High Local Strain Research Articles

Microstructural evolution and deformation mechanisms of Khao Kho Fault, Thailand

Lithologies and microstructures of fault rocks from the Khao Kho Fault in Thailand are investigated to understand fault behavior and underlying deformation mechanisms. Fault gouges in the shear zone are mainly composed of kaolinite, illite-mica, and illite-smectite and exhibit weak preferred orientation, ranging from 1.2 to 2.3 m.r.d. These fault cores are characterized by high localized strain and intense deformation due to cataclasis and frictional granular flow mechanisms, which produce transgranular fractures, rounded quartz grains, and an ultrafine clay matrix. In contrast, wall rocks show stronger clay preferred orientation, up to 3.9 m.r.d., despite having less clay content than the fault gouges. Much larger fault-related bent and sheared clay grains are present in the wall rocks, suggesting less severe deformation. The discrepancy between fault cores and wall rocks indicate that the development of fabrics is related to faulting and authigenic processes. Weak fabrics in the slip zone are attributed to the comminution of grains and neomineralization, allowing clay platelets to grow and rotate in many directions. In addition, the abundance of metal oxides indicates extensive, multiple fluid alteration processes, which further influence microstructures and fault properties.

Termination of local strain concentration led to better tensile ductility in multilayered 2N/4N Al sheet

Multilayered aluminum composites (ARB6-2N/4 N) with alternating layers of commercially purity aluminum (2N–Al) and high-purity aluminum (4N–Al) were fabricated by six cycles of accumulative roll bonding (ARB) processing. The ARB processed specimens were annealed at different temperatures. It was found that heterogeneous lamellar microstructures composed of fine-grained 2 N (hard) layers and coarse-grained 4 N (soft) layers formed in the specimens. The specimen with the heterogeneous layered structure showed a large tensile elongation with a strength following the rule of mixture. Digital image correlation analyses revealed that the deformation bands with high local strains formed in the 4 N layers but were terminated at the interfaces between the 2 N and the 4 N layers in the multilayered aluminum composites, accommodating the deformation between the hard (2 N) layers and the soft (4 N) layers during tensile deformation. Termination of the strain localization delayed macroscopic necking of the materials, leading to a better combination of high strength and good ductility in the multilayered aluminum composites.

Quantifying the 2D anisotropic displacement and strain fields in graphite-based electrode via in situ scanning electron microscopy and digital image correlation

During charge and discharge of the lithium-ion batteries, deformation in the graphite-based electrode can induce high local strains and severe mechanical degradation. However, little quantitative method are available to help understand the mechanisms of deformation evolution at the microscopic scales. This work reports a combined method via in-situ scanning electron microscopy and digital image correlation technology to characterize the two-dimensional displacement and strain fields of the electrode throughout operation. It is found that 50% irreversible swelling in the initial cycle and reversible anisotropic deformation in the following cycles. The average strain in the vertical direction is 4.94%, which is about 18 times higher than that in the horizontal direction of electrode. This new combined method based on in-situ scanning electron microscopy and digital image correlation can be used to quantify the evolution of the displacement and stain fields in the electrode.

Cracking Behavior of Coating during Hot Tensile Tests of AlSi-Coated Press Hardening Steel

For industrial hot stamping applications, press hardening steel is usually coated with Al-10wt.%Si, in order to prevent substrate decarburization and oxidation at elevated temperatures. However, during hot stamping, the AlSi coating layer fractures, causing severe tool wear, substrate oxidation and increased friction coefficient between the tool and stamped part. The initiation of coating fracture can largely be attributed to the formation of several intermetallic compounds (i.e., FeAl & Fe2Al5) via Fe-diffusion, which also results in void formation throughout the coating layer. These intermetallics are formed mainly during the heating stage, with decreasing Fe-content from the coating-substrate interface. Due to distinctive thermo-mechanical properties of intermetallics compared to the steel substrate, the interaction between different intermetallics, including voids, causes high strain localization around the voids, leading to coating fracture. The goal of this study is to detect the initiation of cracks in a ~45 μm coating layer during uniaxial tensile deformation of a 1.5 mm AlSi-coated press hardening steel. For this purpose, isothermal tensile tests were performed at elevated temperatures. The coating cracks were detected by means of acoustic emission (AE) sensors during deformation. The distribution of coating cracks at hot stamping condition was examined via optical measurements. The tensile strain was measured from a strain grid on the sample gauge. The experiment involves heating the coated steel in a furnace to 920°C for 6 minutes, followed by uniaxial tensile deformation (at 600°C and 800°C), and finally quenching at ambient air. The first AE signal from the sample was observed during the tensile deformation at 600°C, indicating that tensile strain initiates fracture in the coating layer. At cooling stage, the temperature change with time triggered more AE signals, which may correspond to substrate phase transformation and additional fractures in the coated steel; the latter is owing to thermal expansion mismatch between the intermetallics in the coating layer, and steel substrate. Interestingly, no AE signals were observed during the heating stage; i.e., no coating cracks occur prior to deformation and quenching.

Dynamic fracture of CrN coating by highly-resolved nano-impact

Nano-impact testing has provided localized high strain rate deformation information in either (i) repetitive impact for fatigue of brittle materials or (ii) high-resolution data acquisition of single impacts for energy dissipation and dynamic hardness in ductile materials. The current study focusses on whether high-resolution data acquisition can be used to determine fracture occurring during a single impact event. A PVD CrN coating on 42CrMo4 high speed steel (HSS) was impacted with a cube corner indenter over a range of different acceleration forces. By analyzing the high precision depth-time curves during the first contact phase of the impact, an abrupt depth change was observed at and above 30 mN acceleration force. Focused ion beam technology confirmed through-thickness coating fracture without delamination on tests where the step was observed. The step was absent at lower force or on the more ductile HSS throughout the load range. The fracture dissipated energy and fracture toughness of the coating was estimated under 50 mN acceleration force based on the analysis of impact energy. The calculated dynamic fracture toughness of CrN coating is in the range of 2.75–7.74 MPa·m1/2, depended on the indenter geometry constant, which is comparable with the reported value of about 3.13 MPa·m1/2.

Spatially Regulated Multiphenotypic Differentiation of Stem Cells in 3D via Engineered Mechanical Gradient.

Within the osteochondral interface, cellular and extracellular matrix gradients provide a biomechanical and biochemical niche for homeostatic tissue functions. Postnatal joint loading is critical for the development of such tissue gradients, leading to the formation of functional osteochondral tissues composed of superficial, middle, and deep zones of cartilage, and underlying subchondral bone, in a depth-dependent manner. In this regard, a novel, variable core-shell electrospinning strategy was employed to generate spatially controlled strain gradients within three-dimensional scaffolds under dynamic compressive loading, enabling the local strain-magnitude dependent, multiphenotypic stem cell differentiation. Human mesenchymal stem cells (hMSCs) were cultured in electrospun scaffolds with a linear or biphasic mechanical gradient, which was computationally engineered and experimentally validated. The cell/scaffold constructs were subjected to various magnitudes of dynamic compressive strains in a scaffold depth-dependent manner at a frequency of 1 Hz for 2 h daily for up to 42 days in osteogenic media. Spatially upregulated gene expression of chondrogenic markers (ACAN, COL2A1, PRG4) and glycosaminoglycan deposition was observed in the areas of greater compressive strains. In contrast, osteogenic markers (COL1A1, SPARC, RUNX2) and calcium deposition were downregulated in response to high local compressive strains. Dynamic mechanical analysis showed the maintenance of the engineered mechanical gradients only under dynamic culture conditions, confirming the potent role of biomechanical gradients in developing and maintaining a tissue gradient. These results demonstrate that multiphenotypic differentiation of hMSCs can be controlled by regulating local mechanical microenvironments, providing a novel strategy to recapitulate the gradient structure in osteochondral tissues for successful regeneration of damaged joints in vivo and facile development of interfacial tissue models in vitro.

Microstructure refinement and mechanical properties improvement of Al-Si-Fe alloys by hot extrusion using a specially designed high-strain die

A heteromorphic extrusion die with right-angle corners in the die hole was designed in this study to give a high local strain to the extrudate metal during hot extrusion of Al-Si-Fe alloys, and the strain distribution in the extrudate metal was evaluated using the finite element method. It was demonstrated that the change in metal flow direction at the right-angle corners is effective for generating high strain in the extrudate metal and refining brittle inclusion particles such as primary Al 9 Si 2 Fe 2 (β phase) particles and eutectic Si particles. The refinement of these particles led to significant improvements in tensile elongation and plastic deformability in cold rolling of the alloys.

A CONSTITUTIVE MODEL FOR BOTH LOW AND HIGH STRAIN NONLINEARITIES IN HIGHLY FILLED ELASTOMERS AND IMPLEMENTATION WITH USER-DEFINED MATERIAL SUBROUTINES IN ABAQUS

ABSTRACT Strain energy functions (SEFs) are used to model the hyperelastic behavior of rubberlike materials. In tension, the stress–strain response of these materials often exhibits three characteristics: (i) a decreasing modulus at low strains (<20%), (ii) a constant modulus at intermediate strains, and (iii) an increasing modulus at high strains (>200%). Fitting an SEF that works in each regime is challenging when multiple or nonhomogeneous deformation modes are considered. The difficulty increases with highly filled elastomers because the small strain nonlinearity increases and finite-extensibility occurs at lower strains. One can compromise by fitting an SEF to a limited range of strain, but this is not always appropriate. For example, rubber seals in oilfield packers can exhibit low global strains but high localized strains. The Davies–De–Thomas (DDT) SEF is a good candidate for modeling such materials. Additional improvements will be shown by combining concepts from the DDT and Yeoh SEFs to construct a more versatile SEF. The SEF is implemented with user-defined material subroutines in Abaqus/Standard (UHYPER) and Abaqus/Explicit (VUMAT) for a three-dimensional general strain problem, and an approach to overcome a mathematically indeterminate stress condition in the unstrained state is derived. The complete UHYPER and VUMAT subroutines are also presented.

Comparison of Predicted Forming Limit Curves with Measured Experimental Data on Hot-Dip Zinc-Coated Cold-Rolled Steel by Incremental Forming Process

Single Point Incremental Forming (SPIF) is a die-less forming process with advantages of high-flexibility, low-cost and short lead time. The high local strains that are applied to the metal sheet, often exceeding the conventional formability limit. This paper is focused on comparison of predicted forming limit curves with measured experimental data on Hot-Dip Zinc-Coated Cold-Rolled sheet, with 0.20 mm thick is studied in single point incremental forming. Truncated square pyramid and cone are formed to study the formability of blank sheets at room temperature. It was found that both Formulation of plastic instability criteria and Keeler’s formula gives the lowest FLC. FLDs have predicted failures in forming process consistently with the real experiments. The experimentally obtained cracking limit differ from analytical one and empirical one by about 3.398 and 2.135 true strain respectively at FLD0, the corresponding plane strain values.

FeMnNiAl Iron-Based Shape Memory Alloy: Promises and Challenges

Among all shape memory alloys, the iron-based FeMnNiAl has emerged as one of the most promising compositions with a huge superelasticity temperature window (> 400 °C). In this article, we first point to the high local transformation strains (> 10%) and high transformation stress levels (500–700 MPa) that result in a large work output. When subjected to either tensile or compressive loading, the transformation stress exhibits very small temperature dependence (Clausius–Clapeyron slope less than 0.2 MPa/°C in compression and 0.5 MPa/°C in tension) and an extremely small adiabatic temperature rise (less than 1 °C) during transformation. The complexity in transformation behavior associated with the presence of grain boundaries (GBs) is discussed. In particular, the work provides insight in the localization occurring at GBs due to transformation front–GB interactions and the potential cracking that can degrade fatigue performance. Overall, this work provides a deeper insight into the deformation response, advantages, and drawbacks of FeMnNiAl SMA. The comprehensive handling of various aspects of this alloy system paves the way for the development of future iron-based shape memory alloys.

Subsurface Dynamic Deformation and Nano-structural Evolution in 40Cr Steel Under Dry Sliding Wear

Dry sliding tribological properties of normalized 40Cr steel pins against quenched and tempered GCr15 discs were investigated under various sliding distances through disc-on-pin contact configuration. Microstructural feature evolution and strain-hardening behavior of worn subsurface in 40Cr pins were systematically analyzed using scanning electron microscopy and transmission electron microscopy. It is found that based on the variation of wear rate, three stages can be divided throughout the friction process: initial wear, slow wear and stable wear. The corresponding microstructures beneath contact surface are subjected to significant strain-hardening effect and structural rearrangement. Nano-grains of 40–100 nm are produced in the topmost subsurface layer and the nanocrystallization mechanism during friction process is further elucidated. Pronounced vortex structures are generated in the localized subsurface zones due to high strain localization and shear instability. With the prolongation of sliding distances, vortex structures suffer dynamic embrittlement and exfoliation from the wear surface as a result of periodically excessive hardening effect, leading to the high wear rate in stable wear stage.

Dynamic Tensile Behavior of Fiber Laser Welds of Medium Manganese Transformation-Induced Plasticity Steel

During a crash situation, tailor-welded blanks (TWBs) are subjected to local high strain rate (HSR) deformations. Hence, understanding the effects of fiber laser welding on the microstructure and subsequent dynamic (1 to 100 s−1) tensile properties of the recently developed medium-Mn transformation-induced plasticity (TRIP) steel is essential. Electron backscatter diffraction (EBSD) analysis indicated that Mn segregation during rapid solidification of the FZ resulted in retention of austenite within interdendritic regions. Moreover, it was found that the lower stability of retained austenite within the intercritical heat-affected zone (HAZ) resulted in double yielding behavior of the welds due to discontinuous yielding (Luders banding). However, by increasing the strain rate, the second yield point decreased and shifted to higher strains. In the dynamic loading range, the ultimate tensile strength (UTS) and elongation of welds showed positive strain rate sensitivity (SRS). In addition, high-speed thermal camera results indicated that the average temperature decreased with increasing strain rate as a consequence of the balance between TRIP effect suppression and HSR adiabatic heating. The overall increase in elongation of the welds at higher strain rates was attributed to the higher localized softening during pre-failure nonuniform deformation.

Study of saw-tooth chip in machining of Inconel 718 by metallographic technique

The present research work aims to understand the chip formation mechanisms in Inconel 718 machining through metallographic technique. The influence of machining speed on shear band and chip shape have been analyzed. Characterization of the shear band has been conducted by analyzing the shear band thickness, strain, strain rate, stress and micro-hardness. Additionally, chip reduction coefficient parameters and chip segmentation ratio parameters have been analyzed to understand the type of chip formation at various machining speeds. The observations and analysis of the same indicated that the formation of the saw-tooth chip is due to high-strain localization within the chip and crack initiation at the free surface of the chip. Thinner shear bands are observed at higher machining speed (120 m/min), and thicker shear bands are observed at lower machining speed (40 m/min). The magnitude of chip segmentation parameters indicates that crack generation increases with increase in machining speed.

Lath martensite plasticity enabled by apparent sliding of substructure boundaries

Lath martensite is widely present in advanced high strength steels as the key strengthening phase. Unexpectedly high ductility of lath martensite has been reported in the literature in both single-phase and multi-phase steels, however, without systematic identification of the underlying plasticity mechanisms. In this study, first, well-defined micro-tensile tests are carried out on fully martensitic steel with a clean large substructure and a variety of substructure boundary orientations with respect to the loading direction. Two deformation mechanisms of lath martensite were identified, namely, intra-lath crystallographic slip and apparent substructure boundary sliding, that compete with each other to carry the overall plasticity. The condition under which these two mechanisms are active has been clarified. It is found that boundary sliding is more easily activated than intra-lath crystallographic slip. In addition, for dual phase steel, as an example of multi-phase steels, the probability for sliding of lath martensite boundaries was estimated by boundary orientation characterization and micro-tensile tests. The results suggest that the apparent boundary sliding is also important for lath-martensite-containing multi-phase steels, which would explain prior reports in the literature of unexpectedly high local strains in the martensite regions.

Machine-learning based temperature- and rate-dependent plasticity model: Application to analysis of fracture experiments on DP steel

Slow, intermediate and high strain rate experiments are carried out on flat smooth and notched tensile specimens extracted from dual phase steel sheets. A split Hopkinson pressure bar testing system with load inversion device is utilized to achieve high strain rates. Temperature dependent experiments ranging from 20 °C to 300 °C are performed at quasi-static strain rates. The experimental results reveal a non-monotonic effect of the temperature on the stress-strain curve for DP800 steel. For the temperatures considered, the lowest and highest curves are obtained for 120 °C and 300 °C, respectively. A modified Johnson-Cook plasticity model is developed to capture the observed unconventional effect of the strain rate- and temperature on the hardening response. It includes a von Mises yield surface, a non-associated Hill’48 flow rule, a Swift-Voce reference stress-strain curve describing the rate-independent behavior at room temperature, and a neural network function depending on the plastic strain, strain rate and temperature. This model is implemented into a material user subroutine and identified using a combination of analytical formulas along with a resilient back propagation algorithm. It is found that the obtained machine-learning based Johnson-Cook plasticity model can describe all experimental data with high accuracy, including both force-displacement and local surface strain measurements. Using a hybrid experimental-numerical approach, the loading paths to fracture are determined. It is found that the temperature not only affects the plasticity of the DP800 steel in a non-monotonic manner, but also its strain to fracture, with ductility loss of up to 30% when increasing the temperature from 20 to 120 °C, followed by a gain in ductility when increasing the temperature further to 300 °C.

Microscopic origin of highly enhanced current carrying capabilities of thin NdFeAs(O,F) films.

Fe-based superconductors present a large variety of compounds whose physical properties strongly depend on the crystal structure and chemical composition. Among them, the so-called 1111 compounds show the highest critical temperature Tc in the bulk form. Here we demonstrate the realization of excellent superconducting properties in NdFeAs(O1−xFx). We systematically investigated the correlation between the microstructure at the nanoscale and superconductivity in an epitaxial 22 nm NdFeAs(O1−xFx) thin film on a MgO single crystalline substrate (Tc = 44.7 K). Atomic resolution analysis of the microstructure by transmission electron microscopy and atom probe tomography identified several defects and other inhomogeneities at the nanoscale that can act as extrinsic pinning centers. X-Ray diffraction and transmission electron microscopy displayed a broad variation of the a-axis lattice parameter either due to a partially strained layer at the interface to the substrate, high local strain at dislocation arrays, mosaicity, or due to composition variation within the film. The electrical transport properties are substantially affected by intrinsic pinning and a matching field corresponding to the film thickness and associated with the Bean–Livingston surface barrier of the surfaces. The thin film showed a self-field critical current density Jc(4.2 K) of ∼7.6 MA cm−2 and a record pinning force density of Fp ≈ 1 TN m−3 near 35 T for H‖ab at 4.2 K. These investigations highlight the role of the microstructure in fine-tuning and possibly functionalizing the superconductivity of Fe-based superconductors.

Dislocation arrangements within slip bands during fatigue cracking

Microstructures and dislocations associated with the slip band (SB) fatigue cracks were investigated in Cu single crystal and bicrystal. It is found that there are high strain localizations with abundant dislocation cells around the cracked SBs. Due to the high local strain and slip irreversibility, the lattice rotates severely leading to large misorientation and hence low- or high- angle grain boundaries could form between the neighboring cells. Moreover, the dislocation walls can serve as preferential sites for nucleation and propagation of fatigue cracks with high density of dislocations, vacancies and high strain incompatibility.

Effects of heat treatment on microstructure and creep properties of a laser powder bed fused nickel superalloy

Nickel-based superalloy C263 has been consolidated with Laser Powder Bed Fusion (LPBF) with two perpendicular build orientations and exposed to either of two heat treatment programmes. This study analyses the effects of build orientation and heat treatment on the resulting microstructures produced in LPBF C263 variants, evaluated against a cast equivalent. Results show that although a strongly anisotropic microstructure was present in standard heat-treated (HT1) LPBF material, this was eradicated following an alternate heat treatment regime (HT2) through recrystallisation, aided by high local strain. Subsequently, their mechanical properties have been assessed by means of the Small Punch (SP) creep test. A contrasting presence of Σ3 formations was observed between the two LPBF heat treatment programmes with the resulting random grain boundary network (RGBN) revealing shorter potential intergranular crack paths in the HT2 material, although grain boundary carbides were found to be the dominant strengthening mechanism for improved creep resistance. Adapted Wilshire equations have been implemented to predict the long-term creep lives of the C263 variants and their apparent activation energies have been determined.

Effect of SR-microCT radiation on the mechanical integrity of trabecular bone using in situ mechanical testing and digital volume correlation

The use of synchrotron radiation micro-computed tomography (SR-microCT) is becoming increasingly popular for studying the relationship between microstructure and bone mechanics subjected to in situ mechanical testing. However, it is well known that the effect of SR X-ray radiation can considerably alter the mechanical properties of bone tissue. Digital volume correlation (DVC) has been extensively used to compute full-field strain distributions in bone specimens subjected to step-wise mechanical loading, but tissue damage from sequential SR-microCT scans has not been previously addressed. Therefore, the aim of this study is to examine the influence of SR irradiation-induced microdamage on the apparent elastic properties of trabecular bone using DVC applied to in situ SR-microCT tomograms obtained with different exposure times. Results showed how DVC was able to identify high local strain levels (> 10,000 µε) corresponding to visible microcracks at high irradiation doses (~ 230 kGy), despite the apparent elastic properties remained unaltered. Microcracks were not detected and bone plasticity was preserved for low irradiation doses (~ 33 kGy), although image quality and consequently, DVC performance were reduced. DVC results suggested some local deterioration of tissue that might have resulted from mechanical strain concentration further enhanced by some level of local irradiation even for low accumulated dose.

The effect of off-axis angles on the mesoscale deformation response and failure behavior of an orthotropic textile carbon-epoxy composite

The deformation response and failure behavior of an orthotropic textile carbon-epoxy composite were investigated under off-axis tensile loading. Digital image correlation (DIC) was utilized to effectively capture the full-field and mesoscale strain distribution. The macroscale mechanical performance was strongly sensitive to the fiber bundles orientation relative to the loading direction. Based on the experimental data, a quantitative relation between the rotation angle and off-axis angle was established, and a negative correlation between the failure strength and the rotation angle was observed. The underlying failure mechanisms of the specimens with different off-axis orientations were analyzed using scanning electron microscopy (SEM) and DIC techniques. The load-bearing mechanisms were different between the on- and off-axis cases. High local shear strain eventually resulted in the brushy shear-type fracture in the off-axis case, and the local micro cracks developed during the loading caused the reduction and imbalance of local load-bearing capacity.