D. T. Spasic

On Viscoelastic Compliant Contact-Impact Models

We study dynamics of a mass, moving on a straight line, and impacting against the rigid wall through a deformable body, that we model as a straight rod of negligible mass. The chosen constitutive model of the viscoelastic body comprises fractional derivatives of stress and strain and the restrictions on the coefficients that follow from Clausius Duhem inequality. We show that the dynamics of the problem is governed by a single differential equation of real order The obtained equation was solved numerically. The comparison is made to the solution obtained by the Laplace transform and Posts inversion formula. The predictions of the model concerning the duration of the impact, maximal values of the impacting force and deformation as well as the restitution coefficient are determined for several values of system parameters.

A model for plane elastica with simple shear deformation pattern

SummarySeveral existing theories of elastic rods that take into account shear effects are classified. Then, a new model for the influence of shearing force, based on simple shear of finite amount, is proposed. The properties of the model are examined on the stability problem for a heavy elastic rod. It is shown that the rod may exhibit sub- and super-critical bifurcation patterns at the trivial equilibrium configuration. A comparison is made between the results for the critical load of the rod predicted by the new and the three different existing models.

Optimizing the electrodynamical stabilization method for a man-made Earth satellite

We consider the problem of stabilizing the angular position of a man-made Earth satellite based on an electrodynamic method that uses the electromagnetic interaction of the satellite with the geomagnetic field. We propose an optimization of this satellite stabilization method that allows to reduce energy expenditure on board the satellite.

Risk factors for accelerated atherosclerosis in young women with hyperprolactinemia

Prolactin is a metabolic hormone. The hypothesis is that hyperprolactinemia can cause metabolic and inflammatory changes which are associated with accelerated atherosclerotic process, but the treatment of hyperprolactinemia with dopamine agonists, leads to reversibility of these processes. The first aim of this study was to determine whether hyperprolactinemia in premenopausal women is accompanied with the increase in body mass index (BMI), changes in body composition, lipid disturbances, the presence of inflammation and changes in systolic and diastolic blood pressure as risk factors for the development of early atherosclerosis. The second aim was to know whether the therapy of hyperprolactinemia and prolactin normalization lead to improvement of the observed parameters. Twenty female patients with prolactinomas, before and during treatment with dopamine agonists and 16 healthy controls were evaluated. Prolactin, BMI, total body fat, free fat mass, total body water, total cholesterol, triglycerides, high density lipoprotein (HDL), low density lipoprotein (LDL) and fibrinogen as well as systolic and diastolic blood pressure were measured at baseline and during the therapy. Hyperprolactinemic patients had pathologic and significantly higher levels of prolactin (PRL) than the controls (p=0.000). The BMI, body fat, total body water (TBW), total cholesterol, triglycerides, LDL were in normal range and higher in the patients than in the controls. HDL was lower in hyperprolactinemic females than controls. The difference was significant only for body fat (fat % p=0.006; fat kg p=0.009). Fibrinogen was slightly increased in patients compared with the controls. Hyperprolactinemic patients had normal, but increased levels of systolic and diastolic blood pressure compared with the controls. The difference with border significance was found in diastolic blood pressure (p=0.065). The correlation of PRL with all the observed parameters was positive apart from HDL, but relatively significant only with diastolic blood pressure (r=0.31). The therapy with dopamine agonists caused the decrease of all the observed parameters, but significant decreases was achieved only in BMI (p=0.028), total cholesterol levels (p<0.001) and LDL (p<0.002). Changes in BMI, body composition, serum lipids and lipoproteins, fibrinogen level and blood pressure confirm our hypothesis about the possible role of hyperprolactinemia in developing adverse metabolic disturbances which are reversible after treatment.

ON IMPACT SCRIPTS WITH BOTH FRACTIONAL AND DRY FRICTION TYPE OF DISSIPATION

We study the dynamics of a mass, sliding on a dry surface and impacting against a rigid wall through a viscoelastic body, that we model as a straight rod of negligible mass. The problem comprises a constitutive model of the viscoelastic body with fractional derivatives of stress and strain, restrictions on the coefficients that follow from Clausius–Duhem inequality, and discontinuous inequality constraint conditions imposed by the Coulomb friction model. We show that the dynamics of the problem is governed by a single integro-differential inclusion. By use of the slack variable algorithm the problem was solved numerically. The predictions of the model concerning the duration of the impact, maximal values of the impacting force and deformation, as well as the restitution coefficient are determined for several values of system parameters. Depending on the dry friction coefficient three different impact scripts are identified: rebound after the impact, capture in the approaching phase, and capture in the reb...

Clinical characteristics and type of thrombophilia in women with pregnancy-related venous thromboembolic disease.

Background: Normal pregnancy is characterized by numerous changes in the hemostatic system, creating the hypercoagulable state which increases the risk of venous thromboembolic event (VTE) occurrence. The risk is further increased by the presence of inherited or acquired thrombophilia. Objective: In this study, we aimed to determine the prevalence of different types of thrombophilia in women with pregnancy-related VTE, and to investigate the possible connection between the type of thrombophilia and localization of VTE as well as the gestational age of VTE occurrence. Participants and Methods: Two hundred and two women with the first episode of pregnancy-related VTE and 130 controls were investigated. The antithrombin, protein C and protein S activity, APC resistance, FVG1691A, and FIIG20210A were determined. None of the investigated women was pregnant at the time of thrombophilia testing, and none was using oral contraceptives. Results: Thrombophilia was diagnosed in 95 patients (47%) and 7 controls (5.4%). The prevalence of FV Leiden, FIIG20210A mutations, antithrombin, PC and PS deficiencies taken together and combined thrombophilia was 22.3, 10.4, 6.9 and 6.9%, respectively. Significantly more frequent antepartum occurrence of VTE (11 vs. 3, p < 0.05) was found in women with natural coagulation inhibitor deficiency. Pulmonary embolism occurred more frequently in nonthrombophilic women (25 vs. 3, p < 0.001). Conclusion: Inherited thrombophilia was found to be considerably more frequently present in women with pregnancy- and puerperium-related VTE compared to healthy controls. Women with thrombophilia are at higher risk of developing thromboses localized in the iliacofemoral region, and women without thrombophilia are at higher risk of developing pulmonary embolism. Deficiency in natural coagulation inhibitors is associated with antepartum VTE occurrence.

Multivalued fractional differential equations as a model for an impact of two bodies

Impact models of two colliding bodies are usually generated by fundamental physical and geometrical principles. Initially, the number of unknown variables is not the same as the number of equations expressing the principles, so the models should be complemented with an appropriate set of constitutive equations which contain enough information on the physical properties of the system and therefore allow accurate predictions of its behavior. Along these lines we here discuss a problem when a block, moving along a line on a dry surface, impinges against another block being at rest, through a deformable straight rod of negligible mass. Among the variety of all possible choices that can be used, we suggest the constitutive model of the viscoelastic body with fractional derivatives of stress and strain, restrictions on the coefficients that follow from Clausius–Duhem inequality, and the Coulomb friction law given in the set-valued form. Owing to the presence of dry friction and the proposed fractional model, known as the fractional Zener model, the problem belongs to the class of set-valued fractional differential equations (or multivalued differential equations of arbitrary real order) leading to the equivalent Cauchy problem given in terms of two coupled integro-differential inclusions, for which the existence result ensuring the contractible solution set exists. By use of the combinatorial analysis of the problem, we identify 11 imaginable scripts and separate 10 feasible ones that were confirmed numerically by a procedure that combines nonlocal and nonsmooth modules.

On the Impact of Elastic Bodies with Adhesive Forces

We analyze the problem of an elastic sphere impacting against another elastic body for the case when adhesive forces act between the bodies. Depending on the parameter describing the (relative) influence of the adhesive forces, the bodies will either separate or continue to be in contact and perform an oscillatory motion after the rebound. The value of the adhesive forces parameter defining separation and capture is calculated.