A Distortion Modelling Method for Aerospace Wing Structures

Distortion in large monolithic aircraft wing rib components is a recurring issue for aerospace manufacturers globally. Excessive distortion in these components has resulted in high levels of concession, rework and scrap due to failing geometric tolerance limits on inspection. Vast business capital is lost through scrap or corrective treatments to bring distorted components back in line with the design intent. Distortion in machined components is caused by the redistribution of bulk residual stresses after material removal and the introduction of machining-induced residual stresses due to the high thermal and mechanical loads imparted by the cutting action. Modelling of distortion due to residual stresses can be achieved using numerical tools that account for these sources of residual stress.

A detailed literature review has been conducted to understand the sources of residual stresses and their influence on distortion. Additionally, the review focused on the current state-of-the-art modelling of residual stress-related machining distortion. From the review of previous work, a series of objectives were conceived to address the gaps in knowledge on the impact of machining strategy on residual stress formation and part distortion and to develop a modelling technique capable of simulating both residual stress-related distortion and distortion-related part quality issues.

The developed modelling concept was designed as a multi-step simulation process with machining-induced stress defined as a function of the tool path strategy. The simulation concept can account for the bulk and machining-induced stress influence on inter-process and post-process distortion. Experimental trials were conceived and conducted to explore the influence of machining sequencing on machining-induced residual stress formation, where no influence on the final machining-induced stress was found under the developed test regime. Furthermore, machining trials were conducted to understand tool path strategy selection and the influence of inter-process machining conditions on machining-induced stress formation. It was determined that tool path strategy significantly influences the machining-induced stress state in the component, and machining-induced residual stresses vary according to local cutting condition variations, although only in very localised regions for the trialled cutter path. The developed modelling method has been validated against other methods from literature and against experimental trials where the machining of a representative component has been conducted and the distortion measurements captured for comparison to the numerical results.