Laser shock peening treatment to control and moderate fatigue crack growth in aircraft structure based on residual stress engineering approach

Laser Shock Peening (LSP) is a surface enhancement treatment which induces a significant layer of beneficial compressive residual stresses of up to several mm underneath the surface of metal components in order to improve the detrimental effects of the crack growth behavior rate in it. The aim of this thesis is to predict the crack growth behavior in metallic specimens with one or more stripes which define the compressive residual stress area induced by the Laser Shock Peening treatment. The process was applied as crack retardation stripes perpendicular to the crack propagation direction with the object of slowing down the crack when approaching the peened stripes. The finite element method has been applied to simulate the redistribution of stresses in a cracked model when it is subjected to a tension load and to a compressive residual stress field, and to evaluate the Stress Intensity Factor (SIF) in this condition. Finally, the Afgrow software is used to predict the crack growth behavior of the component following the Laser Shock Peening treatment and to detect the improvement in the fatigue life comparing it to the baseline specimen. An educational internship at the “Research & Technologies Germany – Hamburg” department of AIRBUS helped to achieve knowledge and experience to write this thesis. The main tasks of the thesis are the following: •To up to date Literature Survey related to “Laser Shock Peening in Metallic Structures” •To validate the FE model developed against experimental measurements at coupon level •To develop design of crack growth slowdown in Centered Cracked Tension specimens based on residual stress engineering approach using laser peened strip transversal to the crack path •To evaluate the Stress Intensity Factor values for Centered Cracked Tension specimens after the Laser Shock Peening treatment via Finite Element Analysis •To predict the crack growth behavior in Centered Cracked Tension specimens using as input the SIF values evaluated with the FE simulations •To validate the results by means of experimental tests

Gene expression responses to acute and chronic heat stress in the common reef-building coral Pocillopora verrucosa

Global climate change is impacting coral reefs worldwide, with approximately 19% of reefs being permanently degraded, 15% showing symptoms of imminent collapse, and 20% at risk of becoming critically affected in the next few decades. This alarming level of reef degradation is mainly due to an increase in frequency and intensity of natural and anthropogenic disturbances. Recent evidence has called into question whether corals have the capacity to acclimatize or adapt to climate changes and some groups of corals showed inherent physiological tolerance to environmental stressors. The aim of the present study was to evaluate mRNA expression patterns underlying differences in thermal tolerance in specimen of the common reef-building coral Pocillopora verrucosa collected at different locations in Bangka Island waters (North Sulawesi, Indonesia). Part of the experimental work was carried out at the CoralEye Reef Research Outpost (Bangka Island). This includes sampling of corals at selected sites and at different depths (3 and 12 m) as well as their experimental exposure to an increased water temperature under controlled conditions for 3 and 7 days. Levels of mRNAs encoding ATP synthase (ATPs) NADH dehydrogenase (NDH) and a 70kDa Heat Shock Protein (HSP70) were evaluated by quantitative real time PCR. Transcriptional profiles evaluated under field conditions suggested an adaptation to peculiar local environmental conditions in corals collected at different sites and at the low depth. Nevertheless, high–depth collected corals showed a less pronounced site-to-site separation suggesting more homogenous environmental conditions. Exposure to an elevated temperature under controlled conditions pointed out that corals adapted to the high depth are more sensitive to the effects of thermal stress, so that reacted to thermal challenge by significantly over-expressing the selected gene products. Being continuously exposed to fluctuating environmental conditions, low-depth adapted corals are more resilient to the stress stimulus, and indeed showed unaffected or down-regulated mRNA expression profiles. Overall these results highlight that transcriptional profiles of selected genes involved in cellular stress response are modulated by natural seasonal temperature changes in P. verrucosa. Moreover, specimens living in more variable habitats (low-depth) exhibit higher basal HSP70 mRNA levels, possibly enhancing physiological tolerance to environmental stressors.