Numerical and Experimental Studies on Hydro-Forming of a Thin Metallic Disc

This paper deals with the hydro-forming of a flat thin metallic disc to achieve a forward domed disc which will be subsequently adopted to manufacture a rupture disc. The plastic deformation induced by the hydraulic energy is numerically simulated through an isotropic hardening plasticity model using a non-linear explicit finite element analysis (FEA). The variation in disc’s central deformation, thickness, equivalent plastic stress and equivalent plastic strain with respect to the applied hydraulic pressure are determined from FEA simulations. The hydro-forming setup is then designed and manufactured, and the metallic disc is experimented under hydro-forming process. The reduction in thickness due to stretching of the thin disc is evaluated from experiment and simulation and a close agreement is found. This research attempt helped in finalizing the hydro-forming fluid pressure, the feasibility and the accuracy of practically achieving the desired geometry of the metallic disc. The near-fixidity effects on abrupt variation in sheet thickness and plastic strain are well captured through simulations which are very difficult to be studied through hydro-forming experiments.

Numerical Constraints on Folding and Thrusting in Jiudong Basin: Implication for the Northeastward Growth of Qinghai–Tibetan Plateau

Under regional tectonic shortening in the northern margin of Qinghai–Tibetan Plateau, the fold-and-thrust belts composed of four thrust faults (North Qilian-Shan, North Yumu-Shan, South Heli-Shan, and North Jintanan-Shan) formed from southwest to northeast discontinuously sequentially in the Jiudong Basin area during Late Cenozoic. Meanwhile, the North Qilian-Shan, Yumu-Shan, and Heli-Shan ranges were formed successively, as the Earth's local surface was unequally uplifted. In this study, based on geological and geophysical observations, a simple two-dimensional elastic-plastic numerical finite element method model for a southwest-northeast section in Jiudong Basin is successfully established to simulate the spatiotemporal evolution of the local fold-and-thrust belts. Results show that the computed equivalent plastic strain concentration zones and the four observed thrust faults are consistently correlated in spatial position orientation and time sequence. The simulated upper-surface deformation is congruent with the observed topographic peaks and uplift sequences of the North Qilian-Shan, Yumu-Shan, and Heli-Shan ranges. This study provides a geodynamic basis for understanding the growth mechanism of the northern margin of Qinghai–Tibetan Plateau under tectonic horizontal shortening. Also, we provide a thorough sensitivity analysis for the model parameters of this particular geologic setting. Our sensitivity simulations, considering systematic case variations about the regional geometrical-material parameters, suggest the manifestation of three different possible evolution patterns of fold-and-thrust belts for a wedge above a decollement layer, with wedge plastic deformation migrating from 1) thick to thin end (well-known), 2) thin to thick end, and 3) both ends to middle. Finally, our results suggest that in this region, further growth of mountain ranges is expected to continue in the future.

Finite element modeling of sub-surface damage while machining aluminum based metal matrix composites

Sub-surface damage during machining of aluminum-based metal matrix composites (MMCs) has been modeled using finite element models. These models are based on reinforcement particles size and volume fractions and particles are distributed uniformly in the metal matrix. In order to simulate particle debonding cohesive zone elements (CZE) have been incorporated along the parting line. In addition, failure criteria based on brittle fracture have been added for ceramic particles to simulate particle fracture. To reduce computational time and simplify the model both CZE and particle fracture is limited to the reinforced particles along the parting lines facing the tip of the cutting tool. The damage depth beneath the machined surface is measured by using the non-zero plastic strain values in the equivalent plastic strain contours obtained from the FE models. The results were compared against published experimental data and found to be in good agreement.

Influence of Crystal Plasticity Parameters on the Strain Hardening Behavior of Polycrystals

The effective mechanical properties of a polycrystal depend directly on the single-crystal properties of each grain and its crystallographic orientation with respect to the load axis. While the micromechanical approach has been used quite extensively to study the influence of grain shape and crystallographic texture on the resulting mechanical behavior of a polycrystal, the influence of the crystal plasticity parameters, which describe the constitutive behavior of the single crystal, requires to be investigated systemically because, typically, these parameters are fitted to describe a given material behavior. In the current research, this gap is filled by systemically studying the effect of changes in crystal plasticity parameters on the effective mechanical properties of polycrystals. The numerical model employed here consists of a representative volume element of 100 grains, and the material properties are described by using a non-local crystal plasticity model. A proper homogenization technique was used to homogenize the micromechanical results to an effective macroscopic material response. The equivalent stress versus equivalent plastic strain curve was obtained numerically by introducing the Voce-type hardening law, mimicking the material behavior in uniaxial tensile tests. The four parameters of the Voce-type hardening law were fitted to the macroscopic stress-strain curves, and the correlation between the crystal plasticity parameters and the Voce parameters has been studied, which is an efficient way to study the influence of microscopic material descriptions on the macroscopic behavior of polycrystals.

Energy Absorbing Properties Analysis of Layers Structure of Titanium Alloy Ti6Al4V during Dynamic Impact Loading Tests

This paper presents the testing methodology of specimens made of layers of titanium alloy Ti6Al4V in dynamic impact loading conditions. Tests were carried out using a drop-weight impact tower. The test methodology allowed us to record parameters as displacement or force. Based on recorded data, force and absorbed energy curves during plastic deformation and sheet perforation were created. The characteristics of the fractures were also analyzed. The impact test simulation was carried out in the ABAQUS/Explicit environment. Results for one, two, and three layers of titanium alloy were compared. The increase in force required to initialize the damage and the absorbed energy during plastic deformation can be observed with an increase in the number of layers. The increase in absorbed energy is close to linear. In the simulation process, parameters such as Huber–Mises–Hencky stress value, equivalent plastic strain, temperature increase, and stress triaxiality were analyzed.

Force Controlled Low Cycle Fatigue (LCF) Life Evaluation Methodology Based on Unstabilized Material Properties

Low Cycle Fatigue (LCF) is a prominent failure mechanism in many design components; therefore, an evaluation of cycles to failure in this regime is of high importance. Most international standards recommend a closed loop strain-controlled mode specimen testing in this regime. However, the ꜪN data obtained from this test is not suitable for life evaluation of parts enduring force-controlled history during actual service without correction for control mode. Many existing procedures, which accounts for cyclic strain stabilization during force-controlled loading may significantly complicate the finite elements analysis (FEA) at solving or post processing stages and are often an inherent source of uncertainty. A heuristic, cost effective and sufficiently accurate approach for LCF life estimation is advocated. The method involves only two force loading FEA simulations, one of the actual parts and the other of the test specimen, using initial unstabilized stress strain curve, followed by a limited number of force-controlled specimens testing. Actual part and specimen life correlation performed using first loading unstabilized equivalent plastic strain value Ꜫp1 under locality and similitude assumptions. Unstabilized strain vs. number of cycles to failure curve Ꜫp1N is constructed and discussion regarding specimen geometry considerations for providing sufficient accuracy is included. Method validation and crack propagation study are provided.

Analysing the Performance of Hollow Core Slabs Strengthened with CFRP

External bonding with CFRP (Carbon Fiber Reinforced Polymers) has been investigated over the last decade, as it represents a viable technique of strengthening existing prestressed concrete structures, including hollow-core slabs (HCS) with non-circular voids. The high performance of these carbon fibers has been validated through a large volume of experimental and numerical research and yet there are a few issues which remain controversial in simulating their behaviour with the finite element modelling. Although the CFRP mechanical properties are provided by the manufacturers, they are not satisfactory for a complete understanding of the analysis and design approach of HCS strengthened with CFRP. The present research is conducted on prestressed HCS with non-circular voids. The strengthening method consisted in the application of the composite material on the slab’s end internal regions of the voids, on a 500mm length: 1 layer and 2 layers. The objective of this study is to emphasize the effect of damage in the CFRP strips and moreover the interface effectiveness on the CFRP strengthened HCS. Damage is predicted using Hashin’s initiation criteria and the cohesive behaviour in the interface is used to analyse the epoxy resin which bonds the CFRP sheets to the hollow-core units. A plastic damage model was used for modelling the concrete, after a parametric study regarding the dilatancy angle and viscosity parameter was conducted for the most appropriate choice of concrete damage plasticity parameters. The overall procedure consists of numerical FE modelling in Abaqus software. Two different modelling possibilities of CFRP-to-concrete interface were studied: a tie constraint connection was first used and secondly the contact bonding was defined with the cohesive behaviour option of the contact interaction property. The results are provided in terms of load-displacement response, equivalent plastic strain and distribution of Von Misses stresses in the CFRP strips.

Experimental verification of computational and sensitivity analysis on substrate deformation and plastic strain induced by hollow thin-walled WAAM structure

Purpose This paper aims to numerical and experimental analysis on substrate deformation and plastic strain induced by wire arc additive manufacturing. Design/methodology/approach The component has the form of a hollow, rectangular thin wall consisting of 25 deposition layers of SS316L on an SS304 substrate plate. Thermo-mechanical finite element analysis was applied with Goldak’s double-ellipsoidal heat-source model and a non-linear isotropic hardening rule based on von Mises’ yield criterion. The layer deposition was modelled using simplified geometry to minimize overall pre-processing work and computational time. Findings A new material modelling of SS316L was obtained from the chemical composition of the evolved component characterized by scanning electron microscope/energy dispersive X-ray and further generated by an advanced material-modelling software JMatPro. In defining heat-transfer coefficients, transient thermometric analysis was first performed in the bead and on the substrate, which was followed by an adjustment of the heat-transfer coefficients to reflect the actual temperature distribution. Based on the adjusted model and boundary conditions, sensitivity analysis was conducted prior to the ultimate simulation of substrate deformation and equivalent plastic strain. Furthermore, this simulation was verified by conducting a series of automated wire + arc additive manufacturing tests using robotic gas Metal arc welding with distortion measured by coordinate-measurement machine and equivalent plastic strain measured by optical three-dimensional-metrology measurements (Gesellschaft für Optische Messtechnik). Originality/value It can be concluded that a proper numerical computation using the adjusted model and property-evolved material exhibits a similar trend with acceptable agreement compared to the experiment by yielding an error percentage up to 30% for deformation and up to 21% for equivalent plastic strain at each individual measurement point.

A Coupled Elastoplastic Damage Dynamic Model for Rock

Rock dynamic constitutive model plays an important role in understanding dynamic response and addressing rock dynamic problems. Based on elastoplastic mechanics and damage mechanics, a dynamic constitutive model of rock coupled with elastoplastic damage is established. In this model, unified strength theory is taken as the yield criterion; to reflect the different damage evolution law of rocks under tension and pressure conditions, the effective plastic strain and volumetric plastic strain are used to represent the compressive damage variable and the equivalent plastic strain is used to represent the tensile damage variable; the plastic hardening behavior and strain rate effect of rocks are characterized by piecewise function and dynamic increase factor function, respectively; Fortran language and LS-DYNA User-Defined Interface (Umat) are used to numerically implement the constitutive model; the constitutive model is verified by three classical examples of rock uniaxial and triaxial compression tests, rock uniaxial tensile test, and rock ballistic test. The results show that the constitutive model can describe the dynamic and static mechanical behavior of rock comprehensively.

ON MULTISCALE DAMAGE MODELLING OF HETEROGENEOUS MATERIALS USING NONLOCAL CONTINUUM THEORY

An overview of the modelling of quasi-brittle as well as ductile damage is given. The multiscale procedure employing the nonlocal continuum theory is described in more detail. The softening is introduced at the microlevel in the microstructural volume element and after that the homogenization procedure state variables are mapped at the macrolevel material point via the scale transition approach. In the case of quasi-brittle softening the C1 continuous finite element discretization is applied at both micro- and macrolevel. At the modelling of ductile damage response, the macrolevel is also discretized by the C1 finite element formulation, while the microscale utilizes quadrilateral mixed finite elements employing the nonlocal equivalent plastic strain and gradient-enhanced elastoplasticity. All approaches presented are verified in the standard examples.