Miniature specimens for fracture toughness evaluation of dental resin composites

A Bilayer Method for Measuring Toughness and Strength of Dental Ceramics

The ever-increasing usage of ceramic materials in restorative dentistry necessitates a simple and effective method to evaluate flexural strength σF and fracture toughness KC. We propose a novel method to determine these quantities using a bilayer specimen composed of a brittle plate adhesively bonded onto a transparent polycarbonate substrate. When this bilayer structure is placed under spherical indentation, tunneling radial cracks initiate and propagate in the lower surface of the brittle layer. The failure analysis is based on previous theoretical relationships, which correlate σF with the indentation force P and layer thickness d, and KC with P and mean length of radial cracks. This work examines the accuracy and limitations of this approach using a wide range of contemporary dental ceramic materials. The effect of layer thickness, indenter radius, load level, and length and number of radial cracks are carefully examined. The accuracy of the predicted σF and KC is similar to those obtained with other concurrent test methods, such as biaxial flexure and 3-point bending (σF), and bending specimens with crack-initiation flaws (KC). The benefits of the present approach include treatment for small and thin plates, elimination of the need to introduce a precrack, and avoidance of dealing with local material nonlinearity effects for the KC measurements. Finally, the bilayer configuration resembles occlusal loading of a ceramic restoration (brittle layer) bonded to a posterior tooth (compliant substrate).

On the fracture behavior of molar teeth with MOD cavity preparation

Mesial-occlusal-distal (MOD) cavity preparations are commonly used to restore damaged teeth. While numerous in vitro cavity designs have been devised and tested, no analytical frameworks for assessing their resistance to fracture seem to exist. This concern is addressed here by resorting to a 2D slice specimen cut from restored molar teeth with rectangular-base MOD cavity. The evolution of damage due to axial cylindrical indentation is followed in situ. The failure begins with a rapid debonding along the tooth/filler interface and continues with unstable cracking from the cavity corner. The debonding load q_d is fairly fixed while the failure load q_f is insensitive to the presence of filler, increasing with cavity wall thickness h and reducing with cavity depth D. The growth of the corner crack is studied using a 2D fracture analysis in conjunction with the FEM technique. The ratio h = h/D emerges as a viable system parameter. A simple expression for q_f given in terms of h and dentin toughness K_C is developed that predicts well the test data. In vitro studies on full-fledged molar teeth with MOD cavity preparation show that the fracture resistance of filled cavities often exceeds by a large margin that of unfilled ones. Indications are that this may reflect load sharing with the filler. Thus, the fracture resistance of the unfilled cavity provides a lower bound to a compromised MOD filling after long-term aging in the mouth. This bound is well predicted by the slice model. Finally, it is recommended that MOD cavities be prepared, if applicable, such that h > D regardless of the tooth size.

Mechanical behaviour of prosthodontic CAD/CAM polymer composites aged in three food-simulating liquids

OBJECTIVES

This study investigated the effect of ageing in three food-simulating liquids (FSLs) on mechanical properties of three prosthodontic CAD/CAM polymer composites intended for construction of implant-supported frameworks.

METHODS

Materials investigated were: (i) a carbon fibre-reinforced composite (CarboCAD 3D dream frame; CC), (ii) a glass fibre-reinforced composite (TRINIA; TR), and (iii) a reinforced PEEK (DentoKeep; PK). Filler contents and microstructural arrangements were determined by thermo-gravimetry and tomography (µ-CT), respectively. Flexural properties (FS and E_f) were measured by 3-point bending (3PB) of 1 mm and 2 mm thick beam specimens. Fracture toughness (K_IC) was measured by single-edge-notched-bending (SENB). All measurements were made at baseline (dry) and after 1-day and 7-day storage at 37 ℃ in either water, 70 % ethanol/water (70 % E/W) or methyl ethyl ketone (MEK). Failed specimens were examined microscopically. Statistical analyses included four-way ANOVA, two-way ANOVA and multiple Tukey comparison tests (α = 0.05). Multiple independent t-tests were performed regarding thickness effects on FS and E_f (α = 0.05).

RESULTS

At baseline, the mechanical properties increased in the sequence: PK< TR< CC (p < 0.001). FS ranged from 192.9 to 501.5 MPa; E_f from 4.2 to 18.1 GPa; and K_{IC f}rom 4.9-12.4 MPa.m^0.5. Fibre-reinforced composites (CC and TR) were significantly stronger than PK. However, all properties of CC and TR reduced after 1 d storage in 70 % E/W and MEK with FS ranging from 58.6 to 408 MPa; E_f from 1 to 15.4 GPa; K_IC from 6.87 to 10.17 MPa.m^0.5. Greater reductions occurred after 7 d storage. MEK was more detrimental than 70 % E/W and water on fibre-reinforced composites.

SIGNIFICANCE

Mechanical properties of each CAD/CAM composite were strongly dependent upon media and ageing. Although the mechanical properties of PK were initially inferior, it was relatively stable in all FSLs. All three materials exhibited sufficient mechanical properties at 1 mm thickness, but thicker specimens were more tolerant to ageing.

Mechanical behavior of restorative dental composites under various loading conditions

Mechanical engineering and its scientific principles constitute an essential core in medical science. Currently, different composite resins are widely used as restorative dental materials. However, their lack of adequate strength and toughness has led to research that is aimed at improving the mechanical properties of dental composites. In the present study, the behavior of three different dental materials is investigated under static and dynamic loading conditions. In the experimental tests, a split Hopkinson pressure bar is utilized which corresponds to the most commonly used experimental setup for examining material behavior under a high rate of loading. The examined dental composites experience impacts during their service life and also during car accidents or sport injuries. Hence, in the study, impact loading is modeled in an experiment. A series of compression tests is conducted from low to high strain rates up to 40s^-1, and the dynamic elastic moduli of three different dental composites are measured. Furthermore, studies on the compressed surface of the dental composite specimens are performed to improve the analysis with respect to the hardness of the materials. The responses of the examined composites to dynamic loadings verify the impact resistance of the materials. The results indicate the load-carrying capabilities of the dental composites. These results can be used for materials development and existing computational models.