Compact Specimens Research Articles

Effect of length-to-height ratio on fracture properties of asymmetrical single-edge notched beam (ASENB) specimen made of ceramic under full range mixed mode I/II loading state

Asymmetrical single-edge notched beam (ASENB) specimen is among the suitable specimens for measuring full mode I/II fracture parameters. However, in lack of a standard or common method, researchers used this specimen with different geometers, which have proven can affect the results. This research evaluated the effect of ASENB’s geometry on fracture parameters numerically and experimentally. First, the finite element method determined the geometry factors (YI and YII) and non-singular (T-stress) fracture parameters. Then, the experimental fracture tests were conducted using ceramic material. Results show it is more reasonable to express the YI and YII based on S1/L and S2/H, instant S1/L, and S2/L. In other words, the geometry factors can be expressed better based on the height of the specimen and not length. So, for ASENB specimens tested in conventional S1/L of 0.7 to 0.9, the pure mode-II condition was generated in 0.05 < S2/H < 0.14. The modeling showed that the non-singular term of fracture (T-stress) was significant compared to fracture toughness, so the Biaxiality was measured as 0.5 to −2.5, more significant for pure mode-II and almost regardless of the a/H ratio. As experimental tests show, the relative length of the ASENB specimen has an insignificant effect on measured fracture toughness, so a more compact specimen with L/H of about 2 to 4 can suggested for tests.

A Relationship between Fracture Toughness Kc and Energy Release Rate Gc According to Fracture Morphology Analysis

This study investigated the relationship between fracture toughness (Kc) and energy release rate (Gc) through fracture morphology analysis, emphasizing the critical role of fractal dimensions in accurately characterizing fracture surfaces. Traditional linear elastic fracture mechanics (LEFM) models relate Gc to Kc by combining energy principles with the nominal area of the fracture surface. However, real materials often exhibit plasticity, and their fracture surfaces are not regular planes. To address these issues, this research applied fractal theory and introduced the concept of ubiquitiform surface area to refine the calculation of fracture surfaces, leading to more accurate estimates of Gc and Kc. The method was validated through standard compact tensile specimen tests on a nickel-based superalloy at 550 °C. Additionally, the analysis of fractal dimension differences and dispersion in various fracture regions provides a novel perspective for evaluating the fracture toughness of materials.

Characterization on fibre kinking fracture of laminated composites under combined compression and shear at high loading rate

This paper presented a novel method to characterize the fracture toughness and cohesive law of laminated composites under combined compression and shear at high loading rate. Compact compression specimens with off-axis fibres which introduce the in-plane shear stresses were conducted on the uniaxial bidirectional electromagnetic Hopkinson bar system and the displacement field and strain field were recorded by the high-speed camera for the digital image correlation analysis and J-integral calculation. The results reveal that the in-plane shear stresses bring the increase of fracture toughness at crack initiation and propagation and advance the damage initiation of the specimens. The fracture surfaces indicate that the shear stresses cause the fibre bundles' shear failure during the formation of the kink band, accompanied by more energy dissipation with the increase of off-axis angle.

Investigation on fracture resistance behaviour of dissimilar metal weld joint of modified 9Cr–1Mo steel and AISI 316LN SS

Fracture behavior of dissimilar metal weld joint consisting of mod. 9Cr–1Mo steel, AISI 316LN stainless steel, Inconel 82 butter layer and Inconel 182 weld has been investigated through experiments and FE simulations. The metallographic studies on weld joint and its interfaces have been carried out to determine the influence of microstructure on tensile and fracture properties. Tensile properties of different weld regions viz., weld metal, butter layer and HAZ have been evaluated using sub-size specimens. Fracture resistance behavior of weld joint has been studied through testing of compact type specimens with initial cracks machined at different locations. The plastic η factors for estimation of J-R curve have been obtained using FE analysis for crack in different regions viz., weld metal, butter layer and HAZ. Crack in weld metal exhibits lower fracture resistance as compared to butter layer and HAZ. Further, the constraint acting on crack and its susceptibility for deviation have been discussed in terms of stress triaxiality ahead of crack tip.

Dynamic propagation of moving cracks in brittle materials by field-enriched finite element method

We propose a field-enriched finite element model to simulate the dynamic growth of moving cracks in brittle solids. The paper introduces the dynamic model and verifies its accuracy by analyzing dynamic stress intensity factors during the propagation of a mode-I moving crack. We also simulate crack bifurcation using a specific criterion. Our numerical method successfully reproduces experiments, including the Kalthoff-Winkler experiment, three-point bending test under impact load, and compact compression specimen (CCS) experiment. Finally, we investigate how the location of a circular hole influences the propagation paths of a pre-existing straight crack using a numerical model including both the hole and the crack (COSSCC). Several numerical examples demonstrate the ability of dynamic field-enriched finite element method in simulating dynamic moving cracks.

Modelling of crack propagation in miniaturized and normal SENB specimens based on local failure criterion

The use of miniaturized specimen testing methods is a promising way to solve the problem of limited materials in RPV monitoring programs. The use of miniature specimens allows the evaluation of fracture toughness from other specimen materials used. In particular, the small-size compact tensile test specimen (0.16T CT) is promising for the determination of fracture toughness, as it can be produced from the standard size Charpy specimen that has already been tested. However, if we have only 0.16T CT test, we cannot investigate the dimensional response and also have only one restricted deformation state, which may pose problems in verifying geometry independence and determining local parameters for state-of-the-art analyses. It is therefore recommended to prepare at least two tests with two different restricted deformation specimens. Therefore, the testing of mini single edge notched bending (SENB) is also required and can be worked out from the Charpy specimens. The paper presents the determination of fracture toughness for these miniaturized specimens by modifying the virtual crack closure technique (VCCT) simulation method using GTN parameters instead of energy release as the driving force. This allows the calculation of the J-integral to proceed in parallel with the crack propagation.

On the application of numerical differentiation methods to the determination of the fatigue crack growth rate

Using a sample of test results from 68 compact eccentric tensile specimens made of titanium alloys, nickel alloys and steel, the effect of the choice of numerical differentiation method (secant method and the method of differential polynomials on three, five and seven points) used to calculate the fatigue crack growth rate on characteristics of the linear section of the kinetic diagram of the fatigue failure. The purpose of the study is to determine the advantages, disadvantages and consistent patterns of the considered methods. The coefficient of determination R 2, integral criterion χ which characterizes the difference between the predicted and actual number of cycles corresponding to the section of stable crack growth, and correlation between the logarithms of the Paris constants for alloys of the same class were used as criteria for the correct choice of the method of numerical differentiation. The main results and conclusions of the study: the use of the method of differential polynomials over three points compared to the secant method slightly increases the correlation between the logarithms of the fatigue crack growth rate and the range of the stress intensity factor (an increase in R 2) and increases the difference between the calculated and experimental number of cycles corresponding to stable crack growth (an increase in χ). However, when determining the fatigue crack growth rate by the method of differential polynomials for five and seven points, a more significant smoothing of the experimental data is observed, accompanied by a significant increase in R 2 and a decrease in χ; proximity to zero of the integral accuracy parameter χ is a necessary but not sufficient criterion for good agreement between the test result and the mathematical model that describes it, while the combination of parameters χ and R 2 uniquely forms this criterion; the choice of the method of numerical differentiation does not affect the correlation of the logarithms of the constants of the Paris equation.

Unified theoretical solutions for describing the crack-tip stress fields of mode-II cracked specimens under the fully plastic condition

Potential novel singular solutions to the crack-tip stress fields beyond the asymptotic solution frame are discussed in this study. It is believed that one of them is not only suitable for the crack-tip stress fields of mode-I cracked specimens but also for those of mode-II cracked specimens. The crack-tip stress fields of mode-II cracked specimens are investigated based on the singular solution and the stress factor of mode-II cracked specimen derived from energy density equivalence principle. Further, special functions for describing the coefficients of normalized stress distributions of mode-II cracked specimens are proposed. The method to determine the parameters in the theoretical solutions are introduced. To give an example, one set of parameters for compact shear specimen under plane-strain conditions are determined by the finite element analysis of a few typical materials and geometries conditions. Finally, comparisons of radial and circumferential stress distributions and contour lines of equivalent stress are completed for the compact shear specimens and Arcan-shaped shear specimens of power-law plastic materials under plane-strain and plane-stress conditions. And the results show that the applicability and reliability of the proposed unified theoretical solutions are verified.

Crack growth behaviour of P91 steel under trapezoidal loading at high temperature

The increased incentives to utilize renewable energy sources so as to supplement the energy requirements of the present civilization pose a challenge to the power plants running on fossil fuels, as they are required to operate in a ‘flexible’ manner instead of operating at a constant load as intended during their design and inception. This flexible mode of operation at elevated temperature causes a combined creep and fatigue type of loads to be applied on the components. This allows for a comprehensive and effective understanding of synergistic creep-fatigue crack growth behaviour to assess long term failure of components and accessories in plant. The use of P91 steel has been widely prevalent in power plant components owing to its high creep resistance, superior mechanical properties and excellent thermal conductivity, cost-effectiveness, and minimal coefficient of thermal expansion when compared to austenitic steels and Ni-based alloys. The American Society for Testing and Materials (ASTM) has proposed a standard for experimentally evaluating creep-fatigue crack growth, E 2760-19, wherein compact specimens, C(T) are tested under load-controlled conditions. The creep-fatigue crack growth regime and mode of fracture undergone by P91 steel at 600 °C were studied using C(T) specimens at two different force ratio 0.1 and 0.5 and with two different hold time 10 and 600 sec. The Creep fatigue crack growth behaviour has been characterized using stress intensity factor range parameter ∆K and (Ct)avg parameter to study the effect of hold time and force ratio.

Моделирование упругопластического разрушения компактного образца

The strength of a compact specimen under normal fracture (fracture mode I) is studied within the framework of the Neuber-Novozhilov approach. A model of an ideal elasto-plastic material with limiting relative elongation is chosen as the model of the deformable solid. This class of materials includes low-alloy steels that are used in structures operating at temperatures below the cold brittleness threshold. The crack propagation criterion is formulated using the modified Leonov-Panasyuk-Dugdale model, which uses an additional parameter, i.e., the diameter of the plasticity zone (the width of the prefracture zone). In the case of stress field singularity occurring in the vicinity of the crack tip, a two-parameter (coupled) criterion for quasi-brittle fracture is developed for type I cracks in an elastoplastic material. The coupled fracture criterion includes the deformation criterion attributed to the crack tip and the force criterion attributed to the model crack tip. The lengths of the original and model cracks differ by the length of the prefracture zone. Diagrams of the quasi-brittle fracture of the specimen under plane strain and plane stress conditions are constructed. The constitutive equations of the analytical model are analyzed in terms of the characteristic linear dimension of the material structure. Simple formulas applicable to verification calculations of the critical fracture load and pre-fracture zone length for quasi-brittle and quasi-ductile fractures are obtained. The parameters in the presented model of quasi-brittle fracture are analyzed. It is proposed to select the parameters of the model according to the approximation of the uniaxial tension diagram and the critical stress intensity factor.

A novel monitoring system for fatigue crack length of compact tensile specimen in liquid lead-bismuth eutectic

Fatigue strength of the structural materials of lead-cooled fast reactors (LFRs) and accelerator-driven systems (ADS) may be degraded in liquid metal (Lead or lead-bismuth eutectic (LBE)) environments. The fatigue crack growth (FCG) data of structural materials in liquid LBE are necessary for damage tolerance design, safety assessment and life management of key equipment. A novel monitoring system for fatigue crack length was designed on the compliance method and the monitor technology of crack opening displacement (COD) of CT specimens by the linear variable differential transformers (LVDT) system. It can be used to predict the crack length by monitoring the COD of CT specimens in harsh high-temperature liquid LBE using a LVDT system. The prediction accuracy of this system was verified by FCG experiments in room temperature air and liquid LBE at 150, 250 and 350 °C. The first results obtained in the FCG test for T91 steel in liquid LBE at 350 °C are presented.

In-situ investigation of fatigue crack growth behavior of 34CrNi3Mo high-strength steel

In this study, the short fatigue crack growth behavior of 34CrNi3Mo high-strength steel was investigated through in-situ fatigue testing under scanning electron microscopy. An innovative method was proposed to determine the fatigue crack growth threshold by a changing load strategy, which shows high consistency with the standard test for compact tensile specimens. The difference in misorientation along distinct directions at the crack tip has been identified as a factor controlling crack deflection regardless of slip system types.

Open material database for tensile test properties of additive manufacturing materials

In recent years, the investigation of material properties within additive manufacturing, also known as 3D printing, has gained significant research attention. The intricate interplay between numerous fabrication parameters and the resultant material properties of 3D-printed components has become crucial, particularly for enabling effective topology optimization. Considering this, we propose the establishment of an accessible open database. This repository stores a comprehensive collection of fabrication files corresponding to each distinct material and printer combination, accompanied by the outcomes of meticulous tensile testing. To support the research community, our initiative extends to the inclusion of material provider datasheets, facilitating comprehensive result comparisons. A standardized approach utilizing consistently applied strain rates is recommended, focusing on a compact dog bone specimen design. This pioneering attempt encompasses an expansive array of data derived from 25 distinct materials and 9 diverse printers, meticulously capturing the inherent variability within the samples. The database catalogues the complete spectrum of tensile test data, encompassing various essential measurements such as mass, and crucial material properties including Young’s modulus, yield stress, fracture strain, and absorbed energy. These recorded metrics can be seamlessly correlated against density, manufacturing time, or cost parameters, enabling the generation of insightful plots and analysis. Through this collaborative effort, we aim to provide researchers with a robust foundation for informed decision-making and advancements in additive manufacturing.

Effect of material inhomogeneity on constraint loss measured with subsized C(T) fracture specimens of a reactor pressure vessel steel

In this study, the fracture behavior of the low-alloy reactor pressure vessel JRQ steel was investigated in the ductile-to-brittle transition region with two different subsized compact tension fracture specimens: 0.18T C(T) and 0.5T C(T). The specimens were extracted from two thin plates (surface and middle) of the large JRQ steel plate, which is well-known to be macroscopically inhomogeneous in terms of brittleness. In particular, the reference temperature T0 as defined in the ASTM-E1921 standard was calculated for the two plates with the 0.18T C(T) and 0.5T C(T) specimens. A significant difference in the reference temperature T0 of more than 90 °C as determined with the 0.5T C(T) data was found between the two plates indicating a clear macroscopic inhomogeneity between the two depths. The measured toughness of 0.18T C(T) specimens and corresponding T0 evaluation showed that the effect of constraint loss was more pronounced for the middle plate due to its lower strength and that the constraint loss effect can be confounded with inhomogeneity. A local approach to fracture model was applied to account for the constraint loss by calibrating a criterion based on critical stressed volume V* and critical stress σ* obtained by finite element simulations. A big difference in the local fracture stress σ* between the two plates was found and based on the deduced local criterion it was shown that the measured 0.18T toughness can be effectively predicted within the temperature range of the ductile-to-brittle transition.

Numerical Study of the Plastic Zone at the Crack Front in Cylindrical Aluminum Specimens Subjected to Tensile Loads.

Cylindrical specimens are of great interest in analyzing mechanical elements' behavior and investigating phenomena with biaxial loads. It is necessary to identify the behavior of the crack front along the thickness to interpret these results, which are usually based on the hypothesis of a straight crack and the observation of the outer face of the crack front. Based on the work carried out on compact tension type specimens, this work proposes adapting this methodology to cylindrical specimens, adapting the previous finite element models. Cylindrical specimens provide an asymmetric behavior influenced by the radius, where the CT (compact tensile) specimen can be considered the extreme infinite radius case. Combinations of the load level and radius values help us simulate the crack's behavior under intermediate hypotheses between a plane crack theory and a three-dimensional one. The plastic strain around the crack front will be analyzed as a function of the thickness and the load level applied. The results allow us to validate the numerical methodology and establish the differentiated behaviors of the plastic zones close to the outer and inner radii.

Experimental Analysis of Fatigue Crack Path for Strained Functionally Graded Materials

This paper experimentally discusses crack paths in strained Functionally graded materials under constant loading conditions and vibration fatigue. The objective is to synthesize the body of knowledge on the stability of FGM. The article presents the development, production, and characterization of multi-Al (aluminum), Ni (nickel), and Titanium alloys. FGM it was successfully modified using the powder metallurgical technique. Five-layer FGM samples frequently utilize (Al, Ni), on one end and (Al-Ni-Ti) on the first. FGM sample mechanical characteristics have been investigated using wear and Fatigue testing. The Al/Ni/Titanium FGM compact specimen approaches the yield stress and ultimate stress values, which is regarded as a significant improvement in mechanical qualities with less weight. The test findings are for a constant amplitude load fully reversed with zero mean stress. The stress life approach examined three samples' fatigue characteristics and natural frequency under random vibration.

Adaptive construction approach for virtual samples of hot mix asphalt based on 3-D structural characterization of laboratory compact specimens

Marshall and Superpave gyratory compactor (SGC) methods are widely applied in pavement engineering. Laboratory samples derived from the two methods have great difference in aggregate structure, which should be adequately characterized and considered to generate virtual samples of Marshall and Superpave mixture. To characterize the aggregate structure in realistic samples of asphalt mixture, six laboratory-compacted samples, three Marshall and three SGC, are prepared under SMA-13 gradation and then three-dimensionally (3-D) reconstructed. Next, the statistical indicators of the aggregate structure are extracted, analyzed, and characterized to develop an algorithm for adaptive construction of virtual samples conforming to the structural characteristics of realistic samples. Four virtual samples, two Marshall and two SGC, are generated to verify the control effect of the proposed approach on the aggregate structure to simulate the internal structure of realistic samples. Results indicate that virtual samples capture close structural characteristics with laboratory samples of Marshall and SGC, which validates the effectiveness of the proposed approach.

Influence of the moisture content on the fracture energy and tensile strength of hardwood spotted gum sawn timber and adhesive bonds (gluelines)

This study aims to measure the fracture properties, including crack initiation and propagation, of Australia’s native forest grown spotted gum ([SPG], Corymbia citriodora) sawn timber and associated adhesive bonds at different moisture content levels. The collected data were used as input values to develop a numerical model to understand the delamination of SPG glulam beams when exposed to a wetting and drying process. Thus, Mode I and Mode II fracture energies for crack propagation along radial and glueline directions were experimentally investigated under various moisture content levels (8%, 12% and 16%). Single-end notched beams and compact shear specimens were used to capture the Mode I and Mode II fracture energies, respectively. For crack initiation, the tensile strength perpendicular to the grain and the shear strength (taken as the maximum stress from the Mode II fracture tests) were also measured. In total, 200 experimental tests were performed. One-way analysis of variance statistical analyses showed that the fracture energies and shear strengths were independent of the range of moisture content levels investigated. In addition, the collected data were compared with the limited published fracture properties of other hardwood species.

An experimental analysis of crack terminating perpendicular to the bimaterial interface under varying mode mixities

SIFs for crack terminating perpendicular to the interface of a bimaterial subjected to varying mode mixities are evaluated with a novel bimaterial specimen using the experimental technique of photoelasticity. A simple form of multiparameter equation defining the stress field for the crack terminating at the interface at an arbitrary angle along with the details on interfacial SIF and combined SIF for a bimaterial problem are brought out. The stress field equation is incorporated in an in-house developed PSIF software as a separate problem module for the determination of crack tip stress field parameters using a non-linear least squares methodology. A compact bimaterial Brazilian disc specimen with crack in one of the material sides terminating perpendicular to the interface and at an arbitrary angle to the point of loading, is considered for the study. SIFs of two such specimens, a polymer–polymer pair, and a metal-polymer pair with two different bimaterial constants are determined. The normalized SIF values are evaluated as a function of crack orientation angles. The SIF values for the metal-polymer pair were found to be higher compared to the polymer–polymer interface. The peak values of SIFs are observed at a crack orientation angle of 30°. At orientation angles close to 0° and 90°, the SIF values were less pronounced.

Advancement of thermoelectric performances through the dispersion of expanded graphene on p-type BiSbTe alloys

ABSTRACT We demonstrated the systematic investigation of the graphene addition on the microstructural behaviour and concurrent thermoelectric properties of water-atomized p-type BiSbTe (BST)- based alloy using mechanical ball milling, and followed through the spark plasma sintering to fabricate a bulk compact specimen. X-ray diffraction results confirm the single phase of BST. The fracture surface of both specimen exhibits the homogenous distribution of grains with irregular shapes in a random alignment. The graphene in the BST provides an extra carrier transport leading to an increment in the carrier concentration (n) resulting in higher electrical conductivity (σ) for the BST + Graphene composite. The interface between two-dimensional (2D) graphene and bulk BST provides a larger degree of phonon scattering leading to a maximum reduced thermal conductivity (к) of 0.8 W m−1K−1 compared to pristine- BST (0.9 W m−1K−1) results in a maximum figure of merit (ZT) 1.1 at 400 K.