J.B. Jordon

Strain-Controlled Low-Cycle Fatigue Properties of Extruded 6061-T6 Aluminum Alloy

One of the most commonly extruded aluminum alloys is the 6061 alloy largely because of its good formability and high specific strength (Ref 1). The ability of the 6061 aluminum alloy to be extruded or otherwise formed into complex geometries at relatively low cost is the reason for its widespread use in aerospace, construction, transportation, and many other industries. In fact, 6061 aluminum alloy has been produced in various forms including plate, extrusion, foil, sheets, pipes, forgings, and structural forms (Ref 2). Since most load bearing components fail due to cyclic loading, analytical prediction and finite element modeling of fatigue damage of such a prevalent alloy is critical for safe designs. A review of literature regarding fatigue in 6061 aluminum alloys reveals most experimental studies characterized fatigue behavior based on load-control cyclic tests. However, for applications where low-cycle fatigue (LCF) is dominant with variable-amplitude histories and sequence effects, strain-controlled fatigue tests better characterize the fatigue behavior compared to stress-controlled tests (Ref 3). However, only a few data sets on strain-controlled fatigue of 6061 aluminum alloy exist and are not easily located, with the one data set published in an obscure journal (Ref 4), and the other published in a conference proceeding (Ref 5). The purpose of this paper is to provide readily available LCF properties of an extruded 6061-T6 extruded aluminum alloy and to validate the results of Ref 5. Furthermore, the cyclic stress-strain behavior, including initial and stabilized hysteresis loops, is not reported in literature and thus will be presented here. The material used in this study was an extruded 6061-T6 aluminum alloy. Taber Extrusion, LLC (Russellville, AR) provided the extruded panels. The nominal composition in atomic weight of the 6061 aluminum alloy is given in Table 1 (Ref 6). For comparison purposes, Table 2 lists the monotonic properties of the extruded 6061-T6 aluminum alloy. Cylindrical specimens were machined from the extruded alloy so that the loading axis was parallel to the extrusion direction (ED). The fatigue specimens were designed following ASTM Standard E.606 specifications (Ref 7) with a nominal 27-mm gage length and diameter of 6.35 mm. Prior to testing, all specimens were hand ground in the loading directionwith 800-grit silicon carbide paper to remove residual stresses and any machining marks. Fully reversed cyclic (R = 1) tests were performed using a servo-hydraulic MTS load frame at room temperature with relative humidity level of 45%. The fatigue specimens were tested to failure under strain-controlled conditions at 5 Hz frequency at the following strain amplitudes: 0.002, 0.003, 0.004, 0.005, 0.006, and 0.007. The specimen tested at 0.002 strain amplitude was fatigued at 5 Hz to 20,000 cycles, and then the test was stopped and resumed in load control at 30 Hz to failure. A fatigue-rated MTS extensometer with a 25.4-mm length gage was used to directly measure the axial strain induced in the gage length. Final failure of the specimen was defined as a 50% drop in peak load during the test, as recommended by ASTM Standard E.606 (Ref 7). Figure 1 shows the results of the strain-controlled fatigue tests conducted on the extruded 6061-T6 aluminum alloy. Note that the 0.002 amplitude shown in Fig. 1 did not fail but is plotted here as a run-out. Also shown in Fig. 1 is the strain-life equation (Ref 3), including the elastic and plastic strain amplitudes, where the total strain-life is the summation of the elastic and plastic strain components. In strain-controlled cyclic deformation, the elastic strain amplitude is more dominant in small strains or longer lives, and the plastic strain amplitude is particularly dominant in large strains or short lives (Ref 3). The total fatigue life is shown in Eq 1

A Fatigue Model for Discontinuous Particulate-Reinforced Aluminum Alloy Composite: Influence of Microstructure

In this paper, the use of a microstructure-sensitive fatigue model is put forth for the analysis of discontinuously reinforced aluminum alloy metal matrix composite. The fatigue model was used for a ceramic particle-reinforced aluminum alloy deformed under conditions of fully reversed strain control. Experimental results revealed the aluminum alloy to be strongly influenced by volume fraction of the particulate reinforcement phase under conditions of strain-controlled fatigue. The model safely characterizes the evolution of fatigue damage in this aluminum alloy composite into the distinct stages of crack initiation and crack growth culminating in failure. The model is able to capture the specific influence of particle volume fraction, particle size, and nearest neighbor distance in quantifying fatigue life. The model yields good results for correlation of the predicted results with the experimental test results on the fatigue behavior of the chosen aluminum alloy for two different percentages of the ceramic particle reinforcement. Further, the model illustrates that both particle size and volume fraction are key factors that govern fatigue lifetime. This conclusion is well supported by fractographic observations of the cyclically deformed and failed specimens.

Microstructure-Sensitive Fatigue Modeling of AISI 4140 Steel

A microstructure-based fatigue model is employed to predict fatigue damage in 4140 steel. Fully reversed, strain control fatigue tests were conducted at various strain amplitudes and scanning electron microscopy was employed to establish structure-property relations between the microstructure and cyclic damage. Fatigue cracks were found to initiate from particles near the free surface of the specimens. In addition, fatigue striations were found to originate from these particles and grew radially outward. The fatigue model used in this study captured the microstructural effects and mechanics of nucleation and growth observed in this ferrous metal. Good correlation of the number of cycles to failure between the experimental results and the model were achieved. Based on analysis of the mechanical testing, fractography and modeling, the fatigue life of the 4140 steel is estimated to comprise mainly of small crack growth in the low cycle regime and crack incubation in the high cycle fatigue regime.

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.

Finite Element Analysis of Self-Pierce Riveting in Magnesium Alloys Sheets

Conventional fusion joining methods such as resistance spot welding have been demonstrated to not be effective for magnesium alloys. Therefore, self-pierce riveting (SPR) has been presented as an attractive joining technique for these lightweight metals. However, SPR must be performed at elevated temperatures because of the low ductility of magnesium alloys at room temperature. Even though the SPR joining process has been established on magnesium alloys, this joining process is not optimized. As such, this study establishes the first attempt at simulating the SPR of magnesium alloys through the use of the finite element method. An internal state variable (ISV) plasticity and damage material model was employed with results in good agreement to experimental data. The results of this study show that the ISV material model is ideally suited for modeling the SPR process in magnesium alloys.

Fatigue Behavior of Friction Stir Linear Welded Dissimilar Aluminum-to-Magnesium Alloys

In this paper, we present the results of fatigue testing and analysis of friction stir linear welded dissimilar aluminum-to-magnesium alloys in lap-shear configuration. The overlap linear welds were created by joining AA6022 aluminum alloy to AM60 magnesium alloy. In general, the test data exhibited significant scatter in the fatigue life results and the corresponding failure modes. In fact, observations from fractography analysis revealed two distinct modes of failure. In the first mode of failure observed, fracture occurred when the dominant fatigue crack propagated into either the magnesium or aluminum sheet in a kinked crack formation. Interestingly, frettinglike debris was observed at the initiation sites for this failure mode. In the second mode of failure observed, fracture occurred by interfacial weld separation. In this mode, fractography analysis suggests that the fatigue cracks initiated at weld defects and then propagated through the intermetallic phase.

Fatigue Evaluation of Friction Stir Spot Welds in Magnesium Sheets

In this study, the fatigue behavior of a magnesium AZ31 alloy in friction stir spot welds is experimentally characterized. Load control cyclic tests were conducted on single weld lap-shear coupons to determine fatigue crack characteristics. Cyclic failure modes included weld nugget pullout and full coupon separation. Good correlation was obtained between fatigue life and linear elastic fracture mechanics. In addition, in-situ fatigue tests were conducted to determine crack growth rates of the friction stir spot welds via an automated digital microscope. The experimentally measured crack growth rate suggests that the assumption of constant crack growth employed in literature is reasonable.

A Multi-stage Approach for Predicting Fatigue Damage in Friction Stir Spot Welded Joints of Mg AZ31 Alloy

In this work, we propose a model for predicting fatigue damage in friction stir spot welded (FSSW) joints made of Mg AZ31 alloy. In this modeling approach, an attempt is made to capture failure mechanisms due to the influence of variation in welding parameters including tool plunge depth, tool rotation speed, and tool pin diameter. As such, the fatigue model presented here is a deterministic approach, where fatigue lifetimes are estimated based on specific geometrical and micro structural information. In particular, the model addresses the observed variation in failure mechanisms commonly observed in Mg FSSW coupons under a range of applied loading. Further, a distinction is made between fatigue crack incubation, micro structural small and physically small fatigue crack growth, and finally long crack growth of the coupon. The fatigue model presented here showed good correlation for fatigue lifetimes for variation in welding conditions.

Effect of Weld Structure on Fatigue Life of Friction Stir Spot Welding in Magnesium AZ31 Alloy

In this paper the fatigue behavior in friction stir spot welded coupons of magnesium AZ31 alloy manufactured under different welding conditions are investigated. Two sets of lap-shear coupons were welded based on varying the plunge depth and tool geometry. Metallographic analysis of the untested lap-welds revealed differences in microstructural and geometrical features. Results from the load controlled cyclic tests showed that one set of welds exhibited better fatigue performance compared to the other set. Optical fractography of the failed fatigue coupons revealed that fatigue cracks initiated at the weld interface in both sets of coupons. However, the fracture mode showed variability between the two sets of coupons. As such, the main conclusion of this study is that the effective top sheet thickness, which is largely determined by the shoulder plunge depth, plays a significant role in the fatigue behavior of the friction stir spot welds in magnesium alloys.

Experiments and Modeling of Fatigue Damage in Extruded Mg AZ61 Alloy

In this study, structure-property relations with respect to fatigue of an extruded AZ61 magnesium alloy were experimentally quantified. Strain-life experiments were conducted in the extruded and transverse orientations under low and high cycle conditions. The cyclic behavior of this alloy displayed varying degrees of cyclic hardening depending on the strain amplitude and the specimen orientation. The fracture surfaces of the fatigued specimens were analyzed using a scanning electron microscope in order to quantify structure-property relations with respect to number of cycles to failure. Intermetallic particles were found to be the source of fatigue initiation on a majority of fracture surfaces. Finally, a multistage fatigue model based on the relative microstructural sensitive features quantified in this study was employed to capture the anisotropic fatigue damage of the AZ61 magnesium alloy.