Weinong (Wayne) Chen - Research Interests
Current research is focused on the dynamic behavior of advanced engineering materials.
Research Interests are:
Dynamic Response and Failure Behavior of Intact and Damaged Armor Ceramics.
The goal of this US Army sponsored program is to determine the dynamic compressive responses and failure behavior of vehicle and personnel armor ceramics as a function of loading rates, damage levels, and lateral confinement. To determine the dynamic properties under loading conditions simulating those encountered in ceramic armors subjected to impact, a novel dynamic compressive experimental technique modified from a split Hopkinson pressure bar (SHPB) is employed to load the ceramic specimen by two consecutive stress pulses. The first pulse determines the dynamic response of the intact ceramic material and then crushes the specimen. The second pulse determines the dynamic compressive constitutive behavior of the damaged ceramic that is still interlocked. The results we obtained so far show that the compressive strengths of damaged ceramics are insensitive to strain rates in the range studies once the damage level exceeds a critical value. This indicates that while the damage in the ceramics accumulated incrementally, the load-bearing capabilities of the ceramics drop suddenly at a critical damage level, rather than gradually.
Dynamic Response of Textile Materials
The current focus of this Collaborative Program with US Army Research Laboratory (ARL) is on the mechanical behavior and the rate effects of single Kevlar® KM2 fibers. Use of Kevlar® KM2 fabrics in personnel protection applications creates an urgent need to develop a better scientific understanding of the damage of the fibers caused by transverse compressive impact. In this research, a new experimental facility is developed to measure the transverse compressive behavior of transversely isotropic high-performance fibers of a diameter of 12 micrometers. The effects of the damage caused by transverse deformation on the load-bearing capabilities of the fiber have been fully investigated. Phenomenon similar to the Mullins effect observed in rubber materials also exists in the transverse response of the Kevlar® KM2 fibers. Besides, the transverse compressive behavior of the Kevlar® KM2 fibers is insensitive to loading rates. However, the longitudinal tensions can stiffen the transverse behavior at large deformation although this effect is insignificant at small deformation.
Dynamic Behavior of Biological Tissues.
The current focus of this Collaborative Program with ARL is on the mechanical behavior and the associated rate and aging effects of porcine muscles. The dynamic mechanical response of biological tissues to impact loading has been an important aspect in the impact-induced injury prediction and the subsequent effective design of personnel protection systems. However, the rate effects on the mechanical response of soft tissues have not been well studied. Furthermore, the effects of the tissue status (temperature, aging, freezing, etc.) on the its high-rate mechanical response are not understood. In this research, the compressive stress-stain behavior of a porcine muscle is investigated experimentally, at quasi-static strain rates (0.04/s~4.0/s) with a conventional material testing system and at dynamic strain rates (400/s~1500/s) with a modified split Hopkinson pressure bar (SHPB). The experimental results reveal a significant dependence of the stress-strain behavior on strain rates. The effects of specimen status depend on the loading orientation with respect to the axial direction of the muscle. Experimental data are analyzed using a phenomenological material model with loading-rate effects.
Dynamic Behavior of Soft Materials
In a continuous collaboration with Sandia National Laboratories, novel experimental techniques have been developed to investigate the dynamic behavior of polymeric foams, silicone rubbers, particle- and fiber-reinforced rubbers. Current focus is on the dynamic compressive responses of a sytectic epoxy foam and a low-density closed-cell epoxy foam under wide ranges of strain rates, environmental temperatures, and stress states. A series of particle-filled rubbers are also being investigated.
Impact Response of LIGA Structures
This Sandia sponsored research project investigates the mechanical responses of miniturized structural components manufactured by LIGA process. The components are slightly larger than MEMS components. Modified split Hopkinson bars are employed to load the components at desired accelerations ranging from 5,000 to 200,000 g. A high-speed, high resolution digital camera with a framing rate up to 2,000,000 frames per second is employed to record the deflection history of the small components subjected to controlled shock loading. Analysis of the shock and deflection histories reveals the high-rate behavior of the materials of the LIGA structure.
Dynamic Hysteretic Behavior of Shape-Memory Alloys
A novel dynamic experimental technique has been developed to determine the dynamic compressive hysteretic loops of shape-memory alloys. Pulse-shaping techniques were developed for both the loading and unloading paths of a split Hopkinson pressure bar (SHPB) experiment to obtain valid dynamic stress-strain loops for engineering materials. Front and rear pulse shapers in association with a momentum trap were used to precisely control the profiles of loading and unloading portions in the incident pulse. The modifications ensure that the specimen deforms at the same constant strain rate under dynamic stress equilibrium over not only loading but also unloading stages of an experiment so that dynamic stress-strain loops can be accurately determined. Dynamic constant-strain-rate stress-strain loops for a NiTi SMA and a PMMA were determined with the modified SHPB in this study. A modified momentum trap prevents repeated loading on specimen without affecting the amplitude of the desired loading pulse and without damaging the bar at high stress levels.
Impact Responses of Nano-composites
In this research collaborating with ARL and USDA, the dynamic response of a series of nano-composites have been determined. The failure behavior were examined with TEM. The materials investigated so far incluse a PMMA and polycarbonate reinforced nano-particles of various concentrations and three naturally degradable nanocomposites which were made of epoxidized soybean oil (ESO) polymer and nanoclay of 0%, 5%, and 8% weight. Nearly constant strain-rate deformation in specimen under dynamic stress equilibrium through pulse-shaping in the dynamic experiments yielded valid stress-strain curves at high strain rates. A new one-dimensional material model, combining a rate-dependent strain-energy function and a relaxation function in a viscoelastic framework, has been developed to accurately describe the strain-rate dependent behavior of the three soybean materials at both large and small strains and at both high and low strain rates.
Development of Impact Energy Dissipation Materials via Heterogeneous Interfacial Fracture Characterization and Modeling
The goal of this ARO-proposed research is to develop a fundamental understanding of the mechanisms of mechanical energy dissipation during the dynamic fracture of an interface in hierarchical heterogeneous materials under stress wave loading. The approaches of maximizing impact energy dissipation through tailoring interfaces between different phases of the materials will also be explored. The investigation of this phenomenon and the resultant new high-energy absorbing materials are critical to the design of future lightweight material systems for human and vehicle protection.
- Song, B., Chen, W., and Cheng, M., 2004, "A Novel Model for Uniaxial Strain-Rate-Dependent Stress-Strain Behavior of EPDM Rubber in Compression or Tension," Journal of Applied Ploymer Science, vol. 92, pp. 1553-1558.
- Song, B. and Chen, W., 2004, "Dynamic Compressive Behavior of EPDM Rubber Under Nearly Uniaxial Strain Condition," Transaction of the ASME, Journal of Engineering Materials and Technology, vol. 126, no. 2, pp. 213-217.
- Chen, W. and Luo, H., 2004, ?Dynamic Compressive Responses of Intact and Damaged Ceramics from a Single Split Hopkinson Pressure Bar Experiment, Experimental Mechanics, vol. 44, No. 3, pp. 295-299.
- Song, B. and Chen, W., 2004, "Dynamic Stress Equilibrating Process in a Rubber Specimen during a Split Hopkinson Pressure Bar Experiment," Experimental Mechnics, vol. 44, no. 3, pp. 300-312.
- Song, B., Chen, W., and Frew, D. J., 2004, "Quasi-Static and Dynamic Compressive and Failure Behaviors of An Epoxy Syntactic Foam," Journal of Composite Materials, vol. 38, no. 1, pp. 915-936.
- Luo, H. and Chen, W., 2004, ?Dynamic Compressive Responses of Intact and Damaged AD995 Alumina,? International Journal of Appied Ceramic Technology, vol. 1, issue 3, pp. 254-260.
- Cheng, M. Chen, W., and Song, B., 2004, "Phenomenological Modeling of the Stress-Strain Behavior of EPDM Rubber with Loading-Rate and Danage Effects," International Journal of Damage Mechanics, vol. 13, no. 4, pp. 371-381.
- Cheng, M., Chen, W., and Weerasooriya, T., 2004, "Experimental Investigation of the Transverse Mechanical Properties of a Single Kevlar KM2 Fiber," International Journal of Solids and Structures, vol. 41, issues 22-23, pp. 6215-6232.
- Wang, B., Lu, H. B., Tan, G. X., and Chen, W., 2003, ?Strength of damaged polycarbonate after fatigue,? Theoretical and Applied Fracture Mechanics, Vol. 39, No. 2, pp. 163-168.
- Cheng, M., Chen, W., and Sridhar, K. R., 2003, ?Biaxial flexural strength distribution of thin ceramic substrates with surface defects,? International Journal of Solids and Structures, Vol. 40 No. 9, pp. 2249-2266.
- Chen, W., Song, B., Frew, D. J., and Forrestal, M. J., 2003, "Dynamic Small Strain Measurement with a Split Hopkinson Pressure Bar," Experimental Mechanics, Vol. 43, No. 1, pp. 20-23.
- Song, B. and Chen, W., 2003, "Dynamic Compressive Behavior of EPDM Rubber," Transaction of the ASME, Journal of Engineering Materials and Technology, vol. 125, pp. 294-301.
- Cheng, M. and Chen, W., 2003, "Experimental Investigation of the Stress-Stretch Behavior of EPDM Rubber with Loading Rate Effects," International Journal of Solids and Structures, vol. 40, pp. 4749-4768.
- Song, B. Chen, W., and Weerasooriya, T., 2003, "Quasi-static and Dynamic Compressive Behaviors of a Glass/Epoxy Composite," Journal of Composite Materials, vol. 37, Issue 19, pp. 1723-1743.
- Rojas, R. R., and Chen, W., 2003, ?Instrumented Low-speed Penetration into Granular Alumina,? Instrumentation, Measurements, and Metrology, vol. 3, No. 3-4, pp. 213-236.
- Chen, W., Lu, F., and Cheng, M., 2002, ?Tension and Compression Tests of Two Polymers Under Quasi-static and Dynamic Loading? Polymer Testing, vol. 21, No.2, pp. 113-121.
- Chen, W., Lu, F., and N. A. Winfree, 2002, "Dynamic Compressive Response of Polyurethane foams of Various Densities," Experimental Mechanics, Vol. 42, No. 1, pp. 65-73.
- Frew, D. J., Forrestal, M. J., and Chen, W., 2002, "Pulse-shaping techniques for testing brittle materials with a split Hopkinson pressure bar," Experimental Mechanics, Vol. 42, No. 1, pp. 93-106.
- Chen, W., Lu, F., Frew, D. J., and Forrestal, M. J., 2002, "Dynamic compression testing of soft materials," Transaction of the ASME, Journal of Applied Mechanics, Vol. 69, No. 3, pp. 214-223.
- Forrestal, M. J., Frew, D. J., and Chen, W., 2002, "The Effect of Sabot Mass on the Striker Bar for Split Hopkinson Pressure Bar Experiments," Experimental Mechnics, Vol. 42, No. 2, pp.129-131.
- Cheng, M., Chen, W., and Sridhar, K. R., 2002, "Experimental Method for a Dynamic Biaxial Flexural Strength Test of Thin Ceramic Substrates," Journal of the American Ceramic Society, Vol. 85, No. 5, pp 1203-1209.
- Zhang, B., Poirier, D. R., and Chen, W., 2002, ?Effects of hipping and strontium modification on the fatigue behavior of A356.2 aluminum alloy,? Transactions of the American Foundrymen's Society, Vol. 110, pp. 393-405.
- Chen, W., Song, B., Frew, D. J., and Forrestal, M. J., 2002, "Recent Developments in Split Hopkinson Bar Technique," Ceramic Transactions, Vol. 134, pp217-224.