Cracks In Materials Research Articles

Structural Features of Fatigue Crack Propagation of a Forging Die Made of Chromium-Molybdenum-Vanadium Tool Steel on Its Durability.

The paper presents the results of tests on a die insert made of non-standardised chrome-molybdenum-vanadium tool steel used during pre-forging, the life of which was 6000 forgings, while the average life for such tools is 8000 forgings. It was withdrawn from production due to intensive wear and premature breakage. In order to determine the causes of increased tool wear, a comprehensive analysis was carried out, including 3D scanning of the working surface; numerical simulations, with particular emphasis on cracking (according to the C-L criterion); and fractographic and microstructural tests. The results of numerical modelling in conjunction with the obtained results of structural tests allowed us to determine the causes of cracks in the working area of the die, which were caused by high cyclical thermal and mechanical loads and abrasive wear due to intensive flow of the forging material. It was found that the resulting fracture initiated as a multi-centric fatigue fracture continued to develop as a multifaceted brittle fracture with numerous secondary faults. Microscopic examinations allowed us to evaluate the wear mechanisms of the insert, which included plastic deformation and abrasive wear, as well as thermo-mechanical fatigue. As part of the work carried out, directions for further research were also proposed to improve the durability of the tested tool. In addition, the observed high tendency to cracking of the tool material used, based on impact tests and determination of the K1C fracture toughness factor, led to the proposal of an alternative material characterised by higher impact strength.

Metode Peningkatan Ketahanan Retak Resin Polyester Terhadap Penambahan Serat Sekam Padi

Polymer materials have been widely developed as alternative materials to replace metal materials due to various advantages such as having low density and an easy manufacturing process. Still, polymers have many disadvantages, including not having high mechanical strength, easy to crack when hit by impact. One of the materials studied is an unsaturated polyester polymer widely used for composite matrices in vehicle structural components, aircraft and ship bodies, and vehicle components. One of the studied fibers is rice husk particle fiber. The study of cracks is essential because it causes the material to no longer be able to support the load according to the previous plan and causes failure to occur more quickly. One way to overcome material failure due to cracks in the composite material is to prevent crack propagation by adding reinforcing material. In this study, a composite material was made using rice husk fiber to increase the crack resistance of the polyester composite matrix. From the results of crack testing, there is a tendency to increase the percentage of rice husks by 5%, 10%, 15%, and 20%. The value expected to be obtained for determining the crack resistance of the material is the value of the stress intensity factor (K1). The most significant K1 value was obtained at a 15% rice husk percentage variable of 1,558 MPa.m0.5. This price could increase the value of the pure polyester stress intensity factor K1 by 0.667 MPa.m0.5, indicating an increase of 233.58%.

Systematical calibration and validation of discrete element models for fiber reinforced cement treated aggregates

Cracking of pavement materials affects the service performance of pavement structures. It is widely investigated by discrete element method (DEM) because the DEM can present the cracking behavior and mechanism from the mecroscopic perspective. However, improper calibration method is time-consuming and the simulation results are not applicable for different loading conditions. To address this issue, a systematical calibration method is proposed to establish a more reasonable DEM model, and it is used to explore the cracking behavior and mechanism for pavement materials. The fiber reinforced cement treated aggregates (F-CTA) used to construct the pavement base are selected as an example to elaborate the process. First, contacts within the F-CTA were divided into five groups according to physical components. Then, laboratory tests were performed to obtain macro mechanical properties including strength and modulus. Next, the corresponding virtual tests were established and the contact model parameters were calibrated from mono-component to mixture. This calibration method was validated by the three-point bending test incorporating all contact parameters obtained from the above calibrations and the process of cracking evolution was analyzed. Results show that the simulated mechanical responses and crack distribution based on the systematical calibration are in good agreement with laboratory measurements and observations, exhibiting well reliability for calibrating contact parameters of composite material. Young's modulus of the F-CTA is sensitive to the mecro effective modulus in the virtual indirect tensile test while Young's modulus and tensile strength are vulnerable to the mecro cohesion in the virtual uniaxial compression test. The mortar-aggregate and mortar-aggregate interface is relatively weak where cracks are prone to appear. The fibers can optimize the distribution of the crack network. The cracking evolution contains three stages and the second stage has a significant influence on the cracking behavior of the F-CTA.

Numerical simulation of mesoscopic cracking of cement-treated base material based on random polygon aggregate model

Numerical simulation of mesoscopic cracking of cement-treated base material based on random polygon aggregate model

A comparison index in mortar repair treatments by microbiologically induced carbonate precipitation and its evaluation by a non-destructive technique

Cracks in cement-based materials are a common issue to be solved in construction and civil engineering. Although in the last 20 years microbiologically induced carbonate precipitation (MICP) has been investigated successfully to recover and improve the physical properties of cracked concrete, the assessment of the effectiveness of treatments is a very important challenge to solve. In this article, a non-destructive methodology is reported to monitor the progress of crack sealing and calcium carbonate (calcite) deposition during MICP. An ureolytic bacteria was applied to treat mortar specimens for 30 wk, in which the effectiveness of calcite precipitation was evaluated. For this, two-dimensional X-ray tomography was used to characterize the calcite filling progress, showing that the MICP treatment had a higher CaCO3 filling effect when compared to control. It was estimated that MICP activity allowed to fill mortar holes with CaCO3 by 18–27%, after 8 wk. As a comparison indicator during repair treatments of cement-based materials using microorganisms that perform MICP, a Relative Treatment Effect (RTE) index was proposed. It is concluded that X-ray tomography can be used as an effective non-destructive technique to track the progress of MICP during calcite deposition and crack repair. HIGHLIGHTS An experimental study was carried out to monitor progression of mortar healing by ureolytic bacteria by non-destructive methods. X-ray tomography technology was feasible to monitor mortar healing in real time. Relative effect (RTE) is proposed as a comparison index in the repair treatments of cement-based materials by Microbiologically Induced Carbonate Precipitation (MICP).

An inversion scheme for sizing crack from signals of the motion-induced eddy current testing method based on a new formula of signal gradient of ferromagnetic materials

In this paper, an inversion scheme for the profile reconstruction of a fatigue crack in rails from the motion-induced eddy current testing (MIECT) signals is proposed based on the conjugate gradient (CG) optimization method and the fast MIECT forward simulator developed by authors. To calculate the gradient of MIECT signals with respect to the crack parameters, a simplified analytical gradient formula is deduced, which enables a high efficiency crack reconstruction from MIECT signals for a crack in ferromagnetic material. The validity of the inversion schemes is demonstrated at first by reconstructing cracks with numerical simulated signals for ferromagnetic materials with an oblique elliptic crack model. The good reconstruction results reveal that the new signal grandient formula is efficient for reconstructing crack in ferromagnetic material with the CG algorithm. Finally, sizing of artificial cracks are carried out with use of measured MIECT signals due to cracks in plates of both non-magnetic and ferromagnetic materials. The reconstructed crack parameters are in good agreement with the true values, which experimentally proved the validity and efficiency of the proposed inversion scheme and the new signal gradient formula for MIECT inversion.

Capacity Degradation and Aging Mechanisms Evolution of Lithium-Ion Batteries under Different Operation Conditions

Since lithium-ion batteries are rarely utilized in their full state-of-charge (SOC) range (0–100%); therefore, in practice, understanding the performance degradation with different SOC swing ranges is critical for optimizing battery usage. We modeled battery aging under different depths of discharge (DODs), SOC swing ranges and temperatures by coupling four aging mechanisms, including the solid–electrolyte interface (SEI) layer growth, lithium (li) plating, particle cracking, and loss of active material (LAM) with a P2D model. Additionally, the mechanisms causing accelerated capacity to drop near a battery’s end of life (EOL) were investigated systematically. The results indicated that when the battery operated with a high SOC range, the capacity was more prone to accelerated degradation near the EOL. Among the four degradation mechanisms, li plating was mainly sensitive to the operation temperature and SOC swing ranges, while the SEI growth was mainly sensitive to temperature. Furthermore, there was an inhibitory interaction between li plating and SEI growth, as well as positive feedback between LAM and particle cracking during battery aging. Additionally, we discovered that the extremely low local porosity around the anode separator could cause the ‘knee point’ of capacity degradation.

Microstructural Analysis of Thermally Treated Geopolymer Incorporated with Neodymium.

The following investigation presents the thermal treatment of geopolymer at 300 °C, 600 °C and 900 °C. We investigated what happens to the geopolymer base when incorporated with 1% and 5% of neodymium in the form Nd2O3. A total of six samples were synthesized. Geopolymer 1 contained 1% and geopolymer 2 contained 5% Nd2O3, and these samples were treated at 300 °C; then, samples geopolymer 3 and geopolymer 4 also had the same percentage composition of Nd2O3 and were treated at 600 °C, while samples geopolymer 5 and geopolymer 6were treated at 900 °C. Physical and chemical changes in the aluminosilicate geopolymer matrix were monitored. The incorporation of rare earths into the polymer network of aluminosilicates has been proven to disrupt the basic structure of geopolymers; however, with increased temperatures, these materials show even more unusual properties. Diffuse reflectance infrared Fourier transform (DRIFT) analysis showed that the intensity of the vibrational band decreases with the increase in temperature during thermal treatment, suggesting alterations in the chemical structure of the geopolymers. Transmission electron microscopy (TEM) analysis showed that the diameter of the nanoparticles containing Al2O3 is in the range 5-10 nm, while larger crystallites range from 30 to 80 nm. Scanning electron microscopy (SEM) analysis revealed that the temperature of the thermal treatment increases to 300 °C and 600 °C; the porosity of geopolymer increases in the form of the appearance of large pores and cracks in material. X-ray photoelectron spectroscopy (XPS) analysis was used to investigate the surface chemistry of geopolymers, including the chemical composition of the surface, the oxidation state of the elements, and the presence of functional groups. The UV/Vis spectra of the synthesized geopolymers doped with Nd3+ show interesting optical properties at 900 °C; the geopolymer matrix completely disintegrates and an amorphous phase with a rare-earth precipitate appears.

The strain gradient viscoelasticity full field solution of mode-III crack problem

The size and viscosity effects are noticeable at the micro-/nano scale. In the present work, the strain gradient viscoelastic solution of the mode-III crack in an infinite quasi-brittle advanced material is proposed based on the strain gradient viscoelasticity theory using the Wiener–Hopf method. The solutions to the gradient-dependent viscoelastic crack problem are obtained directly by using the correspondence principle between the strain gradient viscoelasticity and strain gradient elasticity in Maxwell’s standard linear solid model. In this model, the stress near the crack tip is time-dependent and size-dependent. Besides, the stress near the crack tip is more significant than that based on gradient elasticity theory. Compared with the elastic strain gradient effect, the viscous gradient effect makes the stress field at the crack tip harden. The location and the value of maximum stress change with time, which differs from the case in strain gradient elasticity theory. The time that the normalized stress takes to stabilize also changes with the distance from the crack tip. When the viscosity effect is neglected or time tends to infinity, the strain gradient viscoelasticity theory can be reduced to the classical strain gradient elasticity theory.

Effect of surface machining on the environmentally-assisted cracking of Alloy 182 and 316L stainless steel in light water reactor environments: results of the collaborative project MEACTOS

Abstract The main objective of the EU-funded project mitigating environmentally-assisted cracking through optimisation of surface condition (MEACTOS) was to gain knowledge on the ability of different surface machining procedures to mitigate environmentally-assisted cracking (EAC) in typical light water reactor structural materials and environments. Surfaces of cold-worked (CW) type 316L austenitic stainless steel and nickel-based weld metal Alloy 182 flat tapered tensile specimens were machined using different processes. EAC initiation susceptibility of these specimens was evaluated using constant extension rate tensile (CERT) tests under simulated boiling water reactor (BWR) and pressurized water reactor (PWR) conditions and assessed using constant load experiments. More than a hundred tests were performed covering about 10 years of autoclave testing time. Only minor or no measurable improvements in EAC initiation susceptibility as a function of surface treatments (grinding or advanced machining) compared to the standard industrial face milling were demonstrated. In most cases, the stress thresholds for EAC initiation determined in constant load tests confirmed the trend obtained from CERT tests. This paper summarises the most important results and conclusions concerning the EAC initiation behaviour for the CW 316L and Alloy 182 under reducing PWR and oxidizing BWR conditions.

Suppressing the formation of OP4 phase in P2-Structured Na0.67Ni0.1Co0.1Mn0.8O2 by in-situ formed NiF2 layer

The P2-structured sodium transition metal oxides suffer from an irreversible phase transition to OP4, which results in the -1.3 Å shrinking of lattice c and locks the following electrochemical cycles between OP4 and P2’, seriously aggravating the cracking of cathode materials. Therefore, strategies to suppress the phase transition from P2 to OP4 are currently pursued to alleviate the destructively volumetric changes. Here, the DFT calculations reveal that the transition to OP4 phase is not driven by thermodynamical mechanism but is induced by structural defects. To verify this prediction, an in-situ formed NiF2 layer via electrolyte additives is coated on the P2-structured Na0.67Ni0.1Co0.1Mn0.8O2 during initial charging, which mitigates the generation of structural defects from the side reaction of electrolyte with active sites of cathode particles. The sample protected with NiF2 layer presents a continuous, moderate and reversible structural evolution, free from phase P2’. Therefore, the coated sample delivers 230.1 mAh/g at 0.1C and retains 83.4% capacity at 0.5C after 150 cycles, significantly higher than the values, 174.5 mAh/g and 63.4%, respectively, of the pristine materials. This work demonstrates the severe phase transition in P2-type cathode can be effectively alleviated by a simple surface modification.

A hybrid direct FE2 method for modeling of multiscale materials and structures with strain localization

Strain localization is a common phenomenon existing in various multiscale materials or structures, e.g., the bulking bands of thin-walled structures, the local collapse of porous materials, and the crack in solid materials, etc. However, this phenomenon cannot be captured by the conventional homogenization methods, such as mean-field homogenization (e.g., Mori–Tanaka method), first-order or high-order computational homogenization (e.g., the Finite Element Square method), etc., due to the high strain gradient associated with strain localization. Aiming at this, a hybrid Direct FE2 method is proposed by combining the D-FE2 (Direct FE2) and the traditional FE methods, while the multiscale structure is modeled using the D-FE2 method in the region exhibiting low deformation gradient, and the other region displaying high deformation gradient is modeled using the traditional FE method. Moreover, a node displacement constraint and an overall node displacement constraint derived from the multilevel equilibrium equations using the Gauss–Ostrogradsky theorem are respectively prescribed to the interface between the D-FE2 model and the FE model of the multiscale structure, to enforce the energy equilibrium and deformation continuity. The proposed hybrid D-FE2 method is then applied to predict the strain localization behavior of multiscale materials or structures, including local bulking of honeycomb structures, in-situ crack propagation, and localized plastic deformation in fiber reinforced composites, etc. Comparison of the simulation results obtained from the hybrid D-FE2 method and the traditional FE method validates the accuracy, efficiency and ease of numerical implementation of the proposed hybrid D-FE2 method.

PD-FEM chemo-thermo-mechanical coupled model for simulation of early-age cracks in cement-based materials

The hydration process during the hardening of early-age cement-based materials can lead to a considerable volume change, consequently causing considerable deformation, and even fracturing. In the present study, an efficient PD-FEM chemo-thermo-mechanical coupling approach is proposed with the aim of modeling fracturing in early-age concrete structures, taking into account coupled processes, such as thermal transfer, hydration heat, creep, evolution of strength, fracturing as well as hindering effect of cracks on the temperature and hydration fields. The coupled chemo-thermo-mechanical problem is solved in a staggered manner within the classical finite element (FE) and the non-ordinary state-based peridynamics (NOSBPD) framework. By doing so, fracturing in early-age concrete structures can be easily accommodated by NOSBPD featuring an elegant treatment of discontinuities. The accuracy of the proposed approach is evaluated by a benchmark problem with experimental and existing numerical solutions. The capability of the proposed approach for simulating the complex crack initiation and propagation in early-age ring and T-shaped concrete structures is demonstrated. The interaction and competition between different stress inducing mechanisms of hydration heat, autogenous shrinkage, and creep are analyzed.

Повышение качества магнитов из сплава ЮНДКТ5БА на этапе литья их заготовок

Metallographic analysis of the magnet material made from the Fe-Al-Ni-Co made it possible to determine that melt excessive overheating before casting reduced the product target characteristics due to formation of various internal defects. Inaccessibility was shown of high microchemical inhomogeneity and internal stresses that led to formation of cracks in the casting material and destruction of the columnar grains deteriorating the finished magnets performance.

Dynamically propagating cracks in anisotropic plates subjected to hyperbolic thermal shock

ABSTRACT In this paper, dynamically propagating cracks in anisotropic plates exposed to a generalised thermal shock are investigated. Here, the governing equations are considered according to the Lord-Shulman (LS) model as a fully coupled generalised thermoelasticity theory. The crack is modelled in the context of the eXtended Finite Element Method (XFEM). To evaluate Stress Intensity Factors (SIFs), a technique based on the J integral and crack tip opening and sliding displacements, previously applied for stationary cracks, is developed for a dynamically propagating crack in orthotropic solids. Besides, a variety of functions for enriching the displacement field are derived relying on the behaviour near the tip of a dynamically propagating crack in anisotropic materials. The maximum hoop stress criterion is also modified by using the asymptotic stress field for a dynamically propagating crack. In addition, the effect of the angular variation of fracture toughness in orthotropic materials is accounted in computing the crack propagation speed. In some examples, dynamic crack growth in complex cracked orthotropic structures subjected to LS thermal shock is studied in detail. The impacts of fibre orientations and LS relaxation time are also investigated in these examples. In conclusion, applying non-Fourier thermal shock for modelling dynamically propagating cracks in orthotropic structures is recommended to obtain more realistic results. In addition, by increasing the LS relaxation time, the crack grows less oscillatory and is more aligned along the material fibres.

Initiation and propagation of a single internal 3D crack in brittle material under dynamic loads

Different defects are widely distributed inside natural rock masses, and the development of these defects poses a potentially severe threat to the entire structural stability and safety. Most studies at present are concentrated on the propagation of two-dimensional (2D) penetrated cracks under static loads. In this paper, a series of tests were carried out on cubic glass specimens containing a closed pre-crack using a split Hopkinson pressure bar (SHPB) system, aiming to investigate the internal three-dimensional (3D) crack growth under dynamic compression. The results indicate that the dynamic compressive strength of the cubic specimen is influenced by the inclination direction of the 3D pre-crack. Theoretical analysis of the circumferential stress field at the pre-crack tip reveals the mechanism behind dominant crack initiation and favorable direction, which are also affected by non-singular T stress along and perpendicular to the pre-crack plane, as well as friction between crack planes. Through the post-mortem examination, the mode III fracture characteristics, crack bifurcation and Wallner lines are observed in fracture morphology. The terminal length of wrapped wing cracks subjected to dynamic loads is sensitive to pre-crack inclination angle, while it is approximately 1–1.5 times the pre-crack radius under static compression.

XFEM for semipermeable crack in piezoelectric material with Maxwell stress

In this work, extended finite element method (XFEM) is implemented to model the semipermeable cracks in the piezoelectric material with Maxwell stresses. This Maxwell stress is induced due to the presence of an electric field at the crack faces and boundary of the piezoelectric material. To model the semipermeable crack with Maxwell stress, the crack tip is enriched with six-fold enrichment functions. The electro-mechanical interaction integral is modified and implemented for several crack configurations to compute the stress intensity factor (SIF). The accuracy of the XFEM-based semipermeable crack model in the presence of Maxwell stresses is demonstrated by validating with the analytical solution. The effects of crack length, far-field loading, Maxwell stress, and poling direction are investigated on the normalized SIF.

A chemo-thermo-mechanical coupled phase field framework for failure in thermal barrier coatings

The failure process in a thermal barrier coating (TBC) system can be complicated due to complex microstructure, material property mismatch, high temperature change, chemical reactions during the failure process, etc. It brings considerable difficulties to the modeling technique. A complete failure process in TBC contains multiple failure types, e.g., bulk fracture, delamination of TC (top coating)/TGO (thermally grown oxide) interface and TGO/BC (bond coating) interface, and the interaction among them. However, existing approaches mainly focus on a single failure type and thus are faced with limits on exploring the complete failure mechanism. In this work, we propose a chemo-thermo-mechanical coupled phase field model based on thermodynamic laws, such that the complete TBC fracture process can be captured. The present contribution provides a step forward in comparison with the existing phase field models. Couplings among thermal conduction, material oxidation and damage are considered, and the fundamental differential equations are derived strictly follow the thermodynamic laws. The proposed model is first validated through several benchmarks, and then followed by several simulations of the failure process of a TBC system. The simulations and experiments show that the bulk fracture occurs when the TGO is thin at the cooling process, the growth effect of TGO layer causes damage initiation at the interface and finally transformed into the interfacial delamination, the interaction of interface cracking and bulk material cracking is captured. The proposed model is proven to be a powerful tool in solving the failure process of a TBC system and is promising to handle other similar problems.

Fatigue Crack Growth in a Monocrystal and Its Similarity to Short-Crack Propagation in a Polycrystal of Nickel

Short fatigue cracks in polycrystalline materials are very important from both practical and basic aspects, yet they are very difficult to observe. Therefore, it is suggested to emulate some properties of short cracks with long fatigue cracks in monocrystals. Indeed, such experiments in pure nickel crystals prove that the three peculiar properties of short fatigue cracks in polycrystalline metals are observed in long cracks in monocrystals, i.e., a lower threshold of the stress intensity factor (ΔKI)th, a higher crack propagation rate at low ΔKI regimes, and the fact that different cracks exhibit different growth behaviors. The fatigue experiments in monocrystals reveal interesting details of the slip activity at the front of fatigue cracks, including a selection rule for the active slip systems above and below the crack, the slip behavior under conditions of steep strain gradients, and the activation of a new slip system.

Mixed-mode fracture analysis of FGMs using [formula omitted]-integral: Formulation and FE implementation

This paper introduces a computational method based on the Jk-integral for mixed-mode fracture analysis of isotropic functionally graded materials (FGMs), subjected to mixed-mode loading. The present development is implemented by means of the finite element method (FEM), by couplingofAnsys and Matlabsoftware’s. The continuous variations in the material parameters of the functionally graded medium are treated by specifying the material properties at the centroid of each finite element. The Jk-integral implementation is validated numerically by considering two different problems, the edge-cracked FGM plate and the four-point bending specimen with crack parallel to material gradation. The presented results illustrate the influences of material property gradation and crack geometry on the mixed-mode SIFs. The comparisons reported by available reference solutions indicate also that the numerical results obtained by present method have a high precision and that the expanded form of the Jk-integral is independent of the domain.