Jianghui Mao

This paper presents an alternative method of material damage evaluation based on the X-ray computer tomography-detected microdefects and multiscale computer simulation. This is achieved by developing a method of the digital diagnosis and full-field numerical calculation of material degradation in macroscopic material test specimens. The method comprised three basic components: (a) digital detection and processing of micro/mesoscale material defects of macroscopic material test specimens; (b) multilevel meshing and multilevel finite element analysis for evaluating local/global material degradation; and (c) synchronized experimental and numerical determination of material damage. The unique contributions of the proposed approach include (a) a multilevel finite element meshing and analysis scheme that makes the full-field estimation of material degradation in macroscopic test specimens computationally tractable on regular workstations, (b) full-field exploration of mesoscale material d...

This article presents development of a damage-coupled viscoplastic constitutive model with temperature consideration. The model is subjected to an explicit nonlocal treatment within the characteristic length that is not limited to one local element as conventional ones. Temperature dependence of material behavior is incorporated into the model to account for the material property degradation at elevated temperature. A test program to determine the correlation between material parameters and temperature is also presented. The nonlocal damage-coupled viscoplastic material model is implemented in a commercial finite element program ABAQUS through its user-defined material subroutine UMAT using a semi-implicit time integration scheme. The model is applied to predict the behavior of 63Sn37Pb soldered structure. The mesh sensitivity of the model is discussed, as well as the efficiency of deduced consistent tangent modulus.

A proposed damage model is used for investigating the deformation and interfacial failure behavior of an adhesively bonded single-lap thick joint made of S2 glass/SC-15 epoxy resin composite material. The bonding material is 3M Scotch-Weld Epoxy Adhesive DP405 Black. Continuum damage mechanics models are used to describe the damage initiation and final failure at or near the interface. The effect of adhesive overlap length, thickness, and plasticity on the interfacial shear and normal stresses is studied. Experimental and analytical data are used to validate the proposed damage models.

A proposed damage model is used for investigating the deformation and interfacial failure behavior of an adhesively bonded single-lap thick joint made of S2 glass/SC-15 epoxy resin composite material. The bonding material is 3M Scotch-Weld Epoxy Adhesive DP405 Black. Continuum damage mechanics models are used to describe the damage initiation and final failure at or near the interface. The effect of adhesive overlap length, thickness, and plasticity on the interfacial shear and normal stresses is studied. Experimental and analytical data are used to validate the proposed damage models.

The ability to quantify the material damage at different length scales iscritical in the multiscale analysis of material behavior from nanoscale to macroscale. In this article, on the basis of the equivalence of complementary elastic energy wepropose a multiresolution rule that transforms different levels of material defects to the equivalent degradation of material properties. It facilitates a sequential memory efficient processing of massive material defects in a multiresolution framework, and also supports a functionality of partial damage conversion to serve different needs insubsequent numerical analyses. Numerical simulation was conducted with differentsettings of material defects. The analysis results indicate the efficacy of the proposed method, offering a potential (i) to interface between multiscale material defects and(ii) as an effective method of homogenization for the determination of the damage variable in continuum damage mechanics.

This article presents the development of a generalized nonlocaldamage-coupled material model. The model introduces the concept of cumulativedamage gradient through a set of damage evolution equations within the irreversiblethermodynamics framework. The proposed material model is implemented in a commercial finite ele-ment code ABAQUS (Version 6.5) via its UMAT subroutine. The implementation of this model on ABAQUS is described with a focus on the nonlocal treatment together with the derivation of the consistent tangent modulus (Jacobian). As a numerical example, the nonlocal damage model is applied to center-cracked specimen made of aluminum alloy 2024-T3. Comparison is made between the computed results andexperimental ones. The validity of the proposed model is examined, and its effectiveness for engineering application is elucidated.

Based on the theory of Damage Mechanics, the damage of material is considered as an internal state variable. It describes the deterioration of material upon loading. If it is locally defined, then, the damage of a material point is completely determined by the state of its own. It is also known as local material damage model, a popular way in current modeling of material damage. On the other hand, the nonlocal damage model provides an alternative way to define the material damage of a point through certain domain on where it lays. This approach yields a more accurate stress description than that of the local model in describing high stress/damage gradient structures, such as an observed crack in a structure.

To describe the damage characteristics in high stress/damage gradient structures, a nonlocal damage model is developed in this work. The nonlocal damage is treated as weighting average over a prescribed domain in terms of explicitly expressed damage gradient. The proposed damage model together with the constitutive model of aluminum alloy are implemented in a commercial finite element code ABAQUS (Version 6.5) via its UMAT subroutine. Full implicit integration scheme is used to solve for internal variables of integration point (IP) at each time interval. Then the weighting average is carried out over a particular domain characterized by Characteristic Length for the processing IP. Finally, consistent tangent modulus (Jacobian) of this IP is provided to ABAQUS to assemble global stiffness matrix. A center- cracked specimen made of AL2024-T3 is chosen as a typical example to illustrate the advantage of introducing nonlocal damage and also to provide a validation of the proposed constitutive model. In addition, the effects of Characteristic Length and strain-hardening coefficient on the proposed nonlocal model are investigated and discussed.

One major consideration on the development of the nonlocal damage model is the stress/damage gradient observed in a loaded structure. As illustrated in chapter 2, there is no difference in the computed stress/damage distribution based on the nonlocal damage model and corresponding local ones in a homogeneous structure where no gradient exists when loaded. To examine the effect of stress/damage gradient on structures, notches of varying radii are designed to examine notch sensitivity on the nonlocal model.

Unlike a typical strain hardening material of AL2024-T3, 63Sn37Pb exhibits strain-softening behavior. The proposed nonlocal damage model is coupled into the constitutive model of 63Sn37Pb to show its applicability on strain softening materials. Again, the nonlocal damage model together with the constitutive model of 63Sn37Pb is incorporated into ABAQUS via its UMAT subroutine. Similar to the strain hardening material AL2024-T3, the mesh-sensitivity is remarkably reduced, demonstrating that the proposed nonlocal damage model is capable of being applied to both strain hardening materials and strain softening ones.