Crack Initiation Energy Research Articles

Assessment of Dimensional Stability, Biodegradability, and Fracture Energy of Bio-Composites Reinforced with Novel Pine Cone.

In this investigation, biodegradable composites were fabricated with polycaprolactone (PCL) matrix reinforced with pine cone powder (15%, 30%, and 45% by weight) and compatibilized with graphite powder (0%, 5%, 10%, and 15% by weight) in polycaprolactone matrix by compression molding technique. The samples were prepared as per ASTM standard and tested for dimensional stability, biodegradability, and fracture energy with scanning electron micrographs. Water-absorption and thickness-swelling were performed to examine the dimensional stability and tests were performed at 23 °C and 50% humidity. Results revealed that the composites with 15 wt % of pine cone powder (PCP) have shown higher dimensional stability as compared to other composites. Bio-composites containing 15–45 wt % of PCP with low graphite content have shown higher disintegration rate than neat PCL. Fracture energy for crack initiation in bio-composites was increased by 68% with 30% PCP. Scanning electron microscopy (SEM) of the composites have shown evenly-distributed PCP particles throughout PCL-matrix at significantly high-degrees or quantities of reinforcing.

Microstructure and Fracture Characteristics of Heat‐Affected Zone in Shipbuilding Steel Plates with Mg Deoxidation after High Heat Input Welding

To clarify the microstructure and fracture characteristics of heat‐affected zone in shipbuilding steel plates with Mg deoxidation after high heat input welding, two steels with 0.0002 (2Mg) and 0.0049 (49Mg) wt% Mg are prepared and investigated with metallography and crystallography. Compared with 2Mg, TiN particles are incrementally refined in 49Mg, making the prior austenite grain (PAG) size decrease. In the large‐sized grains of 2Mg, there are preferentially formed coarse grain boundary ferrites (GBFs) and ferrite side plates (FSPs), followed by intragranular bainite ferrites (IBFs). The small PAGs of 49Mg make fine‐grained polygonal ferrites (PFs) dominate the microstructure. With Mg addition, the effective grain with θ > 50° refines and the high‐angle grain boundary density increases. The brittle transgranular fracture occurs around little‐deformed coarse GBFs, FSPs, and IBFs with θ < 15° in 2Mg, showing the weak crack resistance. However, in 49Mg, the ductile fracture occurs around fine PFs and IAFs with large plastic deformation. Although there is transgranular fracture in coarse PFs, it can be soon deflected and arrested by small‐sized PFs with θ > 50° in 49Mg. Thus, for 49Mg, the crack initiation and propagation energies increase with the toughness at −40 °C increasing from 20 to 167 J.

Analysis of fracture energy in drop weight tear testing of API X65 gas pipeline steel

In this research, the effect of low velocity impact (LVI) on different components of fracture energy of API X65 gas pipeline steel is investigated using drop weight tear test (DWTT) specimens. For this purpose, twenty one test specimens (arranged in seven similar test groups) were used. The LVI tests were carried out first on six test groups by releasing the machine hammer from different heights from 0.05 m to 0.3 m. Then, full fracture DWTTs were conducted on all seven test groups using 2 m drop height. The fracture energy of tested specimens and its different components were calculated from test data using an instrumented hammer. These were compared to similar energies of virgin test specimens. It was found that by increasing LVI energy, yield energy increased up to 26% due to work hardening. However, crack initiation energy decreased significantly (down to 48%) due to plastic deformation at crack tip and crack tip blunting. By considering energy consumption, a novel test technique was presented for exact determination of crack initiation point.

Mechanism study of preventing crack propagation of fractured rockunder dynamic loads

To deeply understanding dynamic fracture properties and preventing crack propagation of fractured rock mass under dynamic loads, impact experiments were conducted using TWSRC (tunnel with single radial crack) samples, and sandstone were selected as the raw material to manufacture fractured rock samples. The crack initiation, propagation and obstructing behavior were measured by using a drop hammer impact test device and crack propagation gauge measuring system. The mechanism of preventing crack propagation and failure behavior during dynamic fracturing process was focused, and then the corresponding numerical simulation was conducted by using the finite difference code, which can be used to accurately estimate the experiment result. The results indicate that the whole dynamic fracturing process of fractured rock under dynamic loads is composed of the cyclic process of crack initiation, high-speed crack propagation, slowly deceleration, preventing crack propagation. In addition, the period of crack obstruction was approximate the microsecond level. The ratio of transgranular (TG) fracture at the crack obstruction point of fractured rock was smaller than that of the crack initiation point, and the ratio of TG fracture of green sandstone during the dynamic fracturing process was larger than that of black sandstone. The fracture energy for crack initiation again after crack obstruction was much less than the fracture energy required for the initial initiation of pre-existing crack.

Study on the intrinsic factors determining impact toughness of TC21 alloy

Ti-6Al-3Mo-2Sn-2Zr-2Nb-1.5Cr-0.1Si (TC21) alloy exhibits high specific strength and superior fracture toughness, making it attractive in the aerospace field. However, its fracture resistance under impact loading was not fully understood. Therefore, the impact toughness of TC21 alloy with various microstructures was studied using instrumented Charpy impact testing. Combining microstructure characterization, crystallographic orientation analysis, nanoindentation test, fractography observations with analysis of kernel average misorientation (KAM) distribution near crack path, the heterogeneity of deformation at small length scales, as well as crack initiation and propagation mechanisms were revealed. Analysis of load-displacement curves indicated that the crack initiation energy accounts for most of the total impact energy. The crack initiation energy is proportional to the degree of plastic deformation of the microstructures in the crack initiation region. The thick lamellar structure either in bimodal or fully lamellar structure can generate large plastic deformation, thus promote the crack initiation energy. In contrast, the KAM value in crack propagation region was much lower than that in crack initiation region, suggesting that impact energy consumed by plastic deformation during crack propagation was lower than during crack initiation. The crack propagation energy increased with the increasing degree of the tortuosity of propagation path. Especially in fully lamellar microstructure, the high angle α colony boundaries deflected the crack propagation direction and thus increased the crack propagation energy

Microstructure tailoring and impact toughness of a newly developed high strength Ti-5Al-3Mo-3V-2Cr-2Zr-1Nb-1Fe alloy

To achieve preferable impact performance of a newly designed high strength Ti-5321 alloy, samples with three kinds of microstructures, i.e., bimodal microstructure (BM), tri-modal microstructure (TM) and lamellar microstructure (LM) were prepared for instrument Charpy tests at room temperature. Results of impact tests indicated that LM presented the higher impact toughness of 37.5 J/cm2 than that of BM (11.25 J/cm2) and TM (25 J/cm2). Moreover, both crack initiation energy and crack propagation energy of LM were obviously raised. Further analysis containing fracture surfaces and the cross-sectional microstructures of the impact samples by the usage of SEM and EBSD provided insights into the influence of microstructure on crack initiation and propagation during impact process. The largely plastic deformation of primary α near the V-notch tip impeded crack initiation and enhanced the crack initiation energy. The interweaved α colonies with high angle boundaries deflected the crack propagation direction and thus increased the crack propagation energy. Besides, TEM analysis showed that the typical deformation characteristic under impact test with high strain rate. Except for dislocation lines, the pile-up of highly dense dislocations, twinning, dislocation cells, sub-grains, shear bands and fragmentation of α phase can be found. These approaches presented in this study could provide certain insights into fabricating typical microstructure in the high strength titanium alloys to ameliorate impact properties.

Modifying of microstructure and toughness in the weld metal prepared by welding wire containing nanosized titanium oxides

Abstract Titanium oxides are beneficial to the weld metal for their nucleation effect on acicular ferrite. The present study puts forward a new method to introduce fine titanium oxides into the weld metal by using welding wire containing nanosized titanium oxides. In the study, a special way was introduced and it was adopted to prepared solid welding wire containing nanosized titanium oxides. The W1 weld metal was obtained by using this welding wire after multi-layer multi-pass welding; as a contrast, the W2 weld metal was prepared by using conventional welding wire of similar composition under the same welding condition. Microstructures of the two weld metals were studied by OM and SEM; nanosized titanium oxides in the welding wire and inclusions in each weld metal were investigated by SEM and TEM. Toughness of the two weld metals was measured through instrumented impact test; fracture and crack behaviour was analyzed with SEM observation. Through microstructure characterization, it is concluded that nanosized titanium oxides in the welding wire are transitted into the W1 weld metal to form Ti3O5 inclusions and size of the inclusions is larger than the nanosized titanium oxides in the welding wire due to cluster of the titanium oxides in the weld metal. The inclusion size just locates in the size range that is favorable for acicular ferrite nucleation. Through comprehensive analysis of the instrumented impact test, fracture and secondary crack, it is revealed that W1 weld metal shows higher crack initiation energy, higher crack propagation energy and higher impact toughness compared with the W2 weld metal, which results from finer inclusions in the W1 weld metal prepared by the special way.

Analysis of SA 387 Gr. 91 welded joints crack resistance under static and impact load

This paper presents the results of experimental testing of crack resistance of specimens taken from a welded plate made of steel SA-387 Gr. 91, while taking into account different filler materials and welding procedures that were used. This type of steels is typically used in pressure vessels, pipelines and gas installations in the chemical and petrochemical industry, as well as in thermal installations. Since it is operating under extreme conditions, which include elevated temperatures and/or corrosion, the mechanical properties of SA-387 Gr. 91 will deteriorate over time, especially its welded joints. For this reason, it is very important to thoroughly analyse the behaviour of welded joints, taking into account the possibility of cracks initiating in any of the three welded joint regions, the parent material (PM), the weld metal (WM) and the heat affected zone (HAZ). This can be achieved by determining the total impact energy of Charpy specimens with V-2 notches in each of the three regions, along with their components, crack initiation and crack propagation energy, as well as by measuring the fracture toughness.

Toughening effects of Mo and Nb addition on impact toughness and crack resistance of titanium alloys

Ti-6Al, Ti-6Al-2Mo and Ti-6Al-3Nb alloys were prepared to investigate the toughening effects of β stabilizers Mo and Nb on impact toughness and crack resistance of titanium alloys. Instrumented Charpy impact tests showed that the total impact absorbed energy of Ti-6Al-2Mo and Ti-6Al-3Nb (∼64 J) were two times higher than that of Ti-6Al (∼30 J), indicating the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys. Analysis of load-displacement curves revealed the similar crack initiation energy of Ti-6Al, Ti-6Al-2Mo and Ti-6Al-3Nb (15.4 J, 16.1 J and 15.0 J, respectively). However, the higher crack propagation energy of Ti-6Al-2Mo and Ti-6Al-3Nb (46.7 J and 48.3 J, respectively) were about three times higher than that of Ti-6Al (14.4 J), indicating the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb. Post-mortem analysis of impact samples demonstrated that the increased dislocation density and deformation twinning were mainly responsible for the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb. Due to the invisibility of dislocation activation and deformation twinning during the Charpy impact process, a mathematical model has been proposed to evaluate the effects of Al, Mo and Nb elements on dislocation mobility based on the Yu Rui-huang electron theory. Addition of Mo and Nb elements significantly improved the dislocation mobility in Ti-6Al-2Mo and Ti-6Al-3Nb compared to that in Ti-6Al alloy. Therefore, more dislocations were activated in Ti-6Al-2Mo and Ti-6Al-3Nb which supplied the larger plastic deformation under impact loading. A dislocation-based model also has been proposed to interpret the nucleation and propagation of deformation twinning under the impact loading. Dislocation pileup at α/β interfaces provided potential sites for nucleation of deformation twinning in Ti-6Al-2Mo and Ti-6Al-3Nb. Furthermore, deformation twinning facilitated the dislocation motion in α grains with hard orientations. The increased dislocation mobility and deformation twinning were responsible for the stronger crack resistance as well as the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys.

Thermal shock resistance properties of refractory castables bonded with a CaO-free binder

The potential of hydratable magnesium carboxylate compounds (HMC) combined with MgO powder as a binder of castables was evaluated. The green mechanical and thermal shock resistance properties of HMC-bonded castables (HMCC) and calcium aluminate cement-bonded castables (CACC) were compared. The green mechanical strength of the HMCC reached the mechanical strength of the CACC. The residual strength ratio of the HMCC is higher than that of the CACC and increases first and then decreases with the MgO/HMC ratio. The residual strength ratio's highest value is 56.8% (MgO/HMC ratio is 2/3), which is 1.8 times that of CACC. Interestingly, the microcracks in the HMCC after thermal shock are radial, while those in CACC are linear. The micromechanical testing on the matrix of castables after thermal shock showed that the ratio of the energy for crack initiation to the work of fracture of the HMCC is lower than that of the CACC, but its fracture toughness is higher than that of the CACC, indicating that the microcracks in the HMCC initiate easily but propagate hardly.

Scale levels of quasi-static and dynamic fracture behavior of Ti-6Al-4V parts built by various additive manufacturing methods

Comparison of the microstructure and the phase composition of the Ti-6Al-4V alloy parts built by various additive manufacturing technologies was carried out by optical and transmission scanning electron microscopy, as well as X-ray diffraction analysis. It was shown that the martensitic αʹ phase, formed due to the melt pool fast cooling rate, determined the accommodation mechanisms of shear deformation of the as-built Ti-6Al-4V AM fabricated samples under uniaxial quasi-static tension and impact bending (and, accordingly, their mechanical properties). The effect of microstructure on impact toughness, ultimate impact strength, as well as the crack initiation and propagation energy was investigated by the impact bending tests with recording impact load diagram (force–displacement graph). It was concluded that impact toughness of the Ti-6Al-4V samples built by wire-feed electron beam additive manufacturing was significantly higher than that of ones manufactured by selective laser melting and electron beam melting of a powder. Matching the mechanical properties and SEM micrographs of the fracture surfaces of the as-built Ti-6Al-4V AM fabricated samples enabled to reveal the crack initiation and propagation mechanisms during their quasi-static and dynamic loading. Discussion of the results was carried out using the concept of scale levels of plastic deformation.

Enhancing mechanical properties of a V + Ti microalloyed bainitic forging steel via tailoring microstructure through controlled forging

To obtain a refined and homogenized microstructure as well as enhanced combination of strength and toughness, the influence of controlled forging on the microstructure and mechanical properties of a Mn–Cr-type bainitic forging steel microalloyed with V + Ti elements was explored. Microstructural characterization shows that the final microstructure of the as-forged steel changed from primarily granular bainite (GB) to primarily lower lath-bainite (LLB) through controlled forging of mainly finish-forging temperature and post-forging cooling rate. The result of physical–chemical phase analysis shows that almost all the added Ti is in the (V,Ti)(C,N) precipitates, while only ~ 7% of the added V is in the precipitates, and there is only a slight increase in the amount of the (V,Ti)(C,N) precipitates with changing forging condition, and their precipitation strengthening is ~ 40 MPa. The tensile properties except the total elongation increase significantly from the GB sample to the LLB sample, and the latter exhibits identical or even higher mechanical properties than conventional quenched and tempered 42CrMo steel except the yield strength. Instrumental impact tests revealed that the crack initiation energy, propagation energy and the ability of arresting crack growth of the LLB sample are significantly higher than those of the GB sample. The reduction and/or even elimination of large blocky M/A, the increase in high-angle boundaries fraction and microstructure refinement are primarily responsible for this enhancement of doubled impact toughness. It is thus suggested that suitable controlled forging is a promising strategy to regulate and control the microstructure and mechanical properties of bainitic forging steel.

Influence of Cooling Time t<sub>8/5</sub> on Impact Toughness of P460NL1 Steel Welded Joints

The results of impact testing of welded joint specimens taken from a welded plate made of P460NL1 steel are presented in this paper, and analysed with regards to the cooling time t8/5, that was previously calculated. The aim was to determine how the cooling times that were measured, some of which were below the minimum required values, affected the toughness, in terms of total impact energy and its components, crack initiation and crack propagation energy. In addition, this analysis included the effects of temperatures measured at the opposite ends of the plate during the welding process, since this had also affected the cooling times for each welding pass. After observing the differences in total, crack initiation and propagation energy between the tested specimens taken from different parts of the welded plate, it was determined that the specimens from the part where the higher temperatures were measured had shown better, more uniform results, whereas the average total impact energy for specimens from both groups were very similar. It was also noticed that the ratio of crack propagation to crack initiation was more favourable (greater) in the case of specimens from the second group (with higher temperature), as the values of crack initiation energy decreased slightly and the crack propagation energy increased.

Evaluation of UV‐induced embrittlement of PE‐HD by Charpy impact test

AbstractThe impact fracture behavior of two common high‐density polyethylene grades for container applications were intensively studied by the instrumented Charpy impact test after well‐defined exposure to UV‐irradiation. Individual stages of the impact event, such as crack initiation and crack propagation energy as well as maximum impact load, were investigated from the recorded load–deflection curves. UV‐induced material property changes were further investigated by infrared spectroscopy, differential scanning calorimetry, and dynamic‐mechanical analysis as well as density measurements. Based on the results of the Charpy impact test, three indicators were identified to describe the extend of photooxidation on high‐density polyethylene: (a) a reduced Charpy impact strength—at least to half of its initial value for a distinctly brittle impact fracture, (b) a marked decrease in the crack propagation contribution to the impact strength, and (c) an increase of the brittle features of the fracture surface.

Characteristics of the mechanical behavior of Ti–6Al–4V multilayer laminate under impact loading

Multilayer laminates comprising thirteen layers of Ti–6Al–4V alloy were produced by diffusion bonding. The influence of the processing conditions on the microstructure and mechanical properties of laminates were determined. The multilayer laminates were tested at room and liquid-nitrogen temperatures under impact bending tests with recording of dynamic loading diagrams. The influence of the interfaces pores on the total energy of fracture, the energy of crack initiation and the energy of crack propagation was analyzed. The impact strength value of the laminate decreased by 1.7 times for the crack divider orientation, and it increased by 14 times for the crack arrester orientation with the relative length of pores increasing from 1 to 30%.

Improvement of fracture toughness of Ti+Nb stabilized microalloyed and interstitial free steels processed through single phase regime control multiaxial forging

Objective of the current study is to enhance the mechanical properties, with a special emphasis on fracture toughness, of Ti + Nb stabilized interstitial free and microalloyed steels through microstructural modification by single-phase controlled multiaxial forging at large cumulative strains. Analysis of fracture toughness was executed through calculating KQ (conditional fracture toughness), Kee (equivalent energy fracture toughness) and J-integral (crack initiation energy) values from single-edge bend test data of the forged specimens. The effect of strain hardening rate and strain hardening exponent on deformation behavior were examined to correlate the yield strength (YS) and uniform elongation. Also, theoretically calculated YS (obtained from analysis of strengthening mechanisms) was correlated well with the experimentally obtained results. The quantitative measurement of grain size, low- and high-angle grain boundaries and their distribution in the deformed state were investigated through EBSD/TEM analysis. Superior combinations of the YS, ductility (%El.) and fracture toughness were obtained through intercritical (α+γ) phase regime (~Ar1) control 15 cycles multiaxially forged (MAFed) microalloyed steel (YS = 1027 MPa, %El. = 8.3% and Kee = 90 MPa√m) and pure α-ferritic region (<Ar1) control 18 cycles MAFed IF steel (YS = 881 MPa, %El. = 11.2% and Kee = 97 MPa√m) specimens. Enhancement of the fracture toughness is ascertained to the formation of uniformly distributed nanosize fragmented cementite (Fe3C) particles (~35 nm size) within the submicron size (~280 nm size) ferritic microstructure in the microalloyed steel; whereas in case of the IF steel, this is attributed to the formation of ultrafine ferrite grain (~320 nm) along with dense dislocation substructures. These dislocation cells and fine substructures as well as nanosize Fe3C could effectively block the crack initiation and propagation and thereby enhance the fracture toughness.

Fracture toughness characteristics of ultrafine grained Nb–Ti stabilized microalloyed and interstitial free steels processed by advanced multiphase control rolling

Aim of the current study is to analyze the fracture toughness values along with other mechanical properties and correlating the microstructures of ultrafine grained (UFG) microalloyed and interstitial free (IF) steels produced through advanced 3-steps control multiphase rolling. The analysis of fracture toughness was carried out through computing KQ (conditional fracture toughness), J-integral (crack initiation energy) and Kee (equivalent energy fracture toughness) values from 3-point bend test data of rolled specimens. Microstructural analysis was performed through transmission electron microscopy (TEM) along with selected area electron diffraction (SAED) and Electron backscatter diffraction (EBSD). The quantitative measurement of low and high angle grain boundaries and their distribution in the deformed state were determined through EBSD analysis. The good combinations of fracture toughness, yield strength (YS) and percent elongation (%El.) (i.e. ductility) were achieved through innovative 3-phase control rolling (microalloyed steel: Kee = 68.9MPa√m, J = 81.4 kJ/m2, YS = 923MPa, %El. = 13.6; IF steel: Kee = 72MPa√m, J = 87.7 kJ/m2, YS = 623Mpa and %El. = 19). This is ascribed to the development of homogeneously distributed submicron size (0.69μm) ferritic + martensitic structure in the microalloyed steel and submicron size (0.83μm) ferritic grains along with high density dislocation substructure in the IF steel. These dislocation cells and substructures could effectively block the crack initiation and propagation. The development of UFG microstructure has been analyzed in the light of deformation induced ferrite transformation (DIFT) and dynamic recrystallization (DRX) mechanisms. Superior fracture toughness of the UFG steels along with better combination of mechanical properties is very demanding for high strength structural applications.

Effect of simulated cooling time on microstructure and toughness of CGHAZ in novel high-strength low-carbon construction steel

Influences of cooling time (welding heat input) on microstructure, impact toughness and the fracture mechanism of the weakest CGHAZ (coarse-grained heat-affected zone) in a novel high-strength low-carbon microalloyed construction steel were studied for the purpose of laying a theoretical foundation for developing welding support technologies. When the cooling time (t8/5) was increased, the microstructure changed from dot shape M-A constituents and lath martensite/bainite to slender and blocky M-A constituents and coarse granular bainite. Accordingly, the impact toughness deteriorated. Large blocky M-A constituents seriously reduced the impact absorbed energy during crack initiation. For coarse bainite, the high-misorientation boundary almost disappeared. Therefore, crack initiation energy determines the cleavage fracture micromechanism of high heat input construction steel.

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.