María Jesús Lamela

Stress relaxation behaviors of articular cartilages in porcine temporomandibular joint

In this study, we tested the compressive stress relaxation behaviors of the mandibular condylar and temporal cartilages in the porcine temporomandibular joint (TMJ). The aim was to determine the quantitative and qualitative similarities and differences of compressive stress relaxation behaviors between the two cartilages. Ten porcine TMJs were used; the articular surface was divided into 5 regions: anterior, central, posterior, lateral and medial. Compressive relaxation test was carried out at a strain level of 5% in each region of the two cartilages. The stress relaxation was monitored over a period of 5min. In all the regions of the two cartilages, the time-dependent stress relaxation curves showed a marked drop in stress within the initial 10s, which can be fitted by a standard linear viscoelastic model. The instantaneous moduli in the temporal cartilage were dominantly larger than those in the condylar cartilage, while the condylar cartilage had slightly larger relaxation moduli than the temporal cartilage except for the medial region. The both cartilages showed the regional differences in the compressive stress relaxation behavior, and in the temporal cartilage the lateral and medial regions revealed the largest values for the instantaneous and relaxation moduli. The present results demonstrate that the viscoelastic properties of compressive stress relaxation in both cartilages are region-specific, which might have an important implication for stress distribution and transmission along with the TMJ disc.

The region-dependent dynamic properties of porcine temporomandibular joint disc under unconfined compression.

In this study, the dynamic compressive properties in five different regions of the porcine temporomandibular joint (TMJ) disc are investigated over a wide range of loading frequencies. The aim was, thus far, to evaluate the regional difference and the frequency-related effect of the applied load on these properties. Eleven porcine TMJ discs were used; each disc was divided into 5 regions, anterior, central, posterior, lateral and medial. Sinusoidal compressive strain was applied with an amplitude of 1.0% and a frequency range between 0.01 and 10Hz. The dynamic storage and loss moduli increase with frequency, the highest values being attained at the posterior region, followed by the central and anterior regions. Loss tangent, tanδ, ranged from 0.20 to 0.35, which means that the disc is primarily elastic in nature and has a small but not negligible viscosity. The present results suggest that the dynamic viscoelastic compressive modulus is region-specific and depends on the loading frequency, thus having important implications for the transmission of load to the TMJ.

A Design Model for Glass Elements Based on the Statistical Distribution of Crack Sizes

A probabilistic model for the design of building glazing plates of monolithic glass in buildings is presented here. The model is implemented for practical use in Windesign, a computer program developed by the authors. A novel contribution of the model is the consideration of maximal crack sizes as a reference parameter to predict material fracture instead of the critical strengths of the materials, as is usually the case. A sensitivity analysis is also included to assess the relative influence of the different parameters on the model.

Analysis of Compressive Properties of Porcine Temporomandibular Joint Disc

Since the temporomandibular joint (TMJ) disc material exhibits a non-homogenous and viscoelastic structure, the compressive properties in five different regions of eleven porcine TMJ discs were investigated over a wide range of loading frequencies. The results obtained suggest that the dynamic viscoelastic compressive modulus is region-specific and depends on the loading frequency, thus having important implications for the transmission of load in the TMJ. The dynamic storage and loss moduli increase with frequency, the highest values being attained at the posterior region, followed by the central and anterior regions. Loss tangent, tan δ, ranged from 0.20 to 0.35, which means that the disc is primarily elastic in nature and has a small but not negligible viscosity.

Strength Characterization of Glass by Means of the Statistical Theory of Confounded Data

A central point in a probabilistic model for designing brittle materials is how to analytically determine the statistical distribution of the critical strength from experimental data. The cutting process used in the preparation of specimens is frequently the source of edge defects causing failures at stresses generally lower than those that have originated from micro-cracks at the surface. Since this kind of failure occurs almost exclusively in certain testing procedures but is not present in the real, edge-polished glazing elements, adequate treatment of this data is needed to derive the cumulative distribution function (cdf) characterizing the surface strength of the material. Neglecting data associated with the edge failures results in an erroneous estimation of the cdf. This paper describes the statistical procedure for the evaluation of this kind of data (known as confounded data) that permits the correct estimation of the cdf of the surface strength taking into account all the results obtained in the tests, irrespective of their origin. The theoretical model is applied to experimental data resulting from 4-point bending tests on glass specimens. Introduction It is commonly accepted that the brittleness of glass elements is caused by the presence of superficial micro-cracks formed during the manufacturing or manipulating processes. Any probability model used in design requires the statistical distribution of the maximum crack size [1] or of the critical stress [2], as a possible alternative. This characterization succeeds experimentally when testing circular glass plates until failure under concentrated central load, or small glass beams, tested under 4or 3-point bending loads, extracted from on-going production. The latter kind of test appears to be easy to perform but presents the inconvenience that some of the failures are originated from the non-polished edges, as can be easily ascertained by mere observation of the fractures. Polishing the specimen edges substantially reduces the number of edge failures but is generally questionable because of the increasing costs involved. The cdf obtained is related to the specific specimen size and stress distribution, which means that the size effect has thereby to be considered. On the other hand, disregarding the results originating from the edges is statistically unacceptable, since this would lead to a spurious cdf of the maximum crack size [3]. This situation can also be extended to ceramics, the design of which is more complex that of glass due to the need to discriminate between fractures originating from surface cracks and fractures caused by volume cracks. The distinct maximum crack size observed, depending on which glass side (tin or air) has been exposed to tension under bending, affords us a better understanding of the problem. For instance, in the glass quality that motivated this study, only 12% approx. of the failures resulting from the tests conducted with the tin side under tension could be assigned to edge flaws, whereas 58% of failures of this kind occurred in the case of air side under tension. Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 1923-1926 doi:10.4028/www.scientific.net/KEM.264-268.1923