Effect of fiber position and orientation on fracture load of fiber-reinforced composite

Abstract

Objectives. The aim of this study was to determine the effect of fiber position and orientation on the initial and final fracture loads of fiber-reinforced composite (FRC).

Methods. Test specimens made of two indirect particulate composites (BelleGlass HP, Kerr, Orange, CA) or (Targis, Ivoclar Vivadent, Amherst, NY) were reinforced with ultra high molecular weight polyethylene (UHMWPE) fiber ribbon (Connect, Kerr, Orange, CA), woven E-glass fibers (Vectris Frame, Ivoclar Vivadent, Amherst, NY) or unidirectional R-glass fibers (Vectris Pontic, Ivoclar Vivadent, Amherst, NY). Fibers were placed with different positions, orientations or geometry into the rhombic test specimens (2×2×25 mm³). Control specimens did not contain fiber reinforcement. The test specimens (n=6) were stored in distilled water for 1 week at 37 °C before testing in a three-point loading test to determine the initial and final fracture load values.

Results. Initial failure loads varied from 22.6 to 172.1 N. The lowest value resulted from one UHMWPE reinforcement fiber located in diagonal orientation and the highest from two unidirectional glass fiber reinforcements, one located on the tension side and the second on the compression side.

Significance. Position and fiber orientation influenced the load to initial and final failure, and specimen deflection. Tension side reinforcement was most effective in increasing the load to initial and final fracture.

Introduction

Composite materials are a combination of two or more distinct components forming a new material with enhanced properties. While many combinations exist, the most common composites in engineering are composed of strong fibers held by a binder or matrix. Unlike traditional materials, the properties of composites can be designed simultaneously with structural aspects. This allows composite designers to manipulate material properties by changing fiber orientation, fiber content, and geometry. Additionally, the most common types of matrix materials are polymers [1].

Attempts have been made to reinforce dental polymers with several types of fibers for various treatment modalities during the past 30 years. Studies have tested polyethylene fibers [2], carbon/graphite fibers [3], [4], [5], or glass fibers [6], [7], [8], [9]. There exist potential applications for fiber-reinforced composites (FRC) in prosthodontics, periodontics, and orthodontics. Several in vitro studies have been conducted to find out and understand the factors influencing dental FRC properties [10], [11], [12], [13], [14], [15]. Important factors influencing the mechanical properties of FRCs include: (1) inherent material properties of fibers and polymer matrices, (2) fiber surface treatment (sizing) and impregnation of fibers with resin, (3) adhesion of fibers to the polymer matrix, (4) quantity of fibers [16], (5) direction of fibers, (6) position of fibers [17], [18], [19] and (7) water sorption of FRC matrix [16].

Previous dental FRC research on position and orientation has focused upon the effects of the question of fiber reinforcement directionality (i.e. random or longitudinal orientations) [20], [21]. It is widely accepted that directional orientation of the fiber long axis perpendicular to an applied force will result in strength reinforcement. Forces that are parallel to the long axis of the fibers, however, produce matrix-dominated failures and consequently yield little actual reinforcement. Design strategies are on occasion employed to provide multi-directional reinforcement, to minimize the highly anisotropic behavior of unidirectional fiber reinforcement. Multidirectional reinforcement, however, is accompanied by a decrease in strength in any one direction when compared with unidirectional fiber, as described by Krenchel [22]. In most instances in the dental literature, fiber reinforcement has been positioned in the center of a composite specimen [20]. Yet from engineering applications, it is known that the position and orientation of the reinforcement within a construction influences mechanical properties [23].

For a small sized construction, such as a dental prosthesis, the quality and characteristics of the FRC are important and demand careful attention. Fiber reinforcement should be optimal when designing prostheses and their components. As an example, the components (e.g. connector, pontic, retainer) of a FRC fixed partial denture (FPD) need to be designed to withstand masticatory loading [24]. While it is known that tension side fiber reinforcement strengthens a loaded construction, the effect of varying the cross-sectional design in a FRC structure is not fully known. Respectively, all factors relating to design and failure of FRC structures should be investigated and better understood.

Questions exist whether ultra high molecular weight polyethylene (UHMWPE) fibers can be used to fabricate a high quality dental composite structure. Criticism has been focused on findings that interfacial adhesion between polyethylene fibers and dental polymers is not adequate [25], [26]. The use of resin pre-impregnated silanized glass fibers instead of non-impregnated polyethylene fibers results in the highest mechanical properties according to the majority of recent research [27], [28].

A subtle and under-reported attribute of FRC is a description of the fracture failure. A specimen may catastrophically fail in an instant while another may simply bend under increasing load. In 1975, Craig and Courtney described three modes of failure that can occur when characterizing FRC in a tension type test [29]. The three failure modes are described as instantaneous (Fig. 1, Curve A), statistical (Fig. 1, Curve B), and stepwise (Fig. 1, Curve C). Instantaneous failure occurs after a load causes a strain concentration in a narrow region sufficient to break the composite structure. A strain concentration distributed to a wide region may require further load or elongation for continued fracture. Thus leading to what is denoted as stepwise (more bending type) or statistical failure (series of small intense fractures which recover before complete failure and require more load to progress). Analysis of the mode of failure can give insight on how and why failure occurs. Accurate reporting should therefore include when the fracture begins (initial fracture), how it progresses, and when it finishes (final fracture) [30].

The aim of this study was to compare the effect of various positions, orientations, and geometries of glass and polyethylene fibers in a dental particulate composite test specimen upon the initial and final fracture load. It was hypothesized that there would be no difference in load to failure among designed specimen groups.