Experimental Characterization, Modeling, and Numerical Evaluation of a Novel Friction Damper for the Seismic Upgrade of Existing Buildings.

The paper presents the experimental characterization, the formulation of a numerical model, and the evaluation, by means of non-linear analyses, of a new friction damper conceived for the seismic upgrade of existing building frames. The damper dissipates seismic energy through the friction force triggered between a steel shaft and a lead core prestressed within a rigid steel chamber. The friction force is adjusted by controlling the prestress of the core, allowing the achievement of high forces with small dimensions, and reducing the architectural invasiveness of the device. The damper has no mechanical parts subjected to cyclic strain above their yield limit, thereby avoiding any risk of low-cycle fatigue. The constitutive behavior of the damper was assessed experimentally, demonstrating a rectangular hysteresis loop with an equivalent damping ratio of more than 55%, a stable behavior over repeated cycles, and a low dependency of the axial force on the rate of displacement. A numerical model of the damper was formulated in the OpenSees software by means of a rheological model comprising an in-parallel system of a non-linear spring element and a Maxwell element, and the model was calibrated on the experimental data. To assess the viability of the damper for the seismic rehabilitation of buildings, a numerical investigation was conducted by performing non-linear dynamic analyses on two case-study structures. The results highlight the benefits of the PS-LED in dissipating the largest part of seismic energy, limiting the lateral deformation of the frames, and controlling the increase in structural accelerations and internal forces at the same time.

An opto-structural method to estimate the stress-strain field induced by cell contraction on substrates of controlled stiffness in vitro.

PURPOSE: Mechanical properties of the extra-cellular matrix (ECM) such as stiffness mediate cell signaling, proliferation, migration, and differentiation. Within this context, we developed a method to estimate in vitro the stress-strain field induced by contraction of cardiovascular progenitor cells on substrates of controlled stiffness. METHODS: Two alginate-agarose hydrogels were polymerized and mechanically characterized under compression. The hydrogels showed different levels of stiffness, mimicking either normal or pathologic ECM of the cardiac tissue, with an average compressive equilibrium modulus of 3 and 25 kPa, respectively. To estimate substrate deformation induced by the adhering cells, fluorescent microspheres were included under the surface layer of the hydrogels as displacement trackers. The hydrogels were polymerized in multiwell plates and seeded with cells that were allowed to adhere for 24 hours. On the softer substrate, images of the substrate surface and the cells were acquired using time-lapse fluorescence microscopy. Image processing enabled tracking the microsphere movements and mapping local substrate deformation because of tensile stresses produced by the cells. The resulting tensile stresses could then be calculated from measured stiffness. RESULTS AND CONCLUSIONS: The substrate strains ranged between a maximum contraction of -26.5% to a maximum stretching of 19.8%. The calculated stresses ranged between a maximum compression of -0.53 kPa to a maximum tension of 0.4 kPa (nN/µm²). These results may help to interpret experimental findings, showing important differences in cell morphology and expression of phenotypic markers, induced by culturing cells on substrates with different mechanical properties.

In vitro wear performance of standard, crosslinked, and vitamin-E-blended UHMWPE.

Crosslinked vitamin-E-stabilized polyethylene acetabular cups were compared with both commercially available conventional and custom-crosslinked polyethylene acetabular cups in terms of wear behavior, in a hip joint simulator for five millions cycles, using bovine calf serum as lubricant. We correlated the wear experiments results with the chemical characterization of the investigated materials: Fourier transformed infrared (FTIR) spectroscopic analyses, differential scanning calorimetry, and crosslink density measurements were used to assess the chemical characteristics of the pristine materials. In addition, further FTIR analyses and cyclohexane extraction were carried out after the simulator experiments. Lipids absorption was observed in all tested specimens and it has been shown to strongly affect the results of the wear test. Corrected gravimetric wear measurements showed that vitamin-E blended, crosslinked polyethylene wore more than the traditional crosslinked polyethylene but exhibited a much lower wear than conventional ultrahigh-molecular weight polyethylene. The chemical analyses showed that the addition of vitamin E reduced the crosslinking efficiency. Given the correlation between crosslink density and wear resistance, this gave an explanation for the observed wear performances.

Thermal stabilization of highly crosslinked UHMWPE: a comparative study between annealed and remelted resins.

PURPOSE: An important issue related to the use of highly crosslinked ultra-high molecular weight polyethylene (HXLPE) in arthroplasty concerns the long-term oxidation of the polymer and related degradation of its end-user properties. Although in very recent years several procedures have been introduced into the manufacturing of prosthetic components to overcome this problem, the risk of long-term oxidation has not been completely eliminated. The aim of this study is to compare the effects on the physical and mechanical properties of HXLPE from two different thermal treatments used to promote oxidative stabilization. It also gives a description of the effects of oxidative degradation on the mechanical performance of HXLPE. MATERIALS AND METHODS: Virgin medical grade UHMWPE GUR 1020 was irradiated at 100 kGy and submitted to heat treatments at either 110 degrees C (annealing) or 150 degrees C (remelting). Oxidation analysis, Wear tests, Tensile tests and Charpy impact tests were carried out. RESULTS: The temperature of the thermal treatment affects both oxidation resistance and impact strength of HXLPE, whereas wear resistance is not affected. CONCLUSION: The study provides a confirmation that oxidative degradation is a serious issue for the long-term performance of HXLPE prostheses.

Behavior of the bone-titanium interface after push-in testing: a morphological study.

Fourteen titanium dental implants (Tioblast) were implanted singly in the proximal tibia of New Zealand rabbits for 120 days. A bone defect was surgically produced and filled with Bio-Oss around six of these implants. After the animals were sacrificed and their organs harvested, bone segments were fixed and methacrylate embedded after the push-in test had been performed. Microradiography was performed on longitudinal sections of the implants, whereas scanning electron microscope analysis was performed on the remaining embedded half-implants using secondary electrons only. The results showed that the implants were apically and coronally surrounded by bone, whether Bio-Oss was used or not. Fractures were evident through the newly formed bone and between the pre-existing and newly formed bone. Some fracture lines propagated through the bone and stopped at the implant surface without continuing along the bone-titanium interface. Detachment between the implant and the bone occurred at the coronal extremity of the implants and along its cervical region. These results highlight the fact that the bone-titanium interface has a high resistance to loading. It exhibited greater resistance than the newly formed bone and seems to behave in a manner similar to the cement lines of osteons.

An in vitro methodology for evaluating the mechanical properties of aortic vascular prostheses.

The main problem in the replacement of pathological segments of the aorta with vascular prostheses consists of matching the fluid admittance of the host artery and the graft. This mismatch results from the different compliance between natural and prosthetic vessels and from the plastic dilatation of the prosthesis diameter that occurs after implantation. An experimental procedure was set up for evaluating the mechanical properties of aortic vascular prostheses. An MTS 858 MiniBionix testing machine was equipped with a purposely designed testing apparatus, which allows loading a ring-shaped prosthesis specimen with forces that can be related easily to the transmural pressure acting on the prostheses in vivo. The reference pressure waveforms are simulated from a lumped parameter model of the cardiovascular system. Preliminary tests on 3 different (woven, warp knitted, and carbon-coated warp knitted fabric) aortic prostheses point out a good reproducibility of the results. The fabric strongly affects the circumferential elasticity and the dimensional stability of the graft. Simulation of hypertension promotes larger diameter dilatation and reduction in compliance. Agreement between in vitro and clinical diameter measurements has been assessed for 8 prosthesis samples and found to be adequate. This method is thus a potentially useful means for preclinical evaluation of compliance of vascular prostheses for the purpose of matching to native vessels.

A discrete-time nonlinear Wiener model for the relaxation of soft biological tissues.

The present paper is devoted to introducing discrete-time models for the relaxation function of soft biological tissues. Discrete-time models are suitable for the analysis of sampled data and for digital simulations of continuous systems. Candidate models are searched for within both linear ARX structures and nonlinear Wiener models, consisting of an ARX element followed in cascade by a polynomial function. Both these discrete-time models correspond to sampling continuous-time exponential function series, thus preserving physical interpretation for the proposed relaxation model. The estimation data set consists of normalized stress relaxation curves drawn from experiments performed on samples of bovine pericardium. The normalized relaxation curves are found to be almost insensitive to both the magnitude of strain and the loading direction, and so a single model for the whole relaxation curves is assumed. In order to identify the parameters of the Wiener model an iterative algorithm is purposely designed. Over the ARX one, the nonlinear Wiener model exhibits higher capability of representing the experimental relaxation curves over the whole observation period. The stability of the solution for the iterative algorithm is assessed, and hence physical interpretation as material properties can be attached to the parameters of the nonlinear model. Suitable features of the Wiener model for computational application are also briefly presented.

Experimental procedure for the evaluation of the mechanical properties of the bone surrounding dental implants.

The mechanical stability of the fixture in bone is one of the most important factors for the long-term reliability of dental implants. This paper focuses on an experimental procedure to evaluate the mechanical properties of the bone surrounding dental implants. The procedure is based on a surgical animal model followed by mechanical tests. The experimental mechanical testing has been used for preliminary investigations on the role played by different parameters such as the healing time and the surgical technique (standard or with regenerative material). The procedure has been evaluated in some preliminary tests on a few specimens. Microradiographic analyses have been performed on the bone surrounding the implants in order to give an interpretation of the bone properties on the basis of the bone morphology and to distinguish the newly formed bone from the pre-existing bone. The preliminary results relevant to 10 threaded titanium implants are presented and discussed. Our findings show that the mechanical properties of the bone surrounding the implant improve with the increase in the healing time from 24 to 45 days. The ultimate loads recorded during mechanical tests arise from 395 N to 2665 N in case of coronal defects filled with bone regenerative and from 2200 N to 5700 N in case of standard technique.