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The purpose of this paper is to address the problems of interaction of tensile stress-waves with polyurea/fused-silica and fused-silica/polyurea interfaces, and the potential for the accompanying interfacial decohesion.

The problems are investigated using all-atom non-equilibrium molecular-dynamics methods and tools. Before these methods/tools are employed, previously determined material constitutive relations for polyurea and fused-silica are used, within an acoustic-impedance-matching procedure, to predict the outcome of the interactions of stress-waves with the material-interfaces in question. These predictions pertain solely to the stress-wave/interface interaction aspects resulting in the formation of transmitted and reflected stress- or release-waves, but do not contain any information regarding potential interfacial decohesion. Direct molecular-level simulations confirmed some of these predictions, but also provided direct evidence of the nature and the extent of interfacial decohesion. To properly model the initial state of interfacial cohesion and its degradation during stress-wave-loading, reactive forcefield potentials are utilized.

Direct molecular-level simulations of the polyurea/fused-silica interfacial regions prior to loading revealed local changes in the bonding structure, suggesting the formation of an interphase. This interphase was subsequently found to greatly affect the polyurea/fused-silica decohesion strength.

To the authors’ knowledge, the present work is the first public-domain report of the use of the non-equilibrium molecular dynamics and reactive force-field potentials to study the problem of interfacial decohesion caused by the interaction of tensile waves with material interfaces.

Assembling items to achieve bigger parts seems to be the solution to counterbalance the dimension limits of 3D printing. This work aims to propose an approach to achieve optimal assembling.

Acrylonitrile butadiene styrene polymer samples were printed using fused deposition modelling (FDM). These samples were assembled and the precise contribution of interfacial shearing and tension was measured using simple tensile experiments.

The results achieved show the correlation between the printing orientation and the assembling angle. It could be proved that rupture by an interfacial decohesion mechanism of glued parts can be avoided by simple adaptation of the assembling junction.

Design of large parts using FDM is no more a limitation if assembling configurations are adapted based on the knowledge gained about the interfacial phenomena occurring at the junction position.

The unbalanced contribution of shearing and tension at the interface defines new assembling profiles that exclude flat junctions.

A model for the decohesion of aggregates of suspended particulate material in a binding matrix is developed. In the model cohesive zones which envelop each particle individually are introduced at the particulate/binder interface. During progressive loading, the deterioration of the cohesive zones is initiated if constraints placed on the microstress fields are violated. In order for the material behavior to be energetically admissible, the deterioration of the material at a point is in the form of a reduction of the elasticity tensor’s eigenvalues at that point. The material within the cohesive zones deteriorates until the constraints are met. In order to isolate and study the effects of interfacial deterioration, outside of the cohesive zones, the material is unaltered. Mathematical properties of the model, as well as physical restrictions, are discussed. Numerical simulations are performed employing the finite element method to illustrate the approach in three‐dimensional applications.

The purpose of this paper is to model a nacre-like composite material, consisting of tablets and polyurea tablet/tablet interfaces, B₄C. This composite material is being considered in the construction of the so-called backing-plate, a layer within a multi-functional/multi-layer armor system.

Considering the basic functions of the backing-plate (i.e. to provide structural support for the ceramic-strike-face and to stop a high-velocity projectile and the accompanying fragments) in such an armor system, the composite-material architecture is optimized with respect to simultaneously achieving high flexural stiffness and high ballistic-penetration resistance. Flexural stiffness and penetration resistance, for a given architecture of the nacre-like composite material, are assessed using a series of transient non-linear dynamics finite-element analyses. The suitability of the optimized composite material for use in backing-plate applications is then evaluated by comparing its performance against that of the rolled homogeneous armor (RHA), a common choice for the backing-plate material.

The results obtained established: a trade-off between the requirements for a high flexural stiffness and a high ballistic-penetration resistance in the nacre-like composite material; and overall superiority of the subject composite material over the RHA when used in the construction of the backing-plate within multi-functional/multi-layer armor systems.

This study extends the authors previous research on nacre-mimetic armor to optimize the architecture of the armor with respect to its flexural stiffness and ballistic-penetration resistance, so that these properties could be increased over the levels attained in the current choice (RHA) for the backing layer of multi-functional/multi-layer armor.

The purpose of this paper is to understand the sliding wear response of a cast iron as influenced by applied load and changing concentration of solid lubricant (graphite) particles in oil lubricant, and operating material removal mechanisms in different sets of experimental conditions.

The sliding wear response of a grey cast iron has been examined as a function of test environment and load. Properties evaluated were wear rate, friction coefficient and frictional heating. The wear behaviour of the samples has been substantiated through the characteristics of their wear surfaces, subsurface regions and debris particles.

The wear rate and frictional heating increased with load while friction coefficient was affected in an opposite manner. The presence of oil lubricant led to a substantial improvement in wear response (in terms of decreasing wear rate, friction coefficient and frictional heating) while the presence of graphite particles in the oil lubricant proved to be still better. A critical content of graphite in the oil lubricant becomes most effective towards improving the wear response of the samples. Formation of dark patches on the wear surface, substantial subsurface deformation and fine debris led to improved wear response.

The study enables one to understand the wear behaviour of a cast iron as influenced by the changing concentration of solid lubricant (graphite) particles in the oil lubricant. It also enables one to understand the operating material removal mechanisms responsible for the observed wear characteristics of the samples under varying test conditions. The investigation helps one to see that only a critical concentration of the solid lubricant particles in oil can lead to the best wear performance of materials.

From a practical standpoint, the observations made here gain importance from the fact that solid lubricants are added frequently in oil in engineering applications but it becomes imperative to understand that only a critical concentration can lead to the best wear behaviour of materials.

Nacre is a biological material constituting the innermost layer of the shells of gastropods and bivalves. It consists of polygonal tablets of aragonite, tessellated to form individual layers and having the adjacent layers as well as the tablets within a layer bonded by a biopolymer. Due to its highly complex hierarchical microstructure, nacre possesses an outstanding combination of mechanical properties, the properties which are far superior to the ones that are predicted using techniques such as the rule of mixtures. Given these properties, a composite armor the structure of which mimics that of nacre may have improved performance over a monolithic armor having a similar composition and an identical areal density. The paper aims to discuss these issues.

In the present work, an attempt is made to model a nacre-like composite armor consisting of B₄C tablets and polyurea tablet/tablet interfaces. The armor is next tested with respect to impact by a solid right circular cylindrical (SRCC) rigid projectile, using a transient non-linear dynamics finite-element analysis. The ballistic-impact response and the penetration resistance of the armor are then compared with that of the B₄C monolithic armor having an identical areal density. Furthermore, the effect of various nacre microstructural features (e.g. surface profiling, micron-scale asperities, mineral bridges between the overlapping tablets lying in adjacent layers, and B₄C nano-crystallinity) on the ballistic-penetration resistance of the composite armor is investigated in order to identify an optimal nacre-like composite armor architecture having the largest penetration resistance.

The results obtained clearly show that a nacre-like armor possesses a superior penetration resistance relative to its monolithic counterpart, and that the nacre microstructural features considered play a critical role in the armor-penetration resistance.

The present work indicates that for a given choice of armor material, penetration resistance may be improved by choosing a structure resembling that of nacre.

This paper aims to investigate the effect of printing temperature on the thermal and the mechanical behaviour of polylactic acid (PLA)-polyhydroxyalkanoate (PHA) blend printed using fused deposition modelling (FDM).

Because of the use of an infra-red camera, thermal cycling during the laying down is quantified. In addition, X-ray micro-tomography is considered to reveal the microstructural arrangement within the three-dimensional printed material. Tensile loading conditions are used to derive Young’s modulus, tensile strength and fracture toughness, and relate these to the printing temperature. Finite element computation based on three-dimensional microstructure information is used to predict the role of defects on the tensile performance.

The results show a remarkable cohesive structure of PLA-PHA, particularly at 240°C. This cohesive structure is explained by the ability to ensure heat accumulation during laying down as evidenced by the nature of thermal cycling. The printing temperature is found to be a key factor for tuning the ductility of the printed PLA-PHA allowing full restoration of tensile strength at high printing temperature.

This study reports new results related to the thermo-mechanical behaviour of PLA-PHA that did not receive much attention in three-dimensional printing despite its potential as a candidate for pharmacological and medical applications. This study concludes by a wide range of possible printing temperatures for PLA-PHA and a remarkable low porosity generated by FDM.

Polyurethane concrete (PUC), as a new type of steel bridge deck paving material, the bond-slip pattern at the interface with the steel plate is not yet clear. In this study, the mechanical properties of the PUC and steel plate interface under the coupled action of temperature, normal force and tangential force were explored through shear tests and numerical simulations. An analytical model for bond-slip at the PUC/steel plate interface and a predictive model for the shear strength of the PUC/steel plate interface were developed.

The new shear test device designed in this paper overcomes the defect that the traditional oblique shear test cannot test the interface shear performance under the condition of fixed normal force. The universal testing machine (UTM) test machine was used to adjust the test temperature conditions. Combined with the results of the bond-slip test, the finite element simulation of the interface is completed by using the COHENSIVE unit to analyze the local stress distribution characteristics of the interface. The use of variance-based uncertainty analysis guaranteed the validity of the simulation.

The shear strength (τ_f) at the PUC-plate interface was negatively correlated with temperature while it was positively correlated with normal stress. The effect of temperature on the shear properties was more significant than that of normal stress. The slip corresponding to the maximum shear (D₁) positively correlates with both temperature and normal stress. The interfacial shear ductility improves with increasing temperature.

Based on the PUC bond-slip measured curves, the relationship between bond stress and slip at different stages was analyzed, and the bond-slip analytical model at different stages was established; the model was defined by key parameters such as elastic ultimate shear stress τ₀, peak stress τ_f and interface fracture energy G_f.

Traditionally, an armor-grade composite is based on a two-dimensional (2D) architecture of its fiber reinforcements. However, various experimental investigations have shown that armor-grade composites based on 2D-reinforcement architectures tend to display inferior through-the-thickness mechanical properties, compromising their ballistic performance. To overcome this problem, armor-grade composites based on three-dimensional (3D) fiber-reinforcement architectures have recently been investigated experimentally. The paper aims to discuss these issues.

In the present work, continuum-level material models are derived, parameterized and validated for armor-grade composite materials, having four (two 2D and two 3D) prototypical reinforcement architectures based on oriented ultra-high molecular-weight polyethylene fibers. To properly and accurately account for the effect of the reinforcement architecture, the appropriate unit cells (within which the constituent materials and their morphologies are represented explicitly) are constructed and subjected to a series of virtual mechanical tests (VMTs). The results obtained are used within a post-processing analysis to derive and parameterize the corresponding homogenized-material models. One of these models (specifically, the one for 0°/90° cross-collimated fiber architecture) was directly validated by comparing its predictions with the experimental counterparts. The other models are validated by examining their physical soundness and details of their predictions. Lastly, the models are integrated as user-material subroutines, and linked with a commercial finite-element package, in order to carry out a transient non-linear dynamics analysis of ballistic transverse impact of armor-grade composite-material panels with different reinforcement architectures.

The results obtained clearly revealed the role the reinforcement architecture plays in the overall ballistic limit of the armor panel, as well as in its structural and damage/failure response.

To the authors’ knowledge, the present work is the first reported attempt to assess, computationally, the utility and effectiveness of 3D fiber-reinforcement architectures for ballistic-impact applications.

The purpose of this paper is to optimize the design of the advanced combat helmet (ACH) currently in use, by its designers in order to attain maximum protection against ballistic impacts (fragments, shrapnel, etc.) and hard-surface/head collisions. Since traumatic brain injury experienced by a significant fraction of the soldiers returning from the recent conflicts is associated with their exposure to blast, the ACH should be redesigned in order to provide the necessary level of protection against blast loads. In the present work, augmentations of the ACH for improved blast protections are considered. These augmentations include the use of a polyurea (a nano-segregated elastomeric copolymer)-based ACH external coating/internal lining.

To demonstrate the efficacy of this approach, instrumented (unprotected, standard-ACH-protected, and augmented-ACH-protected) head-mannequin blast experiments are carried out. These experimental efforts are complemented with the appropriate combined Eulerian/Lagrangian transient non-linear dynamics computational fluid/solid interaction analysis.

The results obtained indicated that: when the extent of peak over-pressure reduction is used as a measure of the blast-mitigation effectiveness, polyurea-based augmentations do not noticeably improve, and sometimes slightly worsen, the performance of the standard ACH; when the extent of specific impulse reduction is used as a measure of the blast-mitigation effectiveness, application of the polyurea external coating to the standard ACH improves the blast-mitigation effectiveness of the helmet, particularly at shorter values of the charge-detonation standoff distance (SOD). At longer SODs, the effects of the polyurea-based ACH augmentations on the blast-mitigation efficacy of the standard ACH are inconclusive; and the use of the standard ACH significantly lowers the accelerations experienced by the skull and the intracranial matter. As far as the polyurea-based augmentations are concerned, only the internal lining at shorter SODs appears to yield additional reductions in the head accelerations.

To the authors’ knowledge, the present work contains the first report of a combined experimental/computational study addressing the problem of blast-mitigation by polyurea-based augmentation of ACH.