Micromechanisms Of Crack Growth Research Articles

Structure, Mechanical Properties and Features of Crack Growth Micromechanism of Maraging Steel Obtained by Hybrid Additive CMT Surfacing

Structure, Mechanical Properties and Features of Crack Growth Micromechanism of Maraging Steel Obtained by Hybrid Additive CMT Surfacing

Fracture toughness of wood and transparent wood biocomposites in the toughest LT-direction

Fracture toughness and mechanisms of crack growth are characterized for transparent wood polymer biocomposites and compared to native wood, with the crack normal to the fiber direction (LT fracture plane). Side-grooved specimen geometries generated pure mode I crack growth, whereas previous investigations commonly report 90° crack path deflection. Crack growth micromechanisms were analyzed by experimental fracture tests and in-situ microscopy observations. Large damage zones around the crack tip with fiber bundle bridging and pull-out were observed in the crack wake, justifying more advanced cohesive zone modeling suitable for composite materials design. The polymer matrix resulted in much higher fracture energy of the biocomposites compared to native wood due to increased local cohesive strength. This strength increased from the polymer contribution and more homogeneous stress distribution in the wood fibers.

The behavior of cutting discs for mechanized tunneling under cyclic loading conditions

In the case of a heterogenous ground condition, tunneling tools are exposed to cyclic loading. The cyclic loads can exceed the materials' permanent load-bearing capacity, which is associated with fatigue crack growth. Subsequently, the coalescence of fatigue cracks can cause spalling of material from the tool surface, leading to the deterioration of their functionality. The problem of fatigue damage on tunneling tools has lately been recognized. However, the underlying micromechanism of subcritical crack growth and the counteraction of fatigue by advanced material concepts still require fundamental research. In this work, miniaturized cutting discs made of different tool steels are subjected to a laboratory scale fatigue test and subsequently analyzed concerning fatigue damages and the material response to cyclic loading. The hot work tool steel X40CrMoV5-1, widely used for cutting discs in industrial practice, showed fatigue crack growth and significant plastic deformation of the cutting edge under cyclic loading, leading to chip formation and spalling at the cutting edge. An improvement was found by using X153CrMoV12 ledeburitic cold work tool steel, which offers a higher strength due to the coarse chromium carbides distributed in the microstructure. Due to their high young modulus, the carbides can inhibit the plastic deformation of the softer metal matrix at the cutting edge, thus increasing the edge retention and suppressing chip formation.

Micromechanics of fatigue, creep-fatigue interaction and thermo-mechanical crack growth of XH73M nickel alloy

This study presents interpretation and evaluation of a range of isothermal experimental crack growth data on polycrystalline XH73M nickel-based alloy generated by two type tests carried out by stress-controlled pure fatigue and creep-fatigue interaction conditions. The tests have been carried out using cycles with a triangular and trapezoidal waveform and a temperature range of 23-750°C. It was found that from the crack growth rate point of view, the following order of arrangement of fatigue fracture diagrams is formed: isothermal creep-fatigue interaction, isothermal pure fast (f=10 Hz) and slow (f=1 Hz) fatigue. The ordering of the crack growth rate curves is supported by detailed fractographic analysis which shows transgranular or intergranular mechanism of fatigue crack growth depending on thermo-mechanical test conditions. The regularities of fatigue crack propagation were established by fractographic analysis. It was found for example that the intergranular crack growth mechanism become dominant at 750°C for the pure fatigue and at 550°C for creep-fatigue interaction conditions. The secondary crystals on the grain boundary of primary grain at intergranular fracture of specimen tested at 650°C under creep-fatigue interaction conditions appeared.

Micromechanisms and modelling of crack initiation and growth in tool steels: role of primary carbides

AbstractMicromechanisms of damage initiation and crack growth in high speed and cold work steels are investigated using scanning electron microscopyin situexperiments. The role of primary carbides in initiation and growth of cracks in tool steels is clarified. It is shown that initial microcracks in the steels are formed in primary carbides and then join together. A hierarchical finite element model of damage initiation, which included a macroscopic model of the deformation of the specimen under real experimental conditions and a mesomechanical model of damage in real microstructures of steels, was developed. Using the hierarchical model, the conditions of local failure in the steels have been obtained. In order to study the effect of carbide arrangement on fracture, numerical simulations of fracture in steels with different ideal carbide arrangements were carried out and compared with each other. It is found that the heterogeneous arrangements of primary carbides can lead to strong deviations of a crack from the mode I path and, therefore, to a significant increase of the fracture energy of the steels.

Creep crack growth of GH3535 alloy at elevated temperatures

The behavior and micromechanism of creep crack growth (CCG) of GH3535 alloy at 650 to 750 °C have been investigated experimentally. The relationships among the effective stress intensity factor, fracture parameter C ⁎ , and CCG rate were obtained and the effect of temperature on the crack initiation time were analyzed. The results show that it is suitable to use the fracture parameter C ⁎ to correlate the CCG rate at different temperatures and that the CCG rate and fracture parameter C ⁎ have a good linear relationship in a narrow band in the log-log scale. This linear relationship does not depend on temperature when using the fracture parameter C ⁎ to correlate the crack initiation time. During crack propagation, intergranular propagation and dimple transgranular fracture occurred successively. The crack growth is found to be accompanied by oxidation. • Creep crack growth behavior of GH3535 allov used as structural material of TMSR at different temperatures was investigated. • The relationships between crack gowth rate crack initiation time and C* parameter are independent of temperature. • The propagation mechanism of GH3535 at different temperatures was found by observing the fracture and crack tip morphology. • Oxidation promotes the crack growth and the interaction between crack growth and oxidation is described.

Structure, Mechanical Properties, Elemental Composition of Phases and Micromechanism of Crack Growth in Alloy AK9Ch Subjected to Homogenizing Treatment and Modifying

The structure and mechanical characteristics of alloy AK9ch are studied after different treatments involving homogenization and modification of the melt. The micromechanism of crack growth in alloy AK9ch after the treatments is investigated. The composition of the phases is studied quantitatively with plotting maps of the distribution of elements in them. The changes in the morphology of the individual phase components due to the modifying and due to joint homogenizing and modifying are determined.

Structural and Fractographic Features of Formation of Splits in Low-Alloy Steel Subjected to Thermal Deformation Treatment

The structure and micromechanism of crack growth in steel 09G2S are studied after heat and thermal deformation treatments involving cold radial forging (CRF) with 55% total deformation and subsequent annealing at 300 and 600°C. A method for electron microscope panoramic X – Y – Z joining of images of a fracture surface with level difference up to 3 mm is developed and tested. The structure under a fracture surface of steel 09G2S after CRF and annealing at 300°C is studied. The positive role of bands of adiabatic shear forming in the structure under cold radial forging in dispersion of the steel is demonstrated. Special features of the micromechanism of crack growth due to formation of splits are determined.

Combining Fractography with High-Energy x-Ray Diffraction to Study Fatigue Crack Growth in Ti-6Al-4V

Fatigue crack growth in Ti-6Al-4V has been explored through the combined use of high-energy x-ray diffraction (HEXD) analysis, electron backscatter diffraction analysis, and fractography to draw a connection between the driving forces and micromechanisms of fatigue crack growth. Samples were prepared from a forging with two configurations, viz. with the crack plane either perpendicular or parallel to the primary extension direction of the forging. Crack growth was examined in situ by mapping the lattice strain fields using HEXD for loading at R = 0.1 and R = 0.7. The crack growth as assessed by HEXD was contrasted with predictions of linear elastic fracture mechanics. The variation in the lattice strain field with crystallographic orientation was assessed. At the microscale, a clear correlation between the availability of c-axes along the loading direction and facet formation was found.

Structure, Mechanical Properties and Fracture Surface Features of Structural Steels Subjected to Deformation-Heat Treatment

A comparative study is made of the structure, mechanical properties and crack growth micromechanisms of 09G2S, 25 and 35X steel pipes after different versions of deformation and heat treatment, including cold plastic deformation of highly tempered material by radial forging and subsequent annealing. It is established that the maximum strength of the test steels is achieved after cold radial forging (CRF) with a degree of deformation of 55% and subsequent annealing at 300 °C. It is shown that CRF and annealing at 600 °C leads to formation of an ultra-fine-grained structure as a result of which reliability properties are achieved similar to a highly tempered condition with retention of improved strength properties. Features of the structure are revealed that is formed during CRF. The micromechanism of crack growth after heat and deformation-heat treatment is studied. Analysis is provided for elements of the failure surface after dynamic tests.

Experimental and numerical simulation analysis on creep crack growth behavior of CLAM steel

The experiment and numerical simulation were performed to analyze the creep crack growth behavior of China Low Activation Martensitic (CLAM) steel under temperatures ranging from 500 °C to 600 °C with a step of 50 °C. The creep crack growth micro-mechanism under multiaxial state of stress was investigated with the compact tension (CT) specimens. The creep crack growth rates were simulated based on the Nikbin-Smith-Webster (NSW) model. The difference of the crack propagation behavior is correlated with the evolution of two types of precipitates: Cr-rich M23C6 and W-rich M6C phases. The coarsening of M23C6 carbides and the precipitation of M6C phase particles lead to dissolving of the solid solution elements and deteriorating the creep properties during the testing processes.

Micromechanisms of fatigue crack growth in polycarbonate polyurethane: Time dependent and hydration effects.

Polycarbonate polyurethane has cartilage-like, hygroscopic, and elastomeric properties that make it an attractive material for orthopedic joint replacement application. However, little data exists on the cyclic loading and fracture behavior of polycarbonate polyurethane. This study investigates the mechanisms of fatigue crack growth in polycarbonate polyurethane with respect to time dependent effects and conditioning. We studied two commercially available polycarbonate polyurethanes, Bionate® 75D and 80A. Tension testing was performed on specimens at variable time points after being removed from hydration and variable strain rates. Fatigue crack propagation characterized three aspects of loading. Study 1 investigated the impact of continuous loading (24h/day) versus intermittent loading (8-10h/day) allowing for relaxation overnight. Study 2 evaluated the effect of frequency and study 3 examined the impact of hydration on the fatigue crack propagation in polycarbonate polyurethane. Samples loaded intermittently failed instantaneously and prematurely upon reloading while samples loaded continuously sustained longer stable cracks. Crack growth for samples tested at 2 and 5Hz was largely planar with little crack deflection. However, samples tested at 10Hz showed high degrees of crack tip deflection and multiple crack fronts. Crack growth in hydrated samples proceeded with much greater ductile crack mouth opening displacement than dry samples. An understanding of the failure mechanisms of this polymer is important to assess the long-term structural integrity of this material for use in load-bearing orthopedic implant applications.

Fatigue crack propagation under biaxial fatigue loading with single overloads

The crack propagation behavior and the governing crack growth micromechanisms in aluminum alloy under in-plane biaxial fatigue loading with single overloads, of different magnitudes and occurring at different fatigue crack lengths, is investigated. The microscale fracture mechanisms governing crack growth behavior under these conditions are identified through detailed fractography. Crack growth retardation behavior observed due to the occurrence of single overloads is correlated with overload magnitude, instantaneous fatigue crack length, crack-tip plasticity and fracture surface morphology. The results obtained provide insight into the relationship between macroscale crack growth behavior to microstructural mechanisms, which is essential to understanding the fatigue behavior of metallic materials under variable amplitude biaxial loading scenarios.

Structure, Dynamic Cracking Resistance, and Crack Growth Micromechanism in Pipe Billets after Thermomechanical Treatment

The structure, dynamic cracking resistance, and micromechanisms of crack growth in initially highly tempered pipe billets made of structural carbon steel are studied after thermomechanical treatment, including cold plastic deformation by radial forging followed by annealing, under various conditions. The strength is found to be maximum after cold radial forging followed by annealing at 300°C. Cold radial forging and annealing at 600°C are shown to cause the formation of an ultrafine-grained structure with an average grain/subgrain size of ~900 nm. The structural features formed in both the axial and the transverse direction after cold radial forging have been revealed. The mechanism of crack growth after heat treatment and thermomechanical treatment has been studied. The fracture surface elements formed during dynamic-crackingresistance tests have been qualitatively analyzed.

In-situ SEM and optical microscopy testing for investigation of fatigue crack growth mechanism under overload

In this paper, the in-situ scanning electron microscope (SEM) and optical microscopy experiments are performed to investigate the crack growth behavior under the single tensile overload. The objectives are to (i) examine the overload-induced crack growth micromechanisms, including the initial crack growth acceleration and the subsequent retardation period; (ii) investigate the effective region of single overload on crack growth rate. The specimen is a small thin Al2024-T3 plate with an edge-crack, which is loaded and observed in the SEM chamber. The very high resolution images of the crack tip are taken under the simple variable amplitude loading. Imaging analysis is performed to quantify the crack tip deformation at any time instant. Moreover, an identical specimen subjected to the same load condition is observed under optical microscope. In this testing, fine speckling is performed to promote the accuracy of digital imaging correlation (DIC). The images around the crack tip are taken at the peak loads before, during and after the single overload. After that, the evolution of local strain distribution is obtained through DIC technique. The results show that the rapid connection between the main crack and microcracks accounts for the initial crack growth acceleration. The crack closure level can be responsible for the crack growth rate during the steady growth period. Besides that, the size of retardation area is larger than the classical solution.

Model for predicting fatigue life and limit of steels based on micromechanics of small crack growth

A model for predicting the fatigue life and limit of steels with banded ferrite-pearlite microstructures is proposed based on the micromechanics of small crack behavior. The model was simplified to a two-step 2D problem: (i) determination of crack initiation site on the surface plane; (ii) fatigue life evaluation by simulating crack growth on the inside plane. A multiscale model synthesis approach was used to integrate three elemental models of (i) macroscopic finite element analysis, (ii) geometries and spatial distributions of ferrite grains and pearlite colonies, and (iii) crack initiation and growth. The required input data consisted of only (i) microstructural information, (ii) monotonic tensile properties, and (iii) test conditions. Model validation was performed by comparing the experimental results using steels with various volume fractions of pearlite phases and grain sizes. The predicted fatigue lives and limits were in good agreement with the experimental data for all the steels. In addition, the model could accurately simulate the crack growth behavior observed by optical microscopy. The framework of the proposed model may serve as a basis for predicting the fatigue lives of metallic structural materials.

High cyclic fatigue performance of Al–Cu–Mg–Ag alloy under T6 and T840 conditions

High cyclic fatigue (HCF) behavior of an AA2139 alloy belonging to Al–Cu–Mg–Ag system in T6 and T840 conditions was examined. The T840 treatment involving cold rolling with a 40% reduction prior to peak ageing provides an increase in tensile strength compared with the T6 condition. However, fatigue lifetime for two material conditions was nearly the same since there is weak effect of thermomechanical processing on micro-mechanisms of crack initiation and growth.

Comparative Analysis of Zones of Plastic Strain, Dynamic Crack Resistance, Structure and Micromechanisms of Crack Propagation in Structural Steels 09G2S, 25 and 40 in High-Toughness Condition

Comparative analysis of zones of plastic strain, dynamic crack resistance, structure, and micromechanisms of crack propagation in structural steels 09G2S, 25 and 40 in high-toughness condition is performed. The structure, the micromechanisms of crack growth, and the dynamic crack resistance of steels 09G2S, 25 and 40 are studied. Complete zones of plastic stain (CPSZ) under fracture surfaces are plotted after quenching and high tempering at 650°C. The levels of microhardness in the CPSZ are mapped for specially-designed specimens with additional 1-mm-deep side notches and relative crack length of 0.4 – 0.5. The sizes of the zones of plastic strain in the starting region are determined. Special features of the distribution of microhardness in local volumes of the CPSZ are determined. The structure under fracture surfaces of steels 09G2S, 25 and 40 is studied over the whole of the path of propagation of a dynamic crack.

Effect of Structure on the Dynamic Crack Resistance and Special Features of the Micromechanism of Crack Growth in Steel 35Kh After Cold Radial Forging

The structure, dynamic crack resistance, strength and micromechanisms of crack growth are studied in tubular billets from steel 35Kh after different variants of deformation and heat treatment including cold plastic deformation by radial forging followed by annealing. The radial forging with subsequent post-deformation annealing is shown to affect positively the strength and the dynamic crack resistance of the metal. Quantitative analysis of the micromechanism of crack growth is made after tests for dynamic crack resistance. Special features of the formation of macro-splits are determined.

Damage based model to study the effect of notch introduction technique on the J-integral value of PolyOxyMethylene

Experimental data on a CT specimen made of PolyOxyMethylene were enriched by in situ laminographic observations in 3D. The observed micro-mechanisms of crack initiation and growth have been simulated by using a damage based model. A critical value of J-integral was determined. The same model was utilized to simulate an initially heterogeneous porosity in the CT specimen to mimic the state of the material after a cutting process. A notch tip touching tool led to a less compliant specimen and a higher value of critical Jcr whereas non touching conditions, induced a damaged layer around the notch tip leading to a decrease in Jcr.