Strength Testing and Analysis of Fatigue Crack Growth in Selected Aircraft Materials

The study has been intended to determine the most essential mechanical and fatigue properties as well as impact strength of the 30HGSNA steel, to gain own data on the above-mentioned characteristics of materials to be used further on in numerical analyses of life estimates of aeronautical structural components. The scope of the study comprised the following assignments: determination of the most fundamental mechanical properties and impact strength of materials, low-cycle fatigue testing and evaluation of the Manson-Coffin curves, high-cycle fatigue testing and evaluation of the Wohler curves, investigation into fatigue crack growth rates at constant and variable load-cycle amplitudes (determination of curves da/dN = f(ΔK, R), coefficients in Paris and NASGRO equations, coefficients in the Wheeler models of delay, the value of Kth(R)), crack toughness testing under the plane-state-of-strain conditions at room temperature (determination of the KIc(R)). The strength/fatigue testing was carried out in the Laboratory for Materials Strength Testing of the AFIT’s Division for Aeronautical Systems Reliability and Safety, the lab being accredited by the Polish Centre for Accreditation (Accreditation Certificate No.: AB 430). Preparing for the testing work The 30HGSNA is a constructional alloy steel intended for the manufacturing of structural components exposed to exceptionally high loads. The steel purchased was in the form of a bar of the diameter ∅ = 60 mm. Table 1 shows properties of the material of the bar manufactured at the Batory Steelworks according to the Engineering Acceptance Report No. 212/P/89, and specifications for the steel following the Polish Standard PN-72/H-84035. Unauthenticated Download Date | 5/22/15 10:08 AM Strength testing and analysis of fatigue crack growth in selected aircraft materials 75 Tab. 1. Properties of the material of the bar according to the Engineering Acceptance Report, and specifications following the Polish Standard PN-72/H-84035 Chemical composition [%] C Mn Si Pmax Smax Cr Ni 0.28÷0.30 1.16 1.08 0.024 0.011 0.93 1.58 PN-72/H-84035 0.27÷0.34 1.00÷1.30 0.90÷1.20 0.030 0.025 0.90÷1.20 1.40÷1.80 Mechanical properties (after thermal treatment: quench hardening 900 C 20’ oil, tempering 250 C 3 h air) Rm [MPa] Re [MPa] A [%] Z [%] KCU2 [J/cm] HB 1725÷1775 1505÷1535 13÷14 45÷47 90÷95 2.8 PN-72/H-84035 min. 1620 min. 1370 min. 9 min. 45 min. 60 The following specimens were prepared for the tests: cylindrical ones for strength and static tensile testing, sand-clock ones for low-cycle fatigue (LCF) testing, cylindrical ones for high-cycle fatigue (HCF) testing, round compact tension (RCT) ones for testing both fatigue-crack propagation rate and crack toughness. Shapes and dimensions of the specimens have been shown in Figs 1 ÷ 4. The quench hardening was carried out in oil, starting from 900 C; the tempering was carried out at temperature ranging from 240 to 250 C for three hours. After that, the specimens were cooled in steady air. What followed the thermal treatment was grinding of the specimens. Fig. 1. Shape and dimensions of an exemplary specimen for strength testing Fig. 2. Shapes and dimensions of specimens to examine fatigue-crack propagation rates and crack toughness Unauthenticated Download Date | 5/22/15 10:08 AM Sylwester Klysz, Janusz Lisiecki 76 Fig. 3. Shapes and dimensions of specimens for low-cycle fatigue (LCF) testing Fig. 4. Shapes and dimensions of specimens for high-cycle fatigue (HCF) testing Findings on mechanical properties of the 30HGSNA steel Nr d0 L0 R0,05 R0,2 Rm R m rz A n K E probki [mm] [mm] [MPa] [MPa] [MPa] [MPa] [%] [ ] [MPa] [MPa] 2/05/1 6.95 50 420 450 795 910 0.19