Ductile Fracture Behavior Research Articles

High Temperature Mechanical Properties of a Vented Ti-6Al-4V Honeycomb Sandwich Panel.

For aerospace applications, honeycomb sandwich panels may have small perforations on the cell walls of the honeycomb core to equilibrate the internal core pressure with external gas pressure, which prevent face-sheet/core debonding due to pressure build-up at high temperature. We propose a new form of perforation on the cell walls of honeycomb sandwich panels to reduce the influence of the perforations on the cell walls on the mechanical properties. In this paper, the high temperature mechanical properties of a new vented Ti-6Al-4V honeycomb sandwich panel were investigated. A vented Ti-6AL-4V honeycomb sandwich panel with 35Ti-35Zr-15Cu-15Ni as the filler alloy was manufactured by high-temperature brazing. The element distribution of the brazed joints was examined by means of SEM (scanning electron microscopy) and EDS (energy-dispersive spectroscopy) analyses. Compared to the interaction between the face-sheets and the brazing filler, the diffusion and reaction between the honeycomb core and the brazing filler were stronger. The flatwise compression and flexural mechanical properties of the vented honeycomb sandwich panels were investigated at 20, 160, 300, and 440 °C, respectively. The flatwise compression strength, elastic modulus, and the flexural strength of the vented honeycomb sandwich panels decreased with the increase of temperature. Moreover, the flexural strength of the L-direction sandwich panels was larger than that of the W-direction sandwich panels at the same temperature. More importantly, the vented honeycomb sandwich panels exhibited good compression performance similar to the unvented honeycomb sandwich panels, and the open holes on the cell walls have no negative effect on the compression performance of the honeycomb sandwich panels in these conditions. The damage morphology observed by SEM revealed that the face-sheets and the brazing zone show ductile and brittle fracture behaviors, respectively.

3D Printing of Well Dispersed Electrospun PLGA Fiber Toughened Calcium Phosphate Scaffolds for Osteoanagenesis

Although the toughening of Calcium phosphate (CaP) scaffold by the addition of fiber has been well recognized, integrated mechanical, structural and functional considerations have been neglected in the design and fabrication of CaP scaffold implant. The emerging 3D printing provides a promising technique to construct CaP scaffold with precise size and elaborate microstructure. However, the most challenge is to extrude smoothly the CaP paste containing fibers for frequently-used extrusion-based 3D printing. In this study, frozen section and chemical dispersant (Pluronic F127, F127) were employed jointly to prepare non-aggregated polylactic-co-glycolic acid (PLGA) fibers. The injectability of CaP pastes with well dispersed PLGA fibers was more than 90% when the content of PLGA fibers was no more than 3 wt%. Meanwhile rheological property of CaP pastes with well dispersed fibers showed shear thinning, which were both beneficial to extrude CaP paste with well dispersed fibers for 3D printing. Moreover, these CaP scaffolds showed ductile fracture behavior due to the pullout and bridging effect of PLGA fibers. The cell proliferation and alkaline phosphatase (ALP) activity indicated that 3D printed CaP scaffold containing PLGA fibers possesses excellent biocompatibility and facilitate osteogenic differentiation ability. Thus, it was feasible to print CaP pastes with well dispersed PLGA fibers to construct toughening CaP scaffolds with the higher shape fidelity and complex structures, which had significant clinical potentials in osteoanagenesis due to their higher toughness and excellent biocompatibility.

Effect of Rapid Quenching on the Solidification Microstructure, Tensile Properties and Fracture of Secondary Hypereutectic Al-18%Si-2%Cu Alloy

In this work, the effect of rapid quenching from the partially liquid and solid condition is studied on the as-cast microstructure, tensile properties and fracture features of a secondary hypereutectic Al-18%Si-2%Cu alloy. For comparison purposes, the same ingots of 50 mm in diameter and 300 mm in height were also fabricated using conventional chill casting. The microstructure of the samples was subjected to detailed characterisation using scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDX) analyses. The tensile properties and fracture were also evaluated. A significant grain refinement of the eutectic constituents (α (Al) + Si), as well as primary silicon and intermetallics accompanying coarse plate and Chinese script with a well-dispersed morphology transition for the intermetallics, was observed in the alloy subjected to the rapid quenching. The connection between primary and eutectic silicon was shown metallographically, confirming the fact that primary silicon served as a nuclei site for eutectic silicon. The microstructure refinement, together with a favourable morphology transition, resulted in greatly enhanced tensile properties and the more ductile fracture behaviour of the studied alloy.

Dynamic behavior and microstructure characterization of TaNbHfZrTi high-entropy alloy at a wide range of strain rates and temperatures

The TaNbHfZrTi refractory high-entropy alloy (RHEA) is suction casted by vacuum arc melting furnace. The mechanical behavior of the TaNbHfZrTi RHEA is systematically investigated through material testing machine and Split Hopkinson Pressure Bar (SHPB) system at strain rate up to 2600s-1 and temperatures from 77K to 873K. Significant increases in the yield strength and interesting changes in strain rate sensitivity at high strain rates and high temperatures are observed. During the dynamic tests, the strain rate sensitivity rises from ~5 to ~76 with the temperature increases from 293K to 673K, demonstrating that high temperature has a significant influence on the strain rate effect of the TaNbHfZrTi RHEA. The comprehensive analysis of electron backscatter diffraction (EBSD) and Kernel Average Misorientation (KAM) technique shows that the pronounced homogeneous deformation microstructures exist in the sample even at the true strain of about 0.34, only very short adiabatic shear band (ASB) initiates. The fracture morphology investigated by scanning electron microscopy (SEM) indicates both ductile and brittle fracture behavior of the TaNbHfZrTi RHEA at 77K. The improved Johnson-Cook (J-C) model is proposed to describe the deformation behavior, in which both strain rate hardening and temperature softening terms are expressed as a function of strain and strain rate, respectively.

Fundamental differences between plane strain bending and far-field plane strain tension in ductile plate failure

The objective of this paper is to investigate the ductile fracture behavior of thin metal plates under nominally plane strain conditions, with an emphasis on the differences between far-field plane strain tension and plane strain bending. Recent experimental results show that the fracture strain obtained using far-field plane strain tension tests is significantly lower than that obtained under plane strain bending conditions. Investigating the source of these differences is of critical importance for multiple industries since it affects formability, crashworthiness and ductile failure of thin metal plates. The present work provides critical insights into this issue based on the results of a detailed numerical investigation of both tests to failure, using the micro-mechanics based Gurson material model. The numerical simulations reveal many similarities between plane strain bending and far-field plane strain tension until the onset of localization. In the far-field plane strain tension case, after Considère thinning, a through-thickness shear localization occurs, followed by a micro-crack formation in the center of the plate. This is accompanied by an increase of the stress triaxiality well above the nominal plane strain tension level (i.e., ~ 0.57 for thin plates). In contrast, the absence of thinning in plane strain bending yields a stress triaxiality that equals nominal plane strain tension, which results in a significantly lower void growth rate (in reference to far-field plane strain tension). A surface instability (undulations) eventually forms on the tension side of the plate in plane strain bending, followed by multiple shear bands. The crack initiates inside the dominant shear band and propagates from the surface into the plate. These differences in the localization mechanisms and local stress states are responsible for the difference in the macroscopic fracture strain obtained under far-field plane strain tension and plane strain bending.

Ductile fracture behavior and flow stress modeling of 17-4PH martensitic stainless steel in tensile deformation at high temperature

The ductile fracture behavior of 17-4PH martensitic stainless steel at high temperature was investigated based on the experimental data of hot uniaxial tensile test. The elongation and reduction of area indicated that the ductility was mainly enhanced during the deformation at the temperature 1150 °C–1200 °C with high strain rate more than 0.1 s−1. The hot flow stress curves were obtained from the tensile experiments, and the true strain of necking initiation was determined by analyzing the contours of fractured specimens. Then, the true stress-true strain data of the deformations before necking were adopted to establish the tensile flow stress model. In order to express the combined influences of the strain, strain rate and temperature on flow stress, a modified Johnson-Cook model was proposed to describe the hot constitutive behavior of the presented steel. It indicates that reasonable agreements between the model-predicted results and the experimental data were achieved.

Shear behaviors and failure mechanisms of 2D C/SiC pins prepared by chemical vapor infiltration

Connecting and fastening large-scale all-C/SiC airfoils to the fuselage of hypersonic vehicle requires C/SiC fasteners of high-performances under the re-entry environment. In this paper, 2D C/SiC pins prepared by chemical vapor infiltration (CVI) were sheared off to demonstrate their strength sensitivity to the nonuniform microstructures, such as porosity and ply orientation. Results showed that their average shear strengths increase linearly from 85.8 to 134.8 MPa as their average total porosity decreases from 23.2% to 14.5%. The shear strengths of 2D C/SiC pins are almost the same as the in-plane shear strengths of 2D C/SiC composites, under the condition that both have identical total porosity. In contrast, the variances of the pin shear strength are much higher than the in-plane shear strength, which is mainly ascribe to the effects of the loading angle between the shear plane and the ply orientation. The porosity of the 2D C/SiC pin is the main microstructural factor influencing the pin shear strength. Matrix shear cracking, debonding between matrix and fibers, crack deflection along fibers and interlayer sliding are the main shear failure mechanisms. Due to the secondary bending deformation, the interlayer sliding and the corresponding fiber pull-out mechanisms dominate the final ductile fracture behavior.

On the prediction of ductile fracture in ship structures with shell elements at low temperatures

Abstract This paper proposes a practical method for addressing the effects of low temperatures on the ductile fracture behavior of high tensile strength steel plates and modeling using large shell finite elements. The test data of marine high-tensile strength steel grades, DH32, DH36, and EH36, at sub-zero temperatures were utilized. The tensile tests at sub-zero temperatures were simulated using fine solid elements to identify hardening law parameters. Simple localized necking loci were derived using the hardening curves of the steel grades considered. By defining the onset of localized necking as a failure condition, a simple fracture model for large shell finite elements that consider the low temperature effect was proposed. Several case studies were included to demonstrate the application and limitations of the proposed model.

Influence of microstructure on elastocaloric and shape memory effects in Mn50Ni32Sn7Co11 alloys

Abstract Magnetic shape memory alloys have received much attention due to their intriguing physical and multifunctional properties, including magnetostrain, magnetoresistance, elastocaloric effect (eCE), and shape memory effect (SME). In order to develop improved materials, it is necessary to study the effect of microstructure on the material properties in greater depth. In this work, Mn50Ni32Sn7Co11 bulk alloys were obtained by copper mold suction casting, directional solidification, and quenching with different cooling rates. The effects of cooling rate on the microstructure and the corresponding eCE/SME were investigated. It was found that the content of γ phase (fcc structure) increased and martensitic transformation temperature decreased with decrease in the cooling rate. The solidification process of γ phase and the matrix phase was determined by studying the solid-liquid interface. Mn–Ni–Sn–Co alloys with γ phase exhibited ductile fracture behavior and better mechanical properties. The adiabatic temperature change was −2.6 K upon unloading. Moreover, shape memory behavior of as-quenched specimen was investigated by in situ digital image correlation technique in non-oriented polycrystalline materials. It was found that heterogeneous strain distribution reduced the SME. Therefore, this work provides a novel strategy for designing magnetic shape memory alloys with better ductility and multi-functional properties.

Microstructure and mechanical properties of low power pulsed Nd:YAG laser welded S700MC steel

Thermomechanically controlled processed steels have gained attention increasingly by many industries. In this research S700MC steel is successfully welded using low power pulsed Nd:YAG laser and the microstructure and mechanical properties are investigated. It is shown that the average power and overlapping factor both affect the weld geometry. Full penetration with double-sided welding achieved on 2 mm thick plates autogenously. Optical metallographic methods and SEM/EDS were used to evaluate the resulting microstructures. The evaluations revealed that the weld metal microstructure contains different morphologies of ferrite such as acicular, allotriomorphic and Widmanstatten as well as bainaite and martensite structure in the weld zone. Also, no noticeable heat affected zone was detected near the fusion zone of the weldments. In addition to microstructures investigation, micro hardness and tensile tests were performed to evaluate mechanical properties. Hardness measurement results exhibit higher hardness values in weld zone than that of in the base metal. The tensile test revealed a ductile fracture behavior which happened in the base metal, due to proper weld zone microstructure. The strength and elongation of the prepared joints were 774 ± 14 MPa and 26.5 ± 2.5%, respectively.

Numerical Investigation of Strength Mismatch Effect on Ductile Crack Growth Resistance in Welding Pipe

The effect of strength mismatch (ratio between the yield stress of weld metal and base metal, My) on the ductile crack growth resistance of welding pipe was numerically analyzed. The ductile fracture behavior of welding pipe was determined while using the single edge notched bending (SENB) and single edge notched tension (SENT) specimens, as well as axisymmetric models of circumferentially cracked pipes for comparison. Crack growth resistance curves (as denoted by crack tip opening displacement-resistance (CTOD-R curve) have been computed using the complete Gurson model. A so-called CTOD-Q-M formulation was proposed to calculate the weld mismatch constraint M. It has been shown that the fracture resistance curves significantly increase with the increase of the mismatch ratio. As for SENT and pipe, the larger My causes the lower mismatch constraint M, which leads to the higher fracture toughness and crack growth resistance curves. When compared with the standard SENB, the SENT specimen and the cracked pipe have a more similar fracture resistance behavior. The results present grounds for justification of usage of SENT specimens in fracture assessment of welding cracked pipes as an alternative to the traditional conservative SENB specimens.

Prediction of crack formation in the progressive folding of square tubes during dynamic axial crushing

Experimental and numerical investigations at both lab and structure scales on the plasticity and failure behavior of an automotive dual-phase steel sheet (DP1000) are performed in this study. On the lab level, an extensive experimental program considering the temperature and strain rate effects on the plastic deformation behavior and the stress state dependency of the ductile fracture behavior is designed. On the structure level, dynamic square tube crushing tests are performed on a drop tower. The extended hybrid damage mechanics model is formulated in the study to describe the deformation and damage/fracture behavior of DP1000 considering the influence of temperatures, strain rates, and loading histroy. The plasticity and fracture description at the lab scale is used to calibrate the material parameters of the model, which is further applied to predict the crashworthiness of the square tube crushing tests. The proposed model has been proven to show very good predictive capability at both lab and structure scales. It is further found that for the modern high-strength steels short cracks could be developed in the tube crushing tests and the crack formation mechanics is shearing and local second-level bending caused by the self-contact between folds.

Investigation on the ductile fracture of high-strength pipeline steels using a partial anisotropic damage mechanics model

A hybrid experimental and numerical investigation has been conducted to comprehensively characterize the anisotropic plasticity and ductile fracture behavior of a high-strength pipeline steel. Tensile tests have been performed on various flat specimens along three different loading directions to collect the experimental mechanical data covering a wide range of stress states. For numercial modeling, the anisotropic plastic deformation is described by the evolving non-associated Hill48 (enHill48) plasticity model considering anisotropic/distortional hardening and evolution of r-value. Based on the enHill48 model, in this study, an anisotropic damage mechanics model with consideration of the evolution of anisotropy and stress states has been proposed and calibrated to predict the anisotropic damage and fracture of the investigated material. It is concluded that the anisotropic hardening is critical for an accurate prediction of the ductile fracture. The proposed model has achieved good predictive capability for anisotropic fracture behavior.

Numerical modelling of dynamic ductile fracture propagation in different lab-scale experiments using GTN damage model

Initiation and propagation of ductile fractures are a major consideration during the design of high-pressure pipelines. Consequences of a pipeline failure can be catastrophic thus structural integrity must be ensured over several decades. Traditional lab-scale experiments such as the Charpy V-Notch (CVN) and Drop Weight Tear Test (DWTT), impact experiments on a notched three-point bending sample, are widely used to measure the fracture toughness of a material. However, with increasing wall thickness and the transition to high-grade steels in the pipeline industry, the size-effect of the specimen and inverse fracture became prominent issues. A new testing methodology called the Dynamic Tensile Tear Test (DT3) is currently investigated as to address the issues presented by the current state of the art. In this study, a numerical investigation is conducted on the CVN, DWTT and DT3 experiments to compare the modelling of dynamic ductile fracture propagation in three different testing scales using the Gurson-Tvergaard-Needleman (GTN) damage model. X70 and X100 pipeline steel grades are used to model material behaviour. For each considered lab-scale experiment, the dynamic ductile fracture behaviour was successfully reproduced using the GTN damage model.

DUCTILE FRACTURE BEHAVIOR OF WELD METAL FOR Q345 STEEL UNDER MONOTONIC LOADING

To study the ductile fracture behavior of weld metal for Q345 steel, nine specimens with different distribution of stress triaxiality and Lode angle were tested under monotonic loading. Swift, Voce and mixed swift-voce hardening models were used to fit the full stress-strain curves up to fracture. in comparison, the mixed Swift-Voce hardening model showed the best accuracy in simulating the load-displacement curves of the specimens. In addition, three kinds of fracture prediction models, namely, VGM, modified SWDM and Lou models, were utilized. The material parameters were calibrated by the UVARM subroutine and applied to the finite element numerical simulations. The accuracy of these prediction models were compared and analyzed. The results showed that the VGM model presented a relatively large error in predicting the fracture of rectangular notches and grooved plate specimens when they were near the plane strain state. However, the modified SWDM and Lou models produced accurate predictions when the Lode angle were considered to describe the deviatoric stress state. Based on the fracture prediction results, the modified SWDM and Lou models were effective for specimens of various stress states.

Microstructure and Mechanical Properties of Carbon/Carbon Composites Infiltrated with Ti–6Al–4V Titanium Alloy

In this work, chemical vapor infiltration (CVI) was combined with reactive melt infiltration (RMI) using Ti–6Al–4V titanium alloy powder to prepare Cf/C–TiC composites. The microstructure and composition of Cf/C–TiC composites were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The flexural properties of the composites were also analyzed. The results indicated that the Ti–6Al–4V titanium alloy infiltrated the Cf/C preform and reacted with the pyrolytic carbon (PyC) to form a TiC–VC and Al4C3 matrix, and no residual Ti, Al, or V was detected. Moreover, Al4C3 was concentrated and independently distributed, whereas Ti and V reacted with C to form a TiC–VC solid solution. The porosity was 6.75%, and the bulk density of Cf/C–TiC was 1.96 g/cm3. The flexural strength, flexural modulus, and failure strains were 256 ± 18 MPa, 89 ± 9 GPa, and 0.93 ± 0.13%, respectively. The work of fracture of the Cf/C–TiC composite was about 6.8 ± 0.38 KJ/m2. Due to the propagation and deflection of cracks, as well as debonding and fiber pullout, the Cf/C–TiC composite showed ductile fracture behavior.

Temperature Dependence of Plastic Flow, Anisotropy and Ductile Fracture

As one of the most common extrinsic features, temperature has an essential impact on the mechanical properties as well as the controlling mechanisms of materials. Therefore, it is aimed in this study to investigate the effect of temperature on the mechanical properties of several steels in an extensive temperature range from -150 °C to 300 °C. Three basic and essential mechanical properties are considered, the plastic flow behavior, anisotropy, and ductile fracture behavior. The temperature influence on plastic flow behavior has been well studied in the literature. With this broad temperature range in the current study, we intend to create a large database to characterize the material behavior as well as to reveal the deformation mechanism change from low to high temperatures. Except for the typical dislocation slipping, the other possible involved mechanisms are twinning and dynamic strain aging. In addition to this, we are particularly focusing on the temperature effect on anisotropy, which has been not covered yet by the existing literature. Based on the experimental results for two pipeline steels, it is shown that the temperature effect on anisotropy could be negligible or significant, depending on the materials. There have been also many investigations of the effect of strain rates on the ductile fracture; however, the temperature effect has not received obvious attention yet. In this study, we set the scope to quasi-static loading condition but vary the loading temperature from -50 °C to 300 °C. The stress-state influence under these temperatures is also considered by employing various sample geometries. Two automotive steel sheets are employed for this study and the results show that a strong temperature effect on the ductile fracture exists for the elevated temperature mainly due to the dynamic strain aging effect, while the less pronounced but non-negligible effect of the temperature is revealed for the lowered temperatures.

Hybrid Approach to Describe and Understand the Influence of Inclusions on the Transition Behavior of X65 Pipeline Steel

Within the last decade, the European steel industry has made impressive progress in providing pipeline steels with excellent mechanical properties. Continuous improvement of cleanness and tailored thermomechanical treatments lead to damage tolerant microstructures. These steels offer high strength as well as impressive impact toughness properties with upper shelf values of more than 250 J. However, some of these steels show special fracture phenomena such as inverse fracture, seperations and a contracted transition area. The last of the three mentioned phenomena is characterized by the simultanious occurrence of upper and lower shelf values at the same testing temperature during charpy impact test. This unusual toughness behavior subsequentelly leads to difficulties in the component safety assessment and needs to be fundamentally understood. In this study, a combined experimental and numerical method is developed to investigate the microstructural reasons of such a fracture behavior. The methodology is demonstrated on a X65 pipeline steel that features a bainitic microstructure and shows a very narrow transition behavior. Following the characterization of the mechanical property profiles of the pipeline steel by different experiments on the meso-scale (e.g. Charpy impact test), the underlying fracture mechanisms were studied. It can be noted that inclusions are identified to be crack initiating for both ductile and brittle fracture behavior. In order to be able to understand the influence of the shape and location of the different inclusions on the fracture behavior, representative volume element (RVE) simulations were carried out. Through the integration of a crystal plasticity (CP) model, the material behavior is simulated at microstructure level and inclusion-related stress concentrations can be discussed. In the future, this approach can be applied to better understand microstructure-related phenomena as well as to couple scale-bridging simulation methods for a material-oriented component design.

Evaluating the Resistance Performance of the VAEPC and the PAFRC Composites against a Low‐Velocity Impact in Varying Temperature

This paper aims to evaluate the resistance performance of the vinyl acetate ethylene polymer cement (VAEPC) composite and the polyvinyl alcohol fiber‐reinforced cement (PAFRC) composite against a low‐velocity impact in varying temperature. Their impact resistance performances are analyzed and compared with plain mortar after 28 days of age. Low‐velocity impact tests were carried out under the various room temperatures of −70°C, 70°C, and 140°C. Also, an INSTRON CEAST 9350 drop‐tower system has been used to get the impact load, fracture energy, and displacement of the specimens while loading low‐velocity impacts. From these tests, the failure pattern, shape, and strength of each test specimen were evaluated for the VAEPC, the PAFRC composite, and the plain mortar. Those test results showed that the flexural strength of both the VAEPC and the PAFRC composites has increased compared to that of the plain mortar. However, the compressive strength of the PAFRC composite decreased slightly after 28 days, while its flexural strength increased by 24.4% compared to that of the plain mortar. In addition, the drop test results show that PAFRC composite specimens have the highest impact fracture energy compared to other specimens at −70°C, 70°C, and 140°C, whereas plain mortar specimens have their lowest. This is because the PVA fiber included in the PAFRC acts as a bridge to suppress crack propagation and to improve energy absorption performance, which helps it resist relatively better against impact. It is also known that while the VAEPC composite and the plain mortar were destroyed in a form of being perforated, the specimens of PAFRC composite were observed to some extent to suppress the perforation failures. Therefore, under a load of low‐velocity impact, the resistance performance of the VAEPC composite and the plain mortar was proven to show brittle fracture behavior, while the PAFRC showed ductile fracture behavior in virtue of PVA fiber reinforcement which improved its flexural performance. According to the SEM observation which followed the tests, the PAFRC composite as a fiber‐reinforced material of the hydrophilic material was found to show the most excellent interfacial bond adhesion compared to the other composite and the plain mortar. The PAFRC composite manufactured in the study has been proven to be very useful as a reinforcement material in both high and low temperature environments.

Influence of Temperature and Strain Rate on Microstructure and Fracture Mechanism of Mild Steel

This paper presents the results of experimental research about the influence of temperature and strain rate on microstructure and fracture mechanism of C25 mild commercial steel. This type of material has applicability in the construction of machines domain (for example: shafts and gears).In the paper, there were done the following experimental determinations: hot tensile stress tests were investigated over wide ranges of forming temperature and strain rate; hardness tests and fractographic examination were carried out by scanning electron microscopy. According to the results obtained, the flow stress increases with the strain rates at constant temperature, because of the dynamic recovery and dynamic recrystallization have enough time to complete at low strain rate. The appearance of necking and its location induce the ductile fracture behavior, the voids was generated by the intercrystalline fracture highlighted by images made with scanning electron microscope (SEM).