Online fatigue crack growth monitoring with clip gauge and direct current potential drop

Fatigue is a well-known failure phenomenon which has been and still is extensively studied. Often structures are designed according to the safe-life principle so no crack initiation occurs. Nowadays there is a high emphasis on cost-efficiency, and one might rather opt for a fail-safe design. Therefore a certain amount of crack growth can be allowed in structures, but then a good knowledge of stresses and related crack growth rates is needed. To this end, extensive studies are done to obtain a material’s Paris law curve. Within the framework of research for offshore wind turbine constructions, tests were done to determine the crack growth rate of a high strength low alloy (HSLA) steel. A dedicated LabVIEW program was developed to be able to determine an entire Paris law curve with a single specimen, by controlling the stress intensity factor range (ΔK). The program is controlled by the readings of a clip gauge, which make it possible to plan the amount of crack growth per ΔK block and thus plan an entire test in advance. The potential drop technique was also applied in order to obtain the Paris law curve. Clip gauge results were compared with direct current potential drop monitoring. This comparison was done by means of an a/W-N diagram and the resulting Paris law curves. The results show a very good correlation between both methods and with the visual confirmation

Microbial community analysis in soil (rhizosphere) and the marine (plastisphere) environment in function of plant health and biofilm formation

In this thesis, high-throughput sequencing methods were used to study microbial communities in two environments: the plant’s rhizosphere and on plastic debris. To prevent plant diseases and maintain a good crop productivity, crop cultivation often relies on high pesticide and fertilizer uses. There is a great interest in reducing this high use, and the effect of application of specific environment-friendly substrate amendments, such as biochar and chitin, to the soil or substrate is studied in this respect. These amendments can have a direct effect on the micro-organisms in the rhizosphere, the narrow zone of soil surrounding the plant root. This shift in rhizosphere microbiome should be directed towards beneficial micro-organisms such as plant-growth promotors and biocontrol agents. Micro-organisms in the ocean are less-extensively studied compared to a soil environment. Nonetheless they can reach high numbers, especially when they can attach to substrates, and perform similar crucial roles as for soil environments. With an expected amount of 8 million tons entering our marine ecosystem yearly, the major contaminator of our oceans and seas is probably plastic. The microbial colonization of plastic debris in the marine environment, also referred to as “the plastisphere”, has been studied since the seventies, but the dynamics of this colonization and impact on plastic degradation, the marine ecosystem and animal and human health is currently poorly understood.In the first part of this thesis, the effect of biochar and chitin on the rhizosphere microbiome in relation to crop growth, disease development or survival of human pathogens is studied. We showed that in nutrient-limiting conditions, biochar was able to change the physicochemical properties of soil and substrates and induced major changes in the bacterial composition of strawberry plants, redirecting the rhizosphere community towards a higher relative abundance of plant-growth promoters and biocontrol agents. No effect of biochar addition to peat was seen on the fungal composition of the strawberry rhizosphere. Both the increase in nutrient stock and the shift in bacterial community composition could be related to an increase in strawberry crop growth, a higher strawberry yield and an increase in resistance towards the fungal pathogen Botrytis cinerea. In addition, this aboveground infection also affected the rhizosphere bacterial community. This research indicates that upon biochar incorporation in peat, plants recruit rhizosphere bacteria that may help them in their defense and plant growth promotion.In contrast, chitin addition to potting soil altered both the bacterial and fungal community composition of the lettuce rhizosphere, redirecting the microbiome towards higher abundances of chitin-degraders and plant growth promotors. These effects were correlated with an increase in lettuce growth and a reduction in the survival of Salmonella enterica on the leaves. Especially the consumption of contaminated leafy vegetables, such as lettuce, can be problematic for human health and reported to be the cause of S. enterica outbreaks. The use of chitin can thus be tested further as an interesting supplementary strategy for sustainable control of this zoonotic pathogen in the food chain. In the second part of this thesis, the major contributors and dynamics of the bacterial and fungal colonization of marine plastic debris located at the seafloor in the Belgian part of the North Sea were studied. We showed that environmental properties, plastic-related properties and biofilm formation stages are probably the most important factors influencing the bacterial colonization on plastic. Dependent on the environment, this bacterial biofilm formation can go through progressive temporal stages, reaching a more or less stable community after a few months. In addition, we studied for the first time the fungal community on plastic debris, which was also quite diverse for different plastics, even when they were located at the same site. This microbial colonization could have major influences on the marine ecosystem. We showed that compared to seawater and sediment, other bacterial groups were attached on plastic, indicating that plastic could serve as a transport vector for micro-organisms to other environments. In contrast, the microbial colonization could also be beneficial in terms of biodegradation. Despite plastic-degradation has not been shown in this study, a Mycobacterium species was identified which is probably able to degrade the pigments of beach-located resin pellets, indicating that biodegradation of plastic-related chemicals is possible. The use of high-throughput sequencing techniques made it possible to give insight in the microbial community composition. Here we focussed mainly on taxonomic identification using amplicon sequencing, but already a first step was taken towards functional annotations using shotgun metagenomics in function of chitinase detection. Future studies should elaborate more on this also including e.g. shotgun metagenomics, metatranscriptomics and stable-isotope probing in order to identify specific biochar-, chitin- and even plastic-metabolizers and related functions

Evaluation of fatigue crack propagation in steel ESET specimens subjected to variable load spectra

This paper reports on an experimental study of fatigue crack growth in steel specimens. First, block loading tests (sequences of low and high stress intensity factor ranges ΔK) are discussed. Limited crack growth retardation occurs at transitions from low to high load ranges; significant retardation or crack arrest are observed in high-low transitions. Next, semi-random load spectra are created, processed using a peak-and-valley analysis and further reduced by removing the load ranges below the stress intensity factor threshold ΔKth. Rainflow counting is performed to obtain load profiles consisting of a sequence of blocks with constant ΔK. For the semi-random and the (reduced) peak-and-valley spectra no significant load interaction is observed. Pronounced crack growth retardation is observed in an ordered spectrum obtained by rainflow counting. The strong reduction in number of cycles of the (reduced) peak-and-valley spectra allows for exploration of accelerated fatigue testing. Experimental results of fatigue crack propagation are compared to results of calculations using a Python based numerical framework

Tapping into the maize root microbiome to identify bacteria that promote growth under chilling conditions

Background When maize (Zea mays L.) is grown in the Northern hemisphere, its development is heavily arrested by chilling temperatures, especially at the juvenile phase. As some endophytes are beneficial for plants under stress conditions, we analyzed the impact of chilling temperatures on the root microbiome and examined whether microbiome-based analysis might help to identify bacterial strains that could promote growth under these temperatures. Results We investigated how the maize root microbiome composition changed by means of 16S rRNA gene amplicon sequencing when maize was grown at chilling temperatures in comparison to ambient temperatures by repeatedly cultivating maize in field soil. We identified 12 abundant and enriched bacterial families that colonize maize roots, consisting of bacteria recruited from the soil, whereas seed-derived endophytes were lowly represented. Chilling temperatures modified the root microbiome composition only slightly, but significantly. An enrichment of several chilling-responsive families was detected, of which the Comamonadaceae and the Pseudomonadaceae were the most abundant in the root endosphere of maize grown under chilling conditions, whereas only three were strongly depleted, among which the Streptomycetaceae. Additionally, a collection of bacterial strains isolated from maize roots was established and a selection was screened for growth-promoting effects on juvenile maize grown under chilling temperatures. Two promising strains that promoted maize growth under chilling conditions were identified that belonged to the root endophytic bacterial families, from which the relative abundance remained unchanged by variations in the growth temperature. Conclusions Our analyses indicate that chilling temperatures affect the bacterial community composition within the maize root endosphere. We further identified two bacterial strains that boost maize growth under chilling conditions. Their identity revealed that analyzing the chilling-responsive families did not help for their identification. As both strains belong to root endosphere enriched families, visualizing and comparing the bacterial diversity in these communities might still help to identify new PGPR strains. Additionally, a strain does not necessarely need to belong to a high abundant family in the root endosphere to provoke a growth-promoting effect in chilling conditions

Shifts in the rhizobiome during consecutive in planta enrichment for phosphate-solubilizing bacteria differentially affect maize P status

Summary Phosphorus (P) is despite its omnipresence in soils often unavailable for plants. Rhizobacteria able to solubilize P are therefore crucial to avoid P deficiency. Selection for phosphate‐solubilizing bacteria (PSB) is frequently done in vitro; however, rhizosphere competence is herein overlooked. Therefore, we developed an in planta enrichment concept enabling simultaneous microbial selection for P‐solubilization and rhizosphere competence. We used an ecologically relevant combination of iron‐ and aluminium phosphate to select for PSB in maize (Zea mays L.). In each consecutive enrichment, plant roots were inoculated with rhizobacterial suspensions from plants that had grown in substrate with insoluble P. To assess the plants’ P statuses, non‐destructive multispectral imaging was used for quantifying anthocyanins, a proxy for maize’s P status. After the third consecutive enrichment, plants supplied with insoluble P and inoculated with rhizobacterial suspensions showed a P status similar to plants supplied with soluble P. A parallel metabarcoding approach uncovered that the improved P status in the third enrichment coincided with a shift in the rhizobiome towards bacteria with plant growth‐promoting and P‐solubilizing capacities. Finally, further consecutive enrichment led to a functional relapse hallmarked by plants with a low P status and a second shift in the rhizobiome at the level of Azospirillaceae and Rhizobiaceae

Chemically versus thermally processed brown shrimp shells or Chinese mitten crab as a source of chitin, nutrients or salts and as microbial stimulant in soilless strawberry cultivation

Brown shrimp (Crangon crangon) shells and Chinese mitten crab (Efiocheir sinensis) were chemically demineralized and deproteinized (denoted as M1 to M4 for the shrimp shells and MS to M7 for the Chinese mitten crab), and shrimp shells were torrefied at 200 to 300 degrees C (denoted as R200, R255, R300), and were compared with a commercially available chitin source (denoted as reference chitin). Based on their chemical characteristics, a selection of chitin sources was tested for their N mineralization capacity. The N release was high for the chemically treated shrimp shells and Chinese mitten crab, but not for the Dandled shrimp shells with or without acid treatment, indicating that treatment at 200 et or higher resulted in low N availability. Interaction with nutrients was tested in a leaching experiment with limed peat for three thermally and two chemically processed shrimp shells and the reference chitin source. The K concentrations in the leachate for the chemically treated shrimp shells and the reference chitin were lower than for limed peat during fertigation. Irreversible K retention was observed for one source of chemically treated shrimp shells, and the reference chitin. The thermally treated shrimp shells had a significantly higher net release of P. Na and CI than the treatment without chitin source. Three shrimp shell based materials (M4, R200 and 8300) and the reference chitin were tested in a greenhouse trial with strawberry at a dose of 2 g/L limed peat. A very positive and significant effect on Borryris cinerea disease suppression in the leaves was found for the reference chitin, M4 and R200 compared to the unamended control. The disease suppression of the 3 chitin sources was linked with an increase of the microbial biomass in the limed peat with 24% to 28% due to chitin decomposition and a 9-44% higher N uptake in the plants