Weinong Chen

Reilly Professor of Aeronautics and Astronautics & Materials Engineering

Telephone: (765) 494-1788
Email: wchen@purdue.edu
More about Weinong Chen


Graduate Students

Benjamin Claus
School: Aeronautics and Astronautics
Graduated: December, 2018
Project/Thesis: Controlling Dynamic Torsion Loading

Jonathan Drake
School: Aeronautics and Astronautics
Expected Graduation: May, 2020
Project/Thesis: Dynamic Response of Polymer Bonded Explosives under Impact Loading

Zherui Guo
School: Aeronautics and Astronautics
Expected Graduation: May, 2020
Project/Thesis: Design Optimization of Multi-ply Soft Armor Targets Based on Failure Modes under Projectile Normal Impact

Michael Harr
School: Aeronautics and Astronautics
Graduated: May 2017
Project/Thesis: Hot-spot Formation in PBX under Impact Loading

Nesredin Kedir
School: Materials Science and Engineering
Expected Graduation: December, 2020
Project/Thesis: Particle Impact Damage on Coated Structure

Nicholas Kerschen
School: Mechanical Engineering
Graduated: May, 2018
Project/Thesis: Response of PBX under Shock Loading

Cody Kirk
School: Aeronautics and Astronautics
Expected Graduation: May, 2022
Project/Thesis: Response of Liquid Energetic Materials under High-speed Impact

Amelia Leenig
School: Materials Science and Engineering
Expected Graduation: July, 2020
Project/Thesis: Effects of Defects on the Damage and Failure of Energetic Materials under Dynamic Loading

Naranjan Parab
School: Aeronautics and Astronautics
Graduated: May, 2017
Project/Thesis: Fracture of Spherical Particles under Compression

Shane Paulson
School: Aeronautics and Astronautics
Expected Graduation: May, 2021
Project/Thesis: Mechanisms of Dynamic Interlaminar Delamination in Fiber-reinforced Composites

Andrew Roginski
School: Aeronautics and Astronautics
Expected Graduation: December, 2021
Project/Thesis: Impact Response of Polymer Bonded Explosives at elevated temperatures

Kerry-Ann Stirrup
School: Materials Science and Engineering
Expected Graduation: May, 2023
Project/Thesis: Effects of Temperature on the Impact Damage and Failure of Energetic Materials


Recent Publications

High Speed X-ray Phase Contrast Imaging of Energetic Composites under Dynamic Compression

Parab, N.D., Roberts, Z., Harr, M., Mares, J., Casey, A., Gunduz, I., Hudspeth, M., Claus, B., Sun, T., Fezzaa, K., Son, S. and Chen W., 2016,
“High Speed X-ray Phase Contrast Imaging of Energetic Composites under Dynamic Compression,” Applied Physics Letters, 109: 131903


Fracture of crystals and frictional heating are associated with the formation of “hot spots” (localized heating) in energetic composites such as polymer bonded explosives (PBXs). Traditional high speed optical imaging methods cannot be used to study the dynamic sub-surface deformation and the fracture behavior of such materials due to their opaque nature. In this study, high speed synchrotron X-ray experiments are conducted to visualize the in situ deformation and the fracture mechanisms in PBXs composed of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) crystals and hydroxyl-terminated polybutadiene binder doped with iron (III) oxide. A modified Kolsky bar apparatus was used to apply controlled dynamic compression on the PBX specimens, and a high speed synchrotron X-ray phase contrast imaging (PCI) setup was used to record the in situ deformation and failure in the specimens. The experiments show that synchrotron X-ray PCI provides a sufficient contrast between the HMX crystals and the doped binder, even at ultrafast recording rates. Under dynamic compression, most of the cracking in the crystals was observed to be due to the tensile stress generated by the diametral compression applied from the contacts between the crystals. Tensile stress driven cracking was also observed for some of the crystals due to the transverse deformation of the binder and superior bonding between the crystal and the binder. The obtained results are vital to develop improved understanding and to validate the macroscopic and mesoscopic numerical models for energetic composites so that eventually hot spot formation can be predicted.


Perforation of Aluminum Armor Plates with Fragment-Simulating Projectiles

Guo, Zherui ; Forrestal, Michael ; Martinez-Morales, Stephenie ; Chen, Weinong
Journal of Dynamic Behavior of Materials, 2019, Vol.5(4), pp.409-415


Experiments with fragment-simulating projectiles (FSP) and aluminum plates are conducted to evaluate the performance of various aluminum alloys and plate thicknesses to resist perforation against fragments. Ballistic-limit velocity data for several aluminum alloys and plate thicknesses are presented in several US Army Research Laboratory (ARL) reports. In this study, we present additional ballistic-limit data for plates thinner than the plates reported by ARL. In addition, we present an equation that predicts the ballistic-limit velocity for fragment-simulating projectiles (FSP) that perforate aluminum armor plates. The ballistic-limit equation is presented in terms of dimensionless parameters so that the geometric and material problem scales are identified. Predictions and data from two different fragment-simulating projectiles and two different strength aluminum alloys show the range of plate thicknesses for reasonable model predictions.


Effect of replacement strike-face material on the ballistic performance of multi-ply soft armor targets

Guo, Zherui ; Chen, Weinong ; Zheng, James
Textile Research Journal, March 2019, Vol.89(5), pp.711-725


In this study, the impact-face material of a multi-ply soft armor system was varied to different ratios and tested for the effects on the ballistic performance. It is known that the first few layers of multi-ply soft armor material typically fail inelastically near the system ballistic limit and can be replaced with a “sacrificial” material with other more desirable properties. Previous studies have determined that the ballistic performance of these hybrid systems is largely dependent on the amount of high-performance backing material. However, the extent to which the high-performance fabric can be replaced has yet to be fully quantified and examined. Materials of different properties, namely stainless steel mesh, Makrolon® polycarbonate sheets, and cotton, were used as replacement frontal material for 840 d Twaron® panels, and the hybrid panels were impacted by O1 tool steel right-circular cylinder projectiles fired using a single-stage smooth-bore gas gun. Results show that the ballistic performance is maintained up to a frontal material ratio of about 40%, and off-axis material properties play a role in energy dissipation.


A semi-empirical design parameter for determining the inelastic strike-face mass fraction of soft armor targets

Guo, Zherui ; Chen, Weinong ; Zheng, James
International Journal of Impact Engineering, March 2019, Vol.125, pp.83-92


At the ballistic limit velocity of a soft armor target pack, the impact response was shown to be decoupled in the thickness direction, with the initial few plies behaving in an inelastic fashion while the remaining plies dissipate energy via elastic strain modes. Since these initial plies only contribute to energy absorption via inelastic kinetic energy transfer, these plies may be replaced with another material with desirable properties, such as low costs or high shear strengths. The behavior of these diphasic armors has been shown in previous works to be varied depending on the type and the amount of strike face material that was replaced. However, there is a limited amount of published literature investigating this phenomenon. In this study, a framework is proposed to estimate the inelastic strike-face mass ratio as a function of the overall system areal density ratio to provide a preliminary design tool for diphasic armors.


X‐Ray Phase Contrast Imaging of the Impact of a Single HMX Particle in a Polymeric Matrix

Kerschen, Nicholas E. ; Sorensen, Christian J. ; Guo, Zherui ; Mares, Jesus O. ; Fezzaa, Kamel ; Sun, Tao ; Son, Steven F. ; Chen, Weinong W.
Propellants, Explosives, Pyrotechnics, April 2019, Vol.44(4), pp.447-454


A complete understanding of the mechanisms by which high explosives (HEs) are shock initiated, especially at the particle scale, is still in demand. One approach to explain shock initiation phenomenon is hot spot theory, which suggests that distributed energy in energetic material is localized due to shock or impact to generate the high temperatures for ignition. This study focuses on the impact response of a HE polycrystalline particle, specifically HMX, in a polymer matrix. This represents a simplified analog of a traditional polymer‐bonded explosive (PBX) formulation. A light gas gun, together with high‐speed x‐ray phase contrast imaging (PCI), was used to study the impact response of a single particle of production‐grade HMX in a Sylgard‐184® matrix. The high‐speed x‐ray PCI allows for real‐time visualization of HE particle behavior. The experiments revealed that, at impact velocities of ∼200 m s, the energetic particle was cracked and crushed. When the impact velocity was increased to 445 m s, a significant volume expansion of the particle was observed. This volume expansion is considered to be the result of chemical reaction within the HE particle.


Improved quasi-static twin-fiber transverse compression of several high-performance fibers

Guo, Zherui ; Chen, Weinong ; Zheng, James
Textile Research Journal, May 2019, Vol.89(9), pp.1595-1613


The method of determining the quasi-static transverse compressive response of several high-performance polymer fibers was improved upon from a previous twin-fiber transverse compression setup in order to detect small initial high compliance signals while maintaining consistent diametral compression. Two fibers were laid parallel between two polished tool steel platens, and the fibers were subsequently compressed using a piezo-electric actuator at quasi-static rates. The new experimental setup ensures that the compression cycle begins when extremely small load signals are detected so that initial elastic transverse moduli may be more accurately measured. Nominal stress–strain curves were obtained for several types of high-performance fibers. The results show good agreement with previously obtained measurements. S-glass fibers exhibited a vastly different mechanical response compared to the polymer fibers.


In-situ X-ray observations of ultrasound-induced explosive decomposition

Mares, Jo ; Roberts, Za ; Gunduz, Ie ; Parab, Nd ; Sun, T ; Fezzaa, K ; Chen, Ww ; Son, Sf ; Rhoads, Jf
Applied Materials Today, 2019 Jun, Vol.15, pp.286-294


Ultrasound is used to study “hot spot” formation and explosive initiation. This work details observations of the heating and decomposition of an explosive. Interfacial friction is shown to be a dominant heating mechanism. High-strain mechanical loading of polymer-bonded explosives can produce significant stress concentrations due to microstructural heterogeneities, resulting in localized thermal “hot spots”. Ultrasound produces similar effects and has been proposed as a tool to study the thermomechanical interactions related to explosive initiation. Detailed observations of the processes governing the generation of heat in these materials are severely lacking, yet they are vital for identifying salient physics, improving the modeling tools used to predict mechanical response, improving explosives safety, and providing insight into the initiation mechanisms of explosion. Here we report on high-speed, high-resolution in-situ observations, obtained via synchrotron X-ray phase contrast imaging and diffraction, of the heating and decomposition of an explosive material under ultrasonic excitation. We demonstrate that interfacial friction is a dominant heating mechanism and can lead to a violent reaction in the explosive particles. Furthermore, sub-surface particle temperatures are estimated via diffraction.


Localized impact stress concentrations in soft armors due to microscale projectile edge geometries

Guo, Zherui ; Chen, Weinong ; Zheng, James
Textile Research Journal, August 2019, Vol.89(16), pp.3234-3243


Although extensive focus has been placed on the ballistic performance of projectiles with certain macroscale geometries and dimensions, the microscale geometries are not as rigorously standardized. The localized stress concentrations arising from microscale geometries introduce multiaxial and locally concentrated stress states within the constituent material of the soft-armor target, which can result in premature failure that is not predicted with existing models. In this study, the microscale edge/corner geometries of right circular cylinder (RCC) projectiles are varied, and their respective ballistic performance was determined via experiments to examine the effects of the localized stress concentrations. Target panels were examined post-mortem and the effects of these localized stress concentrations on the failure modes were quantified. Experiments results indicate that stress concentrations drastically reduce the ballistic performance of the soft armor targets, and the fabric targets appear to fail without significant strain energy absorption.


The effect of the particle surface and binder properties on the response of polymer bonded explosives at low impact velocities

Dandekar, Akshay ; Roberts, Zane A ; Paulson, Shane ; Chen, Weinong ; Son, Steven F ; Koslowski, Marisol
Computational Materials Science, August 2019, Vol.166, pp.170-178


Polymer bonded explosives are designed to initiate under controlled conditions. However, accidental ignition leading to a deflagration, and even detonation, may occur during manufacturing, handling and transport. Understanding how ignition depends on microstructural features, such as cracks and voids in the particles, and on the adhesive and mechanical properties of the binder through predictive numerical simulations and modeling will help to improve safety. Finite element simulations and experiments of a single high energetic material particle embedded in polymer binders are performed to investigate the effect of the material properties of the binder and the particle surface properties, on damage and temperature at an impact velocity of 10 m/s. Particles with low and high quality surface properties, and two different binders are analyzed. The simulations with the lower stiffness binder do not show a significant increase in temperature after impact. A polymer with higher stiffness induces more particle damage under impact contributing to a larger temperature rise. Furthermore, high quality surface and higher adhesion strength induces larger stresses and increase the temperature rise.


Observation of Damage During Dynamic Compression of Production and Low-Defect HMX Crystals in Sylgard® Binder Using X-Ray Phase Contrast Imaging

Paulson, S.C. ; Roberts, Z.A. ; Sorensen, C.J. ; Kerschen, N.E. ; Harr, M.H. ; Parab, N.D. ; Sun, T. ; Fezzaa, K. ; Son, S.F. ; Chen, W.W.
Journal of Dynamic Behavior of Materials, Oct. 2019


Polymer bonded explosives (PBX) have many applications in both the military and civilian sectors, making their safety and behavior predictability of the utmost importance. Most explosive devices are typically initiated by some external stimulus; however, initiations can also occur via localized mechanical conversion of energy during impact, called ‘hot spots’. These unintended loads can lead to crystal fracture and frictional heating, amongst other mechanisms, in the energetic crystals of a PBX. In order to visualize the behavior of these crystals, high-speed phase contrast imaging experiments were conducted using synchrotron X-ray radiation to observe the internal crack behavior of simplified PBXs subjected to low velocity impact. The PBX samples used in these experiments were composed of single production-grade and recrystallized octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) crystals embedded in a Sylgard® 184 binder doped with iron (III) oxide. We observed a clear distinction in the qualitative behavior of production-grade versus recrystallized ‘low-defect’ HMX crystals which lacked significant internal voids. Production grade crystals exhibited consistent cracking behavior in the crystals, while the recrystallized crystals exhibited debonding from the surrounding binder material and cracked much less frequently. We assert that there is a clear effect of crystal quality on the behavior of PBX, which should influence future insensitive munition formulation design choices.


Using mechanoluminescence as a low-cost, non-destructive diagnostic method for transient polymer impact processes

Guo, Zherui ; Chen, Weinong
Measurement, February 2020, Vol.151


At ballistic velocities, certain polymers such as nylon and polyethylene have been shown to exhibit mechanoluminescence (ML) upon impact. This ML event results in the emission of photons in the visible wavelength spectrum and may occur on transient timescales as short as a sub-microsecond. In this study, we take advantage of this ML phenomenon to design a low-cost diagnostic tool by recording the ML emissions. The design consists of photodiodes of spectral range in the near-infrared to visible spectrum located radially in order to record the luminescence emissions around the circumference of the ML footprint during impact. Ultra-high-molecular-weight polyethylene rods were fired at velocities between 200 and 450 m/s as preliminary proof of concept experiments. Recorded signals were post-processed to yield information such as the projectile’s time-of-arrival, approximate impact location, and approximate attitude during initial impact. The experimental setup, measurement and post-processing techniques, and reconstruction results are detailed and discussed in this work.


Projectile strength effects on the ballistic impact response of soft armor targets

Guo, Zherui ; Martinez-Morales, Stephenie ; Chen, Weinong
Textile Research Journal, February 2020, Vol.90(3-4), pp.282-293


Upon impact with a target panel, a portion of a projectile's striking kinetic energy is dissipated via heat loss or deformation. Typical ballistic performance determination standards require strict projectile hardness values of Rc 29 ± 2 for consistency and repeatability, but it is of interest to examine if these required hardness values give a lower bound where the ballistic performance determination is independent of the strength of the projectile. In this study, a large range of yield strengths of metallic right-circular cylinders were used to test the effects on the ballistic response of a multi-ply soft body armor. The results show that with an increase in projectile yield strength, the ballistic limit velocity decreases. This degradation in ballistic performance of the soft armor target levels off at higher yield strengths to about 75% of the expected ballistic performance for Rc 29, indicating that there may be a minimum projectile strength after which the influence of strength is no longer significant. The degree of deformation of projectiles during impact is related to the striking velocity and the off-axis failure of the soft armor target material.


Void Collapse in Shocked β ‐HMX Single Crystals: Simulations and Experiments

Duarte, Camilo A. ; Hamed, Ahmed ; Drake, Jonathan D. ; Sorensen, Christian J. ; Son, Steven F. ; Chen, Weynong W. ; Koslowski, Marisol
Propellants, Explosives, Pyrotechnics, February 2020, Vol.45(2), pp.243-253


Heat generation in the vicinity of a void during shock compression plays a key role in the initiation of energetic materials. The shock response of a single ‐HMX crystal with a single void is studied with simulations that include plasticity and heat transport. The numerical results are validated with an experiment in which a 500 m void is machined in an HMX single crystal and impacted. Experiments and simulations of the dynamical evolution of the morphology of the void during the collapse and the rate of the area are in very good agreement for weak shocks.


In-situ observation of cutting-induced failure processes of single high-performance fibers inside a SEM

Gao, Jinling ; Nie, Yizhou ; Lim, Boon Him ; Zhai, Xuedong ; Kedir, Nesredin ; Chen, Weinong
Composites Part A, April 2020, Vol.131


Insight into the detailed fracture processes of single high-performance fibers improves understanding for the nature of their unique resistances to external loads. Here, we show the in-situ observation of entire fracture processes of an uncoated single Kevlar® KM2 Plus and Dyneema® SK76 fiber cut by a razor blade at various angles in a Scanning Electronic Microscope (S), including initial contact, deformation, crack initiation and propagation until final failure. The effects of gauge length, sputter coating of the platinum, fiber type and cutting angle on the fiber failure were investigated and discussed. The mass-efficient cutting resistance of a single fiber was evaluated by the specific energy. The diversity of the failure modes and cutting resistance were analyzed and attributed to the specific fiber nanostructure and different cutting angles.


X‐ray Phase Contrast Imaging of the Impact of Multiple HMX Particles in a Polymeric Matrix

Kerschen, Nicholas E. ; Drake, Jonathan D. ; Sorensen, Christian J. ; Guo, Zherui ; Mares, Jesus O. ; Fezzaa, Kamel ; Sun, Tao ; Son, Steven F. ; Chen, Weinong W.
Propellants, Explosives, Pyrotechnics, April 2020, Vol.45(4), pp.607-614


The initiation of high explosives (HEs) under shock loading lacks a comprehensive understanding: particularly at the particle scale. One common explanation is the hot spot theory, which suggests that energy in the material resulting from the impact event is localized in a small area causing an increase in temperature that can lead to ignition. This study focuses on the response of HMX particles (a common HE) within a polymer matrix (Sylgard‐184®), a simplified example of a polymer‐bound explosive (PBX). These PBXs consist of multiple HMX particles in a single polymer‐bound sample. A light gas gun was used to load the samples at impact velocities above 400 m/s. The impact events were visualized using X‐ray phase‐contrast imaging (PCI) allowing real‐time observation of the impact event. The experiment used two different types of samples (multi‐particle and two crystals) and found evidence of cracking and debonding in both sample types. In addition, it was found that the multiple particle samples showed similar evidence of damage at lower velocities than that of single particle samples. This is an expected result as the multiple particles add additional interfaces for stress concentration and frictional heating.


Dynamic stress-strain response of high-energy ball milled aluminium powder compacts

Justice, A.W ; Beason, M.T ; Gunduz, I.E ; Chen, W ; Son, S.F
Mechanics of Materials, April 2020, Vol.143


Ball milling is a bulk powder manufacturing process used in the creation of dispersion strengthened and nanostructured materials. Fundamentally, these powders have not been dynamically characterized in a green state prior to hot consolidation. The understanding of high strain-rate compaction on void collapse and particle interaction for such systems can help the development of predictive models for impact events of porous metallic structures that may be employed as energy absorbers, reactive structures, and intermetallic materials. This study investigates high strain-rate impact of porous green compacts of as-received and high-energy ball milled (HEBM) aluminium powders characterized under dynamic compression using a split-Hopkinson pressure bar (SHPB) in a passive confinement configuration. The plastic deformation of the powder compacts and crush up were shown to be strain-rate insensitive within the strain rate range of 1000–2100 s−1 and as a result, were modelled adequately with a second order P-α model. The as-received aluminium and HEBM aluminium powders appear to have the same strain-hardening coefficient and strength index as solid aluminium after yielding. The respective stress-strain responses of green compacts follow the same trend but differ only in strength as result of porosity and pre-strain experienced prior to dynamic compression. The HEBM powder was found to be twice as strong as the untreated as-received aluminium powder.


A merit parameter to determine the stacking order of heterogeneous diphasic soft armor systems

Guo, Zherui ; Chen, Weinong
Composite Structures, 01 June 2020, Vol.241


The effects of stacking order on the ballistic performance may be detrimental if the order is improperly chosen. When the frontal material is constrained transversely by the rear material, it results in sub-optimal performance compared to the alternate configuration where both layers can freely deform. In this study, we examine the possibility of using the Cunniff velocity as a merit parameter in determining the optimal stacking order of heterogeneous diphasic soft armor systems by reviewing the results from previous studies. Experiments were performed on heterogeneous systems comprising ballistic-grade polyurea, Twaron® fabric, and Dyneema® UD laminate plies. Results show that the two constituent materials should be ordered such that the material with a higher Cunniff velocity is placed at the rear to minimize interference. The use of the merit parameter is then analyzed via existing models to examine the effects of changing various parameters. We further discuss the idea of “ballistically-thin” materials in relation to the concept of membrane strain energy dissipation efficiency of a soft armor target.