Fracture toughness of chairside CAD/CAM materials – Alternative loading approach for compact tension test
Introduction
Ceramics are inorganic products with nonmetallic characteristics. These compounds are fired at higher temperatures to attain desirable properties [1]. Dental ceramics have excellent esthetic properties and are highly biocompatible [2], [3], [4], [5], [6], [7]. New handling and processing technologies have led to a wider range of available modern ceramic materials for CAD/CAM machining [4], [8], with a more easy fabrication procedure [9].
Poor longevity of earlier ceramic materials due to increased fracture rates was a main complication of these materials [10]. Ceramics are considered to be brittle in nature having increased susceptibility to fracture under tension. This brittleness results in the development of cracks with subsequent crack propagation and finally catastrophic failure [11], [12]. Moreover, ceramic restorations present in the oral cavity are subject to thermal, chemical and mechanical influences, which concentrate stresses on minute surface areas. These concentrated stresses cause strain(s) to develop [13], [14], [15].
Dental restorations must be mechanically stable and durable during function [16], in order to resist deleterious effects of the harsh oral environment. A significant factor affecting strength and mechanical behavior of ceramic materials is the distribution of existing flaws [17]. During CAD-CAM milling procedure, machining and grinding of the blocks creates surface damage in the form of micro-cracks and flaws [8] which may propagate slowly causing failure of the restoration. To counteract this side effect of grinding, polishing or glazing of the outer surfaces of the restoration is routinely performed [9], [13], [18].
Fracture toughness (KIC) is an intrinsic property of a material that relates to its resistance to crack propagation which finally causes its failure [19], [20], [21], [22], [23], [24]. This property is concerned with the critical stress intensity, K, at the crack tip [24], [25]. The critical stress intensity depends on the tri-axial strain conditions and crack instability that occurs under plane strain conditions at a minimum K to be referred to as fracture toughness (KIC) [24].
A restorative material with high fracture toughness (KIC) shows better fracture resistance and longevity in service as compared to materials with lower fracture toughness (KIC) [14], [24], [25]. For brittle materials, fracture toughness (KIC) is one of the most significant mechanical properties that is independent of specimen shape, flaw size and stress concentration [19].
Numerous techniques have been commonly used for testing fracture toughness (KIC). These include: the indentation strength (IS), indentation fracture (IF), the single-edge-notched beam (SENB), single-edge pre-cracked beam (SEPB), compact tension (CT), chevron notched short rod/chevron notched short bar (CNSR/CNSB) and the double torsion double cantilever beam (DCB) [19].
The single-edge-notched beam (SENB) and compact tension (CT) test geometries are suggested for dental materials, because both tests require a smaller specimen size to fulfill plane strain conditions as compared to configurations of specimens of other tests [26].
Traditionally, the compact tension (CT) test was used for determining fracture toughness (KIC) of resilient materials, such as dentin and resin composites [24], [27], [28], [29], [30], [31], [32]. It has not as yet been used for testing brittle dental ceramic materials.
This study aimed to apply the compact tension (CT) test design in a modified form to measure fracture toughness (KIC) of a group of restorative CAD/CAM materials including ceramics and nanoceramic resin composites.
Section snippets
Materials and methods
Machinable CAD/CAM blocks representative of 5 different material types were cut into rectangular-shaped specimens (n = 10 specimens/material) being 4 mm thick. The specimens conformed to the dimensions prescribed for the compact tension test, ASTM E-399 [32], however, no holes were drilled. The materials comprised a lithium disilicate glass ceramic, IPS e.max CAD (Ivoclar Vivadent) tested before and after crystallization; a fully-crystallized zirconium-reinforced lithium silicate glass ceramic,
Results
Means and standard deviation values of KIC for tested materials are presented in a bar chart (Fig. 4). ANOVA revealed a significant difference among the means (p < 0.05). Highest mean KIC values were recorded for CD and E-max specimens, 2.65 (0.32) and 1.88 (0.62) MPa m1/2, respectively. These values were significantly higher than those of all other groups according to Tukey's test. Furthermore, mean KIC value for CD was found to be significantly higher than that of E-max (p < 0.0001).
Discussion
Fracture toughness (KIC) is an intrinsic material property [24] that determines a material's ability to resist crack propagation, as well as its resistance to fracture [25]. According to ASTM E399 [32], a state of plane strain prevails with a sample thickness ≥2.5 (KIC/σYs)2, where σYs is the yield strength of the material. To calculate the correct thickness of the specimen, fracture toughness of Lava (9 MPa m1/2) as the highest KIC value of a ceramic and yield strength (Ys) of enamel (330 MPa)
Conclusions
Within the limitations of this in vitro study, the following conclusions could be drawn:
- 1.
Highest KIC values were recorded for fired/crystallized glass-ceramic materials (CD/E-max, respectively).
- 2.
Glass-ceramic materials without firing or crystallization were associated with significantly lower mean KIC compared to their fired/crystallized counterparts.
- 3.
As a first report, the modified test arrangement was easy to follow and simplified specimen preparation process.
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