Shihua Nie

An Irreversible Thermodynamics Theory for Damage Mechanics of Solids

The entropy production is a non-negative quantity based on irreversible thermodynamics and thus serves as a basis for the systematic description of the irreversible processes occurring in a solid. In this paper, a thermodynamic framework has been presented for damage mechanics of solid materials, where entropy production is used as the sole measure of damage evolution in the system. As a result, there is no need for physically meaningless empirical parameters to define a phenomenological damage potential surface or a Weibull function to trace damage evolution in solid continuum. In order to validate the model, predictions are compared with experimental results, which indicates that entropy production can be used as a damage evolution metric. The theory is founded on the basic premise that a solid continuum obeys the first and the second laws of thermodynamics.

Influence of Interfacial Bond Strength on Fatigue Life and Thermo-Mechanical Behavior of a Particulate Composite: An Experimental Study

Experimental studies conducted on a particular cast acrylic composite demonstrate the significant influence of the interfacial bond strength between filler particles and the polymer matrix on the fatigue life, and mechanical properties. The composite studied in this project is composed of a ductile matrix, which is lightly cross-linked poly-methyl methacrylate (PMMA) and hard, brittle alumina trihydrate (ATH) agglomerate particle filler. In the study, high, moderate, and low levels of interfacial adhesion between the matrix and the filler are investigated, while all the other material properties are kept constant. Monotonic tension and fatigue tests are conducted at different temperatures. Material degradation is presented in terms of elastic modulus degradation, load-drop parameter, and plastic strain range.

Time Dependent Behavior of a Particle Filled Composite PMMA/ATH at Elevated Temperatures

Creep behavior of particle filled acrylic composite materials become a major concern when they are used at elevated temperatures. Therefore, for elevated temperature finite element simulations any constitutive modeling requires time— temperature dependent material properties. Unfortunately, this type of data is very difficult to come across in the literature, due to a very long time needed to conduct creep testing. In this study, the creep properties of acrylic casting dispersion PMMA/ ATH were obtained experimentally and the observed characteristics of this material are presented with the experimental data. The underlying deformation mechanisms and the steady-state creep response are also discussed.

Failure Mechanisms in PMMA/ATH Acrylic Casting Dispersion

Acrylic casting dispersion is used to fabricate particulate composites such as poly-methyl methacrylate (PMMA) filled with a fine dispersion alumina trihydrate (ATH). This composite is subjected to severe temperature variations during in-service conditions, giving rise to high thermal stresses which lead to failure by cracking. The influence of the interfacial bond strength between a particle and the matrix on the failure mechanism of acrylic casting dispersion has been investigated using in situ observations during tensile and compressive loadings. Experiments show that the failure in pure tension occurs differently than in flexure loading where the failure process is believed to be more complex. During tensile loading, it is observed that macroscopic fracture is initiated in the clusters of the reinforcing particles because of the strong interfacial bonding strength between the filler particles and matrix. For weak interfacial bond strength, the macroscopic fracture is initiated by separation of filler agglomerates from the matrix.

Experimental study of failure mechanisms in particle filled acrylic composites

Particle filled solid surface composites are used to fabricate kitchen countertops and sinks which may be subjected to severe temperature variations, giving rise to high thermal stresses. These stresses may lead to failure by cracking in regions subjected to large temperature variation. An aim of this paper is to investigate mechanisms of failure in solid surface materials using in situ observations during tensile, compressive and fatigue loading and to define test configurations that give meaningful measurements of material properties. Experiments show that the failure in tension occurs in several stages. In flexural loading the failure process is more complex. Consequently, flexural testing should not be used as a substitute for the measurement of ultimate tensile strength in a particle filled solid surface composite. The application of conventional damage mechanics to describe the failure of test specimens is also discussed.

Irreversible Thermodynamics for Damage Mechanics of Solid Materials

In this paper a thermodynamic framework has been presented for damage mechanics of solids materials. Traditional damage mechanics theory uses damage potential function to trace damage evolution. In this framework entropy production is used as a measure of damage in the system. As a result there is no need for physically meaningless empirical material parameters to define a damage potential function. It is assumed that entropy production is non-negative for solids.Copyright

Experimental Damage Mechanics of Microelectronic Solder Joints Under Fatigue Loading

Fatigue damage is a progressive process of material degradation. The objective of this study is to experimentally qualify the damage mechanism in solder joints in electronic packaging under thermal fatigue loading. Another objective of this paper is to show that damage mechanism under thermal cycling and mechanical cycling is very different. Elastic modulus degradation under thermal cycling, which is considered as a physically detectable quantity of material degradation, was measured by Nano-indenter. It was compared with tendency of inelastic strain accumulation of solder joints in Ball Grid Array (BGA) package under thermal cycling, which was measured by Moire interferometry. Fatigue damage evolution in solder joints with traditional load-drop criterion was also investigated by shear-strain hysteresis loops from strain-controlled cyclic shear testing of thin layer solder joints. Load-drop behavior was compared with elastic modulus degradation of solder joints under thermal cycling. Following conventional Coffin-Manson approach, S-N curve was obtained from isothermal fatigue testing with load-drop criterion. Coffin-Manson curves obtained from strain controlled mechanical tests were used to predict fatigue life of solder joints. In this paper it is shown that this approach underestimates the fatigue life by an order of magnitude. Results obtained in this project indicate that thermal fatigue and isothermal mechanical fatigue are completely different damage mechanism for microstructurally evolving materials.© 2002 ASME