Crack Propagation Energy Research Articles

Effect of martensite–austenite constituents on impact toughness of pre-tempered MnNiMo bainitic steel

The martensite-austenite constituents are known to deteriorate the impact toughness of alloyed steel. In order to reduce this adverse effect, the pre-tempering process at low temperatures was introduced before the tempering of intercritical heat treatment process of a MnNiMo bainitic steel. The effects of pre-tempering temperature on the decomposition of martensite-austenite constituents and impact toughness properties of the pre-tempered specimens were investigated. The results of instrumented Charpy V-notch impact testing show that the impact toughness of the materials can be significantly improved by the pre-tempering process, especially for the condition of pre-tempering at 400 °C. After pre-tempering at 400 °C, more martensite-austenite constituents are decomposed into ferrite and small carbides, resulting in the reduction of microcracks nucleation sites. The residual martensite-austenite constituents become mainly the island-like, which can effectively hinder the propagation of secondary cracks. In addition, the pre-tempering at 400 °C increases the fraction of high-angle grain boundaries and refines the size of blocks, which can effectively prevent or deflect the propagation of microcracks. All of those microstructure changes are beneficial to the crack propagation energy. The results suggest that the application of the pre-tempering process is a feasible method to enhance the impact toughness of the studied alloyed steel.

Fine-grained limestone concrete with improved dissipative properties

Crack propagation in concrete is known to be partially stipulated by the process of energy dissipation. In this study the usage of complex admixture including limestone fines and superplasticizer which enhance crack propagation energy dissipation rate is discussed. Energy dissipation rate and crack propagation are assessed by comparing cracking strains and stress intensity ratios for two sets of specimens, with and without limestone fines. Limestone semi-inert filler acting in a synergetic way with superplasticizer is revealed to enhance crack propagation energy dissipation rate and cracking strains, which makes fine-grained limestone concrete the effective crack-resisting material.

A novel temperature-dependent yield stress model for ceramic materials under indentation

In this study, based on the force-heat equivalence energy density principle of ceramic materials subjected to fracture, a novel temperature-dependent crack propagation energy model is proposed. Subsequently, based on the energy balance between plastic deformation and surface development, a new temperature-dependent yield stress model is proposed for ceramic materials subjected to indentation. We focused on developing this model because there are very few experimental and no theoretical studies on the relationship between the yield stress of ceramic materials and temperature, which is necessary for designing high-performance materials that can function at high temperatures. The simple theoretical model proposed in this study is expressed in terms of Young's modulus, melting point, and critical damage size. A surprising agreement was observed between the predicted and experimental results. Our analysis indicates that the temperature dependence of the yield stress of ceramic materials is controlled by their Young's modulus and the ratio of the size of critical damage at room temperature and test temperature.

Effective Microstructure Unit in Control of Plasticity during Strain‐Controlled Fracture

Strain‐controlled fracture is investigated in 20CrNi2Mo steel using the Hall–Petch relationship, electron backscatter diffraction (EBSD), and other techniques. An interesting result is that the ductility of the tested steel increases with a coarsening of prior austenite grains. This finding is in contrast to conventional understanding. Meanwhile, the strain after necking is approximately 2/3 of the total strain, and the value of UC (crack propagation energy per unit volume) reaches 95% of UK (static toughness). The ductility of 20CrNi2Mo steel also depends on crack propagation. Furthermore, the Hall–Petch relationships between ductility and multilevel microstructure demonstrate that lath is the effective control unit of the plasticity of the tested steel. Lath also plays a key role in crack propagation that results from the rotating, bending, and shearing of lath, as confirmed by fracture analysis and EBSD.

Effect of controlled rolling and cooling process on microstructure and properties of X65 pipeline steel

Effect of rolling and cooling process on microstructure and properties of X65 pipeline steel was studied by means of tensile test at room temperature, oscilloscopic impact test and metallographic microscope. And the impact fracture process of samples with different microstructures was analyzed by oscillographic impact curve. The results indicate that controlled cooling process has important influence on the microstructure of the sample. The strength and toughness of sample with acicular ferrite microstructure are better than this with ferrite and pearlite microstructure. The crack initiation energy of the acicular ferrite microstructure is similar to that of ferrite and pearlite microstructure. However the crack propagation energy of the acicular ferrite microstructure is different from that of ferrite and pearlite microstructure.

Cleavage fracture and microstructural effects on the toughness of a medium carbon pearlitic steel for high-speed railway wheel

The fracture behavior and microstructural effects on the toughness of a medium carbon wheel steel were systematically investigated by using instrumented impact tests. The microstructure of the steel consisted of lamellar pearlite and a small amount of proeutectoid ferrite. The results showed that with the refinement of prior austenite grain size, the pearlite block size decreased and the ferrite content increased, while the pearlite interlamellar spacing kept almost unchanged. The impact toughness was improved in the temperature range from −80 °C to room temperature. Observation of fracture surfaces revealed that the fracture occurred in a brittle manner, which was characterized by cleavage and a small proportion of ductile fracture. However, the toughness was positively correlated to the ductile fracture area. Cleavage cracks were found to be initiated at the sites of proeutectoid ferrite, pearlite colony or pearlite/ferrite interface. The values of macroscopic cleavage fracture stress measured were similar (~1833 MPa) for the specimens with different pearlite block size and ferrite content but the same pearlite interlamellar spacing. The toughness improvement by microstructural refinement in the transition temperature region was attributed to the increase in cleavage crack initiation energy. The cleavage crack propagation energy was, however, relatively low and mainly unchanged. With decreasing the test temperature, the difference in crack initiation energy was decreased, causing the difference in total impact absorbed energy to decrease. The results also suggested that in the lower shelf temperature region where complete cleavage fracture occurred, the toughness was insensitive to the microstructural parameters.

Effects of HfC Particles on Crack Initiation and Propagation Behavior of WC/Co Composites

Tungsten carbide composites were prepared by cold-pressing and hot-pressing sintering; fracture toughness and bending strength of the specimens were tested. The microstructures of HfC/WC/Co composites were observed with the SEM. The mathematical models were established to investigate the relationship between stress intensity factors of crack straight-through, crack deflection, and crack bifurcation with crack length, based on the crack propagation energy release rate. The simulation software ABAQUS was used to verify the four crack propagation methods of crack straight-through, crack deflection, crack bifurcation and crack pinning. The simulation results show that adding appropriate amount of HfC can effectively improve the fracture toughness and bending strength of the composites. The homogeneous distribution of HfC and Co in the matrix has a significant effect on the improvement of the strength and toughness of the composites, and the improvement mechanism is to disperse or transfer the stress at the crack tip to HfC by crack deflection, crack bifurcation, crack pinning, transcrystalline fracture, etc. As a result, the stress concentration at the crack tip in the matrix is reduced, and the toughness of the composites is improved.

Energy Separation and Explicit Dynamic Analysis of Low Temperature Impact Toughness of Transmission Tower Material Q420B

To determine the physical significance of the impact toughness parameters and accurately characterize the low temperature impact toughness of transmission tower material Q420B, the finite element model of Charpy impact test is established on the basis of experiment. The simulation and test results are verified, and the specimen fracture is analyzed by scanning electron microscope. The formation and growth mechanism of the crack are dynamically analyzed. On this basis, energy separation method is used to investigate the effect of low temperature on impact toughness. The results show that the simulation and test results are in good agreement, and the ductile-brittle transition temperature of Q420B is about −50°C. The breaking process of the specimen is divided into the crack formation and propagation. When temperature drops from 20 to −60°C, the crack propagation energy decreases from 51.0 to 11.9 J, the crack formation energy reduces from 39.9 to 15.8 J, and the fracture time of the material drops from 1.8 to 0.6ms.

Influence of Inter-Pass Cooling on Microstructural Evolution and Plastic Deformation of Heavy EH47 Plates.

Herein, the influence of inter-pass cooling (IC) and conventional two-stage rolling (CTR), on microstructural evolution and plastic deformation behavior of ultra-heavy EH47 plates, is demonstrated. It is reported that the deformation amount and deformation rate, in every deformation pass during rough rolling, at 1/4- and 1/2-thickness of IC steel were higher than the CTR steel. The volume fraction of ferrite and acicular ferrite was 45% and 18%, at 1/4-thickness, and 35% and 50% at 1/2-thickness of IC steel, respectively, whereas the sum of both ferrite phases was smaller than 25% in the CTR steel. The austenite grain boundary area and high-angle grain boundary fraction in the IC steel were higher than the CTR steel. The high density of fine and shapeless pearlite has been observed in IC steel, whereas large-size carbides, with hexagonal structure, have been observed in CTR steel. Compared to the CTR steel, the density of precipitates was apparently lower in IC steel. Two kinds of Nb containing precipitates, such as (Ti, Nb)(C, N) and (Nb, Ti)C, were observed in the tested steels. Total ductility and uniform elongation of the IC steel were higher than the CTR steel. During the tensile process, the crack initiation energy and crack propagation energy of the IC steel were higher than the CTR steel. Moreover, the volume fraction of retained austenite (FCC) was reduced from 7.71% to 0.42% near the tensile fracture in IC steel at 1/4-thickness. In additon, the strain of synergetic plastic deformation of the IC steel was higher than the CTR steel. Meanwhile, compared to the CTR steel, the synergetic plastic deformation of the IC steel occurred at low stress after the yield point, which can be ascribed to the presence of fewer microcracks in the IC steel. Hence, a delayed fracture has been observed in the IC steel plate.

Dependency of mesh size and loading history on crack propagation energy of cyclic ductile fracture model

Ductile fracture is one of the main failure modes of metal structures, especially under ultra-low-cycle fatigue (ULCF) loading. To date, it is still difficult to accurately predict ULCF failure of welded steel structures. This paper aims to investigate mesh size and loading history dependency effects of a previously proposed cyclic ductile fracture model. The ductile fracture model is employed to simulate experimental results of welded T-joints under cyclic bending. A method to mitigate the mesh size and loading history dependency effects is proposed in this study, and is also employed to simulate the whole cracking processes of the specimens. The proposed method can predict the crack initiation with good accuracy, and there are still some deviations for the crack propagation simulation. In addition, it is found that the size dependency effect decreases rapidly as the characteristic length of the elements increases, and the loading history has a great effect on the fracture energy parameter in the cyclic ductile fracture model.

Reflective cracking reduction by a comparison between modifying asphalt overlay and sand asphalt interlayer: an experimental evaluation

ABSTRACT Reflective cracking is a frequent type of distress in asphalt concrete (AC) pavements that decreases pavement service life in a newly constructed AC overlay. In this study, natural zeolite and hydrated lime were added to crumb rubber modified (CRM) asphalt binder in order to assess their effects on reducing propagation of reflective cracks in composite pavements. CRM asphalt binder containing these additives was used in both AC overlay and sand asphalt interlayer to investigate crack propagation in the composite specimen under dynamic cyclic loading. The results show that asphalt interlayer modification is more effective than AC overlay modification (using CRM binders containing additives) in controlling reflective cracks. In other words, sand asphalt interlayer modification is more capable of absorbing the energy of crack propagation and, consequently, delaying the crack propagation. The results also indicated that simultaneous modification of both the interlayer and overlay significantly decreases reflective cracking potential. With respect to the significant role of additive type in crack propagation delaying, the results show that CRM binders containing natural zeolite are more efficient in crack mitigation compared to CRM binders containing hydrated lime.

Analysis of heat-affected zone microstructures of steel P92 after welding and after post-weld heat treatment

Modern thermal power plant operating on fossil fuels has to be highly efficient and should emit low levels of CO2. One of possible ways to fulfil these requirements is to increase thermal power plant peak service temperature. P92 steel enables service temperature of up to 650 °C. It is stronger, more resistant to creep and enables higher service temperature than P91. During welding of steam boiler components made of P92, it is important to consider proper preheating, welding and post-weld heat treatment parameters in order to avoid cracks and diminish sensitivity to creep in service. The research focused on analysis of simulated heat-affected zone (HAZ) microstructures and distribution of carbides. The FG HAZ (fine-grain heat-affected zone) (Tmax is 950 °C) is the most problematic when it comes to occurrence of type IV cracks, as in this zone, the average representation of carbides on PAGB is 72%, while the average representation of carbides on PAGB in the P21 steel is 58%. After the heat treatment, maximum hardness is recorded in the CG HAZ (coarse-grain heat-affected Zone) (Tmax is 1300 °C), where the values reached 248 HV, which was below the maximum allowable value of 250 HV. Other areas of HAZ expressed the values from 218 to 227 HV. Charpy impact test at service temperatures of 650 °C showed that the CG HAZ was the most critical immediately after welding as the energy for crack initiation was 30 J. However, after obligatory post-weld heat treatment, FG HAZ appeared to be more problematic than CG HAZ, so this research showed that the crack initiation energy value increased due to the fine-grain microstructure, yet the crack propagation energy reduced.

Experimental Study on Crushing Resistance of Square CFRP Frusta under Axial Loading

Fibre-reinforced composite materials and structures have been extensively used as energy dissipating components in industries. The crashworthiness of square and circular CFRP (Carbon Fiber Reinforced Plastics) tubes have been widely studied. This paper investigates the deformation and energy absorption of square CFRP frusta under axial quasi-static crushing. The effects of various geometry parameters such as height-to-mean width (H/B) and thickness-to-mean width (t/B) ratios on the crushing resistance for square CFRP frusta with an apical angle of 10 ° have been examined experimentally. Finally the crashworthiness metrics of square CFRP frusta and square tube are compared and evaluated. The results show that the experimentally observed deformation mode of CFRP square frusta can be classified into three types, namely progressive folding collapse; splaying crushing mode characterized by a combination of progressive splaying and transverse shearing failure; and transition mode respectively. The initial peak force, mean crushing force and energy absorption of square CFRP frusta deformed in splaying mode increase with the increasing of wall thickness and width. For the desirable progressive splaying mode, three energy dissipated mechanisms have been identified as crack propagation, transverse shearing and friction energy.

Effects of microstructure on crack resistance and low-temperature toughness of ultra-low carbon high strength steel

The effects of microstructure on crack resistance and toughening mechanism of an ultra-low carbon steel were investigated. The microstructures were controlled via thermal-mechanical control processing (TMCP) and heat-treatments. Distribution of stress concentration, microcracks formation and propagation during Charpy impacting were investigated in detail. The results indicate that the lath martensitic structure provided a higher yield stress together with a better impact property, compared to the polygonal ferritic structure. The high strength can be attributed to the high density of dislocations in the lath martensitic structure introduced by quenching. The instrumented Charpy impact results indicated that the crack initiation energy in the lath martensitic structure was similar to that in the ferritic structure while the crack propagation energy was significantly greater than that in the ferritic structure, leading to the high toughness of the steel with the lath martensitic structure. Local stress concentration distributed uniformly in lath martensitic structure, leading to the homogeneous nucleation of microcrack. The high crack propagation energy in the lath martensitic structure can be attributed to the high fraction of high angle grain boundaries and fine effective grains, which deflected the cleavage crack propagation direction.

Irradiation effect on impact fracture behavior of A508-3 steel in ductile-to-brittle transition range

Abstract The mechanical performance of reactor pressure vessel (RPV) materials is an important factor in determining the safety and economics of the operation of a nuclear power plant. The ductile-to-brittle transition temperature (DBTT) tested by Charpy impact test is an important parameter for evaluating the RPV embrittlement. In this paper, the Charpy impact test of RPV steel prior and after irradiation was carried out at different temperatures. The fracture morphology was observed by scanning electron microscopy. The irradiation effect on the impact fracture behavior was analyzed in the DBTT temperature zone. Results show that in the DBTT range, the crack initiation and propagation energy are obviously reduced for the irradiated material. After irradiation, the dimple area on the fracture surface is significantly reduced, and the distance from the crack initiation source to the gap is decreased. For the RPV steel with low Cu steel content, matrix damage becomes the main factor on material irradiation embrittlement.

An Experimental Study on Determination of Crack Propagation Energy of Rock Materials under Dynamic (Impact) and Static Loading Conditions

The Charpy impact test, a widely applied impact strength determination test for various materials such as metals, polymers and cementitious materials was performed to evaluate the crack propagation energy of 13 different granite type rock materials under the impact load condition. Additionally, crack propagation energies of the granite materials were determined under the static load condition to compare the results with those of the Charpy impact test. The energy levels measured from static load tests were significantly lower than those obtained from the dynamic load test that the ratio of energy level under the dynamic loading to energy level under static loading condition was measured to change between 39 and 200 for different 13 type of granite materials tested in this study. The crack propagation time for the chevron-notched specimens under static loading was also measured using professional sound recording systems. As results of this study have not indicated that the crack propagation speed and energy values measured from different granite materials have a direct relationship, energy-dependent crack propagation speed was found to be an inherent property of rock materials. The Charpy impact test was assessed usable for being a sensitive crack propagation energy determination method for rock materials. In the context of improvement of the Charpy impact test for rock materials, some issues were pointed out in this study

Experimental determining of fracture behaviour of P460NL1 steel welded joint specimens

This paper is focused on the experimental measuring of impact energy as a part of the doctoral dissertation involving the fatigue behaviour of welded joints, made of fine grain normalised micro-alloyed low carbon high strength steel P460NL1 [1,2], typically used for pressure vessels working at subzero temperatures. VAC 65 [3] was used as filler material, and two plates were welded using the MAG welding procedure (with 82% Ar + 18% CO2 shielding gas).The specimens were taken from two opposite ends of the welded plate, taking into account the measured values of groove edge temperature in these locations, wherein the temperatures in Location 2 (near the end of the weld) were higher than those in Location 1 (near the weld’s beginning). Based on their specific position, in terms of temperature and notch position (weld root or face), the specimens were divided into four groups of three. This testing was performed using a SCHENCK-TREBEL instrumented Charpy pendulum. The total impact energy was determined, along with its components, crack initiation and crack propagation energy.The obtained results have confirmed that the test specimens were of high toughness, with total impact energy ranging from 165-200 J, at room temperature. The crack propagation energy was the dominant component, being several times greater than the crack initiation energy, as was expected.

Crack dynamics and crack tip shielding in a material containing pores analysed by a phase field method

Many naturally occurring materials, such as wood and bone, have intricate porous micro-structures and high stiffness and toughness to density ratios. Here, the influence of pores in a material on crack dynamics in brittle fracture is investigated. A dynamic phase field finite element model is used to study the effects of pores with respect to crack path, crack propagation velocity and energy release rate in a strip specimen geometry with circular pores. Four different ordered pore distributions are considered, as well as randomly distributed pores. The results show that the crack is attracted by the pores; this attraction is stronger when there is more energy available for crack growth. Crack propagation through pores also enables higher crack propagation velocities than are normally seen in strip specimens without pores (i.e. homogeneous material), without a corresponding increase in energy release rate. It is further noticed that as the porosity of an initially solid material increases, the crack tip is increasingly likely to become shielded or arrested, which may be a key to the high relative strength often exhibited by naturally occurring porous materials. We also find that when a pore is of the same size as the characteristic internal length then the pore does not localise damage. Since the characteristic internal length only regularises the damage field and not the strain end kinetic energy distributions, crack dynamics are still affected by small pores.

Microstructural evolution and its influence on toughness in simulated inter-critical heat affected zone of large thickness bainitic steel

Simulated inter-critical heat affected zones (ICHAZ) of a large thickness bainitic steel were prepared with three different peak temperatures (Tp) of 750 °C, 780 °C and 800 °C. The microstructure and crystallographic feature were investigated. Toughness of simulated ICHAZ specimens was assessed using instrumental Charpy impact test at − 40 °C. It was found that the volume fraction of newly formed bainite increased and the effective grain size decreased with the increase in inter-critical Tp. The toughness decreased slightly with the Tp of 750 °C compared with the base metal (BM), while the toughness improved sharply when the Tp exceeded 780 °C. The network-like martensite was primarily responsible for the low crack initiation energy with the Tp of 750 °C. In contrast, the fine bainite formed with the Tp above 780 °C was effective in increasing the crack nucleation resistance, resulting in the high initiation energy. The matrix softening as well as the grain refinement played important roles in improving the crack propagation energy. Furthermore, it was confirmed that the presence of a multi-phase microstructure can lead to the scattering of toughness.

Improvement of Strength and Impact Toughness for Cold-Worked Austenitic Stainless Steels Using a Surface-Cracking Technique

For cryogenic applications, materials must be cautiously selected because of a drastic degradation in the mechanical properties of materials when they are exposed to very low temperatures. We have developed a new technique using a cold-working and surface-cracking process to overcome such degradation of mechanical properties at low temperatures. This technique intentionally induced surface-cracks in cold-worked austenitic stainless steels and resulted in a significant increase in both strength and fracture at low temperatures. According to the microstructure observations, dissipation of the crack propagation energy with surface-cracks enhanced the impact toughness, showing a ductile fracture mode in even the cryogenic temperature region. In particular, we obtained the high strength and toughness materials by a surface-cracking technique at 5% cold-worked specimen with surface-cracks.