Dominik Kukla

Infrared thermographic analysis of shape memory polymer during cyclic loading

In this paper we present the effects of thermomechanical couplings occurring in polyurethane shape memory polymer subjected to cyclic tensile loadings conducted at various strain rates. Stress–strain characteristics were elaborated using a quasistatic testing machine, whereas the specimen temperature changes accompanying the deformation process were obtained with an infrared camera. We demonstrate a tight correlation between the mechanical and thermal results within the initial loading stage. The polymer thermomechanical behaviour in four subsequent loading-unloading cycles and the influence of the strain rate on the stress and the related temperature changes were also examined. In the range of elastic deformation the specimen temperature drops below the initial level due to thermoelastic effect whereas at the higher strains the temperature always increased, due to the dissipative deformation mechanisms. The difference in the characteristics of the specimen temperature has been applied to determine a limit of the polymer reversible deformation and analyzed for various strain rates. It was shown that at the higher strain rates higher values of the stress and temperature changes are obtained, which are related to higher values of the polymer yield points. During the cyclic loading a significant difference between the first and the second cycle was observed. The subsequent loading-unloading cycles demonstrated similar sharply shaped stress and temperature profiles and gradually decrease in values.

EVALUATION OF STRAIN DISTRIBUTION FOR THE P91 STEEL UNDER STATIC LOAD USING ESPI SYSTEM

Abstract The goal of the research was to evaluate the change of displacement/strain phases in the P91 steel under static load conditions. Undertaken tests were aimed at estimation and analysis of the impact of the material state, which was subjected to loading conditions, on the distribution of stress pattern using ESPI system. Specimen made of high temperature creep resistant steel X10CrMoVNb9-1 (P91) used as a construction material for boiler steam feed heaters, vapor tanks, pressure vessels and vapor pipelines, is used in the service conditions of temperature range up of 650°C. Test samples were taken from two P91 steel pipes. One sample came from a segment of a pipeline transporting fresh vapor in time 80 000 h, under the pressure of 8.4 MPa and temperature 540 °C. The second sample was the same material but in the delivery state.

Evaluation of the fatigue damage development using ESPI method

Fatigue damage process developing in structural materials under long-term cyclic loading is still an unsolved problem of modern engineering. Attempts to assess a degree of materials degradation under fatigue conditions on the basis of changes in the areas of local strain concentration determined by optical methods can be treated as the promising contemporary research direction of majority of scientific centers in the world. In most cases, fatigue damage has a local character and it is based on damage development leading to generation of cracks appearing around structural defects or geometrical notches. An identification of these areas and their subsequent monitoring requires a full-field displacement measurements performed on the objects surfaces. It seems that modern contemporary optical methods for displacement components measuring on the surfaces of structural elements or tested specimens are suitable for such purpose. Digital Image Correlation (DIC) and Electronic Speckle Pattern Interferometry (ESPI) are nowadays the most widely used testing methods in this area. Both of them enable capturing of displacement and strain components distributions. This paper presents an attempt to use the ESPI method for fatigue damage evaluation and its monitoring on specimens made of the aluminide coated nickel super-alloys. Flat specimens were subjected to cyclic loading. The fatigue tests were interrupted several times in order to perform a static loading during which the optical measurements were carried out. An analysis of the results captured by the ESPI system allowed indication of places of the greatest stress concentration and demonstration of the damage development process as a function of the increasing number of cycles.

Influence of Aluminide Layer on the Dynamics of the Development of Fatigue Damage in Ni-Cr-Co Alloy

Thermomechanical conditions in which the elements of modern aircraft engines work, require the use of protective coatings. The coatings increase the creep strength and also the local thermal stress in the near-surface areas due to the differing thermal expansion of particular material layers. For this reason it is necessary to develop a method for evaluating the operating properties of nickel superalloys with the aluminide layer, taking into consideration the surface processes related to the thermo-mechanical fatigue, taking place during the operation. In the presented work the assessment of the influence of the aluminium-coated layer, deposited on the nickel alloy specimens in the chemical deposition process (CVD) on the changes of the damage parameter in cyclic load conditions was carried out. The damage parameter was defined as a total strain in consecutive load cycles. The dynamics of damage development was analysed for two specimen lots (4 with the layer and 4 without it) displaying axial symmetry with a narrowing in the measured section. The results obtained were correlated with the results of fractography studies using SEM. The results obtained were used for the determination of the relationship between the damage parameter being the sum of the average strain and the strain amplitude, and the number of cycles, until the specimen is destroyed.

Ocena rozwoju uszkodzenia zmęczeniowego na podstawie zmian odkształcenia i parametrów prądowirowych w kolejnych cyklach obciążenia; Fatigue damage growth evaluation based on changes in the strain and eddy current parameters in the subsequent cycles load

W pracy zaprezentowano wyniki badań nad opracowaniem metodyki ilościowej oceny rozwoju uszkodzenia zmęczeniowego wysokostopowej stali X10CrMoVnb9-1 z wykorzystaniem nieniszczącej techniki diagnostycznej – metody prądów wirowych. W oparciu o procedurę charakteryzowania stopnia degradacji zmęczeniowej na podstawie zmierzonych wartości odkształceń średnich oraz odkształceń plastycznych w kolejnych cyklach obciążenia zdefiniowano odkształceniowy współczynnik uszkodzenia zmęczeniowego (Φ) opisujący dynamikę zmian odkształcenia w kolejnych cyklach. Wyznaczone w ten sposób parametry uszkodzenia skorelowano z wynikami pomiarów kąta fazowego sygnału prądowirowego mierzonego w ustalonych etapach procesu zmęczenia, przy amplitudzie naprężenia w zakresie 360÷420 MPa. na podstawie opracowanych procedur pomiarowych uzyskano wyniki wskazujące na możliwość identyfikacji i lokalizacji uszkodzenia zmęczeniowego we wczesnym etapie jego rozwoju, jak również na ilościową ocenę stopnia degradacji w oparciu o zdefiniowany parametr uszkodzenia oraz wyniki uzyskane za pomocą nieniszczącej techniki diagnostycznej.

Digital image correlation utilization in pipeline oriented residual stress estimation

The aim of the paper is to present an idea of the utilization of Digital Image Correlation (DIC) method for industrial pipelines residual stress oriented investigation. For this purpose results of tests performed in laboratory and industrial conditions are presented. Obtained results showed that DIC method gives reliable near drilled hole strain/displacement distribution maps which may be used for accurate residual stress calculations.

Evaluation of Microstructural Changes of S235 Steel after Rolling on the Basis of Microscopic Observations and Eddy Current Non-Destructive Method

Abstract The aim of the study was to determine the microstructure and residual stress changes which appears in the S235 steel (which is designed for operating in elevated temperatures) during cold rolling. The changes of orientation, both morphological and crystallographic, are occurring as a result of the rolling process. Analysis of the results allowed to determine the change in grain morphology (size, shape), determination of grain morphology of the rolling direction and determination of crystallographic texture. The dislocation density present in the material before and after the cold rolling process was estimated on the basis of dislocation structure images obtained via transmission electron microscope. The observed microstructural changes were correlated with the results of nondestructive testing using eddy current method. They allowed for the identification of the state of stress measured on three different surfaces of the rolled sheet (parallel and perpendicular to the rolling direction). As the result the usability of using the non - destructuve techniques of stress level determination was proved.