Higher Total Strain Research Articles

Effect of Higher Strain Range Cycling on Near Fatigue-Limit Fatigue Life

The objective of this study is to clarify the effects of pre-cycling above the fatigue limit on near the fatigue-limit fatigue behaviors of SA508, low alloy steel for nuclear pressure vessels, correlating fatigue lifetime with microstructural changes in the bulk. It has been observed that dislocation cell structure is well developed in an as-received SA508 sample and the misorientation among cells increases with fatigue accumulation during fatigue tests. The cell to cell misorientation was measured and statistically quantified by the Selected Area electron beam Diffraction (SAD). It was shown that a fatigue crack started growing abruptly when the SAD value (the mean misorientation among cells) exceeded a critical angle, 4-5 degrees. Fatigue tests were performed for SA508 samples, first at a high total strain range (0.62%) to 10, 100, and 6000 cycles followed by cycling at near the fatigue-limit stain range (0.40%). The sample with pre-cycling for 100 and 6000 cycles failed while ones with pre-cycling for 10 cycles and without pre-cycling did not rupture till about 10 6 cycles where the tests were terminated. Small surface cracks were found all samples pre-cycled at the high strain range but the cracks were arrested for a long time at the near fatigue limit cycling followed by an abrupt growth at the failure. It was found by the SAD that samples failed at the fatigue limit when the average misorientation among cells exceeds the critical angle regardless of pre-cycling histories. The SAD value changed even during cycling below the fatigue limit, and the change in SAD value was strongly influenced by the number of pre-cycling above the fatigue limit. These changes well agreed with fatigue lifetime of the samples.

The shearing characteristics associated with equal-channel angular pressing

Equal-channel angular (ECA) pressing is a processing method for introducing an ultra-fine grain size into a material. In practice, it is a procedure that may be used to achieve high total strains by subjecting a sample to repetitive pressings. There is experimental evidence showing that the nature of the microstructural evolution in ECA pressing depends upon whether the sample is rotated between each passage through the die. This paper examines the shearing characteristics associated with ECA pressing for six different processing routes and reaches conclusions concerning the optimum processing procedure and the development of texture.

Optimizing the rotation conditions for grain refinement in equal-channel angular pressing

Equal-channel angular (ECA) pressing is rapidly becoming an established procedure for inducing microstructural refinement in polycrystalline materials through the process of intense plastic straining. To date, there have been numerous reports of the use of the ECA pressing procedure to produce grain refinement down to the submicrometer, or occasionally the nanometer, level in a range of metallic materials[1–10] and in some intermetallic compounds.[11,12] In ECA pressing, a sample is pressed through a die having two channels, equal in cross section, which intersect at an angle of F, with simple shear occurring as the sample moves through the angle subtended at the intersection of the two channels.[13,14] Since the cross section of the sample remains unchanged on passage through the die, the procedure is readily amenable to repetitive pressings of the same sample in order to introduce high total strains. Pressings are generally conducted using dies having F . 90 deg, and under these conditions the equivalent strain introduced on each passage through the die is approximately equal to 1.[15] In practice, experiments have demonstrated that the microstructural characteristics introduced by ECA pressing, including the structural homogeneity, the average shapes of the individual grains, and the misorientation angles of the boundaries between adjacent grains, are all dependent upon experimental conditions such as the number of passages through the die,[5,9,10,16] the shearing directions in each separate passage as manifested by any rotation of the sample between separate pressings,[5,9,16] and the temperature at which the straining is conducted.[10] When repetitive pressings are undertaken, it is a standard procedure to identify three different routes, designated A, B, and C, in which the pressings are conducted without any rotation of the sample, with rotation about the longitudinal axis by 90 deg between each pressing and with rotation by 180 deg between each pressing, respectively. Two recent reports have compared directly the efficiency of these different processing routes for establishing a homogeneous structure consisting of an array of equiaxed grains. In experiments by Ferrasse et al.[9] on Cu and Al alloys, it was concluded that processing by route C produces a more ho-

ＡＣ２Ｂアルミニウム合金鋳物の熱疲労寿命に及ぼすポロシティの影響

Automobile engine materials require higher thermal fatigue resistance with increasing combustion temperature. Porosity in aluminum alloy castings is a serious defect which decreases the strength of the castings. In this study, the effect of porosity on thermal fatigue lives was investigated using JIS AC2B-T6 alloy castings. The thermal fatigue lives were evaluated by a newly-developed test method using restraining holders composed of low-thermal-expansion material. In a high total strain range, the maximum pore area on a fracture surface correlated to the thermal fatigue life. In a low total strain range, on the other hand, the total porosity area on a fracture surface correlated to the thermal fatigue life. It has been clarified that the fracture mechanism was changed by the total strain range, affecting the correlation between the porosity and the thermal fatigue lives.

Kinematic evolution of the Miller Range Shear Zone, Central Transantarctic Mountains, Antarctica, and implications for Neoproterozoic to Early Paleozoic tectonics of the East Antarctic Margin of Gondwana

High‐grade ductile tectonites of the Precambrian Nimrod Group in the central Transantarctic Mountains form the Miller Range shear zone (MRSZ). With no exposed boundaries, this zone has a minimum structural thickness of 12–15 km. Shear‐sense indicators record consistent top‐to‐the‐SE, or left‐lateral, shear within the NW striking, moderately SW dipping zone. Cylindrical folds with axes normal to elongation lineation (Le) are kinematically consistent with other shear indicators. They may represent early stages in the development of subordinate noncylindrical sheath folds, which indicate locally high bulk ductile strain and a moderate strain gradient. Pervasive, open to tight cylindrical folds with axes parallel to Le formed during shear and may reflect a component of constrictional strain. Quartz c axis fabrics from micaceous quartzites show asymmetric single girdles evident of dominantly rhombohedral slip, with limited basal‐plane slip, affirming both the consistency of shear sense and high‐grade syn‐kinematic conditions. Deformation did not persist during subamphibolite facies cooling, as shown by (1) a lack of basal‐plane slip in ductilely deformed quartz, (2) a lack of quartz subgrains and grain shape‐preferred orientation, and (3) the presence of oriented muscovite “fish” included within polygonal quartz grains, which show that quartz grain boundaries migrated and annealed under static conditions following ductile shear. From the uniform Le orientation and consistent shear sense, we interpret that ductile deformation resulted from a single, kinematically simple, left‐lateral (top‐to‐the‐SE) shear event. Together, the scale, high total strains (γ ≥ 5), fabric uniformity, and the widespread presence of asymmetric microstructures formed at high temperatures, all indicate that strain rates within the MRSZ were high and that it represents a major crustal structure. Orogen‐parallel displacements within this zone during the latest Neoproterozoic to Early Cambrian were at a high angle to penecontemporaneous orogen‐normal contraction in outboard supracrustal rocks, suggesting that the Neoproterozoic to early Paleozoic plate margin of Antarctica was characterized by left‐oblique convergence in which strain within the orogen was partitioned into deep‐level strike slip and shallow‐level contraction.

Perspirocyclopropanated [3]Rotane—A Section of a Carbon Network Containing Spirocyclopropane Units?

The increase in strain in branched triangulanes has hardly any influence on the bond lengths. This is shown by the comparison of the structures of twofold (1), fourfold (2), and sixfold cyclopropanated (3) [3]rotanes. In 3, which is prepared in six steps from perspirocyclopropanated bicyclopropylidene (4), the bonds of the central three-membered ring are significantly longer than in [3]rotane. In spite of its high total strain, 3 is stable up to 250 °C.

Stress-induced foliation in the terminus of Variegated Glacier, Alaska, U.S.A, formed during the 1982–83 surge

AbstractTwo sets of bubble-density foliation, one vertical and transverse to flow and one vertical and parallel to flow, developed in the ice of the terminus lobe of Variegated Glacier, Alaska, during the passage of the final stages of the 1982–83 surge in June and July 1983. The transverse foliation is ubiquitous in the lower part of the terminus lobe affected by the surge, but disappears abruptly over a 5 m wide zone at the boundary of the surge, as delineated by measurable strain rates during the surge. The ice at the outermost limit of the transverse foliation experienced compressive total strains no greater than −0.10 (logarithmic), but did briefly experience high strain rates, corresponding to compressive stresses in excess of 0.6 MPa. It is concluded that the transverse foliation was developed as a consequence of high compressive stress rather than high total strains. Experimental evidence of Kamb indicates that bubble migration occurs under similar tectonic conditions to those in the terminus lobe during the surge, and that bubble migration is related to stress rather than finite deformation or rotation of pre-existing structures. Bubble migration may be the mechanism by which the transverse foliation developed during the passage of the surge front.

Stress-induced foliation in the terminus of Variegated Glacier, Alaska, U.S.A, formed during the 1982–83 surge

AbstractTwo sets of bubble-density foliation, one vertical and transverse to flow and one vertical and parallel to flow, developed in the ice of the terminus lobe of Variegated Glacier, Alaska, during the passage of the final stages of the 1982–83 surge in June and July 1983. The transverse foliation is ubiquitous in the lower part of the terminus lobe affected by the surge, but disappears abruptly over a 5 m wide zone at the boundary of the surge, as delineated by measurable strain rates during the surge. The ice at the outermost limit of the transverse foliation experienced compressive total strains no greater than −0.10 (logarithmic), but did briefly experience high strain rates, corresponding to compressive stresses in excess of 0.6 MPa. It is concluded that the transverse foliation was developed as a consequence of high compressive stress rather than high total strains. Experimental evidence of Kamb indicates that bubble migration occurs under similar tectonic conditions to those in the terminus lobe during the surge, and that bubble migration is related to stress rather than finite deformation or rotation of pre-existing structures. Bubble migration may be the mechanism by which the transverse foliation developed during the passage of the surge front.

Isothermal fatigue of low tin lead based solder

Low tin lead based solder fails by intergranular and/or transgranular modes depending upon experimental conditions. At low frequency and in tests with hold times separation of grains is the main mode of fracture. In the 5 to 100 °C temperature range at high frequency (> 10−2 Hz) and at high total strain range (0.75 pct) the failure mode is mixed transgranular-intergranular; at a low total strain range (0.3 pct) the mode of failure is intergranular. Change in failure mode leads to a bend in the Coffin-Manson plot. Tensile hold time and combined tensile and compressive hold times are found to reduce dramatically the fatigue cycles to failure of this solder. A simple mathematical relation between the fatigue life of the solder and ramp time, tensile, and compressive hold times is developed.

Effect of the finish rolling and cooling conditions on the properties of microalloyed plate steels

The processing parameters of finish rolling and cooling of three HSLA steels were varied in a wide range applying the plane strain hot compression simulation test on the hot deformation simulator (Wumsi). Evaluating the flow curves of the deformation steps, some metallurgical phenomena in austenite during hot deformation could be determined. The results of material testing after the simulation experiments exhibit a strong correlation between the finish rolling and cooling conditions and the final mechanical properties of the steels tested. As a way to increase strength, lowering finishing temperatures and/or accelerating cooling after finish rolling proved to be most effective. To improve notch toughness, on the other hand, a high total finishing strain should be applied as well as low finishing temperatures, however, above Ar3.

Low cycle fatigue behavior of Ti-Mn alloys: Fatigue life

The effect of morphology, particle size, β grain size and volume fraction of β, from 0.025 to 1.0, on the low cycle fatigue life of α-β Ti-Mn alloys, have been studied under total strain control. In general, Widmanstatten plus grain boundary (W+GB) α structures show shorter fatigue lives than equiaxed (E) α structures, and this has been ascribed to the formation of much larger surface cracks and ease of transfer of slip from α to β. For Eα structures, fatigue life increases with decreasing α particle size and when the alloy is single phase β fatigue life increases with decreasing grain size. At high total strains the nearly all α alloy had the longest fatigue life and at lower strains the β alloy, with the higher yield strength, had the longest fatigue life. Fatigue life was correlated with strain hardening. The nearly all α alloy which had the highest strain hardening, over the plastic strains encountered, had the highest fatigue life, while the β alloy, with the lowest strain hardening, had the lowest fatigue life. For a portion of the fatigue life curves, it was found that as the average Baushinger strain (ABS) increased, the Coffin-Manson exponentc decreased. The results are discussed.