SCIENTIFIC RESEARCH
Novel Methodology for Experimental Characterization of Micro-Sandwich Materials
Materials Journal
by S. Hammarberg, J. Kajberg, S. Larsson, R. Moshfegh, P. Jonsén
Lightweight components are in demand from the automotive industry, due to legislation regulating greenhouse gas emissions, e.g., CO2. Traditionally, lightweighting has been done by replacing mild steels with ultra-high strength steel. The development of micro-sandwich materials has received increasing attention due to their formability and potential for replacing steel sheets in automotive bodies. A fundamental requirement for micro-sandwich materials to gain significant market share within the automotive industry is the possibility to simulate manufacturing of components, e.g., cold forming. Thus, reliable methods for characterizing the mechanical properties of the micro-sandwich materials, and in particular their cores, are necessary. In the present work, a novel method for obtaining the out-of-plane properties of micro-sandwich cores is presented. In particular, the out-of-plane properties, i.e., transverse tension/compression and out-of-plane shear are characterized. Test tools are designed and developed for subjecting micro-sandwich specimens to the desired loading conditions and digital image correlation is used to qualitatively analyze displacement fields and fracture of the core. A variation of the response from the material tests is observed, analyzed using statistical methods, i.e., the Weibull distribution. It is found that the suggested method produces reliable and repeatable results, providing a better understanding of micro-sandwich materials. The results produced in the present work may be used as input data for constitutive models, but also for validation of numerical models.
A new cracking resistance index based on fracture mechanics for high strength sheet metal ranking
IOP Conference Series: Materials Science and Engineering
by D. Frómeta; S.Parareda; A.Lara; I.Tarhouni (Eurecat); D.Casellas (Eurecat and Luleå University of Technology)
Driven by current safety and weight reduction policies in the automotive sector, the development of new high strength sheet metal products has experienced unprecedented growth in the last years. With the emergence of these high strength materials, new challenges related to their limited ductility and higher cracking susceptibility have also raised. Accordingly, the development of new fracture criteria accounting for the material’s cracking resistance has become unavoidable. In this work, a new cracking resistance index (CRI) based on fracture mechanics is proposed to classify the crack propagation resistance (i.e. the fracture toughness) of high strength metal sheets. The index is based on the fracture energy obtained from tensile tests with sharp-notched specimens. The procedure is very fast and simple, comparable to a conventional tensile test, and it may be used as routine testing for quality control and material selection. The CRI is investigated for several advanced high strength steel (AHSS) sheets of 0.8- 1.6 mm thickness with tensile strengths between 800 and 1800 MPa. The results show that the proposed index is suitable to rank high strength steel sheets according to their crack propagation resistance and it can be correlated to the material’s crashworthiness and edge cracking resistance.
Design of a non-destructive test for validating models of hydrogen migration
IOP Conference Series: Materials Science and Engineering
by M Beghini; G Macoretta; B D Monelli; R Valentini; L Bertini (University of Pisa)
High-strength steels, despite their excellent mechanical properties in normal conditions, can be susceptible to hydrogen embrittlement. Due to the service loads or residual stresses, hydrogen migrates within the component and accumulates in the regions where the highest tensile hydrostatic stress occurs. As a consequence, component brittle failure can occur even if the initial or mean hydrogen concentration is lower than the critical value. The availability of models predicting the hydrogen diffusion within the component is a crucial task for the design. Several diffusive models have been presented in the literature and some general-purpose finite element codes have already implemented some of them. However, the validation of those models is still an open issue due to the difficulty in performing accurate local measurements of the hydrogen concentration. This study deals with the design of a test potentially able to validate hydrogen migration models. In the test, a four-point bending configuration is applied to a properly shaped hourglass specimen, previously charged with hydrogen, extracted from thin high-strength steel sheets. The specimen geometry and the loading configuration were designed to obtain a central region in which the stress and strain field is uniform in plane and exhibits a quasi-uniform gradient in the thickness direction. As a consequence, it is expected a large enough central region of the specimen in which the Hydrogen can migrate only in the thickness direction during the typical duration of the test. The local hydrogen concentration is evaluated by measuring the flux leaving the tensile surface of the specimen by a solid-state hydrogen sensor.
Numerical Investigations on Thermal Forming Limit Testing with Local Inductive Heating for Hot Forming of AA7075
Materials Journal
by F. Reuther; T. Lieber; J. Heidrich; V. Kräusel (Fraunhofer Institute for Machine Tools and Forming Technology IW)
Forming 7000-series aluminum alloys under elevated temperatures is particularly attractive due to their increased formability. To enable process design by finite element simulation for hot forming, strain-based criteria, such as temperature-dependent forming limit diagrams (TFLD), can be consulted to assess forming feasibility. This work numerically investigates the extent to which in-plane experimental concepts with partial inductive heating are suitable for detecting discrete failure points in TFLD. In particular, an alternative to the currently widely used thickness-reduced specimen geometries was created for cruciform specimens under biaxial tension. First, the temperature-dependent and strain-rate-dependent flow behavior was investigated for AA7075 under uniaxial tension. A heat source model for partial inductive heating was inversely parameterized based on heating experiments. Subsequently, the test procedures were simulated with different specimen geometries under discrete strain conditions. Different concepts were discussed for deriving a suitable specimen shape for the biaxial tension case, and the influence of different notch and slot forms were shown. The simulations showed that partial inductive heating was suitable to induce failure situations, thus creating TFLDs. For the biaxial tension case, a sufficiently large temperature gradient was required to use cruciform specimens without thickness reduction.
Identification of fracture toughness parameters to understand the fracture resistance of advanced high strength sheet steels
Engineering Fracture Mechanics
by D. Frómeta, S.Parareda, A.Lara, S.Molas (Eurecat), D.Casellas (Eurecat and Luleå University of Technology), P.Jonsén (Luleå University of Technology) and J.Calvo (Polytechnic University of Catalonia).
The fracture toughness of four advanced high strength steel (AHSS) thin sheets is evaluated through different characterization methodologies, with the aim of identifying the most relevant toughness parameters to describe their fracture resistance. The investigated steels are: a Complex Phase steel, a Dual Phase steel, a Trip-Aided Bainitic Ferritic steel and a Quenching and Partitioning steel. Their crack initiation and propagation resistance is assessed by means of J-integral measurements, essential work of fracture tests and Kahn-type tear tests. The results obtained from the different methodologies are compared and discussed, and the influence of different parameters such as specimen geometry or notch radius is investigated. Crack initiation resistance parameters are shown to be independent of the specimen geometry and the testing method. However, significant differences are found in the crack propagation resistance values. The results show that, when there is a significant energetic contribution from necking during crack propagation, the specific essential work of fracture (we) better describes the overall fracture resistance of thin AHSS sheets than JC. In contrast, energy values obtained from tear tests overestimate the crack propagation resistance and provide a poor estimation of AHSS fracture performance. we is concluded to be the most suitable parameter to describe the global fracture behaviour of AHSS sheets and it is presented as a key property for new material design and optimization.
Investigation of Mechanical Tests for Hydrogen Embrittlement in Automotive PHS Steels
Metal Journal, Special Issue “Hydrogen Embrittlement of Metallic Materials: Past, Present and Future”
by R. Valentini (University of Pisa); M. Maria Tedesco (Centro Riserche Fiat); S. Corsinovi; L. Bacchi and M. Villa (Letomec).
The problem of hydrogen embrittlement in ultra-high-strength steels is well known. In this study, slow strain rate, four-point bending, and permeation tests were performed with the aim of characterizing innovative materials with an ultimate tensile strength higher than 1000 MPa. Hydrogen uptake, in the case of automotive components, can take place in many phases of the manufacturing process: during hot stamping, due to the presence of moisture in the furnace atmosphere, high-temperature dissociation giving rise to atomic hydrogen, or also during electrochemical treatments such as cataphoresis. Moreover, possible corrosive phenomena could be a source of hydrogen during an automobile’s life. This series of tests was performed here in order to characterize two press-hardened steels (PHS)—USIBOR 1500® and USIBOR 2000®—to establish a correlation between ultimate mechanical properties and critical hydrogen concentration.