FATIGUE TEST WITH RANDOM AND PROGRAMMED LOAD SEQUENCES, WITH AND WITHOUT GROUND-TO-AIR CYCLES, A COMPARATIVE STUDY ON FULL-SCALE WING CENTER SECTIONS.
Abstract
Fatigue test were carried out on 13 full-scale tension skins of a wing center section (material 7075-T6). Variable-amplitude tests included two tests for each of the following load sequences: (1) random load, (2) programmed load, (3) random load with GTAC (ground-to-air-cycles) and (4) programmed load with GTAC. Constant-amplitude tests included two tests with GTAC, and three tests with different gust amplitudes for establishing a part of the S-N curve. The main objectives were to investigate (1) the equivalence of random and programmed loading, (2) the damaging effect of the GTAC and (3) recommendations on planning load sequences for full-scale tests. The main results are summarized below. Cracks were found at the same locations in random and in programmed tests, both with and without GTAC. The fatigue lives under program loading were some 10 - 30% larger than under random loading. The GTAC reduced the fatigue lives by about 50%. Average values of epsilon n/N varied from 1.4 to 2.5. The GTAC caused only a small reduction in epsilon n/N. The crack propagation mainly occurred at the higher load levels. The crack rate was not very much different for random and programmed loading. Small cracks from 1 to 2 inches led to complete failure of the skin at a stress level of about 35,000 psi. At larger cracks unstable crack extension occurred at lower stresses, but the specimens did not rupture. Open holes and free sheet edges were locations prone to fatigue crack nucleation. For similar notches in one specimen, an average standard deviation of log life was 0.085. (Author)
Document Details
- Document Type
- Technical Report
- Publication Date
- Oct 01, 1966
- Accession Number
- AD0810654
Entities
People
- A. Nederveen
- D. Broek
- J. Schijve
- P. De Rijk
- P. J. Sevenhuysen