Epoxy Specimens Research Articles

A comparative machinability analysis of aged and freshly manufactured epoxy resins through orthogonal machining experiments

Applications of thermoset epoxy resins in load-bearing fibre-reinforced polymer (FRP) composites are decisive, mainly due to their excellent material properties, low viscosity before hardening and good adhesion with the reinforcing fibres. Although numerous experiences on the machinability of FRPs have been published, these experiences can be only indirectly adapted to pure epoxy resins. Reflecting on the lack of knowledge on the machinability of epoxy resins, the main aim of the present study is to compare the machinability of aged and freshly manufactured epoxy resins. Half of the epoxy specimens were naturally aged in a continental climate environment for a year, while the other half was manufactured prior to the orthogonal machining experiments. The experiments were conducted in a dry condition in a Kondia B640 machining centre. The cutting speed and the uncut chip thickness were varied systematically. The cutting force was measured by a KISTLER 9257B dynamometer, and the machined surfaces were characterised by a Mitutoyo SJ400 surface tester and a Keyence VR-5000 3D profilometer. The experimental results prove that the aged epoxy degraded significantly; thus, the specific cutting force is significantly smaller than that of freshly manufactured epoxy. However, the surface quality was not significantly influenced by the polymer degradation.

Post-creep residual tensile properties of multi-walled carbon nanotube/epoxy nanocomposites

Epoxy resin as a thermoset polymer is vulnerable to creep loading even at room temperature due to its viscoelastic nature. This study investigated the effect of reinforcing epoxy resin with different functionalized multi-walled carbon nanotubes (MWCNT) contents on the creep response and post-creep residual tensile properties of nanocomposites. The creep tests were performed on the nanocomposite specimens containing different filler contents and the neat epoxy specimen at 40°C under a constant load level of 200 N. It was found that the nanocomposites containing 0.3 wt% MWCNTs experienced 29.6%, 69.1%, and 74.1% decreases in the elastic strain, creep strain, and steady-state creep strain rate, respectively, compared to the neat epoxy. Furthermore, the tensile strength and stiffness of the neat epoxy and nanocomposite specimens were evaluated before and after a partial creep test (at a load level of 200 N for 150 min) by conducting tensile tests. The nanocomposites containing 0.3 wt% MWCNTs demonstrated considerable improvements of 35.9%, 41.2%, 27.9%, and 28.1% in strength, residual strength, stiffness, and residual stiffness, respectively, compared to the neat epoxy. Furthermore, scanning electron microscopy assessment was utilized to investigate the fracture surfaces of the nanocomposite specimens.

Investigation of viscoelastic behavior of Abaca-reinforced epoxy composites

Natural fiber-based composites demonstrate excellent and comparable static and dynamic mechanical properties to conventional materials, such as steel and aluminum. They also extend their applications to aeronautical, sports equipment, and marine fields. This experimental study aims to find the effect of untreated and treated Abaca-reinforced epoxy composites on the viscoelastic behavior and the optimum combinations of fiber and resin to produce better bonding efficiency. The different specimens used for this study were pure epoxy, untreated, and chemically treated composite specimens. The four weight percentages of Abaca fibers are 10%, 20%, 30%, and 40% used to prepare composite specimens. Similarly, four different sodium hydroxide (NaOH) concentrations, 4, 6, 8, and 10 wt. %/vol. %, have been used for the chemical treatment of fibers. The storage modulus of Abaca-reinforced epoxy composite specimen has been investigated with respect to temperature and fiber content. The result shows that the 30% weight fraction of fibers with chemically (8 wt. %/vol. %) treated fiber-reinforced epoxy specimen produces 41.67% higher storage modulus than the 10% weight fraction of fibers content of composite specimens. Fourier-Transform Infrared Spectroscopy (FTIR) broad transmittance has been used to distinguish the raw and chemically treated fibers. FTIR results reveal the removal of functional groups after NaOH treatment.

Surface modification for improving interfacial, mechanical and thermal performance characteristics in epoxy composites: Electroless nickel enhancement of dendritic copper particle-reinforced epoxy

While studies on the incorporation of metal particles into the epoxy matrix are quite common, this research introduces a novel approach by incorporating electroless Ni-coated Cu particles with functional features into the epoxy matrix. Thus, it presents an innovative investigation of epoxy composites with enhanced both thermal and mechanical properties. Therefore, three samples of pure epoxy, dendritic Cu particle reinforced (20 wt%), and electroless Ni-coated dendritic Cu particle reinforced (20 wt%) epoxy composites were fabricated by mixing followed by casting into the mold. The epoxy and composite specimens underwent phase analysis through XRD, and morphological examinations were conducted using SEM equipped with an EDS analyzer, respectively. The hardness, tensile, and bending tests were conducted for mechanical characterization. To evaluate the thermal properties, DSC and TGA tests were employed to assess the impact of the Cu and the electroless Ni-coated Cu particles on the thermal stability of the composites. The mechanical analysis results showed that the pure epoxy sample had tensile and flexural strengths of 23.2 MPa and 43 MPa, respectively. These values increased significantly by 90 % and 311 % in the presence of Ni-coated dendritic Cu particles, reaching 54 MPa and 177 MPa, respectively. Thermal analysis results indicated that the Tg increased with the incorporation of dendritic Cu particles and Ni-coated dendritic Cu particles. Furthermore, the highest thermal conductivity was observed in the Cu-reinforced composites.

Static and Fatigue Characterization of Large Composite T-Bolt Connections in Marine Hygrothermal Environments

Fiber-reinforced polymer composites have been highlighted as ideal candidates for structural applications in marine renewable energy devices, such as tidal turbines and wave energy converters. It is well understood that harsh marine environments can cause strength degradation of composite laminates, which has been extensively researched at the coupon scale; however, no research has investigated how this translates into larger-scale composite structures. This paper presents a subcomponent-scale study which investigates the effects of hygrothermal aging and subsequent static and fatigue characterization of thick composite T-bolt connections as part of a large, multilaboratory materials research effort. Of the glass-reinforced epoxy and vinylester-epoxy matrix composites tested, both showed measurable static strength degradation (4–36%) after being hygrothermally aged, even though the composite specimens were only partially saturated with water. Under tension–tension fatigue loading, the epoxy specimens performed very well in their dry states but exhibited significant degradation after hygrothermal aging. In comparison, the vinylester-epoxy specimens had much shorter fatigue lives in their dry states but exhibited no degradation after hygrothermal aging. Overall, this research demonstrates that hygrothermal aging can have significant effects on the ultimate strengths and fatigue lives of even partially saturated thick composite T-bolt connections, indicating that degradation of the outer plies on thick composite laminates can have pronounced effects on the whole structure. It discusses the challenges of building an understanding of the effects of harsh marine environments in large-scale composite structures.

Adaptive Prognostics: A reliable RUL approach

Prognostic methodologies have found increasing use the last decade and provide a platform for remaining useful life (RUL) predictions of engineering systems utilizing condition monitoring data. Of particular interest is the reliable RUL prediction of engineering assets that either underperform or outperform due to unexpected phenomena that might occur during the operational life. These assets are often referred as outliers and the prediction of their RUL is a challenging task. The challenge is to accurately predict the RUL of an outlier without taking into account outlier’s condition monitoring data in the training process but just in the testing process. As a result, the lifetime of the testing asset is shorter (left outlier) or longer (right outlier) than the training process’ lifetimes.
 This study addresses this challenge by proposing a new adaptive model; the Similarity Learning Hidden Semi Markov Model (SLHSMM), which is an extension of the Non-Homogenous Hidden Semi Markov Model (NHHSMM). The SLHSMM uses a similarity function, such as Minkowski distances, in order firstly to quantify the similarity between the testing asset and each training asset and secondly to adapt the trained parameters of the NHHSMM. To demonstrate the effectiveness of the proposed adaptive methodology, composite structures have been used as a validation engineering asset. In particular, the training data set consists of strain data collected from open-hole carbon–epoxy specimens, which were subjected to fatigue loading only, while the testing data set consists of strain data collected from specimens that were subjected to fatigue and in-situ impact loading, which can be considered as an unexpected phenomenon and unseen event regarding the training process. 
 Utilizing the aforementioned strain data the SLHSMM RUL predictions and the NHHSMM RUL predictions were compared, so as to verify that the SLHSMM provides better prognostics than the NHHSMM. SLHSMM provides better predictions in comparison to the NHHSMM for all the test cases, demonstrating its capability to adapt to unexpected phenomena and integrate unforeseen data to the prognostics course.

Study of crack extension and branching behavior of layered materials under impact loading

This study utilized a digital laser dynamic caustics experimental system to conduct three-point bending impact experiments on layered epoxy specimens with different values of angle α between the bedding and the prefabricated crack, as well as the epoxy specimen without bedding. A high speed camera was employed to record the dynamic caustic spot at the crack tip during specimen extension and branching, analyze the dynamic stress intensity factor for crack tip for Mode I and II, crack extension speed, and exploration of the influence of angle α on crack extension and branching behavior. Numerical simulations were performed using the cohesive element method. The findings indicate an increase in crack initiation toughness as the angle α increases. After the initiation of the main crack, the failure mode was Mode I-dominated damage. When it extended near the bedding, the failure mode of the main crack changed to Mode II-dominated damage due to the influence of the bedding, resulting in the formation of branching cracks and microcracks. The main crack again exhibited Mode I-dominated damage after branching. Notably, the branching cracks primarily exhibited Mode II-dominated damage. The occurrence of branching cracks and microcracks intensifies with higher α. The variation of α significantly influences the speed of crack extension.

Prediction of Mechanical Behavior of Epoxy Polymer Using Artificial Neural Networks (ANN) And response Surface Methodology (RSM)

The aim of this study is to analyze the effect of different geometries and sections on the mechanical properties of epoxy specimens. Five tensile tests were carried out on three types of series. The experimental results obtained were 1812.21 MPa, 3.90% and 41.91 MPa for intact specimens, 1450.41 MPa, 2.16% and 21.28 MPa for specimens with hole and 750.77 MPa, 2.77% and 11.89 MPa for specimens with elliptical -notched for Young's modulus, strain and stress respectively. In addition, the experimental results indicated that the mechanical properties of both (Young's modulus value and stress value) were higher in an intact specimen. Afterwards, the nonlinear functional relationship of input parameters between epoxy sample geometries and sections was established using the response surface model (RSM) and the artificial neural network (ANN) to predict the output parameters of mechanical properties (Young's modulus and stress). In addition, the design of experiment was developed by the Analysis of the Application of Variance (ANOVA). The results showed the superiority of the ANN model over the RSM model, where the correlation coefficient values for the model datasets exceed ANN (R2 = 0.984 for Young's modulus and R2 = 0.981 for the constraint).

Length-scale discrepancy in the properties of epoxy resin specimens

In studies of the fibre-matrix interphase with microscale single fibre methods, the dependence of results on conversion of the thermoset resin – or degree of cure as it is often called – remains an issue. In the microbond method specifically, the curing of picolitre volume drop-on-fibre systems differs significantly from that of macroscale resin batches. The surface-to-volume ratio and vapour pressure can cause volatile components of the resin to evaporate, potentially limiting the degree of cure. Atomistic scale modelling along with experimental thermal analysis were used to understand the curing process and how it translates to resin properties, while nanoindentation was used to experimentally compare the mechanical performance of samples prepared in different length-scales. The evaporation is experimentally verified. Comparable variation in mechanical properties is shown in atomistic scale models of the epoxy network with no evaporation. The origin is in the network morphologies created by varying the curing process. Thus attributing the length-scale discrepancy solely to conversion is likely an oversimplification and understanding the network morphology from different curing conditions is also needed.

Preparation of polymeric composite for antiskid flooring purpose

Three polymeric blends were prepared using epoxy, nitrile butadiene rubber (NBR), and polyurethanes (PU) to develop antiskid surfaces with the required mechanical properties. The optimum mixing ratio was decided according to the results of the coefficient of friction and impact tests. Mechanical properties, including wear, bending, compression, impact, hardness, and water uptake, were measured. Scanning electron microscopy (SEM) was used to investigate the microscopic features. The results showed variations in the values of coefficient of friction for the three blends, where the epoxy/NBR specimens showed the highest value of 0.81, followed by specimens of epoxy blended with the two types of commercial polyurethanes, Sikaflex™ and Sikaswell™ of 0.73 and 0.69, respectively. The highest wear rate was noticed for the epoxy/NBR specimen, while the epoxy+Sikaflex specimen showed the lowest wear rate compared to other specimens. The epoxy/Sikaflex also showed the highest values for hardness, compared with epoxy/Sikaswell and epoxy/NBR, respectively. The resultant blends showed improved mechanical properties with high slipping resistance, which makes the prepared blends potential alternatives compared to traditional flooring materials.

Effect of imperfect cure on tensile bond measurement

This paper quantifies variability in tensile bond measurement due to incomplete mixing within the nozzles of manual gun epoxy dispensers. Controlled pull-off tests were conducted using an MTS machine and steel substrates were tested to minimize variations in bond line thickness. The pull-off tensile force was evaluated after 1, 2, 3, 7, 14 and 28 days. In each of these six series, five identically prepared specimens were tested. The average coefficient of variation was below 0.10; where it was higher, investigations showed that the epoxy had not mixed thoroughly inside the nozzle. The average 7-day cure tensile bond from identically tested hand mixed epoxy specimens was over 19% higher with lower variability. Hand mixing is recommended in preparation and bonding of pull-off dollies.

Identification of invisible fatigue damage of thermosetting epoxy resin by non-destructive thermal measurement using entropy generation

To quantify carbon fiber-reinforced plastic (CFRP) fatigue, herein, we investigate the relationship between fatigue and an epoxy resin used in CFRPs. Generally, fatigue is related to the entropy, which comprises the mechanical entropy, calculated from the dissipated energy and temperature, and thermal entropy, calculated from the relationship between specific heat capacity and temperature. According to previous studies, mechanical entropy generation and thermal entropy generation are equal. Herein, 100 cyclic loading tests are conducted on epoxy resin specimens consisting of 4,4’-DDS and bisphenol a diglycidyl ether. The dissipated energy is determined based on stress – strain curves, and mechanical entropy generation is quantified. An equation for the relationship between the specific heat capacity and temperature is developed based on the Debye model, and the increase in specific heat capacity is calculated for equal mechanical and thermal entropy generations. Generally, differential scanning calorimetry is used for specific heat capacity measurements; however, because these measurements are performed by cutting the specimen, a nondestructive measurement method is required. In this study, the specific heat capacity is measured using lock-in thermography (LIT), and the measured and estimated values are comparable. Thus, fatigue can be estimated by quantifying the thermophysical properties, and the lock-in thermography method is a suitable thermophysical property measurement method for this application.

Effect of impactor nose form on the impact behavior of reinforced composite materials

Abstract The goal of the present study is to investigate the influence of impactor shape on the low-velocity impact behavior of the composite panels manufactured with different reinforcement materials at the same thicknesses, experimentally. Kevlar, carbon, S-2 glass woven fabric and epoxy matrix have been used to manufacture thermoset composites with the vacuum-assisted resin infusion molding method. The Fractovis plus test machine with a 12.7 mm diameter hemispherical and two different conical impactor noses was used to perform impact tests. The effects of impactor shapes on the low-velocity impact behavior have been compared for 20 J, 40 J, and 60 J energy levels. Although at the same impact energy level, S-2 glass fiber-reinforced epoxy specimens have a higher perforation threshold than carbon and Kevlar fiber-reinforced epoxy. Because to achieve the same thickness, the number of S-2 glass layers was greater than the number of carbon and Kevlar layers. Additionally, the form of the impactor greatly influenced the perforation threshold.

Time-dependant degradation of highly cross-linked epoxy due to hygrothermal ageing at three different temperatures

Laminating grade, highly cross-linked, epoxy specimens were hygrothermally aged by immersing in de-ionized water baths maintained at 75°C, 50°C and room temperature respectively for a period of 75 days. An increase in mass which exacerbated with temperature of ageing was observed for all the aged specimens and the rate of increase became sluggish as ageing progressed without attaining a saturation in mass. The diffusion coefficient (D) increased with hygrothermal ageing temperature. A fraction of the absorbed water (RF) was retained by the epoxy and it increased with ageing temperature and time. This absorption led to hydrolysis reaction in the epoxy matrix as inferred from the Fourier-transform infrared spectroscopy (FTIR) spectrum where we observe an increase in the absorbence peak of C = O (carboxylic), -OH (bending carboxylic) and C-O (stretching alcohol), which are the reaction products of hydrolysis. Differential scanning calorimetry (DSC) indicated a decrease in the glass transition temperature (TG) with severity of ageing till 50°C but showed only a slight decrease in TG for the specimens aged at 75°C possibly due to the formation of a secondary cross-link network by the retained water molecules which are predominantly Type II bonded water. Uniaxial tensile strength decreased with ageing time and temperature accompanied by a decrease in ductility. Tensile fracture surface showed a crack initiation region usually at an edge, a crack stretch region followed by a catastrophic failure region which encompassed the balance region of the fracture surface. The spacing of the crack stretch marks, formed due to crazing, increased with severity of ageing due to strain localization. However, uniaxial compression tests showed an increase in strength with hygrothermal ageing.-

Synergistic influence of MWCNTs/RGO on low-velocity impact response and mechanical properties of carbon fiber/epoxy composite

Nanoparticles can be used to enhance and improve the mechanical properties and low-velocity impact response of carbon fiber-reinforced polymer (CFRP) composites both simultaneously and individually. Also, the synergistic influence of two nanoparticles can be improved the mechanical properties and low-velocity impact response of CFRP composites. In this paper, the effects of reduced graphene oxide (RGO) and multi-walled carbon nanotubes (MWCNTs) on the mechanical properties, low-velocity impact response and damage area of epoxy/fiber carbon composites are investigated both simultaneously and individually. Composite specimens are fabricated with 0.4 weight percentages (wt.%) of RGO, 0.06 wt.% of MWCNTs individually, and a combination of RGO and MWCNTs with 0.6 and 0.06 wt.%, respectively. For comparison of the results, the neat epoxy specimens are fabricated and also tested. Direct homogenization technique is applied for preparation of nanocomposite mixture and then each layer of carbon fiber reinforced nanocomposite is fabricated using a hand lay-up process. Tensile modulus, tensile strength, variations of load, displacement, velocity, and absorbed energy of specimens versus time are obtained. For specimens with MWCNTs/RGO tensile modulus and tensile strength increased by about 21.30% and 17.12%, respectively, and the load peak increased by 15.15% at 1 J, 13.35% at 2 J, 39.62% at 3 J, and 39.62% at 3 J. It is concluded that the synergistic influence of MWCNTs and RGO on the results is more significant and has a higher effect on the impact responses. After the impact tests, Optical microscopy and SEM method are used to analyze fracture surfaces.

The investigation of wear properties of nanoparticle-reinforced epoxy composite material surfaces

This study added nano-sized Al2O3, Boron, and TiO2 powders to the epoxy polymer at 0.5% and 1% ratios. Abrasive wear resistance properties of nanoparticle-reinforced epoxy polymers were investigated. First cylindrical specimens with and without additives were prepared for realizing the experimental research. Pin-on discs were used for the wear test of epoxy samples. The mass losses were measured via a precision scale. According to the results, the boron nanoparticles have increased the epoxy specimens’ resistance. As a result of the experimental studies, it was observed that the wear resistance of the epoxy composite increased with each nano-sized powder added to the epoxy. SEM and optical profilometry investigated the composites’ friction coefficient and surface morphology. As a result of friction coefficient and wear weight loss tests, the highest wear resistance was obtained in 1% boron powder nano-reinforced epoxy composites. It was observed that the epoxy friction coefficient was in the range of 0.4–0.6, which decreased to the range of 0.2–0.4 with the addition of nano boron. The surface roughness value after epoxy wear was measured as 1.4 μm. With the addition of nano boron, this value was measured as 0.32 μm. Optical profilometry and SEM imaging results also support these values.

Synthesis of a Sustainable and Bisphenol A‐Free Epoxy Resin Based on Sorbic Acid and Characterization of the Cured Thermoset

AbstractIn the present study, an epoxy compound, 1,2‐epoxy‐6‐methyl‐triglycidyl‐3,4,5‐cyclohexanetricarboxylate (EGCHC) synthesized from sorbic acid, maleic anhydride, and allyl alcohol is proposed. Using commodity chemicals, a bio‐based carbon content of 68.4 % for the EGCHC resin is achieved. When cured with amine hardeners, the high oxirane content of EGCHC forms stiff cross‐linked networks with strong mechanical and thermal properties. The characterization of the epoxy specimens showed that EGCHC can compete with conventional epoxy resins such as DGEBA. A maximum stiffness of 3965 MPa, tensile strength of 76 MPa, and Tg of 130 °C can be obtained by curing EGCHC with isophorone diamine (IPD). The cured resin showed to be decomposable under mild conditions due to the ester bonds. The solid material properties of EGCHC expose its potential as a promising bisphenol A, and epichlorohydrine free alternative to conventional petroleum‐based epoxies with an overall high bio‐based carbon content.

The effect of aligning graphene‐oxide nanoplatelets on moisture absorption and thermal stability of polymeric nanocomposites

AbstractThis article investigates the effect of aligning graphene‐oxide nanoplatelets (GONPs) on the moisture absorption and thermal stability of polymeric nanocomposites. Various nanocomposite specimens were fabricated using different GONP contents to improve water uptake resistance and thermal stability by aligning GONPs using an AC electric field during manufacturing. The results showed that aligning 0.3 wt% GONPs resulted in the most significant improvements, with the reductions of 76% and 54% observed in the water uptake of nanocomposites at the initial and saturation stages, respectively. The thermal stability of the nanocomposite specimens was also investigated using thermogravimetric analysis, in which nanocomposites containing 0.3 wt% randomly dispersed GONPs demonstrated improved performance compared with the neat epoxy specimen. However, aligning GONPs resulted in a marginal effect on the thermal stability parameters of nanocomposites. Moreover, Fourier transform infrared spectroscopy was conducted to investigate the chemical nature and hydrophilicity of the specimens. In addition, the freeze‐fractured surfaces of the neat and nanocomposite specimens were investigated using scanning electron microscopy. The results indicated the role of nanoparticle alignment in enhancing the performance of nanocomposites under hygrothermal conditions in terms of water uptake, which is of high importance in the marine industry.

Hygrothermal degradation of MWCNT/epoxy brittle materials under I/II combined mode loading conditions: An experimental, micro structural and theoretical study

The effects of carboxylated multi-walled carbon nanotubes (MWCNTs) are studied experimentally and theoretically on mixed-mode (I/II) fracture behavior of epoxy resin at the presence of hygrothermal aging conditions. In this paper, the mixed-mode fracture of MWCNT/epoxy is experimentally measured using Short Beam Bending (SBB) samples with different crack inclination angles (α) ranging from pure mode I (i.e., α = 0°) to pure mode II (i.e., α = 39°). To examine the effects of hygrothermal aging conditions and water barrier properties of MWCNTs, four groups of samples including a group of neat epoxy specimens, and three groups of MWCNT/epoxy specimens (with 0.1, 0.3, and 0.5 wt% of MWCNTs) were manufactured and immersed into hot deionized water for 15 days at 60 °C temperature and similarly four groups were placed at room temperature conditions (26 °C and 25–35% relative humidity) for 15 days. According to the experimental results, toughening the epoxy resin by incorporating nanotubes with 0.1, 0.3, and 0.5 wt% of MWCNTs improved the fracture toughness at both environmental conditions, but the highest fracture toughness was obtained for composition containing 0.3 wt% nanoparticles. The variations of fracture loads were dependent on the ambient environment, MWCNTs content, and mode mixity. The relationship between the obtained experimental results, microstructure and morphology of fracture in the tested samples was also investigated. In addition, the load-carrying capacity of the SBB samples made of the neat and MWCNTs modified epoxy material was also predicted well using a two-term based fracture model called Generalized Maximum Tangential Stress (GMTS) criterion.

Quasi-static compressive behaviour of epoxy composites reinforced with crumb rubber

Quasi static compressive response of crumb rubber-epoxy composites was examined by varying the crumb rubber composition (0, 10, 20 and 30 vol.%). All the composites, irrespective of the strain rates, depict elastic regions tailed by a wide plateau area that is credited with the densification of rubber particulates. Higher strains to failure of composites were revealed as compared with neat epoxy signifying higher energy absorption ability of constituents. The modulus of elasticity of composites was noted to be lower than neat epoxy specimens, irrespective of the strain rates. Irrespective of strain rates, the strength of all the composites were inferior to neat epoxy specimens. Energy absorption of EC-30 was higher compared to EC-20 and EC-10 and noted to be increased in the range of 6–14% for 0.1 mm/min strain rate while it increases in the range of 5–9% for 0.01 mm/min strain rate, respectively. Rubber-toughening mechanism was credited with the increase in energy absorption of composites. Higher energy absorption of composites was mainly due to higher strain realisation indicating more deformation ability. Fracture features of specimens were analysed by scanning electron microscope.