On the Influence of Cross-Section Size on Measured Strength of SLM-Produced AlSi10Mg-Alloy

The freedom in choice of geometries in additive manufacturing (AM) favors the use of structures with large surface and small cross-section such as lattice structures and thin-walled hollow profiles. On the other hand, the practices of strength testing of metals require a certain bulk of the material to be printed to be able to produce a sample and test material properties. The size of the sample cross section might influence the strength and up to 30% decrease in strength for small struts was reported in the literature. Understanding the influence of the cross-section size on the strength of SLM-produced metal is crucial to be able to relate the strength determined through tensile testing and the strength of an SLM-produced component with complex geometry. This article deals with effect of cross-section size on the measured strength of the SLM-produced AlSi10Mg-alloy. It is demonstrated how the decrease in strength can be explained by the difference between measured and actual cross-section area induced by surface roughness rather than by the difference in microstructure between the samples of different sizes.publishedVersio

Effect of the Joint Strength on the Performance of Ordinary Moment-resisting Frames Under a Progressive Collapse Situation

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Mathematical models for assessment of vehicle crashworthiness: a review

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Vehicle crashworthiness performance in frontal impact: Mathematical model using elastic pendulum

Vehicle occupant injuries due to collisions cause many fatalities every year. Safe vehicle design plays a critical role in averting serious injuries to occupants and vulnerable road users in the event of a crash. In this paper we study a full frontal vehicle crash against a rigid barrier introducing a Lumped Parameter Model (LPM) inspired by the elastic pendulum motion. The model uses polar coordinates to simplify the problem and the governing equations have been defined using Lagrangian formulation. The Simulink model has been validated against Finite Element (FE) data demonstrating good correlation with pitching angle and maximum crush of the vehicle. These parameters are crucial for designing vehicles which efficiently protect occupants.publishedVersionPaid Open Acces

Crash Response of a Repaired Vehicle-Influence of Welding UHSS Members

Author's accepted manuscriptAutomakers generally recommend not to weld structural parts after a vehicle crash, and these should be replaced as a whole part in case of a crash event. Sectioning of these members is also not recommended and use of the repair manual is mandatory in case of fracture of such parts. However, repair shops may not adhere to these instructions and use incorrect repair procedures on these members which would modify their strength properties. This study analyses the impact of welding structural members in a vehicle like the A-pillar which use Ultra-High Strength Steels (UHSS) for reducing the weight of the vehicle and improving the crashworthiness of the structure. The research conducted in this paper highlights the differences in the crash performance of a repaired vehicle as opposed to baseline injury values for the vehicle. The performance of the modified vehicle when tested for different loadcases shows reduced crash performance as compared to the baseline performance and it can be concluded that welding or sectioning the UHSS parts would influence the crashworthiness of a vehicle. This paper only focuses on structural integrity of the repaired vehicle in a crash event. The performance of the vehicle in occupant injury is kept out of scope for this study.acceptedVersio

Modeling of Modified Vehicle Crashworthiness using a Double Compound Pendulum

Vehicle crash modeling has been a challenge for researchers for several decades. Occupant injury prevention and prediction is a critical area within vehicle safety design. The modeling of material failure in structural members during a full frontal crash has been presented in this paper. This study presents a Lumped Parameter Model (LPM) with an elastic double compound pendulum replicating the impact kinematics. The model defined using Lagrangian formulation; presents a novel methodology to represent material fracture caused due to heat affected zones or welding in Ultra High Strength Steels (UHSS) in a non-linear crash event. The material fracture leads to rotation of the vehicle; presented in the form of torsional springs in the LPM developed in this study. The Simulink model has been validated with a finite element simulation and shows good correlation to predict parameters crucial to design for occupant protection in a vehicle crash.publishedVersio

A Novel Technique for Modeling Vehicle Crash using Lumped Parameter Models

This paper presents a novel technique for modeling a full frontal vehicle crash. The crash event is divided into two phases; the first until maximum crush and the second part when the vehicle starts pitching forward. This novel technique will help develop a three degrees of freedom (DOF) lumped parameter model (LPM) for crash and support in the vehicle development process. The paper also highlights the design process for reducing vehicle pitching in occupant protection load cases. The model has been validated against a finite element (FE) simulation of a full frontal crash of a Chevrolet Silverado developed by the National Highway Traffic Safety Administration (NHTSA), and the LPM shows good correlation with the FE test data.publishedVersio

Experimental investigation of effect of printing direction and surface roughness on the mechanical properties of AlSi10Mg-alloy produced by selective laser melting

The additive manufacturing has initially gained popularity for production of non-loadbearing parts and components or in the fields where the material strength and ductility are less important such as modelling and rapid prototyping. But as the technology develops, availability of metal additive manufacturing naturally dictates the desire to use the produced components in load-bearing parts. This requires not-only a thorough documentation on the mechanical properties but also additional and independent research to learn the expected level of variation of the mechanical properties and what factors affect them. The presented paper investigates strength, ductility, hardness, and microstructure of the AlSi10Mg alloy produced by the selective laser melting (SLM). The mechanical properties were determined through a series of uniaxial tension tests and supplementary hardness tests and rationalized with the microstructure evolution with regard to printing direction and heat treatment. The paper also addresses the effect of surface roughness on the mechanical properties of the material, by comparing the machined and net shape tension samples. As expected, the as-manufactured AlSi10Mg-alloy appears to be a semi-brittle alloy, but its microstructure can be altered, and ductility increased by a proper heat-treatment. The effect of surface layer removal on the measured mechanical properties is of particular interest.publishedVersio

Formability of aluminium alloy subjected to prestrain by rolling

The purpose of this thesis was to improve the understanding and the accuracy of the description of metal sheet formability following the needs of the automotive industry. In particular, the effects of plastic anisotropy and prestrain by rolling on the formability of AA6016 sheets were investigated. The formability of metal sheets is traditionally described by strain-based Forming Limit Curves (FLCs) which are known to be path-dependent. In this thesis the merits of the alternative descriptions by means of stress-based and equivalent plastic strain based FLCs were investigated. First, the mechanical properties of the AA6016 sheets were examined through a series of material tests. Uniaxial tension tests in seven different directions in the plane of the sheet along with the disc compression test were used for calibration of the plasticity model. In addition, plane-strain tension, in-plane shear and cyclic shear tests were performed for validation of the plasticity model. Then, the formability properties were investigated by means of Marciniak-Kuczynski and Nakazima tests. The investigated material showed a tendency to form multiple local necks in biaxial tension making the standard method for experimental detection of forming limit strains inapplicable. In order to enable the construction of FLCs a method capable of handling multiple local necks had to be developed. The new method improved the accuracy and robustness of experimental detection of forming limits. In addition, this method was developed to differentiate between local necking and fracture strains without involvement of the user. It was shown that for the tested AA6016 the sheet formability in biaxial tension is limited by local necking. The possible influence of fracture on the forming limits was excluded both by improving the experimental procedure to differentiate between local necking and fracture strains and by applying predictions of calibrated fracture models. The experimentally detected FLCs at necking were compared with the predictions of the Marciniak- Kuczynski model. The comparison revealed that the investigated material displayed significantly higher anisotropy in formability than could be explained by the plastic anisotropy alone. The formability proved to be affected by roping phenomenon. Roping is likely to be the cause of the multiple local necking that made accurate experimental detection of the forming limit strains challenging in the first place. The effect of prestrain by rolling on the forming limit strains proved to be similar to the effect of prestrain in plane-strain tension. On the other hand, the forming limit stresses and the equivalent plastic strain at necking proved to be much less affected by the prestrain. This confirmed the advantages of the description of formability by means of stress-based and equivalent strain based FLCs in comparison with the traditional strain-based FLCs

On the Effect of Various Heat Treatments on Microstructure of AISI 4130 Steel Used in Sour Service Pipes

Author's accepted manuscript.acceptedVersio