Fracture Toughness Characteristics Research Articles

Deep learning for characterizing fracture toughness from the nanoindentation image of a complex heterogeneous medium

Fracture toughness is a fundamental property characterized using nanoindentation, and it typically requires elastic modulus, applied load, and crack length. This study demonstrates that deep learning can predict fracture toughness using only nanoindentation images. A deep-learning model is designed, incorporating a pretrained Visual Geometry Group model with 16 layers (VGG16) and fully connected layers. The study augments 3,546 original nanoindentation images of shale to increase them to 21,276 images and employs the adaptive momentum (Adam) solver with a learning rate of 0.0002. The nanoindentation images contain complex patterns distinct from the simple topologies of homogeneous media, such as pure silica. Results show that the model accurately determines normalized fracture toughness, with a mean squared error (MSE) of 0.0014, indicating that the model effectively learns to interpret the underlying features. Additionally, once trained, the model predicts fracture toughness much faster than the existing approach based on K-means clustering. More importantly, this study suggests that nanoindentation images of complex porous media convey crucial information, including elastic modulus, applied load, and hardness. The results and the proposed model have applications in characterizing heterogeneous media with complex structures.

Fracture toughness in SPCC/CFRP hybrid laminates: Mode I and mode II perspectives

Hybrid composites using adhesive joints are gaining popularity in various industries as their various material combinations can create certain advantages. This research focuses on examining the mode I and mode II fracture toughness characteristics of the combined laminate between Steel Plate Cold Rolled Commercial (SPCC) metal and Carbon Fiber Reinforced Polymer (CFRP) composite processed using adhesive bonding. SPCC/CFRP hybrid laminate composites were manufactured with a variety of manufacturing options (A, B, and C) to find the most optimal method using two types of epoxy and cyanoacrylate adhesives, with two different bonding processes: secondary bonding and co-bonding. Fracture toughness values were measured through the Double Cantilever Beam (DCB) test for mode I and the End Notched Flexure (ENF) test for mode II. The test results showed that epoxy adhesives in the SPCC/CFRP adhesive joints provided better fracture toughness performance, especially when combined with the co-bonding technique. Specimens from manufacturing option C showed the highest GIC and GIIC values of 83.05 and 254.94 J/m2, respectively. These values increased by 58.98 % and 80.48 % compared to manufacturing options A and B in Mode I. Additionally, the GIIC value for manufacturing option C increased by 78.00 % and 66.76 % compared to manufacturing options A and B. The SPCC/CFRP hybrid laminates showed adhesive failure on the SPCC surface when using epoxy adhesive, whereas adhesive failure occurred on the CFRP surface with cyanoacrylate adhesive for the different manufacturing options.

Translaminar fracture toughness characterisation for a glass fibre/polyamide 6 laminated composite by a novel approach based on fictitious material concept

This paper proposes a novel approach for the characterisation of the material fracture toughness associated with translaminar tensile failure, consisting in the application of the Fictitious Material Concept (FMC) by using the testing configuration of the Modified Two Parameter Model (MTPM). The main advantage of such an approach is to avoid the use of nonlinear fracture mechanics for material fracture toughness characterisation. The novel approach is applied to compute the translaminar fracture toughness of a unidirectional glass fibre (GF)/polyamide 6 (PA6) laminated composite. Moreover, the experimental campaign carried out is numerically simulated by means of a micromechanical finite element model, and the fracture toughness is computed by employing two different approaches, that is, the novel one and the MTPM. The present study proves, for the first time, that the Fictitious Material Concept can be applied by considering both experimental and numerical structural responses since it provides, in both cases, quite satisfactory accuracy in term of laminated composite fracture toughness. Therefore, the great advantage is that, when a validated numerical model is available, experimental campaigns may be avoided, saving time and money. Moreover, it is proved that the FMC can be used to investigate specimen size effect on fracture toughness.

Experimental Study on Fracture Properties of Self-Compacting Concrete Containing Red Mud Waste and Different Steel Fiber Types

This study aims to investigate the effect of integrating red mud (RM) waste and different types of steel fibers on the fracture toughness characteristics of self-compacting concrete (SCC). A total of 24 specimens consisting of notched SCC beams with various steel fibers (measuring 100 × 100 × 500 mm) are subjected to a three-point bending test. This study examines five various fiber types characterized by varying shapes and aspect ratios. These fiber types include the hook-end fiber with lengths of 60 and 30 mm, the long straight fiber with lengths of 21 and 13 mm, and the flat-end fiber. Six concrete mixtures, each incorporating fibers with 1% of the volume percentage, are examined. RM is used at a replacement rate of 20% of the mass of cement. Another objective of the study is to analyze the mechanical and fresh characteristics of concrete. The findings indicate that the incorporation of steel fiber has an adverse effect on the fresh concrete characteristics of SCC. The presence of steel fiber results in enhanced mechanical properties, peak loads, and deflection at the point of failure, in addition to an increase in the crack mouth opening displacement. The fracture toughness of SCC mixtures is also influenced by the presence of steel fiber.

Experimental Characterization and Phase-Field Damage Modeling of Ductile Fracture in AISI 316L

(1) Modeling and characterization of ductile fracture in metals is still a challenging task in the field of computational mechanics. Experimental testing offers specific responses in the form of crack-mouth (CMOD) and crack-tip (CTOD) opening displacement related to applied force or crack growth. The main aim of this paper is to develop a phase-field-based Finite Element Method (FEM) implementation for modeling of ductile fracture in stainless steel. (2) A Phase-Field Damage Model (PFDM) was coupled with von Mises plasticity and a work-densities-based criterion was employed, with a threshold to propose a new relationship between critical fracture energy and critical total strain value. In addition, the threshold value of potential internal energy—which controls damage evolution—is defined from the critical fracture energy. (3) The material properties of AISI 316L steel are determined by a uniaxial tensile test and the Compact Tension (CT) specimen crack growth test. The PFDM model is validated against the experimental results obtained in the fracture toughness characterization test, with the simulation results being within 8% of the experimental measurements. (4) The novel implementation offers the possibility for better control of the ductile behavior of metallic materials and damage initiation, evolution, and propagation.

Enhancement of tensile strength and fracture toughness characteristics of banana fibre reinforced epoxy composites with nano MgO fillers

Natural fibre reinforced polymer composites have drawn the attention of researchers in the creation of novel materials suitable for industrial applications. These polymer composites have favourable mechanical characteristics. Epoxy polymer's versatility and diversity make it suitable for a wide range of industrial applications. The mould casting technique approach was utilised in this work to develop polymer composites using alkali-treated banana fibres and filled with MgO nanoparticle. The study was conducted on the characteristics of Nano MgO, as well as its mechanical qualities such as fracture toughness and surface morphology. The incorporation of Nano MgO enhanced the fracture toughness of the polymer composites with the addition of 1 % MgO. The scanning electron microscopy analysis of the surface morphology revealed the presence of brittle fracture in the specimen.

The mode I fracture toughness and acoustic emission characteristics of hot dry rock under temperature effects

The application of liquid nitrogen fracturing technology exhibits promising potential in the exploitation of hot dry rock (HDR). Therefore, studying the variation in granite's fracture characteristics under rapid cooling with liquid nitrogen is highly significant. In this study, the mode I fracture characteristics and acoustic emission (AE) characteristics of thermally treated granite under liquid nitrogen cooling were investigated experimentally. The results indicate that as the temperature increases, both the natural cooling group and liquid nitrogen cooling group exhibit a gradual decrease in granite's fracture toughness, and the cooling effect of liquid nitrogen will further damage the granite. When heated from 25 °C to 600 °C, the natural cooling fracture toughness decreased from 1.089 MPa·m1/2 to 0.1631 MPa·m1/2, with a reduction of 85.0 %; the liquid nitrogen cooling fracture toughness decreased from 1.089 MPa·m1/2 to 0.1493 MPa·m1/2, with a reduction of 86.3 %. Additionally, both the peak load and fracture energy are reduced; intragranular fracture behavior is observed within the granite, resulting in a rougher fracture surface. The integration of a crack classification method based on the rise angle (RA) and average frequency (AF) of AE parameters with the kernel density estimation (KDE) method effectively validates the fracture mechanism of semi-circular bend (SCB) specimens. Meanwhile, based on the AE results during granite fracturing, a method is proposed to discriminate crack types using RA and AF thresholds.

Fracture toughness testing of metallic materials based on scratch tests

As a crucial parameter for determining a material's resistance to crack propagation, the convenient characterization of fracture toughness has attracted much attention. The scratch test is an alternate method for determining fracture toughness because it does not require notches or pre-existing cracks and only requires a minimal sample volume. In this study, the product of strength and elongation of materials is incorporated into a modified model based on the energy size effect law to calculate the fracture toughness of metallic materials using scratch tests. To validate the proposed model, scratch tests of DP800, 18CrNiMo7-6, 4140, and 4340 are performed to characterize their fracture toughness. The fracture toughness of these four metallic materials determined from scratch tests utilizing the proposed model, Akono's linear elastic fracture mechanics (LEFM) model, Akono's microscopic energetic size effect law (MESEL) model, Hubler's MESEL model, and Liu's MESEL model are compared with the results from a compact tensile test or single edge notched beam test. Our results indicate that our modified approach is applicable to metallic materials. The proposed model could be reduced to the linear elastic fracture mechanics model. This study offers an efficient way of characterizing the fracture toughness of materials using the scratch method.

Enhancing carbon fiber reinforced aluminum laminates with cellulose paper interlayers: experimental characterization of tensile, flexural, and interlaminar fracture toughness

ABSTRACT The mechanical properties of fiber metal laminates (FML) are influenced by several various factors. Interface adhesion plays a particularly crucial role in interlaminar strength. Enhancing the interlaminar strength of carbon fiber reinforced aluminum laminate (CARALL) composites present a persistent challenge due to inherent weaknesses between metal and composite elements. Therefore, this study focuses on improving the interlaminar performance of CARALL composites by introducing cellulose paper interlayer at the metal/composite interface. The cellulose paper interlayer offers the advantage of being cost-effective and sustainable. Cellulose paper-interleaved CARALL composites were fabricated by vacuum bagging method and exhibited noteworthy improvements in mechanical properties. Comparative analysis with pristine samples revealed substantial enhancements, including a 15% increase in tensile strength, a remarkable 42% improvement in flexural strength, and a significant enhancement in mode-I fracture toughness by 65%. Furthermore, the cellulose paper interleaving played a crucial role in stabilizing fracture formation at the fiber-matrix interface, with mode II fracture toughness witnessing a 3% increase. Visual examination revealed the underlying toughening processes occurring in the interfacial area. This innovative approach of interleaving laminated composites with cellulose paper emerges as a sustainable and effective strategy, demonstrating the potential to fortify and toughen the interlaminar zones of CARALL composites.

Critical analysis of the mini-CT geometry for fracture toughness characterization of the 73W high-copper submerged-arc weld in the unirradiated and irradiated condition

Fracture toughness measurement using miniaturized specimens such as the mini-CT is gaining a lot of acceptance worldwide. This was recently acknowledged by the multiple organizations participating to the EU H2020 FRACTESUS project that SCK CEN is coordinating. One of the key materials used within this project and that will serve as a benchmarking material is the 73W weld of the 5th HSSI program performed at ORNL in the late 90′s for an extensive well-controlled characterization in both unirradiated and irradiated condition. From the remaining broken specimens, a large number of mini-CT specimens were fabricated and tested. This paper addresses a number of key questions, among which the effect of geometry, testing conditions and master curve evaluation as compared to the original data obtained on large CT specimens at ORNL. The test results are analyzed for master curve evaluation according to the ASTM E1921-22 standard and further discussed to assess the applicability of this mini-CT geometry as an irradiation damage metrics but also for evaluating the lower bound fracture toughness curve.

Effect of Temperature and Ageing on Fracture Toughness of Alloy 617M

In recent years, Advanced Ultra Super Critical (AUSC) plants have used Ni-Cr-Co-Mo based alloy 617M for high-temperature components like steam piping and rotors, where material resistance against fracture after prolonged exposure in a hostile environment is a matter of concern to the designers. To this end, fracture toughness characterization of the material in as-received and aged conditions using an elastic-plastic fracture mechanics (J-R curves) approach can provide a significant input towards the component integrity assessments at service conditions. In this study, J-R curves of the alloy 617M have been determined for the as-received and differently aged (up to 20000 h at 710 °C) conditions using compact tension (CT) specimens following ASTM E 1820-13. Repeated tests have been carried out at temperatures 27 °C, 650 °C, 710 °C and 750 °C. The fracture toughness (JIC) thus determined, has shown a declining trend with test temperatures. This attributes to the ease of the crack initiation process owing to the γ′ induced matrix hardening and subsequent ageing-inducing precipitations of M23C6 carbides at the grain boundaries.

Leak-before-Break (LBB)-Based Safety Verification of Reverse Cyclic Loading for 316L Stainless Steel: A Study Using Flat ESG Specimens

The leak-before-break design concept is based on J-R curves, which are obtained by J-R tests on various types of specimens and are known to be dependent on the cyclic load history. The J-R curves of standard specimens suggested by the American Society for Testing Materials are determined based on quasi-static tensile loading. However, seismic loading induces a reverse cyclic loading that alternately applies a tensile and a compressive load to nuclear plant piping. Therefore, it is very important to obtain the fracture toughness characteristics under reverse cyclic loading for the integrity estimation of nuclear plant piping. The objective of this paper is to study the effects of reverse cyclic loading on the fracture toughness characteristics of SA312 TP316L stainless steel, which is a nuclear plant piping material. J-R tests on a flat, equivalent stress gradient specimen with varying incremental displacement were carried out. The test results were reviewed by comparing the J-R test results under quasi-static loading. In addition, the safety margin of the nuclear plant piping was evaluated using a crack driving force diagram method. For the SA312 TP316L stainless steel, the results showed that the J-R curves were decreased with a decrease in the incremental displacement. When the incremental displacement was set to 0.25 mm, the unstable crack growth point value was about 73.0% of those for the quasi-static loading conditions.

The effect of long-term operation on fatigue and corrosion fatigue crack growth in structural steels

Operational degradation of steels under a combined action of mechanical loading and aggressive hydrogenating environments is a crucial problem for structural integrity. Among the mechanical properties, the characteristics of brittle fracture resistance, namely, impact strength and fracture toughness, as well as the plasticity characteristics, are highly sensitive to the operational degradation of structural steels. However, in the case of exposing of structures beside environmental influences to cyclic mechanical loading scenarios, their reliability depends on the degradation degree due to fatigue and corrosion fatigue. Tendency of in-service changes in the fatigue characteristics (fatigue crack growth rate and fatigue strength) can be unambiguous. In-service lifetime of structure due to the non-monotonic character of operational degradation of certain mechanical characteristics is divided into two stages: deformation strengthening (stage I) and in-bulk dissipated damaging (stage II). The operational degradation of steels is accelerated under influence of corrosive hydrogenating environments as a result of intensification of dissipated damaging. The effect of environments on acceleration of fatigue crack growth rate is mainly revealed in the mid-region of stress intensities, more significantly for in-service degraded metal, indicating its susceptibility to stress corrosion fatigue.

Effect of highenergetic electrons irradiation on the fracture toughness characteristics of Si and Si+2at.%Ge wafers

Effect of highenergetic electrons irradiation on the fracture toughness characteristics of Si and Si+2at.%Ge wafers

Effect of size and anisotropy on mode I fracture toughness of coal

Obtaining the accurate fracture toughness of coal considering bedding angle and independent of size is of great significance to the evaluation of the initiation pressure in the hydraulic fracturing of coal seam. In this study, the newly proposed modified semi-circular bending test was used to conduct mode I fracture testing of coal samples with different sizes and bedding angles. A four-dimensional lattice spring model (4D-LSM) was established based on the cohesive zone model considering bedding planes to study the effects of specimen size and bedding angle the fracture toughness and fracture pattern of coal. The size effect and anisotropy characteristics of fracture toughness of coal were analyzed by using linear elastic fracture mechanics and Bazant size effect law (SEL) theory. The results show that the fracture toughness of coal has obvious size effect and anisotropy. In quasi-brittle materials, the fracture toughness increases with the increase of specimen size due to the fracture process zone (FPZ). However, the fracture toughness of coal affected by microcracks generally decreases with the increase of specimen size. The size effect of fracture toughness of coal is jointly determined by the FPZ with increasing effect and the microcrack with reducing effect. The fracture toughness of coal increases with the increase of bedding angle. The bedding angle has no influence on the fracture toughness size effect of coal, but the microcracks associated with the bedding plane have a greater influence on the fracture toughness size effect. The specimen size only affects the fracture toughness value, but has no obvious effect on the fracture toughness anisotropy. Based on Bazant SEL theory, a fracture toughness size effect model considering microcracks and bedding angles was established. The research results have reference significance for crack initiation and propagation in hydraulic fracturing of coal seam.

Study on interlaminar fracture toughness of unidirectional composites

Carbon fiber composite is widely used in the nuclear industry and other fields due to their lightweight and high strength. There are various forms of failure in composite, including resin cracking, fiber fracture, and resin fiber interface debonding. At the initial stage of damage, they all exhibit the form of microcracks. As cracks propagate and fuse, they ultimately lead to macroscopic fracture of composite components. For safety reasons, it is necessary to study the fracture toughness of composite. This article uses a combination of winding method and double cantilever beam method to study the interlayer fracture toughness characteristics of high-strength (or high modulus) fiber epoxy resin composite under type I load. Choose a more suitable propagation GIC as a measurement metric. The results indicate that under the same resin matrix, the interlayer fracture toughness of high-strength fiber composite is much higher than that of high modulus fibers, which is related to the high surface chemical activity of high-strength fiber monofilament. Finally, combined with finite element analysis, its application in laminated structures was briefly analyzed.

Degradation and anisotropy characteristics of fracture toughness in Ni-based superalloys after phase coarsening

The effect of phase coarsening on the crack propagation characteristics and the corresponding fracture toughness in Ni-based superalloys is investigated by integrating with the finite element method simulations and related mechanism analysis, and the relationship between the macroscopic mechanical properties of alloys and the evolution in their microstructures are discussed in detail. Related analysis shows that the fracture toughness of alloys degraded considerably with phase coarsening. Moreover, the fracture toughness also demonstrates an anisotropic characteristic. Specifically, when the volume fraction of precipitate is low or the degree of phase coarsening is relatively low, the fracture toughness of alloys increases first and then decreases gradually along with increasing the loading angle, showing an inverted "V" shape. In comparison, in the case of a higher volume fraction of precipitate or higher degree of phase coarsening, the fracture toughness displays an opposite variation trend, and reaches the minimum value at the loading direction of 45°. The corresponding variation characteristics of fracture toughness are correlated with the evolutions of precipitate shape, size, and distribution within the alloys. The change in the microstructure or that in the loading direction affects remarkably the initiation and growth of micro-voids within the alloys, and it further affects the crack propagation characteristics, resulting in the degradation and anisotropy characteristics of macroscopic fracture toughness in the alloys. Related work is of great significance for the structure design of Ni-based superalloys components, the mechanical property evaluation and the life prediction during their service process.

Fracture behavior of PH15-5 stainless steel manufactured via directed energy deposition

The tensile properties, fracture toughness, and fatigue crack growth (FCG) characteristics of a directed energy deposited precipitation hardened stainless steel (grade: PH15-5; age hardening heat treatment condition: H900) were examined. In the as-fabricated condition, the alloy contains microfissures that are oriented parallel to the build direction, whose appearance was difficult to be detected using optical microscopy. Due to their relative orientation w.r.t. the loading direction, significant anisotropy in tensile and FCG behavior was noted, with the properties being particularly lower when the loading direction is perpendicular to the crack orientation. Despite the presence of microfissures, the alloy’s fracture initiation toughness is comparable to (or in some cases exceeds) those manufactured using either conventionally techniques or laser powder bed fusion. Activation of the extrinsic toughening mechanisms, such as crack deflection when its mode I direction is perpendicular to the microfissures and a combination of crack bridging and deflection when it is parallel, are the micromechanical reasons for the observed high toughness. The efficacy of such mechanisms is observed to depend on the plastic zone size relative to the microfissure spacing. The understanding developed in this study enables the development of strategies for enhancing the damage tolerance of additively manufactured alloys.

Multi-Parameter Methodology for Assessing Quality Indicators of Nanomodified Fiber-Reinforced Concrete for Construction Site

Nanomodified fiber-reinforced concrete is a building material for which the required characteristics of fracture toughness are a distinctive feature. Determination of the stress intensity factor of fiber-reinforced concrete makes it possible to correctly assess the resistance of the material during the formation and development of cracks. The proposed multi-parameter methodology for assessing the quality indicators of nanomodified fiber-reinforced concrete makes it possible to evaluate the quality of a fiber-reinforced concrete structure in construction and laboratory conditions. To carry out control at the construction site, modern and long-used methods of non-destructive testing are used: ultrasonic sounding, ultrasonic tomography, elastic rebound, separation with chipping. For laboratory studies, the technique provides for the manufacture of prism samples that can be molded or cut from the body of the structure. This methodology makes it possible to obtain in laboratory conditions such material parameters as tensile strength in bending, tensile strength in splitting, critical stress intensity factor for normal separation, critical stress intensity factor for transverse shear, energy consumption for individual stages of deformation and destruction of the sample, as well as to evaluate the uniformity of distribution fibers. Moreover, it is provided to obtain all the parameters on one sample from the series, which eliminates errors and inaccuracies in the quality indicators of the material associated with different conditions of hardening, molding, inaccuracies in duplicating the composition.

On adhesively bonded joints with a mixed failure mode—An experimental and numerical study

Cohesive zone model (CZM) has been extensively used to study interfacial failure behaviors such as fracture of adhesively bonded joints (ABJs). Accurate and efficient identification of traction–separation law (TSL) in CZM signifies an important issue for design and analysis of adhesive structures. In this study, a modified Arcan fixture was developed and employed to investigate the mixed mode failure behaviors of ABJs for carbon fiber reinforced plastic (CFRP) substrates under a tensile-shear coupling load, in which the double cantilever beam (DCB) test was carried out to characterize fracture toughness for benchmark data. With the help of digital image correlation (DIC) technique, the interfacial displacement field was obtained. The virtual crack closure technique (VCCT) was adopted to decouple the mode mixture at different tension-shear angles of the Arcan specimens. Based on the traction–separation law for each mixed ratio of failure mode, a refined CZM was established. In this study, the Arcan and DCB specimens bonded with a flexible adhesive of 3M DP190 were prepared and tested. The experiments were carried out for the ABJs with two adhesive layer thicknesses of 0.3 mm and 1 mm, respectively, to investigate the influence of bonding thickness. Based on the extracted CZM parameters, a finite element (FE) analysis model was developed and validated with the experimental results. Unlike the traditional predetermined damage evolution law, a new CZM was established without a priori cohesive damage evolution; and the effectiveness of the proposed model for the TSL extraction was verified. By using this method, the mechanical response of the bonding interface under complex stress state was well predicted and the FE modeling of ABJ’s fracture was accurately achieved. The study is anticipated to gain new understanding and fundamental data on mixed failure mode, thereby enabling better design of ABJs for engineering applications.