Tensile Stage Research Articles

In situ digital image correlation study on the mechanical properties of GH4169 at different temperatures

AbstractGH4169 is a precipitation‐strengthened nickel‐based high‐temperature alloy, and its mechanical behavior at different temperatures is of significant importance for practical applications. Here, an in situ study on the mechanical properties and microstructural evolution of the GH4169 alloy at different temperatures was conducted using scanning electron microscopy in combination with digital image correlation. The results demonstrate the existence of a linear relationship between the local average strain and the macroscopic strain. At temperatures of 20 °C, 100 °C, 180 °C, and 260 °C, the local average strain is 1.96, 2.13, 2.26, and 2.30 times the macroscopic strain, respectively, which shows a clear trend. This indicates that the temperature has a significant influence on the plasticity of the alloy. Additionally, the strain is concentrated below the notch, whereby at a macroscopic strain of 1.7 % at 20 °C and 260 °C, the local maximum strain at the notch is 52 % and 83 %, respectively. Furthermore, when the macroscopic strain reaches 2.12 %, the local maximum strain at the notch is 72 % and 103 %, respectively. At this point, cracks start to initiate and gradually propagate. In situ observations show that at the beginning of the tensile plasticity stage, the plastic strain rate in the grains is twice the rate at the grain boundaries; subsequently, both rates tend to stabilize, and a high‐strain zone appears inside the grains and is coordinately transferred to the surrounding grains through the grain boundaries. The final stage of damage of the alloy consists of perforation fracture, and the presence of a large number of cracked carbides at the fracture results in specimen cracking.

Multimodal Non-Destructive In Situ Observation of Crystallinity Changes in High-Density Polyethylene Samples with Relation to Optical Parameters during Tensile Deformation

Abstract: Many non-destructive optical testing methods are currently used for material research, providing various information about material parameters. At RECENDT, a multimodal experimental setup has been designed that combines terahertz (THz) spectroscopy, optical coherence tomography (OCT), infrared (IR), and Raman spectroscopy with a tensile test stage. This setup aims to gather material information such as crystallinity and optical parameters of high-density polyethylene (HDPE) during a tensile test. The setup compares common IR and Raman spectroscopy and the less common optical methods THz and OCT. Complementarity is achieved through different frequency ranges and measurement approaches, resulting in different measured optical material parameters and depths. During tensile testing, HDPE samples with varying crystallinity were analysed, and the determined optical parameters such as refractive index, birefringence, scattering coefficient of decay, and penetration depth can be correlated with the change in crystallinity. These findings demonstrate that the optical methods and their outcomes can be interconnected. With further optimization of the experimental setup, it would be possible to observe the alignment of fibres in fibre composite panels and the stress distribution of polymers effectively. This opens interesting possibilities for polymer characterization in the future, including quality control during moulding processes and material testing.

In-situ investigation of coordinated deformation behavior of Ti-6242S alloy with duplex microstructure

In this work, the coordinated deformation behavior related to slip activation, slip transfer and microcrack growth of Ti-6242S alloy with duplex microstructure was investigated through the in-situ tensile testing. It was found that the deformation process of duplex microstructure can be divided into three stages: slip deformation, the formation of slip band and microcrack nucleation, crack propagation and fracture. And single slip was the dominant slip mode, and multiple slip was the auxiliary slip mode. During the initial tensile stage, the slip lines appearing within colony α were shallower than those within primary α owing to the hindering of α/β interface. The slips penetrated across the entire colony region by straight line. This was because there existed a Burgers orientation relationship between lamellar α and β layer, and the orientation of lamellar α was similar. Meanwhile, the common crystal plane between adjacent grains can still provide good condition for slip transfer when the grain boundary was greater than 15°. Moreover, the slip transfer between adjacent slip systems was more prone to occur when there existed good geometric compatibility and high Schmid factor of exit slip system, which provided coordinated deformation between adjacent grains and avoided stress concentration. However, due to the existence of incompatible deformation, the cracks were generated at the stress concentration areas and then grew along the grain boundary or localized slip band.

In Situ Study of Precipitates' Effect on Grain Deformation Behavior and Mechanical Properties of S31254 Super Austenitic Stainless Steel.

Grain boundary (GB) precipitation-induced cracking is a significant issue for S31254 super austenitic stainless steel during hot working. Investigating the deformation behavior based on precipitate morphology and distribution is essential. In this study, continuous smaller and intermittent larger precipitates were obtained through heat treatments at 950 °C and 1050 °C. The microstructure evolution and mechanical properties influenced by precipitates were experimentally investigated using an in situ tensile stage inside a scanning electron microscope (SEM) combined with electron backscatter diffraction (EBSD). The results showed that continuous precipitates at 950 °C had a stronger pinning effect on the GB, making grain rotation difficult and promoting slip deformation in the plastic interval. Continuous precipitates caused severe stress concentration near GB and reduced coordinated deformation ability. Additionally, the crack propagation path changed from transcrystalline to intercrystalline. Furthermore, internal precipitates were a crucial factor affecting the initial crack nucleation position. Interconnected precipitates led to an intergranular fracture tendency and severe deterioration of the material's plasticity, as observed in fracture morphology.

In-situ study of microstructure and mechanical properties of GH3536 alloy manufactured by selective laser melting at 750 °C

Nickel-based GH3536 alloy exhibits high strength but low ductility when produced by selective laser melting (SLM), which hinders its widespread application in aerospace. This paper employed a ‘two-step’ method of heat treatment to optimize the microstructure of the SLM GH3536 alloy and investigated the mechanical properties. Tensile tests were conducted on SLM GH3536 alloy before and after heat treatment at 750 °C, using an in-situ high-temperature tensile stage. The relationship between the microstructural evolution and deformation behavior of the alloys was investigated by combining the in-situ tensile experiments and crystal plasticity finite element method (CPFEM). The results suggested that the elongation can be increased by 80% without a significant reduction in tensile strength. The improved elongation is attributed to the complete recrystallization, formation of annealed twins and serrated grain boundaries after heat treatment. The study also investigated the deformation behavior and fracture mode of the alloy before and after heat treatment. Discussion was done on the influence of the microstructure, particularly the twin and serrated grain boundaries on the alloy's plastic deformation. This study enhanced the understanding of plastic deformation of SLM GH3536 alloy and provided a valuable reference for the design and post-treatment of superalloy.

Effect of N on microstructure evolution and strain hardening of high-Mn austenitic steel during tensile deformation

The present study investigated the microstructure evolution and strain hardening of two types of high-Mn austenitic steels, i.e., Fe−18.5Mn–7Cr−0.6C (00 N) and Fe−18.5Mn–7Cr−0.6C−0.21 N (21 N), during tensile deformation. The results revealed that the addition of N increased the yield and tensile strength of high-Mn austenitic steel with almost no loss of elongation. The strain hardening of the 00 N and 21 N steels was closely related to the microstructure evolution. At the initial tensile deformation stage, dislocation slip was the dominant deformation mechanism of the two test steels. Due to the addition of N, the dislocation cross-slip in the 21 N steel was suppressed, leading to a higher strain hardening rate. With the increase of true strain, the mechanical twins became active. At a true strain of 0.18, a unique network structure of mechanical twins and dislocation cells was formed in the 00 N steel, and the thickness and spacing of mechanical twins were lower, which increased the resistance of dislocation motion and made its strain hardening rate slightly higher than that of the 21 N steel. However, at a true strain of 0.37, the thickness and spacing of mechanical twins in the 21 N steel decreased rapidly, which increased the difficulty of dislocation movement, resulting in a higher strain hardening rate than that of the 00 N steel. In short, the increased resistance of dislocation motion was the fundamental reason for the enhanced strain hardening rate, rather than mechanical twins.

Study on the Stability of Accumulation Using a Slope Shaking Table Test during Earthquake Action

To examine the effects of different peak accelerations on the stability of the accumulation slope and the effectiveness of anti-slide piles under seismic loads, this paper used the Fanlingqian landslide as the main research object and combined it with digital image correlation (DIC) technology in order to carry out a shaking table test. Then, the acceleration response, displacement field, strain field, the bending moment distribution of the 0.05–0.3 g ground motion accumulation slope, and the anti-slide pile reinforcement were studied. The results of the test show the following: the amplification coefficient of the measuring points A1–A6 of the accumulation slope reaches the maximum at a peak acceleration of 0.2 g, and its values are between 1.25 and 1.3, respectively. Finally, it shows a decreasing trend at a peak acceleration of 0.3 g, and its corresponding values are, respectively, between 1.1 and 1.2. In the anti-slip pile reinforcement test, due to the obstruction of the anti-slip pile, the damping of the soil around the pile increases. As the peak value of the seismic wave input increases, the amplification factor shows an overall decreasing trend. A1–A6 correspond to a peak acceleration of 0.3 g. The amplification factors are all close to 1. During different peak accelerations, the accumulation slope mainly experienced the earthquake-induced stage, tensile failure stage, creeping deformation stage, and overall instability stage. In the anti-slide pile reinforcement test, under the same conditions, the slope mainly experienced the earthquake-induced stage, tensile failure stage, lower sliding surface formation stage, and soil shedding stage in front of the pile. At the same time, the displacement and strain fields of each stage of the two groups of tests are compared, and it is found that the displacement and strain values of the accumulation slope test are greater than those of the anti-slide pile reinforcement test, and the horizontal displacement difference at the top of the slope is the most significant, reaching 2.3 times at the maximum. The bending moment of the anti-slide pile first increases and then decreases with the increase in acceleration, the reverse bending point of the pile appears at 5 times the pile diameter below the soil surface, and the maximum bending moment of the middle pile, corresponding to a peak acceleration of 0.05–0.3 g, is between 7.5 N·m and 47 N·m, respectively, while the maximum bending moment of the outer pile is between 6.5 N·m and 52 N·m, respectively. It is important to apply DIC image processing technology to the monitoring of landslide structure and the evaluation of slope stability in practical engineering.

Dynamic recrystallization-dependent high-temperature tensile properties and deformation mechanisms in Al-Mg-Sc-Zr alloys

This research elucidates discontinuous dynamic recrystallization (DDRX)-dependent high-temperature tensile properties and deformation mechanisms in simply extruded Al-7Mg alloys tailored by Sc/Zr ratios. The bimodal-grained Al-7Mg-0.4Sc alloy presents a high elongation to failure of ∼540% at 400 °C and 5 × 10−4 s−1. However, such superplastic behavior has not been observed in both coarse-grained Al-7Mg-0.1Sc-0.3Zr and Al-7Mg-0.3Sc-0.1Zr alloys. Thermally stable dispersed nano-sized Al3(Sc,Zr) particles strongly retard DDRX, accompanied by the retention of heterogeneous grain structure in Al-7Mg-0.1Sc-0.3Zr till fracture. The predominance of dislocation slip is proved by analyzing the evolution of texture and dislocations. In contrast, the bimodal grain structure evolves into a homogeneous fine one in Al-7Mg-0.4Sc with tensile deformation proceeding since coarsening Al3Sc precipitates play a weak role in restricting DDRX. The superplasticity in Al-7Mg-0.4Sc is achieved by cooperated mechanisms, i.e., grain boundary sliding (GBS) and dislocation slip as dominant mechanisms, which is accommodated by DDRX in the initial tensile deformation stage, followed by dislocation slip-accommodated GBS in the late stage.

In-situ observation of micro-cracking in a SiC/BN/SiC ceramic matrix composite under tension

Crack spacing and crack opening displacement values are critical for CMC damage and oxidation modeling. A tensile stage used with light and scanning electron microscopy (SEM) was utilized to evaluate micro-cracking under load in SiC/BN/SiC ceramic matrix composites (CMCs). Surface micro-crack spacing and crack composition, a novel measurement introduced in this work, were measured with light microscopy at room temperature. Crack spacing decreased with increasing load, with a majority of cracks being in contact with BN fiber coatings. Crack opening displacement (COD) measurements were taken at room and elevated temperature (600 °C) with scanning electron microscopy. CODS increased nearly linearly with increased load; CODs ranged from 1 to 3 μm at 200–350 MPa at room temperature and at 600 °C ranged from 0.5 to 2 μm at 175–350 MPa. CODs were also related to local microstructure, with CODS being lower near longitudinal fiber tows. Implications on use of the data for CMC oxidation models are discussed.

Enhancement of metal surface micro-jet by nanoscale helium bubbles under supported and unsupported shocks

By using molecular dynamics, we have investigated the effect of nanoscale helium (He) bubbles on the formation of micro-jets and the various physical mechanisms under supported and unsupported shock wave loading. Our simulations suggest that the micro-jet is primarily influenced by the local dynamics of the nano-He bubbles, as the velocity of the shock wave in copper–helium (Cu–He) system is slightly slower than in pure Cu. The expansion of He bubbles can accelerate the velocity of the jet head, but this effect disappears during the released tensile stage. We categorize the behavior of nano-He bubbles into three types: Type A bubbles are in the micro-jet forming region, and their expansion increases the velocity and rupture of the jet. Type B bubbles are located between the micro-jets, and their compression and rapid bursting accelerate the free surface. Type C bubbles are situated far from the free surface and mainly affect the propagation of the shock wave and the released damage process. The global effects of the He bubble are similar under both supported and unsupported shock wave loading. However, the evolution of Type C He bubbles is significantly different under unsupported shock wave loading, with pressure-atom volume and density attenuated to zero and temperature reduced to the initial temperature due to the strong tensile effect. Overall, our study has revealed the differences in the evolution process of He bubbles and their dynamic effects during the formation of micro-jets under different compressed and released paths.

Fracture properties of thin brittle MTM clay coating on ductile HEC polymer substrate

Thin clay coatings can be deposited from water dispersions for the purpose of improved gas barrier properties and fire retardancy of polymeric materials. Mechanical properties of the coatings are difficult to assess, since they are very thin (≈1µm). In-situ tests using a micro tensile stage in a scanning electron microscope reveal a thickness-dependent microcracking mechanism, and Weibull parameters for coating fracture are extracted. Complex fracture events are identified, related to a weak clay coating-polymer substrate interface. A micromechanical finite element formulation provides values of 5 MPa for interfacial shear strength and 1 J/m2 for interfacial fracture toughness. From the multiple cracking behavior of the clay coating, a clay strength ≈ 225 MPa is estimated by the Weibull strength parameter from fragmentation diagrams. The method may be extended to other combinations of brittle coating-ductile substrates.

Microstructural evolution and twins formation under tensile test of Ni-free Mn-N austenitic stainless steel using laser powder bed fusion method

At the initial uniaxial tensile stage, predominant planar dislocation slip occurs to generate dislocation pile-ups and dipole bundles. Low-energy Taylor lattice structure is then formed by co-planar slip bands along the primary slip system along [001]γ and [111]γ directions to form primary twins. Increasing the straining and generation of numerous micro-bands and crystallographic defects led to the strong restriction for dislocation motion, enhancing the formation of secondary twins with stacking fault near the deformation twin's energy. Decreasing the mean slip band and micoband spacing and configuration of complex dislocation arrangement would cause the dislocation slip, generating an extensive number of dislocation walls and cell blocks, resulting in forming a noncrystalline structure with multi-orientation. Molecular dynamics (MD) analyses revealed that deformation began by nucleation of Shockley partial dislocations with a burger vector of 1/6 〈112〉 from none-periodic boundaries. Afterwards, dislocations junctions created Hirth dislocations with a Burger vector of 1/3 〈100〉, which causes strain hardening in the system. At this point, Shockley dislocations nucleated from Hirth dislocations and strain hardening increases.

A tensile stage for high resolution synchrotron-based infrared microspectroscopy at MIRAS

For the study of thin films and fibres under load, a uniaxial tensile stage has been developed for synchrotron-based polarized Fourier-transform infrared (FTIR) microspectroscopy. One of the advantages compared to commercial available stages is its compact design at the sample position (<20 mm thickness) and the large field of view on the sample for transmission and reflection geometry. In addition, the stage is mounted on a base plate, which can be rotated between -15° and +193° in the sample plane, in order to rotate the sample relative to the inherent polarization of the incoming infrared light from the synchrotron light source. Preliminary in situ tensile load experiments conducted at MIRAS beamline of the ALBA synchrotron were done on 3D printed thermoplastic polyurethane (TPU) polymer thin films. The samples could be mapped in transmission geometry under tensile load achieving high spatial resolution up to 10 micros using the intense IR source of the synchrotron light. Making use of polarized synchrotron-based infrared light, it was possible to show the alignment of different vibrational bands parallel and orthogonal to the stretching direction. The v(C=O) absorbance bands decrease upon stretching using parallel polarized infrared light, while the v(C=C) bands are increasing in intensity, revealing the orientation of v(C=O) bonds orthogonal to the stretching direction during stretching. The experiments highlight the unique instrumentation capabilities of the tensile stage for in situ measurement of molecular distributions and chemical bond orientations as a function of sample displacement and applied load.

Synergistic tensile response of reinforced ultra high performance concrete with low fiber contents

From the perspectives of ductility, economic and construction, reinforced ultra high performance concrete (UHPC) structures with low fiber contents have a significant potential in applications. This paper studied the tensile behaviors and damage mechanisms of reinforced UHPC with low fiber contents based on the direct tensile tests accompanied with acoustic emission (AE) detection system. The test results show that the high strength steel bar can prolong the tensile multiple-cracking stage of reinforced UHPC with low fiber contents in some extent at the serviceability stage and further improve its tensile ductility via inducing additional longitudinal cracks at the failure stage. It is found that the low fiber contents and steel bar grades have marginal effect on the tension stiffening at the first cracking of reinforced UHPC. Non-negligible tension stiffening effect still remains at yielding of steel bar and it is more obvious for normal strength steel bar and UHPC with a higher fiber content. The AE weighted peak frequency can identify the tensile damage model of reinforced UHPC with low fiber contents: no internal damage occurs during stage Ⅰ; matrix cracking occurs accompanied by marginal fiber pull-out during stage Ⅱ; matrix cracking and fiber pull-out are dominated during stage III and stage IV, respectively.

Plastic Damage Assessment in 316 Austenitic Steel Using the Misorientation Parameters from an In Situ EBSD Technique

Plastic damage assessment in 316 austenitic steel was performed in this research by using the misorientation parameters derived from an in situ EBSD technique. With the increase in plastic strain, the misorientation parameters, such as the Grain Reference Orientation Deviation (GROD), Grain Orientation Spread (GOS), the Grain Orientation Spread over the grain Diameter (GOS/D), and Geometrically Necessary Dislocation (GND) density presented a growing trend. Nevertheless, the variation in GROD did not show a monotony trend, and the relative increase in the amplitude of GOS and GND density was less in the late plastic stage. Compared with the above parameters, the (GOS)/D exhibited a near-linear increase during the plastic tensile stage. As the specimen was stretched to a strain of 56.99%, the (GOS)/D increased 8.9 times compared with the original specimen. The results showed that the (GOS)/D parameter has the potential of becoming an indicator for the assessment of plastic damage in 316 steel.

Study on Tensile Mechanical Properties of GAP/CL‐20/HMX Propellant

AbstractTo study the tensile properties of GAP/CL‐20/HMX (GCH) propellant, a new generation high energy solid propellant, uniaxial tensile test and dynamic mechanical analysis (DMA) test were performed at different temperatures (233.15–333.15 K) and strain rates (0.000476–0.9524 s−1). The experimental results show that, unlike Hydroxyl‐terminated Polybutadiene (HTPB) and Nitrate Ester Plasticized Polyether (NEPE) propellants, GCH propellant have special arc section in the initial tensile stage at room temperature and high temperature. This phenomenon may be caused by the stretching of the curled molecular chains. The variation of mechanical properties shows that the maximum tensile strength and initial elastic modulus increase with the decrease of temperature and the increase of strain rate. The variation of maximum elongation is complex, which increases with the increase of strain rate at room temperature and high temperature, and first increases then decreases with the increase of strain at low temperature. The constitutive model of the GCH propellant was established by introducing the chain extension and the debonding damage thresholds. Both of which show a linear double logarithmic increasing trend with the increase of strain rate at room and high temperature. The latter shows a linear double logarithmic decrease trend with the increase of strain rate at the low temperature.

Acoustic and thermal energy evolution of AZ31B magnesium alloy under static tensile deformation

Based on the complexity of material damage and fracture behavior, the tensile deformation process of AZ31B magnesium alloy is characterized by infrared thermography (IR) and acoustic emission (AE) these two non-destructive technologies, and using in-situ EBSD technology to explore microstructure evolution under different tensile strains simultaneously. The results show that in the process of tensile deformation, the degree of strain hardening increases continuously with the deformation, and the deformation mechanism of the material changes from basal slip to pyramidal slip and {10–12} tensile twinning. The tensile deformation stage presents different signal characteristics. Using fracture mechanics and the energy release law of materials, combined with temperature and AE signal fluctuations, the transformation of the deformation mechanism, the degree of work hardening, and the dangerous zone during the entire tensile process can be effectively captured.

Deformation and damage characteristics of copper/honeycomb-graphene under shock loading

Three-dimensional graphene networks have been promising structures for improving the mechanical properties of metallic composites. This work investigated the deformation and damage characteristics of a single-crystalline and polycrystalline copper matrix with a honeycomb graphene network based on atomistic simulations. The graphene interface exerts a minor influence on the shock Hugoniot relation compared with that of the crystalline orientation of the copper matrix. Shock-induced dislocation nucleation occurs at the junction of the graphene interface and then propagates along the (111) plane of the copper matrix, and the graphene interface can be penetrated by copper atoms under strong shock. The graphene interface will dramatically reduce wave transmission and convert some shock energy into heat at the grain boundary, forming local hot spots and disordered regions. With shock wave releasing to the tensile stage, the graphene interface undergoes wrinkling, folding, and even fracture. Differing from the classical spall, the composites easily spall at the graphene interface under the release wave due to the weak Cu-C interfacial interaction, which significantly reduces the damage degree inside grains and leads to a special failure mode of grain flying. In addition, this fracture dominated by interface separation will produce a complex fracture interface and lead to a nonuniform distribution of spall strength on the fracture surface, with obvious local spallation waves, which makes the estimation of spall strength based on the acoustic approximation method difficult. The average strength of the graphene/single-crystalline copper composite is close to that of polycrystalline copper but higher than that of the graphene/polycrystalline copper composite.

In situ observation of the effect of the twin boundary orientation on the mechanical properties of single crystalline Ni

The construction of twin boundaries (TBs) in materials is a remarkable way of promoting their strength and ductility. However, the effects of TB orientation on the mechanical properties have not been reported experimentally so far. Using a state-of-the-art in situ tensile stage equipped in a transmission electron microscope, uniaxial tensile tests were performed on three single-crystalline Ni samples with TB parallel and perpendicular to the tensile direction and no TB. The results showed that the uniform tensile elongation strongly depends on TB, 120% for the perpendicular TB sample, 99% for the parallel TB sample, and only 55% for the no TB sample. In addition, dislocation interaction before reaching the perpendicular CTB contribute to cross-slip and dynamic formation of dislocation jogs, thereby improving strain hardening and resulting in a large uniform tensile elongation.

Experimental Investigation on the Influence of Confining Pressure on Thermal Aged CMDB Propellant

AbstractWith the purpose of investigating the influences of aging time and confining pressure on the mechanical properties of a composite modified double‐based (CMDB) propellant, uniaxial tension tests of the propellant with various accelerated aging times were conducted under various confining pressure conditions (relative atmospheric pressure 0, 0.5±0.05, 2±0.05 and 6±0.05 MPa). The stress‐strain behaviors at various experimental conditions were analyzed. The results show that stress‐strain curves can be divided into three stages, the initial tensile stage, the elastic stage, and the damage growth stage. Additionally, the key mechanical parameters (e. g., the maximum tensile strength) show a strong dependence on aging time and confining pressure. The ultimate strain decreases with increasing aging time, while the tensile strength first decreases and later increases with aging time. It was observed that the maximum tensile strength and elastic modulus of the CMDB propellant decrease with confining pressure. Furthermore, the damage evolution curves can be divided into two stages. The later nonlinearity of the stress‐strain curves is mainly induced by the initiation and propagation of macro‐cracks. It is believed that these findings will help to accurately analyze the structural integrity of CMDB propellant grains during ignition pressurization loading after a long storage time.