Toughness Measurements Research Articles

Measurement of fracture toughness in high-strength alloys via modified limit load analysis using flat-end cylindrical indenter

In this paper, fracture toughness (KJ) was measured for high strength rail steels and AL2024-T351 via chamfered cylindrical flat-end indentation. The indentation loading focused on applying the J-integral approach to curves of load versus indentation depth up to the crack initiation point based on a modified limit load via multiple indenter sizes. To promote single indenter size for practical use, virtual indenter sizes were proposed based on geometrical similarities, where the stress intensity factors according to J-integral approach were extrapolated to minimize the contribution of the plastic component of J-integral (JP). However, when the indentation method for KJ is applied to high strength rail steels, a consideration for the modification of the J-integral approach is suggested with the inclusion of stress triaxiality effect to accommodate the pressure sensitivity experienced in compression-based testing for some materials. The KJ values were seen to agree well with fracture toughness from conventional testing (KIC) for all materials in the study showing a relative difference below 5% except the JP rail steel, which showed only a relative difference of 16%. This is based on the condition that pressure sensitivity effect is present for the rail steels and not for AL2024-T351. The study also compares different indenter sizes, which show similar pressure and normalized depth profiles consequently offering the potential for a non-destructive means to measure mechanical properties and fracture toughness via micro-sized indenters. This opens an opportunity for further studies in material characterization capabilities across a wide range of industries in the future.

Comparison of destructive and non-destructive fracture toughness measurements for Q235 steel butt-welded joint

As a critical component of a welded structure, the integrity of the welded joint significantly affects the safety and reliability of the structure in service. Therefore, it is essential to investigate the integrity of welded joints. However, given the harshness of standard measurements renders them unsuitable for application to small volume zones, welded structures or in-service structures. In this study, the fracture properties of a Q235 steel butt-welded joint was evaluated by boundary effect modelling (BEM) method and the ball indentation (BI) method. The results demonstrate that the fracture toughness distribution obtained by the two methods are consistent, with the highest fracture toughness in the weld metal, followed by the heat-affected zone and the base metal, and the lowest fracture toughness in the fusion zone, which is in agreement with the results of the metallographic testing. Furthermore, the relative errors of the zones obtained by the ball indentation method and the boundary effect modelling method ranging from 10.16 to 19.05%, which indicates safety margin ought to be considered for employing the BI method to determine fracture properties of in-service structures.

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Dual-phase (DP) steels are a subset of high-strength, low-alloy steels that exhibit a unique balance of strength and ductility due to their dual microstructure of ferrite and martensite. This study investigates the effects of varying martensite volume fractions on the mechanical properties of DP steel by preparing samples with martensite contents from 20% to 50%, achieved through intercritical annealing at temperatures between Ac1 and Ac3, followed by quenching in ice-brine. The mechanical properties of these samples were analyzed using optical microscopy, hardness tests, and impact toughness measurements. Results reveal that increasing the martensite volume fraction enhances both hardness and impact toughness, indicating that adjusting heat treatment parameters can optimize DP steels for strength-critical applications. This study provides insight into the microstructural factors influencing DP steel properties and highlights effective processing routes for tailoring material performance. Keywords: Dual-Phase Steel, Microstructure, Heat treatment, Mechanical properties.

Evaluation of the 10-item Mental Toughness Questionnaire (MTQ10): cross-cultural assessment and scrutiny of method effects

The 10-item Mental Toughness Questionnaire (MTQ10) is an easy to administer, global measure of mental toughness. Previous analysis established that the MTQ10 was psychometrically superior to the preceding, widely used, 18-item version. Nevertheless, the MTQ10 is potentially undermined by a method effect arising from the inclusion of negatively phrased items. Noting this, the present paper further assessed the measurement properties of the MTQ10 via cross-cultural comparisons. This involved assessing the factor composition in samples from the UK (N = 596), Greece (N = 1230), and Italy (N = 425). Rather than a strict method effect, analyses found effects specific to pairs of negative items. The emergent model demonstrated partial invariance across countries. National variations in mental toughness scores were ascribable to societal differences. Convergent validity was demonstrated using theoretically related variables (Dark Triad and Life Satisfaction). Overall, results supported the use of the MTQ10 as a global mental toughness measure. Additionally, outcomes suggested that further cross-cultural comparison would provide useful insights into the nature of mental toughness.

“There Is Only One Childhood”: A New Interpretation of China’s “Crackdown” on Education Companies

In recent years, the Chinese government has implemented a series of dramatic crackdowns on various business sectors in China, one of which, beginning in July 2021, targeted the private tutoring industry. This article offers a new interpretation of this crackdown on private education companies, arguing that it is part of a new comprehensive education policy called the “Double Reduction” (DR) policy. Although the policy’s main goal has been to reduce students’ educational burden, this article also links the policy to China’s core objective of advancing indigenous innovation to enable national rejuvenation. The analysis of the DR policy also reveals two contradictions that currently characterize China’s educational path: first, the contradiction inherent in the authoritarian and paternalistic state using tough measures to enforce parental compliance with policies aimed at fostering more democratic parenting styles; and second, the contradiction inherent in policies aimed at fostering happy childhoods for the purposes of global competitiveness.

Improving shear behavior of rubberized concrete beams through sustainable integration of waste tire steel fibers and treated rubber

This study investigates the innovative and sustainable use of crumb rubber (CR) and waste tire steel fibers (WTSF) in reinforced concrete beams. By addressing the concern of reduced shear capacity in rubberized reinforced concrete (RC) beams, the paper presents a sustainable approach that integrates WTSF and a hybrid treatment of CR involving temperature and fly ash. Six different concrete mixtures were developed, incorporating 15 % and 25 % CR as a replacement for natural fine aggregates, both with and without treatment, along with the addition of WTSF. The experimental program assessed the workability, mechanical properties, including compressive and split-tensile strength, and conducted four-point loaded flexural tests on rubberized RC beams designed to fail in shear, in order to investigate their shear behavior.A numerical study further analyzed this hybrid treatment approach across various parameters, including concrete mixture compositions, types of tensile reinforcement (conventional steel or Glass Fiber Reinforced Polymer, GFRP), transverse reinforcement types (conventional steel or GFRP), longitudinal reinforcement ratios, and the combined effects of treated rubber and WTSF. Key findings include a 10 % higher compressive strength in the 25 % treated rubberized concrete mixture (TRC25) compared to the 15 % untreated rubberized concrete (URC15). Tensile strengths were significantly enhanced, with treated rubberized concrete beams containing WTSF at 15 % and 25 % showing tensile strengths 195 % and 180 % higher than normal concrete (NC), respectively. The inclusion of treated rubber and WTSF significantly improved shear capacities, with both 15 % and 25 % rubber content mixtures demonstrating substantially higher shear capacities than normal concrete. Toughness measurements indicated that treated rubberized concrete beams with WTSF at 15 % and 25 % rubber content showed toughness levels 15 to 18 times higher than NC. Beams reinforced with GFRP showed up to a 31 % higher load capacity and greater deflection before failure compared to those with conventional steel reinforcement. The study highlights a beneficial shift from shear to flexural failure modes due to the combined effects of treated rubberized concrete and WTSF incorporation.

“Suck it up, go play”: Mental health stigma in college coaches and their use of mental illness microaggressions

Objective: This study explored the extent to which college athletic coaches endorse mental illness microaggressions toward their student-athletes and the importance of mental toughness in sports, and how these impact support for help-seeking among student-athletes. Methods: Fifty-eight coaches at Northeastern U.S. colleges in the National Collegiate Athletic Association completed an online survey, including measures of mental illness microaggressions, mental toughness, and questions about vignettes portraying scenarios with a physically injured athlete and an athlete with anxiety. Results: Multivariate analyses revealed that endorsement of mental illness microaggressions was negatively related to willingness to refer an athlete with anxiety to counseling services and positively related to willingness to allow a physically injured athlete to return to play. However, mental toughness was not predictive of microaggressions or vignette responses. Conclusions: Endorsement of mental illness microaggressions appears to be related to how coaches respond to athletes experiencing a mental health issue or physical injury.

Determination of Fracture Mechanic Parameters of Concretes Based on Cement Matrix Enhanced by Fly Ash and Nano-Silica.

This study presents test results and deep discussion regarding measurements of the fracture toughness of new concrete composites based on ternary blended cements (TCs). A composition of the most commonly used mineral additive (i.e., fly ash (FA)) in combination with nano-silica (NS) has been proposed as a partial replacement of the ordinary Portland cement (OPC) binder. The novelty of this article is related to the fact that ordinary concretes with FA + NS additives are most often used in construction practice, and there is a decided lack of fracture toughness test results concerning these materials. Therefore, in order to fill this gap in the literature, an extensive evaluation of the fracture mechanic parameters of TC was carried out. Four series of concretes were created, one of which was the reference concrete (REF), and the remaining three were TCs. The effect of a constant content of 5% NS and various FA contents, such as 0, 15%, and 25% wt., as a partial replacement of cement was studied. The parameters of the linear and nonlinear fracture mechanics were analyzed in this study (i.e., the critical stress intensity factor (KIcS), critical crack tip opening displacement (CTODc), and critical unit work of failure (JIc)). In addition, the main mechanical parameters (i.e., the compressive strength (fcm) and splitting tensile strength (fctm)) were evaluated. Based on the studies, it was found that the addition of 5% NS without FA increased the strength and fracture parameters of the concrete by approximately 20%. On the other hand, supplementing the composition of the binder with 5% NS in combination with the 15% FA additive caused an increase in all mechanical parameters by approximately another 20%. However, an increase in the FA content in the concrete mix of another 10% caused a smaller increase in all analyzed factors (i.e., by approximately 10%) compared with a composite with the addition of the NS modifier only. In addition, from an ecological point of view, by utilizing fine waste FA particles combined with extremely fine particles of NS to produce ordinary concretes, the demand for OPC can be reduced, thereby lowering CO2 emissions. Hence, the findings of this research hold practical importance for the future application of such materials in the development of green concretes.

An efficient and quantitative approach for fracture toughness measurement of Micron-Thick hard coatings based on crack spacing

Micron-thick hard coatings find widespread applications, with fracture toughness being a crucial property that ensures their usability. However, measuring the fracture toughness of hard coatings is time-consuming and costly due to the susceptibility to cracking and small sample size. Thus, developing a simple, effective and precise measurement method is necessary. This work presents an analytical model that calculates KC from crack spacing induced by uniaxial tension. By measuring the variation of crack spacing, this model determines the fracture toughness from the energy difference during the coating fracture, and residual stress can also be estimated. This simple method overcomes the limitations of uniaxial tension methods, does not require any in-situ observations, and offers low-cost and user-friendly testing. The KC of TiN coatings with 2 μm thickness were tested and found to match the micro-cantilever beam test results from the literature. This model used data from other literature to calculate the toughness of diamond and a-C:H thin coatings, showing its generality. Additionally, our model’s estimation of residual stress in coatings, as reported in the literature, is accurate to within 0.1 ∼ 1 GPa.

Thickness Effect of 2195 Al-Li Alloy Friction Stir Weld Fracture Toughness.

For damage tolerance design in engineering components, the fracture toughness value, KIC, of the material is essential. However, obtaining specimens of sufficient thickness from stir friction welded plates is challenging, and often, the experimental test values do not meet the necessary criteria, preventing the experimental fracture toughness, Kq, from being recognized as plane strain fracture toughness KIC. The fracture toughness Kq of 2195 Al-Li alloy welding seams with different thicknesses was measured on the forward and backward sides. Microstructure characterization was conducted by scanning electron microscope (SEM). The results indicated minimal significant differences in grain size between the advancing and retreating sides of the weld nugget zone. In specimens of the same thickness, fracture toughness measurements along the normal direction of the joint cross-section showed a high similarity between the advancing and retreating sides of the weld nugget zone. Utilizing the quantitative relationships between fracture toughness and sample thickness derived from both the fracture K and G criteria, it is possible to predict the fracture toughness of thick plates using thin plates. This study employs these relationships to calculate the fracture toughness KIC of 2195 aluminum-lithium alloy friction stir welds. The KIC values obtained are 41.65 MPa·m1/2 from the fracture K criterion and 43.54 MPa·m1/2 from the fracture G criterion.

Validity of Toughness Measurements From Miniature Specimens Failing in Different Fracture Modes

Abstract Using miniature compact tension (mini-C(T)) (4 mm thick, 0.16T) specimens to determine toughness in reactor pressure vessel (RPV) steels permits the ductile-to-brittle transition temperature to be derived from small amounts of material and allows more effective use of surveillance specimens. However, questions have been raised as to whether the failure mechanisms are the same in miniature and large specimens, something that must be ensured when transferring fracture results obtained in mini-C(T) specimens to larger components. This work, performed within the FRACTESUS project, presents toughness measurements and detailed fractography on both a homogeneously brittle base metal and a relatively ductile, inhomogeneous weld to assess the transferability of fracture data. The fractography shows that brittle fracture initiates within the part of specimen experiencing small-scale yielding (SSY), so long as the toughness measurement is valid. Similarly, although the precrack front asymmetry appears more marked in smaller specimens, as long as the deviation from planarity is within the American Society for Testing and Materials (ASTM) E1921 limits, the asymmetry does not affect the location of the initiation site. For materials showing a variety of fracture modes, the fracture modes observed at the initiation sites are consistent with those observed in larger specimens. Where data are available, the stress and strain conditions at the initiation sites are also found to be consistent in mini-C(T) and larger specimens. These observations support the thesis that toughness measurements made on mini-C(T) specimens reflect the same material characteristics and failure mechanisms as those made on larger specimens.

Optimal PLA+ 3D Printing Parameters through Charpy Impact Testing: A Response Surface Methodology

Additive manufacturing (AM) has revolutionized the manufacturing sector, particularly with the advent of 3D printing technology, which allows for the creation of customized, cost-effective, and waste-free products. However, concerns about the strength and reliability of 3D-printed products persist. This study focuses on the impact of three crucial variables—infill density, printing speed, and infill pattern—on the strength of PLA+ 3D-printed products. Our goal is to optimize these parameters to enhance product strength without compromising efficiency. We employed Charpy impact testing and Response Surface Methodology (RSM) to analyze the effects of these variables in combination. Charpy impact testing provides a measure of material toughness, while RSM allows for the optimization of multiple interacting factors. Our experimental design included varying the infill density from low to high values, adjusting printing speeds from 70mm/s to 100mm/s, and using different infill patterns such as cubic and others. Our results show that increasing infill density significantly boosts product strength but also requires more material and longer processing times. Notably, we found that when the infill density exceeds 50%, the printing speed can be increased to 100mm/s without a notable reduction in strength, offering a balance between durability and production efficiency. Additionally, specific infill patterns like cubic provided better strength outcomes compared to others. These findings provide valuable insights for developing stronger and more efficient 3D-printed products using PLA+ materials. By optimizing these parameters, manufacturers can produce high-strength items more efficiently, thereby advancing the capabilities and applications of 3D printing technology in various industries.

Properties of a Pressureless Sintered 2Y-TZP Material Combining High Strength and Toughness

Yttria stabilized zirconia materials are frequently used in mechanical engineering and biomedical applications. Demanding loading conditions require materials combining a high level of strength and fracture toughness. A ready-to-press alumina doped 2 mol% yttria-stabilized zirconia powder was shaped by axial pressing and sintering in air at 1250–1500 °C for 2 h. At 1350 °C the best combination of strength (1450 MPa) and toughness (7.8 MPa√m) was achieved. Materials sintered in the middle of the chosen temperature range combine full density, high transformability and small grain size. Toughness measurements by direct crack length measurements delivered unrealistically high fracture toughness values.

Determining the Cohesive Length of Rock Materials by Roughness Analysis

In this research, the cohesive length of various rock types is measured using quantitative fractography alongside a recently developed multifractal analysis. This length is then utilized to gauge material cohesive stress through the theory of critical distances. Furthermore, the fracture process zone length of different rings sourced from identical rocks is assessed as a function of ring dimensions and experimental measurements of fracture toughness, in accordance with the energy criterion of the finite fracture mechanics theory. Subsequently, employing the stress criterion within coupled finite fracture mechanics, the failure stress corresponding to the fracture process zone is determined for various rings. Ultimately, through interpolation, the critical stress corresponding to the cohesive length, quantified via quantitative fractography, is approximated. Remarkably, the cohesive stress values derived from both methodologies exhibit perfect alignment, indicating the successful determination of cohesive length for the analyzed rock materials. The study also delves into the significant implications of these findings, including the quantification of intrinsic tensile strength in quasi-brittle materials and the understanding of tensile strength variations under diverse stress concentrations and loading conditions.

Mechanical and biological behavior of double network hydrogels reinforced with alginate versus gellan gum

Alginate and gellan gum have both been used by researchers as reinforcing networks to create tough and biocompatible polyethylene glycol (PEG) based double network (DN) hydrogels; however, the relative advantages and disadvantages of each approach are not understood. This study directly compares the mechanical and biological properties of polyethylene glycol di-methacrylate (PEGDMA) hybrid DN hydrogels reinforced with either gellan gum or sodium alginate using PEGDMA concentrations from 10 to 20 wt% and reinforcing network concentrations of 1 and 2 wt%. The findings demonstrate that gellan gum reinforcement is more effective at increasing the strength, stiffness, and toughness of PEGDMA DN hydrogels. In contrast, alginate reinforcement yields DN hydrogels with greater stretchability compared to gellan gum reinforced PEGDMA. Furthermore, separate measurements of toughness via unnotched work of rupture testing and notched fracture toughness testing showed a strong correlation of these two properties for a single reinforcing network type, but not across the two types of reinforcing networks. This suggests that additional notched fracture toughness experiments are important for understanding the full mechanical response when comparing different tough DN hydrogel systems. Regarding the biological response, after conjugation of matrix protein to the surface of both materials robust cell attachment and spreading was supported with higher yes associated protein (YAP) nuclear expression observed in populations adhering to the stiffer gellan gum-PEGDMA material. This study provides valuable insights regarding how to design double network hydrogels for specific property requirements, e.g., for use in biomedical devices, as scaffolding for tissue engineering, or in soft robotic applications.

In situ TEM nanomechanical study on enhanced toughness of graphene-nanoparticle nanocomposite

Although theoretical studies and experimental research have shown outstanding mechanical properties of graphene, its brittleness and low toughness restrict its widespread industrial use. Here, we conducted a study to examine the impact of encapsulated Ag nanoparticles on the mechanical properties of graphene through in situ nanomechanical testing in an aberration-corrected transmission electron microscope (TEM). The presence of Ag nanoparticles was found to decrease crack propagation speed, enabling the TEM observation of crack deflection around the nanoparticles at nanoscale, as well as real-time measurement of the concomitant increase in stress. Using our nanomechanical testing data, we were able to quantitatively estimate the stress intensity factor, which serves as a measurement of fracture toughness for graphene, and then compared our results with previously reported values. We further discuss the role of nanoparticles in the propagation of cracks and their potential to enhance the toughness of materials. These findings have practical implications for the development of high-performance graphene composite materials.

The anomalous crack growth behaviour of an elastic-brittle octet-truss architected solid

Strongly rising R-curves are observed for octet-truss architected material specimens comprising ∼1 million unit cells and made from an elastic-brittle parent material. The measurements are supported by finite element (FE) calculations where the octet-truss is modelled discretely and show that the pseudo-ductility is not related to the usual toughening mechanisms such as crack bridging or crack-tip plasticity. Rather, the toughening is attributed to the three-dimensional (3D) structure of the octet-truss specimen: remarkably, no R-curve effect is observed in a quasi two-dimensional (2D) octet-truss specimen. This anomalous behaviour is clarified via FE calculations of the indentation stiffness of the octet-truss which reveal a strong indentation size effect in 3D with the stiffness decreasing with decreasing indenter size relative to the cell size of the octet-truss. In the 3D octet-truss, the size independent continuum stiffness is only attained for indenter sizes greater than about 20 unit cells while the size effect is almost absent in the quasi-2D octet-truss. The strong elastic softening in the 3D octet-truss in the presence of strain gradients gives rise to a large elastic crack-tip process zone. This in turn implies that a specimen geometry independent fracture toughness (i.e., a material property) can only be measured in specimens with large numbers of unit cells. We report a fracture mechanism map that reveals that measurements of specimen geometry independent fracture toughness can only be made in specimens with more than ∼10 million unit cells. This map is also used to rationalise the measured rising R-curves in the 3D specimens with ∼ 1 million unit cells. We end with a discussion on the implication of these findings for the laboratory measurement of the fracture toughness of both architected materials as well as biological cellular materials such as bone.

Effect of temperature on apparent crack arrest toughness of Eurofer97 measured by a newly developed small specimen test technique

Neutron embrittlement of in-core and out-of-core components made of ferritic steels is one of the key parameters limiting the life of power plants. Owing to the small volume of available irradiation facilities and the limited number of neutron-irradiated specimens, techniques to measure initiation fracture toughness with small specimens have been developed over the years. Although arrest toughness is of equal importance regarding safety assessment, the emphasis has been put on the effect of specimen size on initiation toughness and only a few small specimen test techniques can be found for arrest toughness measurements. The objective of the current work was to develop an experimental technique for measuring the apparent arrest toughness of ferritic steels with miniaturized specimens. Small cantilevers made of the 9Cr Eurofer97 equiaxed ferritic steel were produced with a brittle thin laser-treated layer, which enables crack initiation. The cracks were successfully arrested in the more ductile equiaxed ferritic matrix in a series of tests that were carried out from -123 °C to room temperature with different test configurations. The mechanical tests showed that brittle fracture could be triggered and stopped inside the specimen with a width of 2 mm over a temperature range of more than 100 °C. An apparent arrest toughness was calculated and its temperature dependence was related to the plastic work dissipated by a propagating crack.

Static and dynamic fracture toughness of graphite materials with varying grain sizes

Graphite materials play critical roles as moderators, reflectors, and core structural components in high-temperature gas-cooled nuclear reactors. During the reactor operation, graphite materials may experience a variety of loads, including thermal, radiation, fatigue, and dynamic loads, potentially leading to crack initiation and propagation. Therefore, it is imperative to investigate their fracture properties. Despite this, there remains a paucity of comprehensive studies on the fracture toughness of graphite materials with varying grain sizes, particularly concerning dynamic fracture toughness. This study addresses this gap by employing a digital-image-correlation-based virtual extensometer to analyze crack propagation in graphite materials of different grain sizes, enabling precise measurement of crack propagation length and fracture toughness. Findings reveal that static fracture toughness increases with larger grain sizes, while under dynamic loading, smaller grain sizes exhibit greater fracture toughness. Additionally, dynamic fracture toughness shows a near-linear increase with impact speed. Scanning electron microscopy analysis of fracture surface morphology highlights the impact of grain size and impact speed on fracture toughness. Nuclear graphite specimens with larger grain sizes have more irregular grain and pore distributions, enhancing crack deflection and propagation resistance, thereby increasing fracture toughness. The observed loading rate dependence of dynamic fracture toughness is attributed to a gradual transition from intergranular to transgranular fracture modes with increasing impact speed.

Designing the geometry of compact tension specimens for easy fracture toughness measurement using reinforcement learning

Abstract For composite laminates, a rising R-curve is observed for their fracture toughness under Mode I stress, which is important for a comprehensive failure analysis of the materials. Since it is laborious to measure the R-curve due to its dependence on both the load and the crack extension, we put forward a novel compact tension specimen by modifying its geometry to eliminate the relation between fracture toughness and crack extension, so as to simplify the experimental process of the R-curve measurement by only recording the load history. Two machine learning models were developed for the optimum sample design based on the finite element analysis of the effect of sample geometries on the R-curve. A simple neural network model was built for designing tapered specimen and a reinforcement learning model was created for further finding the best design from a broader design space. The results showed that, in contrast to the specimens with a tapered shape, which only ensure the independence between the R-curve and crack extension in the case of a small extension, the design provided by the reinforcement learning provides such independence across a wider range of crack length and an improved accuracy.