Dr. Zhou maintains a constant component of undergraduate research in his group. These opportunities can take a variety of forms, including funded research through research grants, funded research through the Georgia Tech SURF (Summer Undergraduate Research Fellowship) program, the FACES program, and the GT work study program. In addition, research projects for special course credit are also offered. All these opportunities provide real hands-on research experience with faculty and graduate students. Motivated students should contact Dr. Zhou through email or office visit.
Dr. Zhou currently has openings for 2-3 new M.S. and Ph.D. students to conduct thesis research in the areas of deformation and fracture. Projects concern computational modeling of microscopic fracture and experimental characterization of fracture resistance of ceramics and composite materials. Specific projects include
Finite Element Simulations of the Dynamic Behavior of Polymer-Bonded Explosives
Polymer bonded explosives (PBX) refers to a class of composites consisting of crystals of explosives bound together by polymer binders and a small percentage of additives such as plasticizers and antioxidants;
The mass fraction of crystals may vary over a range of 60% to 95%;
Young’s modulus of elasticity of the crystal is typically an order of magnitude higher than that of the binder. As a result, the deformation characteristic of the composite is primarily influenced by the mechanical properties of the binder itself. Upon application of load, the binder deforms and absorbs most of the energy of deformation;
Heating resulting from plastic deformation of the binder generates local hot spots and serves as an ignition mechanism;
The model analyzed here consists of crystals of HMX evenly dispersed in a matrix of polymer HTPB. HMX granules are essentially elastic under the conditions analyzed although our model is capable of capturing any plastic deformation as well. A range of loading rate, initial temperature and volume fraction is considered to quantify the variation of deformation, stress and temperature in the microstructure;
Microstructure image acquired from X-ray microtomographic scan of simulant QRX030 by C.R. Siviour et al., Proc. R. Soc. A (2008);
Experimentally measured curves form C.M. Cady et al., Polymer Engineering and Science, 2006 are used to calibrate thermo-viscoplastic parameters;
Stochastic Analyses of Dynamic Fracture in Composite Ceramic Microstructures
Variations in constituent properties, phase morphology, and phase distribution cause deformation and failure at the microstructural level to be inherently stochastic. This paper focuses on the stochasticity of fracture processes that arises as a result of measurement uncertainties in the properties of the constituents in the heterogeneous microstructures of an Al2O3/TiB2 ceramic composite system. A micromechanical cohesive finite element framework with explicit resolution of arbitrary fracture patterns and arbitrary microstructural morphologies is used. A deterministic analysis and a stochastic analysis are carried out simultaneously. The combination of determinism and stochasticity is achieved by integrating a perturbation analysis of the influence of stochastic property variations around their mean values and a deterministic analysis for the microstructure with the mean values of the constituent properties. Calculations are carried out for actual and idealized microstructures of the Al2O3/TiB2 material system. It is found that for the system analyzed the variations in the crack surface area generated and the variations in the energy release rate are of the same order as the variations in constituent properties.
Explicit Finite Element Simulation of Impact Damage in Composite Laminates
Fiber-reinforced composite laminates undergo internal damage under low velocity impact. Expeiments show that when impact energy exceeds a threshold, damage occurs in the forms of matrix cracking and inter-ply delamination. Interlaminar bonding, inter-ply layup and loading modes are found to have significant influences on failure behavior and energy dissipation associated with impact damage.
A cohesive finite element method (CFEM) is used to provide explicit modeling of the damage process. This model combines a cohesive crack surface relation and orthotropic material constitutive characterization. The effects of interlaminar bonding strength, laminate layup, loading mode and loading rate on the damage initiation and evolution under low-velocity impact are analyzed. Explicit CFEM simulations track the time-resolved history of damage initiation and growth. Finite deformation kinetics and a full dynamic framework of analysis are used. Details of the formulation can be in papers on the Publications list.
The following movie illustrates the process of the dynamic failure development in a composite beam under three-point bend loading due to low-velocity impact. The laminate consists of three plies. The top and bottom layers are 0-degree plies and the middle layer is a 90 degree ply. Damage initiates in the 90-degree ply and subsequently propagates and causes interlaminar delamination.
Time-Resolved Analysis of the Rate-Dependent Behavior of PZT Ceramics
Ferroelectric ceramics have a wide variety of applications, ranging from ultrasonic devices to pressure sensors. In many of these applications, the ferroelectric material is subjected to transient mechanical loading and its electro-mechanical response is of critical importance. There is a lack of characterization for the responses of Lead Zirconate Titanate (PZT) ceramics to stress pulses with magnitudes below 1 GPa. A split Hopkinson pressure bar (SHPB) apparatus is used to subject PZT specimens to transient stress pulses of 50 to 500 MPa in magnitude. The duration of the stress pulse is approximately 150 microseconds, corresponding to load frequency of 6.7 kHz if repeated. A time-resolved analysis of the electro-mechanical response is carried out using high-speed digital oscilloscopes with a resolution of 500 nanoseconds. For comparison purposes, quasistatic experiments are also conducted. The materials tested are four different hard and soft PZT ceramics. These materials allow the effects of composition and grain size on behavior to be characterized.