3-point Flexure Research Articles

The Effect of Ozone on Bone Strenght in Animal Model of Rheumatoid Arthritis

Objective: Periarticular and systemic osteoporosis are more common in rheumatoid arthritis (RA) than normal population. In this study, we aimed to investigate the effectiveness of ozone on bone strength in Freund’s complete adjuvant (FCA) arthritis, which is considered as the animal model for RA. Materials and Methods: In this study, 28 male Wistar rats were used. Saline was injected into the hindpaws of 14 rats, and Freund’s complete adjuvant was injected into the hindpaws of the other 14 rats, subcutaneously. At the end of two weeks, 40 μg/ml ozone was administered intraperitoneally to 7 of the rats in each group for 6 times totally within duration of three weeks. At the 6th week, serum interleukin-1 (IL-1), IL-6 and tumor necrosis factor-alpha (TNF-α) levels were measured. Right femurs were separated for 3-point flexure test. Results: TNF-α levels of FCA arthritis were significantly higher than that of the control group (p 0.05). Serum levels of IL-1 and IL-6 were not statistically significant among all groups (p>0.05). Maximum force and moment of inertia tended to increase in FCA arthritis-ozone group compare to the FCA arthritis group (p>0.05). The stiffness and toughness were similar in the all groups (p>0.05). Conclusion: To the best of our knowledge, this is the first study in which the effects of ozone on bone strength of RA were investigated. It is determined that ozone is not effective enough, but not harmful on bone strength of FCA arthritis. It is clear that further studies are required with ozone treatment and its use in RA when administrated in different doses and time courses.

Influence of temperature of thermal treatment on surface densification of spruce

Spruce (Picea abies L. Karst) wood lamellae, thermally treated at 170, 190, 210 and 230 °C were surface densified by compression at a temperature of 150 °C to three degrees of compression. Immediate springback, set recovery, mechanical properties in 3-point flexure, Brinell hardness and density profiles measurements were used to determine the effect of thermal treatment on the properties of surface densified wood. The highest immediate springback occurred in wood specimens thermally treated at the highest temperature (230 °C) and decreased with decreasing thermal treatment temperature. The untreated samples had the highest set recovery, which decreased with the temperature of thermal treatment. The surface densification increased hardness and bending strength. The highest increase was in the case of untreated wood and decreased with the temperature of thermal treatment. The modulus of elasticity (MOE) and modulus of rupture (MOR) of surface densified wood decreased with increasing thermal treatment temperature. The trend was similar for specimens which were thermally treated but not surface densified. Surface densification increased the density of the specimens in the first few millimetres below the surface. The highest density was achieved in untreated specimens and the lowest in specimens thermally treated at the highest temperature.

A modular cantilever fixture and test methodology for thermoplastics: An alternative bending load case for validation of CAE material models

Several conventional CAE material models account for only uniaxial tensile behavior of the material; however, the said models may be required to predict component-level response in a complex loading scenario. Therefore in developing input files for such models, it becomes critical to evaluate their performance in alternative validation loading scenarios such as 3-point bending (flexure). Simulating 3-point flexure requires optimization of contact parameters, thereby introducing an extraneous factor that adds uncertainty to the validation of the material model in bending scenarios.This paper targets to evaluate the opportunity to substitute the conventional 3-point flexure with a cantilever bending exercise as the method of choice for bending load case validation. The cantilever bending test is expected to alleviate several challenges associated with CAE validation using 3-point flexure. We describe a novel test fixture for carrying out cantilever bending at low velocities; the fixture can be conveniently mounted on commercial Universal Testing Machines (UTM), and is capable of generating precise load–displacement information. Cantilever and flexure simulations in LS-DYNA employing piecewise-linear-plasticity formulation (*MAT_024) highlight the relative simplicity of validation in bending using cantilever loading. Further, the equivalence of the bending kinematics during cantilever and 3-point flexure is assessed through investigation of stress–strain information.

The influence of preload and boundary conditions on pre-damaged composite plates subject to soft-body impact

Abstract This research investigates the influence of preload and boundary condition compliance on low-velocity impact damaged CFRP T800S/3900-2B plates subject to secondary higher-velocity soft-body impact (50 J, 75 J, 100 J). A bilinear cohesive element based delamination model combined with ply-based composite material failure was developed using LS-DYNA 971. Double Cantilever Beam (DCB), 3-point End-Notched Flexure (ENF), and Fixed Ratio Mixed Mode Bending (FRMMB) simulations validate the accuracy of the delamination model when compared with theory. The low-velocity impact simulations were shown to compare well with the results of drop-tower impact experiments and ultrasonic inspection data in terms of maximum impact force and projected delamination area. An investigation of boundary condition compliance in the direction of impact showed a reduction in peak impact force, interlaminar delamination and intralaminar failure with increasing coupon translation. High-velocity impact simulations based on the model with initial damage showed a reduction in interlaminar delamination damage with tensile preload when compared to compressive preload and unloaded cases for all impact energies.

Study of the mechanical damping behavior of SBR-modified cement pastes by dynamic mechanical analyzer

Improvement in mechanical damping of SBR-modified cement pastes had been evaluated by dynamic mechanical analyzer. Specimens were fabricated and tested under 3-point flexure clamp at frequencies of 0.5–50 Hz or temperatures of −30 to 70 °C. Significant improvement in damping was observed in cementitious-SBR composite specimens when SBR latex to cement ratio was 0.12, which is hypothesized to occur due to improvements in viscosity of cement paste. Furthermore, the SBR-modified cement pastes showed a decreased damping variation tendency with an increase of frequency. They also showed a peak damping variation tendency under the effect of the glass transition temperature. Based on the three element model, mechanical parameters are calculated by fitting the dynamic modulus of SBR-modified cement pastes.

Effect of thermal treatment on the bending strength of glass ceramic/resin cement

Objectives: To investigate the effect of the thermal treatment on the bending strength of ceramic bars (VM7) treated by the cementation protocol. Material and Methods: Forty ceramic bars (20 x 4 x 1.2 mm) were constructed and randomly distributed into 2 groups (n=20): NT - without treatment/control and T – with treatment (50°C above the glass transition temperature, for 15 min). All specimens underwent the cementation process in which one surface of the ceramic bar was etched (20 s) with 10% hydrofluoridric acid, followed by air-water jet washing (60 s), drying (30 s), silane agent application and resin cementation. After storage (distilled water/seven days at 37 ºC), the specimens were submitted to 3-point flexure test at 100% humidity/37 ºC (v=1 mm/min, load cell of 10 kgf). Results: The bending strength values and standard deviations (MPa) were: NT: 19.98 (3.17)a; T: 20.45 (4.67)a (one-way ANOVA and Tukey test, ?=0.05). Conclusions: The thermal treatment did not influence on the bending strength mean values of the ceramics studied.

Slate flexural and anchorage strength considerations in cladding design

This paper describes an experimental investigation on the flexural and anchorage strength of slate for cladding. The study has been conducted on sawed slate specimens, all showing the same surface finishing. Slate flexural strength was compared for two distinct situations: (i) using a 3-Point flexure loading configuration in batches of materials with larger cross-sectional specimen dimension (50 × 30 mm 2); and (ii) using a 4-Point flexure loading configuration in the same batch of materials but with smaller cross-sectional dimensions (30 × 25 mm 2). The 4-Point bending specimens were tested in three different directions considering slate anisotropy planes. Load was applied along the direction perpendicular to the planes of schistosity; and also along two directions parallel to the planes of schistosity. Slate anchorage strength has been determined on slate slabs with 400 × 200 × 30 mm 3 with dowel anchorage in 8 mm diameter cylindrical holes with 35 mm depth. Test load was applied perpendicularly to the schistosity planes. Cladding stone in building facades and its supporting systems must be compatible with the behaviour and performance of other interfacing systems, such as curtain walls and superstructure frames. In this sense, a properly executed dimensional stone cladding should be designed and installed within the capabilities and limitations of the slate’s support system to resist all active forces or actions. The results of this work reveal the importance of complementary characterization techniques for dimension stone cladding, particularly for anisotropic rocks as slates. From the results it is possible to conclude that schistosity planes have an utter influence on either anchorage or flexural strength.

Analysis of Strength Scaling Effect in Portuguese Limestone: Comparison between Three- and Four-Point Bending Tests

This paper presents a comparative study between 3- and 4-point bending tests applied to five Portuguese limestones. The study has been conducted on sawed limestone specimens, all showing the same surface finishing. The materials were compared for two distinct situations: i) using a 3-point flexure loading configuration in batches of materials with larger cross sectional specimen dimension (50 × 30 mm2); and ii) using a 4-point flexure loading configuration in the same batch of materials but with smaller cross sectional dimensions (30 × 25 mm2). In all situations, the materials have broken due to intrinsic volume defects. Formulae for the effective volumes and effective surfaces for rectangular beam specimens loaded in flexure were reviewed in order to analyse the strength scaling effect. The results show the applicability of the Weibull statistics to explain the differences in the results of the 3-point and 4-point bending tests, even when different cross sectional sizes are employed. Among other important remarks, in all the different limestone specimens used it was possible to confirm that the strength values determined experimentally via 3-point bending are of the same order as those estimated for the same loading configuration but via experimental data of 4-point bending tests using the Weibull strength scaling approach, even if employing a different cross-sectional dimension.

Effects of Different Exposure Times and Concentrations of Sodium Hypochlorite/Ethylenediaminetetraacetic Acid on the Structural Integrity of Mineralized Dentin

Introduction This study tested the null hypothesis that there is no difference between the use of 1.3% NaOCl/17% ethylenediaminetetraacetic acid (EDTA) and 5.25% NaOCl/17% EDTA irrigation regimens on the collagen degradation and flexural strength reduction in mineralized dentin. Methods Dentin powder and mineralized dentin sections were immersed in 1.3% or 5.25% NaOCl for 10–240 minutes and then rinsed with 17% EDTA as the final irrigant for 2 minutes. Untreated mineralized dentin powder/sections served as controls in the respective experiments. Dentin powders were examined by using Fourier transform infrared (FT-IR) spectroscopy to analyze their relative subsurface intact collagen content with the apatite/collagen ratio. Hydrated dentin sections were subjected to 3-point flexure under water for determining their flexural strengths. Results Collagen degradation was significantly increased and the flexural strength of mineralized dentin was significantly reduced after the use of 5.25% NaOCl as the initial irrigant for more than 1 hour ( P < .05). Conversely, changes were insignificant when 1.3% NaOCl was used as the initial irrigant for up to 4 hours (Kruskal-Wallis analysis of variance, n = 10, P < .05). Conclusions The null hypothesis was rejected. The deleterious effects attributed to the use of NaOCl on dentin are concentration-dependent and time-dependent and are not associated with the demineralization caused by the use of EDTA as the final active irrigant.

Processing and performance of nanophased braided carbon/epoxy composites

Abstract A systematic study has been carried out to investigate mechanical properties of 2D nanophased braided carbon/epoxy composites. SC-15 epoxy with three types of braided fabrics: ±45°, 0/±45°, and 0/±60° was used to fabricate composite laminates using vacuum assisted resin infusion molding (VARIM) process. Low-velocity impact (LVI), ultrasonic nondestructive evaluation (NDE) and 3-point bend flexure studies were carried out on biaxial and triaxial braided samples. Impact parameters like peak load and absorbed energy were calculated. All the LVI tested samples were then subjected to ultrasonic c-scan testing to determine the damage size. From the results it was seen that laminates sustained the impact load without any damage at 10 J, a little damage at 20 J and more damage at 30 J. From the ultrasonic tests it was seen that the biaxial ±45° laminates had lowest damage. Flexural test showed the highest flexural strength and stiffness for triaxial 0°/±45°. An investigation was also carried out to improve the properties of the braided laminates by introducing Nanomer® I-28E nanoclay, a surface modified montmorillonite mineral, into SC-15 epoxy matrix. Different weight percentages of nanoclay were dispersed in SC-15 epoxy. Highest properties were obtained for samples with 1% by weight of nanoclay reinforcement.

Calcium Requirements of Growing Rats Based on Bone Mass, Structure, or Biomechanical Strength Are Similar

Although calcium (Ca) supplementation increases bone density, the increase is small and the effect on bone strength and fracture risk is uncertain. To investigate if bone mass, morphology, and biomechanical properties are affected by deficient to copious dietary Ca concentrations, the long bones (tibia and femur) of growing female Sprague-Dawley rats (8/group) were assessed after 13 wk of consuming 1, 2, 3, 4, 5, 6, or 7 g Ca/kg of a modified AIN-93G diet. Dietary phosphorous (P) and vitamin D remained constant at recommended concentrations. The assessment included mineralization, density, biomechanical properties of breaking by a 3-point flexure test, and morphological properties by microcomputed topography scanning of trabecular bone of the proximal tibia metaphysis. Dietary treatment did not affect food intake, weight gain, renal and muscle Ca concentrations, and bone hydroxyproline. All bone parameters measured were significantly impaired by Ca deficiency in rats fed the diet containing 1 g Ca/kg. Modest impairments occurred with some parameters (bone density, biomechanical bending moment, modulus of elasticity, and stress) in rats fed 2 g Ca/kg, but all parameters stabilized between 2 and 3 g/kg diet, with no differences between 3 and 7 g/kg. The results suggest that a threshold response in bone Ca retention or bone mass at ∼2.5 g Ca/kg diet is associated with similar threshold responses in bone breaking strength and related biomechanics as well as trabecular structural properties. There was no evidence of a relative P deficiency or of improved or impaired bone strength and structure as Ca intakes increased beyond those needed to maximize bone density.

Flexural and fatigue strengths of denture base resin

Mechanical properties of denture acrylic resins are important for the clinical success of multiple types of prostheses. Acrylic resins must be strong and resilient so as to withstand impact. Few studies utilize cyclic loads to characterize material response to repeated stress. The purpose of this study was to evaluate static and dynamic flexure properties of a variety of acrylic resins utilized in the fabrication of prostheses: (1) heat-polymerized polymethyl methacrylate (PMMA), powder-liquid type, and (2) a newly introduced, visible light-polymerized urethane dimethacrylate dough type. Twenty rectangular bars each of 4 PMMA acrylic resin materials (Diamond D, Fricke HI-I, Lucitone 199, Nature-Cryl Hi-Plus) and 1 urethane dimethacrylate (Eclipse) were fabricated and stored in 100% humidity for 30 days. Half of the specimens for each group were submitted to a static 3-point flexure test. The other half was submitted to cycling loading at 5 Hz for 10(4) cycles. Data were analyzed using 1-way ANOVA and 2-way ANOVA, followed by Tukey HSD or Bonferroni post hoc tests when necessary (alpha=.05). Mean static flexure strength (SDs) in MPa were: Eclipse, 127.11 (5.83); Diamond D, 84.92 (5.10); Lucitone 199, 83.96 (11.96); HI-I, 79.54 (5.84); and Nature-Cryl Hi-Plus, 75.82 (6.96). Mean flexural strengths (SDs) in MPa postcycling were: Eclipse, 113.36 (31.29); Diamond D, 88.26 (5.46); Nature-Cryl Hi-Plus, 81.86 (4.93); HI-I, 79.18 (6.60); and Lucitone 199, 74.34 (4.95). The visible light-polymerized urethane dimethacrylate resin (Eclipse) showed greater flexure strength than all PMMA heat-polymerized resins for both static and cycled groups (P<.001). Yet the Eclipse material had lower load limits, and demonstrated brittle-type behavior and greater standard deviations. The heat-polymerized PMMA materials did not significantly differ from each other after static or cyclic testing.

Investigations on the thermal and flexural properties of plain weave carbon/epoxy-nanoclay composites by hand-layup technique

The matrix of carbon fiber/SC-15 epoxy composites was modified with Nanomer® I-28E nanoclay, a surface modified montmorillonite mineral, to determine the effects of particle reinforcement on the response of these materials to flexural and thermomechanical loading. Different weight percentages of nanoclay were dispersed in SC-15 epoxy using sonication route. The nanophased epoxy was then used to manufacture plain weave carbon/epoxy nanocomposites using hand-layup process followed by vacuum bagging. Control samples of woven carbon fiber/epoxy were fabricated for comparison purposes. Effect of post curing on these samples was also investigated. 3-point bend flexure and Dynamic Mechanical Analysis (DMA) studies were carried out on 8- and 3-layered samples respectively. Results of flexural tests indicate significant improvements in flexural strength and modulus for nanoclay reinforced composites as compared to the control samples. DMA studies also showed enhancement in thermomechanical properties especially in storage modulus though no appreciable change was noticed in glass transition temperature, Tg. Scanning electron microscopy (SEM) studies were carried out to comprehend the effect of nanoclay on the microstructure and the failure modes.

A comparison of debonding strengths of four metal-ceramic systems with and without opaque porcelain

When performing an adjustment on metal-ceramic restorations, opaque porcelain may become exposed, particularly on the lingual surface of maxillary anterior teeth. It is generally believed that exposed opaque porcelain can be more abrasive and destructive to an opposing dentition than body porcelain, and that these teeth may require restoration as a consequence. This study compared the debonding strengths of 2 types of porcelain, with and without opaque porcelain, to 2 types of dental casting alloys. Two porcelain systems, Ceramco3 and Vita 900, and 2 metal alloys, a high noble (Encore) and a base metal (Duceranium U), were used to fabricate and test 56 flexure bars in accordance with ISO 9693:1999(E): Metal-Ceramic Dental Restorative Systems. Half of the bars received opaque porcelain prior to body porcelain additions, and the other half did not. The metal-ceramic debonding strength was determined by using a 3-point flexure apparatus and a mechanical testing device (Instron). A center load was applied at a crosshead speed of 1.5 mm/min(-1) until debonding occurred. In addition to the load-versus-displacement curve, a precision measurement microphone was used to assist in ascertaining the point in time when debonding occurred. Since the sound analysis and the mechanical test were started simultaneously, the debonding load could be more accurately determined. Data were statistically analyzed using 3-way analysis of variance, and all pairwise multiple comparisons were made with the Tukey HSD test (alpha=.05). The difference in mean debonding strength values for opaqued and non-opaqued flexure bars were statistically significant (P=.028). The mean debonding strength values (MPa) for each metal-ceramic system were as follows: Encore-Opaque-Ceramco3 (EOC), 31.43 +/- 6.92(a); Encore-Opaque-Vita 900 (EOV), 30.37 +/- 3.25(a); Duceranium U-No Opaque-Ceramco3 (DNC), 29.20 +/- 6.97(a); Duceranium U-Opaque-Ceramco3 (DOC), 26.61 +/- 4.98(a); Duceranium U-Opaque-Vita 900 (DOV), 26.15 +/- 4.29(a); Encore-No Opaque-Vita 900 (ENV), 25.45 +/- 4.04(a); Encore-No Opaque-Ceramco3 (ENC), 23.96 +/- 4.14(a); Duceranium U-No Opaque-Vita 900 (DNV), 22.88 +/- 6.15(a). Identical superscript letters denote no significant difference among groups. The precision measurement microphone resulted in selection of a debonding strength/crack initiation load that was lower than the peak load recorded during strength testing. Initial debonding during crack initiation strength testing may not always correspond to the peak load recorded, but rather to the point on the load-versus-displacement curve beyond which the relationship is no longer a straight line. Presence of opaque porcelain generally increased the debonding strength for metal-ceramic systems; however, opaque porcelain may not be necessary for a clinically acceptable metal-ceramic bond for some metal-ceramic systems.

Tailoring of the practical adhesion between polyethylene and galvanised steel

The practical adhesion of maleic anhydride grafted polyethylene (MAH-PE) to galvanised steel was studied using 3-point flexure tests, before and after hydrothermal ageing. Before bonding, the electro-galvanised steel was treated with γ-aminopropyltriethoxysilane (γ-APS). The influence of the silane coating thickness and deposition pH on the practical adhesion of MAH-PE to steel was investigated. FT-IR spectroscopy and microscopy enabled to gain understanding of the interphase formation between the silane and the metal substrate. It was found that, at the natural pH of the γ-APS, Zn ions dissolved in the silane coating with subsequent formation of crystals. This interphase could be held responsible for the better durability of the bonds than for silane coatings applied at quasi-neutral pH, for which dissolution of Zn ions was not observed.

On the Displacement Field for Unidirectional Off-Axis Composites in 3-point Flexure - Part II: Numerical and Experimental Results

The displacement field calculated in Part I of this work is numerically analysed for specimens of IM7-8552 epoxy matrix based carbon fiber reinforced unidirectional composite material. Three-point tests have been performed in 5 specimens with different fiber orientation, using 10 spans for each specimen. Theoretical displacement fields obtained in Part 1 have been used for showing level curves on supports and load cylinder lines, in order to analyse the suitability of assumptions concerning boundary conditions and load application. Middle point load-displacement curve slopes obtained from displacement field equations and from Classical Laminated Beams Theory are compared with experimentally obtained results. On the other hand, in-plane shear modulus has been calculated in different ways, knowing the other elastic constants of the material and the experimental load-displacement curve slope. The agreement between numerical and experimental results is rather good in all cases.

Bone development and age-related bone loss in male C57BL/6J mice

The objective of this study was to examine changes in the long bones of male C57BL/6J mice with growth and aging, and to consider the applicability of this animal for use in studying Type II osteoporosis. Male C57BL/6J mice were aged in our colony between 4 and 104 weeks ( n = 9–15/group). The right femur and humeri were measured for length and subjected to mechanical testing (3-point flexure) and compositional analysis. The left femurs were embedded and thick slices at the mid-diaphysis were assessed for morphology, formation indices, and bone structure. In young mice, rapid growth was marked by substantial increases in bone size, mineral mass, and mechanical properties. Maturity occurred between 12 and 42 weeks of age with the maintenance of bone mass and mechanical properties. From peak levels, mice aged for 104 weeks experienced decreased whole femur mass (12.1 and 18.6% for dry and ash mass, respectively), percentage mineralization (7.4%), diminished whole bone stiffness (29.2%), energy to fracture (51.8%), and decreased cortical thickness (20.1%). Indices of surface-based formation decreased rapidly from the onset of the study. However, the periosteal perimeter and, consequently, the cross-sectional moments of inertia continued to increase through 104 weeks, thus maintaining structural properties. This compensated for cortical thinning and increased brittleness due to decreased mineralization and stiffness. The shape of the mid-diaphysis became increasingly less elliptical in aged mice, and endocortical resorption and evidence of subsequent formation were present in 20–50% of femurs aged ≥78 weeks. This, combined with the appearance of excessive endocortical resorption after 52 weeks, indicated a shift in normal mechanisms regulating bone shape and location, and was suggestive of remodeling. The pattern of bone loss at the femoral mid-diaphysis in this study is markedly similar to that seen in cortical bone in the human femoral neck in Type II osteoporosis. This study has thus demonstrated that the male C57BL/6J mouse is a novel and appropriate model for use in studying endogenous, aging-related osteopenia and may be a useful model for the study of Type II osteoporosis.

On the Displacement Field for Unidirectional Off-Axis Composites in 3-Point Flexure — Part 1: Analytical Approach

The displacement field of a unidirectional off-axis specimen in 3-point flexure test is obtained in a new way, by applying the Second Castigliano's theorem and the unitarial load method. The displacement field is calculated for two cases: in the first one, lift-off between specimen and fixture support occurs caused by bending-twisting coupling, and specimen is supposed to contact with the test fixture support at two diagonally opposite points. In the second case, lift-off does not occur and specimen is supposed to contact with the fixture support at four corners. The limit between two cases is established by the calculation of the critical value of the span-to-width ratio for lift-off to occur. The displacement field calculated includes shear effects and satisfies displacements compatibility and equilibrium conditions, except in the vicinity of point reactions and applied load. Neither displacement field solutions for composite plates with point boundary conditions, nor solutions based on the Second Castigliano's theorem concerning composite plates have been encountered in the literature survey.

45 Flexure Test for Measurement of In-Plane Shear Modulus

A new method to obtain the in-plane shear modulus GLT for unidirectional oriented composite materials is proposed. The method is based on an original analytic way for calculating middle point displacement in a 3-point flexure test. The bending–twisting coupling effects in such a test induce the lift-off of the specimen at the fixture supports for some geometrical conditions. Thereby, contact points are located at two opposite points of the specimen. Consequently, new bending moments along the width of the specimen and twisting moments appear. By supposing resultant moments and shear forces per unit length are uniformly distributed, these distributions are calculated for static conditions along longitudinal and transverse cross sections of the specimen. After having expressed strain energy as a function of resultant moments and resultant shear forces per unit length, Second Castigliano’s theorem is applied in order to calculate the middle point displacement. No similar analytic way has been encountered in classical laminated beams theory or in classical laminated plates theory. The displacement obtained in this work and the one obtained from classical laminated beams theory are particularised to the case of 45 fibre orientation. GLT expressions have been derived from those displacement expressions in three ways: two of them from the solution of this work, not considering and considering shear effects, respectively, and the third one from displacement obtained from classical laminated beams theory. Experiments have been made for different geometric conditions in order to test the influence of geometric parameters in experimental results. For span-to-width ratios up to two, the values obtained are quite constant and agree well with the in-plane shear modulus value obtained by the material manufacturer using 45 tensile test.

Strength and fatigue of polyacid-modified restorative materials (compomers).

The purpose of this study was to compare the fatigue behavior of a hybrid composite, four compomers, and two viscous glass ionomers after short- and long-term soaking in distilled water. Bars with dimensions of 30 mm x 2 mm x 2 mm were formed in stainless steel molds, finished with 600 grit SiC, then soaked for either 24 h or one year, and tested in 3-point flexure at stressing rates between 0.001 and 2500 MPa s(-1). Data were plotted as fracture stress vs. stressing rate, and the exponent N in the power law for crack growth rate was computed from the slope of these plots. All compomer and resin composite materials tested exhibited subcritical crack growth (i.e. a reduction in strength with a decrease in stressing rate). Soaking lowered the slope for all materials, that is, increased the crack propagation rate. F2000 exhibited an increase in fracture strength while Hytac and Compoglass exhibited a decrease following long-term soaking in distilled water. Crack-growth exponents obtained from these fatigue data were used to estimate the stresses which would result in a five-year lifetime for these materials.