-Endorphin Modulates the Effect of Stress on Novelty-Suppressed Feeding

Although stress is implicated in the pathophysiology of mood and anxiety disorders, not all individuals who suffer stressful life events develop psychopathology. Differential susceptibility to stress may be influenced by genetically mediated differences in hypothalamic-pituitary-adrenal (HPA) axis activity and moderation of the stress response by the opioid peptide beta-endorphin (beta-E). The present study investigated genetic contributions to coping behavior by examining anxious behavior of transgenic mice with varying capacities to synthesize beta-E [B6.129S2-Pomc(tm1Low)/J; regulated by insertion of a premature stop codon into one or both copies of the proopiomelanocortin (POMC) gene], both under normal conditions and following 3 min of forced swim (FS). Ten minutes after this stress exposure or a control manipulation, acutely food-deprived female and male transgenic mice were subjected to a novelty-suppressed feeding (NSF) test, during which their interaction with an almond slice located in the center of an open field box was measured. There was an interaction between genotype and stress for latency to approach the almond and whether or not the almond was approached, such that mice with low or absent beta-E displayed a stronger aversion to novelty-feeding after stress exposure than did mice with normal levels. These data provide evidence for a moderating effect of beta-E on the behavioral response to stress. Genotypic differences in anxious behavior emerged when mice were stressed prior to behavioral assessment, suggesting that beta-E plays a role in coping behavior. These findings indicate that genetic variability in sensitivity of the beta-E system to stress may contribute, at least in part, to heritable differences in stress reactivity as well as vulnerability to stress-related psychopathology.

The Effect of Chronic Estrogen Administration on Same-Sex Social Behavior and Central Oxytocin Levels in Prairie Voles (Microtus ochrogaster)

The purpose of our study is to investigate the effects of chronic estrogen administration on same-sex interactions during exposure to a social stressor and on oxytocin (OT) levels in prairie voles (Microtus orchrogaster). Estrogen and OT are two hormones known to be involved with social behavior and stress. Estogen is involved in the transcription of OT and its receptor. Because of this, it is generally thought that estrogen upregulates OT, but evidence to support this assumption is weak. While estrogen has been shown to either increase or decrease stress, OT has been shown to have stress-dampening properties. The goal of our experiment is to determine how estrogen affects OT levels as well as behavior in a social stressor in the voles. In addition, estrogen is required for many opposite-sex interactions, but little is known about its influence on same-sex interactions. We hypothesized that prairie voles receiving chronic estrogen injections would show an increase in OT levels in the brain and alter behavior in response to a social stressor called the resident-intruder test. To test this hypothesis, 73 female prairie voles were ovariectomized and then administered daily injections of estrogen (0.05 ¿g in peanut oil, s.c.) or vehicle for 8 days. On the final day of injections, half of the voles were given the resident-intruder test, a stressful 5 min interaction with a same-sex stranger. Their behavior was video-recorded. These animals were then sacrificed either 10 minutes or 60 minutes after the conclusion of the test. Half of the animals (no stress group) were not given the resident-intruder test. After sacrifice, trunk blood and brains were collected from the animals. Videos of the resident-intruder tests were analyzed for pro-social and aggressive behavior. Density of OT-activated neurons in the brain was measured via pixel count using immunohistochemistry. No differences were found in pro-social behavior (focal sniffing, p = 0.242; focal initiated sniffing p = 0.142; focal initiated sniffing/focal sniffing, p = 0.884) or aggressive behavior (total time fighting, p= 0.763; number of fights, p= 0.148; number of strikes, p = 0.714). No differences were found in activation of OT neurons in the brain, neither in the anterior paraventricular nucleus (PVN) (pixel count p= 0.358; % area that contains pixelated neurons p = 0.443) nor in the medial PVN (pixel count p= 0.999; % area that contains pixelated neurons p = 0.916). These results suggest that estrogen most likely does not directly upregulate OT and that estrogen does not alter behavior in stressful social interactions with a same-sex stranger. Estrogen may prepare the animal to respond to OT, instead of increasing the production of the peptide itself, suggesting that we need to shift the framework in which we consider estrogen and OT interactions.

Delayed response and lack of habituation in plasma interleukin-6 to acute mental stress in men

Acute mental stress induces a significant increase in plasma interleukin (IL)-6 levels as a possible mechanism for how psychological stress might contribute to atherosclerosis. We investigated whether the IL-6 response would habituate in response to a repetitively applied mental stressor and whether cortisol reactivity would show a relationship with IL-6 reactivity. Study participants were 21 reasonably healthy men (mean age 46+/-7 years) who underwent the Trier Social Stress Test (combination of a 3-min preparation, 5-min speech, and 5-min mental arithmetic) three times with an interval of 1 week. Plasma IL-6 and free salivary cortisol were measured immediately before and after stress, and at 45 and 105 min of recovery from stress. Cortisol samples were also obtained 15 and 30 min after stress. Compared to non-stressed controls, IL-6 significantly increased between rest and 45 min post-stress (p=.022) and between rest and 105 min post-stress (p=.001). Peak cortisol (p=.034) and systolic blood pressure (p=.009) responses to stress both habituated between weeks one and three. No adaptation occurred in diastolic blood pressure, heart rate, and IL-6 responses to stress. The areas under the curve integrating the stress-induced changes in cortisol and IL-6 reactivity were negatively correlated at visit three (r=-.54, p=.011), but not at visit one. The IL-6 response to acute mental stress occurs delayed and shows no adaptation to repeated moderate mental stress. The hypothalamus-pituitary-adrenal axis may attenuate stress reactivity of IL-6. The lack of habituation in IL-6 responses to daily stress could subject at-risk individuals to higher atherosclerotic morbidity and mortality.

Association between cannabis use and sexual risk behavior among young heterosexual adults

To study the association between cannabis use and frequent sexual risk behavior, we tested the hypothesis of a situational influence of cannabis use in sexual encounters using a combination of global association study and event-level analysis and examined possible mediator variables, including the personality trait of hedonism/risk preference, psychosocial stress, and HIV-related beliefs, using mediation models. The results of a computer-assisted telephone interview of a random sample of 2790 heterosexual men and women aged 16-24 years showed that risky sexual behavior was more frequent in cannabis-using men and women than in non-using persons. The results did not support a situational effect of cannabis intoxication on sexual risk behavior. The more frequent sexual risk behavior among cannabis users was mediated by decreased intentions to use HIV protection, by lower HIV-self-efficacy, and higher risk preference/hedonism. Only among women psychosocial stress was a partial mediator. The findings show that HIV prevention programs for cannabis-using young adults should emphasize the role of person variables instead of situation variables.

COMPRESSION BEHAVIOR AT HIGH STRAIN RATE FOR AN ULTRA HIGH PERFORMANCE CONCRETE

The need for a stronger and more durable building material is becoming more important as the structural engineering field expands and challenges the behavioral limits of current materials. One of the demands for stronger material is rooted in the effects that dynamic loading has on a structure. High strain rates on the order of 101 s-1 to 103 s-1, though a small part of the overall types of loading that occur anywhere between 10-8 s-1 to 104 s-1 and at any point in a structures life, have very important effects when considering dynamic loading on a structure. High strain rates such as these can cause the material and structure to behave differently than at slower strain rates, which necessitates the need for the testing of materials under such loading to understand its behavior. Ultra high performance concrete (UHPC), a relatively new material in the U.S. construction industry, exhibits many enhanced strength and durability properties compared to the standard normal strength concrete. However, the use of this material for high strain rate applications requires an understanding of UHPC’s dynamic properties under corresponding loads. One such dynamic property is the increase in compressive strength under high strain rate load conditions, quantified as the dynamic increase factor (DIF). This factor allows a designer to relate the dynamic compressive strength back to the static compressive strength, which generally is a well-established property. Previous research establishes the relationships for the concept of DIF in design. The generally accepted methodology for obtaining high strain rates to study the enhanced behavior of compressive material strength is the split Hopkinson pressure bar (SHPB). In this research, 83 Cor-Tuf UHPC specimens were tested in dynamic compression using a SHPB at Michigan Technological University. The specimens were separated into two categories: ambient cured and thermally treated, with aspect ratios of 0.5:1, 1:1, and 2:1 within each category. There was statistically no significant difference in mean DIF for the aspect ratios and cure regimes that were considered in this study. DIF’s ranged from 1.85 to 2.09. Failure modes were observed to be mostly Type 2, Type 4, or combinations thereof for all specimen aspect ratios when classified according to ASTM C39 fracture pattern guidelines. The Comite Euro-International du Beton (CEB) model for DIF versus strain rate does not accurately predict the DIF for UHPC data gathered in this study. Additionally, a measurement system analysis was conducted to observe variance within the measurement system and a general linear model analysis was performed to examine the interaction and main effects that aspect ratio, cannon pressure, and cure method have on the maximum dynamic stress.

Effects of depressive and anxious symptoms on norepinephrine and platelet P-selectin responses to acute psychological stress among elderly caregivers

BACKGROUND: Caring for a spouse with Alzheimer's disease is associated with increased psychological distress, impaired immunity, and heightened cardiovascular risk. Hyperreactivity of sympathetic and platelet activation responses to acute psychological stress, or the failure to recover quickly from stressful events, may constitute an important pathway linking stress and negative affect with cardiovascular disease (CVD). OBJECTIVES: (1) To evaluate associations between negative affect (i.e., depressive and anxious symptoms) with increased norepinephrine and P-selectin responses to an acute psychological stress task. (2) To establish whether these associations are augmented among elderly spousal caregivers (CG) compared to non-caregivers (NC). METHODS: Depressive (DEP) and anxious (ANX) symptoms from the Brief Symptom Inventory were assessed among 39 CG and 31 NC. Plasma norepinephrine levels (NE) and percent platelet P-selectin (PSEL) expression were assayed at three time-points: rest, immediately following a laboratory speech test (reactivity), and after 14 min of recovery. Results: Among CG, but not NC, increased symptoms of depression and anxiety were associated with delayed NE recovery (DEP: beta=.460, p=.008; ANX: beta=.361, p=.034), increased PSEL reactivity (DEP: beta=.703, p<.001; ANX: beta=.526, p=.002), and delayed PSEL recovery (DEP: beta=.372, p=.039; ANX: beta=.295, p=.092), while controlling for age, gender, aspirin use, antidepressant use, and preexisting CVD. Bivariate correlations showed delayed NE recovery was also associated with increased PSEL reactivity (r=.416) and delayed PSEL recovery (r=.372; all ps<.05) among CG but not NC. DISCUSSION: Among chronically stressed caregivers, increased levels of depressive and anxious symptoms are associated with prolonged sympathetic activation and pronounced platelet activation. These changes may represent one pathway linking caregiving stress to cardiovascular risk.

Aspirin, but not propranolol, attenuates the acute stress-induced increase in circulating levels of interleukin-6: a randomized, double-blind, placebo-controlled study

Psychosocial stress might increase the risk of atherothrombotic events by setting off an elevation in circulating levels of the proinflammatory cytokine interleukin (IL)-6. We investigated the effect of aspirin and propranolol on the responsiveness of plasma IL-6 levels to acute psychosocial stress. For 5 days, 64 healthy subjects were randomized, double-blind, to daily oral aspirin 100mg plus long-acting propranolol 80 mg, aspirin 100mg plus placebo, long-acting propranolol 80 mg plus placebo, or placebo plus placebo. Thereafter, all subjects underwent the 13-min Trier Social Stress Test, which combines a preparation phase, a job interview, and a mental arithmetic task. Plasma IL-6 levels were measured in blood samples collected immediately pre- and post-stress, and 45 min and 105 min thereafter. The change in IL-6 from pre-stress to 105 min post-stress differed between subjects with aspirin medication and those without (p =0.033; eta p2=0.059). IL-6 levels increased less from pre-stress to 105 min post-stress (p <0.027) and were lower (p =0.010) at 105 min post-stress in subjects with aspirin than in subjects without aspirin. The significance of these results was maintained when controlling for gender, age, waist-to-hip ratio, mean arterial blood pressure, and smoking status. Medication with propranolol was not significantly associated with the stress-induced change in IL-6 levels. Also, aspirin and propranolol did not significantly interact in determining the IL-6 stress response. Aspirin but not propranolol attenuated the stress-induced increase in plasma IL-6 levels. This suggests one mechanism by which aspirin treatment might reduce the risk of atherothrombotic events triggered by acute mental stress.