Fiber Failure Modes Research Articles

Failure Modes of New Fiber Post Systems Using Stereomicroscope: An in Vitro Study

Objective: Failure of the glass posts is a major and the main concern of practical dentistry. This study conducted to evaluate the mode of failure among prefabricated bundle fiber posts, prefabricated taper fiber posts, and short fiber-reinforced composite as a post and luting material using a stereomicroscope. 
 Methods: Thirty mandibular premolars were divided into three groups (n=10). Group 1: prefabricated taper fiber post cemented with a resin core, group 2: prefabricated bundle fiber post cemented with a resin core and group 3: short fiber-reinforced composite used instead of post and cement. Each root specimen cut into three slices of 2 mm thickness from the cervical, middle, and apical parts of the posts. Push-out tests were performed using a universal testing machine at three sites in each root at a crosshead speed of 0.5 mm/min. Then, the specimens evaluated under stereomicroscope 40X to classify the failure mode.
 Results: Sixty two specimens failed adhesively between dentin and cement in all groups, adhesive failure between the post and cement was in the second group, prefabricated taper fiber post (GC fiber post) followed by prefabricated bundle fiber posts (Rebilda GT) (17 sections), and the mixed failure was second for prefabricated bundle fiber posts (Rebilda GT) and third for prefabricated taper fiber post (GC fiber post) (11sections). 
 Conclusions: Adhesive failure between cement and dentin is dominant in all post systems in three sites of the roots (cervical, middle, and apical). The second most frequent failure is adhesive between post and cement in all regions in post systems.

A particle swarm optimization–support vector machine hybrid system with acoustic emission on damage degree judgment of carbon fiber reinforced polymer cables

The feasibility of machine learning in damage degree judgment of carbon fiber reinforced polymer cables was first verified by the improved b-value method and wavelet packet spectrum analysis. Then, a hybrid system with support vector machine classification and particle swarm optimization algorithms was proposed to realize the prediction. The b-value calculated with all acoustic emission events has better performance when noise cannot be avoided. The 1/b-value has almost the same trend with acoustic emission signal cumulative energy, which can meet the preliminarily needs of health monitoring. The particle swarm optimization clustering algorithm works by using nine characteristic parameters of acoustic emission signals. It demonstrates that the characteristic parameters of acoustic emission signals are closely related to the failure mode of the carbon fiber reinforced polymer cable. This indicates their correspondence to the cable’s damage degree and their ability to work as training data for machine learning. With particle swarm optimization, the trained support vector machine can reach at least 77% accuracy of a single acoustic emission signal when predicting the corresponding current damage degree. In addition, using the voting mechanism can promote the performance of support vector machine. This demonstrates the practicability of applying acoustic emission combined with machine learning as a damage degree judgment method for carbon fiber reinforced polymer cables.

Experimental study on the interfacial bond between short cut basalt fiber bundles and cement matrix

Bonding performance of basalt fiber reinforced cementitious composites (BFRCC) was studied through analyses on pull-out tests and scanning electron microscope (SEM) results. It is concluded that: (1) The rupture failure occurred for dispersed and flexible-type fibers, the pull-out failure controlled cluster-type fibers, and failure modes of alkali-resistant fibers are related to their embedded lengths (le). (2) As le increases, peak pull-out load (Npmax) increases and then becomes unchanged. Appropriate le is within 15–18 mm. (3) As specimen ages, Npmax first increases and starts to decrease at 7-day. (4) Comparisons of bonding strength among different fiber types: dispersed > flexible > alkali-resistant > cluster. (5) SEM analyses are consistent with the macro bonding performance of BFRCC.

Research on Material Removal Mechanism of C/SiC Composites in Ultrasound Vibration-Assisted Grinding.

C/SiC composites are the preferred materials for hot-end structures and other important components of aerospace vehicles. It is important to reveal the material removal mechanism of ultrasound vibration-assisted grinding for realizing low damage and high efficiency processing of C/SiC composites. In this paper, a single abrasive particle ultrasound vibration cutting test was carried out. The failure modes of SiC matrix and carbon fiber under ordinary cutting and ultrasound cutting conditions were observed and analyzed. With the help of ultrasonic energy, compared with ordinary cutting, under the conditions of ultrasonic vibration-assisted grinding, the grinding force is reduced to varying degrees, and the maximum reduction ratio reaches about 60%, which means that ultrasonic vibration is beneficial to reduce the grinding force. With the observation of cutting debris, it is found that the size of debris is not much affected by the with ultrasound vibration. Thus, the ultrasound vibration-assisted grinding method is an effective method to achieve low damage and high efficiency processing of C/SiC composites.

Research on energy-absorption and failure of carbon fiber reinforced epoxy resins double cone structure

One of the research hotspot in strategic missile is to develop a multifunctional and lightweight composite launcher, which has a certain bearing capacity and anti-fragment penetration capability. On this basis, this paper designs the launcher’s wall by using carbon fiber reinforced epoxy resin composite double cone structure (CFREPDCS) as sandwich core layer. Due to obtain the high-speed impact energy-absorption and failure characteristic of the CFREPDCS, the quasi-static collapse experiment and the LS-DYNA finite element simulation were used. The experimental results and the scanning electron microscope show that the failure mode of the carbon fiber reinforced epoxy resin composite (CFREP) circular tube is progressive failure, which includes matrix fragmentation and fiber fracture. The CFREP circular tube with larger wall thickness has higher specific energy-absorption (SEA), approximately 58.6 J/g. The coupled relationship between SEA and mid-diameter height shows that the maximum SEA occurs while mid-diameter height reaches 8 mm. On the other hand, the change of the mid-diameter height also reflects the influence of loading direction on SEA. This study provides the significant guide to design and manufacture of sandwich carbon fiber composite launcher.

A computational model for simulation of steel fibre reinforced concrete with explicit fibres and cracks

A new formulation is herein proposed to embed discrete fibres and cracks in finite elements. The main features of the fibre behaviour are automatically accounted for, including bond and snubbing effects. Fibres can interact with cracks and are modelled without the need for additional degrees of freedom. A comprehensive constitutive law directly models the two possible fibre failure modes: pull-out and tensile rupture. The bridging effect of fibres crossing active cracks follows a pull-out relation that depends on the current crack width, fibre orientation, relevant mechanical and geometric fibre properties, and quality of surrounding cement-based matrix. Fibres can be randomly positioned inside a structural member using a non-uniform distribution. The effect of crack openings is transmitted to the neighbouring material as rigid body movement. Numerical tests of material and structural scale are simulated to assess the performance of the technique. The model is shown to adequately simulate the complete behaviour of the members without the need for fitting material parameters to capture complex failure mechanisms. The model is also used for a rational justification about the loss of ductility when fibres are used in concrete beams flexurally reinforced with small amounts of longitudinal reinforcement.

Effect of fiber orientation angles on the material removal behavior of CFRP during cutting process by multi-scale characterization

CFRP composites have been widely used in many fields due to their excellent comprehensive performance. Precision machining of CFRP is the premise to ensure the efficient assembly of CFRP components. Therefore, it is important to clarify the material removal behavior for optimizing the process scheme and improving the machined surface quality. In this presented paper, the 3D finite element models including macro and micro models for CFRP cutting are found and verified by experiment. Based on four typical fiber orientation angles (FOAs), the micro-failure mechanism of CFRP and the formation mechanism of macro-chips under cutting load are studied. Finally, the machined surface quality of CFRP is studied using 3D nano system and scanning electron microscope (SEM). Results show that the damage of fiber and matrix on micro scale and the chip formation on macro scale are greatly affected by FOAs. Under different FOAs, the failure modes of fibers and matrix and the removal behavior of material are different, so that the machined surface quality is various. In this study, the relatively good surface quality is obtained when FOA is 45° or 90°, but a poor surface quality when FOA is 0°or 135°.

An Energy-Based Concept for Yielding of Multidirectional FRP Composite Structures Using a Mesoscale Lamina Damage Model.

Composite structures are made of multidirectional (MD) fiber-reinforced polymer (FRP) composite laminates, which fail due to multiple damages in matrix, interface, and fiber constituents at different scales. The yield point of a unidirectional FRP composite is assumed as the lamina strength limit representing the damage initiation phenomena, while yielding of MD composites in structural applications are not quantified due to the complexity of the sequence of damage evolutions in different laminas dependent on their angle and specification. This paper proposes a new method to identify the yield point of MD composite structures based on the evolution of the damage dissipation energy (DDE). Such a characteristic evolution curve is computed using a validated finite element model with a mesoscale damage-based constitutive model that accounts for different matrix and fiber failure modes in angle lamina. The yield point of composite structures is identified to correspond to a 5% increase in the initial slope of the DDE evolution curve. The yield points of three antisymmetric MD FRP composite structures under flexural loading conditions are established based on Hashin unidirectional (UD) criteria and the energy-based criterion. It is shown that the new energy concept provides a significantly larger safe limit of yield for MD composite structures compared to UD criteria, in which the accumulation of energy dissipated due to all damage modes is less than 5% of the fracture energy required for the structural rupture.

MICROMECHANICAL MODELING OF TENSILE STRENGTH OF SHORT RANDOM CARBON FIBER REINFORCED CONCRETE

The accurate prediction on the response of the composite material is difficult to achieve due to the complexity of its mechanical properties. However, such complexity can be understood well using the micromechanical analysis. One of the most important goals of micromechanical analysis is to predict the failure and strength of the composite material on the basis of the geometries and properties of the matrix and the fibers. This study aims to develop a simplified micromechanical model that predicts the direct tensile strength of a randomly oriented short Carbon Fiber Reinforced Concrete (CFRC) using the modified rule of mixtures based on the assumptions that CFRC will fail by fiber failure mode and with the perfect interfacial bond between the matrix and the fibers. PAN-based High Tensile short carbon fibers distributed randomly in 3D with low fiber volume fractions (Vf) of 0.10%, 0.15%, 0.20%, 0.25% and 0.30% were used in this study. Fiber lengths (Lf) of sizes 19mm, 30mm, and 38mm were used. The designed compressive strength considered for each fiber volume fraction and fiber length was 21MPa, 28MPa, and 35MPa. There were three samples of specimens considered for each case. Each case was tested for its cylindrical compressive strength and its direct tensile strength. Test results showed that the tensile strength of CFRC was optimum at Vf = 0.10% and Lf = 38mm. Finally, good agreement has been observed between the experimental tensile strength and predicted tensile strength using the micromechanical model.

Effects of loading rates on mechanical property and failure behavior of single-lap adhesive joints with carbon fiber reinforced plastics and aluminum alloys

In this paper, the effects of loading rate on mechanical property and failure mode of carbon fiber reinforced plastic (CFRP) composite and aluminum alloy (Al) single-lap adhesive joints were investigated. Four loading rates (2 mm/min, 4 m/s, 8 m/s, and 12 m/s) were chosen to carry out the shear tests. Digital image correlation (DIC) technique was used to analyze the strain evolution of the joints. Results showed that the shear strength increased with the increase of loading rate. When the loading rate increased from 2 mm/min to 12 m/s, the average shear strength of the joint increased from 19.3 to 29.2 MPa. The load-displacement curves under dynamic loading had two peaks, which is mainly caused by local failure and progressive failure of the joints. There were stress concentrations at both ends of the bonding area, which caused cohesive failure and fiber-tear failure. At both ends of the bonding area, the failure mode was mainly cohesive failure of the adhesive, whereas the failure mode in the middle of the bonding area was mainly resin matrix failure of the CFRP. Wherein the peak loads had a certain correspondence with failure modes and failure regions.

Numerical Modelling of Glass Fiber Reinforced Polymer (GFRP) Cross Arm

Composites are not isotropic like their metal counterparts, e.g. steel and aluminum, as they are made of two distinctive phases known as the matrix and the reinforcing phases. In addition, weight, fiber direction, fiber composition and even the manufacturing process are all critical factors in determining the strength, stiffness and the behaviour of a composite member. All of that create more challenging designing and manufacturing approaches. This paper shows how to model a GFRP cross arm using SOLIDWORKS to create the 3D geometrical model because it has an intuitive and easy to use user interface, and ANSYS to create the numerical model and the analysis for its great and comprehensive capabilities in the finite element analysis. The cross arm was found to be safe against the failure modes of fiber, matrix, in-plane shear, out-of-plane shear and delamination under all load cases which satisfies the ultimate limit state requirements but the concern was on the serviceability limit state which had a deflection of 34 mm.

Effect of steel and polypropylene fibers on the quasi-static and dynamic splitting tensile properties of high-strength concrete

The influence of steel-polypropylene hybrid fibers on the tensile properties of high-strength concrete is investigated by quasi-static and dynamic Brazilian disc splitting tests. The failure modes of fiber reinforced concrete specimens and the influence of different fiber mixing ratios on the splitting tensile strength of the specimens are analyzed. The research results indicate that the plain concrete and single-doped polypropylene fiber concrete specimens show straight cracks along the loading line during the experimental process, showing the brittle failure characteristics; while the single-doped steel fiber concrete and hybrid fiber concrete specimens show more bifurcated micro-cracks along the loading line, showing the ductile failure characteristics. The splitting tensile strength of concrete specimens increases with the increase of steel fiber content, and increases first and then decreases with the increase of polypropylene fiber content. The mixed use of steel fiber and polypropylene fiber not only improves the splitting tensile strength of concrete specimens, but also reduces the risk of brittle failure of concrete. Especially when the steel fiber content is S2 (2.5%), the hybrid of steel-polypropylene fibers has a great improvement effect on the splitting properties of concrete specimen. In addition, it is found that the splitting tensile strength of fiber reinforced concrete specimens increases with the increase of force loading rate, showing the enhancing effect of loading rate. Comparing the rate-enhancing effect of specimens with different fiber content, it is found that the change of fiber content has a certain influence on the loading rate-enhancing effect of specimens.

Mechanical properties and failure modes of hybrid fiber reinforced polymer composites with a novel liquid thermoplastic resin, Elium®

In virtue of recyclability purposes, higher impact and damage tolerance properties, efficient and ease of fabrication, a new liquid Methyl Methacrylate (MMA) thermoplastic resin, Elium® 188, has been recently developed. This new resin can replace (epoxy) thermoset-based resins for the fabrication of laminates at room temperature with equivalent mechanical properties. Plain weave ultra-high molecule weight polyethylene fiber (UHMWPE), carbon fiber, and their hybrid systems are fabricated by Vacuum Assisted Resin Infusion (VARI) method at ambient temperature. ASTM standard tests of tensile, compression, and shear (both intralaminar and interlaminar) are conducted to determine the mechanical properties with the aim of comparing the results with those of thermosets. Fractographic analyses are performed to have a better understanding of the behavior of these new thermoplastic-based laminates. The test results show the mechanical properties of thermoplastic-based laminates are comparable to those thermoset-based but with the aforementioned advantages, leading to wider applications in the industry.

A high-efficient model for interface debonding analysis of carbon fiber-reinforced polymer composite

ABSTRACTDue to the difficulties in micro stress calculation of interface, most failure analysis models of macro composite structure mainly focus on the failure modes of fiber and matrix, which have not taken interface debonding into consideration. In this paper, a high-efficient failure analysis model is proposed to realize quick investigation of interface debonding as well as fiber and matrix failure in macro composite structure under complex stress condition. Stress amplification factors are adopted to realize the rapid calculation of micro stress at interface according to the macro stress of composite. Three-point bending tests with elaborate designed stacking sequences are carried out to extract the critical interfacial normal strength and shear strength used in this model. In the failure analysis of micro unit cell this model shows high accuracy in the prediction of interface debonding both in the failure modes and failure locations compared with cohesive zone model. To further inspect its capability in the analysis of macro composite structures, open-hole compression tests of quasi-isotropic laminates are carried out. Good agreements in the interface debonding mechanisms are obtained between numerical prediction and experimental results.

FLEXURAL CAPACITY ASSESSMENT OF BRICK AGGREGATED PRE-CRACKED RC BEAM STRENGTHENED WITH CARBON FIBER POLYMER

Ageing and improvements to design code has led to many existing RC structures made of locally available brick aggregates are now found structurally deficient and are in need of rehabilitation. This research emphases on flexural capacity assessment and investigation of failure modes of Carbon Fiber Reinforced Polymers (CFRP) strengthened brick aggregated RC beams. Flexural performance of the RC beam specimens are evaluated using four point bending method. Six RC beams (initially cracked) with CFRP strengthening were tested by varying (i) type of CFRP, (ii) reinforcing area, (iii) anchorage type; and (iv) number of CFRP layers. Two beams were tested as control specimens. Unidirectional carbon fiber sheet (Tow Sheet) and individually hardened continuous fiber strands woven into sheet form (Strand Sheet) were used. Simple flexure failure was obtained for unstrengthened RC beams while end plate and interfacial debonding were observed for the initially cracked CFRP strengthened RC beams. Strengthening of pre-cracked beams using Strand Sheet gave better performance compared to Tow sheet. Overall flexural strength improvement of CFRP strengthened beams varied from 12% to 34% with respect to unstrengthened beams depending on strengthening methods.

FLEXURAL CAPACITY ASSESSMENT OF BRICK AGGREGATED PRE-CRACKED RC BEAM STRENGTHENED WITH CARBON FIBER POLYMER

Ageing and improvements to design code has led to many existing RC structures made of locally available brick aggregates are now found structurally deficient and are in need of rehabilitation. This research emphases on flexural capacity assessment and investigation of failure modes of Carbon Fiber Reinforced Polymers (CFRP) strengthened brick aggregated RC beams. Flexural performance of the RC beam specimens are evaluated using four point bending method. Six RC beams (initially cracked) with CFRP strengthening were tested by varying (i) type of CFRP, (ii) reinforcing area, (iii) anchorage type; and (iv) number of CFRP layers. Two beams were tested as control specimens. Unidirectional carbon fiber sheet (Tow Sheet) and individually hardened continuous fiber strands woven into sheet form (Strand Sheet) were used. Simple flexure failure was obtained for unstrengthened RC beams while end plate and interfacial debonding were observed for the initially cracked CFRP strengthened RC beams. Strengthening of pre-cracked beams using Strand Sheet gave better performance compared to Tow sheet. Overall flexural strength improvement of CFRP strengthened beams varied from 12% to 34% with respect to unstrengthened beams depending on strengthening methods.

A study of energy dissipating mechanisms in orthogonal cutting of UD-CFRP composites

This paper presents a three-dimensional thermo-mechanical finite element (FE) model on the micro-scale for the orthogonal cutting simulation of unidirectional carbon fiber reinforced polymer (UD-CFRP) materials. Fiber, matrix and the fiber-matrix interface are modeled separately to simulate failure mechanisms associated with fiber breakage, matrix cracking and fiber-matrix debonding. Experiments on orthogonal cutting of UD-CFRP workpieces with various fiber orientations have been conducted and compared to the micro-scale FE simulations. The good correlation between the experimental and numerical results with respect to cutting forces, chip formation and surface quality enables for an in-depth FE energy analysis to quantify dominating failure mechanisms in different fiber orientations. In addition, the contribution of each fiber failure mode during orthogonal cutting is evaluated. It is found that dissipated energies associated with various failure modes and friction vary with the fiber orientations and that the simulation provides insight into the characteristic fiber orientation-dependent quantities observed in the experiments such as chip formation and surface morphology. Finally, a sensitivity analysis is carried out to assess the variation of the simulated cutting response with respect to input parameters with high uncertainty including friction coefficients or fiber strength values.

Fabrication and mechanical behaviors of carbon fiber reinforced composite foldcore based on curved-crease origami

A carbon fiber-reinforced composite foldcore based on curved-crease origami was designed and fabricated using a hot press molding method. The fabrication process based on curved-crease origami reduces the abrupt change in fiber direction and is more suitable for continuous fiber-reinforced composites. The analytical models based on differential and integral method for predicting the compressive stiffness and strength of curved-crease origami foldcores were developed first and a three-dimensional failure mechanism map was constructed. The compressive properties and failure modes of carbon fiber reinforced composite curved-crease origami foldcores were investigated through experiments and finite element analysis (FEA). A reasonable agreement among the analytical models, experiments, and FEA results was observed. The dominant failure mode in the curved-crease origami foldcores could change from buckling to crushing with the wall thickness of foldcores increasing when subjected to compressive loads. The novel curved-crease origami design with open core channels has potential to applications for lightweight multifunctional structures, such as fuselage panels of airplane.

Influence of Polypropylene Fiber Reinforcement on Tensile Behavior and Failure Mode of Tailings Cemented Paste Backfill

Tailings were used to prepare cemented paste backfill (CPB) reinforced with polypropylene (PP) fiber, and fiber content and fiber length were 0-20 % and 3-12 mm, respectively. The Brazilian indirect tensile strength tests, macrostructural and microstructural (SEM) failure mode analyses and nuclear magnetic resonance (NMR) tests were performed. The results demonstrate that PP fiber with high tensile strength can substantially enhance the tensile strength and ductility of CPB specimens. The fiber-reinforced CPB exhibits superior performance during the pre-peak and post-peak stages and can retain its residual strength after being broken. The fiber-reinforced CPB provided a higher secant Young's modulus and strain at peak stress than that of the unreinforced CPB. From the failure mode analysis, fiber was used to bridge the CPB microelements and combine these effectively with calcium silicate hydrate gelling. The governing failure mode of fiber is pulled out or pull off, through which fiber absorbs the tensile stress and energy. However, fiber is also believed to have confusion distribution in CPB because fiber cannot reach their full strain capacity, especially at the later curing stage. And the weak structural layers in the CPB can also be formed, and the higher tensile strength is not achieved with the additional fiber content or longer fiber length. The optimal fiber content in this study is recommended to be 0.15 %, and it's better to choose 6 or 9 mm fiber. Moreover, fiber can reduce the original porosity by 10.72 %, but only has a slight influence on the pore size. The decrease in porosity helps increase the tensile strength of the fiber-reinforced CPB.

Pullout behavior of straight and hooked-end steel fibers in UHPC matrix with various embedded angles

The study provided a systematic evaluation of the pullout behavior of straight and hooked-end steel fibers embedded in ultra-high performance concrete (UHPC) with various angles using a single fiber pullout test. Three steel fibers, including one straight fiber and two hooked-end fibers, were aligned at 0°, 30°, and 45° with respect to the loading direction. The fibers were pulled out from non-fiberous UHPC matrix with a compressive strength of 151.5 MPa. Experimental results showed that when the fiber embedded angle increased from 0° to 30° and 45°, the average bond strengths of straight fibers were increased by 19.2% and 52.9%, while the average bond strengths of hooked-end fibers were increased by 10.3–13.6% and 16.2–26.1%, respectively. The percentage of end hook contribution to total pullout energy ranged from 50% to 56.8%, indicating significant mechanical anchorage contribution. Two types of fiber failure modes, including fiber pullout failure and fracture failure, were found, depending on fiber angles. From Scanning Electronic Microscopy (SEM) observation, interfacial transition zone failure, fiber surface scratching, and matrix failure were identified. Two new indices, namely energy dissipation index and bond strength index, were proposed to evaluate the fiber-matrix bond behavior. It was found that hooked-end fibers with a smaller diameter could be a better choice for structural application according to the proposed evaluation indices. The proposed indices offered an alternative and practical method to assess the fiber-matrix bond behavior.