Seminar Series - 2002-2003

Dynamic Tribological Response of Fractured and Shear-Damaged Surfaces

Hongfa Huang - Doctoral Dissertation Defense
Advisor:  Dr. Ruqiang Feng

Date:  Wednesday, April 23, 2003
Time:  7:30 a.m.
Place:  W106 Nebraska Hall


When deformed under confining stresses, materials may undergo shear cracking/damage with the fractured/damaged surfaces being closed, e.g., failure of ceramic armors under ballistic impact. The friction contact of the fractured/damaged surfaces plays an important role in the response of the damaged materials. Although microcracking models considering friction have been proposed, tribological study on the fractured/damaged surface pairs is lacking. In this research, a novel experimental method to characterize the dynamic tribological response of fractured or shear-damaged surfaces has been developed. The method consists of a new dynamic tribometric experiment based on the torsional Kolsky bar technique and an integrated optical profilometric examination that enables comparison of the initial and tested surfaces.

Using this method, the dynamic tribological response of aluminum alloy (Al) and silicon carbide (SiC) fracture surface pairs and shear-damaged SiC surface pairs have been studied in detail including the dependencies on roughness, wear evolution and wear debris. The experimental investigation covers the shear/sliding velocities 0.04~6.50 m/s and the contact stresses 0.15~1.9 GPa. In addition, empirical modeling has been performed to evaluate the key factors affecting the tribological behavior of Al fracture surface pairs. Realistic fracture surface modeling and finite element simulations have been carried out to elucidate the possible micromechanisms governing the tribological response of closed fracture surfaces.

It has been found that the tribological response of closed Al fracture surface pairs is highly nonlinear and softens exponentially with increasing wear. In contrast, the tribological response of initially flat SiC surface pairs displays substantial hardening while the surfaces are roughened via microscopic shear damage. The transient response of shear-damaged SiC surface pairs may display hardening or softening depending on whether or not the debris from prior damage is removed. The steady-state response is, however, consistently Coulombic and can be characterized with a nominal kinetic frictional coefficient  . Surprisingly, the steady-state tribological response of closed SiC fracture surface pairs gives a significantly lower nominal value  . Further numerical study suggests that shear dilatancy occurred during the tribometric tests resulting in a significantly smaller true contact area than the nominal area used in calculation.


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