Al.Th. Kermanidis

Strain energy density prediction of fatigue crack growth from hole of aging aircraft structures

Fatigue crack growth rate depends not only on the load amplitude, but also on the morphology of crack path. The strain energy density theory has the ability to analyze crack growth rate. A strain energy density crack growth model is proposed. It can predict the lifetime of fatigue crack growth for mixed mode cracks while an equation for mode I crack is also obtained. The validity of the model is established with two cases: a center-crack panel and cracks emanating from the edge of a hole. The stress intensity factor expression for the former case is analytical while that of the latter is calculated numerically using finite elements. The results are compared with the testing data. Good agreement shows that the proposed model is useful.

Crack-growth analysis code for assessing fatigue life of 2219 T851 aluminium specimens under aircraft structure service spectra

Abstract A new crack growth analysis code for assessing crack growth curve and fatigue life of specimens subjected to aircraft structure service spectra is proposed. It makes use of the strip plastic zone approximation to include material hardening effect by means of yield stress variations along the path of prospective crack growth. Calculation of degree of plastification has been made manageable by simulating aircraft service spectra by equivalent sequences of full distinguished stress cycles. For the spectrum simulation a modified rainflow counting method is introduced to account also for non-linearity in fatigue damage accumulation and load sequence effects. Computed results are correlated to test data obtained from 2219 T851 aluminium specimens and to analytical results obtained by applying six different wide use fatigue crack growth codes.

CORROSION AND HYDROGEN EMBRITTLEMENT OF THE 2024 AIRCRAFT ALUMINIUM ALLOY

The present work aims to provide a better understanding of the effects of corrosion and the associated corrosion-induced hydrogen embrittlement on the mechanical behaviour of the 2024 aircraft aluminium alloy. Evidence is provided of the corrosion-induced hydrogen embrittlement of the material and the tensile, fatigue and damage tolerance behaviour of precorroded specimens are discussed under the viewpoint of a synergistic effect of corrosion and corrosion-induced hydrogen embrittlement. The results presented rely on an extensive experimental investigation involving mechanical testing, metallographic and fractographic analyses as well as measurements of the hydrogen uptake during the corrosion process.

Effect of corrosion on the mechanical behaviour of aircraft aluminum alloys

Abstract: A brief overview of aircraft aluminum alloys, along with a discussion of their susceptibility to corrosion and the various types of corrosion damage is provided. The significance of the effect of corrosion on mechanical behaviour under static and fatigue loading conditions is demonstrated. For this purpose, experimental results concerning the tensile and fatigue behaviour of pre-corroded aluminum specimens are presented. The results, which are supported by metallographic observations, are discussed in terms of the synergetic effect of corrosion damage and corrosion-induced hydrogen embrittlement of the material. The fatigue crack growth and fracture behaviour of pre-corroded aluminum alloys is also examined. Experimental results demonstrate the essential influence of prior corrosion exposure on the material’s damage tolerance performance. Corrosion, being a time-dependent and diffusion-controlled process degrades the material properties in a local scale. To describe the fracture behaviour of pre-corroded aluminum alloys, the concept of local fracture toughness is introduced. A mechanical model for assessing the local fracture toughness is presented and incorporated into a fatigue crack growth code for fatigue life assessment of pre-corroded material under irregular loading.