Analysis of crack interactions at adjacent holes and onset of multi-site fatigue damage in aging airframes

Abstract

Focusing on the geometry of one hot spot in airframes, this paper discusses the onset of the interaction of two collinear cracks at adjacent holes and defines the onset as a criterion for multi-site fatigue damage failure. The finite element method is used to calculate the stress intensity factors at the tips of two collinear cracks at adjacent holes growing towards each other. The stress intensity factor is found to increase rapidly at the onset of interaction. Since a rapid increase in stress intensity factor results in a rapid and unstable growth of the crack, the onset of the interaction is proposed as the point where the multi-site fatigue damage starts. A criterion to avoid multi-site fatigue damage locally is then established based on the separation distance of two crack tips at the onset of the interaction. To speed up the simulation of crack growth under multi-site fatigue damage with the finite element method, a semi-empirical criterion is derived to determine the time at which the stress intensity factors at the tips of the cracks correlate. The numerical examples show that the proposed criterion saves simulation time while incurring negligible relative error in the computation of the final crack length.

The Application of DIC Technique to Evaluate Residual Tensile Strength of Aluminum Alloy Plates with Multi-Site Damage of Collinear and Non-Collinear Cracks

“…As for the conditions of 21 and 29 mm, there were no actual differences between the average RTS of CC and NCC based on the statistical analysis. The average ultimate tensile strength σ of the specimen without MSD was 456.0 MPa, the average RTS of MSD specimens σ' compared with that without MSD decreased 46.4% (CC21), 43.3% (CC25), 42.0% (CC29), 45.9% (NCC21), 42.1% (NCC25), and 41.0% (NCC29) respectively which were calculated by Equation (5). Moreover, the RTS decrease percentage (pct) of CC21 was larger than that of NCC21, similar decreasing trends were observed in other CC and NCC, which contain the following relationship: pctCC21 > pctNCC21 > pctCC25 > pctNCC25 > pctCC29 > pctNCC29.…”

“…Multi-site damage (MSD) is a special kind of failure that is commonly found in aging aircraft structures, it leads people to realize what is called the aging aircraft problem, which has developed into the main concern for research on airframe strength [4]. Experiments revealed that the design of the fuselage may affect the ultimate failure type of longitudinal lap joints in the case of MSD [5][6][7][8]. The effects of overloading, under-loading and bending on MSD were examined.…”

Experiments and model predictions for fatigue crack propagation in riveted lap‐joints with multiple site damage

“…Among them, Chang and Kotousov recently used the classical strip yield model and plasticity induced crack closure concept to develop and experimentally validate a new computational technique for the evaluation of interactions between through cracks. Wang, Modarres and Hoffman used the Finite Element method to evaluate K at the tips of two collinear through cracks at adjacent holes and proposed a criterion to determine when crack growth is influenced by mutual interactions.…”

Application of artificial neural network for predicting crack growth direction in multiple cracks geometry

“…The trend of operating aircraft up to or even exceeding their design lives has raised concerns on the airframe structural integrity due to fatigue failures, which often involve multiple cracks [31][32][33][34]. Experiments and numerical studies conducted by Iida [35], Iida and Kawahara [36], Wang et al [37], Frise and Bell [38], Leek and Howard [39], and Kamaya [40], which were mainly on long crack coalescence showed that the effects of interaction amongst closely spaced cracks can significantly influence the rate of coalescence, and hence, accelerating the crack growth rates. This will in turn potentially affect the overall fatigue life of engineering structures.…”