Crack Origin Research Articles

A study on fatigue damage and crack initiation in austenitic steel matrix during very high cycle fatigue

Fatigue damage and crack initiation behavior has been studied in an austenitic stainless steel using a novel progressive stepwise load increasing method with a cycle step greater than 108 cycles and investigated with a FIB-SEM. Subsurface crack origin with a fine granular area in the matrix was formed. Heterogeneous plastic deformation causes strain localization and grain fragmentation. Localized plasticity exhaustion induces crack initiation. A crack origin is completed once a short crack reaches the stress intensity factor threshold for stage II crack propagation. This study provides a fundamental discussion how damage and crack initiation in matrix occur during very high cycle fatigue.

Effect of curing agents and hollow glass microspheres on the compression properties of syntactic foams

To investigate the compression properties and failure mechanisms of syntactic foams, two different resin matrices and three types of hollow glass microspheres (HGMs) were used to prepare syntactic foams with different properties. Results showed that the compression strength of syntactic foams increases with the increase in the compression strength of HGMs or resin matrix. Low-strength HGMs will be the first to break down as a source of cracks under stress. When the stress value exceeds the compression strength of the matrix, the matrix will deform and break, leading to the overall destruction of syntactic foam. High-strength HGMs not only act as a lightweight filler but also act as a reinforcing phase to improve the strength of the syntactic foam. The matrix located at the interface will experience initial cracking when subjected to stress, becoming the origin of subsequent cracks. Consequently, the weaker part between the HGMs and the matrix will be the initial point of failure, resulting in the formation of a crack source. The failure of the matrix will ultimately result in the complete destruction of the syntactic foam.

Two-stage framework with improved U-Net based on self-supervised contrastive learning for pavement crack segmentation

After the deep learning method emerged, the automated detection technology of pavement crack images has significantly progressed. The dominant approach is supervised deep learning, which relies on large-scale labeled ground truth. However, the problems are mostly unlabeled original crack images, which are difficult to fully utilize by the supervised deep learning network model. As a representative method of self-supervised learning, contrast learning can learn feature representations from unlabeled data, thus improving the accuracy of downstream tasks. This paper proposes a two-stage framework with improved U-Net based on self-supervised contrastive learning for pavement crack image segmentation. The framework takes improved U-Net as the basic architecture to highlight the significant features of the target segment of fine cracks. U-Net is improved by integrating the residual structure and attention mechanism in the typical U-Net architecture. The framework includes two learning stages: pre-training and fine-tuning. In the pre-training stage, the potential feature representation is learned from the unlabeled crack image. Crack images and pavement background images are used in the training data so that the model learns the distinguishable mapping relationship between crack and its background in the high-dimensional vector space without supervision comparison. In the fine-tuning stage, the network loads the parameters after the pre-training and uses the labeled training data for the retraining. Experimental results show that the proposed two-stage framework significantly improves the performance of crack segmentation accuracy without increasing the number of existing training samples and their labeling.

Effect of Inhibitors on Hydrogen Bond Reduction and Kaolinite Dissolution for Enhanced CO2 Adsorption in Coal.

The application of an inhibitor to the remaining coal in the goaf not only prevents spontaneous combustion of the coal seam in the mining area but also greatly enhances the capacity of coal to adsorb CO2. To investigate the mechanism by which inhibitors improve the CO2 adsorption capacity of the coal seam in the goaf, we conducted swelling experiments, infrared spectroscopy, scanning electron microscopy, and X-ray diffraction analyses to examine the microstructural changes in the adsorption of CO2 before and after inhibition. The results indicate that after inhibition, the number of hydrogen bonds between coal macromolecules decreased, and the samples exhibited approximately 5% swelling. This swelling of the coal macromolecular structure and the increased distance between coal particles create additional space for CO2 sequestration, which is a critical factor contributing to the enhanced CO2 adsorption capacity of coal. The mineral composition of coal consists of 75.6% kaolinite, and inhibition leads to a reduction in kaolinite content by 0.8-7.9%. After inhibition, the swelling and disintegration of kaolinite cause uneven stress, resulting in changes to the pore structure. Closed pores filled with kaolinite transform into open pores, and the original pores crack, forming new pores and pore channels. The dissolution of kaolinite particles increases the porosity of the coal, further facilitating gas adsorption. Among the three inhibitors tested, the most effective in enhancing CO2 sequestration by bituminous coal in the mining area was the urea solution. This study holds significant importance in improving the CO2 sequestration capacity of residual coal in goaves.

Micro-mechanical behavior of lamellar structured Ti6Al4V alloy upon compression via experimental and crystal plasticity study

The micro-mechanical behavior of lamellar structured Ti6Al4V alloy to a small compressional strain of 4.5% was studied. Strain and stress intends to localize along grain boundaries (GBs), triple points (TPs) as well as grain interiors (GIs). The maximum strain and stress is found at some GB-TPs with soft and grain clustering mostly owing to their strong strain incompatibility. Their heterogeneous distribution is highly related to individual crystallographic orientation, their size, shape and grain-grain interactions. The local dislocation behavior forms ledges along with Geometrically necessary dislocations (GNDs) to accomodate strain incompatibility. EBSD and crystal plasticity (CP) simulation disclosed some long range meso-bands (about 45° to loading direction) which crosse multi-grains and some mini-bands within grain interiors (GIs) reflecting a concentrated strain behavior and ductile nature of the Ti6Al4V material. Prismatic slip dominates Ti6Al4V lattice (mainly from surface dislocation activation) determined by its low critical resolved shear stress (CRSS) which is away from some findings of literature. Dislocation is found commenced from some high angle GBs (HAGBs) and extened into GI which can pass through GBs of both low (LA) and high angles given their slip plane coherency. The substructures (α-lamellae) separated by LAGBs within α-colonies, have no significant effect to dislocation movement. Fatigue failure can be inferred from the early stage micro-mechanical behavior through accumulated plastic strain and strong strain incompatability. Fractography reaveals fatigue failure from surface, sub-surface, shearing, cleavage and intergranular modes are presumed highly related to surface defects, compressional residual stress gradient, easy prismatic slip and soft and hard GBs. Multiple crack origins from such combinations are deemed for reduced fatigue life of shot peened samples regardless of the surface compressional residual stress being generated.

Reinforced concrete surface cracks length detection and length estimation by using digital image processing approach

Surface cracks are a common failure that occurs in reinforced concrete structures (RC). With the help of new technologies, access to crack properties should be easier and help the inspector to provide better results. However, most inspectors still prefer manual visual inspection approach, which leads to inconsistent results when investigating this flaw. Moreover, cracks with inconsistent shapes and irregularities are a difficult task for crack extraction, and inspectors overlook the details of cracks. Therefore, in this study, crack detection and thin crack appearance enhancement using various digital image processing algorithms (DSP) were proposed to improve the accuracy of crack length estimation. By using certain DSP on the captured crack images on RC, several algorithms were created and coded in MATLAB via the morphological approach to produce good quality of the original crack images. At the end of this study, the appearance of the thin crack was improved and helped to improve the estimate of the total length of the crack in pixels. The maximum percentage error between the estimated crack length was calculated and compared to the actual length and was 7.10 %. The surface crack detection algorithm and the has the potential as a helpful structural health monitoring (SHM) tool for crack inspection.

Exploring the origins of surface cracks in forged ingots: Impurity element influence and temperature-induced formation of detrimental phases in case hardening steel

Surface hot shortness is a critical defect affecting the performance and reliability of steel components. This study investigates the factors influencing surface hot shortness in DIN 16MnCr5 steel, with a particular focus on the role of impurity elements and temperature. Experimental analysis reveals that copper and zinc, as impurity elements, play a significant role in the formation of detrimental phases and the initiation of surface cracks. Copper enrichment occurs due to its lower affinity to react with oxygen, resulting in its aggregation at the oxide layer/iron interface. Zinc contamination contributes to the formation of intermetallic phases and the initiation of hot shortness, likely due to its low melting point and reactivity. Furthermore, the impact of temperature during hot tensile testing on the ductility and formation of detrimental phases is investigated. Below 950 °C, fractures primarily occur due to proeutectoid ferrite formation at grain boundaries, with limited formation of Cu-rich phases. However, temperatures above 950 °C exhibit increased Cu-rich phase formation, leading to decreased ductility and the formation of hot cracks. The critical temperature range of 1000 °C shows minimum ductility and the highest content of Cu-rich compounds.Grain boundary penetration of Cu-rich melt is identified as a crucial mechanism in the nucleation and propagation of cracks during surface hot shortness. Exceeding a certain copper concentration along grain boundaries leads to the formation of a molten phase at the crack tip, facilitating wetting of the grain boundaries and the subsequent formation of surface cracks. Understanding the influence of impurity elements, such as copper and zinc, as well as the effect of temperature on surface hot shortness is essential for mitigating this defect and improving the performance of steel components. The findings of this study contribute valuable insights toward the development of effective strategies for preventing and controlling surface hot shortness in DIN 16MnCr5 steel.

Origin and Evolution of Cracks in the Glaze Surface of a Ceramic during the Cooling Process.

Because of the significant difference between the thermal expansion coefficients of ceramic blank and glaze, the glaze typically undergoes more pronounced shrinkage than the blank during ceramic cooling, which results in high stress concentrations and cracking. In this study, the mechanical mechanism of glaze cracking is studied, based on the statistical strength theory, damage mechanics, and continuum mechanics. Furthermore, the influence of the glaze layer thickness, heat transfer coefficient, expansion coefficient, and temperature difference on the creation and propagation of inner microcracks is systematically investigated, and the final discrete fracture network of ceramics is discussed at the specific crack saturation state. The results show that (1) a higher heat transfer coefficient will lead to a more uniform distribution of the surface temperature and a faster cooling process of the ceramics, reducing the number of microcracks when the ambient temperature is reached; (2) the thinner glaze layer is less prone to cracking when its thickness is smaller than that of the blank. However, when the thickness of the glaze layer is similar to that of the blank, the increased thickness of the glaze layer will increase the number of cracks on its surface; and (3) when the expansion coefficient of the glaze layer is smaller than that of the blank, cracks will not occur inside the glaze layer. However, as the coefficient of the thermal expansion of the glaze layer continuously rises, the number of cracks on its surface will first increase and then decrease.

Decarburization effects on high-cycle fatigue of uncoated press hardened steels

Abstract The decarburization layer can be detrimental and lead failures in press hardened steels. This work investigates the effect of decarburization layer on the fatigue crack origination and growth on press hardened parts of uncoated 22MnB5 steel. The decarburization layer was analyzed by light microscope, scanning electron microscope, and microhardness measurements. It was found that decarburization layer has two distinct zones indicating complete and partial decarburization. To understand the effect of decarburization on crack origination, high cycle fatigue tests were performed on samples from press hardened parts in decarburized and polished (nondecarburized) conditions. The results have shown that the fatigue performance of press hardened steels in polished condition outperforms the fatigue samples in decarburized condition. In fractographic investigations, it was seen that decarburization is not only responsible for crack origination but also affects crack propagation by having full and partial decarburization zones. The full decarburization zone was acted as surface crack origination source. In the partial decarburization zone, the crack propagation behavior was changed by a complex phase microstructure of bainite, ferrite, and martensite.

Crack instability in deep coal seam induced by the coupling of mining unloading and gas driving and transformation of failure mode

Coal and gas outburst is a violent mechanical instability of gas-containing coal, and its mechanism is crucial for prevention of gas dynamic disasters. The macroscopic instability of gas-containing coal is the result of the expansion and connection of micro cracks driven by unloading induced tensile stress and gas pressure. This study is aimed at solving the problem of crack instability of gas-containing coal during mining unloading. To this end, first, the governing equations of unloading-induced tensile stress and gas pressure evolutions in cracks were built, and the dynamic evolutions and main controlling factors of tensile stress and gas pressure were revealed. Furthermore, the evaluation indexes of crack stability under tensile stress and shear stress were established, and the variation of crack stability during unloading was investigated. Finally, the phase diagram of crack failure mode was drawn, and the transformation of crack failure mode was analyzed. The results show that unloading-induced tensile stress is generated when θ > 54.73°, and it rises gradually with the increase of original stress, unloading rate, unloading time and crack size and dip angle. Overall, gas pressure in cracks increases first and then decreases during unloading. It increases gradually with the increase of initial gas pressure, gas desorption rate and original stress, while it decreases as the unloading rate gets higher (k from 10−4 to 10−2 s−1) and the dip angle gets larger (θ from 50 to 90°). With the increase of unloading ratio, the crack stability declines slowly first and then plunges rapidly until the crack finally becomes unstable. In addition, burial depth of coal seam, unloading rate, gas desorption rate and mechanical strength of coal have significant effects on this process. Different factors have important influences on the crack failure mode. With the increase of burial depth from 100 to 2000 m, the crack failure mode gradually transforms from tensile failure to shear failure, and the higher the unloading rate is, the more easily tensile failure occurs. With the increase of cohesion (0.1–2 MPa), the scope of the shear failure zone shrinks gradually, and the crack is less prone to shear failure. The desorption rate has a notable effect on the scope of the non-failure zone. Under a higher gas desorption rate, the scope of the non-failure zone becomes smaller, which is due to the greater gas pressure in crack for a given time. The research in this paper has certain reference value for further grasping of the instability failure of gas-containing coal and the mechanism of coal and gas outburst.

Compressive strength, pore structure, and hydration products of slag foam concrete under sulfate and chloride environment

In this study, blast furnace slag (BFS) was used instead of partial complex Portland cement (PC) to produce slag foam concrete (SFC), aiming to improve the ion attack resistance and compressive strength of nature foam concrete (NFC). The behavior of compressive strength, pore structure, hydration products, and microstructure of SFC in salt environments (5% Na2SO4, 5% NaCl, 5% Na2SO4+5% NaCl) was investigated. The findings indicate that the negative effect on the compressive strength of sulfate is mitigated by chlorine. In NFC, the loss of compressive strength in the Na2SO4, Na2SO4+NaCl, and NaCl solution at 120 days is 18.06%, 12.28%, and 3.58%, respectively. The optimization of SFC by the addition of BFS first increases with the amount of replaced cement until the positive effect of BFS is overshadowed by its negative effect when the amount replaced reaches 50%. And the salt erosion resistance of SFC with 30% of cement replacement is the best of all mixtures with KSO42- and KSO42-+Cl- of 0.89 and 0.94 respectively. In the beginning, the SO42- and Cl- promote the production of more ettringite (AFt) in the SFC, which results in the filling of the original cracks and a reduction in pore volume. The opposite situation gradually occurs in SFC owing to the expansion of the AFt, as salt curing continues. The content of hydration products varies according to the amount of BFS added, and a decrease in Ca(OH)2 and an increase in the gel are observed in SFC with 30% −50% of cement replacement compared to NFC.

Study of the relationship between the water binder ratio and strength of mixed recycled aggregate concrete based on brick content

High-value recycling of mixed recycled aggregate (MRA) has attracted the attention of researchers in the worldwide, but its influence on the Bowromi formula is still unclear, which hinders its in concrete. Against this background, this research focuses on the basic physical properties of MRA and the effect of brick content on the properties and microstructure of MRA and mixed recycled aggregate concrete (MRAC) under different water binder ratios (W/B). The results showed that the compressive and splitting tensile strengths of MRAC decreased with an increase in the brick content; however, the splitting tensile strength was more sensitive to changes in the brick content than the compressive strength. In microscopic analysis, it was found that although the addition of the recycled brick aggregate could enhance the internal curing effect of the MRAC, it was still insufficient to compensate for the adverse effects of the pore structure deterioration and original cracks in MRA. Finally, based on the above results, this study provides an effective method to predict the compressive strength of MRAC with different brick contents, and optimizes the Bowromi strength and MRAC strength development formulas. This research result is helpful in promoting MRA applications in the real-world construction projects.

On the relationship between the mechanical strength and mineral microstructure of biotite granite under temperature influence

Many studies are currently focusing on the mechanical properties of granite from macroscopic and microscopic perspectives, but only several studies on the influence of rock mineral composition on the mechanical properties using a combined macro-micro assessment. The biotite granite in the Gaoligong Mountains was selected as the research object, and the variation strength characteristics at 20–130°C were obtained through uniaxial compression experiments. The rock’s uniaxial compressive strength and elastic modulus reached peak values at 40°C. The main mineral components of the rock determined by X-ray powder diffraction were quartz, biotite, albite, and potash feldspar. With Material Studio software, molecular dynamics simulations were carried out on the crystal models of quartz, biotite, albite, and potash feldspar at 20–130°C, and relevant mechanical parameters were calculated. The macroscopic and microscopic experimental data were compared and analyzed. The results show that: 1) due to the thermal expansion of minerals in the rock, the original cracks between particles are partially closed, and the sample strength is the largest at 40°C; 2) temperature mostly affects quartz and biotite phases, and the rock strength increases first and then decreases with the temperature increase. The mechanical properties of albite and potash feldspar are less affected by temperature; 3) macroscopic and microscopic experimental results show that the rock strength reaches the peak value in a certain temperature range. The obtained critical temperature is different because the rock sample also contains a small amount of argillaceous rock, siliceous rock, phyllite, and some siliceous and calcareous cement beside the main mineral composition; 4) The structural changes of quartz and biotite are affected by temperature. The bond length of the quartz crystal decreases while that of the biotite crystal first increases and then decreases with the temperature. The mechanism of the temperature influence on the increase of the rock strength is different for these two crystal phases, and it requires further exploration.

Fracture behavior of a high pressure die casting AlSi10MnMg alloy with varied porosity levels

The fracture behavior of a high pressure die casting AlSi10MnMg alloy with varied porosity levels is investigated by synchrotron radiation X-ray computed tomography. Attention is focused on origin of crack and the evolution of porosity in the process of plastic deformation. Different from in-situ 3D tensile test, load-off specimen after plastic deformation is applied for this experiment and it is closer to the real scene. Results show that multiple defects are proposed in high pressure die casting AlSi10MnMg alloy including shrinkage, gas pore, externally solidified primary iron-rich phase (ESC-IMCI) and heterogeneous layers. In a lower-porosity sample, the surface defects like skin layer are a main crack source and a radial river pattern exhibits in crack surface. No new pores generated in the process of plastic deformation. In a higher-porosity sample, no obvious crack source and radial crack propagation path can be identified while the large crack surface fluctuation and tortuous morphology are obtained. The collapse of pre-existing small-sized pores exert a vital role in connecting large-scale pores. The micro-crack initiation and propagation originate from the expansion, consolidation and collapse of pores, especially the irregular pores.

Dynamic Evolution of Cracks in Slag-Modified Soil under Uniaxial Loading Using Real-Time X-Ray Computed Tomography

The cracking behavior of subgrade filling is crucial for the stability of road embankments. Slag-modified soil (SMS), which is commonly used as a reinforcing material for construction in highway and railway engineering, has special mechanical and microstructural characteristics that are easily altered under external disturbances. Although the macroscopic deformation of SMS has been widely examined, research on its cracking behavior at the mesoscale under an external load has received insufficient attention thus far. This study uses real-time X-ray computed tomography (X-CT) as a nondestructive tool for characterizing the microstructure of a SMS specimen subjected to uniaxial compression. Rules for the dynamic propagation of cracks in the specimen during deformation are identified based on two-dimensional (2D) and three-dimensional (3D) visualization and quantitative characterization. The results showed that the SMS can be divided into three components—cracks, soil, and steel slag—based on the density gradients from X-ray CT images. The preprocessed X-ray CT images and reconstructed crack models dynamically demonstrated the expanding and connecting patterns of internal cracks during the entire process of deformation. Values of the crack ratio and the degree of connectivity were positively correlated with the axial strain as expressed by a cubic spline function and a linear function, respectively. Curves of the distribution of the volumes of the cracks suggest that cracks in SMS can be divided into four groups: minicracks, mesocracks, medium cracks, and macroscopic cracks. From quantitative and visual perspectives, a large number of original cracks with small volume and low connectivity were gradually transformed into a main fracture under uniaxial loading. The localized shear band, which existed in the zone adjacent to the main fracture, was generated and obliquely distributed in an irregular, flat strip shape in the middle of the SMS specimen. This study suggests that the real-time X-ray CT scanning technique has broad application prospects for detecting the mesostructure in subgrade filling.

An Improved Crack Breathing Model and Its Application in Crack Identification for Rotors

The crack breathing model and crack identification method for rotors using nonlinearity induced by cracks are studied in this work. Firstly, the finite element method is utilized to model a rotor–bearing system with a response-dependent breathing crack to obtain the numerical data for crack identification. During the modelling, an improved breathing crack model is proposed, focused on the unreasonable assumption about crack closure line in the original crack closure line position (CCLP) model. Compared with the original model, the improved breathing model can reflect the nonlinear behavior of cracks better. Secondly, based on the established model, super-harmonic features at 1/3 and 1/2 of the critical rotating speeds under different crack locations and crack depths are extracted for crack identification. Additionally, the super-harmonic features from two measurement points are used as inputs into an artificial neural network with a Levenberg–Marquardt back-propagation algorithm, corresponding crack positions and depths as outputs. The robustness of the method is tested by examining the identification results under different levels of noise. The results show that the proposed crack identification method is efficient for simultaneous identification of crack depth and position in operating rotors.

Failure Analysis of Fatigue Failed M20 Class 8.8 Galvanized Steel Bolt

An M20 Class 8.8 galvanized steel bolt (derrick bolt) was found to fail after five years of service in an offshore drilling station. A comprehensive failure analysis investigation was carried out to reveal the root cause of the detected failure. Visual inspection coupled with complete microstructural characterization as well as mechanical testing were performed in the investigation. The results showed that the failure was mainly caused by fatigue failure. The initiation of the fatigue crack was supposedly promoted by the existence of non-metallic inclusions that were present at both the surface and the interior of the microstructure. Furthermore, unbalanced preload forces resulted in bolt loosening which caused fretting wear of the galvanized layer thickness. The detected drop in the galvanized layer thickness by 42.20% at the primary origin of the fatigue crack along with the presence of non-metallic inclusions were thought to be the reason for the initiation of the fatigue crack. The crack was then found to propagate through a transitional complete cleavage to quasi-cleavage fracture as appeared from fracture surface topography studied by scanning electron microscopy (SEM). The final stage of the fatigue failure was found to be caused by ductile fracture of the overloaded zone.

Observation of Internal Fatigue Cracks in Repeatedly Induction-Heated S45C Bearing Steel under Rotating Bending Fatigue

Induction-heated steel has hard and soft layers. These layers can cause an internal fatigue crack originating from the boundary of these layers when cyclic stress is applied. Repeated heating is known as a method for improving fatigue strength, and it was applied to induction heating method. Repeatedly induction-heated steel had high fatigue strength compared to single quenching. We performed rotating bending fatigue tests of low carbon steel (JIS-S45C) induction-heated three times, and observed the fracture surfaces and the microstructures of internal fatigue cracks. The internal fatigue cracks originated from the area around the boundary between soft and hard layers surrounding crack origin. Some pearlite and ferrite can be seen. There were pearlite and dimples on the soft layer of internal fatigue crack and clear grains on the hard layer of the crack. From chase-up observation, we revealed that internal fatigue crack originated from soft layer.

On the strain energy release rate and fatigue crack growth rate in metallic alloys

The field of fracture mechanics started with Griffith’s energy concept for brittle fracture in 1920. In 1963, Paris et al. used a fracture mechanics’ parameter to introduce an equation for the fatigue crack growth rate in ductile materials and this equation is now commonly known as the ‘Paris law’. However, the Paris law and the semi-empirical models that followed ever since do not fully account for the main intrinsic and extrinsic properties involved with fatigue crack growth in metallic alloys. In contrast, here a dimensionally correct fatigue crack growth rate equation is introduced that is based on the original crack driving force as introduced by Griffith and the presence of plasticity in a metal to withstand crack propagation. In particular it is shown that the fatigue crack growth rate shows a power law relationship with the cyclic strain energy release rate over the maximum stress intensity factor. The new description corrects for the ratio between the minimum and maximum stress in a cycle during constant amplitude loading and for crack growth retardation under variable amplitude loading. The method has been successfully applied to variable amplitude crack growth with spectra that are representative of different fatigue dominated aircraft locations. As such, it allows for accurate predictions of variable amplitude fatigue crack growth life in aerospace structures.

Formation of fine granular area in a non‐defect matrix of austenitic stainless steel during very high cycle fatigue

AbstractA fine granular area, FGA, is a typical phenomenon observed at the very high cycle fatigue fracture crack origin with a subsurface defect in the material. The FGA has been widely investigated, and different mechanisms have been proposed. In this paper, the formation of FGA in a non‐defect matrix of one austenitic steel during very high cycle fatigue was studied using a progressive stepwise load‐increasing method and electron scanning microscopy/electron channeling contrast imaging (ECCI) technique. A nano rough surface area or FGA at the fatigue crack origin has been observed in the subsurface matrix without any defect. It is a new phenomenon. A mechanism was proposed using the dislocation plasticity theory. The formation of FGA in a non‐defect matrix is a localized plasticity exhausting process by strain localization, grain fragmentation, stress concentration and nano crack initiation and propagation along low‐angle grain boundaries.