Optinen venymämittaus ultralujien terästen särmäys- ja vetokokeessa sekä taivutusvoimien laskenta : SURO2-projektin särmäys- ja vetokokeet

Tässä raportissa on esitetty SURO2 (Superterästen Rakenteen Optimointi) projektissa Kemi-Tornion ammattikorkeakoululla Materiaalien Käytettävyyden Tutkimusryhmässä (MKT) tehtyjen särmäyskokeiden tulokset. Työn tavoitteena oli tutkia vetokokeiden ja särmättävyyden välistä korrelaatiota vertailemalla vetokokeista saatavia materiaaliparametreja särmäyskokeissa näytteestä mitattujen venymäarvojen kanssa ja etsiä niiden välistä yhteyttä särmättävyyteen. Koemateriaaleina olivat 6 mm:n Optim 700 MC Plus ja Raex 400 teräkset, jotka särmättiin erilaisiin kulmiin käyttäen erilaisia painimen säteitä, minkä jälkeen taipeesta mitattiin venymäjakaumat optisella venymämittausjärjestelmällä (ARGUS). Särmäyksessä vallitsevien venymien ja särmättävyyden välistä yhteyttä verrattiin vetokokeissa saatujen tulosten kanssa. Vetokokeissa käytettiin venymien mittauksessa ekstensometrin lisäksi myös optista venymämittausjärjestelmää (ARAMIS). Lisäksi määritettiin joistakin särmätyistä näytteistä neutraaliakselin sijainti (k-arvo) ja paksuusreduktiot särmien poikkileikkauksista tehtyjen mikroskooppitarkastelujen ja kovuusmittausten avulla. Särmäyskokeiden aikana mitattiin myös taivutusvoimia, joiden pohjalta kehitettiin mallia voiman ennustamiseksi

Global formability and bendability of ultra-high strength steels:effect of mechanical properties on the strain distribution and behaviour in air-bending

Abstract Bendability is a key property for ultra-high strength steels, that affects their usability in many industrial applications. Previous research and efforts on improving the bendability of high-strength steels have focused mostly on the minimum bend radius. However, as the minimum bend radius has been deemed insufficient as a measure of bendability, a new approach may be necessary for further advancements in bendability research. In this paper, bendability of nine materials is investigated from a global formability perspective, through bending tests and tensile tests. Digital image correlation is used for strain measurement in both the bending and tensile tests. Linear regression is used for determining the relationships between the obtained tensile test results and bending strain distributions. The findings of this paper show that applying a “local/global formability” approach to bendability could be beneficial for future research, as better description of the bending behaviour can be obtained and the factors affecting certain bending behaviours can be thoroughly investigated

Digital image correlation and optical strain measuring in bendability assessment of ultra-high strength structural steels

Abstract Air-bending is a widely used forming process for ultra-high strength steel because it is fast, cost-effective and flexible way to form material. The bendability of a material can be expressed by minimum bending radius R/tmin, which is the relation of the smallest inner radius to the sheet thickness the material can be bent without damage. Damage usually occurs on the outer surface of the bend in the form of intense strain localization that further progresses to cracking. The minimum bending radius contains no other information that affects the bendability such as the lower tool width or the desired bending angle. Hence, developing more detailed test procedure is critical to better describe the behavior of ultra-high strength steel sheet in bending. In this paper, a method for more detailed assessment of bendability for ultra-high strength structural steel is presented. Using optical strain measuring techniques and digital image correlation coupled with bending tests in a universal tensile test machine one can measure the strain evolution at the outer surface of the bend and determine the critical strains that limit the bendability of these materials

Strain distribution during air bending of ultra-high strength steels

Abstract Air bending is a widely used method for forming ultra-high strength steels (UHSS). However, the limited formability of UHSS poses some challenges for the bending process in the form of strain localisation, surface defects, punch detachment (multi-breakage) and pseudo-polygonal “nut-like” shape of the bend. In this study, the bendability of three UHSS grades (700, 900 and 1100 MPa) is investigated with 3-point bending tests, utilising Digital Image Correlation (DIC) for measuring the strain distributions on the outer curvature. The differences in the extent of multi-breakage and the bend shapes are also studied, and these observations are correlated with the findings from the bending force and strain measurements. The differences between the investigated UHSS grades are significant. The 900 MPa grade produces more localised strain distributions and pronounced multi-breakage compared to the other grades, along with a more polygonal “nut-like” geometry. The reasons and effects of the multi-breakage phenomenon, as well as the causes for the observed differences in the behaviour of the materials are discussed in this paper. The presented results and the measurement data provide more information about the behaviour of the investigated materials in bending, and can be used for improving bending simulation, numerical models, and workshop instructions

The effect of mechanical behavior on bendability of ultrahigh-strength steel

Abstract Bendability is an important property of ultrahigh-strength steels since the typical applications of such materials include structures manufactured by air-bending. Conventional methods to evaluate bendability, such as the bending test according to the standard VDA-238 or the conventional tensile test do not provide sufficient information to evaluate bendability of ultrahigh-strength steels due to the average nature of the material response in these tests. In this study, the mechanical properties were determined using thin tensile specimens cut from the surface of the sheet and the evaluation of bendability was carried out using frictionless bending tests. The results of the experiments and FE-modelling presented in this paper reveal that the mechanical properties of the sheet surface have a significant impact on bendability. Novel ultrahigh-strength steel with better work-hardening capacity at the surface caused by a layer of relatively soft ferrite and lower bainite has good bendability, especially when the bend line is aligned transverse to the rolling direction. Microstructural investigations reveal that in a conventional steel with a relatively hard surface microstructure, the deformation localizes into shear bands that eventually lead to fracture, but similar shear banding was not present in the novel steel surface. This can be attributed to the better work-hardening capacity which delays the onset of shear localization and fracture

Superior bendability of direct-quenched 960 MPa strip steels

Abstract The present paper shows the effect of microstructure on the press brake and frictionless 3-point bending of 6 mm thick ultrahigh-strength steel strips with a yield strength of 960 MPa. With a traditional press brake machine the minimum bending radii of the studied steels varied from 1.3 times the thickness to 3.0 times the thickness for the bend axis perpendicular to the rolling direction and in the range 2.0–3.5 times the thickness for the bend axis parallel to the rolling direction. The frictionless 3-point bending-equipment incorporating rotatable die-rollers has been applied to characterize the material work hardening behavior in a way relevant to the bending process, i.e. by using measured punch force vs. position data to derive the bending moment and the evolution of the flow stress and the strip curvature during the bending process. The main aim of the present paper is to establish an understanding of how bendability can be significantly improved and made more isotropic by modifying the subsurface microstructure to include a relatively soft polygonal ferrite and granular bainite layer and why the subsurface microstructure plays such a dominant role