Fadi Abu-Farha

Dynamic Tensile Behavior of a Quenched and Partitioned High Strength Steel using a Kolsky Bar

Dynamic tension tests were performed on a quenched and partitioned high strength steel grade, QP980, using a direct tension Kolsky Bar method. In this method, the steel incident bar consists of a tube section and a solid section of equal impedance mated through a threaded connection. The striker is pneumatically launched within the tube section into an impact cap to create the tensile loading pulse. The transmission bar, which is constructed of aluminum to improve the force measurement sensitivity, is not impedance matched to the incident bar, and as a result the wave analysis technique was modified accordingly. The sample geometry follows ISO 26203-1:2010. Strain-time histories of the specimens obtained by the wave analysis were compared to high speed DIC strain field measurements, and the latter were used to correct the compliance of the test setup. Material tests were performed parallel to, perpendicular to, and at 45° with respect to the rolling direction. Specimens were taken to failure and to several intermediate strain levels by using momentum traps on the incident and transmission bars. Specimen gauge length, gas pressure and striker bar length were changed to achieve different strain rates, covering the range needed for crash simulations. The dynamic behavior of the material is compared to its quasi-static behavior.

Microstructural Evolution and Its Relationship to the Mechanical Properties of Mg AZ31B Friction Stir Back Extruded Tubes

Friction stir back extrusion (FSBE) was recently introduced as a promising energy-efficient technique for preparing finegrained lightweight alloy tubes; this work focuses on quantifying the effects of the process on the material’s microstructure and exploring the potentials for enhancing its performance. AZ31B-F magnesium round bars are processed into tubes by FSBE at selected process conditions. Microstructural examination is carried out at key locations across the processed tubes, in order to track grain size and texture evolution in the material. FSBE is shown to produce sound tubes with significant grain refinement and homogenisation in the processed material (from a heterogeneous coarse-grained microstructure with a bimodal ~55–115μm, to a homogeneous fine-grained structure with ~5μm average grain size), which is carried on to the walls of the formed tubes. Macro-mechanical testing is carried out on the formed tubes; the changes in the material’s mechanical properties are correlated with its microstructural evolution.

Effects of Microstructure on the Strain Rate Sensitivity of Advanced Steels

The dependence of the strain rate sensitivity of advanced ~1 GPa tensile strength steels on the phases present in their microstructures was studied by testing different steels at 0.005 and 500 s−1. The high strain rate tests were performed using a Kolsky bar setup, while the quasi-static tests were performed using a universal testing machine. The two main steels of interest were the Ferrite-Martensite DP980 and the Ferrite-Martensite-Austenite QP980; the latter being a transformation induced plasticity (TRIP) assisted steel. For comparison, ferritic CR5 mild steel and austenitic stainless steel 201 were also tested under the same conditions. Though the differences in the steel chemistries were not taken into account, the results obtained here suggest a strong relationship between the phase-content of the steel and its response to the changes in the loading rate. The relationships between the observed mechanical behavior and the phases present in the microstructure are discussed.

Friction Stir Back Extrusion of Mg AZ31B-F: a Preliminary Investigation

The limited room-temperature ductility of magnesium and its alloys inhibits their fabrication without the introduction of heat to the process; that’s why magnesium alloys are typically associated with warm or hot forming operations. This work presents a unique approach to heating magnesium alloys during bulk forming by utilising “friction stirring”; the same phenomenon on which friction stir welding (FSW) and friction stir processing (FSP) are based. The proposed friction stirring can be combined with particular forms of forging and back extrusion, thus generating the heat needed to soften the metal and ease its deformation. Moreover, and just like the case with FSW and FSP, the proposed bulk friction stir forming processes have the additional advantage of refining the grainstructure of the processed material, and thus the potential to enhance its performance. A close look is provided here to a particular form of bulk processing, referred to as friction stir back extrusion (FSBE). Magnesium AZ31B-F round bar samples are processed by FSBE at selected set of conditions producing sound tubular specimens. Microstructural examination and microhardness testing are performed at key locations to examine the changes to microstructure of the material due to processing. The preliminary results shown here provide a general validation to the proposed processing concept, showing significant grain refinement in the material.

Opportunities for Inverse Analysis in Dynamic Tensile Testing

Dynamic tensile testing using Kolsky Bar methods are used to assess crashworthiness of new structural materials needed for lightweight automotive design. High speed Digital Image Correlation (DIC) measurements routinely show that the strain experienced by dynamic tensile specimens deviates markedly from what is expected from the original strain wave analysis techniques used in tensile Kolsky bar metrology. Deviations can be manifest either by different average strain values over the gage section, or by departures from strain uniformity, or both. The former can be attributed to plastic yielding in the specimen outside the gauge section, while the latter concerns specimen geometry and material hardening effects. These issues are sometimes difficult to eliminate through simple modifications of the sample or the test design. Finally, it is of interest to make use of the data beyond necking, where the strain state departs significantly from ideal conditions. These metrology issues lend themselves to solution by inverse methods, where full field strain measurements and global load measurement data are available. In this paper we describe typical measurement data and explore methods to identify the constitutive response from dynamic tensile tests.

Revealing Dynamic Banding During High Temperature Deformation of Lightweight Materials Using Digital Image Correlation

In this work, digital image correlation (DIC) is used to investigate the unique banding behaviors of selected steel and aluminium alloy sheets at various temperatures. The materials are deformed in simple tension at different temperatures ranging between −50 and 300 °C. DIC is integrated in the tension testing setup, and the impact of the testing setup (glass and heat) on the accuracy of the measurements is evaluated. DIC reveals the accumulation of localized plastic strains and strain rates, as well as their evolution during testing. Depending on the mechanism of mobility of the material surface during deformation, different dynamic localization bands are noted. Selected examples highlighting a dramatic shift in the dynamic banding behavior in these materials are discussed. The correlations between the bands revealed in DIC measured strain and strain rate maps, and the corresponding macro-mechanical stress-strain behaviors are explored for different types of bands.

Modification of Yoshida-Uemori Model with Consideration of Transformation-Induced Plasticity Effect

Transformation-induced plasticity (TRIP) assisted steels possess improved strain hardening behavior and resistance to necking that are favorable for automotive body applications. However, the TRIP effect causes complex springback behavior of these steels that can hardly be predicted by existing constitutive models for other steels. In this work, the functions in the original Yoshida-Uemori model describing isotropic and kinematic hardening were modified by adding new parameters that can represent the TRIP effect. Cyclic tension/compression experiments were performed on a selected TRIP-steel grade, and the results were used to calibrate the modified model. The modified model was coded via user subroutine into a commercial FE solver. The springback predictions were compared with actual try-out stamping experimental results for highlighting the improvement of predictions with the modified model.