•An improved shear modified GTN model is proposed for ductile fracture prediction of materials under different stress states;•The damage parameters are calibrated using a FE inverse calibration ...method incorporating the Latin hypercube design, Kriging approximate model and NLPQL optimization method;•Influence of each damage parameter on damage evolution under different stress states is analyzed by a unit cell model;•Reasonable identification results are obtained for the aluminum alloy 6061 using the proposed method;•The mechanism of deformation and failure are studied using fracture morphology analyses and damage analyses using FE simulation in microscale and macroscale perspectives.
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To solve the problem that the original GTN model cannot accurately simulate ductile fracture of material under low stress triaxiality, many scholars have made shear improvement to it, but these shear modified GTN models have their own advantages and disadvantages and the parameters are difficult to be determined. An improved shear modified GTN (ISMGTN) model containing two independent damage mechanisms is proposed for ductile fracture prediction of materials under different stress states. The shear damage parameters, tensile damage parameters and the hardening parameters are identified using a FE inverse identification method incorporating the Latin hypercube design, Kriging approximate model and NLPQL optimization method performed in the optimization software ISIGHT. Influence of each damage parameter on damage evolution under different stress states is analyzed by a unit cell model. Accuracy of the ISMGTN model and feasibility of the damage parameters identification method are verified by performing them on a material aluminum alloy 6061 with 0°, 30° and 60° shear tests, plate tensile tests, and notched tensile tests. Additionally, fracture morphology analyses of the fractured specimen and contour plots of the effective tensile damage and effective shear damage from the FE analysis using the identified parameters are performed to study possible mechanism of deformation and failure in microscale and macroscale perspectives, respectively, and a good consistence is obtained.
•The interfacial bond performance of the UHPC-NSC is sufficient for rehabilitation of concrete structures.•Proper surface treatment and moisture degree are the major factor for the bond capacity of a ...UHPC-to-NSC interface.•The bond strength of the UHPC-NSC interface can be evaluated by using the cohesion and friction coefficient.
Ultra-high performance concrete (UHPC) is suitable for the durable rehabilitation and strengthening of deteriorated normal strength concrete (NSC) structures due to its excellent characteristics, such as superior compressive strength, high tensile capacity, and extremely low permeability. However, a successful repair depends on whether the UHPC-NSC interface can provide good bonding performance under various applied loads and working conditions throughout its service life. In this study, the bond characteristics between the NSC substrate and UHPC layer were investigated by applying slant shear, splitting tensile, and direct tensile tests, and the interfacial bond strength and corresponding failure modes were observed. Seven studied factors, the roughness of substrate surface, UHPC age, moisture degree of substrate, curing condition of UHPC, strength of NSC substrate, bonding agent and expansive agent were included to explore their influences on the bond strengths, and the UHPC-NSC interfacial bonding enhancement mechanism. Results indicated that the UHPC overlay achieved superior interfacial bond performance for the rehabilitation of concrete structures, with the appropriate surface roughness and substrate wetness. The UHPC-NSC composite samples met the minimum requirements of the interfacial bond strengths specified in ACI 546-06 and achieved an “excellent” bonding quality. Furthermore, the friction coefficient for calculating interfacial bond strength was back-calculated according to the results obtained from the corresponding direct tensile and slant shear tests.
In this work, mode I fracture parameters of steel fibre reinforced self-compacting concrete (SFRSCC) were derived from the numerical simulation of indirect splitting tensile tests. The combined ...experimental and numerical research allowed a comparison between the stress–crack width (σ–w) relationship acquired straightforwardly from direct tensile tests, and the σ–w response derived from inverse analysis of the splitting tensile tests results. For this purpose a comprehensive nonlinear 3D finite element (FE) modeling strategy was developed. A comparison between the experimental results obtained from splitting tensile tests and the corresponding FE simulations confirmed the good accuracy of the proposed strategy to derive the σ–w law for these composites. It is concluded that the post-cracking tensile laws obtained from inverse analysis provided a close relationship with the ones obtained from the experimental uniaxial tensile tests.
In recent years, additive manufacturing has gained importance, especially since the full melting of the raw material in the selective laser melting process enables the fabrication of directly ...deployable components. However, the multiple directional dependencies involved result in an anisotropic material behavior. The raw material under investigation in this study was the precipitation-hardenable AlSi10Mg alloy, with the main focus on the positioning and inclination effects, which were studied on six characteristic orientations. In addition, the superimposed effects based on the surface condition and thermal post-treatments were taken into account. The examination contained: comprehensive tensile tests with strain gauges, detecting strains in two directions; detailed surface hardness investigations in various conditions; and micro-section investigations. Major direction dependencies were revealed and the tensile strength and the surface hardness results, coupled with annealing procedures, exhibited consistent results, explaining the encountered findings. The Young’s modulus varied between 62.5GPa to 72.9GPa with the Poisson’s ratio fluctuating between 0.29 and 0.36. Regarding the tensile strength, the UTS ranged from 314MPa to 399MPa with breaking elongations spanning from 3.2% to 6.5% in the non-heat-treated condition.
Additive manufacturing is a one of the most promising technology nowadays that offers the advantages not only in building products of complex shapes but also of complex materials. A complex structure ...is characteristic for Functionally Graded Composites, which basics, principles and applicability have been widely investigated over the last years. The present study is focused on the detailed investigation of mechanical and structural properties of FGC consisting of stainless steel 316L and Inconel 718 processed by Blown Powder Directed Energy Deposition system. Mechanical properties within single layers and over layers transitions were investigated with the use of tensile tests and fracture toughness tests. Metallographic and fractographic investigations were carried out. Metallographic investigation revealed the differences in the interfaces between single material layers, nucleation processes and subsequent growth of the grains of the used materials. It has been shown that the formation of transition region between deposited single material layers is dependent on the order of material deposition since different deposition parameters are used for certain material. Evaluation of the tensile properties showed that the mechanical properties of a single material layers are in very good agreement regardless of the deposition height. However, the types of interfaces considering to the results of fractographic observations affect the tensile performance of the Functionally Graded Composite. The fracture toughness test results demonstrate changes in the mechanism of crack propagation at the interface between materials with respect to the type of transition. Furthermore, the material layers interfaces turned out to be the weakest points of the Functionally Graded Composite.
•A great continuation of the Inconel's grains is observed when deposited on 316 L.•316 L grains show rare crystallographic traceability when deposited on the IN718.•Tensile properties are in very good agreement regardless of the deposition height.•Material layers interfaces are the weakest points of the functionally graded material.•The crack propagation mechanism in FGC is dependent on interface type.
For more than 150 years, it has been considered proven that hydrogen generally degrades the mechanical performance of metals. Nevertheless, there is no consensus on the exact mechanisms, how hydrogen ...affects plastic deformation. The strain rate sensitivity of a material results from a thermally activated contribution to the rate-determining deformation process, e.g. to dislocation slip or dislocation grain boundary interaction. In this study, the extent to which hydrogen affects thermally activated dislocation motion and hence the strain rate sensitivity was investigated. For this purpose, specimens were cathodically charged in situ, and subjected to nanoindentation. In addition, macro-tensile tests with strain rate jumps were performed varying the temperature into the cryogenic range, to inhibit effusion, but also to test the effect of hydrogen on the activation parameters of deformation. Hydrogen was shown to increase the strain rate sensitivity of f.c.c. nickel, whereas it is not affected for a structural steel with a b.c.c. lattice. The activation volume for plastic deformation in a direct comparison between nanocrystalline and coarse-grained f.c.c. nickel and the b.c.c. structural steel shows, that the rate-determining deformation mechanism appears to change for f.c.c. but not for the b.c.c. material.
•Hydrogen increases the strain rate sensitivity and thus decreases the activation volume in coarse and nano-grained nickel.•In b.c.c. structural steel the activation volume is not affected by hydrogen.•Combination of tensile and nanoindentation strain rate jumps tests allows also to test materials that suffer from severe hydrogen embrittlement.
The mechanical properties and tensile deformation mechanism of Ti-6.5Al-3.5Mo-1.5Zr-0.25Si alloy (TC11) with different percentages of primary α phase (αp) were studied by conventional and interrupted ...tensile tests, respectively. The results show that the strength of TC11 alloy increases with the increase of the percentage of αp. During tensile deformation, TC11 alloy with different percentages of αp exhibits similar behavior. The deformation mechanism in αp transforms from single slip to multiple slip. While, the microcracks first initiate at the interface of two phases, propagate along it, and then propagate through the transformed β phase (βt) and finally through αp. Finally, a method for predicting the strength of TC11 alloy considering the sizes of αp and βt is proposed.
The present work investigates the influence of hydrogen on the mechanical properties of four multiphase high strength steels by means of tensile tests on notched samples. This was done by performing ...mechanical tests on both hydrogen charged and uncharged specimens at a cross-head displacement speed of 5 mm/min. A considerable hydrogen influence was observed, as the ductility dropped by 8–60%. In order to demonstrate the influence of diffusible hydrogen, some parameters in the experimental set-up were varied. After tensile tests, fractography was performed. It was found that hydrogen charging caused a change from ductile to transgranular cleavage failure near the notch with a transition zone to a fracture surface with ductile features near the centre.
•The effect of hydrogen charging on four high strength steels is evaluated by tensile tests.•TRIP lost 60% of its ductility, DP 54% and FB 37% at a deformation speed of 5 mm/min.•HSLA steel did not suffer from hydrogen embrittlement at a deformation speed of 5 mm/min.•The fracture surface of the H-charged samples showed brittle transgranular cleavage failure.•Decreasing the deformation speed to 0.05 mm/min increased the hydrogen embrittlement.
In this work, a precipitation strengthened high entropy alloy was subjected to thermo-mechanical process in order to homogenize the grain microstructure. Tensile tests from room temperature to ...1000 °C were conducted; microstructures were observed by scanning electron microscope and transmission electron microscope. Formation of cellular precipitate along grain boundaries was observed and could be related to hot tensile ductility drop at 750 °C (1023 K). Experimental analysis has indicated that driving force for the formation of cellular precipitates could be resulted from the chemical instability of supersaturation after annealing and migration of grain boundaries, and this phenomenon could be suppressed either through alloy design to increase gamma-prime solvus, and to hinder the migration of grain boundaries. This study serves as a guideline to design composition and thermo-mechanical process for precipitation strengthened high entropy alloys.
•A precipitation strengthened high entropy alloy was subjected to thermos-mechanical process.•Chemical instability of supersaturation and migration of grain boundaries can cause formation of cellular precipitate.•Tensile ductility drop occurred around 750 °C in present high entropy alloy due to cellular precipitate.•Cellular precipitate does not affect tensile ductility at room temperature.•The formation of cellular precipitate can be suppressed by alloy design and cooling rate during heat-treatment.
In this study, the recycled Al-Mg-Mn-Fe alloys having different Fe levels (0.1%, 0.5%, and 0.8%) were successfully developed by applying squeeze casting without heat treatment. Optical and scanning ...electron microscopy, synchrotron X-ray tomography and radiography, X-ray diffraction, and tensile tests combined with thermodynamic calculations were used to study the correlation between the microstructural evolution and mechanical properties. The results showed that for the alloys with an applied pressure of 75 MPa, as the Fe increase from 0.1% to 0.8%, the yield strength (YS), and ultimate tensile strength (UTS) increased from 122 MPa and 244–146 MPa and 289 MPa, and elongation decreased from 34% to 12%. Even though Fe additions increased the volume fraction of Fe-rich intermetallic phases, it significantly increased the UTS and YS. The synchrotron X-ray tomography and deep-etched results both show that the 3D morphology changed from individual Chinese-script to interconnected plate-like. The 3D morphology of 0.8Fe alloy clearly demonstrate that the hole partially or whole penetrated the entire rectangle-shaped Al6(FeMn) phases, which is due to the close-packed plane growth. In-situ synchrotron X-ray radiography results showed the facet growth behaviour of Al6(FeMn) in 0.8Fe alloys during solidification with a long needle-like shape. The size of primary Al6(FeMn) phases decreases, whereas their number increases with increasing cooling rate. Moreover, the applied pressure was beneficial in refining the size of α-Al grains and Fe-rich phases and reducing the volume fraction of pores, thus contributing to the improvement of strength and elongation. The in-situ tensile test results indicated that the crack initiated in the Fe-rich phases and pores, and the slip lines were blocked by the Fe-rich phases resulting in the strengthening of the secondary phases.
•Fe addition increases the ultimate tensile and yield strength of Al-Mg-Mn alloys.•The 3D morphology of Fe-rich phases is reconstructed by synchrotron X-ray tomography.•The formation mechanism of hollow structure of primary Al6(FeMn) phases is revealed.•Fe-rich phases block the movement of slip lines and improves the strength.
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