Graham Clark

Laser shock processing and its effects on microstructure and properties of metal alloys: a review

The current status of research and development on laser shock processing of metals, also known as laser peening, using Q-switched high power lasers is reviewed. The influence of processing parameters on the laser-induced shock waves in metal components are discussed and analyzed. Special attention is paid to the residual stresses and improved fatigue performance from laser peening, which are compared with conventional shot peening results. Modification of microstructure, surface morphology, hardness, and strength by laser peening is also discussed. Finally, applications of laser peening are addressed. Results to date indicate that laser peening has great potential as a means of improving the mechanical performance of components.

Impact of Mechanical Strain Environment on Aircraft Protective Coatings and Corrosion Protection

Aircraft paint schemes (paint and sealant) are a key element of corrosion preventive measures, and ensuring that such schemes and other coatings are effective and durable under service conditions is essential if corrosion costs and maintenance costs are to be minimized. Coatings can degrade under the influence of environmental factors such as exposure to moisture, high temperature, ultraviolet radiation, and so forth. Coating failure is often particularly evident at joints, and while failure may be accelerated in some regions by local variations in coating thickness and geometry at features such as edges, or by erosion, it is likely that the displacements which occur at these locations under service loads will be a contributing factor. The impact of in-service mechanical loading on coating degradation has so far received little attention, despite clear evidence that coatings tend to fail first at specific sites such as sheet ends and around fastener heads. This paper argues that the magnitude of the applied service loads and the nature of the load history should be considered in predicting and assessing rates of coating degradation, and that development of a thermomechanical history is a more appropriate approach. The likely impact of joint displacements on the protection of aging aircraft is also discussed.

Understanding the Directional Dependence of Intergranular Corrosion in Aluminium Alloys

Intergranular corrosion can lead to significant problems such as sub-critical crack growth or loss in section strength, potentially leading to failure, as well as a substantially increased maintenance burden. This type of corrosion is found in most types of aluminium alloys, but is a particularly significant problem in aerospace aluminium alloys. The form of intergranular corrosion can vary widely, and may depend on alloy composition, product form, environmental conditions and the presence or otherwise of local or global stresses. One notable example is the occurrence of intergranular corrosion due to atmospheric corrosion, in which salts and deposits deliquesce on the surface forming discrete corrosion cells. Intergranular corrosion of aluminium alloys is usually most rapid in the rolling or extrusion direction of wrought alloy. The reasons for this are not fully understood, and may include texture effects that produce highly susceptible grain boundaries, the inhomogeneous distribution of noble constituent particles, and stresses acting at a microscale. This paper will review and discuss the evidence for and against for the different effects mentioned.

A Model for Predicting the Stress Concentration of Intergranular Corrosion around a Fastener Hole

This paper presents a model that predicts the stress field around intergranular corrosion. The stress analysis is conducted in ABAQUS via a Python input script, which is written in Igor Pro. The intergranular corrosion path is described using a Monte-Carlo Markov Chain based on the materials grain size distribution and probability that the corrosion will turn at a grain boundary junction. The model allows a complete analysis of the stresses resulting from intergranular corrosion around a fastener hole of any size. As fatigue initiation is most likely to occur at the highest stress concentration, this model gives an understanding of which of the features of intergranular corrosion are most critical and can allow for the development of beta solutions for crack growth. This model has been applied to 7075-T651 extruded aluminum alloy from a legacy era aircraft but can be readily applied to any material where the microstructure is known and can be described using a statistical distribution.

Influence of Mechanical Loading on Failure of Aircraft Protective Coatings

The degradation and failure of protective coatings (paints and sealants) is a key element influencing the service life of aircraft. Such degradation is influenced by the response of coatings to environmental factors such as high temperatures and exposure to ultraviolet radiation, as well as chemical factors. However, the effect of loading and load history on coating durability has received little attention, despite clearly being a factor in determining failure sites (such as joints) and the rate of degradation. This paper describes the key characteristics of coatings at aircraft joints, and the nature of the strains experienced by coatings in locations influenced by in-service loads. It is first step in assessing the complex strain history at joint strain concentration locations as part of developing a prognostic capability for the service life of aircraft coatings. The configuration of coating layers at different joints is important and this research has considered a simplification of a butt strap joint from a RAAF military aircraft and a generic lap joint; predictions of critical movements/displacements have been made using finite element analysis; the predictions will be tested later as part of an experimental program associated with a full-scale fatigue test.

Safety Aspects of Automotive Child Restraint Systems in Aircraft

The development of a numerical dynamic model for investigating the crashworthiness of automotive child restraints is presented. Civil air regulations in many countries around the world currently allow for infants to be held on the lap of an adult passenger during flight. Research has found that infants carried in this manner are at a significantly increased risk of serious injury or death in the event of an otherwise survivable crash. One solution to this issue is the use of automotive child restraints. These restraints have been shown to provide the infant with a level of safety equivalent to that of an adult passenger. Difficulties in installing the automotive restraints has prevented their widespread use in aircraft, however research has found that a new child restraint attachment method known as ISOFIX has the potential to overcome these difficulties. However, a passenger seated directly aft of an ISOFIX child restraint is potentially at increased risk of head or neck injury. Considerations regarding modeling method, model validation and foam modeling are discussed.

Fatigue behaviour of unitized structures compared to built-up structures

In the last couple of decades, the aerospace industry has been attracted by unitized structures for their potential of economic advantage in terms of low production cost and fuel efficiency from weight reduction due to a reduced number of parts and joints. In recent years, advanced manufacturing technologies, such as additive manufacturing processes and electron beam freeform fabrication, have boosted the feasibility of unitized structure manufacturing. As a result, unitized structures are increasingly becoming common in airframes as they displace conventional ubiquitous built-up structures. One concern with unitized structures is that they may not show the same resistance to fatigue as built-up structures in terms of crack growth rates. For a fatigue crack to cause failure of a built-up structure, it faces the problem of having to jump across the gaps between the different parts of the structure. Those gaps represent natural barriers that slow down crack-growth. However, a unitized structure will likely to have less stress raisers (e.g., fewer holes for fasteners). Hence, experimental as well as numerical analysis approaches are required to determine how a unitized structure compares to an equivalent built-up structure in terms of fatigue crack growth. This paper addresses finite element analysis of modelled unitized structure and equivalent built-up structure under amplitude loading for fatigue crack growth behavior.

Fleet Recovery and Life Extension – Some Lessons Learned

Extending the life of an existing fleet which still has acceptable operational capability can be enormously attractive in economic terms. Ideally, such extension programs will be planned and managed (via an ASI program), although many are urgent “recovery” programs required when substantial problems are discovered. This paper discusses examples of planned and unplanned programs, highlighting the differences in approach required.

Aircraft Joints and Corrosion Control

Corrosion damage in aircraft structure, if undetected and/or left untreated, can undermine safety. Currently corrosion prevention and management in many civil and military fleets still relies strongly on the use of traditional ‘find and fix’ maintenance practices, although this has been refined by the increasing use of Corrosion Prevention and Control Plans (CPCP) which provide a framework for targeted inspections and treatment to help with corrosion management. Teardowns of high-life service aircraft and parts can also be valuable tools to help identify corrosion-prone areas and relative severity of the corrosion. This paper describes research which supports the development of improved prognostic capability for corrosion, by investigating one particular factor which appears to play a significant role in the development of corrosion. The focus of this research is to better understand and predict the deterioration and breakdown of protective paint coatings at aircraft joints, primarily due to the influence of mechanical displacement. The impact of in-service mechanical loading on coating degradation has so far received little attention, despite clear evidence that coating tend to fail first at specific site such as sheet ends and fastener heads. Potential service/performance implications of the joint displacement on the protection of ageing aircraft are discussed. More importantly, it is argued that appropriate corrective actions are required immediately after the paint cracking detected, even if active corrosion is not fully evident.

Aircraft Joints: the Interaction between Corrosion Protection and Structural Performance

Aircraft joints feature prominently in aircraft structural degradation. Fatigue cracking and corrosion damage can reduce joint strength and degrade service life. Corrosion management can include use of paints and sealants and, increasingly, the application of Corrosion Inhibiting Compounds (CICs) which retard corrosion, by penetrating into crevices and cracks, and displacing water. A combination of coatings and CIC use can provide effective corrosion protection, but both interact - in different ways - with structural performance and overall system durability. This paper discusses the interaction between these two corrosion protection measures and fatigue performance of joints. The first issue relates to a reduction in the fatigue life of mechanically-fastened joints after application of CICs (or other lubricants) The lubricating properties of the CICs reduce the friction at the faying surface, which may change the load transfer characteristics of the joint. The paper discusses results from a test program assessing the fatigue life and failure mode of riveted lap joints; the results show a marked reduction in fatigue life for joints containing CICs, and the paper discusses the changes which may be responsible for the reduction. The second issue discussed is the degradation of protective coatings in service. Joints are key locations for coating cracking and failure, since areas such as sheet ends and fastener heads, where displacements are concentrated, will produce concentrated strain in coatings. So far, however, the potential influence of aircraft loading on coating degradation prognostics has received little attention. The paper discusses the role of joint displacement in service as a factor contributing to early degradation of aircraft coatings, and argues that this local strain effect, and indeed structural loading history, needs to be considered in predicting and assessing rates of coating degradation. It describes initial analyses of displacements in aircraft joints, to identify the levels of coating strain and the roles and relative contributions of the various deflections in the joints. The results indicate the potential for very large strains in coatings.