Experimental and computational methods were developed to model three‐dimensional (3‐D) mixed‐mode crack growth under fatigue loading with the objective of evaluating proposed 3‐D fracture criteria. The experiments utilized 7075‐T73 aluminium forgings cut into modified ASTM E740 surface crack specimens with pre‐cracks orientated at angles of 30, 45 and 60° in separate tests. The progress of the evolving fatigue crack was monitored in real time using an automated visualization system. In addition, the amplitude of the loading was increased at prescribed intervals to mark the location of the 3‐D crack front for post‐test inspection. In order to evaluate proposed crack growth equations, computer simulations of the experiments were conducted using a 3‐D fracture model based on the surface integral method. An automatic mesher advanced the crack front by adding a ring of elements consistent with local application of fracture criteria governing rate and direction of growth. Comparisons of the computational and experimental results showed that the best correlation was obtained when KII and KIII were incorporated in the growth rate equations.
A fretting fatigue apparatus was designed, built and tested incorporating independent computer control of fretting fatigue slip distance and fatigue stress. This was accomplished through the utilization of two coaxial servo-hydraulic test actuators controlled in real time by computer. The central hydraulic actuator applies the fatigue load to the test specimen, while the outer concentric hydraulic actuator moves the fretting pin carrier apparatus. Independent control of slip displacement is achieved with the use of a capacitance displacement gage attached to the specimen fret pin carrier in such a manner that relative displacements of <5μm can be controlled. Capacitance gage measurements indicate the relative motion of the fatigue specimen surface caused by loading with respect to the fret pins. The fret pin carrier is subsequently moved to accommodate this motion plus its own-programmed motion. Load cells are provided both above and below the fatigue specimen allowing for measurement, by difference, of the forces applied by the fretting pins. These forces can be used to calculate the dynamic coefficient of friction during test operation. Finally, a 3-D finite element analysis model was constructed of the fatigue specimen and the fret pins to determine analytically the slip occurring at the fatigue specimen surface within the bounds of the test operation.
A comprehensive evaluation of fretting fatigue variables was conducted on shot-peened Ti-6A1-4V forging material in the β-STOA condition in contact with 17-4PH pins, a material couple representative of helicopter dynamic component interfaces. Utilizing test equipment incorporating independent fatigue stress and fretting slip displacement control (as described elsewhere in this symposium), a test matrix spanning slip distances, δ, of 25 ⩽ δ ⩽ 75 μm and contact stresses, σf, of 70 ⩽ σf ⩽ 200 MPa. Fatigue stresses were used which resulted in cycle lives ranging from run out, >107 to 103. A flat against flat contact geometry was used with the contact area covering ~10 mm2. Representative fretting scars through out the test matrix were examined via serial section analysis and the crack number, density, location, length and flank angle noted through the scar volume. A fretting fatigue endurance map of contact stress vs. slip distance shows that slip amplitude dominates mean fretting fatigue strength at 107 cycles under the tested conditions with contact stress playing only a moderate role. The fretting surface could be characterized as moderately pitted with dense third body debris. The debris was determined to be TiO2. Critical cracks formed through a linking of smaller cracks across the fretting scar as evidenced by a number of nucleation sites on the fracture surface. The coefficient of friction, COF, was observed to increase from its initial value of 0.15 to a stable ̃0.75 through the first 103–104 cycles.
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