Fracture Characterization of Rolled Sheet Alloys in Shear Loading: Studies of Specimen Geometry, Anisotropy, and Rate Sensitivity

The final publication is available at Springer via http://dx.doi.org/10.1007/s11340-016-0211-9Two different shear sample geometries were employed to investigate the failure behaviour of two automotive alloy rolled sheets; a highly anisotropic magnesium alloy (ZEK100) and a relatively isotropic dual phase steel (DP780) at room temperature. The performance of the butterfly type specimen (Mohr and Henn Exp Mech 47:805–820, 16; Dunand and Mohr Eng Fract Mech 78:2919-2934, 17) was evaluated at quasi-static conditions along with that of the shear geometry of Peirs et al Exp Mech 52:729-741, (27) using in situ digital image correlation (DIC) strain measurement techniques. It was shown that both test geometries resulted in similar strain-paths; however, the fracture strains obtained using the butterfly specimen were lower for both alloys. It is demonstrated that ZEK100 exhibits strong anisotropy in terms of failure strain. In addition, the strain rate sensitivity of fracture for ZEK100 was studied in shear tests with strain rates from quasi-static (0.01 s−1) to elevated strain rates of 10 and 100 s−1, for which a reduction in fracture strain was observed with increasing strain rate.Cosma International, Automotive Partnership CanadaOntario Research FundNatural Sciences and Engineering Research Council of CanadaCanada Research Chairs SecretariatCanada Foundation for Innovatio

Coil Development for Electromagnetic Corner Fill of AA 5754 Sheet

Electromagnetic (EM) forming is a high-speed forming process that uses the forces induced on a conductive workpiece by a transient high frequency magnetic field to form the workpiece into a desired shape. It has been reported by several researchers that EM forming (EMF) increases the formability of hard-to-form aluminum alloy sheet under certain circumstances. EMF can be combined with conventional forming (e.g. stamping) operations to create a hybrid forming operation that exploits the strengths of each process. One such operation is the “corner fill” operation, which consists in pre-forming sheet using conventional forming and then using EMF to reduce the radii of different features on the part to values that could not be obtained with conventional forming. This paper describes the development of a coil used for a hybrid operation that consisted on pre-forming AA 5754 1 mm into a v-shape with a 20 mm outer radius and then reducing or “sharpening” the radius to 5 mm using EMF. The coil is one of the most important components of an EMF operation, since it is the means of delivering the energy to the workpiece. Coils are subjected to very high stresses and are typically the element of an EMF operation that will fail first. One successful and four unsuccessful coils designs are presented. The successful coil was a single loop design, with the section closest to the part narrowed to increase the current density. The simplicity of the shape was chosen for its current flow characteristics and for its structural strength

Crashworthiness of aluminium tubes ; Part 1 : Hydroforming at different corner-fill radii and end feeding levels

The automotive industry, with an increasing demand to reduce vehicle weight through the adoption of lightweight materials, requires a search of efficient methods that suit these materials. One attractive concept is to use hydroforming of aluminium tubes. By using FE simulations, the process can be optimized to reduce the risk for failure while maintaining energy absorption and component integrity under crash conditions. It is important to capture the level of residual ductility after forming to allow proper design for crashworthiness. This paper presents numerical and experimental studies that have been carried out for high pressure hydroforming operations to study the influence of the tube corner radius, end feeding, material thinning, and work hardening in 76.2 mm diameter, 3 mm wall thickness AA5754 aluminium alloy tube. End feeding was used to increase the formability of the tubes. The influence of the end feed displacement versus tube forming pressure schedule was studied to optimize the forming process operation to reduce thinning. Validation of the numerical simulations was performed by comparison of the predicted strain distributions and thinning, with measured quantities. The effect of element formulation (thin shell versus solid elements) was also considered in the models

Damage Evolution in Complex-Phase and Dual-Phase Steels during Edge Stretching

The role of microstructural damage in controlling the edge stretchability of Complex-Phase (CP) and Dual-Phase (DP) steels was evaluated using hole tension experiments. The experiments considered a tensile specimen with a hole at the center of specimen that is either sheared (sheared edge condition) or drilled and then reamed (reamed edge condition). The damage mechanism and accumulation in the CP and DP steels were systematically characterized by interrupting the hole tension tests at different strain levels using scanning electron microscope (SEM) analysis and optical microscopy. Martensite cracking and decohesion of ferrite-martensite interfaces are the dominant nucleation mechanisms in the DP780. The primary source of void nucleation in the CP800 is nucleation at TiN particles, with secondary void formation at martensite/bainite interfaces near the failure strain. The rate of damage evolution is considerably higher for the sheared edge in contrast with the reamed edge since the shearing process alters the microstructure in the shear affected zone (SAZ) by introducing work-hardening and initial damage behind the sheared edge. The CP microstructures were shown to be less prone to shear-induced damage than the DP materials resulting in much higher sheared edge formability. Microstructural damage in the CP and DP steels was characterized to understand the interaction between microstructure, damage evolution and edge formability during edge stretching. An analytical model for void evolution and coalescence was developed and applied to predict the damage rate in these rather diverse microstructures.Natural Sciences and Engineering Research Council of Canada (NSERC) AUTO21 Network of Centres of Excellence Canada Research Chairs Secretaria

Quantification of Mixed Mode Loading and Bond Line Thickness on Adhesive Joint Strength Using Novel Test Specimen Geometry

The final publication is available at Elsevier via https://doi.org/10.1016/j.ijadhadh.2020.102682. © 2020. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/This study quantifies the effect of mixed mode loading and bond line thickness on adhesive joint strength or automotive structural applications. This research is motivated by the need to address the complex loading that occurs during automotive crash events, as well as the variation in bond line thickness that may occur due to gap variability when joining mass-produced structural components. A newly developed specimen geometry for Mode II and Mixed Mode loading is presented, while a recently published test methodology was used to characterize the Mode I response. Three nominal bond line thicknesses (0.18, 0.30 and 0.64 mm), were investigated for a toughened structural adhesive and steel adherends. The traction-separation response, required for cohesive zone modeling (CZM) of adhesive joints, was determined for each combination of bond line thickness and mode of loading. Mode I loading resulted in higher peak traction and lower critical energy release rates compared to Mode II loading, with the Mixed Mode responses typically falling between Mode I and II, in relation to the loading angle tested. Increasing bond line thickness resulted in a reduction in initial stiffness and peak traction, as well as an increase in critical energy release rate for all modes of loading. Two existing CZM mixed mode failure criteria were assessed and demonstrated a good fit to the tested mixed mode responses, despite the limited ability of the CZM implementation to predict the end of the plateau region of the traction-separation response. The experimental approach described in this study was shown to provide repeatable results that could be directly used to fully define an adhesive CZM, ready for use in finite element modeling without the need for inverse modeling

Experimental fracture characterisation of an anisotropic magnesium alloy sheet in proportional and non-proportional loading conditions

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.ijsolstr.2018.04.010. © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/A comprehensive experimental investigation was performed to characterize the fracture behaviour of a rare-earth magnesium alloy sheet, ZEK100-O, under both proportional and non-proportional loading conditions. This material possesses severe plastic anisotropy and tension-compression asymmetry that evolve with plastic deformation and is an excellent candidate to experimentally evaluate phenomenological fracture modelling strategies. Different types of specimen geometries were fabricated in different orientations with respect to the rolling direction of the sheet to reveal the anisotropic fracture response of the alloy. Moreover, three different types of plane-strain tension tests, namely, v-bend, butterfly, and Nakazima dome tests were conducted and compared in terms of their applicability for fracture characterization of sheet materials. To visualize directional dependency of the fracture response of the magnesium alloy, experimental fracture loci for different orientations were constructed. Furthermore, non-proportional tests were performed in which abrupt changes in stress state were imposed to study the role of the loading history on fracture behaviour of the alloy. The non-proportional tests entailed pre-straining the material in uniaxial and equi-biaxial tension up to a prescribed plastic work level, followed by extreme strain path changes to plane-strain tension and shear states. Non-proportional deformations with such severe strain path variations have not been reported in the literature for materials with complex anisotropic behaviour such as ZEK100-O. The results of which have enabled the direct experimental evaluation of phenomenological damage models without performing an inverse calibration from finite element simulations. Based on the results of the non-proportional tests, it was shown that simple damage indicators were unable to describe the influence of severe changes in the strain path on fracture.Cosma InternationalAutomotive Partnership CanadaOntario Research FundNatural Sciences and Engineering Research Council of CanadaCanada Research Chairs SecretariatCanada Foundation for Innovatio

Experimental Techniques for Finite Shear Strain Measurement within Two Advanced High Strength Steels

This Preprint article has been submitted for consideration.There is a growing need to experimentally characterize local shear deformation in advanced high strength steels (AHSS) for the calibration of stress-state dependent fracture criteria and to better understand sheared edge cracking during secondary forming operations. Planar shear test specimens with digital image correlation (DIC) strain measurement are now commonly performed tests but may not be able to resolve the local strains during the final stage of fracture when the macroscopic shear band collapses to a micro-shear band with intense local strains. Studies of sheared edge stretching of AHSS have shown that the microstructure at the sheared edge experiences extreme local shear deformation with the shear-affected zone (SAZ) that can be much larger than the macroscopic strains reported using DIC on planar shear tests. In this work, two independent experimental techniques are proposed to characterize the residual strain distribution within the shear-affected zone for two AHSS grades with a similar strength level: a complex-phase (CP) steel, CP800, and a dual-phase (DP) steel, DP780. The first method uses finite strain theory to calculate the work-conjugate equivalent strain from grain rotations within the shear bands of interrupted in-plane shear tests. A comparison between the local DIC strain measurements and the grain rotation measurements were found to be in excellent agreement until just prior to failure. The second technique used micro-hardness measurements taken from the interrupted shear tests to develop correlations with the measured equivalent strain from the DIC system. The hardness and grain rotation techniques were then used to characterize the local strain distribution within the SAZ of hole expansion test specimens for punch clearances of 12% and 28%. Both methods provided similar strain distributions with the grain rotation method having the highest strain resolution. The residual strain field within the SAZ of both AHSS was found to be strongly dependent upon the punch clearance. Finally, a homogenization procedure was applied to the SAZ strain distributions to facilitate the length scale transition from the grain-level to length scales appropriate for finite-element modelling of sheet metal forming operations with sheared edges.Natural Sciences and Engineering Research Council of Canada (NSERC) AUTO21 Network of Centres of Excellence Canada Research Chairs Secretaria

Failure parameter identification and validation for a dual-phase 780 steel sheet

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.ijsolstr.2017.06.018 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/A hybrid experimental-numerical procedure was implemented to determine the failure surface of a dual-phase 780 steel sheet as a function of the effective plastic strain, triaxiality, and Lode parameter using butterfly specimens with in situ digital image correlation strain measurement and supporting finite element calculations. A butterfly-type test specimen was employed to experimentally obtain stress states ranging from simple shear to plane strain tension including mixed tensile and shear loading. The numerically-derived failure surface was implemented into the phenomenological GISSMO damage model in the commercial finite element code LS-DYNA and the accuracy of the failure surface was determined using finite element predictions of the characterization experiments. A series of independent validation experiments related to sheet metal forming were performed including a hole tension test, a conical and flat punch hole expansion test, and a hemispherical punch test. The finite element models utilizing the damage model were able to accurately reproduce the load–displacement and surface strains of the sheet material for both the characterization and validation experiments. Prediction of the failure orientation and location compared favorably to each of the validation tests

Constitutive characterization of a rare-earth magnesium alloy sheet (ZEK100-O) in shear loading: Studies of anisotropy and rate sensitivity

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.ijmecsci.2017.04.013 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/Rare-earth magnesium alloys such as ZEK100-O offer improved ductility over other conventional magnesium alloys at room temperature; however, they exhibit significant anisotropy and complex yield behaviour. In this work, a systematic investigation of anisotropy of ZEK100-O rolled sheet in shear loading was conducted at room temperature under quasi-static conditions, as the shear deformation of these alloys is not well understood. Furthermore, uniaxial tensile and compressive characterization of the material was performed to provide context for its behaviour under shear loading. It was revealed that ZEK100-O exhibits strong anisotropy in shear which is markedly different than tensile anisotropy with unique trends that suggest the activation of different deformation mechanisms. To characterize shear anisotropy in HCP materials such as ZEK100-O, the shear response of the material should be investigated in three orientations of 0° (or 90°), 45°, and 135° with respect to the rolling direction. The selection and analysis of these directions is discussed in terms of the principal stress directions and activation of different deformations mechanisms. In order to further investigate this behaviour, the microstructure of the deformed specimens was studied using Electron Backscattered Diffraction (EBSD) analysis to quantify the active twinning systems in different test orientations. Moreover, the CPB06 yield criterion with two linear transformations was calibrated with experimental data to describe the complex anisotropic behaviour of ZEK100-O. It was established that the material exhibits asymmetry not only in tension-compression regions represented by the 1st and 3rd quadrants of yield locus but also in shear regions represented by the 2nd and 4th quadrants. Finally, the strain rate sensitivity of ZEK100-O was studied in shear tests at elevated strain rates of 10s−1 and 100s−1, at which positive rate sensitivity was observed.Cosma International/Automotive Partnership Canada/Natural Sciences and Engineering Research Council of Canada [APCPJ 417811-11]Ontario Research Fund [RE01-054]Canada Research Chairs Secretariat [950-220425]Canada Foundation for Innovation (30337

Metallic Multi-Material Adhesive Joint Testing and Modeling for Vehicle Lightweighting

The final publication is available at Elsevier via https://doi.org/10.1016/j.ijadhadh.2019.102421. © 2019. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/While adhesive bonding has been shown to be a beneficial technique to join multi-material automotive bodies-in-white, quantitatively assessing the effect of adherend response on the ultimate strength of adhesively bonded joints is necessary for accurate joint design. In the current study, thin adherend single lap shear testing was carried out using three sheet metals used to replace mild steel when lightweighting automotive structures: hot stamped Usibor® 1500 AS ultra-high strength steel (UHSS), aluminum (AA5182), and magnesium (ZEK 100). Six combinations of single and multi-material samples were bonded with a one-part toughed structural epoxy adhesive and experimentally tested to measure the force, displacement across the bond line, and joint rotation during loading. Finite element models of each test were analyzed using LS-DYNA to quantitatively assess the effects of the mode mixity on ultimate joint failure. The adherends were modeled with shell elements and a cohesive zone model was implemented using bulk material properties for the adhesive to allow full three-dimensional analysis of the test, while still being computationally efficient. The UHSS-UHSS joint strength (27.2 MPa; SD 0.6 MPa) was significantly higher than all other material combinations, with joint strengths between 17.9 MPa (SD 0.9 MPa) and 23.9 MPa (SD 1.4 MPa). The models predicted the test response (average R2 of 0.86) including the bending deformation of the adherends, which led to mixed mode loading of the adhesive. The critical cohesive element in the UHSS-UHSS simulation predicted 85% Mode II loading at failure while the other material combinations predicted between 41% and 53% Mode II loading at failure, explaining the higher failure strength in the UHSS-UHSS joint. This study presents a computational method to predict adhesive joint response and failure in multi-material structures, and highlights the importance of the adherend bending stiffness and on joint rotation and ultimate joint strength