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The ACL is one of the major ligaments in the knee connecting the femur to the tibia which provides stability by resisting shear in sagittal plane. ACL tears occur in 1 out of 3000, and due to inability to heal, reconstructive surgeries are performed at a rate of 200,000/year. Final graft fixation tension during surgery has been shown to wane due to stress-relaxation which has been correlated with negative clinical outcomes. Therefore, preconditioning, which currently is an isometric load (88N), is performed to remove stress-relaxation after the final tension has been applied in vivo. Three preconditioning protocols, creep, stress relaxation, and none, were tested to show significant differences and variance in graft tension after 30 minutes. The results suggest that the current preconditioning protocol may not be efficient enough to remove stress-relaxation after final fixation, and that a creep protocol causes less variability than the other preconditions performed.

Yes / In constitutive models of polymers, there has been a long history of the use of strain-rate dependent viscous processes, such as the Eyring and Argon models. These are combined with elastic elements to generate viscoplastic models that exhibit typical phenomena such as rate dependent yield, creep and stress relaxation. The Eyring process is one of the most frequently used such mechanisms. It has two significant drawbacks: it implies a temperature dependence of mechanical behaviour that is in an opposite sense to that observed; and it predicts a strain rate dependence of yield stress that is less complex than that observed, leading to the requirement for two or more Eyring processes. In recent years, new ideas for amorphous polymers have been developed that lead to an alternative plastic mechanism that addresses these concerns. In this paper a constitutive model that incorporates this mechanism is developed, and its effectiveness in modelling macroscopic mechanical behaviour of polymers is explored with respect to published data.

Evaluation of the tableting mixtures containing the paracetamolum by the parameters of the equation of compression The diploma thesis deals with effect of paracetamol on the parameters of compression equation. These parameters characterize compression process in the individual phases of compression: precompression phase, phase of elastic deformation and phase of plastic deformation. Paracetamolum was studied in the five mixtures, where it occurred in different ratio with microcrystalline cellulose. Looseness, Carr's index and Hausner ratio were evaluated for each mixture. The sizes of reductions ai, sizes of energies Ei and Ri, "half-pressures" pHi and the velocity constants 1/ti were calculated using three-exponential equation of compression for the individual phases of pressing. The results showed poor flow properties of tableting mixtures and thus its unsuitability for direct compression. The results of the study have revealed that increasing amounts of paracetamolum in the mixture increase reduction of volume a1 and velocity of volume change 1/t1 in dependence on the amount of the extruded air and decrease the required energy due to the smooth surface and less friction during the precompression phase. Reduction of volume a2 decrease with the increase of content of paracetamol due to...

Charles University in Prague, Faculty of Pharmacy in Hradec Králové Department of Pharmaceutical Technology Student: Natália Pribylincová Consultant: Doc. RNDr. Milan Řehula, CSc. Effect of ascorbic acid on the parameters of compression equation Many mathematical models describing the compressing process are widely used in the development of new medicinal products in form of tablets. This process can be evaluated by means of compaction equations or throughout the viscoelastic properties. Compaction equation expresses dependency on height, volume or density of the used material which is being compressed by the applied compacting pressure. Based on the gained parameters, it is possible to describe specifically various stages of the compaction process, to determine characteristic properties of the studied material and accordingly to examine its mechanism. This thesis deals with the impact of the ascorbic acid on the parameters of compaction equation. The paperwork evaluates a mixture consisting of ascorbic acid (AA) and microcrystalline cellulose (MCC) in the ratio of MCC : AA 100:0, 75:25, 50:50, 25:75, 0:100. . The mostly used compression equations are described in the theoretic part. The three - exponential equation by Řehula created at the Department of Pharmaceutical Technology at Charles...

Charles University in Prague, Faculty of Pharmacy in Hradec Králové Department of Pharmaceutical Technology Student: Tisoňová Zuzana Consultant: Doc. RNDr. Milan Řehula, CSc. Compacting process is mathematically expressed by compression equations. It is characterized by various parameters of equations. The equation expresses the dependence of the volume, density and height on compacting pressure. In this paperwork, the parameters of compacting equation are evaluated and the pre-loading phase, the phase of elastic deformation and the phase of plastic deformation are studied. This thesis deals with impact of ibuprofen on the parameters of compaction equation. Five mixtures which included microcrystalline celulose and ibuprofen in different ratio 100:0, 75:25, 50:50, 25:75 and 0:100 were studied. Results were based on the three-exponential equation. They were evaluated thanks to box plots. From the results obtained, it is obvious that with decreasing amount of microcrystalline cellulose, the parameter a1 has increased and the parameter E1 also slightly has increased. The composition of the used material didn't have any huge influence on parameters 1/t1 and pH1. During the phase of elastic deformation, the parameters a2 and E2 with decreasing amount of ibuprofen have decreased. On the other hand, the...

Charles University in Prague, Faculty of Pharmacy in Hradec Králové Department of Pharmaceutical Technology Student: Jakub Šmíd Consultant: Doc. RNDr. Milan Řehula, CSc. Effect of acetylsalicylic acid on the parameters of compression equation Compacting process can be expressed mathematically by compression equations. It is characterized by various parameters. The compression equation expresses the dependence of the volume, density and height on compacting pressure. This paper evaluates the parameters of the compaction equation and study pre-loading phase, the phase of elastic deformation and the phase of plastic deformation. This thesis examines effect of acetylsalicylic acid on the parameters of compaction equation. Tablets were compressed from five mixtures. Mixtures contained acetylsalicylic acid and microcrystalline cellulose in different ratios 0:100, 25:75, 50:50, 75:25 and 0:100. The results were obtained using the three-exponential equation. Evaluation was carried out by using box plots.

Charles University in Prague, Faculty of Pharmacy in Hradec Králové Department of Pharmaceutical technology Supervisor: Doc. RNDr. Milan Řehula, CSc. Title of Dissertation: Validation of a new method for the evaluation of stress relaxation of tablets Keywords: tablets, stress relaxation test, elastic and plastic deformation The theoretical part of this thesis deals with the elastic relaxation of tablets. It describes various methods used in the evaluation of elastic recovery of tablets. The present work explores the factors affecting the elastic relaxation of tablets. The experimental part is focused on the stress relaxation test. Until present only one delay was studied. This work studies the stress relaxation after relieving tablet. The parameters A2, T2 and P2 were reviewed. The relationship between the parameters P2 and compression force relief were studied. For experiments microcrystalline cellulose Avicel PH 102 (MCC) was used. Tablets were compressed in a device used for testing the strength of the material in pressure and tension T1-FRO 50. For the determination of the stress relaxation test, the tablets were compressed with 180 sec first delay. The second delay was always 60 sec but it was measured from certain decrease of pressure (7 kN, 6 kN, 5 kN, 4 kN, 3 kN, 2 kN, 1 kN and 0.1 kN). To...

The objective of this study is to characterize the long-term behavior of an austenitic-ferritic stainless steel-based pretensioned system for strengthening reinforced concrete bridge pier caps in shear. Stress relaxation experiments were conducted on UNS S32101 stainless steel bars subjected to various initial stresses and temperatures within the low homologous temperature (LHT) regime. Data from these experiments were used to develop a viscoplastic constitutive model to describe the long-term time- and temperature-dependent behavior of the stainless steel bars. This mechanics-based approach is integrated with an analytical method based on strut-and-tie analysis to compute the shear strength of reinforced concrete pier caps strengthened with this external pretensioned system.

A new type of computer-controlled instrument has been developed to measure microviscoelastic properties of thin materials. It can independently control and measure indentation loads and depths in situ revealing information about material creep and relaxation. Sample and indenter positions are measured with a specially designed polarization interferometer. Indenter loadings can be varied between 0.5 and 10 grams and held constant to ±41 mg. The resulting indentation depths can be measured in situ to ±1.2 nm. The load required to maintain constant indentation depths from 0.1 to 5.0 microns can be measured in situ to ±3.3 mg and the depth held constant to ±15 nm.