0000000000907786
AUTHOR
Jānis Andersons
A family of weakest link models for fiber strength distribution
It is well known that the most widely used distribution function for fiber tensile strength, the two-parameter Weibull distribution, does not always adequately describe the experimentally observed fiber strength scatter and the strength dependence on fiber length. To remedy this discrepancy, modifications of the Weibull distribution have been proposed that, while providing a good empirical fit to the strength data, sometimes lack the theoretical appeal of the weakest link models. We derive a family of weakest link models based on the assumption of a two-stage failure process incorporating explicitly the probabilities of flaw initiation and the fiber fracture due to the largest flaw (i.e. th…
Reinforcement Efficiency of Cellulose Microfibers for the Tensile Stiffness and Strength of Rigid Low-Density Polyurethane Foams
Rigid low-density closed-cell polyurethane (PU) foams are widely used in both thermal insulation and structural applications. The sustainability of PU foam production can be increased by using bio-based components and fillers that ensure both enhanced mechanical properties and higher renewable material content. Such bio-based foams were produced using polyols derived from rapeseed oil and microcrystalline cellulose (MCC) fibers as filler. The effect of MCC fiber loading of up to 10 wt % on the morphology, tensile stiffness, and strength of foams has been evaluated. For estimation of the mechanical reinforcement efficiency of foams, a model allowing for the partial alignment of filler fibers…
Analysis of thin film cracking and buckling on compliant substrate by fragmentation test
Application of coating fragmentation test for determination of coating properties is considered. A probabilistic model of coating fragmentation under uniaxial tensile loading is applied for coating and interface property identification of thin brittle coating/polymer substrate system. A finite element model is developed to simulate the process of buckle formation in coating strips during fragmentation test. The measured buckle geometry parameters and buckle density evolution as a function of the applied strain is used to estimate the interface toughness.
Rapeseed oil-based rigid polyisocyanurate foams modified with nanoparticles of various type
Polyisocyanurate foams have been developed using a polyol system partially derived from rapeseed oil (RO). The effect of nanofillers with iso-dimensional geometry (zinc oxide), nanofibers (carbon nanotubes), and nanoplatelets (organically modified montmorillonite) on stiffness and strength of the foams has been studied. It was demonstrated that the tensile properties of the filled foams are enhanced by roughly the same proportion as the stiffness and strength of monolithic filled polymers.
Creep and damage accumulation in orthotropic composites under cyclic loading
Experimental results and theoretical prediction of the response of glassfiber-reinforced polyester under quasi-static, static (creep), and cyclic (fatigue) loading are presented. The nonlinear strain component at static loading and the strain amplitude rate at cyclic off-axis loading of an orthotropic composite are shown to follow the associated flow rule with a single-parameter quadratic potential function. The influence of fatigue damage on deformation is considerable due to the reduction in the elastic modulus of the composite and is apparently negligible with respect to its effect on the parameters of the creep kernel.
23.1: Invited Paper: Models and Experiments of Mechanical Integrity for Flexible Displays
Flexible displays present a challenging problem in terms of mechanical integrity, a result of the considerable hygro-thermo-mechanical contrast between the inorganic, brittle device layers and the compliant polymer substrates. This paper reviews the main approaches to study and identify the key factors, which control the mechanical stability of this class of displays. Focus is put on the analyses of residual stress and damage under tensile loading. Novel electro-mechanical methods are used for accurate insight into critical phenomena. An important result is that the thickness and stiffness of the substrate control the critical strain for failure of the device layers.
Estimation of the tensile strength of an oriented flax fiber-reinforced polymer composite
Unidirectional orientation of natural fibers in a polymer composite ensures the highest efficiency of reinforcement. Flax fiber reinforcement is discontinuous due to limited fiber length and heterogeneous due to the presence of elementary fibers and their bundles. In order to assess the upper limit of tensile strength of such slightly misoriented, nominally UD natural fiber composite, a statistical strength model of continuous UD fiber reinforced composites is applied. It is found that the experimental strength of UD flax composites, produced from rovings or manually aligned fibers, approaches the theoretical limit only at relatively low fiber volume fraction ca. 0.2, being markedly below i…
Testēšanas prasmju uzlabošana, pamatojoties uz procesa dokumentāciju
Bakalaura darba tēma ir ”Testēšanas prasmju uzlabošana, pamatojoties uz procesa dokumentāciju”. Latvijā, pēdējos gados, programmatūras izstrādes industrija ir ļoti strauji augusi un augstskolās tiek sagatavoti daudzi šīs jomas speciālisti. Diemžēl ne visās augstskolās tiek pievērsta pietiekama uzmanība testēšanas jomas speciālistu sagatavošanai. Līdz ar to bieži programmatūras izstrādes uzņēmumiem ir pašiem jāsagatavo jaunos darbiniekus produktīvam darbam. Darba mērķis ir definēt un aprakstīt procesus, lai uzlabotu testēšanas prasmju līmeni gan pilnīgi jaunam darbiniekam, kurš vēlas strādāt testēšanas jomā ,gan testētājam ar pieredzi. Darbā tiek sniegts priekšstats par standartu lietojumu p…
Rotor—frame contact in a centrifuge installed on board a ship
The criticality of rotor and stator contact in a large-size marine separator is studied by numerical modelling. Two plausible mechanisms leading to such contact, namely rolling of a ship in steady-state motion and a suddenly occurring unbalance in the rotor, are considered. It is found that contact due to sea motion may be tolerable, while sudden development of unbalance at operating speed can lead to unacceptably high contact forces. This agrees with experience of centrifugal separators operating on board of ships.
CLS Banka: Darbības principi, nozīmība un efektivitāte
Bakalaura darba tēma ir par CLS Banku, tās darbības principiem, nozīmību un efektivitāti, jo tā ir sekmīgi funkcionējoša, visā pasaulē novērtēta organizācija, kas nodarbojas ar risku mazināšanas procesiem globālajā FX tirgū. Galvenais no riskiem – norēķinu risks, kas savu nozīmību irguva 1974.gadā pēc Bankhaus Herstatt krīzes. Darba mērķis ir, pamatojoties uz zinātnisko literatūru, starptautisko un CLS Bankas normatīvo aktu izpēti FX tirgus risku vadības jomā, sniegt ieskatu CLS Bankas darbības pamatprincipos, veikt tās darbības analīzi, nosakot nozīmību un efektivitāti, kā arī izdarīt secinājumus un sniegt priekšlikumus CLS Bankas darbības pilnveidošanai. Izvirzīti šādi uzdevumi: izpētīt g…
Fracture Toughness of PIR Foams Produced from Renewable Resources
Rigid low-density closed-cell polyisocyanurate (PIR) foams are used primarily as a thermal insulation material. Traditionally, they are manufactured from constituents produced by petrochemical industry. Introducing renewable materials in PIR formulation brings definite economical and environmental benefits. Fracture toughness of PIR foams obtained from renewable resources (with the polyol system comprising up to 80% of rapeseed oil esters) and petrochemical PIR foams has been characterized experimentally, by compact tension tests, for mode I crack propagation along the rise direction of the foams.
Empirical model for stress ratio effect on fatigue delamination growth rate in composite laminates
An empirical model is proposed for the stress ratio effect on the fatigue delamination growth rate in composite laminates under single-mode loading, intending to provide a simple and accurate description tool in engineering application. The model is based on heuristic considerations of damage accumulation ahead of the crack. Linear cumulative assumption is used for damage estimation. Comparison of the prediction based on the proposed model with the test results for different carbon, glass, and alumina fibre-reinforced composites at several stress ratios shows reasonable agreement.
Grāmatvedības programmu testēšanas pilnības problēma
Maģistra darbā tiek apskatīta grāmatvedības programmatūras risinājuma testēšanas problēma, analizēta incidentu vēsture. Papildus tiek apskatīta incidentu statiska un veikta to analīze laikā, kad grāmatvedības sistēma tika pielāgota eiro valūtai un ieviesta produkcijā. Darbā tiek meklēts risinājums testēšanas pilnības problēmai un piedāvāti risinājumi testēšanas pilnības uzlabošanai. Darbā piedāvātā metode ir testpiemērus veidot balstoties uz lietojumgadījumu scenārijiem, kas autora pētītajā organizācijā augstas un kritiskas kategorijas incidentus novērstu par 88%, visu incidentu skaitu samazinātu par 70%. Atslēgas vārdi: Testēšanas pilnība, grāmatvedības programmu testēšana, ERP testēšana, …
The Effect of Montmorillonite Type Nanoparticles on Stiffness and Flammability of Rapeseed Oil Based Polyisocyanurate Foams
Polyisocyanurate foams incorporating polyols derived from rapeseed oil are developed, with the polyol system comprising up to 80% of rapeseed oil esters. To enhance the mechanical characteristics and fire resistance of the foams, they are filled with three different types of organically modified clay (organoclay) in the range of loading from 1 to 5% by weight. Marked increase in tensile stiffness of nanocomposite foams is observed.
Robustness of Empirical Vibration Correlation Techniques for Predicting the Instability of Unstiffened Cylindrical Composite Shells in Axial Compression
Thin-walled carbon fiber reinforced plastic (CFRP) shells are increasingly used in aerospace industry. Such shells are prone to the loss of stability under compressive loads. Furthermore, the instability onset of monocoque shells exhibits a pronounced imperfection sensitivity. The vibration correlation technique (VCT) is being developed as a nondestructive test method for evaluation of the buckling load of the shells. In this study, accuracy and robustness of an existing and a modified VCT method are evaluated. With this aim, more than 20 thin-walled unstiffened CFRP shells have been produced and tested. The results obtained suggest that the vibration response under loads exceeding 0.25 of …
Experimental Investigation on Stiffness and Strength of Single-Lap Z-Pinned Joints in a Laminated CFRP Stress-Ribbon Strip
Carbon fiber-reinforced polymer (carbon-polymer) is an advanced lightweight composite material with high strength and excellent resistance to corrosion and fatigue. Over the past decades, application of fiber-reinforced polymers has been spread from the aerospace to other branches of industry such as automobile and civil engineering. Unidirectional carbon-polymers have a high potential for replacing steel in tensile members. Recently, the first carbonpolymer stress-ribbon bridge has been constructed in Germany. The non-laminated strip-loop carbon-polymer thin strips were used as the load bearing components in this bridge. In comparison with the laminated components, the applied cables are c…