Buckling/post-buckling strength of friction stir welded aircraft stiffened panels

Assembling aircraft stiffened panels using friction stir welding offers potential to reduce fabrication time in comparison to current mechanical fastener assembly, making it economically feasible to select structurally desirable stiffener pitching and novel panel configurations. With such a departure from the traditional fabrication process, much research has been conducted on producing strong reliable welds, with less examination of the impact of welding process residual effects on panel structural behaviour and the development of appropriate design methods. This article significantly expands the available panel level compressive strength knowledge, demonstrating the strength potential of a welded aircraft panel with multiple lateral and longitudinal stiffener bays. An accompanying computational study has determined the most significant process residual effects that influence panel strength and the potential extent of panel degradation. The experimental results have also been used to validate a previously published design method, suggesting accurate predictions can be made if the conventional aerospace design methods are modified to acknowledge the welding altered panel properties.

Autoclave Cure Simulation of Composite Structures Applying Implicit and Explicit FE

Simulation of the autoclave manufacturing technique of composites can yield a preliminary estimation of induced residual thermal stresses and deformations that affect component fatigue life, and required tolerances for assembly. In this paper, an approach is proposed to simulate the autoclave manufacturing technique for unidirectional composites. The proposed approach consists of three modules. The first module is a Thermo-chemical model to estimate the temperature and the degree of cure distributions in the composite part during the cure cycle. The second and third modules are a sequential stress analysis using FE-Implicit and FE-Explicit respectively. User-material subroutine is used to model the Viscoelastic properties of the material based on theory of micromechanics.

Virtual Manufacturing of Composite Structures Applying Implicit and Explicit FE Techniques

Virtual manufacturing of composites can yield an initial early estimation of the induced residual thermal stresses that affect component fatigue life, and deformations that affect required tolerances for assembly. Based on these estimation, the designer can make early decisions, which can help in reducing cost, regarding changes in part design or material properties. In this paper, an approach is proposed to simulate the autoclave manufacturing technique for unidirectional composites. The proposed approach consists of three modules. The first module is a Thermochemical model to estimate temperature and the degree of cure distributions in the composite part during the cure cycle. The second and third modules are stress analysis using FE-Implicit and FE-Explicit respectively. User-material subroutine will be used to model the Viscoelastic properties of the material based on micromechanical theory. Estimated deformation of the composite part can be corrected during the autoclave process by modifying the process-tool design. The deformed composite surface is sent to CATIA for design modification of the process-tool.

The Behavior of compressively loaded stiffener runout specimens - Part II: Finite element analysis

The termination of stiffeners in composite aircraft structures give rise to regions of high interlaminar shear and peel stresses as the load in the stiffener is diffused into the skin. This is of particular concern in co-cured composite stiffened structures where there is a relatively low resistance to through-thickness stress components at the skin-stiffener interface. In Part I, experimental results of tested specimens highlighted the influence of local design parameters on their structural response. Indeed some of the observed behavior was unexpected. There is a need to be able to analyse a range of changes in geometry rapidly to allow the analysis to form an integral part of the structural design process.

This work presents the development of a finite element methodology for modelling the failure process of these critical regions. An efficient thick shell element formulation is presented and this element is used in conjuction with the Virtual Crack Closure Technique (VCCT) to predict the crack growth characteristics of the modelled specimens. Three specimens were modelled and the qualitative aspects of crack growth were captured successfully. The shortcomings in the quantitative correlation between the predicted and observed failure loads are discussed. There was evidence to suggest that high through-thickness compressive stresses enhanced the fracture toughness in these critical regions.

Arching or Compressive Membrane Action in Reinforced Concrete Slabs

Arching or compressive membrane action (CMA) in reinforced concrete slabs occurs as a result of the great difference between the tensile and compressive strength of concrete. Cracking of the concrete causes a migration of the neutral axis which is accompanied by in-plane expansion of the slab at its boundaries. If this natural tendency to expand is restrained, the development of arching action enhances the strength of the slab. The term arching action is normally used to describe the arching phenomenon in one-way spanning slabs and compressive membrane action is normally used to describe the arching phenomenon in two-
way spanning slabs. This encyclopedic article presents the background to the discovery of the phenomenon of arching action and presents a factual history of the approaches to the treatment of arching action in the United Kingdom and North American bridge deck design codes. The article summarises the theoretical methodology used in the United Kingdom Design Manual for Roads and Bridges, BD81/02, which was based on the work by Kirkpatrick, Rankin & Long at Queen's University Belfast.