Application of bi-linear log–log S–N model to strain-controlled fatigue data of aluminum alloys and its effect on life predictions

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

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

In this paper, we will give a p-version finite element method for the one-dimensional nonlinear elastoplasticity problem. A family of admissible constitutive laws based on the so-called gauge function method is introduced first, and then a p-version semi-discretization scheme is presented. We show that the existence and uniqueness of the solution for the semi-discrete problem can be guaranteed by using some special properties of the constitutive law. Finally, we show that as the polynomial degree of the elements ${\bf p} \to \infty $ oo the solution of the semi-discrete problem will converge to the solution of the continuous problem. The p-version convergence analysis given here is only for one-dimensional problems with a linear hardening constitutive law hypothesis. However, it also gives us a very useful idea for the study of the hp-version finite element method for the one-dimensional problem with more general constitutive laws or even two-dimensional problems.

A convergence analysis of a P-version finite element method for one-dimensional elastoplasticity problem with constitutive laws based on the gauge function method

The present work encompasses the effect of overaging on the strain-controlled low-cycle fatigue (LCF) behavior of an extruded AA6061 aluminum alloy at varying strain amplitudes. During the T7 aging treatment, the size of precipitates increased from 60 nm under T6 conditions to 220 nm after aging for 48 h at 200 °C, leading to a decrease in the monotonic tensile strength. During the LCF tests, nearly symmetrical hysteresis loops can be observed in the mid-life cycle under all test conditions, whereas the first-cycle hysteresis loops were moderately inflected under long-aging conditions. With increasing aging time, the cyclic peak stresses decreased and the plastic strain increased. Nearly ideal Masing behavior was exhibited under T6 conditions, while it was lost under T7 overaging conditions. The cyclic stress responses were similar under all tested conditions, involving stabilization at low strain amplitudes and softening at high strain amplitudes, with initial hardening for the first few cycles. Compared to the T6 condition, the fatigue life increased with increasing T7 aging time. Various LCF parameters were estimated based on the Coffin-Manson and Basquin relationships and on the LCF experimental results. The relationship between the fatigue life, strength, and microstructure of the investigated AA6061 aluminum alloy under various aging conditions was discussed.

Effect of Overaging on the Cyclic Deformation Behavior of an AA6061 Aluminum Alloy

In order to consider cyclic material behavior in the course of the design process of cyclically loaded components and safety relevant parts, the importance of local strain-based fatigue de...

Fatigue life curve – A continuous Wöhler curve from LCF to VHCF

Monotonic and cyclic strain-controlled fatigue tests were conducted at room temperature on several heat-treatable and non-heat-treatable aluminum alloys of interest to the transportation industry. Extrusion, forging, plate, and sheet product-forms were tested. Various monotonic and cyclic fatigue properties are furnished, and the respective strain-life curves are compared. In general, materials with strong monotonic strain-hardening tendencies are also shown to exhibit marked cyclic hardening.

R. H. Stentz

Papers

Tensile and strain-controlled fatigue data for certain aluminum alloys for application in the transportation industry