Confined Transducer Geometries to Enhance Sensitivity to Thermal Boundary Conductance in Frequency-Domain Thermoreflectance Measurements

Quantifying the resistance to heat flow across well-bonded, planar interfaces is critical in modern electronics packaging architectures, particularly as device length scales are reduced and power demands continue to grow unabated. However, very few experimental techniques are capable of measuring the thermal resistance across such interfaces due to limitations in the required measurement resolution provided by the characterization technique (i.e., Rth < 0.1 mm2·K/W in steady-state configurations) and restrictions on the thermal penetration depth that can be achieved as a result of the heating event that is typically imposed on a sample’s surface (for optical pump-probe thermoreflectance techniques). A recent numerical fitting routine for Frequency-domain Thermoreflectance (FDTR) developed by the authors1 offers a potential avenue to rectify these issues if the transducer’s geometry can be confined. This work utilizes numerical simulations to evaluate the sensitivity of FDTR to a range of thermal boundary resistance (TBR) values as a function of the thermal resistance of adjacent material layers. Experimental measurements are performed across a handful of different material systems to validate our computational results and to demonstrate the the extent to which confined transducer geometries can improve our sensitivyt to the TBR across so-called “buried” interfaces when characterized with FDTR.

A new type of S-N equation and its application to multiaxial fatigue life prediction

The S-N equation is one of the most important equations in fatigue model investigation. A majority of fatigue models, including multiaxial fatigue model and mean effect models, are established on the basis of the S-N equation. Obviously, an accuracy of the S-N equation is very important. Taking into account that the S-N equation is, in fact, an empirical one in which the material constants are determined by numerical fitting fatigue experimental data, in this paper, the S-N equation can be improved, by further processing these fatigue experimental data, to present a new type of S-N equation that is more accurate than the S-N equation. The new type of S-N equation is called a similar S-N equation in this paper. By using a large number of experimental data of metallic materials reported in literature, an accuracy of the similar S-N equation has been proven.

Approximate Calculation Method for Noncentral t-Distribution Quantile

In the process of structural design and structural performance evaluation, the inference of the reliable life of the structure and the representative value of the material strength is a necessary work. The determination of material strength is the presumption of the quantile of the normal distribution, and the determination of the confidence level of the quantile of the normal distribution involves the noncentral t-distribution function. However, the calculation of the quantile is very complicated and is often provided in the form of a numerical table, which often involves multiparameter interpolation calculation, so it is not convenient to apply. The existing approximate calculation methods for noncentral t-distribution quantiles have strict application conditions, and the calculation process is relatively cumbersome. It is still difficult to meet actual needs in terms of fitting accuracy, application range, and convenience. In this paper, a new calculation method for noncentral t-distribution quantiles is proposed by introducing new probability expressions and related approximate distributions, based on theoretical derivation and numerical fitting. The comparative analysis results show that the method not only is convenient for calculation but also has the advantages of higher accuracy and wider application range, and it is more in line with the actual needs of engineering.

A numerical fitting procedure for the embedded atom method interatomic potential and a bridged finite element-molecular dynamics method for large atomic systems

This thesis presents a powerful numerical fitting procedure for generating Embedded Atom Method (EAM) inter-atomic potentials for pure Face Centred Cubic (FCC) and Body Centred Cubic (BCC) metals. The numerical fitting procedure involves assuming a reasonable parameterized form for a portion of the EAM potential, and then fitting the remaining portion to select thermal and elastic properties of the metal. Molecular Dynamics (MD) simulation is used to effect the fitting procedure. The procedure is used to generate an EAM potential for copper, an FCC metal. This resulting EAM potential is used to conduct MD simulations of perfect copper crystals containing voids of different geometries. Following this, a bridged Finite Element-Molecular Dynamics (FE-MD) method is presented, which can be used to simulate large atomic systems much more efficiently than MD simulation alone. The method implements a novel element discretization scheme proposed by the author that is so general that it can be applied to any system of objects interacting with each other via any potential (simple or complex, EAM or otherwise). This bridged FE-MD method is used to reanalyze the voids in the copper crystal lattice. The resulting virial stress increment patterns are found to agree remarkably with the earlier MD simulation results. Furthermore, the bridged FE-MD method is much quicker than the pure MD simulation. These two facts prove the power and usefulness of the bridged FE-MD method, and validate the proposed element discretization scheme

A numerical fitting procedure for the embedded atom method interatomic potential and a bridged finite element-molecular dynamics method for large atomic systems

This thesis presents a powerful numerical fitting procedure for generating Embedded Atom Method (EAM) inter-atomic potentials for pure Face Centred Cubic (FCC) and Body Centred Cubic (BCC) metals. The numerical fitting procedure involves assuming a reasonable parameterized form for a portion of the EAM potential, and then fitting the remaining portion to select thermal and elastic properties of the metal. Molecular Dynamics (MD) simulation is used to effect the fitting procedure. The procedure is used to generate an EAM potential for copper, an FCC metal. This resulting EAM potential is used to conduct MD simulations of perfect copper crystals containing voids of different geometries. Following this, a bridged Finite Element-Molecular Dynamics (FE-MD) method is presented, which can be used to simulate large atomic systems much more efficiently than MD simulation alone. The method implements a novel element discretization scheme proposed by the author that is so general that it can be applied to any system of objects interacting with each other via any potential (simple or complex, EAM or otherwise). This bridged FE-MD method is used to reanalyze the voids in the copper crystal lattice. The resulting virial stress increment patterns are found to agree remarkably with the earlier MD simulation results. Furthermore, the bridged FE-MD method is much quicker than the pure MD simulation. These two facts prove the power and usefulness of the bridged FE-MD method, and validate the proposed element discretization scheme

A modified Sneddon model for the contact between conical indenters and spherical samples

Indentation techniques have proven to be effective to characterize the mechanical properties of materials. For the elastic deformation, the commonly used models are Hertz model and Sneddon model. However, neither of them works for indenting the spherical samples using the pyramid or conical indenter. Therefore, one modified Sneddon model has been developed to determine the Young’s modulus of spherical samples from indentation results. In this study, the effects of sample diameter and indenter angles on indentation tests were investigated by finite element method (FEM). The empirical correction parameters in the new mathematical model were introduced based on dimensional analysis and determined by the numerical fitting to FEM results. Experimental tests with different conical indenters have demonstrated that the new model is capable to reliably determine the Young’s modulus of the spherical samples. The new model can fill the gap of the contact mechanics and enrich the experimental solid mechanics for the interpretation of indentation results. Graphic abstract

Experimental Investigation of Geosynthetic-Reinforced Pile-Supported Composite Foundations under Cyclic Loading

A series of model tests were conducted in this study to investigate the deformation characteristics of geosynthetic-reinforced pile-supported (GRPS) composite foundations under cyclic loading. The effects of the applied load, the number of geogrid layers, and types of piles on the performance of the GRPS composite foundation were studied through 1g physical models of composite foundation with well-planned instrumentation. Furthermore, a numerical fitting method was used to assess the relationship between the foundation settlement and the number of load cycles. The results show that with the increase in the magnitude of cyclic load and the number of load cycles, the settlement of GRPS composite foundations and the strain of the pile and geogrid increased accordingly. Adding rigid piles and increasing the number of geogrid layers both could reduce the settlement of GRPS composite foundations, while adding rigid piles was more effective. The relationship between the foundation settlement and the number of load cycles can be expressed by an exponential regression function. The pile strain varied from place to place that the strain of the upper part of the pile was greater than that of the lower part. The geogrid showed a significant impact on the load transfer mechanism of the composite foundation as the geogrid closer to piles endured larger strain. It is critical to consider the variation of the pile strain and the geogrid strain under cyclic loading in the geotechnical practice of composite foundation. The model test results also suggest that the use of GRPS system can effectively reduce the composite foundation settlement. This paper can provide useful references for developing the theoretical framework and design guides for GRPS composite foundations under cyclic loading.