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Experimental Characterization of Triaxiality Stress State Evolution for Sheet Metal Materials
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.ORCID iD: 0000-0002-5351-9338
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.ORCID iD: 0000-0001-9626-5406
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
Number of Authors: 4
(English)Manuscript (preprint) (Other academic)
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-62474OAI: oai:DiVA.org:ltu-62474DiVA: diva2:1081234
Available from: 2017-03-13 Created: 2017-03-13 Last updated: 2017-04-05
In thesis
1. Material Characterization for Modelling of Sheet Metal Deformation and Failure
Open this publication in new window or tab >>Material Characterization for Modelling of Sheet Metal Deformation and Failure
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Weight reduction is one possibility to reduce fuel consumption and emission of transportation vehicles. Sheet metals are often used in automotive and aerospace applications and therefore the weight reduction achieved by reducing the sheet metals thickness is an important contribution to weight reduction. Increasing the strength of sheet metal materials gives the opportunity to reduce the total weight while maintaining safety. To prove a maintained safety for parts with a decreased weight Finite Element (FE) simulations are commonly used. This leads to a high demand on the simulation precision of sheet metals, where an accurate prediction of the post-necking behaviour of materials is needed. Improved FE simulations are reducing time and costs during the development processes. 

One application to improve the strength of sheet metals in the automotive industry is the usage of ultra high strength steels, which has constantly increased in usage during the last decades. The development of the press hardening process, where sheet metal blanks are formed and quenched simultaneously, brings new design opportunities. Using press hardening tools with zones that uses different cooling rates sheet metal parts can be produced with tailored properties, to improve their performance. Simulating materials based on the microstructure demands high precision on the plasticity modelling for high strain values. 

In this thesis work a method to characterize the elasto plastic post necking behaviour of sheet metal materials, the Stepwise Modelling Method (SMM), is presented. The method uses full field measurements of the deformation field on the surface of tensile specimen. The hardening relation is modelled as a piecewise linear in a step by step procedure. The linear hardening parameter is adapted to reduce the residual between experimental and calculated tensile forces. The SMM is used to characterize the post necking behaviour of a ferritic boron steel and the results are compared with the commonly used inverse modelling method. It is shown that the stepwise modelling method characterizes the true stress, true plastic strain relation in an effective and computational efficient way. Furthermore, the SMM is used to characterize the stress state evolution during tensile testing, which is an important factor for failure and fracture modelling. This method is shown in an aerospace application for the nickel based super alloy Alloy 718. 

The results shows that the stepwise modelling method is an effective and efficient alternative method to characterize the deformation and failure of sheet metals. Based on the results of this method plasticity and fracture models can be characterized in future work.

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet, 2017
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
National Category
Materials Engineering Mechanical Engineering Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-62477 (URN)978-91-7583-845-8 (ISBN)978-91-7583-846-5 (ISBN)
Presentation
2017-05-12, E231, Luleå University of Technology, Luleå, 10:00 (English)
Available from: 2017-03-15 Created: 2017-03-13 Last updated: 2017-04-20Bibliographically approved
2. Method Development for Characterisation of Superalloy used in Containment Design
Open this publication in new window or tab >>Method Development for Characterisation of Superalloy used in Containment Design
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Metodutveckling för karaktärisering av superlegering med tillämpning i containmentdimensionering
Abstract [en]

Due to the trend of increasing environmental demands put on civil aviation, manufacturersof commercial aircraft engines meet increased pressure to reduce weight. Modernturbofan engines represent up to almost one tenth of an aircraft's total weight, meaning areduction of engine component weight of just 30 kg is estimated to reduce CO2 emissionsby 400 tonnes over the lifetime of a medium sized commercial aircraft. At the sametime turbine casings are required to fully prevent debris to escape in the event of bladefailure, to prevent further damage to critical systems. For new designs to be approvedthe Federal Aviation Regulations (FAR) states that the containment capability of a suggesteddesign solution must be experimentally established, a process associated with highcosts and long lead times. The industry therefore more frequently relies on numericalsimulations as part of all stages in the design process. For simulations to replace theexpensive experiments in nding the nal optimum design regarding weight and safety,the accuracy of the used models have to be improved.This thesis aims to provide increased accuracy in the numerical predictions by developingexperimental procedures to test material close to the operational conditions of thecontainment structure. This is realised by performing experiments at high-strain ratesand elevated temperatures in a high-velocity tensile testing machine combined with aninduction heater. Sheet specimens of varying geometries are loaded in tension to achievedierent stress states for covering dierent failure modes. Furthermore, high-speed photographyand Digital Image Correlation are utilised to track in-plane deformations. Theresulting local deformations are then used to derive the stress-strain hardening relationand the evolution of the stress state from initial loading up to fracture. The obtaineddata are nally used to calibrate strain rate and thermal dependent plasticity and fracturemodels. To validate the calibrated models so-called reverse impact testing was used,where the resulting force of a material sample impacting an instrumented target wasquantied. The experiment was straightforward to model numerically since the specimenies freely without constraints, thereby avoiding complex boundary conditions.The characterisation method was developed and performed on nickel based Alloy 718.This material is known for its high strength and good corrosion resistance at high temperaturesand is therefore commonly used in hot parts of aircraft engines, such as thecontainment structures of the low-pressure part of the engine turbine. All material fortesting and validation was supplied from one single heat and batch, aged using the sameheat treatment conditions, to ensure consistent mechanical properties. The results fromthe characterisation procedure showed that the plastic ow of Alloy 718 is moderatelystrain rate and temperature dependent while the fracture is clearly stress state dependent.

Place, publisher, year, edition, pages
Luleå University of Technology, 2017
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Metallurgy and Metallic Materials Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-62799 (URN)978-91-7583-866-3 (ISBN)978-91-7583-867-0 (ISBN)
Public defence
2017-05-24, A109, LTU, Luleå, 09:00 (English)
Opponent
Supervisors
Funder
VINNOVA
Available from: 2017-04-05 Created: 2017-03-30 Last updated: 2017-05-08Bibliographically approved

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