Technical Reports

Fatigue Tests on Trail Hitch at FAST- The FAT II Report

  • 01
  • Feb
  • 1983
AUTHOR: G.J. Moyar, S.K. Punwani
SUBJECT: Fatigue Management
KEYWORDS: Crack Preparation, Fatigue, Flat Car Hitch, Rail Car Dynamics, Strain Gaging, Stress Analysis
ABSTRACT: Cracks appeared in a hitch and its underdeck support on one of the three Trailer-onFlat Cars (TOFC) in the FAST (Facility for Accelerated Service Testing)train consist 3,000 miles after inspection (at 186,000 miles). As a result, a series of static and dynamic (FAST operation} tests was undertaken on a similar car in May of 1981. It was apparent from the static lateral pull tests that lateral loads of the order of 10 kips would cause relatively high (1 ,000 J.lf:), potentially damaging strains if repeated frequently. These high strains occurred in the plate near the vertical strut weld juncture, due primarily to plate bending caused by the particular strut assembly reaction to lateral load. An asymmetry of vertical and lateral loading response was also observed in this test car. Following conventional S-N procedures described in the AAR Fatigue Design Guidelines, a fatigue analysis was performed using the cycle-counted strain history recorded from the dynamic tests. Fatigue cracking in the hitch is predicted in the hitch plate at the critical location. The effects of direction of travel, speed, deck support, and material properties were also studied. Deck support modification appeared to have little effect. From analysis of the dynamic strain and acceleration histories, it is concluded that curve-negotiation forces excite lateral dynamic resonances ("" 7 Hz) of the TOFC combination, causing high strain cycles in the hitch for normal (45 mph) operation. Slower operation (25 mph) over jointed tangent track can i!lso excite another resonance (""1Hz), which contributes to fatigue damage of the hitch.