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Technical Paper

Corrosion Fatigue and Stress Corrosion Cracking of AZ91E-T6 Cast Magnesium Alloy in 3.5% NaCl Solution

The objective of this research was to obtain and compare fracture toughness, stress corrosion cracking and constant and variable amplitude fatigue behavior of AZ91E-T6 cast magnesium alloy in both an air and 3.5% NaC1 corrosive environment. An additional objective was to determine if commonly used models that describe fatigue behavior and fatigue life are applicable to this material and test environments. Fatigue tests included constant amplitude strain-controlled low cycle fatigue with strain ratio, R, equal to 0,−1 and −2, Region II constant amplitude fatigue crack growth with load ratio, R, equal to 0.05 and 0.5 and variable amplitude fatigue tests using keyhole notched specimens. In all fatigue tests, the corrosion environment was significantly detrimental relative to the air environment. The material was also susceptible to stress corrosion cracking. Low cycle fatigue models and the Paris equation properly represented the fatigue data in both environments.
Technical Paper

Fatigue Crack Growth of A356-T6 Cast Aluminum Alloy

Fatigue crack growth behavior was obtained for the SAEFDE Committee's round-robin A356-T6 cast aluminum alloy program with crack growth rates between 10−11 and 10−6 m/cycle for R-ratios equal to 0.1 and 0.5. Three different mold temperatures resulted in secondary dendrite arm spacings (DAS) that varied from approximately 80 to 90 µm, resulting in only coarse microstructure. Threshold levels, ΔKth, and the Paris exponent, m, were approximately twice the values usually found for wrought aluminum alloys. The influence of R-ratio was quite pronounced and crack closure, as measured with a crack mouth COD gage, did not eliminate all threshold and near-threshold R-ratio differences. Roughness-induced crack closure appeared to be more important than plasticity-induced closure.
Technical Paper

Fatigue and Fracture Toughness of Water-Chilled A356-T6 Cast Aluminum Alloy

A continuation of the SAEFDE round-robin fatigue test program was conducted to determine the influence of a finer microstructure on monotonic tension, strain-controlled low cycle fatigue, fatigue crack growth, and fracture toughness of A356-T6 cast aluminum alloy. The finer microstructure castings, referred to as material W, were obtained using a water-chilled sand casting procedure. Material W exhibited more desirable ductile behavior than the previous SAEFDE materials X, Y, and Z. Material W exhibited superior smooth specimen low cycle fatigue resistance at both short and long lives, when compared to materials X, Y, and Z. This was due in part to the higher ductility and lower porosity of material W over materials X, Y, and Z. Material W exhibited similar fatigue crack growth behavior, and slightly higher values of fracture toughness at the same thickness when compared to materials X, Y, and Z.
Technical Paper

Fatigue of High Strength Bolts Rolled Before or After Heat Treatment with Five Different Preload Levels

SI property class 12.9 high strength steel bolts were used to investigate the fatigue behavior of bolt threads rolled before/after heat treatment using two different thread profiles and five different preload values. Bolts were 3/8 UNRC-16 (coarse) and 3/8 UNRF-24 (fine) and preloads were taken as 1, 50, 75, 90, and 100% of roll before heat treatment proof stress. Maximum near surface residual compressive stresses, obtained via x-ray diffraction, ranged from -500 to -1000 MPa. Axial loads were applied through the nut and all fatigue failures occurred at the first thread of the nut/bolt interface. SEM evaluation indicated all fatigue crack growth regions contained multiple fatigue facets, while final fracture regions were ductile dimples.
Technical Paper

Improvements in Empirical Representation of A356-T6 Cast Aluminum Alloy Round-Robin Low Cycle Fatigue Data

The results of the SAEFDE Committee's round robin low cycle fatigue test program with A356-T6 cast aluminum alloy indicated that the conventional low cycle fatigue model was not a satisfactory representation of the data. This occurred because the elastic strain amplitude-life curve was not log-log linear and this yielded a non-conservative fatigue life representation at both extremes of long and short lives. This paper involves a reanalysis of the A356-T6 composite all-laboratory data using two additional empirical models. These models are: 1. linear log-log total strain amplitude-life 2. bilinear log-log elastic strain amplitude-life Both proposed empirical models improve the representation of the data compared to the conventional low cycle fatigue model. The bi-linear log-log elastic equation, however, when added to the plastic equation, yields a discontinuous curve with non-conservatism in the region of the discontinuity.
Technical Paper

Low Cycle Fatigue Behavior and Variable Amplitude Fatigue Life Calculations for an SRIM Polymer Matrix Composite

The objective of this research was to determine the feasibility of applying strain based fatigue life calculation models, which are commonly used for metals, to smooth SRIM polymer matrix composite axial specimens subjected to variable amplitude loading. A thorough investigation of the monotonic and strain controlled constant amplitude low cycle fatigue behavior of this material was conducted, including the effects of mean strains/stresses on the fatigue life of smooth specimens. Using these results, mean stress life calculations were made on the constant amplitude tests, as well as on smooth specimens subjected to strain controlled variable amplitude loading, using the Morrow and SWT mean stress models. These results were compared to experimental data, and it was found that the correlation between experimental and calculated lives was very poor, for both the constant amplitude and variable amplitude tests.
Technical Paper

Low Cycle Fatigue of A356-T6 Cast Aluminum Alloy

Smooth specimen low cycle fatigue behavior of A356-T6 cast aluminum alloy with secondary dendritic arm spacing (DAS) of 36 and 58 µm was obtained under room temperature test conditions. Under strain-controlled axial fatigue conditions, cyclic strain hardening occurred for both DAS values. Hysteresis loops were skewed from 1 to 13% toward the compression side. At higher strain amplitudes the smaller DAS had slightly better fatigue resistance while at the lower strain amplitudes the fatigue resistance was essentially the same for both DAS values. Many test specimens failed outside the test gage length and were thus considered invalid tests. This large number of invalid tests was attributed to surface and near surface discontinuities. Log-log linear low cycle fatigue properties were obtained but these values should be used with caution due to apparent bi-linear log-log elastic strain-life behavior.
Technical Paper

Low Cycle Fatigue of A356-T6 Cast Aluminum Alloy - A Round-Robin Test Program

A round-robin low cycle fatigue test program was conducted by the SAEFDE Committee using A356-T6 cast aluminum alloy. Three different microstructures representative of three solidification rates were sought, but only two significantly different secondary dendrite arm spacings, DAS, resulted. The smaller DAS had slightly greater monotonic yieid and ultimate strengths, greater permanent deformation at fracture and better low cycle fatigue resistance. Under strain-controlled axial low cycle fatigue conditions. A356-T6 was observed to cyclicaiiy strain harden and hysteresis loops were skewed toward the compressive stress from about 1 to 10 percent. Fatigue failures usually initiated at surface or near surface porosity. About 25 percent of the 173 test specimens that were considered valid failed between the strain gage knife edges and the specimen fillet radius.
Technical Paper

Low Cycle Fatigue of A356-T6 Cast Aluminum Alloy Wheels

Axial strain-controlled low cycle fatigue behavior was obtained from smooth specimens machined from spokes of A356-T6 cast aluminum alloy wheels. Two different foundries cast the wheels. Three wheels were used from one production run at one foundry and two wheels were used from two different production runs at the other foundry. Specimens from the three wheels of the same production run had essentially the same monotonic tensile properties and low cycle fatigue resistance. Specimens from the two wheels of the different production runs had different monotonic tensile properties and different low cycle fatigue resistance. All these A356-T6 wheel specimens cyclic strain harden with hysteresis loops typically offset to the compression side by five percent or less. The usual log-log linear model for low cycle fatigue adequately described the low cycle fatigue behavior.
Technical Paper

Low Temperature Fatigue Behavior of Steels - A Review

This paper consists of a comprehensive literature review of constant amplitude fatigue behavior at low temperatures in steels. S-N, low cycle ε-N, and fatigue crack growth da/dN results are included. Unnotched, notched and cracked specimen fatigue behavior is presented. In many cases the constant amplitude fatigue resistance is enhanced at low temperatures. A sufficient number of situations exist, however, to indicate this is not always true. Poor low temperature fatigue behavior in steel is usually due to a change from ductile to brittle fatigue mechanisms. A fatigue transition temperature which can be significantly different than NDT values exists in many steels.
Technical Paper

The Effect of High Tensile Mean Stress on Fatigue of Unnotched and Notched SAE 1045 Steel

The objective of this research was to determine fatigue behavior of SAE 1045 steel subjected to very high tensile mean stress for unnotched, mildly notched, and sharply notched test specimens, and to determine if common S-Nf and ε-Nf mean stress fatigue life models are applicable. High tensile mean stress fatigue tests for R ratios of 0.8 and 0.9 were conducted using unnotched and notched, Kt=1.65 and Kt=3.65, axial loaded SAE 1045 steel specimens with hardness levels of Rc=10, 37, and 50. The monotonic notch strength ratio, NSR, for 5 of 6 test conditions was greater than 1, which allowed many notched cyclic test values of Smax or Sm to exceed the unnotched ultimate tensile strength. Much notched specimen fatigue resistance at these high R ratios was superior to that of unnotched specimens. However, cyclic creep/ratcheting, particularly for Rc=10 and 37, was a predominant cause of failure.
Technical Paper

Variable Amplitude History Editing and Accelerated Testing of SAE 1045 Steel

The objective of this research was to determine if strain gate and SWT parameter history editing techniques can be used to accurately edit a variable amplitude strain history to predetermined damage levels. The log skidder bending strain history with 62 672 reversals was used as the full length variable amplitude history. By editing small damage cycles to 50% and 90% retained damage levels, the edited histories were reduced to between 0.3 and 11 percent of the full length history. Comparisons involving calculated and experimental fatigue lives of smooth uniaxial SAE 1045 steel specimens subjected to full length and edited histories were used to evaluate the editing techniques. Both the strain gate and SWT parameter editing techniques successfully edited the predicted amount of damage. These strain based techniques for removing small damage cycles are thus recommended for practical testing applications.