Fracture Toughness Of Specimens Research Articles

Cryogenic fracturing efficacy in granite rocks: Fracture toughness and brazilian test differences after elevated temperatures and liquid nitrogen exposure

Hot dry rock (HDR) is a significant contingent as a thermal energy source. Due to its low permeability, considerable depth, high temperature (150–650 °C), and high pressure, the stimulation of HDR is challenging. This research work aims to examine the efficacy of cryogenic fracturing on HDR. We performed fracture toughness and indirect tensile stress (Brazilian tests) on granite specimens with three different initial temperatures before exposure to liquid nitrogen (LN2). Fracture toughness tests were implemented on semicircular bend (SCB) specimens containing a pure mode-I fracture, and Brazilian tests were performed with discs. The experimental outcomes show that, overall, both critical load in Brazilian tests and the fracture toughness of LN2-treated specimens of granites both diminished with increasing initial temperature. When the initial temperature exceeded 300 °C (in this instance, 500 °C), the amplitude of thermally induced fractures in granite moderately increased after LN2 treatment, secondary fractures prolonged, and the plastic characteristics increased. Moreover, permeability measurements indicated amelioration with elevated temperatures and subsequent LN2 exposure. SEM analysis showed that an increase in initial temperatures along with LN2 treatment led to further intensified granite microdamage while the quantity and the magnitude of thermal fractures augmented.

Effect of different parameters on mode I fracture toughness of resin samples manufactured by DLP

Abstract Additive manufacturing is spreading rapidly in almost all industries, from household to advanced engineering. Components produced by Digital Light Processing (DLP) are not comprehensively characterized, but exceed the capabilities of many AM processes. Its advantages include the ability to produce highly complex designs, superior precision, fast printing and lower operating costs. The present paper investigates the mode I fracture toughness (KIC) of UV-sensitive resin specimens obtained through DLP process. Single edge notch bending (SENB) specimen were 3D printed and tested according to the ASTM D5045 standard. The influence of printing orientation-PO (0°, 45° and 90°), resin color-RC (white, black and transparent) and corrosive environment-CE (air, water and saline solution) was studied. It was observed that all the studied parameters have some effect on the fracture properties. The highest KIC values were obtained for 45°-PO, white-RC (1.88 MPa•m0.5) and saline solution-CE (2.24 MPa•m0.5). However, the greatest influence is highlighted by RC (∼92%), while CE (∼32%) shows a minimal effect. The fracture surface of the investigated samples is influenced by the printing parameters.

Microscopic characterisation of brittle fracture initiation in irradiated and thermally aged low-alloy steel welds of a decommissioned reactor pressure vessel

Microstructure has a significant effect on material's integrity and in a heterogeneous weld microstructure the discontinuities affect the brittle fracture initiation and propagation and determine the fracture toughness. The knowledge of brittle fracture initiation mechanisms in high-Mn/high-Ni welds is limited. The brittle fracture initiation behaviour of the decommissioned Barsebäck Unit 2 reactor pressure vessel (RPV) welds of high-Mn/high-Ni weld metal from three different locations, the RPV head and the beltline regions, were investigated and compared with specimens from the surveillance program with high fluence. Systematic fractography has been performed on impact and fracture toughness specimens and the main features of the brittle fracture initiation in the component weld are presented and discussed. Two main types of initiators are identified as the weakest links to initiate the cleavage fracture and the initiation mechanism is found independent from the operation condition. The high-fluence surveillance specimens have a larger amount of intergranular cracking. The cleavage fracture initiation appears to be independent of the operation conditions but dependent on the welding process and metallurgical features. The findings aid in the development of improved material-property correlations which will result in better computational tools for predicting aging of welds based on microstructure.

A probabilistic model to predict specimen geometry effects on fracture toughness in ferritic–pearlitic steels

This work describes a probabilistic framework for cleavage fracture of ferritic–pearlitic steels incorporating experimental measurements of microcrack distribution associated with the cracking of the pearlitic microstructure. A central objective of this study is to explore and further extend application of a probabilistic framework incorporating the statistics of microcracks to predict specimen geometry effects on the fracture toughness distribution for a typical ferritic–pearlitic structural steel. Fracture toughness values for an ASTM A572 Grade 50 structural steel derived from fracture tests using conventional SE(B) specimens with varying thickness and a/W-ratios provide the cleavage fracture resistance data needed to assess specimen geometry effects on the probability distribution of Jc-values. The present exploratory study successfully predicts the measured statistical distribution of cleavage fracture toughness in shallow crack specimens and provides further support of the proposed probabilistic model as a more advanced and effective engineering-level procedure in fracture assessment methodologies.

Investigation of the fracture characteristics of mixed-mode I/III crack by using two kinds of sandstone specimens

This study aims to identify specimens that are more suitable for examining the failure properties of I/III mixed-mode cracks under static loads. We employed single-edge notched bending (SENB) and edge-notched disc bending (ENDB) specimens, and used ABAQUS finite element method software to calculate the stress intensity factors for modes I and III at various crack inclination angles θ. Fracture toughness was tested using three-point bending experiments on two types of sandstone specimens. The evolution of mixed-mode I/III fracture morphology and failure patterns was investigated with a monocular microscope. Additionally, the evolution of the strain field and crack tip opening displacement was analyzed using the digital image correlation method. Our findings indicate that the fracture toughness of the ENDB specimens surpassed that of the SENB specimens. The effective fracture toughness of both SENB and ENDB specimens initially increased with crack inclination angle θ but then decreased. The fracture mode correlated with energy release; the energy release rate of the ENDB specimens exceeded that of the SENB specimens, making the former more brittle. For SENB specimens, crack initiation was primarily intergranular, while ENDB specimens exhibited predominantly transgranular fractures. The crack initiation moments for both specimen types increased with higher crack inclination angles. Furthermore, at the same crack inclination angle, ENDB specimens cracked earlier than SENB specimens.

Zirconia specimens printed by vat photopolymerization: Mechanical properties, fatigue properties, and fractography analysis.

The mechanical and fatigue properties of zirconia specimens printed by vat photopolymerization (VPP) were evaluated and compared with those of zirconia specimens milled by computer numerical control (CNC). Bar-shaped specimens were printed by stereolithography (SL) and digital light processing (DLP). CNC-milled specimens were used as control samples. The fracture toughness, hardness, and flexural strength properties of the zirconia specimens were evaluated via single edge V-notch beam tests, Vickers hardness tests, and 3-point bending tests. Dynamic fatigue tests were carried out in distilled water using a step-stress test. After static bending and dynamic step-stress testing, fractography analysis was performed. Statistical analysis was carried out to compare the fracture toughness, hardness, flexural strength, and fatigue cycle results of each group (α = 0.05). The fracture toughness values did not significantly differ among the groups (p>0.05). The flexural strength was 894.10MPa for SL, 831.46MPa for DLP, and 1140.39MPa for CNC. The flexural strength of CNC was greater than that of SL and DLP (p<0.01). The mean fatigue cycles were 23498.07 for SL, 19858.60 for DLP, and 31566.80 for CNC. The mean fatigue failure strength was 643.13MPa for SL, 530.63MPa for DLP, and 903.75MPa for CNC. The fatigue failure strength of CNC was greater than that of SL and DLP (p<0.05). Fractography analysis revealed material defects at the fracture origin for each group. A partially fused structure of the incompletely debonded resin could be observed in SL, and a porous region of incompletely sintered zirconia grains could be observed in CNC. The fracture toughness and hardness of zirconia printed by VPP are comparable to those of zirconia milled by CNC. However, zirconia milled by CNC has superior static flexural strength and dynamic fatigue resistance. Further studies are needed to explore the clinical applications of VPP-printed zirconia.

Prediction of Mode-I Fracture Toughness of the ISRM-Suggested Semi-Circular Bend Rock Specimen Using ANN and Optimized ANN Models

Prediction of Mode-I Fracture Toughness of the ISRM-Suggested Semi-Circular Bend Rock Specimen Using ANN and Optimized ANN Models

Fabrication of Si3N4 ceramics with improved thermal conductivity by carbothermal reduction and two‐step sintering

AbstractSi3N4 ceramics with improved thermal conductivity and mechanical properties were prepared by carbothermal reduction and two‐step sintering. The addition of 2 wt% C led to the development of an intergranular phase YNSiO2 with reduced oxygen content. Notably, setting the pre‐sintering temperature at 1450°C promoted the formation of a rough bimodal morphology within the Si3N4 ceramics. Furthermore, an extended holding period at 1850°C resulted in additional grain growth and the elimination of intergranular phases. The Si3N4 specimens produced at 1450°C for 3 h followed by 1850°C for 8 h, incorporating 2 wt% C, exhibited a notable continuity between Si3N4 grains. The thickness of the grain boundary layer was only about 1 nm, and the N/O atomic ratio of the intergranular phase was measured to be 0.42. Consequently, the resulting thermal conductivity, flexural strength, and fracture toughness of Si3N4 specimens were found to be 101 W·m−1·K−1, 712 ± 29 MPa, and 7.7 ± 0.2 MPa·m1/2, respectively. Compared with samples without C additive and directly sintered at 1850°C for 8 h, the Si3N4 specimens produced in this work showed a significant increase of more than 50% in thermal conductivity and about 17% in fracture toughness.

Experimental and simulation study on quasi-static and dynamic fracture toughness of 2A12-T4 aluminum alloy using CTS specimen

Based on the study of dynamic fracture toughness of mixed mode I/II fracture crack, using compact tensile shear (CTS) specimen and the method of split Hopkinson tensile bar (SHTB) combined with special fixture, which complete the experiments of complex fracture modes of 2A12-T4 aluminum alloy material under quasi-static and dynamic tensile tests. By combining the results of finite element simulation with experimental–numerical method, the feasibility of Richard’s theory formula in dynamic fracture experiment is discussed. The variation law of fracture toughness under quasi-static and dynamic complex fracture under mixed-mode was analyzed by finite element simulation and experiment. The results show that, in the process of quasi-static and dynamic fracture, the fracture toughness of CTS specimen under mixed mode I/II crack loading is the highest. The conclusion is proved by Richard theorical formula and experimental–numerical methods jointly. When the specimen is subjected to pure mode I fracture, the quasi-static fracture toughness is consistent with that is in condition of dynamic experiment. However, when the specimen is subjected to mode II and mixed mode I/II fracture, the dynamic fracture toughness changes greatly compared with the quasi-static fracture toughness. In addition, in the dynamic fracture experiment, the dynamic fracture toughness of mixed mode I/II depends on the initiation time, and the quasi-static fracture criterion is no longer applicable.

A double compliances method for determination of the ELS mode II interlaminar fracture toughness without measuring the growing crack length

Due to crack closure, determination of the interlaminar mode II fracture toughness of composite materials is more challenging than that of mode I. Three data reduction methods (SBT, ECM and CBTE) are recommended in the ISO standard 15114 to calculate the interlaminar mode II fracture toughness using End-Loaded-Split (ELS) specimen. However, the maximum difference in the fracture toughness obtained from these methods is up to 20%. A double compliances method is proposed to determine the interlaminar mode II fracture toughness of an ELS composite specimen, without measurement of the crack growth length. Both virtual and real ELS tests are conducted to verify the accuracy of the present method. It is found that the fracture toughness obtained from the present double compliances method and CBTE are consistent, whereas the crack growth length calculated from the present method is more accurate than that from CBTE. The SBT is very simple, but its accuracy is fair for calculation of the fracture toughness.

Constraint effects on fracture toughness of ductile cast iron in the ductile regime

To assess the effect of constraint loss on fracture toughness of ductile cast iron (DCI), GJS-400–15, in the ductile regime at 0 °C, the fracture toughness was characterised with two different types of fracture toughness specimens, compact tension (C(T)) and single edge tension (SE(T)) specimens. The constraint state in the specimens was quantified using finite element analysis. Overall, the constraint dependence of fracture toughness for DCI is comparable to typical ferritic structural steels. For the obtained J-R curves, the fracture toughness increases as the crack grows, and the crack growth is stable on macroscopic scale up to J = 290 kJ/m2. Independent of the constraint state in the stable crack growth regime, the global failure mechanism is ductile fracture, including occasional local brittle areas.

Effectiveness of artificial neural network for forecasting of fracture toughness of concrete specimens

Effectiveness of artificial neural network for forecasting of fracture toughness of concrete specimens

Prediction of fracture toughness using small specimens based on machine learning

In this study, a neural network was used to predict the fracture toughness of standard specimens of 16MnDR steel in the tough-brittle transition zone. The main factors such as specimen plate thickness and geometry type are taken as inputs to the network and the fracture toughness of the standard specimen is taken as output. A BP neural network is utilized to describe the nonlinear relationship between them. The network was trained and tested by using finite element simulation simulation data based on experimental data. The established neural network model was found to have good accuracy in predicting the experimental data and can be used as a method for predicting the fracture toughness of 16MnDR standard specimens.

Fracture Toughness Investigations of an Ion‐Irradiated Nanocrystalline TiZrNbHfTa Refractory High‐Entropy Alloy

Refractory high‐entropy alloys (RHEAs) show potential for use in extreme environments, such as advanced nuclear reactors, owing to their high melting temperature, and often outstanding combinations of mechanical properties, corrosion resistance, and irradiation‐damage tolerance. This study evaluates the fracture toughness of a TiZrNbHfTa RHEA across different scales and microstructures, with a focus on the impact of He2+‐ion irradiation. Micro‐ and millimeter‐scale specimens with nanocrystalline (NC) microstructures are compared to existing ASTM standard sized coarse‐grained (CG) specimen data, with critical dimensions spanning over three orders of magnitude, from 10 μm to 12 mm. The ASTM standard sized CG specimens exhibit a fracture toughness 41‐fold greater than their NC microscale counterparts (210–5.1 MPa m1/2), while NC millimeter‐scale specimens show a 7.5‐fold higher fracture toughness than NC microscale specimens (38.1–5.1 MPa m1/2). He2+‐ion irradiation leads to a 27% decrease in fracture toughness in the NC microscale specimens. The results highlight the impact of sample dimensional scale, microstructure, and ion irradiation on the fracture toughness of the RHEA, indicating a need for thorough examination of such factors when investigating the mechanical properties of these materials.

Experimental and numerical assessment of fracture toughness variations in steam boiler components after a long period of operation

This study presents an experimental and numerical assessment of the fracture toughness of steam boiler components, which have been in operation for more than 100,000 h. The materials tested are components of two different parts of a steam turbine unit: the pipeline section that connects the headers with turbines, which is made of 13HMF steel, and the valve chamber section, which is made of L17HMF steel grades. Full-size arc-shaped tension specimens (AT) are employed for the experimental evaluation. AT and single-edge notch tension (SENT) specimens are used for numerical assessment. The finite element method (FEM) formulations are developed and examined in Abaqus, which is set up as a shell model with the specimen thickness designated as the mid-surface for the analysis. The notch is modeled using a contour integral definition based on the maximum energy release rate criterion. The fracture toughness is quantified using the stress intensity factor (KI), and the results are compared. The valve chamber is found to have a higher fracture toughness than the pipeline section. The effect of specimen geometry is also noticeable, and the fracture toughness of SENT and AT specimens are compared. The results are useful in assessing the component's durability after long operation.

Dynamic impact experimental and global cohesive element method to shale fracture characterization

The fracture behavior and various of mechanical mechanisms of shale are greatly influenced by the bedding surface. To study the effects of bedding angles on the crack propagation patterns and dynamic fracture properties, the shale notched semi-circular bend (NSCB) specimens were studied by the modified split Hopkinson pressure bar (SHPB) device, three-dimensional digital image correlation (3D-DIC) system with the high-speed (HS) photography and ABAQUS finite element software. The research results showed that at the same impact velocity, the 30°, 45° and 60° beddings played an obvious deflexion effect on the shale crack propagation. However, the 0° and 90° bedding did not deflect the crack propagation. The dynamic fracture toughness of shale NSCB specimens increased with the increase of impact loading rate but the increase magnitude decreased with the higher impact rate. The kinetic energy during specimen fracture was calculated by the extracting displacement data from the 3D-DIC, and it was found that the kinetic energy accounted for less than 6% of the absorbed energy during the dynamic compression of the shale. The crack propagation characteristics of ABAQUS simulation was approximately consistent with the experimental results. It is conducive to the study of shale gas fracturing and the formation of complex seam networks in shale and it is of great research significance for the efficient extraction of shale gas.

Dynamic fracture properties and criterion of cyclic freeze-thaw treated granite subjected to mixed-mode loading

Rock masses in high-elevation or cold regions are vulnerable to the combined effects of freeze-thaw (F-T) weathering and dynamic mixed-mode loading, posing a serious threaten to the safety and stability of geotechnical engineering. In this study, a series of dynamic fracture tests were conducted on notched semi-circular bend (NSCB) granite specimens subjected to different mixed-mode loading and F-T cycles using a split Hopkinson pressure bar (SHPB) test system. The effects of F-T treatment and dynamic mixed-mode loading on the fracture properties of granite, including effective fracture toughness, progressive fracture process, and macroscopic morphology of fracture surface, were comprehensively revealed. The experimental results suggest that the dynamic effective fracture toughness of NSCB specimens is dependent on the loading rate, particularly when the mode I loading is dominant. Additionally, the fracture toughness decreases as the number of F-T cycles increases, with an inflection point at 30 F-T cycles. All granite specimens subjected to mixed-mode loading exhibit a curved fracture path, with faster crack propagation speed and more fine cracks in specimens exposed to higher F-T cycles. Macroscopic morphology of fracture surface obtained using three-dimensional (3D) scanning indicates that the fractal dimension of the fracture surface increases with increasing F-T cycles, and the increment is more pronounced for specimens subjected to a higher mode II loading component. Moreover, this study compared the fracture resistance of F-T treated granite subjected to dynamic mixed loading using the maximum tangential stress (MTS) criterion and the generalized maximum tangential stress-based semi-analytical (SA-GMTS) criterion. Compared with the MTS criterion, the SA-GMTS criterion shows a more reasonable consistency with the experimental results, with a root mean square error within ±7%.

A study of microstructure and mechanical properties of (CoCrNi)82Al9Ti9 high entropy alloy by electrical-thermal-mechanical coupled rapid post-treatment

In this work, we have creatively used electrical-thermal-mechanical coupled rapid post-treatment to improve the properties of (CoCrNi)82Al9Ti9 high entropy alloy (HEA). The effect of the electrical-thermal-mechanical coupled field on the phase, microstructure, microhardness, fracture toughness, and compression properties of SLM specimens are investigated. The results show that the SLM specimen exhibits face-centered cubic (FCC) and body-centered cubic (BCC) phases and the microstructure is a dendrite structure. The SLM specimens exhibit imperfections, including porosity and cracks, as well as low densification. After electrical-thermal-mechanical coupled rapid post-treatment, the FCC phase decreases while the BCC phase increases in the SLM specimen, and the microstructure changes from dendritic to recrystallized. The created specimens are healed of defects and the densification is increased to 99.6%. It can be attributed to the combination of the electrical (Electron wind) and thermal effects (Joule heat) of pulse current. Electron wind promotes the diffusion of atoms and expansion of the movement of dislocations, providing the material basis for defect healing and recrystallization. Joule heat provides the substance conditions. The (CoCrNi)82Al9Ti9 HEA has a remarkable improvement in strength and ductility.

Investigation on interlayer debonding behavior of unidirectional composite laminates under cyclical hygrothermal aging duration effects

AbstractThe composite interlamination is the key part of load transfer and plays a decisive role in the overall mechanical performance of composite structures. The interlaminar property is predominantly influenced by the matrix resin which is easily affected by the hygrothermal environment. This paper delved into the effect of hygrothermal aging duration on the interlayer debonding behavior of unidirectional (UD) laminates. The pre‐cracked specimens were treated by hygrothermal aging with a period of 0, 15, 30, 60, 90, and 120 days. The double cantilever beam (DCB) tests were employed to evaluate the interlaminar property. The effect of hygrothermal aging on load–displacement responses, R‐curves, crack growth speed, and fracture surface topography was analyzed. The results revealed that the load‐bearing capacity of aged specimens presented a reduction of 18.6%–29.0% compared with unaged ones. The interlaminar fracture toughness of specimens subjected to 15 days of treatment decreased significantly compared with that of unaged specimens due to the increase of water absorption caused by hygrothermal aging. With the increase in hygrothermal aging time, the interlaminar fracture toughness peaked at 60 days because of the post‐curing effect, followed by a declining trend influenced by the dominant hygrothermal aging effect. The fiber bridging of aged samples was increasingly obvious, became the most prominent at 60 days, and then gradually reduced. The fiber bridging diminished the crack propagation speed, hindered interlaminar debonding, and contributed to the interlaminar toughening.Highlights The samples underwent hygrothermal aging for 0, 15, 30, 60, 90, and 120 days. Load ability of aged samples was at least 18.6% lower than that of unaged ones. R‐curves rose before the crack length of 79 mm and then tended to stabilize. Fracture toughness first decreased, then increased and reduced with aging time. Fiber bridging could hinder crack growth and improve the interlayer property.

An experimental evaluation on rate sensitivity of fracture toughness in sheet specimens made of SUS304 metastable austenitic stainless steel

Utilizing transformation-induced plasticity (TRIP) steel with strain-induced martensitic transformation (SIMT) on automotive steel plates and developing products which have exceptional energy absorption capabilities can be used to reduce weight. For this purpose, it is important to clarify the fracture toughness of TRIP steels and its strain rate dependency to ensure the use of impact-absorbing members over a wide range of deformation rates. However, inconsistent rate dependent trends were observed in the previous studies, possibly as a result of the transition of the fracture mode. Therefore, the effect of fracture modes and strain rate on the fracture toughness values needs to be examined sufficiently. In the present study, the fracture toughness of the specimens made of SUS304 metastable austenitic stainless steel, which is a kind of TRIP steel, is evaluated by tensile tests at various strain rates. Direct current potential difference and digital image correlation methods are introduced to detect the time of fracture initiation during deformation, especially at high deformation rates by using the developed impact tensile tests based on the split Hopkinson pressure bar. The fracture surfaces of the fractured specimens are observed by using an electron scanning electron microscopy to determine not only the fracture modes and their rate dependent effects but also the reinforcement mechanism of SIMT in mixed modes. The results demonstrate that fracture toughness shows negative strain rate dependency, and mixed-mode fracture can be found near the crack in the case of the impact tensile test.