Composites and Plastics

Determination of Young's modulus of dental composites : a phemenological model,
M. Braem, V.E. Van Doren, P. Lambrechts, G. Vanherle

The Young’s Moduli of isotropic dental restorative composites are determined with a non-destructive dynamic method, which is based on the measurement of the duration of the fundamental period for the first harmonic of a freely oscillating sample. Statistical analysis of these results yields a phenomenological model in which Young’s modulus is given by an exponential rule of mixtures of the matrix phase and the filler phase of the composites. It is found that this phenemenological rule is substantiated empirically.

The impact of composite structure on its elastic response,
M. Braem, P. Lambrechts, V. Van Doren and G. Vanherle

The non-destructive determination of Young’s modulus of dental composites by measn of the fundamental period was found to be reliable and accurate. Post-polymerization effects could clearly be detected. Exponential regression analysis showed a correlation coefficient of 0.92, after logarithmic transformation, with volumetric filler content.
The high accuracy and reliability of the measurments themselves are reflected in low standard deviations. The results are in excellent agreement with those of other investigations. Furthermore, the ease and speed of operation make this new procedure a powerful laboratory tool for material-testing and practical large-scale investigations.

The relationship between test methodology and elastic behavior of composites,
M. Braem, C.L. Davidson, G. Vanherle, V. Van Doren and P. Lambrechts

Comparisons were made of the Young’s moduli obtained with tests that impose static, low-frequency, or high-frequency elastic deformations on dental composite systems.
The frequency of the imposed stress was reflected in the absolute value of Young’s modulus. However, the values obtained at different test frequencies could be compared and understood by taking into account this frequency dependence. It was thus found that the composite structure largely determined the type of reaction to the imposed stress. The fundamental period test permitted the greatest differentiation in the classic behavior of the investigated composites.

Alternate Test Methods for Determining Mechanical Properties of Graphite Epoxy Materials,
Michael J. Viens, Goddard Space Flight Center

He work performed for this memorandum investigated the use of three devices which could potentially reduce both cost and turn around time for testing graphite epoxy (Gr/Ep) tensile coupons. Gr/Ep test coupons are typically tabbed to facilitate load transfer to the test coupon and are strain gauged to determine stiffness and strain at failure of the test coupon. The implementation of the aforementioned devices would eliminate the need for both test tabs and the strin gauges.
The tensile loading of tabless coupons was performed using hydraulic grips with hybrid wedge surfaces to transfer load the test coupons. The strain in the test coupons was monitored using a laser extensometer. The coupon stifness was measured prior to tensil testing with a sonic resonance measurement system. This memorandum reports the results of these tests and compares the strength results to that of prvious tests and the strain results obtained using strain gauges.

Determination of Elastic Moduli of Fiber-Resin Composites Using an Impulse Excitation Technique
Michael J. Viens, Jeffrey J. Johnson

The elastic moduli of graphite/epoxy and graphite/cyanate ester composite specimens with various laminate lay-ups was determined using an impulse excitation/acoustic resonance technique and compared to those determined using traditional strain gauge and extensometer techniques. The stiffness results were also compared to those predicted from laminate theory using unaxial properties. The specimen stiffness interrogated ranged from 12 to 30 Msi. The impulse excitation technique was found to be a relatively quick and accurate method for determining elastic moduli with minimal specimen preparation and no requirement for mechanical loading frames. The results of this investigation showed good correlation between the elastic modulus determined using the impulse excitation technique, strain gauge and extensometer techniques, and modulus predicted from laminate theory. The flexural stiffness determined using the impulse excitation was in good agreement with that predicted from laminate theory. The impulse excitation/acoustic resonence interrogation technique has potential as a quality control test.

Particulate-Reinforced Titanium Alloy Composites Economically formed by Combined Cold and Hot Isostatic Pressing,
Stanley Abkowitz, Paul F. Weihrauch, Susan M. Abkowitz, Dynamet Technology Inc.
Comparison of the Young’s modulus for six reinforced dental materials
Brett I. Cohen, Mark Pagnillo, Barry Lee Musikant, Allan S. Deutsch, Gary Cofrancesco

This in vitro study concerns the modulus of elasticity, Young’s modulus, and compares physical characteristics for six reinforced dental materials. Young’s modulus is the measurement of stress per strain that the material can withstand before catastrophic failure. In a through study by Braem et al. over 40 posterior composites were tested and the Young’s modulus was measured using a non-destructive technique. Feilzer et al. in a similar study tested four posterior composites. In these studies no measurements were performed for core materials or cementation agents.

Development of ZrO2 –ZrB2 composites
B. Basu, J. Vleugels, O. Van Der Biest

Yttria-stabilised tetragonal zirconia (Y-TZP) composites with 30 vol.% ZrB2 are prepared by hot pressing in vacuum for 1 h at 1450°C. Different commercial zirconia starting powders as well as a range of ZrO2 ‘powder mixture’ grades, based on co-precipitated powders were used for the composite production. The measured differences in the mechanical properties of the obtained composites is explained in terms of the microstructure, the residual stresses due to the presence of ZrB2 , and the stabiliser content and its distribution. While the stabiliser content and distribution remained significant, the residual stress in the zirconia matrix is found to be an important additional factor influencing the tetragonal zirconia transformability. Crack deflection by the ZrB2 phase was identified as an active toughening mechanism in the composites.

Toughness Optimisation of ZrO2-TiB2 composites
B. Basu, J. Vleugels and O. Van Der Biest, Key Engineering Materials Vols. 206-213 (2002) pp. 1177-1180

Yttria-stabilised tetragonal zirconia (Y-TZP) composites with 30 vol. % TiB2 are fabricated by hot pressing in vacuum for 1 hour at 1450°C. Commercial co-precipitated zirconia starting powder as well as a range of experimental powder mixture grades of monoclinic and coprecipitated 3Y-TZP powders was used to study the influence of the zirconia matrix composition on the fracture toughness of the composites. The observed variation in mechanical properties of the composites is explained in terms of the microstructure, the residual stresses due to the presence of TiB2 and the stabiliser content. Whereas the overall yttria content is very crucial, the residual stress is found to be an additional important parameter influencing the t-ZrO2 transformability that needs to be taken into account when optimising the toughness of the composites. Beside transformation toughening, crack deflection was identified as an active toughening mechanism in the composites. A simple approach to tailor the toughness of ZrO2-TiB2 composites is proposed.

Dynamic and static moduli of elasticity of resin-based materials
J. Sabbagh, J. Vreven, G. Leloup, Dental materials 18 (2002) 64-71

Objectives: The purpose of this study was to assess and compare the elastic moduli of 34 resin-based materials using a dynamic and static method. The effect of water storage was also studied up to 6 months.
Methods: Five samples of each material were prepared according to ISO-4049. The dynamic moduli we first determined non-destructively from the fundamental period of the vibrating specimen, then the static moduli were determined by a three-point bending test. The percentages of fillers by weight were determined by ashing in air at 900ºC.
Results: Low values were obtained with flowable composites as well as with two packable resin composites. Correlations were found between the static and the dynamic modulus of eslasticity (r=0.94; p=0.0001) as well as between weight percentage of fillers and the moduli of elasticity (r=0.82; p<0.05 for static modulus and r=0.90; p<0.05 for the dynamic modulus) both at 24h. Water storage significantly affected both static and dynamic modulus of elasticity (ANOVA two factors; p<0.05)
Significance: The low moduli of flowable composites do not allow their use in posterior cavities under high stress. However, this does not exclude their use for minimally invasive Class I cavities when the opposing tooth is stabilized to a large amount on the natural enamel. The Grindosonic method is very useful and simple for determining the dynamic moduli although it gives higher values than the static one. The elastic modulus evolution of resin-based materials after water storage is unpredictable since different patterns were observed as a function of time.