Initial Notch Research Articles

An analytical non-linear hinge model for predicting the fracture processes of cementitious composites

A modified non-linear hinge model (NLHM) was proposed to predict the crack propagation of cementitious composites. Based on the toughness criterion and the weight function method, the fracture processes of normal concrete and aligned steel fiber reinforced concrete (ASFRC) were analytically determined. For normal concrete, the P–CMOD curves of the three-point bending beams with different sizes and initial notch ratios were calculated. Compared with traditional NLHM, the results obtained by the proposed model agreed well with those obtained by existing experiments and finite element method. According to the principles of electromagnetics, the ASFRC specimens were prepared. The three-point bending tests of the ASFRC beams with different sizes were conducted. The applicability of the proposed model for fiber reinforced cementitious composites was verified.

Examining the breaking and bending behaviors of the polymethylmethacrylate composites reinforced with hazelnut shell powder

The objective of the present study is to reveal the toughening effect of the hazelnut shell on the brittle matrices by using the bending and breaking methods. The polymethylmethacrylate composites reinforced with hazelnut shell powder with 50µ particle size at the weight concentrations of 5%, 10%, 15%, and 20% were prepared. The polymer beam samples were exposed to heat cure procedure and the initial notches were created at a/W=0.1, 0.2, 0.3, 0.4, and 0.5. The Mode I breaking behaviors of the notched at single edge and non-notched composite samples were examined by using three-point bending test. The critical stress intensity factor (KIC), J-integral, initial notch depth, and compliance methods were used in calculations. Besides them, the bending module and bending stress values were calculated. The microstructures of HNSP/PMMA composites were determined by using XRD (x-ray diffractometer), FTIR (Fouirer Transform Infrared Spektrofotometre), and SEM (scanning electron microscope) analyses. According to the results obtained, it was determined that the bending strength, elasticity module, and fracture toughness values increased at 10% additive ratio for the HNSP (hasel nut shell flour)/PMMA (polymethylmethacrylate) composites.

An arc bending notched specimen for determining the mechanical and fracture parameters of concrete based on the FET

An arc bending notched specimen was used to determine the mechanical and fracture parameters of concrete in this study. Compared with the traditional arc bending notched specimen, the lateral thrust generated at the bottom supports in the specimen can counteract the influence of self-weight on the crack propagation. Therefore, relative stable crack propagation can be obtained. The expressions of the stress intensity factor K and crack mouth opening displacement CMOD of the specimen were derived via the superposition method. Based on the fracture extreme theory (FET), a theoretical method for determining the tensile strength ft and effective fracture toughness KeIC of concrete was established, and only the experimental peak load is required. The rationality and applicability of the proposed theoretical method were verified by comparing the calculated values of ft to experimental data. The sensitivity of KeIC calculated by the proposed method to the initial notch ratio a0/h was analysed.

Methodical approach for studying kinetics of short and long fatigue cracks growth for irradiated reactor materials. Part 1. Statement of problem. The effect of the initial notch acuity on the fatigue crack rate on small-sized specimens

The paper considers methodical issues in the experimental research of fatigue crack growth kinetics when testing irradiated small-sized specimens. The effect of the initial notch acuity is studied on the long crack growth rate. The stress concentration zone sizes are estimated for notches of various types. A brif literature review of the main problems in the study of the growth kinetics of short fatigue cracks has been performed. The tasks of further research are formulated.

Characterization of transverse fracture properties of pultruded GFRP material in tension

This paper presents a study on transverse tensile fracture properties of a pultruded glass fibre reinforced polymer (GFRP) profile, based on compact tension (CT) and wide compact tension (WCT) tests. Through these tests, both the critical energy release rate and the laminate cohesive law were determined. The test specimens presented a variety of initial notch lengths to assess the specimen geometry dependency. As these tests require the completion of some post-processing tasks to determine the fracture properties of the material, the following methods were applied: ASTM E399, J-integral, Compliance Calibration (CC) and Modified Compliance Calibration (MCC). Test results have shown that the CT tests were unsuccessful in reaching the stable propagation stage and provided overestimations of the critical energy release rate. On the other hand, the WCT tests were effective in achieving a stable propagation stage and results presented a good agreement across visually based methods, both in respect to critical energy release rate and laminate level cohesive laws. An overall value of 20 N/mm was determined for the transverse tensile critical energy release rate of the GFRP profile tested. Some variations were documented across initial notch lengths, but these were considered negligible in face of the intrinsic variability typically found in pultruded GFRP materials. Finally, modified compliance calibration results were found to be significantly lower in respect to other methods, with an average critical energy release rate of about 16 N/mm.

Numerical study on fracture behavior of CFRP-strengthened self-compacting concrete beams

This research is dedicated to the numerical investigation of the fracture behavior of notched self-consolidating concrete (SCC) beams which are strengthened by carbon-fiber-reinforced polymer sheets (CFRP). The beams have been constructed with an initial notch in the middle of span and reinforced with a CFRP sheet under the beam. Therefore, the load-carrying capacity of the specimens, shear stresses in CFRP, and crack mouth opening (CMOD) have been documented. Furthermore, specimens have been modeled using the finite-element approach and the nonlinear static analysis. The validity of the numerical modeling has been established through comparing the experimental results with those of numerical simulations. The effect of variation in different parameters such as sheet thickness, concrete mechanical properties, beam height and adhesive strength have been investigated via numerical analyses. The results show that in experimental and numerical models, the load-crack mouth opening (P-CMOD) curves have two peak load points. As the load increases, the first peak value is obtained, which is then followed by a descent in the load-carrying capacity. By further implementing the load, the second peak value is reached on account of the relatively high cohesive impact of the CFRP sheet. Moreover, it is observed that in P-CMOD curves the first and second peak points increase as the concrete strength and the beam height increase, though the rate of this growth is less for $ P_{2\max}$ . While the adhesive strength and thickness of the CFRP sheet have the most influence on the second peak load which indicates the performance of the CFRP sheet.

A Determination of fracture toughness in plain concrete specimens by R curve

Since the last two centuries, concrete has become the number one building material in the world. Therefore, we are interested in studying the mechanical behavior at failure of this material and calculate its toughness. In this paper, we trace the R-curve to determine the critical stress intensity factor and the crack evolution by specifying the three stages: crack stopping stage, stable propagation stage and an unstable propagation stage. The R-curve was obtained from the curve connecting the crack mouth opening to the applied load for prismatic specimens normalized by ASTM and notched laterally (SENB). These specimens are made from concrete with 25 MPa compressive strength and subjected to three-point bending. The dimensionless ratio of the initial notch length and the specimen width is between 0.3 and 0.6. During loading, we can observe an elastic deformation followed by a significant plastic deformation, the average value of the critical stress intensity factor found is 1.05 MPa.m1/2.

Fracture Response of Fine-Grained Cement-Based Composite Specimens with Special Inclusions

This paper concerns research into the importance of the interfacial transition zone around inclusions of selected materials in fine-grained cement-based composite. Tests were performed on eight sets of prismatic test specimens. The sets differed in the inclusion materials used, which were placed at midspan above the initial central edge notch. The first was a reference set without any inclusion, the second contained a steel inclusion, four more contained different types of rock inclusion, the seventh contained an inclusion of extruded polystyrene, and the last contained a space of the same dimensions as that occupied by the inclusions in sets 2 to 7. The test specimens were subjected to three-point bending fracture tests at the age of (usually) 28 days. The fracture response was analysed by means of fracture mechanics theory, and apparent mechanical fracture parameters (modulus of elasticity, fracture toughness and fracture energy) were evaluated. The conclusion shows that a possible relationship exists between the differences in the mechanical fracture parameters of specimens with/without an inclusion and the existence of the interfacial transition zone.

Fracture response of cement-based composite with spherical glass aggregate exposed to high temperatures

This paper concerns an investigation into the effects of high temperatures acting on specimens made from specially designed fine-grained cement-based composite with spherical glass aggregate. The experimental programme was carried out on four sets of beam specimens with the dimensions 20 × 40 × 200 mm. The specimens were thermally loaded to pre-set temperatures of 100, 200 and 400 °C, after which these temperatures were maintained for 60 minutes (one of the sets contained three reference specimens which were placed in a laboratory at a temperature of 20 °C and were not exposed to heat). After the temperature loading had been performed, the specimens were left to cool down to ambient temperature. After that, fracture tests were used to determine the degree of damage caused to the specimens by temperature loading. Three-point bending tests were conducted on the previously loaded beam specimens, which were notched to provide them with an initial stress concentrator using a diamond blade saw before testing. The depth of the initial edge notch on the bottom side of the specimen was approximately 1/3 of the total specimen depth. The span length was 180 mm. Fracture response was evaluated via load versus deflection diagrams. The monitored parameters were modulus of elasticity, effective fracture toughness and specific fracture energy. Informative compressive strength values were also determined. Selected specimens were investigated using X-ray micro-tomography (micro-CT).

Discrete element models of tooth enamel, a complex three-dimensional biological composite

Enamel, the hard surface layer of teeth, is a three-dimensional biological composite made of crisscrossing mineral rods bonded by softer proteins. Structure-property relationships in this complex material have been difficult to capture and usually require computationally expensive models. Here we present more efficient discrete element models (DEM) of tooth enamel that can capture the effects of rod decussation and rod-to-interface stiffness contrast on modulus, hardness, and fracture resistance. Enamel-like microstructures were generated using an idealized biological growth model that captures rod decussation. The orthotropic elastic moduli were modeled with a unit cell, and surface hardness was captured with virtual indentation test. Macroscopic crack growth was also modeled directly through rupture of interfaces and rods in a virtual fracture specimen with an initial notch. We show that the resistance curves increase indefinitely when rod fracture is avoided, with the inelastic region, crack branching, and 3D tortuosity being the main sources of toughness. Increasing the decussation angle simultaneously increases the size of the inelastic region and the crack resistance while decreasing the enamel axial modulus, hardness, and rod stress. In addition, larger contrasts of stiffness between the rods and their interfaces promote high overall stiffness, hardness, and crack resistance. These insights provide better guidelines for reconstructive dental materials, and for development of bioinspired hard materials with unique combinations of mechanical properties. Statement of SignificanceEnamel is the hardest, most mineralized material in the human body with a complex 3D micro-architecture consisting of crisscrossing mineral rods bonded by softer proteins. Like many hard biological composites, enamel displays an attractive combination of toughness, hardness, and stiffness, owing to its unique microstructure. However few numerical models explore the enamel structure-property relations, as modeling large volumes of this complex microstructure presents computational bottlenecks. In this study, we present a computationally efficient Discrete-element method (DEM) based approach that captures the effect of rod crisscrossing and stiffness mismatch on the enamel hardness, stiffness, and toughness. The models offer new insight into the micromechanics of enamel that could improve design guidelines for reconstructive dental materials and bioinspired composites.

Components of the Fracture Response of Alkali-Activated Slag Composites with Steel Microfibers

Knowledge of the mechanical and primarily fracture parameters of composites with a brittle matrix is essential for the quantification of their resistance to crack initiation and growth, and also for the specification of material model parameters employed for the simulation of the quasi-brittle behavior of structures made from this type of composite. Therefore, the main target of this paper is to quantify the mechanical fracture parameters of alkali-activated slag composites with steel microfibers and the contribution of the matrix to their fracture response. The first alkali-activated slag composite was a reference version without fibers; the others incorporated steel microfibers amounting to 5, 10, 15 and 20% by weight of the slag. Prism specimens with an initial central edge notch were used to perform the three-point bending fracture tests. Load vs. displacement (deflection at midspan) and load vs. crack mouth opening displacement diagrams were recorded during the fracture tests. The obtained diagrams were employed as inputs for parameter identification, the aim of which was to transfer the fracture test response data to the desired material parameters. Values were also determined for fracture parameters using the effective crack model, work-of-fracture method and double-K fracture model. All investigated mechanical fracture parameters were improved by the addition of steel microfibers to the alkali-activated matrix. Based on the obtained results, the addition of 10 to 15% of microfibers by weight is optimal from the point of view of the enhancement of the fracture parameters of alkali-activated slag composite.

Plastic eta factor and blunting line for characterization of fracture toughness of dissimilar metal weld

AbstractThis paper addresses the fracture toughness evaluation procedure of different zones of dissimilar metal weld (DMW). Experiments have been conducted on compact tension specimens from DMW joint having initial notch in centre of weld and fusion boundaries. Plastic eta factors (ηp) and slope of blunting line (bl) for above specimens have been evaluated numerically. Experimental load, load line displacement, and crack growth data along with numerically evaluated ηp and bl have been used for calculation of fracture resistance curve and initiation fracture toughness, and finally, it has been compared with that evaluated as per ASTM E1820 and ESIS standard.

Analysing mixed mode (I–II) fracturing of concrete discs including chevron and straight-through notch cracks

Pre-existing cracks in concrete material and discontinuities in concrete structures are seldom subjected to pure tensile loading (mode I); rather, they are subjected to compressive, shear (mode II) or mixed mode loading (mode I-II). Cracked Chevron Notched Brazilian Disc (CCNBD) and Semi-Circular Brazilian (SCB) specimens including chevron notch and straight-through cracks were used in this research to analyse mixed mode fracturing of concrete. Both the experimental and numerical results showed that the crack initiation angle (θ) was found dependent on the crack inclination angle (β). Moreover, CCNBD specimens with chevron notch crack geometry was found more suitable than the SCB geometry with straight-through notch crack to analyse mixed mode fracturing behaviour of plain concrete. However, using CCNBD or SCB specimens straight-through notch cracks may be more useful for testing the fracturing properties of coarse-grained concrete as sharp chevron cracks reduce the possible Fracture Process Zone (FPZ) size in front of the notch crack tip. Further analyses showed that the critical crack length is found as 13 mm (dimensionless notch length (α) = 0.5). Thus, it is understood that the unstable crack propagation starts between the initial chevron notch crack and final chevron notch crack length and it shows all experiments were done accurately. This outcome would be useful for concrete researches to obtain the yield point by using the relative analysis for finding where the unstable crack propagation starts.

Role of prior austenite grain boundaries in short fatigue crack growth in hydrogen charged RPV steel

Short crack propagation under cyclic loading is compared experimentally in hydrogen charged and un-charged SA508 Gr. 3 Cl. I low alloy steel (LAS). This LAS is used in manufacturing of pressure vessels installed in nuclear power plants and is susceptible to hydrogen embrittlement (HE) during operation. Single edge notch tension (SENT) specimens with an initial notch of 85 μm–90 μm are used for this short fatigue crack propagation study. The short crack growth from the notch of SENT specimen subjected to cyclic loading is measured using moving digital microscope fitted on the servo-hydraulic fatigue testing machine. The short fatigue crack growth rate in hydrogen charged SA508 Gr. 3 Cl. I LAS is found to be one order higher as compared to the un-charged subject reactor pressure vessel (RPV) steel. Both trans-granular and inter-granular crack propagation is observed in un-charged and hydrogen charged specimens. In un-charged specimens, inter-granular crack propagation along the prior austenite grain boundaries (PAGBs) occurred only when the short fatigue crack encounters the martensite/austenite (M/A) island present along the PAGB. Whereas, in hydrogen charged specimens, inter-granular crack propagation occurred along the PAGBs of large size prior austenite grains and trans-granular crack propagation through the small size prior austenite grains. Strong resistance to short fatigue crack propagation is provided by PAGBs in un-charged specimens whereas, these PAGBs offered negligible resistance to short fatigue crack growth in hydrogen charged subject RPV steel. A new perspective of studying HE in materials by conducting short fatigue crack growth experiments is demonstrated.

Translaminar fracture toughness and fatigue crack growth characterization of carbon‐epoxy plain weave laminates

An experimental investigation of mode‐I translaminar fracture toughness and fatigue crack growth behavior of a carbon fiber‐epoxy plain weave laminate manufactured by resin infusion under flexible tooling (RIFT) is presented in this article. Pre‐cracked compact tension (CT) specimens were used to perform both quasi‐static and fatigue tests. Different data reduction techniques for fracture toughness calculation were used and compared with each other. The ASTM E399 test method was modified to account for the material orthotropy and specimen geometry effects using a correction function based on a numerical evaluation of the strain energy release rate. The proposed modification shows good agreement against other experimental methods found in the literature and its application was validated for fatigue tests. Fatigue testing shows that failure in undesired modes is likely to occur prior to translaminar fracture, which was attributed to a higher tensile fatigue threshold than compression or shear fatigue threshold presented by the composite in analysis. Increasing the specimens’ initial notch length was a solution for avoiding these types of failure. The experimental results were compiled in the form of a Paris curve, and their particularities were discussed. A fractographic analysis was carried out to define damage patterns and its evolution process in both types of tests. POLYM. COMPOS., 40:3791–3804, 2019. © 2019 Society of Plastics Engineers

Experimental study on acoustic emission characteristic of fatigue crack growth of self-compacting concrete

In order to study the fatigue crack growth (FCG) of self-compacting concrete, in this paper, the 100 mm × 100 mm × 400 mm specimens with a 30-mm initial notch were used for three-point bending test under three different stress levels (0.75, 0.8, and 0.85) and three different loading frequencies (0.5, 1, and 2 Hz), and the acoustic emission (AE) technique was used for monitoring the FCG. According to the results, the changes of the crack mouth opening displacement, the stiffness, and the effective crack length all present three obvious stages with the increase of fatigue cycles. According to Paris law, under the same stress level, the increase of loading frequency results in the increase of fatigue life and the decrease of FCG rate, but the curves of da/dN are approximately parallel under different loading frequencies. Furthermore, the research results of characteristic parameters of AE show that cumulative hits, counts and energy show three stages, which are similar to the change of stiffness, and there is a failure point in the curves of cumulative counts and cumulative energy. The cumulative laws of hits and counts can be used to well describe the FCG, and the b value can be used to analyze the three stages of fatigue fracture. Therefore, the AE technology can be used to monitor the FCG in concrete engineering.

The influence of polygonal cavity on fracture behaviour of concrete

In this work, the influence of polygonal cavity on fracture behaviour of cement composite (concrete) is investigated. Specimens of the nominal dimensions 40 × 40 × 160 mm with polygonal cavity of 8 × 8 × 40 mm were provided with an initial central edge notch with a depth 12 mm, which was made by diamond blade saw. To determine the influence of polygonal cavity on fracture behaviour, fracture tests were conducted via three-point bending. The aim of this work is to analyse the behaviour of such specimen by means of finite element method (FEM) principles in Ansys, Inc. software. For this reason, a simplified 2D model was created for plane strain conditions and based on the fracture test configuration. The crack propagation assessment was based on generalized fracture mechanics approaches using a criterion of an average value of tangential stress determined in dependence on the polar angle θ. Results of numerical analysis indicates that the actual crack depth must be greater than it supposed. In other words, diamond blade saw damage the specimen more than expected.

Effect of hydrogen on short crack propagation in SA508 Grade 3 Class I low alloy steel under cyclic loading

The effect of hydrogen on short crack propagation under cyclic loading in SA508 Grade 3 Class I low alloy steel is investigated. This low alloy steel is used in manufacturing of pressure vessel installed in Indian nuclear power plants. During operation these pressure vessels are subjected to continuous supply of pressurized hot water at 600 K and hence are susceptible to hydrogen embrittlement. In past, research has been conducted on the effect of hydrogen embrittlement on long fatigue crack propagation in this material but, the mechanistic understanding and correlation of hydrogen embrittlement with microstructural features in the material can be understood well by studying the effect of hydrogen embrittlement on short fatigue crack propagation. Short fatigue cracks are of the order of 10 µm to 1 mm and unlike long cracks these short cracks strongly interact with the microstructural features in the material such as grain/phase boundaries. The effect of hydrogen embrittlement on short crack propagation is studied by artificial hydrogen charging of the material through electrochemical process. The single edge notch tension (SENT) specimens with an initial notch of the order of 85 to 90 µm are used to study the short crack propagation. The short cracks in hydrogen charged samples initiated from the notch at lower number of loading cycles as compared to the uncharged notched samples for the same value of applied stress range (Δσ). After initiation, the short fatigue crack in hydrogen charged samples propagated at higher rate as compared to uncharged samples. This dissimilarity in crack propagation behavior is due to the difference in the interaction of short fatigue crack with the microstructural features for a hydrogen charged and uncharged samples.

Fracture behavior of carbon fiber reinforced polymer composites: An optical study of loading rate effects

Crack initiation and growth in single-edge notched unidirectional T800s/3900-2 CFRP laminates are studied under stress wave and quasi-static loading conditions. An optical technique called reflection-mode Digital Gradient Sensing is also extended to study fracture mechanics of CFRP by using it in conjunction with ultrahigh-speed photography to perform full-field measurement of crack-tip deformations in the pre- and post-crack initiation regimes. DGS is capable of measuring two orthogonal surface slopes in the crack-tip vicinity as angular deflection of light rays. A method for extracting crack-tip parameters – the instantaneous crack speed and stress intensity factor (SIF) histories – associated with the stationary and propagating cracks using measured surface slopes is presented. The effect of fiber orientation in the range 0°–60° relative to the initial notch and two loading rates are investigated. Nominally mode-I fracture occurs when the fiber orientation is 0° whereas mixed-mode fractures ensue in others. Besides crack initiation occurrence at higher loads as fiber orientation increases, the SIF histories imply strong fiber bridging at low fiber orientations under quasi-static conditions. Furthermore, this CFRP shows significant loading rate dependence during crack growth. Unlike stress wave loading conditions, an increasing crack growth resistance immediately after crack initiation is seen under quasi-static conditions.

Identification of elasto-plastic and nonlinear fracture mechanics parameters of silver-plated copper busbars for photovoltaics

Silver-plated copper busbars are screen printed onto silicon solar cells and have the key role to collect the electric current produced by the solar cell. Busbars of two adjacent solar cells are then connected by a soldered ribbon made of the same material. Due to mechanical and thermal loads, such a ribbon is subject to axial deformation that, often, causes plasticity and, in some cases, its breakage due to crack growth. A procedure based on the gradient-descent method and particle swarm optimization is herein proposed for the identification of elasto-plastic and nonlinear (cohesive zone model, CZM) fracture mechanics parameters of silver-plated copper busbars. The proposed method requires the experimental determination of the force-displacement curves from uniaxial tensile tests on busbar samples with and without initial notches. The inspection of in situ SEM images during the tests allows also the estimation of the crack opening, which is found to be an important local quantity to assess the reliability of different CZMs in simulating a crack growth process consistent with the real one.