Mohammad Shafinul Haque

Mechanical Behavior of Differently Oriented Electron Beam Melting Ti–6Al–4V Components Using Digital Image Correlation

Mechanical properties of additive manufactured metal components can be affected by the orientation of the layer deposition. In this investigation, Ti–6Al–4V cylindrical specimens were fabricated by electron beam melting (EBM) at four different build angles (0 deg, 30 deg, 60 deg, and 90 deg) and tested as per ASTM E8 Standard Test Methods for Tension Testing of Metallic Materials. With the layer-by-layer fabrication suggesting granting anisotropic properties to the builds, strain fields were recorded by digital image correlation (DIC) in the search for shear effects under uniaxial loads. For the validation of this measuring method, axial strains were measured with a clip extensometer and a virtual extensometer, simultaneously. Failure analysis of the specimens at different orientations was conducted to evidence the recording of shear strain fields. The failure analysis included fractography, optical micrographs of the microstructure distribution, and failure profiles displaying different failure features associated with the layering orientation. Additionally, an experimental study case of how the failure mode of components can potentially be designed from the fabrication process is presented. At the end, remarks about the shear effects found, and an insight of the possibility of designing components by failure for safer structures are discussed. [DOI: 10.1115/1.4040553]

The Stress-Sensitivity, Mesh-Dependence, and Convergence of Continuum Damage Mechanics Models for Creep

The classic Kachanov–Rabotnov (KR) creep damage model is a popular model for the design against failure due to creep deformation. However, the KR model is a local approach that can exhibit numerically unstable damage with mesh refinement. These problems have led to modified critical damage parameters and alternative constitutive models. In this study, an alternative sine hyperbolic (Sinh) creep damage model is shown to (i) predict unity damage irrespective of stress and temperature conditions such that life prediction and creep cracking are easy to perform; (ii) develop a continuous and well-distributed damage field in the presence of stress concentrations; and (iii) is less stress-sensitive, is less mesh-dependent, and exhibits better convergence than the KR model. The limitations of the KR model are discussed in detail. The KR and Sinh models are calibrated to three isotherms of 304 stainless steel creep test data. Mathematical exercises, smooth specimen simulations, and crack growth simulations are performed to produce a quantitative comparison of the numerical performance of the models. [DOI: 10.1115/1.4036142]

Finite-Element Analysis of Waspaloy Using Sinh Creep-Damage Constitutive Model Under Triaxial Stress State

The creep deformation and damage evolution of nickel base superalloy (Waspaloy) at 700 °C are studied using the classic Kachanov–Rabotnov (KR) and a recently developed Sin-hyperbolic (Sinh) model. Uniaxial creep deformation and Bridgman rupture data collected from literature are used to determine the model constants and to compare the KR and the Sinh solutions. Finite-element (FE) simulations on a single eight-node element are conducted to validate the accuracy of the FE code. It is observed that KR cannot predict the creep deformation, damage, and rupture life of nickel base superalloys accurately using one set of constants for all the stress levels. The Sinh model exhibits a superior ability to predict the creep behavior using one set of constants for all the stress levels. Finite-element analysis (FEA) on 3D Bridgman notched Waspaloy specimen using the Sinh model is conducted. The results show that the Sinh model when combined with a representative stress equation and calibrated with experimental data can accurately predict the “notch effect” observed in the rupture life of notched specimen. Contour plots of damage evolution and stress redistribution are presented. It is demonstrated that the Sinh model is less stress sensitive, produces unconditional critical damage equal to unity at rupture, exhibits a more realistic damage distribution around the crack tip, and offers better crack growth analysis than KR.

Exploiting Functional Relationships Between MPC Omega, Theta, and Sinh-Hyperbolic Continuum Damage Mechanics Model

The MPC Omega and Theta models for creep deformation and life prediction have become popular in recent years. Both models offer better prediction than classical constitutive models such as Norton Power law, Bailey-Norton law, and Norton-Soderberg law to name a few. The Omega model uses a strain hardening approach and requires two material constants for creep deformation and life prediction. An analytical solution to the constants are available and it is easy to manipulate and implement numerically. However, the analytical damage of the Omega model predicts an unrealistic linear damage evolution. The Theta model uses a time-hardening approach, and requires four constant that are a function of stress and temperature. For materials under isothermal conditions, with tertiary creep dominant deformation, the Theta model constants can be determined using only two constants. Life prediction using the Theta and Omega models depends on the final creep strain. The final creep strain observed in an experiment is stochastic; dependent on the material, testing conditions, and operator. The statistics of final creep strain must be investigated before the Theta or Omega models can be applied. In literature, some authors add a nonlinear damage variable to the Theta model; however, critical damage at rupture is not unity violating the assumptions of continuum damage mechanics. There is a superior Sin-hyperbolic continuum damage model available in the literature that can be used to overcome these problems. It is hypothesized that a functional relationships exist between the three models and these relationships can be exploited to achieve more accurate and easy to implement creep deformation and life predictions. In this study, the relationships between the constants of MPC Omega, Theta, and a Sin-hyperbolic CDM models are determined analytically. The sin-hyperbolic model incorporates a continuum damage variable in the creep strain rate equation. The damage function exhibits a more realistic elliptical path and is constructed such that damage is always unity at rupture. This function facilitates conversion of one models’ constants to the constants of the other two. The relationships between the constants are identified, while maintaining dimensional homogeneity. Using the derived relationships, the three models can be easily compared and the disadvantages of each respective model can be avoided. Experimental data at four different configurations of stress (6.3 to 36.5 ksi) and temperature (1200 to 1800°F) (sixteen data sets) for Hastelloy X is used to compare the models. Creep rupture data at seven temperature levels (600 to 1000°C) and a wide stress range (5 to 500 MPa) is used to analyze life prediction. The constants for each model are determined. Using one models’ constants and the derived relationships; the predictions of the other two models are generated. It is observed that the relationship generated curves agree with experimental data.Finally, it is demonstrated that using the derived relationships, the most useful aspects of each model can combined. An elliptical damage evolution curve is obtained for the Omega model. The final creep strain rate dependency problem of the Theta model can be avoided. It is observed that the Sinh model becomes more flexible and easy to implement.Copyright

Comparison of a New Sin‐Hyperbolic Creep Damage Constitutive Model with the Classic Kachanov‐Rabotnov Model Using Theoretical and Numerical Analysis

The creep deformation, damage, and life of creep susceptible components are a function of temperature, stress and strain rate. In this study the Kachanov-Rabotnov (KR) creep damage constitutive model and a recently developed Sinh creep damage constitutive model are compared in terms of accuracy, considerations/assumptions, constants evaluation techniques, flexibility in use, and limitations for 304 Stainless Steel (STS). Twenty tests performed at four different configurations of stress and temperature (five repeats for each) are collected from literature and used. It is found that the novel Sin-hyperbolic model exhibits lower constant dependency, is easier to apply, and more accurately models the creep deformations and damage evolution of 304 STS. The Sin-hyperbolic model produces a continuous damage (from zero to unity) by normalizing the experimental data while the KR model produces critical damage values well below unity. It is found that overall the new Sin-hyperbolic model offers more flexibility and prediction accuracy.

A novel sin-hyperbolic creep damage model to overcome the mesh dependency of classic local approach Kachanov-Rabotnov model

All materials include defects or voids. Under load, these defects can serve as a nucleus of crack initiation and propagation. Numerical analysis of notched specimen can provide prediction of creep damage or crack propagation of materials containing defects or initial cracks. However, FEM analyses using the local CDM approach exhibits mesh size dependency, and exhibits localized damaged zone around the crack. In this study, a novel Sin-hyperbolic (Sinh) model is demonstrated to significantly mitigate mesh dependency and exhibits a nonlocal damage distribution around the crack when compared to the classical Kachanov-Rabotnov model. Finite element analysis of circular notched specimen of 304 Stainless-Steel (SS) is performed. Contour plots of damage evolution, and mesh dependency of crack growth rate are discussed in detail. It is demonstrated that the Sinh model offers better creep damage, and crack growth analysis and significantly improves the stress sensitivity, damage localization, and mesh-dependency of numerical results.Copyright