Variability Of Fracture Toughness Research Articles

Assessment of fracture toughness in fibrous concrete via Brazilian test: Experimental study with low-clinker cementitious binder

As the predominant and cost-effective material in the construction industry, concrete is susceptible to tension weaknesses, leading to significant flaws such as cracking and brittle fracturing. Recent advancements in fibrous concrete have emerged as a solution to mitigate these issues. Incorporating steel fibers in concrete has emerged as a promising solution to improve crack resistance and structural integrity. This study focuses on developing eco-friendly concrete using a low-clinker cementitious binder and short steel fibers to enhance fracture toughness and mitigate the environmental impact by effectively utilizing lime sludge. Concrete specimens were prepared with three binder types: treated lime sludge (TLS) at 15 % and 30 % and calcined clay (CC) at 15 % and 30 %, replacing conventional clinker. Short steel fibers were added at 1.5 % by volume to enhance mechanical properties. Fracture toughness was evaluated using notched Brazilian disc specimens, assessing mode I, II, and mixed-mode (I/II) fracture at multiple notch orientations (β = 0°, 15°, 28.83°, 45°, 60°, 75°, and 90°). Microstructural analysis during strength development was performed using scanning electron microscopy and X-ray diffraction. The findings revealed that specimens containing 15 % TLS, 30 % CC, and 1.5 % steel fibers exhibited the highest fracture toughness. Mode II fracture toughness exceeded mode I, indicating improved resistance to crack propagation. The addition of fibers to the specimens under mode II demonstrated improved fracture toughness, ranging from 0.44 to 0.53 MPa·m^1/2 compared to the corresponding non-fibrous specimens. The fibrous specimens showed significantly higher ultimate loads at β = 90° compared to β = 0°, indicating superior crack resistance and structural integrity under perpendicular loading conditions. The Brazilian disc specimens demonstrated variability in fracture toughness across different loading orientations, highlighting their suitability for mixed-mode fracture assessment.

Material Properties of Sediments Steer Burrowers and Effect Bioturbation

AbstractInfaunal organisms mix sediments through burrowing, ingestion and egestion, enhancing fluxes of nutrients and oxygen, yet the mechanisms underlying bioturbation remain unresolved. Burrows are extended through muddy sediments by fracture, and we hypothesize that the cohesive properties of sediments play an important but unexplored role in resisting bioturbation. Specifically, we suggest that crack branching, tortuosity, and microcracking are important in freeing particles from the cohesive matrix, and that the sediment properties that affect these processes are important predictors of bioturbation. We use finite element modeling and simplified, mechanics‐based models to explore the relative importance of sediment mechanical properties and worm behaviors in determining crack propagation paths. Our results show that crack propagation direction depends on variability in fracture toughness, and that applying more force to one side of the burrow wall, simulating “steering” behavior, has surprisingly little effect on crack propagation direction. Burrowers instead steer by choosing among crack branches. Paths created by burrowing worms in natural sediments are mostly straight with some crack branching, consistent with modeling results. Crack branching also requires sufficient stored elastic energy to drive two cracks, and worms can exert larger forces resulting in more stored energy in stiffer sediments. This implies that more crack branching and consequently more particle mixing occurs in heterogeneous sediments with low fracture toughness relative to stiffness. Whether sediments with greater potential for crack branching also experience higher bioturbation remains to be tested, but these results indicate that material properties of sediments may be important in resisting or facilitating bioturbation.

Understanding the Fracture Behaviors of Metallic Glasses—An Overview

Fracture properties are crucial for the applications of structural materials. The fracture behaviors of crystalline alloys have been systematically investigated and well understood. The fracture behaviors of metallic glasses (MGs) are quite different from that of conventional crystalline alloys and have drawn wide interests. Although a few reviews on the fracture and mechanical properties of metallic glasses have been published, an overview on how and why metallic glasses fall out of the scope of the conventional fracture mechanics is still needed. This article attempts to clarify the up-to-date understanding of the question. We review the fracture behaviors of metallic glasses with the related scientific issues including the mode I fracture, brittle fracture, super ductile fracture, impact toughness, and fatigue fracture behaviors. The complex fracture mechanism of MGs is further discussed from the perspectives of discontinuous stress/strain field, plastic zone, and fracture resistance, which deviate from the classic fracture mechanics in polycrystalline alloys. Due to the special deformation mechanism, metallic glasses show a high variability in fracture toughness and other mechanical properties. The outlook presented by this review could help the further studies of metallic glasses. The review also identifies some key questions to be answered.

On the understanding of the effects of sample size on the variability in fracture toughness of bulk metallic glasses

High strength in combination with improvements in failure characteristics and associated gains in fracture toughness have placed bulk metallic glasses (BMGs) among the most damage-tolerant materials to date. Recent studies show, however, that there can be large variabilities in the mechanical performance of these alloys, particularly in their toughness, which are likely associated with sample-size effects or structural variations from differences in processing. Here, we examine the variation in fracture toughness of the Pd-based metallic glass Pd77.5Cu6Si16.5, using single-edge notched bend specimens but in two different sizes. Although all toughness results on this glass were “valid” in terms of the nonlinear-elastic fracture mechanics J-standard, i.e., one would expect a single value of the fracture toughness for this alloy, marked differences were apparent in the toughness values and failure characteristics of the differently-sized samples. Specifically, significantly larger variations in toughness values were measured in larger-sized samples, which all essentially failed catastrophically, whereas none of the smaller-sized samples failed catastrophically yet displayed far less scatter in their measured toughness. Additional in situ tests on the smaller-sized samples in a scanning electron microscope revealed stable crack growth and progressive resistance to crack extension, i.e., rising crack-resistance (R-curve) behavior. Overall, this marked transition from brittle catastrophic failure in large samples, where a size-independent fracture toughness can be measured, to non-catastrophic, more ductile (R-curve), behavior in smaller samples, the latter associated with higher toughness, is related to the distinct size-dependent bending ductility and strain-softening behavior in BMGs.

Test Results from Round Robin on Precracking and CTOD Testing of Welds

Abstract A weld fracture toughness round robin test program was initiated by ASTM to evaluate a proposed annex to E1290-93 on fracture toughness testing of ferritic steel weldments. The focus of the round robin was on specimen preparation procedures and their effect on crack tip opening displacement (CTOD). The local compression procedure was used to reduce the influence of residual stress on precrack curvature. Specimens were tested at −196 °C, which is on the lower shelf for this metal, to minimize variability in fracture toughness and maximize influence of residual stress. Seven labs participated in the round robin and among them conducted 51 tests per E1290-93 to measure CTOD. Statistical analysis of the data showed that crack curvature decreased by 5.2 % ±2.1 % for each 1 % increase in local compression, over the range of compressions tested (0–2.9 % with a nominal target value of 1 %). However, mean values of curvature varied significantly between labs, and when results from all labs are considered, local compression of about 1 % did not reliably reduce crack front curvature. It appears that the procedure used is more important than the amount of local compression, as evidenced by the straight cracks obtained by two labs that exercised tight control on local compression. The statistical analysis revealed that the mean value of measured CTOD for all data with less than 10 % curvature was not statistically different than the mean for less than 20 %, as it is in BS7448 (1997). Perhaps the most surprising observation from this round robin is that there does not appear to be a correlation between CTOD measured at −196 °C and amount of local compression or crack front curvature.

Temperature Dependence and Variability of Fracture Toughness in the Transition Regime for A508 Grade 4N Pressure Vessel Steel

Abstract A508 Grade 4N is a high nickel (3.4%) pressure vessel steel with a relatively high yield strength (585 MPa minimum for Class 1) and a relatively low transition temperature (47 J transition temperature of −29°C or below). Published fracture toughness data on this steel are nonexistent, and so they have not been included in prior assessments of the applicability of the Master Curve approach to pressure vessel steels. The purpose of this study is to compare the temperature dependence and data scatter for this steel with the behavior predicted by the Master Curve. The available database consists of approximately 800 fracture toughness tests on 51 material heats, some of which were given a temper embrittling heat treatment. As an additional basis of comparison, test data on A508 Grades 2 and 3, 1% nickel weld metal, and several high nickel (1.3–3.8%) steels (NiCrMo and NiCrMoV) were included in the assessment. Maximum likelihood techniques were used to fit the toughness data, and Monte Carlo methods were used to assess the observed distribution of Weibull slopes (related to the extent of variability). The results of this assessment show that the Master Curve provides a good description of the shape (temperature dependence) of the toughness curve for moderate and high transition temperatures. However, for heats with low transition temperatures below the range observed in other pressure vessel steels, the shape of the toughness curve deviates from the Master Curve. The lower the transition temperature, the larger the deviation from the Master Curve. Based on the test data, a model describing the behavior of A508 Grade 4N has been developed. This model also accurately describes the toughness behavior of the other pressure vessel steels included in this study. With respect to variability, the Weibull slope of A508 Grade 4N is consistent with the value of four prescribed by the Master Curve. However, the Weibull slope for the other pressure vessel steels included in this assessment is close to 3.4. This lower value of the Weibull slope, which indicates larger variability, is likely the result of material inhomogeneity.

Variability of fracture toughness by the crack tip position in an annual ring of coniferous wood

The effects of the location of the crack tip in an annual ring and the direction of crack propagation on the fracture toughness of the TR crack propagation system of coniferous wood (T, direction normal to the notch plane; R, propagation direction) were analyzed by the finite element method in regard of the changes in elastic modulus and strength within an annual ring. The critical point of the fracture was defined as the state where the stress of a square element (0.125 × 0.125 mm) in contact with the crack tip equals the tensile strength. The distribution of specific gravity was measured by soft X-ray densitometry. The elastic moduli in the T and R directions were estimated by the sound velocity. The tensile strengths in the T and R directions were measured by the tensile test using small specimens of l mm span length. Regarding the variability of fracture toughness (K IC), the experimental and calculated results had the same tendency. Therefore, it was concluded that the variability ofK IC is caused by the (1) heterogeneity of the elastic modulus and strength within an annual ring; and (2) changes in the degree of stress concentration at the crack tips, according to the direction of crack propagation.

Role of microsegregation in fracture of cold-worked Zr-2.5Nb pressure tubes

Results from Zr-2.5Nb pressure tubes removed periodically from CANDU reactors for surveillance tests have revealed significant tube-to-tube variability in fracture toughness. If the microstructural factors responsible for such variations were known, the toughness of future pressure tubes could be improved and their service life extended. Evidence is presented which demonstrates that the variation in toughness for specimens fractured on the radial-axial plane is related to decohesion or fissuring on the transverse-axial plane as a result of microsegregation of Cl and C (as carbide). In the absence of such species and decohesion it is shown that very high toughness can be achieved.

On predicting the temperature and strain rate dependences of the fracture toughness of plain carbon steels

The variability of fracture toughness and its dependence on temperature and strain rate are described by applying a well-developed and accepted model to some recent data found in the literature. At low testing temperatures (⩽ − 60°C), the estimated fracture toughnesses agree well with the measured values when the strain rate increased from 0.0001/s to 1500/s. At room temperature, the calculated fracture toughnesses agree to within 10% or better with the measured values if work-hardening exponents higher than those used at lower temperatures are used in the model. The increase in fracture toughnesses at higher temperatures under higher loading rate conditions is also discussed.

Heat-to-heat variations in the fracture toughness of austenitic stainless steels

The effects of heat-to-heat variations on the fracture toughness behavior for Types 304 and 316 stainless steel (SS) were examined at 24, 427 and 538°C using the multiplespecimen J R- curve procedure. These alloys exhibited considerable variability in fracture toughness, with J c initiation toughness values ranging from 178 to 781 kJ/m 2 and tearing moduli from 272 to 676 at elevated temperatures. The high initiation toughness coupled with the very steep J R curves demonstrate that base metals are exceptionally resistant to unstable fracture. Minimum-expected fracture properties were defined from lower-bound J c and tearing modulus values generated here and in previous studies. Fractographic examination revealed that microvoid coalescence was the operative fracture mechanism. Variations in fracture toughness behavior were correlated with the depth of the microvoids.

Material Considerations in Structural Steel Design

Engineering structures, with very few exceptions, perform safely and reliably. The safety and reliability of these structures has been achieved primarily by adhering to the appropriate specifications that govern the material, design, fabrication and inspection for the particular structure. These specifications undergo periodic reviews and, when necessary, are changed to improve their accuracy and applicability. Deviations from the specifications requirements, misinterpretation of specifications, the use of highly restrained complex welded details and use of new and unproven concepts in design may impair the safety and reliability of a structure. The collapse of a structure or the fracture of a structural component may occur as a result of improper material properties, design, fabrication, inspection, erection and operating conditions. This paper focuses on material considerations and presents a brief discussion of some material factors that should be understood and considered in the fabrication and design of steel structures. The effects of deoxidation practice on segregation in steel ingots, and in the final product are discussed. Also discussed are the variability in the yield and tensile strength properties within a plate or a structural shape and the effects of temperature and rate of loading on these properties. Finally, the paper discusses the variability in fracture toughness and the effects of temperature and rate of loading on this material property. Charpy V-notch test results are used to describe the variability in fracture toughness and the critical stress-intensity factor is used to describe the effects of temperature and rate of loading on fracture toughness. Correlations between CVN test results and the critical stress intensity factor are also presented.