Applied Mechanics

2002;():1-8. doi:10.1115/IMECE2002-33552.

The field of polymer-clay nanocomposites has attracted considerable attention as a method of enhancing polymer properties and extending their utility. Layered silicates dispersed as a reinforcing phase in a polymer matrix are one of the most important forms of such inorganic-organic nanocomposites, making them the subject of intense research. We have recently prepared several thermoset-based nanocomposites with improved thermal and mechanical properties. This paper is primarily focused in studying the effects of nano clay particles such as montmorillonite on improving mechanical and thermal properties of the polymer matrix composite. Epoxy and vinyl ester nanocomposites were prepared by adding different weight percentages (0.5%, 1%, 2%, 5% and 10%) of montmorillonite nano clay particles to epoxy and vinyl ester matrices. The results show significant improvements in mechanical and thermal properties of the nanostructured materials with low loading of organo silicates. Thermal property measurement includes dynamic mechanical analysis (DMA). Mechanical properties such as flexural strength and flexural modulus of polymer matrix were improved in nano structured materials owing to their unique phase morphology and improved interfacial interactions. Molecular dispersion of the layered silicate within the cross-linked matrix was verified using Wide Angle X-Ray Diffraction (WAXD) and Transmission Electron Microscopy (TEM) revealing the intercalated nanocomposites were formed.

Commentary by Dr. Valentin Fuster
2002;():11-19. doi:10.1115/IMECE2002-33575.

Damage in composite laminates affects its overall viscoelastic response. Constitutive equations have been developed for composite laminates considering a fixed damage state. A complete description, however, requires suitable damage evolution laws. This paper is focused on studying damage evolution in viscoelastic laminates using a cohesive finite element approach. A two dimensional, four nodded finite element is developed incorporating a rate-independent traction-displacement cohesive law. This element is used in conjunction with plane strain bulk elements behaving in a linear viscoelastic manner to simulate crack evolution between two existing transverse cracks in symmetric cross-ply laminates. The effects of loading strain-rate, ply constraint and initial crack density are studied. This study shows expected trends in the behavior and indicates the suitability of cohesive zone modeling to study damage evolution in viscoelastic composite materials.

Commentary by Dr. Valentin Fuster
2002;():21-28. doi:10.1115/IMECE2002-33586.

Non-Crimp Fabrics (NCF) and vinylester resins seem to give lower compression strength data than similar prepreg-based composites. However, the lower data turned out to be not only material issue, but a problem associated with testing methodology, namely design of test fixtures and specimen configuration. Specimen geometry optimization study led to research concerning notch sensitivity of NCF. Investigations were performed testing specimens with circular center holes and symmetric sharp edge notches. In all cases reduction of compressive strength, corresponding to the net cross-section, due to introduced notches was not observed. Although NCF composites showed rather low compressive strength if compared with similar prepreg materials, they also exhibit almost complete notch insensitivity. This can be explained by irregular and uneven internal structure of NCF composite. Due to complex internal geometry NCF composite fails close to an internal defect, prior to the failure in vicinity of the notch, where stress concentration takes place.

Commentary by Dr. Valentin Fuster
2002;():29-36. doi:10.1115/IMECE2002-32999.

The primary goal of this study is to model the anisotropic effect of ductile damage in metal forming processes. To represent the ductile metals, an anisotropic ductile plasticity/damage formulation is considered within the framework of continuum mechanics. The formulation is motivated from fracture mechanisms and physical observations in Al-Si-Mg aluminum alloys with second phases. The ductile damage mechanisms are represented by the classical ductile process of nucleation of voids at inclusions, followed by their growth and coalescence. Functions of each mechanism evolution are related to different microstructural parameters. The damage, represented by a second rank tensor, is coupled to the Bammann-Chiesa-Johnson (BCJ) rate-dependent plasticity using the effective stress concept. The constitutive equations are integrated using a fully implicit scheme and implemented into a explicit finite element code. This implementation is used to predict damage during the forward axisymmetric extrusion of an aluminum bar. This example illustrates the applicability of the model to predict the initiation and the evolution of anisotropic damage in metal forming processes.

Commentary by Dr. Valentin Fuster
2002;():37-44. doi:10.1115/IMECE2002-33019.

The paper presents a generalized mixed isotropic-kinematic hardening plastic model coupled with anisotropic damage for sheet metal forming. A nonlinear anisotropic kinematic hardening is developed. For the predication of limit strains at localized necking in stamping under complex strain history, the model and its associated damage criterion for localized necking are established and implemented into LS-DYNA3D by compiling it as a user subroutine. The finite element simulation of LS-DYNA3D based on the present model is carried out. The location of localized necking for sheet metal forming has been successfully identified.

Commentary by Dr. Valentin Fuster
2002;():45-54. doi:10.1115/IMECE2002-32862.

A damage coupling viscoplastic model is developed to predict fatigue life of solder alloy 63Sn-37Pb under stress control. The viscoplastic flow rule chosen employs a hyperbolic sine function. A damage evolution equation is formulated based on three distinct material deformation behaviors: (i) stress rate independent damage evolution; (ii) stress rate dependent cyclic damage evolution; and (iii) stress rate dependent ductile damage evolution. The cyclic stress testing with different stress waveforms was first conducted to investigate their progressive viscoplastic deformations of the solder alloy. The investigation reveals that the material constants used in the model can be adequately determined from the results of standard creep tests. The constitutive model is validated by comparing the predicted and measured ratchetting results of the solder alloy under different forms of stress cycling. The proposed model is found to be capable of satisfactorily describing the viscoplastic deformation and ratchetting failure behaviors of the solder alloy under the conditions of the cyclic stress loading.

Commentary by Dr. Valentin Fuster
2002;():55-60. doi:10.1115/IMECE2002-32867.

In our recent experiments, torsional specimens of PbSn Solder Alloy were cyclically loaded under different loading levels at both room temperature and high temperature (1008C). In these experiments, the Pb-rich phase size and micro-crack damage were also observed using scanning electron microscopy in-between loading cycles. At the moderate strain rate (10−4 ), the growth of Pb-rich phase does not differ much for both temperatures. At room temperature, the damage in the form of micro-cracks along the Pb- and Sn-rich phases are formed during early cycling, and the damage accumulates as the cycling proceeds. At high temperature, the damage does not accumulates as fast as for the similar strain level in room temperature, due to the lowering of stress level in high temperature and dynamic restructuring of Pb-rich phase.

Topics: Alloys , Solders
Commentary by Dr. Valentin Fuster
2002;():61-68. doi:10.1115/IMECE2002-32874.

A thermo mechanical fatigue life prediction model based on the theory of damage mechanics is presented. The damage evolution, corresponding to the material degradation under cyclic thermo mechanical loading, is quantified thermodynamic framework. The damage, as an internal state variable, is coupled with unified viscoplastic constitutive model to characterize the response of solder alloys. The damage-coupled viscoplastic model with kinematic and isotropic hardening is implemented in ABAQUS finite element package to simulate the cyclic softening behavior of solder joints. Several computational simulations of uniaxial monotonic tensile and cyclic shear tests are conducted to validate the model with experimental results. The behavior of an actual Ball Grid Array (BGA) package under thermal fatigue loading is also simulated and compared with experimental results.

Commentary by Dr. Valentin Fuster
2002;():71-78. doi:10.1115/IMECE2002-32885.

Experiments on the eutectic tin-lead alloy were conducted to study the effects of grain boundary sliding on the deformation and damage processes at the microscopic level. The primary objective is to gain mechanistic understandings of solder joint reliability in microelectronic packaging. Bulk specimens were subject to relatively fast deformations of tension, compression and bending, for the purposes of examining the pure mechanical effect without the influence of diffusion related phenomena. Grain realignment and phase redistribution were characterized by microscopy and microhardness indentation. A micromechanical model is proposed to elucidate the observed microstructural changes and progressive damage. This study illustrates the significance of damage in the form of microscopic heterogeneity caused by grain boundary sliding. It also illustrates the possibility of mechanically induced phase coarsening in actual solder joints. High-frequency cyclic shear tests on tin-lead solder joints showed damage along the coarsened band after only a short time, in accord with the proposed effects. Boundary sliding without the influence of atomic diffusion plays an essential role in fatigue damage in solder.

Commentary by Dr. Valentin Fuster
2002;():79-88. doi:10.1115/IMECE2002-39185.

Researchers resort to a wide range of simplified representations at the continuum scale, to model creep-fatigue damage in viscoplastic heterogeneous materials such as Sn-Pb eutectic solders, caused by thermo-mechanical and mechanical cyclic loading (e.g. due to power cycling, environmental temperature cycling, vibration, etc). Typically, in macroscale phenomenological damage models, the cyclic damage is assumed to depend on some loading parameter such as cyclic strain range, work dissipation per cycle, partitioned strain range, partitioned work dissipation per cycle, cyclic entropy changes, cyclic stress range, integrated matrix creep, etc. In many instances, some of these variables are weighted with a factor to account for rate-dependent effects. The task of finding the best damage metric is difficult because of complex microstructural interactions between cyclic creep and cyclic plasticity due to the high homologous temperature under operating conditions. In this study we use insights obtained from microstructural and more mechanistic modeling to identify the most appropriate macro-scale damage metrics. The microstructural models are based on such phenomena as grain boundary sliding, blocking of grain boundary sliding by second-phase particles, grain boundary, volumetric and surface diffusion, void nucleation, void growth and plastic collapse of cavitating grain boundaries. As has been demonstrated in the literature, microstructural models suggest that fatigue damage caused by cyclic plasticity should correlate well with the two most commonly used damage indicators: both cyclic strain range and plastic work dissipation per cycle. This study, however, demonstrates that in the case of damage dominated by cyclic creep, microstructural models developed by the authors indicate closer correlation with creep work dissipation per cycle, than with cyclic creep strain range.

Topics: Alloys , Solders , Modeling
Commentary by Dr. Valentin Fuster
2002;():91-98. doi:10.1115/IMECE2002-39361.

In order to realize the potential of sheet metal forming and take advantage of new process control capabilities, innovative modifications to the traditional sheet metal forming process must be developed. These modifications are particularly important with respect to Tailor Welded Blank (TWB) forming, which offers an excellent opportunity to reduce manufacturing costs, decrease part weight, and improve the quality of sheet metal stampings. However, tearing near the weld seam and wrinkling in the formed wall area and die addendum of the part often occurs when a traditional forming process is used to form a TWB. Research and industrial experience has shown that these forming concerns can be alleviated through advanced forming techniques, for example using a segmented die process or a non-uniform binder force. The difficulty then becomes determining the key process parameters associated with these forming methods. In this paper, a methodology is presented to effectively and easily determine both the location of a segmented die and a non-uniform binder force by evaluating nodal reaction forces provided from FEA simulations. Also, using FEA simulations to determine the process parameters for another advanced forming process, strain path control tooling, is discussed. The advanced forming processes presented in this paper and the use of FEA to determine key process parameters are critical components to the continued evolution of sheet metal forming processes.

Commentary by Dr. Valentin Fuster
2002;():99-106. doi:10.1115/IMECE2002-39362.

To meet the growing demand for rapid, low-cost die fabrication technology in the sheet metal forming industry, easy-to-machine, polyurethane-based, composite board stock is used widely as a rapid tooling material. In practice, it is desirable to terminate die life by wear rather than by catastrophic fatigue. However, the failure mechanisms of the rapid prototyped tools are not clearly understood, thus making the prediction of tool life difficult. This paper presents a method to estimate the fatigue life of a sheet metal forming die fabricated from ATH (aluminum trihydrate)-filled polyurethane. A finite element model of 90° V-die bending process was developed, and the effects of process parameters on stress distribution in the punch and die were investigated through simulation. Mechanical testing was performed to characterize the fatigue properties of the tooling material. The computer-simulated results were verified through experiments using instrumented, laboratory-scale punch and die sets.

Commentary by Dr. Valentin Fuster
2002;():107-112. doi:10.1115/IMECE2002-39363.

As part of a 5-year NSF-sponsored project, a design and fabrication system is being developed for Profiled Edge Laminated (PEL) tooling. The PEL tooling method is a thick-layer Rapid Tooling (RT) approach that offers distinct advantages over both conventional CNC machining of billets and other RT processes. Furthermore, the method is ideally suited for developing large-scale sheet metal forming tools. To date, the following design, fabrication and analysis tools have been completed: details of the ‘front-end’ design and analysis process; valid structural and thermal FEL modeling methods for PEL tooling; and development of the ‘back-end’ PEL tool fabrication process consisting of a CAM software system to allow AWJ cutting of individual PELs based on a CAD model. The front end process has been demonstrated with matched die forming of a 2-dimensional steel part. The back-end process has also been demonstrated using a 3-dimensional hydroformed aluminum part. Future work will include incorporation of variable thickness and orientation algorithms that account for stock lamination thicknesses and part dimensional tolerances, more advanced structural and thermal models, the means to predict the cost and time required for fabrication of PEL tools, an investigation of different lamination bonding methods, and additional industrial case studies.

Commentary by Dr. Valentin Fuster
2002;():115-118. doi:10.1115/IMECE2002-32474.

In this paper, the influence of large particles on the contact force net-work (structure) and their impact on the macroscopic shear resistance of a slowly sheared dense granular medium three dimensional) having a binary size distribution is presented. The evolution of deviator fabric tensor of the ‘strong’ and ‘weak’ contacts in the granular assembly is presented here. The ability of the granular material to establish a strongly anisotropic force net-work of strong contacts is ‘diluted’ by the presence of large particles during shearing. Previous studies [for example, Antony, S. J., 2001, Physical Review E, 63 , 011302 and references there in] have shown that the shear strength of granular materials subjected to slow shearing depends on the ability of the material to built up a strongly anisotropic net work of contacts carrying strong (greater than average) contact normal force.

Commentary by Dr. Valentin Fuster
2002;():119-124. doi:10.1115/IMECE2002-32476.

In the earlier paper, we developed constitutive relations for two kinds of hot mix asphalt, viz., asphalt concrete and sand asphalt using the framework of materials with multiple natural configurations. In the present paper, we apply the framework that we developed for sand asphalt to study compressive creep experiments. Experimental studies of Wood and Goetz (1959) are used to compare with the predictions of the model.

Commentary by Dr. Valentin Fuster
2002;():125-130. doi:10.1115/IMECE2002-32478.

We present a series of experiments to investigate the stability, flow and segregation when liquid bridges or magnetic attraction forces between grains are present. Quantitative data is obtained by high speed and high resolution imaging. First, we measure the angle of repose of a granular pile formed by pouring wet grains in to a quasi-two dimensional silo and compare them to existing models. Second, we measure the size separation of bi-disperse grains as a function of size ratio and viscosity of the liquid. Finally, we introduce a vibro-fluidized system of magnetized particles to study the effect of cohesive forces and inelastic collisions on the formation of clusters and velocity distributions.

Commentary by Dr. Valentin Fuster
2002;():131-140. doi:10.1115/IMECE2002-32489.

Different kinds of hot mix asphalt mixtures are used in highway and runway constructions. Each of these mixtures cater to specific needs and differ from each other in the type and percentage of aggregates and asphalt used, and their response can be markedly different. Constitutive models used in the literature do not differentiate between these different kinds of mixtures and use models which treat them as if they are one and the same. In this study, we propose constitutive models for two different kinds of hot mix asphalt, viz., asphalt concrete and sand asphalt. We use a framework for materials that possess multiple natural configurations for deriving the constitutive equations. While asphalt concrete is modeled as a two constituent mixture, sand asphalt is modeled as a single constituent mixture due to the peculiarity in its makeup. In this study, we present a unified approach for deriving models for these different kind of mixtures. In a companion paper, we compare the predictions of the model for a compressive creep test with available experimental results.

Commentary by Dr. Valentin Fuster
2002;():141-147. doi:10.1115/IMECE2002-32491.

Cohesive forces between grains can arise from a variety of sources – such as liquid bridge (capillary) forces, van der Waals forces, or electrostatic forces – and may play a significant role in the processing of fine and/or moist powders. While recent advances have been made in our understanding of liquid-induced cohesion at the macroscopic level, in general, it is still not possible to directly connect this macroscopic understanding of cohesion with a microscopic picture of the particle properties and interaction forces. In fact, conventional theories make no attempt to distinguish between these modes of cohesion, despite clear qualitative differences (lubrication forces in wet systems or electrostatic repulsion are two good examples). In this work, we discuss several discrete characterization tools for wet (cohesive) granular material with simple, physically relevant interpretations. We examine the utility of these tools, both computationally and experimentally, by exploring a range of cohesive strengths (from cohesionless to cohesive) in several prototypical applications of solid and gas-solid flows.

Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster
2002;():149-154. doi:10.1115/IMECE2002-32492.

Based on the basic balance laws and the second law of thermodynamics, a model for multiphase fluid flows through poro-elastic media is presented. The basic conservation laws. Including the balance of phasic equilibrated forces are are described. Based on the thermodynamics of the multiphase mixture, appropriate constitutive equations are formulated. It is shown that the present theory leads to the extension of Darcy’s law and contains, as its special case, Biot’s (1957) theory of saturated poro-elastic media. The special case of gas-liquid flows in porous media is discussed.

Commentary by Dr. Valentin Fuster
2002;():155-160. doi:10.1115/IMECE2002-32493.

When a coal stockpile is stored in the presence of air, slow oxidation of the carbonaceous materials occurs and heat is released. If the rate of heat generation within the stockpile is greater than the rate of heat dissipation and transportation to the external environment, the self-heating of the coal stockpile ensues. The self-heating of coal stockpiles has a long history of posing significant problems to coal producers because it lowers the quality of coal and may result in hazardous thermal runaway. Precise prediction of the self-heating process is, therefore, necessary in order to identify and evaluate control measures and strategies for safe coal mining, storage and transportation. Such a prediction requires an accurate estimate of the various processes associated with the self-heating which are impossible unless the appropriate phenomenological coefficients are known. This note is to present a simple approach to determine the effective thermal conductivity of a granular porous medium such as a coal stockpile.

Commentary by Dr. Valentin Fuster

Biomedical Technology

2002;():163-197. doi:10.1115/IMECE2002-33517.

Since 1996 the NHTSA has warned of the airbag deployment injury risk to front seated children and infants, during frontal impact, and they have recommended that children be placed in the rear seating areas of motor vehicles. However, during most rear impacts the adult occupied front seats will collapse into the rear occupant area and, as such, pose another potentially serious injury risk to the rear seated children and infants who are located on rear seats that are not likely to collapse. Also, in the case of higher speed rear impacts, intrusion of the occupant compartment may cause the child to be shoved forward into the rearward collapsing front seat occupant thereby increasing impact forces to the trapped child. This study summarizes the results of more than a dozen actual accident cases involving over 2-dozen rear-seated children, where 7 children received fatal injuries, and the others received injuries ranging from severely disabling to minor injury. Types of injuries include, among others: crushed skulls and brain damage; ruptured hearts; broken and bruised legs; and death by post-crash fires when the children became entrapped behind collapsed front seat systems. Several rear-impact crash tests, utilizing sled-bucks and vehicle-to-vehicle tests, are used to examine the effects of front seat strength and various types of child restraint systems, such as booster seats and child restraint seats (both forward and rearward facing), in relation to injury potential of rear seated children and infants. The tests utilized sedan and minivan type vehicles that were subjected to speed changes ranging from about 20 to 50 kph (12 to 30 mph), with an average G level per speed change of about 9 to 15. The results indicate that children and infants seated behind a collapsing driver seat, even in low severity rear impacts of less than 25 kph, encounter a high risk of serious or fatal injury, whether or not rear intrusion takes place. Children seated in other rear seat positions away from significant front seat collapse, such as behind the stronger “belt-integrated” types of front seats or rearward but in between occupied collapsing front seat positions, are less likely to be as seriously injured.

Topics: Wounds
Commentary by Dr. Valentin Fuster
2002;():199-205. doi:10.1115/IMECE2002-33520.

This paper summarizes component models of the human body, from head to foot, developed at WSU over the last decade. All of these models were validated against global response data obtained from relevant cadaveric tests. This report summarizes the capabilities and limitations of these models and points the direction for future developments.

Topics: Simulation , Modeling
Commentary by Dr. Valentin Fuster
2002;():207-233. doi:10.1115/IMECE2002-33523.

This paper presents acceleration data of seventy-eight open-wheel, Indy car type, racecar impacts. These data were collected by the “Impact Sensor Program” conducted jointly by the Ford Motor Company and the Championship Auto Racing Teams (CART), Inc. The seventy-eight impacts consisted of forty-two side impacts, thirty rear impacts, three frontal impacts, and three rollover/flipping of cars. Related crash data were used as input to a CAE model of a racecar driver in a typical CART car to perform computer simulations of the impacts. This model was developed using MADYMO software, and was an enhanced version of one previously published. Enhancements to the model included accurate geometrical representations of the cockpit interior, the seat, and the energy-absorbing collar; a more realistic geometry of the driver’s head and an improved representation of the neck; a highly detailed model of the driver’s helmet; and improved contact algorithms to define the head-helmet, helmet-collar, and head-chin strap interactions. Additionally, data collected from twenty-six drivers were used to improve the seating posture of the driver in the model. Results of simulations performed established the validity of the model in predicting the potential injury risk to the drivers in the head and neck areas. Model predictions of injuries based on the “Head Injury Criterion” (HIC), the Injury Assessment Reference Values (IARVs) of upper neck forces and moments, and a biomechanical neck injury predictor compared well with the actual injuries sustained by the drivers. The model predictions of reversible concussions also compared well with results of recent brain injury risk studies. The present study shows that CAE modeling can be effectively used to predict potential injuries to racecar drivers involved in high “G” impacts, and that the model can be used to evaluate countermeasures to improve safety of CART cars.

Topics: Modeling
Commentary by Dr. Valentin Fuster
2002;():235-248. doi:10.1115/IMECE2002-39706.

Child headform and adult headform impacting the bonnet (hood) of a passenger vehicle are two of the four types of tests conducted by EuroNCAP to rate the pedestrian protection performance of new vehicles. The present work focuses on the dual-peak acceleration pulses of the headform often observed in the tests. A dual asymmetrical triangle function is chosen to model the pulse. Using the analytical function, the kinematical equations of the headform, and the explicit equations for calculating the maximum displacement, rebound time and Head Injury Criterion (HIC) value are derived. A parametric study is conducted to investigate how the wave shape affects the HIC value. Several important characteristics of the dual-peak pulses, including the headform rebound time and the maximum headform displacement, are identified. A spreadsheet tool has also been developed using the dual asymmetrical triangle function to relate the headform kinematics to the mechanical property of the bonnet.

Commentary by Dr. Valentin Fuster

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