Analyse av et kaldformet kvadratisk hulprofil (undertittel: En sammenligning med og uten varmebehandling)

Abstract

Kaldformede hulprofiler er attraktivt for mange ulike konstruksjonsdeler på grunn av sin høye styrke og relativt lave pris. På grunn av produksjonsmetoden inneholder slike profiler ofte store egenspenninger. det er viktig å vite hvordan dette påvirker styrken til materiale. Denne avhandlingen presenterer en eksperimentell studie av ulike materialegenskaper og materialparametere i et kaldformet hulprofil av typen 80x80x6 med ståltypen S420MH. Egenspenningenes innvirkning på den plastiske bøyekapasiteten undersøkes. I tillegg undersøkes materialparameternes bruksverdi og kvalitet gjennom numerisk modellering av tre- og firepunkt bøyetester.

Det ble utført kvasistatiske materialtester hentet fra hjørnet og sidekanten i profilet, i tillegg til kvasistatiske tre- og firepunkts bøyetester for å undersøke virkningen av egenspenningene i en hel bjelke med og uten varmebehandling. Resultatene av UT-testene og bøyetestene ble i begge tilfeller at materialet med egenspenninger har høyere flytespenning, men lavere duktilitet. Dette viste seg spesielt godt i firepunkts bøyetesten, hvor det oppstår to flyteledd. Her hadde profilet med varmebehandling en betydelig bedre evne til å omfordele krefter fra et flyteledd til det andre. Materialkurven ble tatt inn i ABAQUS og brukt til å modellere en elementmodell av de kvasistatiske bøyetestene. Kraft vs. forskyvningskurven ble sammenlignet for modellen og den fysiske testen og samsvaret mellom de to ble vurdert til å være relativt god.

Cold-formed hollow sections are attractive for many different construction parts due to their high strength and relatively low cost. Due to the production method, such profiles often contain large residual stresses. It is important to know how this affects the strength of the material. This thesis presents an experimental study of various material properties and material parameters in a cold-formed hollow profile of the type 80x80x6 with the steel type S420MH. The effect of the residual stresses on the plastic bending capacity is investigated. In addition, the value and quality of the material parameters are examined through numerical modeling of three- and four-point bending tests. Heat treatment in the form of stress relief annealing was performed on a set of profiles to release residual stresses. This made it possible to compare properties and parameters between two profiles, with and without residual stresses. Quasi-static UT-tests performed on specimens from the corner and the middle edge of the profile were performed. A camera made it possible to continuously measure the diameter of the necking in the specimens. The results from these tests showed that the residual stresses increase the yield stress but reduce the ductility. Several larger specimens were also taken out. These were painted with a spreckled pattern and continuously monitored using DIC. Based on this, it became possible to extract true stress and true strain. This formed the basis of the material curve for the profile. To investigate the effect of the residual stresses in an entire beam, quasi-static bending tests were carried out for profiles with and without heat treatment. The two types of profiles were subjected to two different types of tests: three- and four-point bending tests, where one and two plastic hinges, respectively, are to be expected. The results of the bending tests were, as in the UT tests, that the material with residual stresses has a higher yield stress but lower ductility. This was particularly clear in the four-point bending test where two plastic hinges occur, here the profile which had been heat treated showed a significantly better ability to redistribute forces from one plastic hinge to the other. The material curve was taken into ABAQUS and used to model an element model of the quasi-static bending tests. The force vs displacement curve was compared for models and the physical test and the match between the two was considered to be relatively good.