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PubMed1995

An analytical model of intervertebral disc mechanics.

The intervertebral disc is a complex mechanical structure, and it is important to understand the loading of specific structures which might cause damage leading to failure or mechanical impairment. At present it is only possible to model such internal loadings owing to the extreme technical difficulties involved in experimental measurement. The simple analytical model described in this paper makes exact predictions of the loads carried by fibres and also their path within the annulus fibrosus, without pre-defining the fibre configuration. The disc is modelled as an axially symmetric structure comprising a fluid filled centre, retained by a thin, doubly curved, fibre-reinforced membrane under tensile stress. The annulus is taken to consist of two lamellae reinforced by oppositely oriented collagen fibres that are free to follow paths defined by one of two geometrical rules. The predictive power and possible uses of the model are illustrated using boundary conditions experimentally determined from a typical young disc. The model was used to calculate the shape of the membrane surface, fibre path, volume of disc, area of annulus, length of fibre bundle and tension at a point along length of fibre. Equatorial fibre angle could be approximately predicted (to about 5 degrees), since there was only a small range of valid solutions to the model. The predicted surface profiles, fibre loads and angles were found to be in reasonable agreement with published experimental studies. Two examples of how the static model might be used to calculate changes in disc morphology and loading are included to demonstrate how a wide range of experimental data and theoretical behavior might be incorporated. This analytical model is important since it enables exact solutions to be calculated for the forces acting at any point along a fibre, their paths and also the surface geometry, from a small number of physical measurements without the need to estimate the mechanical properties of individual areas of the disc. It facilitates the prediction of the behaviour of the disc under varying load by providing a framework that can be further developed using a wide range and combination of experimental conditions and theoretical relationships.

Adult↗
PubMed1996

Direct mechanics assessment of elastic symmetries and properties of trabecular bone architecture.

A method is presented to find orthotropic elastic symmetries and constants directly from the elastic coefficients in the overall stiffness matrix of trabecular bone test specimens. Contrary to earlier developed techniques, this method does not require pure orthotropic behavior or additional fabric measurements. The method uses high-resolution computer reconstructions of trabecular bone specimens as input for large-scale FE-analyses to determine all the 21 elastic coefficients in the overall stiffness matrix of the specimen, using a direct mechanics approach. An optimization procedure is then used to find the coordinate transformation that yields the best orthotropic representation of this matrix. The method is illustrated here relative to two trabecular bone specimens. The techniques developed here can be used to obtain a complete characterization of the mechanical properties of trabecular architecture. With the development of in vivo reconstruction techniques, even in vivo measurements will be possible.

Algorithms↗
PubMed1995

Highly mobile space suit material optimization.

This paper discusses the factors that control the flexibility of fabric space suit elements by examining a bending model of a pressurized fabric tube. Results from the model are used to evaluate the current direction in highly mobile EVA glove research and suggest that changes are necessary in the suit and glove fabric selection methodology.

Computer Simulation↗
PubMed1993

Impact strength of acrylic resin denture base materials with surface defects.

The first part of this study involved the development of a method for evaluating the effect of small surface defects (3.0 microns-80.0 microns) on the impact strength of acrylic denture base materials. The method was used to compare the properties of 12 denture base materials, some of which are described as "high-impact strength" products. The defects were introduced by drawing the specimens over abrasive papers of varying grit sizes or by notching with a craft knife. Flexural toughness tests were also carried out in order to evaluate the materials under both rapid (impact) and slow (flexural) rate test conditions. The impact resistance of acrylic resins was significantly reduced by the presence of very small surface defects (e.g., < 16 microns), and this was particularly noticeable for so-called high-impact resins. The latter have values of impact strength only slightly higher than standard acrylic products when defects are present. Testing "notched" specimens with small surface defects was a simple method of obtaining reproducible results. This test was sensitive enough to differentiate among several types of currently available denture base resins.

Acrylic Resins↗
PubMed1993

Acrylic resin reinforced with chopped high performance polyethylene fiber--properties and denture construction.

A newly developed incremental mixing technique has been used to incorporate over 30 vol % chopped high performance polyethylene fiber into acrylic denture base resin. The reinforcement produced a substantial improvement in several clinically important properties, namely: 1) stiffness and impact strength were higher; 2) the mechanical properties were insensitive to notches that mimic anatomical features; and 3) samples damaged during bending and impact did not break up into separate fragments. Reinforced complete maxillary and mandibular dentures of good esthetic appearance were successfully manufactured and polished with conventional laboratory techniques.

Acrylic Resins↗
PubMed1994

Mechanical behavior of glass ionomer cements affected by long-term storage in water.

OBJECTIVES: The early strength of glass ionomer cements was investigated previously (Cattani et al., 1993). Using the same set of glass ionomer cements, this study was designed to determine if mechanical property degradation occurred after an extended period of storage in water. METHODS: In the present study, the effect of aging in water on the compressive, tensile and flexural strengths of twelve commercial glass ionomer cements was evaluated at different time intervals after the beginning of the setting reaction. The aging periods varied from 24 h to 12 mon. RESULTS: The results show that the mechanical properties of the tested materials are in some cases affected by long-term aging in water. The evolution of strength follows distinct patterns of change. It has been established that the glass ionomer cements are neither characterized by a continuous increase nor a continuous decrease in strength. Most of the materials tested maintained a constant strength. However, several of them first display an improvement of the mechanical strength followed by a reduction. In this case, the 24h and the 12 mon strengths are usually similar. SIGNIFICANCE: The aging mechanisms of glass ionomers are complex. Strengthening probably results from additional crosslinking and build-up of a silica gel phase, whereas weakening may result from erosion and plasticizing effect of water.

Acrylic Resins↗
PubMed1994

Effect of hydration on the biaxial flexural strength of a glass ionomer cement.

OBJECTIVES: The purpose of this study was to test the hypothesis that hydration during setting enhances the strength of glass ionomer cements. METHODS: Thirty-four groups of eight disks each were prepared and sealed in nonpermeable bags to age for 30 min to 360 d before biaxial flexure testing. At each time interval, one group was tested immediately, while the second group was immersed in water for an additional 48 h prior to the same testing. Six of each group were used for strength testing and the remaining two were placed in a desiccator for determining evaporative water content with respect to initial weight. RESULTS: A major portion of the disk strength developed within the first 2 wk and continued to improve at a much slower pace with time. Tukey's multiple range test indicated that additional 48 h immersion caused significant strength reduction for disks aged less than 30 d. The amount of weight loss in the desiccator decreased with aging time. The profiles of biaxial flexure strengths with respect to weight loss are similar for both groups. It infers that additional water storage introduces surface flaws that result in the strength reduction. SIGNIFICANCE: Glass ionomer cement must be kept from direct water contact as long as possible to reach an acceptable level of strength.

Analysis of Variance↗
PubMed1994

Properties of posterior composite: results of round robin testing for a specification.

OBJECTIVES: Seven sites participated in the round robin testing of five dental composites in order to evaluate specific testing protocols for use in a specification for posterior composites. METHODS: The flexure strength, flexure modulus, solubility in water, and opacity after soaking in water and ethanol, were evaluated for five commercial dental composites at the seven different sites. Samples were either aged for one day or seven days before testing. RESULTS: Although they were not without problems, the results were supportive of including in a specification a test for solubility in water as well as one for flexural strength and flexural modulus. The specification would be similar to that described by ISO 4049, but based upon the results of this study, an increase in the acceptable values for two of these tests was suggested. The results of this study also demonstrated that despite following an identical protocol and using materials from identical batches, significant variations in absolute values were obtained among the Test sites. However, there was generally good agreement among the sites in the relative ranking of the materials. SIGNIFICANCE: These results provide a strong rationale for the inclusion of one or two standard materials in a specification designed to evaluate composites suggested for use in posterior teeth.

Acrylic Resins↗
PubMed1994

Effect of surface treatments on the repair strength of a light-activated denture repair resin using censored data.

OBJECTIVES: The aim of this study was to evaluate the effect of three different chemical surface modifications on the bond strength of light-activated denture repair resin (Triad, Dentsply International) to a heat-cured resin (Lucitone, Dentsply International). METHODS: Transverse flexure test specimens were made, with surface treatment of 1) Triad bonding agent, 2) unreacted Lucitone monomer, and 3) a 1:1 mixture of methylene chloride and monomer, using two different application times (2 and 4 min). For all except one group, 18 specimens were made. The controls were solid single composition bars, 18 for each material. The elastic modulus in transverse flexure was measured for each material. The statistical analysis employed the Weibull distribution. Fracture strength values were obtained in a three-point flexure test. RESULTS: It was noted that many fractures occurred away from the interface and the center member of the test fixture. For those fractures, the observed fracture strength is a censored value of the fracture strength. Algorithms for the statistical treatment of censored data were used to get the maximum likelihood estimates of the fracture strength. A mechanical model showed that under three-point bending of a beam of two joined materials, fractures will occur in the part with the higher elastic modulus. This finding is indeed observed in the data. The 4 min monomer treatment showed the highest estimate of the bond strength (187.8 MPa). SIGNIFICANCE: Censored data can be used to obtain fracture strength estimates, and the censoring indicates that the bond strength may exceed the strength of the bulk material.

Acrylic Resins↗
PubMed1994

Flexural behavior of visible light-cured composites as a function of temperature under water immersion test conditions.

OBJECTIVES: The purpose of this study was to investigate the effect of temperature of the flexural behavior for four visible light-cured hybrid composite materials. METHODS: Light-cured samples were post-cured for 24 h at 37 degrees C in 100% relative humidity prior to testing. Flexural tests were performed at the following water immersion temperatures: 5, 15, 25, 37, 45 and 55 degrees C, as well as in the dry condition which served as control. The following flexural property parameters were obtained as a function of test temperature: flexural strength, sigma fs, flexural modulus, Efs, flexural stress at 0.06% total offset strain, 0.06% (yield stress), and the total displacement at fracture, delta fs. Statistically significant differences (p < 0.01) in flexural parameters were noted for all the materials as a function of test temperature. RESULTS: The yield stress, sigma 0.06% was found to be most sensitive to thermomechanical changes. The results of this study indicate significant softening due to temperature under wet immersion test conditions for all materials. The temperature range at which two of the materials show significant reduction in yield stress compare well with "Wet Tg" values obtained by dynamic thermal mechanical analysis (Culbertson et al., 1990; 1991). SIGNIFICANCE: The effect of damage accumulation on surface or subsurface microstructure due to transient thermomechanical loads needs to be taken into account in the correlation of in vitro and in vivo wear or creep behavior.

Absorption↗
PubMed1994

Tooth stiffness with composite veneers: a strain gauge and finite element evaluation.

OBJECTIVES: This study was conducted to determine the impact of composite veneer procedures on the functional properties of incisors. METHODS: Ten extracted human maxillary central incisors were mounted in pairs in a nylon ring. One strain gauge was bonded along the long axis of each tooth on the center of the lingual surface. Each pair formed half of a Wheatstone bridge circuit and was wired to eliminate all but the voltage resulting from experimentally applied procedures. The teeth were ramp-loaded to 50 N near the incisal edge on the lingual surface. Loading was performed on the unaltered teeth, teeth with preparations and restored teeth. Two-dimensional finite element (FE) models were generated to evaluate each test condition. Relative stiffness, compared with the unaltered tooth, was calculated from measurements with the strain gauge steps and from the FE models. RESULTS: A relative stiffness value of unity represents recovery of stiffness to the level of the unaltered tooth. Both methods of evaluation demonstrated a decrease in mean relative stiffness with each subsequent reduction in tooth structure. The composite restoration increased its mean relative stiffness compared to its corresponding preparation but never to the level of the unaltered tooth. Across all procedures, the two-dimensional FE model correlated well in both direction and magnitude with the experimental strain gauge method (R = 0.83). SIGNIFICANCE: A resin composite veneer does not restore the stiffness to the level of an unaltered tooth.

Analysis of Variance↗
PubMed1995

Elastic responses to longitudinal torsion of single-strand, rectangular, orthodontic archwire segments.

OBJECTIVES: This study was undertaken to characterize elastic responses of orthodontic archwire segments in longitudinal torsion, to compare experimental results with predictions from structural engineering theory, and to examine the potential interaction between flexural and torsional responses of archwires. METHODS: Passively straight and deflected rectangular wire segments were activated in torsion to states beyond their elastic limits. The wire parameters that were controlled included: the alloy, the cross-sectional size, and the gauge length. The research design included 48 cells and 240 separate tests. From torque-twist plots, values of elastic stiffness, elastic range, and unit elastic range were obtained. Raw experimental data were subjected to analyses of variance and means to a Tukey's post-hoc test. Mean stiffness and elastic range outcomes were compared with theoretical values. RESULTS: Most plots were generally characteristic of Hookean materials. All three wire parameters significantly influenced the three dependent variables; few statistical interactions emerged. Theoretical stiffness values were reasonably comparable to those obtained experimentally; however, the elastic range predictions were conservative. Torsion theory predicts unit elastic ranges independent of gauge length; the experimental data displayed a nonlinear relationship. The minor influences of flexural deformations on the responses of wire segments activated in torsion are suggested as clinically inconsequential. SIGNIFICANCE: Few clinically relevant, controlled studies of archwire torsion have been published. A modified or new formula is needed to predict elastic range magnitudes of archwires in torsion. When flexure and torsion exist in an archwire, it may be possible to separate them to determine overall structural response.

Alloys↗
PubMed1995

Properties of heat-treated composites after aging in water.

OBJECTIVES: Post-cure heat treatments have been shown to increase the fracture toughness and elastic modulus of composites. The objective of this study was to determine if the increase remained after the composites were aged in water. METHODS. The fracture toughness (K(lc)), flexural modulus and flexural strength of four experimental and one commercial composite (Z-100, 3M Dental Products) were tested after 1, 7, 30, 60 and 180 d of aging in 37 degrees C water. The four experimental composites were made with a BisGMA/TEGDMA resin and were characterized as follows: Micro = 38 vol% silane-treated silica, Fine = 65 vol% silane-treated quartz of 1-2 micrometer average size, Hybrid = 65 vol% silane treated quartz of a mixture of 1-2 micrometer average and 8 micrometer average size, and Large = 65 vol% quartz of 8 micrometer average size (of which only 75% were silane-treated). All specimens were light-cured (normal-cured; Triad II - 80 s). One set of each composite was further heat-cured at 120 degrees C for 10 min (heat-cured). A third set of the Hybrid was heat-cured with simultaneous light exposure (Elipar, Espe) for the first 3 min. RESULTS: By 30 d, normal-cured and heat-cured specimens showed significant (ANOVA/Tukey's test; p < or = 0.05) reductions in fracture toughness (avg. 16% and 22%, respectively), flexural modulus (avg. 11% and 11%, respectively) and flexural strength (avg. 25% and 29%, respectively). Further aging had little effect. The use of additional light-curing during heating did not affect the properties more than heat-curing alone. SIGNIFICANCE: The improvements in some of the properties of composites produced by heat-treating are of only short-term benefit, and are for the most part negated due to an alteration of the resin matrix as the composite equilibrates with water.

Analysis of Variance↗
PubMed1995

Fracture mechanics principles.

The principles of linear elastic fracture mechanics (LEFM) were developed in the 1950s by George Irwin (1957). This work was based on previous investigations of Griffith (1920) and Orowan (1944). Irwin (1957) demonstrated that a crack shape in a particular location with respect to the loading geometry had a stress intensity associated with it. He also demonstrated the equivalence between the stress intensity concept and the familiar Griffith criterion of failure. More importantly, he described the systematic and controlled evaluation of the toughness of a material. Toughness is defined as the resistance of a material to rapid crack propagation and can be characterized by one parameter, Kic. In contrast, the strength of a material is dependent on the size of the initiating crack present in that particular sample or component. The fracture toughness of a material is generally independent of the size of the initiating crack. The strength of any product is limited by the size of the cracks or defects during processing, production and handling. Thus, the application of fracture mechanics principles to dental biomaterials is invaluable in new material development, production control and failure analysis. This paper describes the most useful equations of fracture mechanics to be used in the failure analysis of dental biomaterials.

Ceramics↗
PubMed1995

In vitro fatigue behavior of restorative composites and glass ionomers.

OBJECTIVES: This in vitro study was conducted to investigate the fatigue behavior of several dental restoratives, including composites, glass ionomers and a resin-reinforced glass ionomer. METHODS: Fatigue was imposed under a reverse stress-controlled regimen, following a staircase approach. Samples were stored and tested under both dry and wet conditions. The following parameters were measured and analyzed: Young's modulus, restrained fracture strength, and flexural fatigue limit. RESULTS: As a general trend, all products showed a decrease in Young's modulus following water sorption. For all products except the resin-reinforced glass ionomer, the same trend was seen in the restrained fracture strength. This is, however, no longer valid for the flexural fatigue limit: the trend is steady-state for the glass ionomers, status quo for the resin-reinforced glass ionomer, and all composites tested show a decrease. SIGNIFICANCE: The diversity in structure of both composites and glass ionomers does not allow findings for one product to be extrapolated to other similar products.

Bisphenol A-Glycidyl Methacrylate↗
PubMed1995

Predicting lifetimes of materials and material structures.

The mechanical strength of brittle materials under stress is of prime importance in applications where allowable design stress, lifetime, and reliability are critical issues. A proper analysis should enable an engineer to select an allowable design stress that will permit a brittle component to function for the expected lifetime with an acceptable low probability of failure. It is the purpose of this paper to provide the background for assessing the mechanical reliability of brittle materials under tensile strength.

Ceramics↗
PubMed1995

Critique of test methods for lifetime predictions.

Failure predictions for ceramics depend on the experimental parameters that measure the strength distribution (m and sigma 0) and the time-dependency of strength (n). These parameters can be determined by measuring strength as a function of stressing rate in a test environment that simulates the service environment. To minimize the uncertainty in these experimental parameters, at least 30 samples per stressing rate should be tested over a stressing rate range of at least 3 orders of magnitude. The uncertainty in these experimental parameters can be taken into account in design calculations by the use of appropriate safety factors. Thus, through well designed experiments coupled with a reliability analysis, rational design decisions can be made that ensure the successful use of ceramics in demanding structural applications.

Ceramics↗
PubMed1995

Cyclic fatigue of a model feldspathic porcelain.

OBJECTIVES: This study was conducted to evaluate the fatigue parameters of a model porcelain based on the Weinstein patent using cyclic fatigue and to compare the parametric values obtained from cyclic fatigue tests with those from dynamic fatigue tests previously reported by Fairhurst et al. (1993). METHODS: Cyclical biaxial flexure of 1 mm thick and 12 mm diameter disks was performed at 37 degrees C in distilled water at a frequency of 4 Hz with constant stressing rates between a minimum and maximum stress. Three groups of samples (50, 40, 40) were tested with a maximum stress of 51, 47, and 43 MPa, respectively. The crack growth exponent, n, and the scaling constant, sigma fo, were derived from the regression constants obtained from a linear regression of the logarithm of the median time to failure with the logarithm of the maximum stress. RESULTS: No significant differences were found between the cyclic fatigue parameters, n and sigma fo, derived from the median time to failure and those obtained from dynamic fatigue data. SIGNIFICANCE: Within the limits of error in this determination, the median cyclic fatigue life can be estimated by the use of fatigue parameters obtained from dynamic fatigue testing.

Analysis of Variance↗