Stages Of Crack Formation Research Articles

Fundamental Drivers of Metal Fatigue Failure and Performance

Understanding fatigue behavior is critical for ensuring the safety and reliability of various engineering components and structures. Fatigue-related failures have historically led to catastrophic events, including aviation accidents and infrastructure collapses, resulting in significant economic and reputational damage. This study delves into the fatigue processes, focusing on crack initiation, growth mechanisms, the impact of microstructure, stress concentrations, surface finish, mean stress, frequency, and environmental conditions on fatigue life. It also explores fatigue principles, stress cycles, S-N curves, fatigue limits, and the stages of crack formation and propagation. By comprehensively analyzing fatigue behavior, engineers can design safer components, optimize materials, and extend product lifespans, thus facilitating technological advancements, predictive maintenance, and risk reduction across industries such as aerospace, automotive, and biomedical sectors.

Ordinary state-based peri‑ultrasound modeling for monitoring crack propagation in plate structures using sideband peak count-index technique

This work presents a nonlocal theory based on ordinary state-based (OSB) peridynamics for numerical modeling of crack propagation under loadings and peri‑ultrasound modeling for elastic wave propagation that interacts with generated cracks in 3-D plate structures. Propagating cracks in plate structures for two types of loading – tensile and shear are considered. For both loading cases, at six different times after the start of the loading the analysis is carried out to investigate how the results vary for these six different stages of crack propagation. An ultrasonic excitation is applied at a point on the surface of the plate to generate guided waves propagating through the plate structure and interacting with generated cracks at different stages of crack formation. The out-of-plane velocity fields are recorded at the receiving material point for further analysis. A relatively new and promising nonlinear ultrasonic (NLU) technique called sideband peak count-index (or SPC-I) is adopted to extract the nonlinear response due to the generated cracks. Results show that when the crack propagation is at its early stage the SPC-I value first increases, and then decreases with time at the advanced stages of crack propagation. This is because for both types of loading (tensile or shear) the newly generated cracks are small cracks (or micro-cracks) in the beginning and then they evolve into macro-cracks under increasing loadings. The observation for SPC-I trends is consistent with other numerical modelings and experimental observations reported in the literature, and such qualitative matching gives confidence in the proposed method. The proposed method combining the peri‑ultrasound modeling and SPC-I technique presented in this paper can provide a new insight for monitoring crack propagation and give some guidance for experimental structural health monitoring (SHM) applications.

Determiantion of energy characteristics of material destruction in the crushing chamber of the vibration crusher

The crushing equipment is characterized by a significant energy-consuming system during the crushing workflow. The current trend in the development of such processes puts forward requirements for the development of new or improvement of existing energy-saving equipment. The essence of the solution to the problem in this work is determined by using resonant modes, which are inherently the most effective. The practical implementation of the resonance mode has been achieved taking into account the conditions for the interaction of the resonant vibration crusher with the material at the stages of its destruction. The degree of the stress-strain state of the material is taken into account, which was a prerequisite for identifying the potential for the development of a vibration load. Composed equations of motion based on a substantiated discrete-continuous model of a vibration crusher and processing material. An approach is applied to determine the stepwise destruction of the material with the determination of the required degree of energy. This methodological approach made it possible to reveal the nature of the process of material destruction, where energy costs at the stages of crack formation, their development and final destruction are taken into account. It was revealed that the greatest energy consumption during the operation of crushers goes into the kinetic energy of the crushing plates and the potential energy of deformation of the springs. The proposed model is common for any design of a vibration machine and its operating modes. The stable resonance mode has made it possible to significantly reduce the energy consumption for the course of the technological process of material grinding. The results obtained are used to improve the calculation methods for vibratory jaw and cone crushers that implement the corresponding energy-saving stable zones of the working process.

Anisotropic crack propagation and self-healing mechanism of freestanding black phosphorus nanosheets

In this study, an indentation simulation is employed to study the anisotropic crack propagation and re-forming mechanism of freestanding black phosphorus (FBP) nanosheets by molecular dynamics simulation. The results indicate that the size of the FBP nanosheet decides the crack direction as well as the von Mises stress concentration. It is found that crack directions are not influenced by temperature. With increasing specimen size, the crack propagation rate is nearly the same as at the first stage of crack formation, while in the later stage, cracking develops very quickly in larger specimens. Especially, small FBP nanosheets almost re-form in a short time at ambient temperature. However, after being destroyed, the larger specimen has no possibility of recovery. Besides, when increasing the number of layers of FBP, the energy stored by the top layer and the system undergoing deformation increases. In addition, the specimen with two fixed edges is less stable, leading to increased stress and decreased Young’s modulus compared with the specimen with four fixed edges.

A Proposed Mechanism of Hot-Cracking Formation During Welding Fan-Shaped Test Specimen Using Pulsed-Current Gas Tungsten Arc Welding Process

Solidification cracking is a significant problem during the welding of fully austenitic stainless steels. The present work is considered as the first trial to investigate and propose a mechanism of hot cracking formation when welding the Fan-shaped cracking test specimen, using the pulsed current gas tungsten arc welding process (PCGTAW). The specimen lateral expansions perpendicular to welding line due to thermal effects, plus the transverse expansion due to crack opening are sensed and recorded to detect the crack behavior with time. The stages of crack formation are filmed by a high-speed photography of the weld pool and solidification process at a speed of about 1000 fps. Additionally, some microscopic examinations using Scanning Electron Microscope (SEM) and Electron Probe Micro-Analyzer (EPMA) are performed on the welds. The results helped in establishing a proposed mechanism for the formation of hot cracks in full–austenitic stainless steel welds done on a Fan-shaped test specimen. The proposed mechanism suggests three stages during hot cracking formation; the crack initiation, propagation, and ceasing. The occurrence of a hot crack during welding mainly depends on the way by which the molten zone solidifies, and which solid phase will primarily solidify. This affects, in turn, the segregation of the chemical elements, which found to have a great role in crack initiation. Moreover, the weld metal structure type, together with the thermal stresses in conjunction with the applied strains on the weld joint play a great role in the crack expansion and ceasing. The present work is considered the first trial done to propose a mechanism of hot-cracking formation during welding the Fan-Shaped test specimen using Pulsed-Current Gas Tungsten Arc welding process.

Computational model of spalling and effective fibers on toughening in fiber reinforced composites at an early stage of crack formation

This work suggests a computational model that takes account of effective fibers on toughening in FRC at an early stage of crack formation. We derived the distribution of pressure provoked by a random inclined fiber in the matrix and calculated stresses through integrating the pressure and tangent stress along the fiber/matrix interface with the Kelvin's fundamental solution and the Mindlin's complementary solution. The evolution of spalling in the matrix was traced. The percentages of effective fibers were evaluated with variations in strength, interface resistance, diameter and elasticity modulus. The main conclusion is that low elasticity modulus combined high strength of fibers raises dramatically the effective fibers, which would benefit toughening.

Laws of fatigue fracture of metallic materials

A kinetic criterion of fatigue fracture of metallic materials is suggested and used for showing the existence of a single fatigue curve for cases of soft (with specified load amplitude) and rigid (with specified strain range in a cycle) loading for the stages of crack formation and propagation. The criterion makes it possible to compute the endurance of elements of a structure and levels of accumulated damage in terms of the ultimate accumulated damage for any loading stage including programmed loading (combination of soft and rigid loading, double-frequency and random loading, and loading with temporary holds under load). The criterion is substantiated by numerous experimental data for various metallic structural materials.

Fluid-inclusion studies on lower crustal gabbroic xenoliths from the Mt.Melbourne Volcanic field (Antarctica): Evidence for the post-crystallization uplift history during Cenozoic Ross Sea Rifting

Fluid inclusions in gabbroic xenoliths from Cenozoic alkaline centres of the Mt.Melbourne Volcanic Field at the margin of the Ross Sea Rift System have been studied in order to derive fluid compositions and conditions of entrapment. Most of the inclusions are secondary, and only few are considered primary with respect to igneous crystallization. Textures indicate that the former inclusions formed from complete necking-down of fluid-filled, healed cracks. Most inclusions studied, primary or secondary, contain almost pure CO 2 , as has been tested by heating/freezing experiments. Other fluid components (N 2 , hydrocarbons) have been shown by Raman spectroscopy to be present in some inclusions. Histograms of homogenization temperatures for large (up to 50 mm) secondary inclusions in plagioclase and pyroxene have maxima between ca. 16° and 26°, 12° and 18°. Inclusions in olivine were measured in one rock only and gave lower values from 0° to 10°. We interpret these results as fluid entrapment and crack healing at different P-T-conditions along the steep geotherm previously documented for the lower crust in the Ross Rift region (Berg et al., 1989), which followed metamorphic equilibration of these rocks at conditions of 900-1000°C and 3-5 kbar (Hornig et al., 1992). Our fluid inclusion data then indicate an uplift path for the lower crust from the early high-T conditions to a later stage of crack formation, fluid migration, and fluid entrapment around 300-400°C, estimated from the maximum temperature for brittle behaviour of plagioclase and the 1-2 kbar derived from fluid inclusion measurements. With the limitation of the method in mind, and considering that the pressure estimate is a minimum value due to additional fluid components, this decompression and cooling is interpreted to be related to uplift and cooling of the lower crust in the Mt.Melbourne area during the formation of the Ross Sea Rift and uplift of the Transantarctic Mountains

Fatigue Life Estimates for Bluntly Notched Members

Methods for estimating the total fatigue lives of bluntly notched members are presented. Analytical estimates are compared to experimental data generated in the Society of Automotive Engineers Cumulative Fatigue Damage Test Program. The crack initiation lives were predicted using strain life concepts and fracture mechanics methods were employed to predict the fatigue crack growth stages. The methods used highlighted the proportions of the total fatigue life spent in crack initiation and propagation. These were found to depend on load amplitude and level of mean load. For the bluntly notched member investigated the initiation and early stages of crack formation formed the major part of the total life, when the loading was predominantly tensile. Conversely for a loading history where most of the cycles were compressive the majority of the fatigue life was expended in crack growth away from the notch.

The origin of embryonic laser cracks

The problem of the initial stage of crack formation in PMMA subjected to laser action is investigated. It is shown that the cracks develop from gas-filled cavities (centers). These centers have been experimentally investigated, and the center-crack transition carefully studied.

Micro and Macro Crack Formation

The formation of cleavage microcracks with a length of the order of one grain diameter is considered to be the initial step in fracture. It is assumed that the stress concentration required for cleavage is supplied by thick slip or twin bands, and the critical width of these yield bands is calculated. For example, in iron with a grain radius of 10−2 cm, the critical slip band width is 2 × 10−5 cm, and this value is compatible with observations in the vicinity of microcracks. The second stage of crack formation involves the semi-continuous propagation of microcracks to form unstable macroscopic cracks. We postulate that plane strain fractures occur under conditions where thick slip bands are formed in the yielded region in front of an advancing crack. Work required to extend the initial crack is used to calculate the crack extension force, GIc, which is required in linear fracture mechanics. In the case of iron, the microcrack extension force, Y, is calculated to be 5 × 103 dynes/cm, and the minimum value of GIc is calculated to be 2.5 × 106 dynes/cm. This approach emphasizes the three conditions required for fracture: 1) a combination of stress and yield band width sufficient to cause local cleavage; 2) sufficient mechanical energy in the system to propagate the crack; 3) the development of a critical value of the initiation stress in order to continue crack extension. These concepts are used to estimate the plane strain transition and the nominal stress for fracture, i.e. plate fracture stress, σn, is estimated from an elastic-plastic stress analysis to be σc/4, where σc is the yield stress at the tensile transition temperature. The tensile transition is chosen as the point at which the yield and fracture stress are about equal, and the plate transition temperature corresponds to the temperature, TC, at which the yield stress has a value of σc/4. We also estimate that crack arrest in steel plates corresponds to an energy absorption, GC = 22.5 × 103 × t/d, where GC is the crack extension force at the transition between plane strain and plane stress (dynes/cm), t is the plate thickness (cm), and d is the grain radius (cm). A reasonably good correlation for GIc and GC calculated and tested, is obtained. We also use tensile transition data to estimate a plate transition temperature and a critical tensile stress for crack propagation. These are combined with a suggested minimum value of the crack extension force, GC (arrest), to provide the basis for a fracture-safe design criterion.

Thermal Effects in Contact Fatigue Under Oscillatory Normal Load

Experiments are reported wherein contact fatigue was brought about by the application of an oscillatory normal load between a ball and a flat specimen. Plots of the flat-specimen temperature versus time showed that a rapid temperature rise occurred in the initial stage of crack formation, and thus provided an early indication of fatigue. Thermal resistances were measured for the apparatus components as well as the specimen contact. Using these, it was possible to apply the measured flat-specimen temperature to obtain estimates of the contact temperature as well as the energy dissipation rate prior to the incidence of fatigue cracks. It was shown that the contact temperature did not rise sufficiently to produce annealing in the test specimens. Thus, toroidal rings of hardened and softened material in the stressed zone could not be attributed to thermal transformation of the bearing steel. It was also shown that energy dissipation due to cyclic loading varied in approximate exponential relationship with dynamic load, and decreased upon increase of static load when dynamic load was maintained constant.