Jafar Albinmousa

Monotonic and Fatigue Behavior of Magnesium Extrusion Alloy AM30: An International Benchmark Test in the "Magnesium Front End Research and Development Project"

ABSTRACT Magnesium alloys are the lightest structural metal andrecently attention has been focused on using them forstructural automotive components. Fatigue and durabilitystudies are essential in the design of these load-bearingcomponents.In 2006, a large multinational research effort, MagnesiumFront End Research & Development (MFERD), waslaunched involving researchers from Canada, China and theUS. The MFERD project is intended to investigate theapplicability of Mg-alloys as lightweight materials forautomotive body structures. The participating institutions infatigue and durability studies were the University of Waterlooand Ryerson University from Canada, Institute of MetalResearch (IMR) from China, and Mississippi StateUniversity, Westmorland, General Motors Corporation, FordMotor Company and Chrysler Group LLC from the UnitedStates. This paper presents the results of benchmark coupontesting that were obtained for monotonic and cyclicconditions on extruded AM30 alloy samples. Tests wereperformed independently in Canada, China, and the US. Ingeneral, the results reported by different institutions were ingood agreement.Microstructure analyses revealed strong material texture witha unique orientation of extension twinning with respect to theinitial basal plane. The cyclic deformation, therefore, wasseen to be dominated by twinning and detwinning. Theunusual asymmetric hysteresis of AM30 observed for fullyreversed cyclic loading is attributed to twinning undercompression in the extrusion direction, detwinning uponunloading from compression and dislocation slip in tension.The monotonic tests were performed under different strainrates and at room temperature or 125°C. Cyclic tests wereperformed under strain controlled conditions. Two strainamplitudes were considered, 0.3% and 0.6% and all fatiguetests were performed under standard laboratory conditions.Raising the temperature from standard laboratory conditionsto 125°C had a significant effect under monotonic loading:both the yield and tensile strength dropped by about 25%,while ductility increased by 300%. Under fatigue loading atroom temperature, extruded AM30 exhibits asymmetricalcyclic behavior at a strain amplitude of 0.6%, whereas thecyclic behavior at 0.3% was symmetric. The material showedsignificant plastic strain recovery, cyclic hardening, and aclear endurance limit.

Investigation on parametric representation of proportional and nonproportional multiaxial fatigue responses

This study aims to investigate fatigue damage resulting from multiaxial fatigue, proportional and/or nonproportional, loading by analyzing stresses and strains over a full domain around a representative stress-strain element. Plain stress-strain transformation of multiaxial hysteresis was performed by varying orientation of plane for 0?fi?2?. Parametric representations for both normal and shear stresses and strains were obtained by plotting them in polar figures. These figures show that depending on the applied loading stresses and strains represent definitive known parametric curves. Parametric representation of fatigue damage parameter such as Fatemi-Socie suggests that fatigue damage shall be calculated as the sum of the damage values on all planes. The proposed technique has been shown to improve fatigue life prediction compared to that obtained from the critical plane method.

Multiaxial fatigue crack path prediction using critical plane concept

Prediction of fatigue crack orientation can be an essential step for estimating fatigue crack path. Critical plane concept is widely used due to its physical basis that fatigue failure is associated with certain plane(s). However, recent investigations suggest that critical plane concept might need revision. In this paper, fatigue experiments that involve careful measurement of fatigue crack were reviewed. Predictions of fatigue crack orientation using critical plane concept were examined. Projected length and angle were used to characterize fatigue crack. Considering the entire fatigue life, this average representation suggests that it is more reasonable to assume the plane of maximum normal strain as the critical plane even though fundamentally the plane of maximum shear strain is more likely to be the critical one at early initiation stage.

Experimental and Numerical Determination of Mixed Mode Crack Extension Angle

Crack initiation angles for mixed mode fracture are measured and predicted using photoelastic and finite element techniques. Different crack inclination angles (0°, 10°, 20°, 30°, and 40°) are considered. The Schroedl and Smith method is used for estimating pure opening mode I stress intensity factor. On the other hand, the Sanford and Dally method is used to estimate stress intensity factors in mixed mode (I and II). The Smith and Smith method for estimating mixed mode SIF is also examined. In addition, the ANSYS finite element code is employed to estimate the pure and mixed mode SIF numerically. The estimated values of the SIF from both finite element analysis and photoelasticity are incorporated into six criteria for crack initiation angle prediction. The applicability of the crack initiation criteria such as maximum tangential stress (MTS), minimum strain energy density (S), maximum dilatational strain (T), maximum triaxial stress (M), modified MTS (MMTS), (R) criterion, and fringe symmetry axis method (FSAM) are investigated. An experimental setup is used to measure the initiation angles at different crack inclinations. The measured crack initiation angles are used to validate the predicted values. The results showed that the initiation angles estimated by different criteria using SIFs determined experimentally and numerically compared well with the measured ones, especially at higher inclination angles.