Materials Performance

AUG 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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55 MATERIALS PERFORMANCE: VOL. 57, NO. 8 AUGUST 2018 more than 80% of the fatigue damage to most civil aviation and navigation aircraft. From the 1970s to the present, people have been adopting a flight-by-flight load spec- trum, which is a standardized load sequence for flight simulation tests on transport air- craft wing structures. The TWIST standard load spectrum provides a load of 4,000 flight cycles (100 circles/cycle), the types of flight mission, and the definition of f light load level . During the f light, the mission is divided into 10 categories, and the load for the various missions is divided into 10 levels. The order of loads at each level is ran- domly generated. The statistical results obtained are shown in Table 1. According to the data in the TWIST standard spectrum of civil transport, and in consideration of the actual aircraft condi- tion, the f light type with a discrete load level (LD) of 1.3 to 1.6 mainly represents the phase when an aircraft is taking off and landing, w hich is accompanied by the instant destruction of material. It does not match with the principle of fatigue failure. In this context, this study represents the gust load with a load level lower than 1.15. By observing the simplified gust load spec- trum, the following results were found: the cycle f luctuation is small when the load level is 0.222, and the cycle f luctuation is relatively small and has little ef fect on fatigue life when the load level is 0.375. Thus, it allows us to carry out a short-cycle- equivalent simplification to obtain the sim- plified load spectrum, S1, which only con- tains load levels of 0.530, 0.685, 0.840, 0.995 and 1.15. Fatigue Life Under the Action of Corrosion/Fatigue Alternation The airborne fatigue plus the ground corrosion alternation process, which is simulated by the fatigue/corrosion alterna- tion cycles in this experiment, is also the actual circumstances faced by aircraft structures in service. The corrosion/fatigue alternation process in this study was com- pleted by multiple rounds of alternation. To study the effect of alternating corrosion and fatigue on the fatigue life of specimens under different corrosion exposure times, six exposure time frames were set up in this study : TABLE 1. OCCURRENCE NUMBER AT DIFFERENT LOAD LEVELS AND FLIGHT TYPES IN TWIST Flight Type (FT) LD (A) 1.6 1.5 1.3 1.15 0.995 0.84 0.685 0.53 0.375 0.222 FT A 1 1 1 4 8 18 64 112 391 900 FT B 1 1 2 5 1 39 76 366 899 FT C 1 1 2 7 22 61 277 879 FT D 1 1 2 14 44 208 680 FT E 1 1 6 24 165 603 FT F 1 3 19 115 512 FT G 1 7 70 412 FT H 1 16 233 FT I 1 69 FT J 25 Total cycle number during 4,000 flights F i 1 2 5 18 52 152 800 4170 34,800 358,665 (A) Discrete load level is referred to as load level, and usually simplified as LD. TABLE 2. CORROSION TIMES AND AVERAGE FATIGUE LIFE FOR EACH SAMPLE GROUP Group Corrosion Time/Loading Cycles for Each Loop (Hours/Cycle) Corrosion Exposure (Times) Fracture Location Fatigue Life (Cycles) Sample 1 Fatigue Life (Cycles) Sample 2 Fatigue Life (Cycles) Sample 3 Fatigue Life (Cycles) Average 1 4/51,920 5 Parallel 249,055 231,586 267,007 249,216 2 8/51,920 4 Parallel 187,145 221,583 198,568 202,432 3 12/51,920 3 Parallel 115,578 135,742 125,276 125,532 4 16/51,920 2 Parallel 101,103 113,287 91,328 101,906 5 20/51,920 2 Parallel 95,134 89,649 80,015 88,266 6 24/51,920 2 Parallel 54,705 62,198 70,012 62,305

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