Effects of several TIG weld repairs on the axial fatigue strength of AISI 4130 aeronautical steel‐welded joints

Abstract

A B S T R A C T Welded joints of airframes critical to the flight-safety are commonly repair welded during its operational live. In this study, the effect of up to three weld repairs by gas tungsten arc welding (GTAW) on the axial fatigue strength of AISI4130 steel used in an airframe critical to the flight-safety was investigated. The tests were performed on hot-rolled steel plate specimens, 0.89 mm thick, with load ratio R = 0.1, constant amplitude, at 20 Hz frequency and room temperature. The results obtained indicated that the axial fatigue strength decreased with the GTAW process itself, and with the subsequent repair cycles, as a consequence of microstructural and microhardness changes and of weld profile geometry factors, which induced high stress concentration at the weld toe.Keywords GTAW; heat-affected zone; high strength low alloy steel; microstructure; repair welding; weld metal; weld toe notch. N O M E N C L A T U R Ea = Peterson's parameter for steel BM = Base metal CE = Carbon equivalent CGHAZ = Coarse-grain heat affected zone da/dN = fatigue crack propagation rate (mm/cycle) DC = Direct Current FGHAZ = Fine-grain heat affected zone GTAW = Gas Tungsten Arc Welding HAZ = Heat affected zone HFC = High fatigue cycles ICHAZ = Inter-critical heat affected zone IIS/IIW = Institut International de la Soudure/International Institute of Welding Kf = fatigue-notch factor Kt = Theoretical stress concentration factor LFC = Low fatigue cycles NDT = Non-destructive testing q = notch-sensitivity factor r = notch-root R = Stress or load ratio R a = Roughness average (μm) S = Stress (MPa)Correspondence: M. P. Nascimento.

Experimental study on mechanical properties and fracture toughness of structural thick plate and its butt weld along thickness and at low temperatures

“…The nonmetallic inclusions inside the steel material are pressed into flakes after rolling, which causes the thick plates become layered and the mechanical properties through-thickness becomes worse. 5,6 Additionally, the huge amount of the welding heat input will induce the steel material adjacent welds become worse and the toughness decrease, 7 and the crack is liable to occur during the cooling process after welding. 2 Furthermore, as the plate thickness increases, the stress along depth in the defect increases, and the defect will be subjected to triaxial tension.…”

Stress intensity factors for three‐dimensional weld toe cracks using weld toe magnification factors

“…Although the Mk factors at the deepest points from Lie et al underestimate the Mk-a/t curves, both values have much in common from the aspect of the tendency and behaviour of the Mk-a/t curve. Judging by these similarities, the Mk factor solutions for the three welded joints permit its use in calculating the stress intensity factors for multiple surface cracks as shown in Eqs (1) and (2).…”

“…The mechanisms associated with the propagation of these cracks involve the mutual interactions and coalescence of two adjacent cracks. When the cracks extend through the thickness of their members, the structural strength of the components subsequently decreases rapidly . An assessment of the fracture mechanics of these weld toe cracks requires accurate the stress intensity factor solutions.…”

“…When the cracks extend through the thickness of their members, the structural strength of the components subsequently decreases rapidly. [1][2][3][4] An assessment of the fracture mechanics of these weld toe cracks requires accurate the stress intensity factor solutions. Particularly in welded components with complex geometrical shapes, evaluating stress intensity factors is a difficult task.…”

Fatigue strength of tungsten inert gas-repaired weld joints in airplane critical structures

“…For example, an investigation on 157 engine-cradle failure reports of Brazilian T-25 Universal aircrafts indicated that all failures occurred at welded joints as a result of fatigue cracks. As a consequence of these successive repair welding operations, the interval between inspections ("Time-Before-Fail") was reduced from 4000 h to 50 h of flight (Nascimento, 2004). Motivated by the high failure incidence on this component, an extensive research program to evaluate the effects of the repair welding on its structural integrity, mechanical properties and microstructure has been developed.…”