Time-dependent Material Properties Research Articles

Crack tip mechanics based on progressive damage of arrow: Hierarchy of singularities and multiscale segments

Crack tip mechanics based on progressive damage of arrow: Hierarchy of singularities and multiscale segments

A robust digital image correlation technique for high-resolution characterisation of microelectronic packaging materials

In this study, we demonstrate a robust Digital Image Correlation (DIC) technique and describe the methods implemented in the DIC code for characterising microelectronic packaging materials. We show how sample preparation may be entirely eliminated by using the natural speckle inherent in specular (rough) surfaces with the help of a robust correlation measure implemented in the code. We use DIC to characterise both time-independent and time-dependent material properties at high spatial and displacement resolution. We measure the Coefficient of Thermal Expansion (CTE) of common metals such as copper and aluminium followed by characterisation of 19 Printed Circuit Board (PCB) coupons. We characterise the CTE of microscale Alumina (Al2O3) specimen using its natural speckle. We calculate the strain and therefore the modulus during mechanical testing of copper, epoxies as well as fibre-reinforced composite materials. Finally, we characterise the time dependent behaviour of a microfibre-reinforced composite (RT/Duroid) at high temperature.

Study of interface mechanical properties between thin-film Au and polymethyl methacrylate

The existing research on interface properties between heterogeneous materials mainly focuses on semiconductor-metal and dielectric materials, but little on organic-inorganic ones. In recent years, the nanoscale phenomena related to the mechanical properties of organic/inorganic material interfaces is gaining a lot of attention and becoming a new area of nanorelated research. Since gold (Au) exhibits excellent optical, electrical, and mechanical properties, it can be applied to nanooptics, mechanics, and electronics. This study explores the nano effect of the mechanical properties between the interface of Au and heterologous polymethyl methacrylate (PMMA) with different side groups, i.e., isotactic-PMMA, syndiotactic-PMMA, and atactic-PMMA. These heterologous PMMA thin films are prepared using a spin-coater to deposit the different side groups of PMMA upon Au thin film. A sputter technique is used to form Au thin films with different thicknesses. An indenter probe is applied by adding different forces on its tip into the PMMA and Au thin films to realize the interface mechanical properties such as hardness and Young's modulus. Finally, the time-dependent properties of viscoelastic materials are evaluated by using this harmonic nanoindentation test.

Modeling Package-Induced Effects on Molded Hall Sensors

The consideration of package-induced effects during the design process of microelectromechanical systems (MEMS) sensors is still of major importance in order to estimate its influence on performance and reliability. A method for predicting the influence of the packaging and assembly process on the performance of a Hall sensor is described. The geometry of the device, the behavior of the packaging materials, as well as the packaging process are regarded using this method. At first, the complex material behavior of adhesives and molding compounds was investigated. It is shown, that for modeling the influence of the packaging on the sensor performance correctly, the nonlinear temperature- and time-dependent material behavior has to be taken into account. Differences of the coefficients of thermal expansion of the packaging materials lead to thermo-mechanical stress in the package. Due to the time-dependent material properties, this results in a change of the offset and the sensitivity of the sensor over time and may lead to a sensor working outside its specification. The piezoresistive and piezo-Hall coefficients of semiconductor Hall plates were measured; thereby the stress- and temperature-dependency of the Hall plates were determined. According to the results, package-induced stresses lead to a change of the offset voltage up to 80% of the full-scale while the sensitivity changes by plusmn4 percentage.

Micro/macro-crack growth due to creep–fatigue dependency on time–temperature material behavior

The implicit character of micro-structural degradation is determined by specifying the time history of crack growth caused by creep–fatigue interaction at high temperature. A dual scale micro/macro-equivalent crack growth model is used to illustrate the underlying principle of multiscaling which can be applied equally well to nano/micro. A series of dual scale models can be connected to formulate triple or quadruple scale models. Temperature and time-dependent thermo-mechanical material properties are developed to dictate the design time history of creep–fatigue cracking that can serve as the master curve for health monitoring. In contrast to the conventional procedure of problem/solution approach by specifying the time- and temperature-dependent material properties as a priori, the desired solution is then defined for a class of anticipated loadings. A scheme for matching the loading history with the damage evolution is then obtained. The results depend on the initial crack size and the extent of creep in proportion to fatigue damage. The path dependent nature of damage is demonstrated by showing the range of the pertinent parameters that control the final destruction of the material. A possible scenario of 20 yr of life span for the 38Cr2Mo2VA ultra-high strength steel is used to develop the evolution of the micro-structural degradation. Three micro/macro-parameters μ ∗, d ∗ and σ ∗ are used to exhibit the time-dependent variation of the material, geometry and load effects. They are necessary to reflect the scale transitory behavior of creep–fatigue damage. Once the algorithm is developed, the material can be tailor made to match the behavior. That is a different life span of the same material would alter the time behavior of μ ∗, d ∗ and σ ∗ and hence the micro-structural degradation history. The one-to-one correspondence of the material micro-structure degradation history with that of damage by cracking is the essence of path dependency. Numerical results and graphs are obtained to demonstrate how the inherently implicit material micro-structure parameters can be evaluated from the uniaxial bulk material properties at the macroscopic scale. The combined behavior of creep and fatigue can be exhibited by specifying the parameter ξ with reference to the initial defect size a 0. Large ξ (0.90 and 0.85) gives critical crack size a cr = 11–14 mm (at t < 20 yr) for a 0 about 1.3 mm. For small ξ (0.05 and 0.15), there results critical a cr = 6–7 mm (at t < 20 yr) for a 0 about 0.7–0.8 mm. The initial crack is estimated to increase its length by an order of magnitude before triggering global to the instability. This also applies ξ ≈ 0.5 where creep interacts severely with fatigue. Fine tuning of a cr and a 0 can be made to meet the condition of t = 20 yr. Trade off among load, material and geometric parameters are quantified such that the optimum conditions can be determined for the desired life qualified by the initial–final defect sizes. The scenario assumed in this work is indicative of the capability of the methodology. The initial–final defect sizes can be varied by re-designing the time–temperature material specifications. To reiterate, the uniqueness of solution requires the end result to match with the initial conditions for a given problem. This basic requirement has been accomplished by the dual scale micro/macro-crack growth model for creep and fatigue.

Determination of viscoelastic material parameters by step-loading nanoindentation

Methodical aspects of studies of the time-dependent mechanical properties of materials by step-loading nanoindentation using a Berkovich indenter have been considered. The possibility is shown to determine the viscosity and relaxation time of viscoelastic materials from experiments on step-loading nanoindentation. The potentialities of step-loading nanoindentation have been demonstrated by the example of the photoplastic effect in chalcogenide glasses.

Nanoindentation study of the time-dependent mechanical behavior of materials

The methodological aspects of studying the time-dependent mechanical properties of materials subjected to nanoindentation under pulsed loading and unloading of a Berkovich indenter are considered. It is shown that the viscosities and relaxation times of viscoelastic materials can be determined from nanoindentation experiments. The advantages of the pulsed nanoindentation technique are demonstrated with the photoplastic effect in chalcogenide glasses.

Surface representation of time-dependent thixotropic material properties

Surface representation of time-dependent thixotropic material properties

A thermo-mechanical viscoelastic analysis of orthotropic materials

Abstract This study introduces a numerical algorithm for nonlinear thermo-mechanical viscoelastic analyses of orthotropic composite materials and structures that follow thermo-rheologically complex behaviors. The algorithm is derived based on implicit stress integration solutions within a general displacement based finite element (FE) framework for small deformations and uncoupled thermo-mechanical problems. The Schapery’s nonlinear single integral form is generalized for modeling viscoelastic responses of an orthotropic medium. The effects of stress and temperature are incorporated in the elastic and time-dependent material properties, which allow prediction of time-dependent responses under general stress and temperature histories. A recursive–iterative method is developed to calculate the current stress state from the given strains and temperature, and the history variables stored at the previous time step. Furthermore, a consistent tangent stiffness matrix is formulated to enhance equilibrium and avoid divergence at the structural level. Available experimental data of nonlinear viscoelastic responses of orthotropic composite materials are used to verify the above numerical algorithm. An integrated recursive–iterative algorithm with FE structural analysis is also presented for creep responses of orthotropic composite plate with a hole.

Windows-Based Top-Down Cracking Design Tool for Florida

Top-down cracking has been found to be a predominant mode of distresses of asphalt pavements in Florida. Therefore, it is important to accommodate top-down cracking in the design of asphalt mixtures and pavement structures. After a multiyear study on top-down cracking supported by the Florida Department of Transportation, the University of Florida developed a top-down cracking model based on hot-mix asphalt fracture mechanics. This paper presents the implementation of the Florida cracking model into a mechanistic–empirical (M-E) flexible pavement design framework. Based on the energy ratio concept, a new M-E pavement design tool for top-down cracking has been developed. In the Level 3 M-E design, a series of semiempirical models were developed for estimation of time-dependent material properties. With incorporation of the material properties models, the design tool is capable of performing pavement thickness design as well as pavement life prediction for top-down cracking in Florida. The thickness design is optimized for different traffic levels, mixture types, and binder selections, and the optimization is an automated process. This design tool has been packed into Windows-based software, making it convenient to use for pavement design engineers.

Design and Control of a Piezoelectric Driven Fatigue Testing System for Electronic Packaging Applications

Failure of solder joints for electronic packaging is an important issue for controlling the reliability of semiconductor devices. However, the complicated coupling between mechanical stressing and temperature and time dependent material properties makes it difficult to explore the fundamental failure control mechanisms using the existing accelerated thermal cycling methods. In addition, the testing speed is also severely restricted by the thermal time constant of the characterization system. In order to decouple the mechanical stress effect from other factors as the first step toward exploring the control mechanisms of failure, a piezoelectric-based fatigue characterization system is developed to replace the thermal cycling and provide fast and purely mechanical stressing cycles. A self-tuning based (STR) adaptive controller is also developed to provide accurate process control during experiments for compensating stiffness variation due to fatigue crack growth. It is found that this STR regulator is more robust than the traditional PID controller. The bandwidth of the system is approximately 70 Hz and is currently restricted by the equivalent time constant of the piezoelectric material. Nevertheless, this speed is sufficient for conducting a successful fatigue testing of solder joints. Finally, preliminary fatigue experiments have been performed on Sn63Pb 37 solders and the reduction of stiffness due to crack growth is clearly visible while the actuation performance is consistent and stable during the entire testing period. In the future, it is possible to operate in conjunction with a temperature control unit and a creep testing scheme to explore both the temperature and time dependent nature of solders in order to fully understand the failure control mechanisms of packaging

On the early breakdown of semisolid suspensions

Semisolid materials are suspensions of metal alloys which are processed while in a mushy state. In this state, the suspensions behave as viscoplastic materials with time-dependent material properties. During processing, the material is injected at high speed into a mold cavity with the process lasting only fractions of a second. The short-term transient material response is thus very important for the understanding of the rheology and the further development of the process. A theory based on the Herschel–Bulkley flow model appropriate for the short-term breakage of welded bonds in semisolid metal slurries is proposed. The “strength” of the slurry due to these bonds is assumed to be a function of a coherency parameter which is proportional to the number of welded bonds that break during the application of shear. The evolution of this parameter is described by a first-order kinetics. Using a novel and simple computational method, well suited for free and moving-boundary problems, we study the flow between two coaxial cylinders and obtain accurate results for the evolution of the structure, the extent of the yielded domain and the evolution of the stress. The results and conclusions apply directly to the design and analysis of rotational rheometer experiments for semisolid metal slurries and suspension systems exhibiting similar behavior.

Lifetime predictions of EPR materials using the Wear-out approach

The Wear-out approach for lifetime prediction, based on cumulative damage concepts, is applied to several ethylene propylene rubber (EPR) cable insulation materials. EPR materials typically follow “induction-time” behavior in which their material properties change very slowly until just before failure, precluding the use of such time-dependent properties to predict failure. In the Wear-out approach, a material that has been aged at its ambient aging temperature T a or at a low accelerated aging temperature is subsequently aged at a higher “Wear-out” temperature T w in order to cause the material to reach its “failure” condition. In the simplest case, which involves the same chemical processes underlying degradation at T a and T w, a linear relationship is predicted between the time spent at T a and the time required at T w to complete the degradation. Data consistent with this expectation are presented for one of the EPR insulation materials. When the degradation chemistry at the two temperatures is different, a linear relationship between the time spent at T a and the time required at T w to complete the degradation is not generally expected. Even so, the Wear-out results for a second EPR material, which has evidence of changing chemistry, are reasonably linear and therefore useful from a predictive point-of-view. The Wear-out approach can therefore be used to transform non-predictive time-dependent material property results into predictive lifetime estimates. As a final example, the Wear-out approach is applied to an EPR insulation that had been aged in a nuclear power plant environment (∼51 °C) for times up to 23 years to show its likely viability for the hundreds of years predicted at this aging temperature from accelerated aging tests on EPR insulation materials.

Cumulative Effects of Load Frequency and Velocity on the Lumbar Spine Response during a Repetitive Lifting Task

Work—related low back disorders arise from a complex interaction of events. One aspect of interest is cyclic or repetitive tasks, which are performed on a daily basis in a variety of situations, and often for extended periods of time. The specific questions to be addressed in this study are the effects of the load frequency and time-dependent material properties on the lumbar spine component behavior. The newly developed and validated, subject specific, finite element model of lumbar spine is used in conjunction with kinematic data imported from human subjects experimental testing of repetitive sagittally-symmetric lifting and lowering tasks. Creep and energy dissipation in the lumbar spine components are used to quantify the effect of lifting frequency and velocity on the potential for injury over an eight-hour workday.

Nanoindentation Study of Polymer Based Nanocomposites

Nanoindentation is a useful technique to measure hardness as well as elastic and timedependent plastic properties of materials with nanometer resolution. The measurement of elastic modulus of polymeric materials remains challenging due to their viscoelastic behavior. Clay reinforced nylon6 nanocomposites are found to have great improvement in the elastic modulus and tensile strength due to exfoliated hybrid structure. However, its mechanical properties have not been well investigated. In the present study, hardness and elastic modulus of nylon6-5wt%clay nanocomposites were investigated using nanoindentation. Creep effects of the nanocomposites on the unloading stiffness, which directly relates to the elastic modulus, were studied under various unloading rates and holding periods. It was found that the elastic modulus and hardness of nylon6-5wt%clay nanocomposites increased by 58% and 80%, respectively, as compared to pure nylon6. Experimental results for both polycarbonate and nylon6-5wt%clay nanocomposites showed that loading rate had no significant effects on the unloading stiffness. However, stiffness decreased to more consistent values after longer holding periods (more than 30 sec) and at faster unloading rates. The results indicated that creep behavior of the polymers affects the measurement of the unloading stiffness and may possibly overestimate the elastic modulus. Errors in the stiffness measurements from nanoindentation could be minimized with appropriate loading, unloading and holding conditions.

Viscoelastic properties of a glass fabric composite at elevated temperatures: experimental and numerical results

Abstract In this work, we characterize the three-dimensional time and temperature dependent material properties of a plain weave, woven roving composite material. The material of interest is an Eglass/vinylester plain weave manufactured using the SCRIMP technology by the Naval Surface Warfare Center, Carderock Division. Experimental data is presented for in-plane loadings of longitudinal tension, transverse tension, and in-plane shear for three different temperatures consisting of 24, 66, and 93 °C (75, 150, and 200 °F). Experimental limitations preclude determining out-of-plane normal and shear behavior for the weave. Therefore, we conducted finite element micromechanics analyses (FEA) to predict linear viscoelastic material properties that could not be determined from experimental testing. The FEA model was developed assuming a balanced weave. Finite element predictions are compared to experimental data whenever possible to lend confidence to the analysis. In general, FEA predictions compared favorably with experimental data.

FINITE ELEMENT SIMULATION OF IN VIVO TOOTH MOBILITY IN COMPARISON WITH EXPERIMENTAL RESULTS

The objective of this study was to characterize the material properties of the periodontal ligament (PDL). Since the PDL undergoes the largest deformations when a load is applied to the tooth crown, its material properties mainly govern the resulting tooth deflection. By comparing experiments on tooth mobility with a Finite Element simulation using individual and realistic geometry models of the measured teeth, information about the mechanical properties of the PDL can be obtained. To investigate in vivo tooth mobility, a special experimental setup has been developed. The experimental results showed highly non-linear and time dependent material properties of tooth deflection as they are known for other soft biological tissues.6 Since in vivo tooth deflection is not an uniaxial tensional experiment, it is not possible to determine material parameters of the PDL. For this reason, a geometry model of the measured tooth was generated using computer tomography data and in a Finite Element simulation tooth deflection under external forces was calculated. A comparison of the simulation with the experimental data lead to an optimized characterization of the PDL in view of its mechanical properties.

Constructability Considerations for Balanced Cantilever Construction

Constructability considerations are of importance in segmental bridge construction as the occurrence of failures of bridge superstructures under construction has highlighted. For a safe and economical construction process, the interactions between construction loads and the permanent structural system, depending on the chosen erection method, need to be evaluated. Segmental bridges can be constructed with methods like balanced cantilever construction, where individual spans are counterweighted about their substructure support. Time-dependent material properties like strength of the newly cast concrete, as well as shrinkage, creep, and relaxation influence the structural system resistance. Resulting stresses in the unfinished bridge structure during construction can even exceed the final stresses during service. This paper makes an educational contribution by illustrating these concepts with the case study of the Wilson Creek Bridge in Virginia. This five-span, cast-in-place bridge was constructed using balanced cantilever construction. Two form travelers were used to construct cantilever arms about the pier tables until the full span was finally connected at midspan; casting cycle duration for a single segment was 7 days. The contractor implemented major constructability changes in both the design and the construction of the bridge to facilitate a more economical construction process.

Performance Evaluation of Jointless Bridges

A recent trend in bridge design has been toward the elimination of joints and bearings in the bridge superstructure. Joints and bearings are expensive in both initial and maintenance costs and can get filled with debris, freeze up, and fail in their task to allow expansion and contraction of the superstructure. They are also a “weak link” that can allow deicing chemicals to seep down and corrode bearings and support components. Because the behavior is unknown and designs are cumbersome, jointless bridges are not widely used despite their enormous benefits. There are no standardized design procedures for these bridges, only a list of specifications and design recommendations are available. The objective of this research on jointless bridges conducted at the Constructed Facilities Center-West Virginia University is to study the behavior of jointless bridges supported on piles and spread footings and subjected to varying load conditions. In addition, time-dependent material properties have also been incorporated in this study. In this paper, the following items are presented: (1) synthesized analytical data that aids in understanding the performance under varying load combinations; (2) effects of primary versus secondary loads, boundary conditions, and system flexibility on induced stresses at various bridge locations; and (3) field testing and monitoring results of a jointless bridge in the state of West Virginia. Based on analytical and experimental results, conclusions are drawn in terms of design alternations.

Improvement in the nanoindentation technique for investigation of the time-dependent material properties

Abstract An improved nanoindentation technique was developed for dynamic investigation of the time-dependent properties of materials under conditions close to real nanocontact interaction during dry friction, abrasive wear, milling, mechanical alloying and so on. For these purposes the duration of the loading- unloading cycle was reduced to 20ms and a nanocontact fatigue test was performed with the number of reloading cycles reaching 103-105.