Mixed-mode Failure Research Articles

Revealing the interlaminar shear failure behavior of unidirectional laminate under combined compression–shear loads

Due to the coupling effects between stresses in different directions, the mechanical behavior of an advanced composite material under multiaxial loading is extremely complex. In this study, the influence of through-thickness compressive stress on the interlaminar shear performance of a carbon fiber-reinforced composite was experimentally investigated. Hollow cylindrical unidirectional laminate specimens were fabricated to conduct combined compression–shear tests, and the fracture morphologies of the specimens were characterized to reveal their failure behavior. The results indicate that a moderate compression load significantly enhanced the shear properties of the laminate by inhibiting crack propagation and improving the friction effect. The shear strength and modulus of a laminate specimen subjected to combined stresses improved up to a maximum of 76% and 231%, respectively, over those of an equivalent specimen subjected to pure shear. However, as the applied through-thickness load approached the compressive strength of the laminate, the specimen shear capacity began to decline owing to the transition of fracture mechanisms. Indeed, the specimens exhibited mixed failure modes corresponding to the different stress states, which were induced by the combined effects of through-thickness compressive and shear stresses. As the applied through-thickness compressive stress increased, the dominant failure mode of the laminate specimen changed from fiber–matrix debonding to fiber shearing and then to fiber breakage, resulting in various shear performances.

Effect of shatterproof polymer film application on the fracture types and strength of glass subject to bending load

Shatterproof polymer films are widely for windows used because they can be easily installed on existing glass windows to improve safety. Applying them to glass plates has been reported to not only prevent fragments from scattering but also increase load-bearing capacity and penetration resistance. However, the clarification of their mechanism and quantitative evaluation are still insufficient because the effect of film application on the strength and failure mode of glass under quasi-static loading has not been investigated. In this study, three-point bending tests and fracture surface observations were conducted on a float glass with a shatterproof polymer film. The stress field formed inside the glass was visualised during the tests using the photoelastic method. By varying the support span of the specimen, the deformation mode was varied to generate three types of failures: bending, shear caused by Hertzian contact stress, and mixed-mode failures. Under the conditions in the present study, the breaking loads of the specimens with and without film were almost the same; however, the fracture surface observation indicated that the area subjected to shear failure caused by Hertzian contact stress was larger with film application. Finally, the effect of the film thickness on the breaking load due to bending deformation was theoretically predicted.

Experimental and numerical investigation of large-scale effect on buckling and post-buckling behavior of tubular structures

In this paper, compressive behavior of tubular structures made of aluminum and multilayer carbon fiber reinforced plastic (CFRP) are studied experimentally and numerically. Results show that current evaluation of aluminum tubes using Johnson–Euler method overestimates the critical stress values of the tubular structure because local failure is ignored. A finite element analysis (FEA) model based on Riks method is proposed to re-evaluate compressive performance and failure modes of aluminum tubes. This model is modified for laminated composites by introducing damage model based on Hashin’s theory, such as matrix tensile/compressive failure, and fiber tensile/compressive failure. Simulation results of CFRP tubular structure show close agreement with experimental results, but also sensitive to geometric constants. Material failure is primarily the failure mode of tubes with slenderness ratios under 21. A mixed failure mode can be found for slenderness ratios of tubes between 21 and 35. For tubes with slenderness ratios above 35, buckling failure is the primary failure mode.

Strengthening of deep RC coupling beams with FRP composites: A numerical study

Currently, most buildings have earthquake-resistant systems such as coupled shear walls and couple beams, designed based on previous code specifications. Also, due to specifications of new codes, diagonal reinforcement details in coupled beams are introduced, which due to the high reinforcement congestion, creates problems in fabrication and pouring. The main goal of this paper is to numerically study retrofitting with polymer composites reinforced with carbon fiber with external bonding for reinforced concrete coupling beams. The previous studies are divided into three categories. The first part concerns using externally bonded composite FRPs and the effect of fiber arrangement and thickness on the coupled beam behavior. The maximum displacement ductility in shear reinforcement patterns belonged to I and 2I patterns, which had the maximum ductility coefficients of 7.04 and 8.87. By comparing 7 CFRP reinforcement patterns, results showed that due to the mixed failure mode (flexural-shear) of the coupling beam, the U-X reinforcement pattern increased the load-bearing capacity by 18% compared with the base sample. By using suitable anchorage and preventing premature debonding, this method could be introduced as an effective method for improving the coupling beam’s behavior. In the second category, reinforcement with composite FRPs was compared with diagonal reinforcement and strengthening by installing steel plates and the accompanying behavior diagrams were given. Compared with other reinforcement methods in terms of load-bearing capacity and ductility, the specimen reinforced with CFRP and steel plates had a 49% and 70.4% increased load-bearing capacity of the control specimen. Among the studied specimens, the best performance belonged to the specimen with diagonal reinforcements, which had an increased load-bearing capacity and ductility of 124% and 4.2% compared with the base specimen. Finally, the third part is about the effective parameters of the coupling beam such as span length to height ratio and longitudinal reinforcement value regarding coupling beam behavior.

Elucidating the deformation behavior of as-cast B2 CoTi intermetallic using in-situ tensile testing

This study deals with microstructural aspects of the deformation behavior of as-cast homogenized CoTi intermetallic by using in-situ tensile testing coupled with EBSD analysis. CoTi exhibits excellent wear resistance and noticeable room temperature ductility. Therefore, it is commonly used as coating material or a reinforcing phase in superalloys. In-situ study of this intermetallic is pertinent as it will provide information about the microstructural evolution during deformation under tensile loading. During the initial stage of deformation, the plastic strain localization was observed to occur at softer grains due to the activation of {011}⟨100⟩ slip systems, which resulted in the accumulation of GNDs and increase in fLAGBs (fraction of low angle grain boundaries). However, fLAGBs did not increase significantly at higher strains. In addition, the contribution of texture towards deformation behavior was found to be weak. The intermetallic exhibited a mixed-mode failure, where both transgranular and intergranular features were observed on the fracture surface. A slightly higher strain to failure in the intermetallic has been attributed to both the increase in the GNDs density at the initial stage of deformation and increase in SSDs at the later stage of deformation, leading to strain hardening.

Bonded connections of pultruded sections for floor system studied using digital image correlation

An experimental investigation of bonded connections of Fiber Reinforced Polymer (FRP) pultruded sections for a new floor design is presented. Flat plates with T-up ribs are bonded to the lower flanges of I-beams and used as a structural form for the concrete floor. Adhesively bonded connections with or without mechanical fasteners of different types and configurations were examined through 48 bond specimens tested in tension. Digital Image Correlation (DIC) was used to capture strain and slip profiles along the connection interface, from which shear and peeling stress distributions were established. The study showed that connections reached 80–100% of the full capacity of the member, governed by the transverse strength of the I-beam. Most samples experienced mixed failure modes. The two highest occurrences were fiber-tear of the ribbed plate, where longitudinal fibers appeared on both surfaces, and adhesive failure, where epoxy appeared on one surface only in certain parts. Using fasteners with the adhesive increased the ultimate capacity by up to 19%. Embedment of fasteners inside the concrete had little effect on ultimate strength. A method was developed to convert DIC measured strains into peeling and shear stresses. The combined test data was used to establish the shear-peeling stress failure envelope, which agreed very well with the theoretical interaction expression. A bond-slip relationship was also deduced from the DIC test results.

Off-Axis Tension Behaviour of Unidirectional PEEK/AS4 Thermoplastic Composites

An experimental method for non-standard off-axis tension tests of unidirectional composites is developed. A new oblique end-tab is designed to eliminate stress concentration and in-plane bending moment induced by off-axis tension loading. Finite element analysis and experiments on Polyetheretherketone (PEEK)/AS4 unidirectional thermoplastic composites (CFRTP) were conducted to evaluate the effectiveness of the proposed testing method. Simulation and test results demonstrate that the use of oblique end-tabs eradicates stress concentration and bending movements. The digital image correlation (DIC) method was used to help investigate the full-field tension/shear coupling deformation response of the off-axis specimen. Test results show significant nonlinear behaviour and inhomogeneous strain distribution under tension/shear combined stresses. A fractographic study was carried out to study the damage mechanisms under a tension/shear combined stress state. Specimens with 30°, 45° and 60° off-axis angles, fail in tension/shear mixed failure mode. Fracture surface morphology indicates that matrix plastic deformation and ductile drawing under tension/shear coupled stress state induced the nonlinear stress-strain response.

Creep behavior and long-term strength characteristics of pre-peak damaged sandstone under conventional triaxial compression

A series of creep tests were carried out on sandstone specimens with different pre-peak instantaneous damage characteristics under different confining pressures. The results revealed that the creep stress was the key factor affecting the occurrence of the three stages of creep, and the steady-state creep rate increased exponentially with increasing creep stress. Under the same confining pressure, the larger the instantaneous damage of the rock specimen was, the more quickly creep failure occurred and the lower the creep failure stress was. For the pre-peak damaged rock specimens, the strain threshold for accelerating creep was the same for a given confining pressure. The strain threshold increased with increasing confining pressure. In addition, the long-term strength was determined using the isochronous stress–strain curve and the variation in the creep contribution factor. The results revealed that the long-term strength decreased gradually with increasing pre-peak instantaneous damage under lower confining pressures. However, the instantaneous damage had little effect on the long-term strength under higher confining pressures. Finally, the macro–micro-failure modes of the sandstone were analyzed according to the fracture morphology observed via scanning electron microscopy. It was found that the macroscale creep failure patterns of the sandstone specimens could be divided into a shear-dominated failure mode under high confining pressures and a mixed shear-tensile failure mode under low confining pressures. At the microscale, as the confining pressure increased, the micro-fracture mode of the sandstone changed gradually from a single brittle fracture to a mixed brittle and ductile fracture mode.

Secondary bonding of CF/PEKK thermoplastic composites with a PEI film adhesive in a press welding process

High-performance poly-ether-ketone-ketone (PEKK) thermoplastic (TP) composites, which are beginning to gain interest in the aerospace industry, require a joining procedure to integrate the individual parts. Amorphous TP bonding has been recently proposed to address the limitations of commonly used joining techniques for composites, such as mechanical fastening and adhesive bonding. In this study, we investigated the adhesive properties of a polyetherimide (PEI) film applied by hot press welding as an amorphous polymer interlayer of PEKK laminate. The temperature range of the bonding process was determined via differential scanning calorimetry analysis. The bonding strength was evaluated through single-lap shear tests. The effect of process temperature on the bonding strength was qualitatively analyzed via fracture surface morphology analysis. The interface between PEKK and PEI was analyzed via optical microscopy. The adhesive strength was as low as about 5 MPa up to the bonding temperature of 280[Formula: see text]C but then started to improve as the temperature increased, which is attributed to the interdiffusion of the PEKK matrix and PEI adhesive film. The maximum bonding strength of 23 MPa was confirmed at a bonding temperature of 340[Formula: see text]C. A mixed failure mode consisting of a relatively high ratio of fiber-tear failure to light-fiber-tear failure is observed at this process temperature. This failure mode can be attributed to the bonding temperature condition at which PEKK begins to melt.

Numerical simulation of fracture in Taylor rod impact problem with stochastically distributed inherent damage

High strain rate deformation behavior studies in structures always hold importance in many areas of science and technology. The high strain rate deformation behavior is generally determined by the Taylor rod impact tests. During the impact process, the front portion of the Taylor rod undergoes a large deformation that results in the evolution of the voids leading to damage/fracture in the rod. A process of fracture phenomenon and mechanics of blunt-shaped projectile impacting a rigid target at high-velocity results in different fracture modes viz. mushrooming, petalling, shear cracks, tensile splitting, fragmentation, or mixed modes of failure. The deformation and evolution of these practically observed fracture modes are investigated due to the introduction of stochastically distributed inherent damage in the flat-faced Taylor rod. The process is simulated in the mild steel Taylor rod using continuum damage mechanics. The evolution of the damage and fracture growth has been presented. The process of deformation, the phenomenon of stress propagation, and the effect of stochastically distributed inherent damage on the fracture mode in the Taylor rod have been discussed. As the stresses initially evolve at the outer edge of the Taylor rod, the damage initially grows at the outer edge leading to the tensile splitting and petal formation. It is found that the introduction of stochastically distributed inherent damage and critical damage inside the Taylor rod changes the fracture modes. The results are found to be consistent with the literature.

Corrosion and tensile behavior of intercritically heat-treated and cathodically polarized prestressing steel wires

The effect of the intercritical heat treatments producing various martensite volume fractions on the corrosion and tensile behavior of the cathodically polarized prestressing steel wires has been investigated. Potentiodynamic polarization and constant extension rate technique (CERT) tests are conducted to clarify these effects and to determine the mode of failure of the prestressing steel wires since the presence of the martensite in the prestressing steel wire microstructure results in the failure and breakage during the corrosion and cold drawing. The electrochemical data measurements reveal that the intercritically heat-treated and cathodically polarized prestressing steel wires containing higher volume fractions of martensite exhibit the lowest corrosion potentials and current densities which indicate higher corrosion rates. A significant decrease in the tensile strength and ductility by the accelerated corrosion tests is observed in the intercritically heat-treated and cathodically polarized prestressing steel wire specimens containing higher martensite volume fractions. The fracture surfaces of the intercritically and cathodically prestressed steel wire specimens containing higher fractions of martensite of 68% exhibit a mixed mode of failure, i.e., quasi-cleavage and microvoid coalescence after potentiodynamic polarization. Moreover, the brittleness due to the residual stresses induced by the formation of martensite and the generation of the galvanic corrosion cell between the martensite and pearlite is noted to be increased with increasing the amount of martensite in the prestressed steel wire specimens.

Multi-layer modelling of masonry structures strengthened through textile-reinforced mortar.

Background Textile-reinforced mortar (TRM) is an innovative strategy for the reduction of the seismic vulnerability of existing masonry buildings consisting in the application on the masonry surface, of a mortar coating with fiber-based grids or textiles embedded. The paper presents the calibration and application of a simplified modelling approach, based on multi-layered elements, for the simulation of existing masonry elements and structures strengthened through TRM. Methods The strengthened masonry is modelled by using 20-nodes brick elements formed by a stacking sequence of layers representing the different material components (the masonry, the mortar coating and the embedded reinforcement). The nonlinear behavior of the materials is considered and calibrated on the basis of experimental characterization tests on individual components available in the literature. The simplified assumption of perfect bond among layers is considered. Results Non-linear static analyses are performed on samples of increasing complexity: elementary panels, structural elements (piers and spandrels) and a pilot building. The results of some tests on TRM strengthened masonry, available in the literature, are considered to assess the model reliability in terms of capacity curves and collapse mode. The model is capable of detecting the typical failure mechanism of both unstrengthened and TRM strengthened masonry, namely the diagonal cracking, the in-plane bending and the out-of-plane bending and is able to detect the activation also of mixed failure modes, that often occur in actual configurations. Conclusions Given the coarse mesh size and the smear plasticization assumption, the model is not suitable for the rigorous reproduction of individual cracks but represents a good compromise between the goal to grasp the structural performances at the wide scale, including failure modes, and the analysis optimization.

Tension and Impact Analysis of Tungsten Inert Gas Welded Al6061-SiC Composite

An aluminum 6061 (Al6061) metal matrix composite (MMC) reinforced with silicon carbide was prepared by stir casting. Specimens of the required dimensions were welded using the tungsten inert gas (TIG) method. ER5356 (Al-5%Mg) was chosen as the appropriate filler material for TIG welding. The input current parameter was varied (150, 170 and 200nA) while maintaining the other welding parameters at constant values. An assessment of the mechanical (tensile and impact strength) and microstructure properties of the TIG-welded Al6061 MMC with 6 wt. % silicon carbide particles was accomplished. An 8.27% improvement was observed in ultimate tensile strength (UTS) for the 150 A TIG-welded sample. UTS and elasticity decreased linearly with an increase in welding current but exhibited higher values than in non-welded specimens. The microstructural analysis of the welded MMCs showed a mixed mode of failure, with equiaxial dimples being dominant in lower-weld-current specimens. Compared to non-welded specimens, a 40% increase in impact strength was observed for the 150 A TIG-welded specimens, which decreased with an increase in the welding current value. SEM analysis revealed ductile striations and continuous river patterns, resulting in mixed failure.

In vitro and practical guide for the analysis of bond strength to ceramics

The purpose of this study was to characterize different test designs applied for analysis of the bond strength between ceramic and resin cement. One lithium disilicate (LD, IPS Emax CAD®, Ivoclar) and one zirconia (YZ, Ceramill, Amann-Girrbach) ceramic were used for specimens’ preparation (n = 10) according to the proposed test: Microtensile, Tensile, Microshear, Shear, Micropush-out, Push-out, and Interfacial Fracture Toughness. A resin cement (Multilink Automix dualcure, Ivoclar) was used to bond the ceramic to the substrate (composite resin, Tetric N-Ceram, Ivoclar) selected according to the test design. During preparation and testing of the specimens, the time required, amount of material and level of difficulty were assessed. Bond strength data were analyzed using one-way ANOVA and Tukey test (P < 0.05). The homogeneity and magnitude of the bond strength values were verified by calculating the Weibull modulus. Failure mode was analyzed by stereomicroscope for all tests. Microshear resulted in the highest (LD: 41.00 ± 14.96 MPa; YZ: 48.95 ± 6.64 MPa) and Interfacial Fracture Toughness in the lowest (LD: 3.54 ± 0.94 MPa; YZ: 4.10 ± 1.65 MPa) bond strength values, regardless of the ceramic. The analysis of the mode of failure showed 100% adhesive failure for Microtensile and Micropush-out samples for both, LD and YZ. Interfacial Fracture Toughness samples of LD also presented 100% adhesive failure. Most of the samples presented mixed failure modes, and only Tensile and Push-out tests presented cohesive failure in the resin cement layer. Overall, the test design affected bond strength results. Weibull modulus indicated that a smaller sample size may be adequate for the analysis of bond strength to ceramic materials. Microtensile test was classified as the most difficult test in terms of preparing the specimen and performing the test.

Experimental–numerical studies of failure behavior of PLC optical splitters under uniaxial tensile loading

This work presents an experimental and numerical study of the failure behavior of planar lightwave circuit (PLC) optical splitters under uniaxial tensile loading. Based on the experimental results, the specific fracture position, ultimate failure load (Pm), failure mode and load-displacement behavior of the PLC optical splitter under uniaxial tensile loading were determined. In order to numerically simulate the real fracture behavior and stress distribution of adhesively bonded joints, a three-dimensional (3-D) explicit finite element analysis method based on material constitutive and tiebreak contact algorithm was proposed, which treats cohesive failure and adhesive/interfacial failure separately. The Pm predicted by the numerical simulation agrees well with the experimentally obtained Pm with a relative error of ∼4.3%, and the predicted failure mode is also similar to the experimentally observed mixed failure mode, which proves the accuracy and rationality of the adopted finite element method and model. This study can provide useful references for the reliability design and optimization of PLC optical splitters.

Investigation of mechanical properties of microwave welded SS304-SS202 lap joints

The joint between dissimilar materials is always challenging to develop through traditional techniques due to the different properties of the parent materials. However, a novel non-traditional route of joining similar and dissimilar materials through energy conversion and absorption has been developed called SMHH (selective microwave hybrid heating). It has gained much attention due to its unique heating abilities. In the present work, the lap joint between SS202-SS304 is developed through SMHH placing nickel powder as an interlayer. The lap joint was characterized mechanically through shear tests and microhardness. Since the sheared area of the joint is large, it is challenging to achieve a good joint. Thus, to develop a solid joint, laser surface texturing (LST) was introduced to the joining surfaces, and the joints were characterized. After LST was introduced, a fully diffused joint interlayer region was observed through scanning electron microscopy (SEM). The EDS results confirmed the complete metallurgical bonding occurred by the diffusion of elements across the weld zone. From the XRD spectrum, the formation of intermetallic compounds like FeNi, FeNi3, C3Fe7, and M23C6 were identified at the joint region. The mean microhardness of 355 ± 10 HV was determined at the joint region of lap joints with LST. A lap joint with LST exhibits 217 MPa of shear strength, which is significantly higher than the joints without LST. Fractography of the shear failure specimens pointed out that partial heating of the interlayer caused incomplete bonding of the base metal and interlayer for without LST lap joints. However, mixed mode failure of SS202 near the overlap region of the LST lap joint occurred. It is concluded that the addition of LST on the lap joint has made a colossal impact on enhancing joint strength and intermetallic bonding. In addition, temperature-time profiling was studied to understand the heating mechanism involved in SMHH using thermocouples.

Microstructure evolution and mechanical property strengthening mechanisms of Cu/Cu NPs/Cu joint fabricated by ultrasonic spot welding

Ultrasonic spot welding, as a high strain rate plastic deformation process, utilizes the high frequency sliding between the metal sheets to generate a solid-state joining. For the ultrasonic spot welded Cu/Cu joint, even with the good metallurgical bonding at the welding interface, it couldn't obtain high T-peel strength and maintain quite high T-peel load bearing capacity before failure. In order to enhance the T-peel strength and bearing capacity, Cu nanopaticles (NPs) interlayer was screen-printed on the Cu sheet surface that was to be welded. It was beneficial to enhance the temperature at the welding interface and the degree of dynamic recrystallization, obtain the nano-scaled equiaxed grains with random orientations at the welding interface, and change the microstructure evolution pattern along the weld thickness. These results are key factors to the improvement of the joint mechanical strength. In comparison with the Cu/Cu joint, the T-peel strength of the Cu/Cu NPs/Cu joint (471.7 N) was increased by 69.4%, the displacement (0.97 mm) corresponding to the T-peel strength was enhanced by 42.7%, the obviously increased fracture energy was achieved, and quite high T-peel load bearing capacity could be maintained after exceeded the T-peel strength. The mixed failure modes of brittle cleavage and quasi-cleavage as well as shallow dimples fracture occurred in the Cu/Cu joint. However, the failure mode for the Cu/Cu NPs/Cu joint was turn into the micro-void coalescence.

Investigation on the transitional micromechanical response of hybrid composite adhesive joints by a novel adaptive DEM model

This work developed a discrete element model to adaptively capture the transitional micromechanical response and failure mode of adhesive joints with dissimilar adherend materials and different configurations. Especially, the development of the model only requires one-time calibration. Loctite EA 9497 epoxy adhesive, aluminium (AL) and polyphthalamide (PPA) were selected to make different types of hybrid adhesive joints for the lab tests. The modelling applicability in simulating Mode I, Mode II cohesive failure and adhesive, mixed failure modes was subsequently validated with the experimental data. The validation shows that the proposed model can accurately capture the observed failure modes and joint performances. It is followed by investigating the ability to adaptively obtain the variation of fracture energies with different adhesive thicknesses. The results agree well with the current reports on the growth trend of fracture energies which see a rise till the thickness reaching 0.8 mm and subsequently decline to a plateau. Finally, key factors including the adhesive thickness, lap length were selected to perform a parametric study to investigate their influences on the failure mechanism and micromechanical response of hybrid joints. It is found that a thickness range of 0.1–0.3 mm is adequate to obtain satisfactory joint strength whilst thicker adhesive over 0.6 mm will decrease the joint strength. This is due to that a thinner adhesive layer can facilitate its cohesive fractures and thus fully use the resistance of adhesive. A range of lap length from 6 mm to 12.5 mm was found to have a higher efficiency of improving the joint strength when AL adherend was used.

A data-driven simple design of single-level nacre-inspired composites with high strength and toughness

Achieving a good strength–toughness match in nacre-inspired composites (NICs) is a difficult issue since it involves an optimum combination of many microstructural parameters. A multi-parameter design rule for the strengthening and toughening of NICs, which is based on extensive finite element (FE) data analysis and experimental verification, is given in this paper. Extensive FE calculations of NICs under uniaxial tension are firstly performed, with which the effects of the aspect ratio of hard phase (ARHP), the volume fraction of soft phase (VFSP) and the hard/soft-phase modulus ratio (HSMR) on the strength and toughness of composites are analyzed. Combinatorial conditions of these parameters for NICs with low strength and low toughness, high strength and low toughness and well-matched strength and toughness are determined, respectively, by comparisons of FE data samples. Furthermore, 3 D-printed NIC specimens consistent with FE models are prepared and tested by tensile experiments. The experimental results agree well with the FE results, both of which disclose the microscopic mechanism behind the strength and toughness of composites. It is found that a mixed failure mode including tensile fracture of hard phase and shear failure of soft phase can be realized by the combination of a large ARHP, a moderate VFSP and a large HSMR, which consequently leads to an excellent strength–toughness match in NICs. Compared with existing nacre-inspired composites with the same hard phase but more complex microstructures such as interlocking interfaces, deflected hard phases and graded soft phases, the present composite not only shows superior strength and toughness but is also easier to produce. Therefore, a highly-efficient and simple method can be provided for the design of strong and tough biomimetic composites.

Bond Strength and Failure Mode of Glass Fiber Posts with Different Surface Treatments Prior to Silanization: An In Vitro Comparative Study.

The use of chemical agents in the surface treatment of glass fiber posts can improve their bond strength to the root canal. The aim of this study was to assess the bond strength and failure mode of glass fiber posts that received different surface treatments prior to silanization. In this cross-sectional and in vitro experimental study, 50 human lower premolar roots were randomly divided into five groups and subsequently prepared to receive the cementation of a fiberglass post prior to silanization. They were distributed as group 1 (with 24% hydrogen peroxide), group 2 (with 37% phosphoric acid), group 3 (with 1.23% acidulated phosphate fluoride for 2 minutes), group 4 (with 1.23% acidulated phosphate fluoride for 6 minutes), and group 5 (without pretreatment). After cementation, the roots were sectioned into two discs for each cervical, middle, and apical region. Bond strength was assessed using the push out technique. Adhesive, mixed, and cohesive failure modes were also assessed. For data analysis, ANOVA and Tukey's post hoc tests were used, as well as Pearson's chi-square test. A significance of P < 0.05 was considered in all statistical analyses. When comparing the bond strength of root regions, significant differences were obtained in groups pretreated with phosphoric acid (P = 0.018) and acidulated phosphate fluoride for 2 and 6 minutes (P = 0.001 and P = 0.000, respectively). Furthermore, significant differences were obtained between posts treated only with silane and those that received phosphoric acid pretreatment (P = 0.006) and acidulated phosphate fluoride for 6 minutes (P = 0.001). Significant association of mixed failure mode was observed with hydrogen peroxide (P = 0.014) and phosphoric acid (P = 0.006) pretreatments. Cohesive failure was significantly associated with acidulated phosphate fluoride pretreatment for 2 minutes (P = 0.032) and with posts that did not receive treatment prior to silanization (P = 0.000). Posts treated only with silane and pretreated with hydrogen peroxide and acidulated phosphate fluoride for 2 minutes presented significantly higher bond strength with respect to those pretreated with phosphoric acid and acidulated phosphate fluoride for 6 minutes. However, acidulated phosphate fluoride for 2 minutes and silane were associated with a better bonding type.