Theoretical and Applied Fracture Mechanics最新文献

The stress ratio effect on fatigue crack propagation threshold has been a subject of extensive investigation for decades in engineering materials. In the work, a modified mean stress correction model is proposed to predict long crack propagation threshold at high stress ratios as an extension of our previous work (Engineering Fracture Mechanics 2020, 223:106787). Moreover, the mean stress modified K-T diagram is derived based on the proposed model. Good accuracy can be achieved in terms of the experimental long fatigue crack propagation thresholds and K-T diagrams.

The flaw tips in brittle rocks are often the sources of crack initiation and growth due to the stress concentration, which commonly governs the rock strength. However, a unified framework identifying the compression-induced crack types, ultimate failure patterns and the cracking levels of brittle flawed rocks under different mechanical confinements is not yet available. This study conducts the laboratory compression experiments with the AE monitoring to explore the failure characteristics of flawed limestone and its confinement-dependency. Four new crack types including loop crack, secondary transverse crack, near-field transverse crack and far-field transverse crack are found experimentally, and then a modified crack type classification strategy is proposed. Four failure patterns including the σ₁-axisymmetric flaw-disturbed spalling for uniaxial compression, the σ₃-transverse-symmetric flaw-disturbed spalling for biaxial compression, the σ₁ −axisymmetric flaw-disturbed shearing for conventional triaxial compression, and the mixed σ₃-transverse-symmetric flaw-disturbed shearing and σ₂-transverse-symmetric flaw-disturbed spalling for true triaxial compression, are documented for the first time. Moreover, an acousto-mechanics-based classification methodology of rock cracking levels is established, as well as an AF (average frequency)-RA (rising angle)-based Kernel density estimation method for interpreting the rock cracking nature and the strength mechanism. This paper gets insights into the mechanical confinement-dependency of the rock failure characteristics incorporating the pre-existing flaws and help interpret the field observations.

In this paper, a new strain-based criterion is suggested for assessing the effects of size and geometry of specimen on the fracture resistance of rocks under mixed-mode (I/II) loading. The new approach named the modified maximum tangential strain (MMTSN) criterion is based on the classical maximum tangential strain (MTSN) criterion, in which the first non-singular term ($T$) of Williams series expansion is considered in addition to the singular terms ($K$). Furthermore, to provide more coherence, the critical distance ($r_c$) from the crack tip is defined according to a new strain-based failure model. Unlike similar strain-based fracture models available in the literature, the critical distance $r_c$ in the MMTSN criterion is assumed to be size-dependent and a semi-empirical formulation is utilized for describing this size-dependency. To assess the ability of MMTSN for considering the size and geometry effects, the experimental data existing in the literature for a number of cracked Brazilian disk (CBD) and semi-circular bend (SCB) specimens manufactured from Guiting limestone are taken into account. It is demonstrated that the MMTSN criterion can predict the experimental data very well by taking into consideration the size and geometry effects without needing to calculate the other higher order terms.

Co-Cr-Mo alloy is crucial for biomedical implants and aerospace components. These parts often exhibit a high level of geometric intricacy. Direct metal laser sintering (DMLS) is ideal for these complex parts. In DMLS, choosing the right scanning strategies is vital, as it significantly affects the fatigue fracture behavior of the printed components. Thus, the present study investigates the effect of different scanning strategies (stripe, meander, and chessboard) on the fracture toughness and fatigue crack growth behavior of DMLS printed Co-Cr-Mo alloy. For each scanning strategy, fatigue crack growth tests have been performed to evaluate the threshold stress intensity factor and Paris law constants. To corroborate the obtained experimental results, microstructure analyses have been performed using electron backscattered diffraction. Further, failure mechanisms have been identified from fractographs obtained using field emission scanning electron microscopy. It is evident from the obtained test results that scanning strategies caused significant variation in fracture toughness and fatigue crack growth behavior. The stripe scanning strategy has exhibited higher resistance to fracture and fatigue crack growth. However, delayed crack initiation has been observed in the case of the chessboard scanning strategy. The present study provide the background for better selection of scanning strategies to mitigate fatigue fracture in DMLS-printed Co-Cr-Mo alloy designed for specific applications.

The Sleipner steel (0.9C-7.8Cr sub-ledeburitic tool steel) is a widely utilized tool steel currently being adopted to produce tools used in fine blanking, shearing, forming, coining, deep drawing, and others. In these branches, tailoring the final mechanical properties, such as hardness and toughness, to specific application is highly appreciated. The Sleipner steel was subjected to sub-zero treatments (at –140 °C for 17 h and 36 h) in the current work. The resulting microstructures, hardness variations, and changes in fracture toughness were analyzed and discussed. It was observed that sub-zero treatments reduced the retained austenite amounts by 14–15 % and slightly refined the martensite. However, the impact of this treatment on carbide count was marginal. The hardness of the sub-zero treated steel increased when tempered at temperatures up to 400 °C, but it decreased after tempering at 520 °C compared to cryogenically treated specimens.

Sub-zero treatment reduced the fracture toughness in the steel tempered up to a temperature of 400 °C, but an increment in this property was found after 520 °C tempering. Nevertheless, the obtained results indicate that it is impossible to simultaneously enhance both the hardness and fracture toughness of this particular steel grade. Therefore, it is necessary to carefully choose the principal goal of the treatment (either hardness or toughness) even before subjecting the tools to the heat/sub-zero treatment.

In flexoelectric materials, strain gradients can induce electrical polarization. However, internal defects such as cracks profoundly affect the electromechanical coupling properties of flexoelectric solids. In particular, anti-plane cracks involve less physical fields, which are easier to study. In this study, we present a comprehensive and innovative investigation of the anti-plane crack problems in flexoelectric materials, including semi-infinite and finite-length anti-plane cracks. For the first time, we formulate a full-field solution for semi-infinite anti-plane cracks in flexoelectric media by applying the Wiener–Hopf technique. Furthermore, the collocation method and the Chebyshev polynomial expansion are used for the first time to derive the full-field hypersingular integral equation solution for finite-length anti-plane cracks in flexoelectric solids. In addition, a comparative analysis between the full-field and asymptotic solutions for semi-infinite cracks is performed, shedding light on the discrepancies in the representation of the electromechanical coupling behavior near the crack tip. The mixed finite element method is used to compare with the full-field solutions of finite-length cracks. The agreement between the numerical results and the full-field solutions demonstrates the rigor of our study. This research advances the knowledge of defects in flexoelectricity and provides significant insight into relevant failure mechanisms.

Martensitic stainless steels (MSS) are known for their high mechanical strength and moderate corrosion resistance across various environments. However, their martensitic structure imposes limitations on fracture toughness. By employing heat treatments like quenching and partitioning, it becomes feasible to augment the presence of residual austenite in the material. This microstructural change enables the material to better absorb energy during fracture, thereby increasing its fracture toughness. The development of high-strength steels with good fracture toughness could influence the project and design of structural components, potentially resulting in reduced structural thickness. Experimental determination of crack growth resistance curves and fracture toughness for thin high-strength steels is challenging because most standardized methodologies were developed for thicker samples. The American Society for Testing and Materials has published a standard test method for the determination of resistance to stable crack extension under low-constraint conditions (ASTM E2472) in terms of critical crack-tip-opening angle (CTOA, ψ_c) and/or critical opening displacement at the original crack tip (δ₅), involving the use of thin compact tension C(T) and middle-tension M(T) specimens. However, this method requires specific instrumentation, relatively large specimens, and additional experimental devices such as anti-buckling guides. In this context, this paper evaluates the applicability of the elastic unloading compliance technique for determining crack growth resistance curves in terms of J-integral of MSS using relatively small, non-standard thin clamped SENT specimens with thickness of 1 mm. The proposed methodology, based on a combination of BS 8571 standard and the compliance, stress intensity factor, and η_pl factor solutions from the literature, has proven to be suitable for evaluating toughness in thin clamped SENT specimens and could be useful for assessing fracture toughness in high-strength steels of small thickness.