Determination Of Toughness Research Articles

Mode I fracture toughness determination and environmental durability evaluation of adhesive bonds by wedge test

This paper presents a study of a method for determining the fracture toughness of adhesive bonds under Mode I loading using the wedge test. The slope of adhesively bonded specimen arms at loading points was used to provide an effective crack length for the wedge test specimen. A simple expression was derived based on beam theory for the Mode I energy release rate. Finite element analyses were conducted on wedge specimens with a range of material and geometrical properties. Good agreement was found between the energy release rates from the beam theory-based expression and the virtual crack closure technique (VCCT). A comparison of various energy release rate expressions showed that the present expression was more accurate than the other expressions available in the literature. The results of this study suggest that the environmental durability of adhesive bonds with composite adherends as well as metallic adherends can be assessed in terms of their fracture toughness using the present method. Experiments under hot and humid conditions were performed on wedge specimens with adherends made of carbon fiber reinforced polymer (CFRP) composite laminate and titanium alloy, and the fracture toughness results were obtained.

Mechanism and Method of Testing Fracture Toughness and Impact Absorbed Energy of Ductile Metals by Spherical Indentation Tests

To address the problem of conventional approaches for mechanical property determination requiring destructive sampling, which may be unsuitable for in-service structures, the authors proposed a method for determining the quasi-static fracture toughness and impact absorbed energy of ductile metals from spherical indentation tests (SITs). The stress status and damage mechanism of SIT, mode I fracture, Charpy impact tests, and related tests were first investigated through finite element (FE) calculations and scanning electron microscopy (SEM) observations, respectively. It was found that the damage mechanism of SITs is different from that of mode I fractures, while mode I fractures and Charpy impact tests share the same damage mechanism. Considering the difference between SIT and mode I fractures, uniaxial tension and pure shear were introduced to correlate SIT with mode I fractures. Based on this, the widely used critical indentation energy (CIE) model for fracture toughness determination using SITs was modified. The quasi-static fracture toughness determined from the modified CIE model was used to evaluate the impact absorbed energy using the dynamic fracture toughness and energy for crack initiation. The effectiveness of the newly proposed method was verified through experiments on four types of steels: Q345R, SA508-3, 18MnMoNbR, and S30408.

Determination of the critical cyclic fracture toughness for the mode II in mixed fracture of structural steels

The developed method of processing experimental data from tests performed according to the four-point asymmetric bending scheme made it possible to establish the coefficient of proportionality between the modes of failure I and II, which for structural steels is in the range of 2,5÷3. The established longevity before the appearance of the critical speed according to the developed models is within the limits of the natural dispersion inherent in fatigue failure, which indicates the effectiveness of the developed algorithm and the correctness of the determined indicators of resistance to failure. The problem of the appearance of an oblique crack during tests on four-point asymmetric bending has been solved. It can be assumed that about 90% of the growth of an oblique crack is caused by the contribution of the mode of failure II.

Determination of mixed-mode I/II fracture toughness and bridging law of composite laminates

Accurate determination of the fracture toughness and the bridging law in a simple way is meaningful for the composite structural design. This study proposes a semi-analytical approach to calculate the mixed-mode I/II fracture toughness, and the crack opening/sliding displacements. Based on the relation between them, the mixed-mode I/II bridging law is obtained by using the J-integral theory. The obtained fracture toughness and distribution of the bridging stress show good agreements with test results, which illustrates the applicability of the semi-analytical approach. The obtained bridging law is further integrated into a tri-linear cohesive zone model for simulating the delamination behavior. Numerical load–displacement responses are consistent with experimental ones, further verifying the semi-analytical approach. Only experimental load and displacement data are necessary in this semi-analytical approach while specific equipment and tedious measurements on the delamination length and the relative crack opening/sliding displacement during the delamination tests are avoided, which makes the proposed approach simple and applicable for engineering application.

Fracture toughness determination for the surface-modified layers of carburized 18CrNiMo7-6 alloy steel via the compact tension method

Anti-fatigue manufacturing technology is the latest manufacturing technology to control surface integrity and modification. Surface modification techniques have been used to construct surface-modified layer (SMLs) on the workpiece surface. In this study, to accurately characterize and measure the fracture toughness of SMLs, a characterization method was proposed for evaluating the J-integral of fracture toughness of SMLs. The elastic–plastic parameters of the SMLs, such as the strain hardening index and yield strength, were obtained via peeling-layer tensile testing. The effect of n was considered when deriving the plasticity factor (ηp). The J-integral of the SML of 18CrNiMo7-6 alloy steel was obtained from single edge notched tension tests and finite element simulations. The results show that the fracture toughness of the SMLs can be accurately characterized using the proposed method.

Experimental Evaluation of KIEAC of a Carbon Steel Using the Pin-Loaded SENT Geometry

Abstract The experimental determination of the threshold stress intensity factor for environment-assisted cracking (KIEAC) is described by several standards, which allow the use of different specimen geometries and methodologies for crack length estimation. In some cases, the combination of structure, specimen size, or both, main loading direction, and crack orientations of the component that need to be characterized limit the use of standardized geometries. Consequently, alternative geometries must be used. In this study, fatigue pre-cracked pin-loaded single edge notched tension specimens as defined by BS 8571:2018, Method of Test for Determination of Fracture Toughness in Metallic Materials Using Single Edge Notched Tension (SENT) Specimens, were applied for the experimental evaluation of the KIEAC of a carbon steel. The specimens were tested in deaerated substitute ocean water solution saturated with carbon dioxide at 40°C and at 1 and 10 bar (100 and 1,000 kPa) under constant load conditions with incremental (step) loading. The crack length during the tests was monitored by direct current potential drop, which was helpful for defining the applied K corresponding to the onset of subcritical crack growth (KIEAC). Additionally, fractographic analysis of KEAC specimens and results from the fracture toughness evaluation of this material in air at room temperature have also been reported.

Evaluation of Precision and Sensitivity of Back Extrusion Test for Measuring Textural Qualities of Cooked Germinated Brown Rice in Production Process.

The textural qualities of cooked rice may be understood as a dominant property and indicator of eating quality. In this study, we evaluated the precision and sensitivity of a back extrusion (BE) test for the texture of cooked germinated brown rice (GBR) in a production process. BE testing of the textural properties of cooked GBR rice showed a high precision of measurement in hardness, toughness and stickiness tests which indicated by the repeatability and reproductivity test but the sensitivity indicated by coefficient of variation of the texture properties. The findings of our study of the effects on cooked GBR texture of different soaking and incubation durations in the production of Khao Dawk Mali 105 (KDML 105) GBR, as measured by BE testing, confirmed that our original protocol for evaluation of the precision and sensitivity of this texture measurement method. The coefficients of determination (R2) of hardness, toughness and stickiness tests and the incubation time at after 48 hours of soaking were 0.82, 0.81 and 0.64, respectively. The repeatability and reproducibility of reliable measurements, which have a low standard deviation of the greatest difference between replicates, are considered to indicate high precision. A high coefficient of variation where relatively wide variations in the absolute value of the property can be detected indicates high sensitivity when small resolutions can be detected, and vice versa. The sensitivity of the BE tests for stickiness, toughness and hardness all ranked higher, in that order, than the sensitivity of the method for adhesiveness, which ranked lowest. The coefficients of variation of these texture parameters were 31.26, 20.59, 19.41 and 18.72, respectively. However, the correlation coefficients among the texture properties obtained by BE testing were not related to the precision or sensitivity of the test. By obtaining these results, we verified that our original protocol for the determination of the precision and sensitivity of food texture measurements which was successfully used for GBR texture measurement.

Determination of extensibility and toughness of wheat-flour dough based on bubbles blown by sheeted dough and airflow-3D imaging technique

The resistance strength (toughness) and extensibility of dough are closely related to the quality of the dough product. In this study, the toughness and extensibility of dough were detected using an airflow-structured light 3D imaging technique combine with bubbles blown out of sheeted dough. The spatial deformation characteristics and point cloud features of each point cloud image were determined using the point cloud data processing algorithms. The frame, height, volume, surface area, projected area of three planes, and three point cloud features corresponding to the maximum height of the bubble were taken as the extracted features. A total of eight features were selected, which were consequently concentrated in the first three principal components (PCs). The first three PCs were fed into three different prediction models. The ELM model achieved a relatively higher prediction accuracy than the other two models. For toughness and extensibility, Rp were 9092 and 0.8950, respectively.

Determination of equivalent fracture toughness and tensile strength of steel fibre reinforced cementitious composite based on boundary effect model

A theoretical method for determining the equivalent fracture toughness and tensile strength of steel fibre reinforced cementitious composite (SFRC) was established based on boundary effect model (BEM). The characteristic parameter Cf and discrete number γ were introduced to the analytical expression of BEM, which considered the influence of orientation, length and diameter of steel fibre on SFRC. The fictitious crack propagation corresponding to the peak load was evaluated by parameter combination γ⋅Cf. SFRC specimens with different depths were analyzed to verify the applicability and feasibility of the proposed method. The full failure curves for SFRC were further established.

Determination of the fracture toughness of a 9Cr ODS steel fuel cladding using the pin loading Tension test

A Pin Loading Tension (PLT) test was developed to evaluate the fracture behavior of thin-walled tubes. The fracture resistance curve (J-R curve) was determined from the load-CMOD curve using the single specimen technique based on the elastic unloading method following the procedure described in the ASTM E1820-22e1 standard [1]. However, the standard does not provide the necessary functions, geometric function f(a/W) and plastic factor ηpl, to apply the methodology to the PLT test. In this work, the geometric functions needed to evaluate the crack length from the unloading compliance, which allows to highlight effects not seen in the past and the elastic and plastic parts of the J-integral were determined using finite element analysis. Special care was taken to consider the effects of contact and friction between the test specimen and the loading device. The methodology was then applied to experimentally study the crack growth resistance of a 9Cr ODS (Oxide Dispersion Strengthened) steel tube, which is a candidate material for fuel claddings of future fast-neutron reactors.

A rate-dependent cohesive zone model for simulating fast crack evolution and growth

This paper proposes a rate-dependent cohesive zone model for simulating the evolution and growth of cracks under dynamic loading. This model takes into consideration the effects of cohesive separation rate and crack growth speed on the cohesive traction-separation relationship to reflect the rate-dependent characteristics of both material viscosity and energy dissipation of micro-crack branches. The evolution algorithm of the cohesive law is presented, wherein separation rate and crack growth speed are evaluated using the backward difference method explicitly in incremental computation, leading to the determination of rate-dependent critical traction and fracture toughness. The proposed model was implemented in ABAQUS software via UEL subroutine and utilized in an example of interfacial crack growth simulation, with satisfactory agreement found among the present results, experimental data, and numerical results from the literature. The effectiveness of the rate-dependent model and numerical algorithm in simulating high speed crack growth is validated through further numerical tests.

Determination of fracture toughness and yield strength of Inconel 718 by milling operation

This paper presents an analytical scheme to determine the fracture toughness and the yield strength of Inconel 718 by the widely used milling operation. It is noted that there exists material fracture at the tool tip and the removed layer of material undergoes plastic deformation along the primary shear plane. The fracture toughness is considered at the tool tip and the yield strength is applied on the primary shear plane. Subsequently, the modified Williams’ model and the modified Williams-minimization model are both proposed on the basis of the geometries and mechanics in the oblique cutting process. The modelling framework indicates that milling can be developed as an alternative method to determine the fracture toughness and the yield strength from the serrated chip thicknesses combined with the corresponding cutting forces. This is because the experimental method by milling is a convenient scheme as it only requires the cutting forces per unit width of cut with respect to a series of uncut chip thicknesses. In addition, the normal shear angle is predicted by applying the principle of energy minimization. Finally, milling experiments are conducted to validate the two proposed methods. Results show that the two analytical methods return similar values of fracture toughness and yield strength, respectively. Besides, it is noted that high values of yield strength are achieved based on Tresca criterion, which would indicate work hardening in the primary shear zone during the serrated chip formation. The proposed methods exhibit advantages of convenience and generality to derive the fracture toughness and the yield strength, since it is still a challenge for the traditional standard mechanical tests for Inconel 718 due to its toughness and ductility.

Determination of fracture toughness of 2.25Cr1Mo0.25V steel based on acoustic emission technique

The correct determination of fracture toughness is dependent on the accurate measurement of the point of crack initiation. In this research work, the AE technique is used to accurately detect the point of crack initiation and determine the initiation fracture toughness value JAE of 2.25Cr1Mo0.25V steel. Moreover, conventional standard procedures such as the J-R curve and stretch zone methods are also used to determine the fracture toughness JQ and JSZW values, respectively. Multiple AE parameters involving the entropy, peak amplitude, count, and energy are extracted from AE signals to identify the damage progression during fracture toughness tests. The results show several critical points associated with the point of plastic yielding, the onset of crack initiation, and the rapid crack growth and final fracture can be accurately identified by AE characteristics. Furthermore, the point of crack initiation recognized by AE is used to determine the JAE value. The fracture toughness JAE values of different specimens are also compared with the corresponding JQ and JSZW values. The JAE values are found to be about 20% lower than the JQ values but are significantly larger than the JSZW(2D) and JSZW(3D) values. It is believed that the fracture toughness parameter JAE determined by the AE technique can provide an important reference for evaluating the fracture toughness of engineering materials.

A Comprehensive Review of Soil Remolding Toughness Determination and Its Use in the Classification of Fine-Grained Soils

The remolding toughness property of fine-grained soil has not been investigated that much, mainly because it has not lent easily to direct measurement, with soil toughness usually qualitatively described. In practical terms, as the plastic limit wP is approached, tougher soils require greater rolling effort during the wP test, such that plasticity and toughness properties can be used to distinguish those plastic soils having greater deformation resistance for various field applications. This state-of-the-art review paper presents a critical appraisal of soil remolding toughness determination and its limited use, to date, in the classification of fine-grained soils. The recent developments reviewed and critically assessed include mechanical thread rolling for nominal toughness measurement during the wP rolling-out procedure, various extrusion approaches, and proposed correlations between toughness and the plasticity index to liquid limit ratio. From statistical analysis of previously reported toughness–consistency limits data, some new correlations are introduced in the present paper. Soil classification using the traditional Casagrande plasticity chart is not entirely accurate for certain soil types in that one can observe soils that present high toughness (something typical of clay) being incorrectly classified as silt soil. From this perspective, a new toughness chart is introduced to augment (or for use instead of) the Casagrande plasticity chart in obtaining more reliable soil classification. This paper concludes with recommendations on future research efforts for routinely obtaining soil toughness measurements.

Ductile Fracture Behavior of ASTM A516 Gr.70 Pressure Vessel Steel by ASTM and ISO Fracture Toughness Standards

Fracture toughness determination is crucial for the design phase of pressure vessels, and, although ASTM E1820 and ISO 12135 fracture toughness standards have existed for some time, some differences have been reported in the determination of this property. This study investigates the ductile fracture behavior of ASTM A516 Gr.70 pressure vessel steel and assesses the differences in estimating both standards. The steel’s tensile properties and initiation fracture toughness (JIC) were evaluated, taking into account the parallel and perpendicular orientations to the rolling direction. The results reveal the properties’ dependence on the rolling direction, mainly attributed to perlite banding. Additionally, as for the JIC determination, the differences were associated with the different blunting line slope estimations on each standard, reinforcing the necessity of a work-hardening-based blunting line for each material assessed.

Artificial Intelligence‐based determination of fracture toughness and bending strength of silicon nitride ceramics

AbstractTwo mechanical properties, fracture toughness (KIC) and bending strength (σ), of silicon nitride (Si3N4) ceramics were determined from their microstructural images via convolutional neural network (CNN) models. The Si3N4 samples used for database were fabricated using various kinds of sintering additives under different process conditions. In total, 330 data sets were prepared and used for building the CNN models for artificial intelligence‐bassed determination of the two mechanical properties and testing the determination accuracy of the trained models. The determination coefficients (R2), which were used as accuracy indices, were approximately 0.85 for KIC and 0.92 for σ. Although both the R2 values were relatively high, the lower value for KIC suggests that it is influenced more by what is little obtained from the microstructural information, such as grain‐boundary characteristics. Furthermore, gradient‐weighted class activation mapping, which can visualize which parts of the image the CNN models focus on, showed that the trained models determined the two mechanical properties based on correct recognition of the microstructural difference among the images.

Determining the fracture toughness of quasi-brittle materials with notched four-point bending tests

Fracture is common failure mode of quasi-brittle materials when suffering tensile loads, the accurate determination of the fracture toughness becomes extremely vital. Notched four-point bending (4-p-b) test is one of main fracture test methods, but is rarely applied in evaluating the fracture behavior of quasi-brittle materials. In this study, on the basis of relevant standards, the specific requirement of 4-p-b fracture test is proposed. Then the closed-form solution is used to establish the relation between the four-point bending test results and fracture toughness on the basis of the Boundary Effect Model. The influence of test conditions, boundary effect and distribution characteristics of fracture properties are discussed in detail. After that, two types of hard rock and concrete are utilized to verify the reliability of proposed methods. The results show that four-point bending test and corresponding analytical solution are suitable for determining the fracture toughness of quasi-brittle materials, regardless of the different initial notch length and material categories.

Determination of fracture toughness and traction–separation relation in Mode I/II of a natural quasi-brittle orthotropic composite using multi-specimen approach

The fracture behavior of quasi-brittle naturally occurring composites is characterized by the presence of fracture process zone (FPZ), which is attributed by crack bridging and micro-cracking in wood. The FPZ limits the use of existing standards for fracture toughness. Also, the repeated loading–unloading approach using single specimen is difficult due to crack face interference. Further, the crack growth monitoring is difficult since the crack tip may be buried inside FPZ. So far, the compliance based beam method (CBBM) has offered a reasonable solution to these challenges. However, the CBBM is not an absolutely independent approach as it uses elastic constant and compliance determined from other experiments. Naturally occurring materials, like wood, exhibits large variations in its properties. Therefore, the use of parameters from multiple other experiments decreases the confidence in measured fracture toughness. Considering this, the present work evaluates the fracture toughness attributed by critical energy release rate of New Zealand Pine wood. Firstly, the research method follows a stand-alone multi-specimen approach to evaluate fracture toughness without any requirement for crack growth monitoring. Secondly, it experimentally establishes the traction–separation relation for both Mode I and II. This work has a general applicability to other natural quasi-brittle orthotropic solids like bone.

Fracture toughness determination and mechanism for mode-I interlaminar failure of 3D-printed carbon-Kevlar composites

Additive manufacturing for continuous fiber composite exhibits great potential for the fabrication of sophisticated structural components. Nevertheless, interlaminar characteristics of printed parts have remained an open research question. This study aims to evaluate the mode-I interlaminar fracture property of 3D-printed continuous fiber reinforced composites with different interlaminar interfaces adopting carbon and Kevlar fiber. Three different double cantilever beam (DCB) configurations are considered, including carbon/carbon, Kevlar/Kevlar and carbon/Kevlar hybrid interface. It is observed that the interlaminar failure of carbon fiber was seriously affected by the void defects caused by 3D-printed. It is further found that Kevlar fiber could greatly improve the interlaminar fracture toughness attributed to the better bonding of interlayers and the denser bridging fiber. Finally, a novel idea for the hybrid interface is provided based on the characterization study of the carbon-Kevlar composites.

Determination of static and dynamic fracture initiation toughness and numerical simulation of dynamic 3-point bend experiments of Al6063-T6

The objective of this research is to find out the static and dynamic fracture initiation toughness of aluminium alloy Al6063-T6. An in-house modified Hopkinson Pressure Bar (MHPB) along with the digital image correlation (DIC) system has been used for 3-point bend experiments under dynamic loadings. However, conventional UTM has been used for quasi-static experiments. Dynamic experiments have been performed at different projectile velocities of 13.37, 14.23, 15.95, and 20.09 m/s, while quasi-static experiments have been performed at a 1 mm/min displacement rate. Dynamic force is calculated by using strain gauge measurement at two-point on the bar while the crack initiation time is calculated using combined results from strain gauge and digital image correlation. Experimental results found that the value of dynamic fracture initiation toughness (DFIT) is higher as compared to static fracture initiation toughness (SFIT). However, DFIT has continuously increased with increasing loading rates. A numerical simulation of pre notched specimen under 3-point bend dynamic loading has been performed in ABAQUS software using the J-C constitutive and fracture model. Simulation results were compared with the experimental results and found good agreement in both these results.