United States Department of Transportation
i
Fatigue performance of brass breakaway light pole couplings.
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2013-04-01
[PDF-2.38 MB]
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Abstract:Breakaway couplings connect light pole bases to foundation anchor bolts and are intended to fracture on impact after vehicle collision to
protect drivers and passengers from severe injury. The coupling consists of an internally threaded hollow hexagon made of half‐hard CDA
360 brass with a thread‐locked stainless steel stud. Several designs of the notched hexagonal shaped coupling were tested in bending by
attaching them to a simulated pole tube and base and then inducing stresses by deflection of the tube by a hydraulic jack. Individual
couplings were also fatigued in axial tension‐compression by a servo‐controlled universal testing machine. Using the data obtained, the
axial tension‐compression fatigue relationship for the brass couplings was ΔS = 433 N –0.153, where ΔS is the alternating stress from
tension to compression. Finite element analysis using ABAQUS® found that stress concentrations in the external notch radius and the
internal threads were accurate to about 20% of the values obtained from experimental strain gage data.
The stress concentration factors for the final design were determined to be Ktt = 10.8 in tension and Ktc = 10.4 in compression. Couplings
were fatigue tested in laboratory air at room temperatures in the range of 104 to 107 cycles. Couplings that were exposed to 1000 hr of
salt fog prior to fatigue testing showed no difference in fatigue life compared to couplings fatigued in air.
Standard structural theory was used to determine the nominal stresses of couplings attached to steel poles subjected to winds of 90 mph.
Using an average stress concentration factor Ktavg of 10.6, the peak coupling stress in various orientations of the pole base with respect to
the wind direction was determined to be 24.1 ksi. When a more accurate segmental wind pressure method of determining moments
acting on the pole base was used, the calculated peak stress was 23.9 ksi. At 90 mph, a +24 ksi to –24 ksi alternating stress develops,
resulting in a 50% mean fatigue life estimated at 1,799,000 cycles.
Variances in notch section were caused by straightness deviations of hexagon stock and from drilling and tapping operations, averaging
4.2% to a maximum of 9%. The fatigue initiation zone was about 30⁰ of the notch circumference. The probability that four couplings
would have only the weakest one facing the maximum wind exposure is 30⁰/360⁰/4, which is about 2% of the time. Maximum wind speeds
recorded in Illinois are less than the 90 mph AASHTO requirement. Decreasing notch depth by 9% would lessen the likelihood of
premature failure due to insufficient load‐bearing section of the coupling notch.
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