Quasi-static Rates Research Articles

A recrystallization heat-treatment to reduce deformation anisotropy of additively manufactured Inconel 718

The aim of the current study is to investigate the suitability of a solutionizing recrystallization (RC) heat-treatment to improve the yield strength of laser powder-bed fusion (L-PBF) Inconel 718 (IN718), while decreasing deformation anisotropy under uniaxial compression. Scanning electron microscopy measurements show an increase in grain size, decrease in texture, and the formation of twins in the RC compared to the as-built (AB) and direct-age (DA) hardened conditions. Transmission electron microscopy measurements of the RC condition revealed the formation of nano-precipitates in the gamma matrix. As a result, the compressive stress-strain behavior showed an increase in the yield strength of the RC specimens compared to the AB condition in the quasi-static and high-strain rate regimes despite having significantly larger grain sizes. Post-mortem in-plane ellipticity measurements show a statistically significant decrease in the deformation anisotropy of the RC specimens compared to the AB and DA conditions. Overall, the solutionizing RC heat-treatment, when compared to the AB condition, resulted in an increase in yield strength and decrease in deformation anisotropy, two desirable outcomes necessary to continue to provide path-ways to transitioning L-PBF parts from the research laboratory setting to commercial implementation.

Revisiting the first Sandia Fracture Challenge with transient deformation heating and strain localization considerations

Here, we present a systematic experimental study and accompanying theoretical analysis of the dependence of ductile fracture on strain localization, strain hardening rates, deformation-induced thermal softening, and transient heat conduction under spatially uniform as well as spatially heterogeneous deformation. Spatially uniform cases are studied via standard dogbone shaped specimens subject to uniaxial tension. Spatially heterogeneous deformation is studied via the so-called Sandia Fracture Challenge (SFC) specimen, which is a standard compact tension specimen modified with three machined holes in front of a blunt notch (Boyce et al. in Int J Fract 186(1–2):5–68, 2014). We utilize the same precipitation hardened martensitic stainless steel used in the first SFC experiments, i.e. 15-5 PH with an H1075 heat treatment. We also study 15-5 PH in Condition A and with the H900 heat treatment, each of which has a different hardening behavior and ductility. We find that the ductility does not correlate with the deformation to first crack initiation in the SFC specimen. Instead, hardening rates are better correlated. Moreover, a re-examination of (Boyce et al. in Int J Fract 186(1–2):5–68, 2014) finds that an accurate calibration of the hardening rates is strongly correlated with accurate blind predictions of ductile fracture in the SFC specimens. Given that thermal softening can greatly affect hardening rates, we provided a thermographic analysis of deformation-induced heating and transient heat transfer in both the dogbone shaped samples and the SFC specimens. Transient heating in stainless steels is found to have first-order effects on the strain at the onset of necking, strain to fracture, as well as strain localization and ductile fracture in complex geometries even under so-called quasi-static loading rates.

Quasi-static loading rate effects on fracture process zone development of mixed-mode (I-II) fractures in rock-like materials

The development of a fracture process zone (FPZ) (microcrack zone) ahead of a fracture tip is a prominent fracture characteristic of rock-like materials. At present, understanding of the quasi-static loading-rate effect on the FPZ development of a mixed-mode (I-II) fracture remains challenging for rock-like materials. In this paper, the centrally cracked Brazilian disk (CCBD) specimens of artificial rock-like materials were tested to create mixed-mode (I-II) fractures at different quasi-static loading rates (0.02–2 mm/min), and the centrally crack in each specimen is prefabricated at 15° (tensile-shearing mixed-mode), 45° (compressive-shearing mixed-mode) and 0° (pure mode-I) to the loading direction. The digital image correlation (DIC) was employed to identify FPZ, with the discontinuous characteristics of DIC-measured displacement and strain field ahead of the fracture tip. Based on test results, a couple of outcomes were obtained. (1) The geometries of mixed-mode (I-II) fractures changed barely at different quasi-static loading rates. (2) At mixed-mode (I-II) fracture tips, the DIC-measured tensile displacement fields presented remarkable discontinuity; in contrast, the DIC-measured sliding displacement fields were nearly continuous. Consequently, at different quasi-static loading rates, the tensile-shearing and compressive-shearing mixed-mode (I-II) just represents the fracture bearing tensile-shearing and compressive-shearing stresses for rock-like materials. Still, at the mixed-mode (I-II) fracture tip, the deformation in FPZ and the generation of the real fracture surface is tensile. (3) With the loading rate increasing, the FPZ length of mixed-mode (I-II) fracture increased roughly from 5 mm to 17 mm, which is similar to the pure mode-I fracturing (FPZ length: 4.5–17.3 mm). The rate-dependent FPZ length of mixed-mode (I-II) fracture follows a power-law, consistent with the mode-I fracture. (4) The peak load (an index indicating the fracture resistance) of mixed-mode (I-II) and pure mode-I fracturing was strengthened with the increasing loading rate, linearly correlated to the increasing FPZ length. It indicates that rate-dependent FPZ is supposed to strengthen the fracture resistance of mixed-mode (I-II) cracks with loading rates increasing. This research can provide a basis for controlling mixed-mode (I-II) fracturing of rock-like materials in underground excavation projects and geo-energy exploitation, such as the deflecting propagation of hydraulic fractures.

Investigation of hydraulic fracturing mechanism by using a coupled continuous-discontinuous hydromechanical model

Fluid-induced fracture nucleation, propagation, and interaction are the essentials for a better understanding of the hydraulic fracturing process in unconventional reservoirs. In this study, a coupled continuous-discontinuous hydromechanical model was established to investigate the hydraulic fracturing propagation under varied conditions. The interactions between induced fractures and natural fractures are investigated and discussed by a series of hydraulic fracturing simulations. Also considered are the influences of bedding joints, in-situ stress ratios, and fluid injection rates on the patterns of hydraulic fractures and the stress field. It was found that hydraulic fracture propagation is controlled by both in-situ stress state and strength anisotropy of the reservoir rock. The simulations indicate that an increase in the fluid injection rate is favorable to the formation of a complex fracture network. More hydraulic fractures were developed when fracture fluids were injected into rock specimens with a faster pressurization rate than a quasi-static pressurization rate. However, higher fluid injection rates could result in higher breakdown pressures for fracture initiation and propagation. In addition, hydraulic fractures tend to extend along the direction of the maximum principal stress or approach this preferred path. Bedding joints are preferred locations and orientations for fracture initiation and propagation in laminated shale reservoirs.

Plasticity and Formability of Annealed, Commercially-Pure Aluminum: Experiments and Modeling.

The plasticity and formability of a commercially-pure aluminum sheet (AA1100-O) is assessed by experiments and analyses. Plastic anisotropy of this material is characterized by uniaxial and plane-strain tension along with disk compression experiments, and is found to be non-negligible (e.g., the r-values vary between 0.445 and 1.18). On the other hand, the strain-rate sensitivity of the material is negligible at quasistatic rates. These results are used to calibrate constitutive models, i.e., the Yld2000-2d anisotropic yield criterion as the plastic potential and the Voce isotropic hardening law. Marciniak-type experiments on a fully-instrumented hydraulic press are performed to determine the Forming Limit Curve of this material. Stereo-type Digital Image Correlation is used, which confirms the proportional strain paths induced during stretching. From these experiments, limit strains, i.e., the onset of necking, are determined by the method proposed by ISO, as well as two methods based on the second derivative. To identify the exact instant of necking, a criterion based on a statistical analysis of the noise that the strain signals have during uniform deformation versus the systematic deviations that necking induces is proposed. Finite element simulation for the Marciniak-type experiment is conducted and the results show good agreement with the experiment.

Solid solution softening in dislocation-starved Mg–Al alloys

Here, we report molecular dynamics (MD) calculations of spall strength in pure magnesium and magnesium alloys with 1–8 at.% aluminum in solid solution. The Mg–Al solid solution alloys are found to have a weaker spall strength than pure magnesium, e.g. the spall strength of Mg-8 at.% Al is found to be 12% lower than pure magnesium. Moreover, we find that the spall strength of Mg–Al alloys monotonically decreases with increasing concentration of Al content. We term this phenomena solid solution softening (SSS) of spall strength, which is contrary to the conventional solid solution strengthening observed in most alloys (including Mg–Al alloys) deformed at quasi-static loading rates. We argue that this transition from solid solution strengthening at quasi-static rates to SSS at extreme loading rates is due to a transition from dislocation glide dominated failure at low rates to dislocation nucleation dominated failure at extreme rates. The distortion of the host lattice by solute atoms is found to retard dislocation glide, but aids dislocation nucleation. We thus conclude that SSS may manifest in any dislocation-starved system, regardless of loading rate. To this end, we propose a theoretical model for spall strength in dislocation-starved bulk and nanoporous alloys that exhibits remarkable agreement with our molecular dynamics calculations. Our theory bares some resemblance to the double kink nucleation enhancement theory for low-temperature SSS in some body centered cubic metals, e.g. refractory metals alloyed with group IV-VIII elements.

Quasi-static and dynamic response of GFRP and BFRP bars under compression

The application of fiber-reinforced polymers (FRP) bars in the building industry has grown intensively over the years, due to their numerous advantages such as corrosion resistance. However, current guidelines in design codes disregard the contribution of FRP bars in compression. The need of investigation of compressive mechanical properties of FRP bars is important, while there is a lack of experimental tests under varying dynamic and quasi-static loading rates. The present paper aims to evaluate the compressive strength and failure modes of glass and basalt fiber-reinforced polymer (GFRP and BFRP) bars. A series of quasi-static and dynamic tests were performed on different FRP bar sizes. Dynamic tests were conducted using the drop hammer procedure and different loading rates were achieved through varying the weight of mass and height of fall. Results showed that GFRP bars attained higher compressive strengths than BFRP bars for the same loading conditions. GFRP bars exhibited compressive strength in the range of 300 MPa to 600 MPa while the maximum compressive strength of BFRP bars was 470 MPa. The increase in the bar diameter resulted in an increase in the compressive strength of most FRP bars. However, Some FRP bars showed different variation in their compressive strengths at high loading rates. Finite element (FE) models were also developed to simulate the FRP bars under dynamic tests by considering the orthotopic material nature of the FRP composites. The FE models were verified with the experiments and successfully described qualitatively the trends observed during the full-scale drop-hammer tests.

Quasi-static and dynamic performance of novel interlocked hybrid metal-composite joints

This study presents a novel hybrid technique for joining composites to metal, employing an array of macro-scale interlocking features on the faying surfaces of adhesively bonded adherends. Single-lap, interlocking adhesive joints (IAJs) and baseline adhesive joints (BAJs), are tested at quasi-static and transient dynamic (0.5 m/s and 3 m/s) loading rates. The joint deformation mechanisms are examined and fractography analysis is performed at the macro and micro scales. Results indicate a 10% increase in lap-shear strength, and 75–120% increase in work to failure for the IAJs compared to the BAJs, at all loading rates. In addition, IAJs exhibit improved damage tolerance compared to adhesive joints, due to reduced joint rotation, more stable adhesive fracture growth, and the ability to sustain load even after cracks have propagated through the adhesive at the ends of the overlap region. The high energy absorption capacity (23–38 J) of IAJs indicates they could be used to significantly improve the crashworthiness performance of multi-material transportation structures.

Pullout behaviors of single yarn and textile in cement matrix at elevated temperatures with varying loading speeds

Textile reinforced concretes (TRCs) have many extraordinary features that make them a future-ready material exposed to complex environmental situations. However, current knowledge related to TRCs under mechanical loading and environmental conditions is not fully explored as compared to conventional fiber reinforced concretes. The present work investigates the quasi-static pullout performance of single-yarn and textile type (AR-glass and basalt fiber) reinforced cement mortars under different temperatures (25 °C, 100 °C, 300 °C and 600 °C). The pullout tests were carried out at loading rates of 0.25, 2.5 and 25 mm/min on an MTS load frame setup. The reinforcement embedded length in the cement matrix was 30 mm. The pullout testing results were evaluated in terms of initial debonding force and maximum pullout (or peak) load, displacement at initial debonding force, and energy absorption. The experimental results indicated that the textile type reinforcement was better than the yarn type reinforcement. The mechanical properties of TRCs were sensitive to loading rate and temperatures. In addition, the scanning electronic microscopy (SEM) images of the fractured samples tested at different loading rates and temperatures highlighted that quasi-static loading rate variations did not affect the fracture pattern. However, the temperature variation highly affected the epoxy resins used for filament bonding or coating in the reinforcements which may critically affect their interfacial performances.

Evaluating FDM Process Parameter Sensitive Mechanical Performance of Elastomers at Various Strain Rates of Loading

To optimize the mechanical performance of fused deposition modelling (FDM) fabricated parts, it is necessary to evaluate the influence of process parameters on the resulting mechanical performance. The main focus of the study was to characterize the influence of the initial process parameters on the mechanical performance of thermoplastic polyurethane under a quasi-static and high strain rate (~2500 s−1). The effects of infill percentage, layer height, and raster orientation on the mechanical properties of an FDM-fabricated part were evaluated. At a quasi-static rate of loading, layer height was found to be the most significant factor (36.5% enhancement in tensile strength). As the layer height of the sample increased from 0.1 to 0.4 mm, the resulting tensile strength sample was decreased by 36.5%. At a high-strain rate of loading, infill percentage was found to be the most critical factor influencing the mechanical strength of the sample (12.4% enhancement of compressive strength at 100% as compared to 80% infill). Furthermore, statistical analysis revealed the presence of significant interactions between the input parameters. Finally, using an artificial neural networking approach, we evaluated a regression model that related the process parameters (input factors) to the resulting strength of the samples.

Geological structures controlled the rupture process of the 2011 M9.0 Tohoku-Oki earthquake in the Northeast Japan Arc

By interpreting the 2D/3D seismic survey data acquired in the surrounding ocean areas of the Northeast (NE) Japan Arc, we clarified the detailed geological structure and demonstrated that the basic structure in the hanging-wall plate of the subduction system consists of many structural blocks (segments) separated by NW–SE trending large transcurrent faults (strike-slip faults). This structural configuration showed a close relationship with the distribution of foreshocks, mainshock, and aftershocks, coseismic slip models of the 2011 M9.0 Tohoku-Oki megathrust earthquake, coseismic slip area of M-7 class earthquakes, quasi-static slip rates, back slip rate, and seismic tomography images. In addition, the coseismic slip models revealed that the trenchward forearc of the structural blocks between the Offshore Hidaka tectonic line and the Honjo-Sendai tectonic line fitted well with the coseismic slip area of the 2011 Tohoku-Oki earthquake. These findings suggest that the structural blocks bounded by these two tectonic lines slipped rapidly trenchward when the mainshock occurred. The M7 earthquakes were also concentrated along these two tectonic lines, thereby suggesting a close relationship between seismic activity and the inherited geological structure of the overriding plate in the NE Japan forearc.

Factors affecting wood-bark adhesion for debarking of shrub willow

Debarking has the potential to make short rotation woody crops more economically attractive by both improving the quality of woody material and creating a new product stream from separated bark. Thus, in this research the ultimate strength of the wood-bark interface was studied relative to the development and optimisation of debarking systems and strategies. Four genetically different cultivars of shrub willow, three harvest times and different moisture levels were tested at a quasi-static extension rate of 0.5 mm min−1. Regression models show that moisture content has a negative correlation with wood-bark bond strength, which is similar to observations made on other species. Harvesting time had a significant effect on wood-bark strength of shrub willow (p

Mode-I behavior of adhesively bonded composite joints at high loading rates

This study investigates the high loading rate behavior of adhesively bonded carbon/epoxy composite joints under mode I loading. A computationally guided experimental setup is developed to study the mode-I behavior of composite joints in the range of quasi-static to high loading rates. A double cantilevered beam specimen with wedge insert type loading setup is used to conduct quasi-static and dynamic experiments. For the dynamic loading, a modified split Hopkinson bar is used to load the sample at high rates. The local deformation field is measured using high Spatio-temporal resolution digital image correlation (DIC). From the experiments, the mode-I energy release rate is calculated from the load, crack extension and crack root rotation data measured using load cell and DIC. A decrease in the initiation fracture toughness with increase in loading rate was observed which is attributed to the strain rate dependent behavior of the epoxy-based film adhesive. For both quasi-static and high loading rates, a mixed adhesive-cohesive failure is observed from the fracture surface analysis.

High-strain rate compressive behavior of Douglas fir and glubam

Understanding the high-strain rate compressive behavior and related energy absorption performances of Douglas fir and glubam is crucial for the application of these materials under impact/blast loading. To this aim, quasi-static tests using a universal testing machine and dynamic tests using an expressly-designed aluminum Split-Hopkinson Pressure Bar (SHPB) apparatus were performed. A total of 90 specimens of Douglas fir and two types of glubam were tested to characterize the quasi-static and high-strain rate compressive characteristics covering a large range of strain rates from 2.2×10-3s-1 to around 1×103s-1. Two different directions were studied: along the fiber (longitudinal) and across the fiber (transverse). The compressive stress-strain curves were analytically described through two uniaxial phenomenological models taken from the literature. Using the fitted parameters of the phenomenological models, it is provided an original definition of the Dynamic Increase Factor (DIF). Finally, energy absorption performances are investigated. The results of this investigation show that Douglas fir and glubam are strain rate sensitive and this sensitivity highly depends on the loading direction.

Mechanical behavior and texture evolution of WE43 magnesium-rare earth alloy in Split-Hopkinson Pressure Bar and Taylor Impact Cylinder Testing

Mechanical behavior and texture evolution of Mg rare-earth alloy WE43 is investigated for strain-rates 10−3/s to upwards of 105/s for the two material conditions - as-cast (AC) and T6 age hardened, rolled plate (RT6). The high strain-rate behavior is tested using both Taylor cylinder impact tests (TC) and split Hopkinson pressure bar tests (SHB) and bulk textures are obtained using neutron diffraction. Unlike the quasi-static strained material, AC and RT6 SHB retained high hardening rates throughout the test, even up to 30% true strain. Moreover, the high strain-rate data revealed that the RT6 material has a much higher strength than the AC material, but similar hardening rates despite significantly different initial texture. The flow stress near yield increased up to 10% for RT6 and up to 30% for AC as the strain-rate increased six orders of magnitude from quasi-static rates 10−3/s to 103/s. Neither material exhibited significant plastic anisotropy over the broad range of strain rates, despite the fact that the RT6 material had a moderately strong initial texture. In the TC tests, the geometric cross-sectional changes and texture along the cylinder from the cylindrical sample foot to head are measured and from the neutron diffraction texture analysis, upper-bound estimates of twin volume fraction are obtained as well as dislocation density from analyzing diffraction peak broadening. Recorded geometrical changes along several loading directions show that the material has deformed homogeneously under impact. Analysis of deformed textures indicates that {101¯2} extension deformation twinning occurred in the RT6 condition over the range of strain rates, with an upper bound estimate of 40% twin volume fraction for approximately 0.10-0.25 true strain. The peak texture components after the impact have their c-axes closely aligned with the impact direction. These observations are presented and rationalized in the paper.

Strain-rate and off-axis loading effects on the fibre compression strength of CFRP laminates: Experiments and constitutive modelling

A series of dynamic longitudinal compression tests have been performed on cross-ply IM7/8552 specimens cut at different off-axis angles to produce different combinations of compression and shear stresses. Together with results from previous quasi-static tests of the same kind, quasi-static and dynamic fibre kinking failure envelopes have been obtained using classical laminate theory. This new experimental data has been compared against predictions from the leading fibre kinking theories, made rate-dependent by using rate-dependent in-plane shear properties, and show that, while they can accurately predict the effects of strain rate on the uniaxial compression strength, they are unable to capture the effects of shear, neither at quasi-static nor dynamic rates. Instead, a simpler more phenomenological approach is proposed to predict the rate-dependent fibre kinking strength of FRP laminates under multi-axial loads until the micromechanics can be more accurately described.

Experimental and numerical characterization of titanium-based fibre metal laminates

The effect of applying a laser surface treatment to the metal plies in a fibre metal laminate (FML) based on a titanium alloy and a fibre reinforced composite has been evaluated and the response compared to that measured on a comparable untreated laminate. It has been shown that applying a laser fluence of 4.54 J/cm2 to the titanium layers in the FML results in a good bond strength between the titanium foil and the glass fibre/PEKK composite. The response of the FMLs under dynamic loading was studied and compared with that measured at quasi-static rates. It has been shown that FMLs based on a titanium alloy exhibit a rate-sensitivity, both in terms of energy absorption and the maximum impact force. An examination of cross-sections removed from samples following testing at dynamic and quasi-static loading rates indicated that the FMLs absorbed energy through plastic deformation, tearing of the metal layers, delamination and fibre fracture. Finite element models were then developed to simulate the response of the FMLs under impact loading. The simulated results were validated against the corresponding experimental data by comparing both the load–displacement traces as well as the resulting failure modes, with good agreement being observed in most cases.

Microstructural and Mechanical Characterization of Square‐Celled TRIP Steel Honeycomb Structures Produced by Electron Beam Melting

The powder‐bed additive manufacturing (AM) technology electron beam melting (EBM) is suitable for the production of metallic components with complex geometries. Therefore, it is an appropriate method of generating lightweight structures to save material and energy, which is of high importance for the transport industry. The microstructure of additive manufactured materials is usually characterized by columnar grains shape, oriented along the building direction. Therefore, the mechanical properties are strongly affected by anisotropy. Herein, the synthesis and the microstructure of CrMnNi transformation‐induced plasticity (TRIP) steel by EBM is examined. This steel is well suited for AM due to the formation of fine‐grained microstructure instead of columnar one through multiple‐phase transformations. Square‐celled honeycomb structures are produced and improved to generate damage‐tolerant struts. The removal of slightly sintered powder from the cavities of square‐celled structures is quantified. Out‐of‐plane compression tests at quasistatic loading rates show that the strength of honeycomb structures rises with increasing energy input during EBM due to lower material porosity. Therefore, an enhanced mechanical energy absorption capacity is achieved which is favorable for the production of cellular structures.

On the kinetics of texture development in Al-Mg alloy under high strain rate tension

In this work, the texture development in Al-4%Mg alloy sheet material under high strain rate (HSR) tensile deformation at 3.1 × 103 s−1 is studied using split Hopkinson tension bar test. Quantitative analysis of texture evolution for three different strain levels is carried out through strain mapping using digital image correlation and subsequent characterization using X-ray diffraction. Critical comparisons to the texture development in the sample deformed at quasi-static rate are made concurrently. The results show that Cube and Copper components strengthen upon tensile deformation under both the loading rates. However, the characteristics of lattice rotation, rotation path, lattice rotation rate and strength of texture components under HSR deformation are significantly different from that under quasi-static loading. Although the deformation texture developed at HSR is relatively weak, Copper component is noted to evolve sharply amongst other texture components. Further, the electron backscatter diffraction measurements reveal that the dislocation density and intragranular misorientation developed under HSR tension are relatively lower than at quasi-static rate for comparable strains. The inclusive analysis of results obtained reveals that texture weakening in Al-Mg alloy under HSR tension occurs as a result of relatively higher strain hardening and absence of dynamic strain aging.

Predicting deformation behavior of α-uranium during tension, compression, load reversal, rolling, and sheet forming using elasto-plastic, multi-level crystal plasticity coupled with finite elements

An elasto-plastic self-consistent (EPSC) polycrystal plasticity formulation is adapted to model deformation of wrought α-uranium (α-U) accommodated by a combination of elasticity, dislocation glide, and deformation twinning. The EPSC model incorporates a strain-path, strain rate, and temperature sensitive dislocation density-based hardening law for the evolution of resistance to slip, twinning, and de-twinning and a slip system-level kinematic back-stress law to influence the driving force for activation. The model is used to interpret the complex deformation behavior of α-U as a function of strain-path and temperature. Samples of α-U with different initial orientation distributions are experimentally evaluated in simple compression, tension, and load reversal at room temperature and in compression and rolling at 573 K under a quasi-static deformation rate. Evolution of texture and twinning is characterized using electron backscattered diffraction and in-situ and ex-situ neutron diffraction during deformation. It is observed that the behavior of the material is highly anisotropic owing to its low-symmetry orthorhombic crystal structure and different activation stresses for crystallographic deformation modes. The model is calibrated and validated under these deformation conditions and predicts the stress-strain responses, amount of twinning, texture evolution, and lattice strains with one set of parameters for the hardening and back-stress evolution laws. Subsequently, the developed model is used as a constitutive law in the implicit finite element (FE) framework to simulate drawing of a hemispherical part from a rolled sheet of α-U. Here, the FE-EPSC model is a two-level homogenization scheme with EPSC relating the grain-level to the polycrystalline aggregate-level response, while the FE framework scales the polycrystalline to the part-level response. The simulation results and insights from the calculations, such as location dependent texture evolution is in good agreement with experiments.