Search

A progressive damage model is presented to investigate the damage and failure behaviors of woven composites. The conformal finite element mesh for the woven composites, used for this analysis, is generated using fabric micro-geometry from DFCA (Digital Element Approach (DEA) Fabric and Composite Analyzer, Kansas State University). The mesh generation strategy is discussed briefly – it does not make any idealized assumptions about yarn geometry and thus can be used for any woven composite micro-geometry. The woven composite domain consists of homogeneous and transversely isotropic yarns, and the surrounding homogeneous isotropic matrix. A recursive stiffness reduction method that allows for combined failure modes is employed to model progressive damage behavior. Two different fabric micro-geometries are studied: a 2D plain-weave laminate and a 3D orthogonal-weave unit cell. To support the validity of the proposed model, numerical results for the mechanical behavior of the two cases are discussed and compared with numerical predictions in literature and available experimental data.

The farmland in Yinchuan is composed of sierozem soil, which is characterized by high sand content and low organic matter content, resulting in poor water-holding capacity and weak soil structure. Humic acid is a natural organic polymer soil amendment. It is critical to study how humic acid affects soil water infiltration in sierozem soil at the microlevel. A one-dimensional vertical infiltration experiment was conducted to explore how adding different amounts of humic acid (0, 1%, 2%, 3% and 4%) affected the infiltration characteristics and hydraulic parameters of the sierozem soil. The results revealed that the wetting front and cumulative infiltration decreased with the increase in humic acid addition. When the infiltration time was 90 min, the wetting front of the 1%, 2%, 3% and 4% treatments was 6.50%, 10.00%, 15.00% and 21.00% lower than CK (0 for CK), and the cumulative infiltration volume was 4.50%, 11.14%, 18.42% and 23.60% lower than CK, respectively. Among the three infiltration models created by Philip, Horton and Kostiakov, the Kostiakov model (R2 > 0.95) could more accurately describe the soil water infiltration process in the study area. After infiltration, the moisture content of each soil layer increased with the increase in humic acid, which improved the water-holding capacity of the sierozem soil. Using Hydrus-1D to calculate soil hydraulic parameters, we found that the humic acid addition affected the hydraulic parameters. With the increase in the amount of humic acid addition, the retention water content θr and saturated water θs were positively correlated with the humic acid addition amount and negatively correlated with the saturated water conductivity Ks and the reciprocal of air-entry α. The results showed that humic acid could increase the water-holding capacity of soil and improve the rapid water loss and poor water-holding capacity of sierozem soil.

Aims: Nutrient resorption affects ecosystem nutrient cycling. Previous studies have described the independent responses of nutrient resorption to climatic factors, plant traits, and soil nutrient status. Little is known about the comprehensive effects of all above factors on nutrient resorption, especially based on observations in situ. Methods: We conducted an eight-year field survey in a semi-arid grassland for exploring interannual variation in foliar nutrient resorption in two herbaceous functional groups (i.e., graminoids and forbs) and the factors driving this variation. Results: Results showed nitrogen and phosphorus resorption efficiencies (NRE and PRE) in graminoids, not in forbs, varied significantly among years, indicating more flexible nutrient resorption strategies in graminoids. There was no significantly direct effect on nutrient resorption because of the insignificant changes of soil nutrients across the selected years. However, NRE was positively related to the N:P ratio in green leaves ([N:P]g) and PRE negatively related, supporting a foliar “stoichiometric control” strategy on nutrient resorption. Moreover, growing-season temperature and precipitation had positive and negative effects on nutrient resorption in graminoids, respectively, via the regulation of [N:P]g. Comparatively, precipitation affected forbs NRE positively and PRE negatively, again acting through regulating [N:P]g. Conclusion: Our study showed growing season hydro-thermal factors control foliar nutrient resorption by foliar [N:P]g, emphasizing the role of both climatic variables and foliar “stoichiometric controls” on foliar nutrient resorption under low nutrient-variability conditions. Meanwhile, more positive responses of graminoids could help explain their current and future dominance in this grassland.

This paper investigates the failure mechanism and fabric ballistic performance of real-size multi-layer 2D woven fabrics impacted by sharp-edge fragment simulating projectile (FSP). First, the relations between digital fiber shear force and bending rigidity are established under the modified digital element approach (DEA) framework. Then, a systematic parametric study was carried out on the ballistic impact of a 4-inch-long single yarn and 4-inch by 4-inch 2D woven fabric at near fiber level to solve for the relations of digital fiber moment of inertia and ballistic limit. The results show that for the same number of digital fibers per yarn model, the simulated ballistic limits are in direct proportion to digital fiber moment of inertia. The increase of the number of digital fibers per yarn, however, decreases the digital fiber moment of inertia effect on ballistic limits. Second, the 1- to 28-layer real-size 2D woven Kevlar KM2 fabrics are simulated at filament level against FSP on the cluster to estimate the V50 zone. The perforation process and failure mechanism of 4-layer fabric is investigated and analyzed in detail. The simulation results demonstrate the deformed fabric shape with respect to time and the damage modes at the impact area. Numerical results are compared with standard ballistic test results.

Environmental pollution of heavy metals in a typical coal industrial city should be paid more attentions nowadays. The spatial distribution and source apportionment of 8 heavy metals (i.e., Cd, Cr, Co, Zn, Ni, Cu, Pb and Mn) from topsoil (158) of Shizuishan city in Ningxia Hui Autonomous Region of China were investigated using principal component analysis/absolute principal component scores (PCA/APCS) receptor model and the geostatistics analysis. These results showed that the mean concentrations of Cd, Cr, Co, Zn, Ni, Cu and Pb were higher than their soil background values in Ningxia. 99.36% of soil was heavily polluted according to analysis of integrated Nemerow pollution index (PN), whereas 81.65% of soil exhibited the highly strong potential ecological risk by ERI (the comprehensive of potential ecological risk index) values. The source apportionment results showed that 8 heavy metals in soil were mainly from natural(32.39%), industrial (26.56%), traffic emission/coal consumption(20.18%) and atmospheric deposition source(12.73%). Typically, Zn, Mn and Ni were derived from natural source, whereas Cr and Co mainly derived from industrial sources. Cu was from the multiple sources, whereas Pb and Cd was weighted primarily from traffic emission/coal consumption source and atmospheric deposition source, respectively. These findings are crucial for the prevention and control of heavy metal pollution in Shizuishan city.

3-D woven textile is generated through the weaving process. Yarn is a non-continuum material domain, the structure of which is determined by inter fiber movement. In this study, the micro-geometry of 3-D orthogonal weave is generated at filament level predicting through the weaving process implementing the digital element approach (DEA). First, the basic concepts and explicit algorithm of DEA is introduced. The method of calculating adjusted digital fiber material property in terms of discretization resolution is proposed. Second, the unit-cell topology of 3-D orthogonal weave is defined by a position matrix. The calculation of potential energy of the cell is derived. At last, a dynamic weaving process is designed to investigate the effect of yarn tension on fabric micro-geometry and cell energy. 4 unit-cells are generated under 4 sets of tension combination. Results show that as the fabric thickness decrease, the filaments of the weft yarns move towards the center. The weft yarns at the top and bottom deform into their final shape first. The applied tension on weaver plays a dominant role in determining fabric thickness and convergence speed. By comparing the numerical results with the microscope pictures taken from the actual specimen, it is concluded that the fabric micro-geometry produced by tension combination 4 closely matches the experimental results.

This paper presents a progressive damage modeling study to investigate the damage and failure behaviors of woven composites using an automated structured conformal meshing model developed by the authors. The composite domain consists of two materials: yarns and matrix. Yarns impregnated by matrix are considered to be homogenous and transversely isotropic. Each yarn element has its local coordinate system oriented according to the yarn’s centroid path. Properties of yarn are obtained using a micromechanics-based homogenization method. The matrix is considered to be homogenous and isotropic. Maximum stress is used as the damage initiation criteria under tensile loading. Damage evolution is governed by the material property degradation method. The meshing algorithm is based on a realistic micro-geometry generated using the Digital Fabric Mechanics Analyzer (DFMA), and thus is applicable to a wide variety of woven architectures. Numerical damage predictions are discussed and compared with previous numerical studies and experimental data to support the validity of the proposed model.

Polymer computational simulations enable the prediction of material properties, exploration of the microscopic and macroscopic phenomenon of materials, and guidance of in vitro macromolecular materials design. This paper presents a mesoscopic level, multiscale approach to generate a complex polymer morphology for future simulations of polymer chains aimed at the design of materials or the study of material properties, such as strain/stress relations and electromechanical properties. To generate a complex polymer morphology, a mesoscopic level model is developed, in which the polymer chains are modeled as one-dimensional homogeneous flexible chains, and the crystalline regions are modeled as rigid continuum bodies. The polymer domain is firstly discretized into a uniform mesh. Location, orientation, and size of crystalline regions can be either pre-defined or randomly distributed inside the polymer domain. Nodes of each cuboid element grid located within crystal units are marked as occupied. Then, a random walking process is applied to generate polymer chains. No overlap is permitted between polymer chains nor between polymer chains and crystalline regions. Chain propagation ceases upon reaching the crystal block surface. Polymer chain properties, such as longitudinal stiffness, bending stiffness, and torsional stiffness, are derived based on molecular structures. Interactions, those between polymer chains, and those between polymer chains and crystalline regions are derived based on Lennard-Jones forces. An explicit dynamic relaxation algorism is applied to minimize potential energy. This approach is much more efficient than MD simulation. The computing time required for this mesoscopic simulation is only 0.5-1% of MD simulation. Yet, nanoand micro-scale geometries of polymer morphology can still be analyzed in detail. Polyvinylidene difluoride (PVDF) is selected to demonstrate the efficiency and efficacy of this numerical model due to its high piezoelectric coefficient.

The digital element approach (DEA) was originally developed to determine the geometry of textile composite preforms. With DEA, a fabric is considered as an assembly of unit cells; a unit cell as an assembly of yarns; and a yarn as an assembly of digital filaments. Each digital filament is discretized into a chain of digital elements. Contact force between digital filaments is calculated based on distance between them. During the past ten years, software, DFMA, has been developed based on DEA to simulate fabric deformation under external force in several areas of textile composite mechanics: determination of fabric microgeometry, simulation of fabric deformation in composite manufacturing processes, such as draping, forming and molding, and generation of conformal FEM meshes for composites with complex shapes. In this paper, principles of DEA are examined first. Applications of DFMA in the following four areas are then reviewed: 1) determine geometries of textile composite preforms with complex shapes, 2) generate conformal FE meshes for textile composites, 3) determine strength and shear characteristics of textile composite preforms and 4) simulate draping, forming and molding processes of textile composite preforms. Multiple examples are presented in detail to illustrate applications from each of the above four areas.

Paper description: Human fetal scleral fibroblasts (HFSFs) are components of the sclera that are involved in eye growth regulation and myopia formation. Nanog homeobox (NANOG) is a key transcription factor essential for the pluripotent and self-renewing phenotypes of embryonic stem cells. NANOG overexpression in HFSFs improves type I collagen expression and increases proliferation rates of HFSFs. The increase in the number of HFSFs and type I collagen may delay sclera remodeling. Our findings contribute to the development of more effictive theoretical support for myopia treatment. Abstract: Human fetal scleral fibroblasts (HFSFs) are components of the sclera and play important roles in its structure and function. In myopia, scleral remodeling reduces collagen fibers and the sclera begins to thin. NANOG is a key transcription factor essential for pluripotent and self-renewing phenotypes of undifferentiated embryonic stem cells. To determine whether NANOG improves human fetal scleral fibroblast quality and the underlying mechanisms in these cells, we established stable NANOG -overexpressing HFSFs. We studied type I collagen ( COL1A 1 ) and Rho-associated coiled-coil protein kinase 1 ( ROCK1 ) expression in transfected cells. We also investigated POU5F1, SOX2, KLF4, MYC and SALL4 expression in NANOG stably-overexpressed fibroblasts. Our data show that NANOG expression increased proliferation rates in fibroblasts. When compared to controls, expression of COL1A 1 in transfected fibroblasts was increased and the expression of ROCK1 was decreased. Similarly, the expression of POU5F1 , SOX2 and KLF4 was downregulated, the expression of MYC was upregulated and there was no significant change in the expression of SALL4 in transfected fibroblasts. Our results suggest that in fibroblasts, NANOG regulates ROCK1 expression and improves COL1A 1 expression to delay scleral remodeling. https://doi.org/10.2298/ABS180711048L Received: July 11, 2018; Revised: September 15, 2018; Accepted: October 11, 2018; Published online: October 23, 2018 How to cite this article: Li X, Yu T, Li M, Wang Y, Meng B, Mu Y. NANOG improves type I collagen expression in human fetal scleral fibroblasts. Arch Biol Sci. 2019;71(1):63-70.

The purpose of this research is to develop a design and manufacturing algorithm of 3-D weaving processes for woven composite preforms with complex shapes, such as integrated woven composite preforms for stiffened structural panels, for tubular components, and for stiffened tubular components. Three design modes are introduced to generate fabric patterns. Warp-yarn paths are defined by two warp matrices, weft-yarn paths by two weft matrices. Fabric topology is then derived from these two sets of matrices. A link is established between fabric topology and weaving matrix, which determines the kinematics of the Jacquard weaving loom. A CAM/CAD tool is developed based on the new algorithm. There are two output files. One relates to fabric topology, which can be applied to determine fabric microgeometry; the other to the kinematics of the Jacquard weaving loom, which can be input into the controller of the 3-D weaving machine. Several detailed design examples are presented.

In this work, we present an innovative simulation method for quasi-static crack propagation in mixed-mode, within a new framework of coupled peridynamics least square minimization and finite element method (PDLSM–FEM). As the inertia effect is neglected for quasi-static problems, the governing equations for PDLSM–FEM can be solved implicitly without iterations. Displacements and stresses from PDLSM–FEM are used to compute the interaction integral, an extended version of the J -integral. The stress intensity factors (SIFs) are derived from the interaction integral and used in the maximum circumferential tensile stress failure criterion to predict the onset of crack propagation and the direction of propagation. New contributions in this work include the following: (1) developing a new framework for coupling PDLSM and FEM; (2) pioneering a quasi-static method for simulating crack propagation in peridynamics; (3) developing an element-based approach for selecting the interaction integral contour; and (4) proposing and implementing an innovative method for handling peridynamic bond breakage and crack growth. Quasi-static crack propagation simulation in this work provides a computationally efficient way to find the shape of the crack, and the current PDLSM–FEM model is straightforward and can be easily introduced into commercial finite element codes. Three numerical examples are carried out, and the results show that the proposed method evaluates SIFs accurately and captures crack growth as expected.

This paper investigates the effect of the inter-fiber friction coefficient on fabric ballistic performance. Firstly, dynamic stress responses within a fabric due to first fiber failure are analyzed. Relations between inter-fiber friction and progressive fiber failure are assessed. Then, ballistic perforation processes of Kevlar KM2 fabrics are simulated using a fiber level micro-approach, the digital element approach (DEA). In this model, each yarn is discretized into many fibers and each fiber is divided into many rod elements. A Monte Carlo process is utilized to assign a unique strength to each element following a bimodal Weibull distribution function. DEA is used to simulate the ballistic perforation processes. A comparative investigation using a broad range of inter-fiber friction coefficients are conducted. Relations of ballistic limits and inter-fiber friction coefficients are presented. Simulation results show that ballistic limit improves as the inter-fiber friction coefficient increases up to a critical value. Beyond that point, ballistic strength decreases slightly as the inter-fiber friction coefficient increases. The inter-fiber friction also changes the ballistic perforation mechanisms.

A procedure is developed to generate a conformal finite element mesh of a textile composite unit cell with a complex micro-geometry with the aim of improving the accuracy of micro-mechanics analysis. A realistic micro-geometry of a textile composite unit cell is initially generated by using a fiber level dynamic relaxation approach. Yarn surfaces are then discretized into triangle elements. An algorithm is established to remove inter-yarn interferences and gaps induced by numerical errors. The unit cell is first divided into a uniform cuboid element mesh, which is later modified and converted to a conformal finite element (FE) mesh through of a process of node shifting and element splitting. In the conformal mesh, the element boundary perfectly matches the yarn-to-yarn interface. Compatibility between elements is ensured. The quality of each element is examined. The mesh can be input to commercial FEM softwares for composite stress analysis.

In recent years, several computer tools, e.g., DFMA, TexGen and WiseTex have been developed to derive realistic yarn-level micro-geometries for textile composites. However, due to numerical errors, the generated micro-geometries by these computer design tools have unavoidably exhibited artificial surface interferences or narrow gaps between yarns. It is therefore problematic to directly input the micro-geometry into a commercial FEM code to generate a conforming element mesh. In this paper, a procedure is developed to generate a conforming FE mesh that matches actual yarn-toyarn and yarn-to-matrix surface inside a textile composite with a complex microgeometry. It improves the accuracy of micro-mechanics analysis. The procedure divides into five steps. Initially, the unit cell domain is discretized into a uniform cuboid finite element mesh and the yarn surface is discretized into triangular plane elements. The second step consists of calculating the intersecting points between yarn surface triangle elements and mesh gridlines in the z-direction. The third step is the removal of numerical error driven artificial surface interferences or narrow gaps between yarns. If the distance between two intersection points from two adjacent yarns is smaller than a specified tolerance, the two adjacent intersecting points are merged to the mid-point. A material type, defined by yarn number, interface or matrix, is assigned to each node. In the fourth step, the initial uniform cuboid finite element mesh is modified so as to match yarn boundaries to the finite element mesh. In the final step, material types/yarn numbers are assigned to each element based on nodal material types. If an element is composed of nodes of two different material types, it is split into two or more elements. As such, a conforming FEM mesh, which matches the element boundary to the yarn-to-yarn or yarn-to-matrix interface, can be generated.

A modified digital element approach (DEA) is applied to simulate fabric perforation process under ballistic impact. The previous version of the DEA is capable of simulating ballistic impact of textile fabric using rigid body spherical and cylindrical projectiles only. Fragment simulating projectile (FSP) and real bullets are not modeled. The subject of this research is to perform ballistic penetration process against projectile of arbitrary shape and validate the modified DEA. A fabric to solid body projectile contact search and contact force calculation algorithm is established. Ballistic impact of textile fabrics against spherical and cylindrical projectiles is performed using the previous DEA and the modified DEA separately. Numerical results are compared to the well published DEA results to investigate the fabric bullet resistant performance.

3-D weaving processes have been advanced within the past decade to manufacture composite components with complex macro- and micro-geometries. Complex composite components are not an assembly of simple unit-cells. At each machine step during the weaving process, the position of each warp yarn must be controlled individually by a Jacquard head. Designing the procedure for the weaving kinematics is labor-intensive. In this paper, a CAD/CAM computational tool is developed to 1) input and define fabric topology and 2) establish a link between fabric topology and weaving kinematics. At each step, the weaving position of the Jacquard head corresponding to each individual warp is calculated, exported in a commercial loom format, and sent to the Jacquard head controller. The 3-D fabric topology can be exported in a format which can be read by the Dynamic Fabric Mechanics Analyzer, the software applied to determine the micro-geometry of fabrics. The geometry of a fabric would thus be available for future mechanical property calibrations and simulations.

A dynamic digital element approach, utilizing a dynamic relaxation procedure, is developed to determine 3-D woven textile micro-geometries. Three yarn/tow structures, including plain yarn, twisted yarn, and twisted tow, are generated by the digital element mesh. An explicit algorithm with a periodic boundary condition is employed to calculate nodal forces, accelerations, velocities, and displacements within the unit cell. Because the majority of the computing time is consumed to detect contacts between fibers, an efficient contact search algorithm is proposed. In addition, a multi-level dynamic relaxation procedure is implemented to further reduce computer time. The yarn-level fabric micro-geometry is also generated in a format that can be read by commercial finite element software. The micro-geometries derived from numerical simulations are compared to the microscopic images of actual fabrics. Good agreement is found between numerical results and experimental results.

A micro-scale computational tool, based upon an explicit digital element method (DEM), has been developed for numerical simulation of ballistic impact and penetration of textile fabrics. In this approach, each yarn is digitized as an assembly of digital fibers. Each digital fiber is further digitized into a short digital rod element chain connected by frictionless pins (nodes). A search is conducted to find contacts between adjacent digital fibers. If a contact is detected, compressive and frictional forces between fibers will be determined, based upon contact stiffness and friction coefficient. Nodal forces are calculated for each time step. Nodal displacements are determined using an explicit procedure. Because the digital element approach operates on a sub-yarn micro-scale, one can determine textile penetration resistance based upon sub-yarn scale properties, such as inter-fiber compression, friction, and fiber strength. Research presented in this paper includes three parts. First, the explicit digital element algorithm used in dynamic simulation is explained. Second, the approach is used to generate 2-D woven fabric micro-geometries and to simulate ballistic penetration processes. Third, numerical results are compared to high resolution experimental impact and ballistic test data.

Textile fabric geometry determines textile composite properties. Textile process mechanics determines fabric geometry. In previous papers, the authors proposed a digital element model to generate textile composite geometry by simulating the textile process. The greatest difficulty encountered with its employment in engineering practice is efficiency. A full scale fiber-based digital element analysis would consume huge computational resources. Two advances are developed in this paper to overcome the problem of efficiency. An improved contact-element formulation is developed first. The new formulation improves accuracy. As such, it permits a coarse digital element mesh. Then, a static relaxation algorithm to determine fabric micro-geometry is established to replace step-by-step textile process simulation. Employing the modified contact element formulation in the static relaxation approach, the required computer resource is only 1–2% of the resource required by the original process. Two critical issues with regards to the digital element mesh are also examined: yarn discretization and initial yarn cross-section shape. Fabric geometries derived from digital element analysis are compared to experimental results.

No-Name Creek Bridge, built in Russell, Kansas, in 1996, is the first all-composite highway bridge in the United States. Its structural panels are built of glass fiber-reinforced polyester sandwich panels with honeycomb cores. The bridge surface is made of polymer concrete. Two static field tests were conducted, one shortly after its completion in 1996, the other in 1997. In September 2004, after 8 years of service, field tests were repeated to examine the environmental effect on the composite materials. Test results were compared with previous results. No significant change of the bridge rigidity was found after 8 years. In addition, dynamic response to moving traffic loads was investigated. An AASHTO Type 3 truck, with a gross weight of 70,340 lb, was used to apply the traffic load to the bridge. Four types of tests were conducted: static loading, crawl speed loading, moving traffic loading, and impact traffic loading. Dynamic factors and natural frequency were measured.

The electrostrictive graft elastomer is a new type of electroactive polymer. Recently developed by NASA, it consists of flexible backbone chains, each with side chains, called grafts. Neighboring backbone grafts physically cross-link and form crystal units. The flexible backbone chain and the crystal graft unit consist of polarized monomers, which contain atoms with electric partial charges, generating dipole moments. When the elastomer is placed into an electric field, external rotating moments are applied to the dipole moment. This stimulates electrostrictive strain in the graft elastomer. In this paper, the deformation of the elastomer under the action of an electric field is explained by means of two dominant mechanisms: crystal graft unit rotation and backbone chain reorientation. A two-dimensional computational model is established to analyze the deformation.

The concept of multi-chain digital element analysis is established. This new approach is a general numerical tool for textile mechanics. It can be used for textile process design and for fabric deformation, strength and failure analyses. In multi-chain digital analysis, fabric is considered as an assembly of yarns; furthermore, a yarn is considered as an assembly of fibers. Each fiber is modeled as a frictionless pin-connected rod element chain, defined as “digital chain”. Once the length of these rod elements approaches zero, the digital chain becomes a fully flexible one-dimensional entity with a circular cross section. It imitates the physicality of the fiber. Since a yarn is composed of many fibers, it is modeled as an assembly of digital chains. Contact between digital chains is modeled by contact elements. A procedure, similar to finite element analysis, is adopted. Displacement of fiber inside a fabric is derived based upon the global stiffness matrix and the boundary conditions. As a result, both detailed yarn paths and cross-section shape can be traced during the textile forming process or fabric deformation. In order to describe the new concept, two numerical examples are presented. The multi-chain digital element approach is used for the simulation of the two-dimensional weaving and three-dimensional braiding processes. One can observe, section-by-section, the yarn cross-section shape inside the woven and braided fabric.

Two types of 2-D nano-scale finite elements, the chemical bond element and the Lennard–Jones element, are formulated based upon inter-atomic and inter-molecular force fields. A nano-scale finite element method is employed to simulate polymer field deformation. This numerical procedure includes three steps. First, a polymer field is created by an off-lattice random walk, followed by a force relaxation process. Then, a finite element mesh is generated for the polymer field. Chemical bonds are modeled by chemical bond elements. If the distance between two non-bonded atoms or monomers is shorter than the action range of the Lennard–Jones attraction (or repulsion), a Lennard–Jones element is inserted between them. Finally, external load and boundary conditions are applied and polymer chain deformation is simulated step by step. During polymer deformation, failed Lennard–Jones bond elements are removed and newly formed Lennard–Jones elements are inserted into the polymer field during each loading step. The process continues until failure occurs. Two examples are presented to demonstrate the process. Stress–strain curves of polymer fields under unidirectional tensile load are derived. Continuum mechanical properties, such as modulus and polymer strength, are determined based upon the stress strain curve. Further, throughout the deformation process one observes polymer chain migration, nano-scale void generation, void coalescence and crack development.

The bottle-neck issues to resolve for numerical simulation of real scale ballistic tests of fabric body armors are computer capacity limitation and prohibitive computational cost. It is not realistic to use micro-level computer simulations for an open end design process. Most numerical simulations are only applicable for small scale parametric analyses, which could facilitate apprehension of fabric failure mechanisms during ballistic impact, but not applicable for the design process. In this paper, a sub-yarn model, the digital element approach, is applied to simulate real scale ballistic tests for soft body armors. In this approach, a yarn is discretized into multiple digital fibers and each fiber is discretized into many digital elements. In order to improve efficiency, two hybrid element mesh concepts are investigated: area based hybrid mesh and yarn based hybrid mesh. The area based hybrid mesh procedure is similar to one utilized in the conventional finite element approach. A fine element mesh is adopted in the area near the impact center; a course element mesh in the area far away. However, numerical simulation results show that the stress wave travels along the principal yarns at the speed of sound immediately after ballistic impact. High yarn stress develops quickly from the impact center to a distance along the principal yarn. As such, the area based hybrid mesh approach fails to obtain improved computer efficiency without loss of accuracy. Because the high stress only develops within principal yarns after a ballistic impact, a yarn based hybrid element mesh procedure is adopted. In this procedure, only principal yarns and yarns near principal yarns are discretized into fine digital fibers; other yarns are discretized into coarse digital fibers. Because only a few principal yarns resist load in a typical ballistic impact, the yarn based hybrid technique could improve simulation efficiency up to 90–95% without sacrificing accuracy. A numerical tool is then developed to generate fabric with a yarn based hybrid mesh. Accuracy of the approach is analyzed. The hybrid mesh technique is applied to simulate real scale ballistic tests of ballistic armors made of 4 to 20 piles of 2-D plain woven fabrics. Numerical results are compared to real scale standard ballistic results.

This paper establishes the concept of a digital-element. A digital-element model was developed to simulate textile processes and determine the micro-geometry of textile fabrics. It models yarns by a pin-connected digital-rod-element chain. As the element length approaches zero, the chain becomes fully flexible, imitating the physicality of yarns. Contacts between yarns are modeled by contact elements. If the distance between two nodes on different yarns approaches the yarn diameter, contact occurs between them. Yarn micro-structure inside the preform is determined by process mechanics, such as yarn tension and inter-yarn friction and compression. The textile process is modeled as a non-linear solid mechanics problem with boundary displacement (or motion) conditions. First, a simple twisting process is simulated, validating the digital-element model. Then, the yarn geometry of a 3-D, four-step braided preform is analyzed. These numerical results reveal the details of yarn paths within the preform. Such results can only be arduously and expensively obtained when achieved through experimental observation, but cannot be generalized; or are inadequately detailed when achieved through existing analytical methods. This makes it possible, therefore, to conduct a quantitative analysis on the distribution of yarn orientation and fiber volume fraction inside a preform. Yet the value of this new model reaches far beyond that of the braiding process. As a general tool, it is advantageous for other textile processes, such as twisting, weaving and knitting and for the investigation of textile preform deformation during the consolidation process. The new numerical approach described here is identified as digital-element simulation rather than as finite-element simulation because of a special yarn discretization process. With the conventional finite-element approach, the element preserves the physical properties of the discretized body. In contrast, with this model the element itself does not preserve physical properties: physical properties are imitated by the element link. The concept is similar to digital discretization. Therefore, the term ‘digital element’ is more appropriate. The size of the element must be very small compared to the size normally employed in finite element analysis.

Because of the worldwide environmental pollution problem with petroleum polymers, soy protein polymers have been considered as alternatives for biodegradable plastics. The objective of this research was to study the curing behavior of soy protein isolates (SPIs) for that application. The molding variables of temperature, pressure, and time and curing quality factors of tensile strength, strain, and water resistance were evaluated. The maximum stress of 42.9 MPa and maximum strain of 4.61% of the specimen were obtained when SPI was molded at 150°C and 20 MPa for 5 min. The water absorption of the specimen decreased as molding temperature and time increased. Glycerol greatly improved the flexibility of the specimen but decreased its strength. For SPI with 25% glycerol added, the maximum stress and strain of about 12 MPa and 140%, respectively, were achieved when the specimen was molded at 140°C for 5 min. Molding temperature, pressure, and time are major parameters influencing the curing quality of soy protein polymers. At fixed pressure, the molding temperature and time had significant interactive effects on curing quality. At high temperature (e.g., at 150°C) it took about 3 min to reach optimum curing quality; however, at low temperature (120°C) it took about 10 min to reach optimum curing quality. The maximum strength and strain of the cured protein polymer occurred at the molding temperature close to its phase transition temperature or about 40°C below its exothermic temperature. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2595–2602, 1999

Fabric systems with through-the-thickness reinforcement have better ballistic strength than those without it. The focus of the current project is to develop the computational capability to evaluate multilayer fabric systems with through-the thickness reinforcement as well as its failure mechanism after ballistic impact. In this project, a numerical model has been established to generate the micro-geometry of fabrics fabricated using TexTech needle-punching technology. A hybrid mesh model has been developed which produces results close to those obtained using full field uniform mesh and yet uses less computer resource by a factor of ten or more. Furthermore, a post-processor has been developed which can display the ballistic perforation process step by step and enables the analysis of the fabric failure mechanism. Based on the work accomplished, we have established the capacity to (1) simulate the fabrication procedure of needle-punching the through-the-thickness fibers into multi-layer fabric systems, (2) conduct ballistic simulations of the TexTech fabric systems with various layer configurations, (3) compare numerical ballistic simulation results to experimental results, and (4) provide information for optimal hybrid design of multilayer fabric systems with through-the-thickness reinforcement.

A new version of low‐noise, high‐stability control, data‐acquisition electronics has been designed and constructed for high‐resolution electron energy loss spectroscopy to meet the requirements imposed by new digital signal processing techniques. The improved electronics feature fast, low‐noise, high‐stability, programable voltage sources with 16 bit precision for the electron optics and a custom‐built 16 bit 100 MHz pulse counter. These electronics, controlled by an IBM‐compatible PC through a user‐friendly, graphical software interface, allow easy computerized tuning and spectral recording.

We have investigated, using electron energy loss spectroscopy, the phonons on the Ru(001) surface that are rendered dipolar active in the specular direction by virtue of ordered p(2×2) and p(1×2) oxygen adatom overlayers. We show that two surface phonons are present in the specular EEL spectra, and we verify, using group‐theoretical arguments that only two surface modes can be dipolar active. This analysis also allows polarizations to be assigned to these modes. Implications for the reconstruction of the Ru(001) surface induced by the oxygen adatom overlayers are discussed.