1

Introduction

Composites reinforced with discontinuous fibers are known as short fiber-reinforced composites (SFRC). Earlier attempts to improve the mechanical properties of composites by incorporating short fibers had limited success. This was due to the difficulty of maintaining their high aspect ratio (l/d), which is important for the high strength performance of the composite and the fiber length above its critical value . Indeed, some SFRCs contain microfiber fillers (60–120 μm) , which are well below the critical fiber length (0.5–1.6 mm) . Due to the insufficient fiber length, micrometer fibers act as microfiller particles instead of fiber filler. This gives the SFRC properties comparable with particulate filler composites (PFCs) .

Interpretation of the properties of SFRCs is more difficult than is for unidirectional fibers and composites reinforced with them. This could be due to the fact that unidirectional fibers are assumed to be straight, perfectly circular at the cross-section, to have same mean diameter and to be closely packed . In contrast, short fibers are often assumed to be randomly distributed, to have a cylindrical shape and an elliptical cross section . Nevertheless, the short fibers could also be transversally aligned ( i.e. in plane). The mechanical properties of SFRCs essentially depend on the fiber orientation distribution (FOD), aspect ratio (l/d) and their volume fraction (Vf) . There is, however, critical l/d for the transversally orientated short fibers above which further improvement of their mechanical properties declines . For engineering SFRC this is between 6 and 9 , whereas the lowest l/d for dental SFRC is 5.2 . With increasing l/d they start behaving as long fibers , which macroscopically are transversally isotropic materials , although in an x-y system they show anisotropic properties . Similarly, transversally aligned short fibers have also been described as macroscopically transversally isotropic materials . By contrast, the properties of randomly orientated discontinuous fibers are mostly affected by their Vf . Thus, these are macroscopically isotropic materials, with most effective mechanical properties at Vf of 10% . Consequently, if the fibers are homogeneously distributed with random orientation, the SFRC is macroscopically homogenous and isotropic, whereas if the fibers are homogeneously distributed with transverse (in plane) orientation, the SFRC is homogeneous and anisotropic i.e. transversally isotropic. Furthermore, in order to evaluate various physicomechanical properties of the SFRCs on macroscopic level, numerous theoretical, computational based models have been proposed. Such models are mechanical-based micromechanical numerical model , energy-based homogenization model and mesostructured with work-energy approach model . These models give an insight on what circumstances initiate fracture in the SFRC structure, what factors influence its progression, what causes the fiber debonding or breakage, how the FOD affects the stiffness of the SFRC system and how FOD interacts with the fiber l/d. These models have shown that in addition to the FOD, l/d and Vf, equally important is the length of the short fibers. Indeed, these factors together determine the fiber debonding from the matrix during loading of the SFRC structure. As an example, the debonding for longer aligned fibers is greater than for random ones, which undergo only limited debonding. In other words, the load bearing capacity of SFRCs is more effective with short random than with longer aligned short fibers . Furthermore, favorably oriented fibers would allow the bridging phenomenon, which is important fiber toughening mechanism. Fiber bridging is most effective for transversally aligned fibers, intermediate for randomly orientated fibers and absent for non-reinforced systems. Fiber breakage, on the other hand, is important as it introduces new length in the system, while the Vf remains unchanged . Apparently, among the factors that determine the mechanical performance of SFRC are the length , type, arrangement and distribution of the fibers . Other factors, known as intrinsic (fiber related) and extrinsic (matrix related) factors, also affect the SFRC’s properties . The fiber related factors of SFRCs could be considered to be the l _f , d _f , Vf, strength as well as the interfacial fiber-matrix bonding, whereas the matrix related factor could be considered to be the FOD . In addition, the random distribution of short fibers could be compromised or intentionally affected during the manipulation process. If discontinuous fibers are long enough, they behave as long fibers giving anisotropic mechanical properties . To date, there exist some research on the mechanical properties of recently introduced millimeter-scale discontinuous fiber-reinforced composite (SFRC) (everX Posterior, GC Corp., Tokyo, Japan) , but none on its fiber related and matrix related factors. Based on its fiber l/d, this SFRC could be classified as high aspect ratio SFRC i.e. millimeter-scale SFRC.

Consequently, the present study was designed to firstly investigate several mechanical properties of this millimeter-scale SRFC in comparison with three conventional PFCs (G-ænial Anterior, GC; Supreme XTE, 3M Espe, MN, USA and Filtek Bulk Fill, 3M Espe) ( Table 1 ) and secondly to evaluate both fiber related and matrix related factors of the SFRC. The selected properties were fracture toughness (K _IC ), flexural strength (FS), flexural modulus (FM), compressive strength (CS), diametral tensile strength (DTS), apparent horizontal shear strength (AHSS) and fracture work (W _f ). The fiber related factors of the millimeter-scale SFRC were analyzed on both a theoretical basis and experimentally. On a theoretical basis, the volume fraction of the short fibers (V _f %) and the theoretical critical fiber length (l _fc ) were calculated. Experimentally determined fiber related factors were the fiber diameter (d _f ), the fiber length (l _f ) and the aspect ratio (l/d), whereas the experimentally determined matrix related factor was the fiber orientation distribution (FOD). The null hypotheses were that the mechanical properties of the millimeter-scale SFRC will not differ from the PFCs’ properties and that short-term water storage would not affect the mechanical properties of the investigated materials.

2

Materials and methods

FS, FM, DTS, CS, AHSS and K _IC were evaluated. Eight specimens (n = 8) were prepared per restorative composite material listed in Table 1 for each test type and each storage condition. The storage conditions were dry and in water at 37 °C for 7 and 30 days, respectively. Testing procedures were performed at room temperature (23 ± 1 °C). The fiber diameter (n = 6) and orientation (n = 6) were determined immediately upon preparation of the specimens. The fiber volume fraction, fiber length, critical fiber length and aspect ratio were additionally computed.

FS and FM were evaluated by the three-point bending test. Specimens were prepared, polymerized and tested following the protocol previously described .

Cylindrical specimens, 6 mm in height and 4 mm in diameter, were used to evaluate DTS and CS. The fabrication and testing procedures of the specimens are described elsewhere . DTS in megapascals (MPa) was calculated using Eq. (1) :

where: DTS is the diametral tensile strength, F is the maximum applied load in newtons (N); D is the diameter of the specimens in mm and l is the length of the specimen in mm. CS was calculated in megapascals (MPa) using Eq. (2) :

where: CS is the compressive strength, F is the maximum applied load in N and D is the diameter of the specimen in mm.

AHSS was determined by the short-beam method, according to ASTM D 4475–96 . A translucent mold with an inner diameter of 3.6 mm and a length of 14.5 mm (±0.2 mm), which is one diameter greater than the test span was used to fabricate the rod shaped specimens. The mold was placed on a Mylar-strip-covered glass slide and the restorative material was packed into the mold. The top surface was flattened with Mylar strip and glass slide and each specimen was polymerized on both sides for 20 s by five overlapping irradiations with a light-curing polymerization unit (Elipar S10, 3M ESPE, St. Paul, MN, USA) at an intensity of 1600 mW/cm ² and wavelength ranging from 430 to 480 nm. The irradiance of the light-curing polymerization unit was measured with dental radiometer (Demetron L.E.D. Radiometer, Kerr, Middleton, WI, USA). Upon removal from the mold, each longitudinal side was further polymerized for 20 s in five overlapping places. The rod specimen was placed with its long axis perpendicular and its midpoint centered to the loading nose. The side supports were adjusted to a span length of 11 mm (±0.2 mm) and specimens were loaded by a universal material testing machine (Lloyd model LRX, Lloyd Instruments Ltd., Fareham, UK) at a crosshead speed of 1.0 mm/min until failure. The diameter of each specimen was measured prior to testing with a digital caliper and the apparent horizontal shear strength in MPa was calculated in accordance with Eq. (3) :

where AHSS is the apparent horizontal shear strength; P is the breaking load in N and d is the specimen diameter in meters (m).

Fracture toughness (K _IC ) was evaluated using single-edge notched bend (SENB) specimens (2.5 × 5 × 25 mm) according to the adapted ISO 20795-1 standard method , and as previously described . The SENB specimens were used for determining the total fracture work (W _f ) in J/m ² , by using Eq. (4) :

where Wf is the fracture work, U is the recorded area under the load-deflection curve and represents the energy required to break the whole specimen. U = ∫PdΔ in newton millimeters (Nmm), where Δ is the measured deflection for load P in N; B is the specimen width in mm, H is the specimen height in mm, and a is the crack length in mm.

Fiber volume fraction (V _f %) was calculated using Eq. (5) :

where W is the weight fraction (wt%), ρ is the density, f represents the fiber and m the matrix. W _f = 8.6 wt% (W _f of the short E- glass fibers in everX Posterior) and ρ _f = 2.56 g/cm ³ . The matrix comprises resin and particulate fillers with ρ _(resin) = 1.23 g/cm ³ , W _(resin) = 23.7 wt%, ρ _{(particulate filler)} = 2.8 g/cm ³ and w _{(particulate filler)} = 67.7 wt%.

The fiber length (l _f ) was measured using the direct technique, by separating the fibers from the matrix, following the technique previously described . Eight batches of millimeter-scale SFRC (everX Posterior) were evaluated. Fibers were deposited onto glass slide and examined with light microscopy (Leica; Leica Mycrosystems, Wild, Heerbrugg, Switzerland). The average fiber length was then defined as L _N = Σ Ni Li/Σ Ni, where L _N is the number average of the fiber length, Ni is the number of fibers with length Li . Four thousand fibers were measured in total (500/batch). When the fiber diameter (d _f ), the ultimate tensile strength (σ _fu ) of the glass fiber and the interfacial shear strength (τ _i ) of the glass fiber reinforced composite are known, then the theoretical critical fiber length ( l _fc ) can be calculated from l _fc = σ _fu d _f /2 τ _i . The tensile strength of E-glass fiber is estimated at 2.0 GPa, whereas the interfacial shear stress (τ _i ) for a glass fiber reinforced composite varies between 17.6 and 33.8 MPa depending on the fiber conditioning and the testing environment , but typically is 20 MPa . The diameter of the individual glass fiber in our study was 17 μm, whereas the interfacial shear stress evaluated by macroscopic short-beam test was 15.6 MPa. This experimentally obtained result was then compared with the typical values presented in the literature .

In the present investigation the fiber orientation distribution (FOD) was visually studied by sectioning the SENB specimens (12 slices obtained from n = 6) along their long-axis plane with a high performance precision cut-off saw at 300 rpm (Struers Secotom 50, Struers, Ballerup, Denmark) and analyzing their images by scanning electron microscopy (SEM, JSM 5500, Jeol Ltd., Tokyo, Japan). The specimens were ground wet (Stuers LabPol-21, Stuers A/S, Copenhagen, Denmark) with silicon carbide papers of decreasing abrasiveness (1000-, 1200-, 4000-grit) and gold sputter coated before the SEM examination. After testing the specimens, their fracture surfaces were examined with SEM.

The fiber diameter (d _f ) was also determined from SEM images, which were processed with the Image J program (National Institutes of Health, USA). A cross section of a fiber bundle was examined with SEM for this purpose. The fiber diameter was determined as a mean value calculated from one hundred measurements per specimen (n = 6).

The data were statistically analyzed with SPSS version 23 (SPSS, IBM Corp.). Homogeneity of variance was analyzed by the Levene test (p = 0.05). Two-way ANOVA (α = 0.05) followed by a Tukey post hoc test was used to determine if the mechanical properties (FS, FM, DTS, CS, AHSS, K _IC , W _f ) were different for the materials investigated in this study (SFRC, MFR, NF, FBF) and for the type of storage condition (dry, water). Pearson correlation was used to analyze the correlation among the mechanical properties.

2

Materials and methods

FS, FM, DTS, CS, AHSS and K _IC were evaluated. Eight specimens (n = 8) were prepared per restorative composite material listed in Table 1 for each test type and each storage condition. The storage conditions were dry and in water at 37 °C for 7 and 30 days, respectively. Testing procedures were performed at room temperature (23 ± 1 °C). The fiber diameter (n = 6) and orientation (n = 6) were determined immediately upon preparation of the specimens. The fiber volume fraction, fiber length, critical fiber length and aspect ratio were additionally computed.

FS and FM were evaluated by the three-point bending test. Specimens were prepared, polymerized and tested following the protocol previously described .

Cylindrical specimens, 6 mm in height and 4 mm in diameter, were used to evaluate DTS and CS. The fabrication and testing procedures of the specimens are described elsewhere . DTS in megapascals (MPa) was calculated using Eq. (1) :

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