Effect Of Specimen Orientation Research Articles

Effect of specimen orientation and welding on the fracture and fatigue properties of 2195 Al–Li alloy

In view of the use of 2195 Al–Li alloy in the construction of super-light-weight external fuel tank of space shuttles, bulkheads of reusable single-stage-to-orbit launch vehicles and in combat ground vehicles, the dependence of tensile properties, fracture toughness and fatigue resistance of this alloy on the specimen orientation and welding is very important and was studied. The T8 base alloy, with primary strengthening precipitates of T1 (Al2CuLi) phase, contained mainly brass-type texture. After welding with AA 4043 filler alloy, the fusion zone (FZ) consisted of T (AlLiSi) phase and in the heat-affected zone (HAZ) T1 phase was replaced by TB (Al7Cu4Li) phase, and micro-cracks were observed. The post-weld heat treatment (PWHT) resulted in the spheroidization of primary T phase and the precipitation of more T particles in the FZ, and the dissolution of TB phase and the re-precipitation of T1 phase in the HAZ. The yield strength, fracture toughness and fatigue threshold of the 2195-T8 alloy was observed to depend on the specimen orientation, with the lowest values obtained at 45° to the rolling direction. Welding resulted in a reduction in the tensile properties and fatigue strength. The post-weld heat treatment enhanced the yield strength, but no increase in fatigue strength was observed. Fracture mechanisms in various cases were evaluated by SEM examination of fracture surfaces and are discussed.

Anisotropic ductile fracture: Part I: experiments

Deformation and fracture are investigated at room temperature in steel through tension and compression tests. Round smooth bars and cylinders are used to characterize the deformation behavior while round notched bars are used to explore stress state and specimen orientation effects on fracture. The microstructure is characterized to infer initial average values of porosity, void aspect ratio and void spacing ratio, all three playing a key role in the fracture process. Interrupted tests are used to determine the spatial distribution of those variables after crack extension thus providing a basis for comparison at the micro-scale with theoretical predictions. All damage stages are found to be inherently anisotropic, including void nucleation and crack propagation. An important finding of this investigation is that, in quantitative modeling of ductile fracture, any void growth model should be supplemented by a physically motivated void coalescence model.

超微細結晶粒Ｐ／Ｍアルミニウム合金における微小疲労き裂の発生および初期進展挙動

Small fatigue crack initiation and growth behavior were investigated for two kinds of ultra fine-grained P/M (Powder Metallurgy) aluminum alloys of which grain size was from 200 to 500nm. Those two materials were denoted as Types M and S according to the chemical composition and manufacturing process. Type M without Si addition was extruded, while Type S with Si was forged. The shape of powders changed through manufacturing processes, where the aspect ratio of elongated powders in Type M was larger than that in Type S. Reversed plane bending fatigue tests were conducted and small fatigue cracks were observed by means of plastic replicas technique. The effect of specimen orientation on fatigue properties in those two materials was also investigated. The specimen orientations evaluated were C-C and L-C for Type M, and C-C and C-L for Type S. It was found that small fatigue cracks initiated at the inclusions for C-C orientation and at the boundary between powders for C-L orientation in Type M, while they initiated at the boundary regardless of specimen orientation in Type S. S-N curves were dependent on the crack initiation mode, and the crack initiation life was well estimated by Kmax calculated with stress amplitude and the size of inclusions or cluster of powders. The fatigue crack growth behavior was almost irrelevant to the materials and specimen orientations.

Fatigue Crack Initiation and Early Growth Behavior of Ferritic Stainless Steel in Laboratory Air and 3%NaCl Solution.

Crack initiation and crack growth behavior were studied on a ferritic stainless steel, SUS444, as a function of specimen orientation. Fatigue tests were conducted in laboratory air and in 3%NaCl solution using electro-hydraulic fatigue testing machine. Specimens with two different orientations, i.e. L and T direction, were prepared. In both orientations, cracks initiated from slip bands within ferritic grains. The effect of specimen orientation on fatigue lives was not observed in both environments. However environmental effect was found on fatigue lives, i.e. fatigue lives were shorter in 3%NaCl solution. In early crack growth region, while no discernible differences were found between both directions, crack growth rates in 3%NaCl solution were faster than in laboratory air. In long crack growth behavior, the anisotropy was only observed at low ΔK region, where the crack growth rates in specimens with L-T orientation were faster than those with T-L orientation. Taking account of crack closure, the crack growth in both orientations exhibited a similar behavior. Environmental effect was also found in the region of ΔK≥15 MPa √m for both orientations, where crack growth was enhanced in 3%NaCl solution.

Anisotropic Fatigue Crack Initiation and Small Crack Growth in Ti-6Al-4V Alloy.

This paper deals with the effects of specimen orientation and microstructure on small fatigue crack growth in Ti-6Al-4V alloy. Fatigue tests have been carried out under axial loading at a stress ratio of-1 using smooth specimens of annealed (AN) and solution-treated and aged (STA) materials. Specimens were cut from the materials so that the loading axis was parallel to (L-orientation) and perpendicular to (T-orientation) the rolling direction. T-orientation showed higher fatigue strength than L-orientation, which was more remarkable in STA material. In both materials, T-orientation also indicated higher resistance to crack initiation than L-orientation. In AN material, there was a distinct difference in small fatigue crack growth between both orientations : L-orientation exhibited faster crack growth rates than T-orientation at the early stage of crack growth, but STA material showed no significant difference. The orientation dependence of small fatigue crack growth was strongly related to stage I facets developed at the crack initiation site. Although stage I facets were not seen clearly in STA material, the sizes of stage I facets for L-orientation in AN material were significantly large compared to those for T-orientation. Since AN material had a texture with the basal plane (0002) parallel to the rolling direction. the preferred orientation led to faster and extended stage I crack growth for L-orientation.

フラクトグラフィ ロータ鋼の各種破壊挙動に及ぼす組織異方性の影響

In order to clarify the anisotropy of fractography at fracture initiation site, monotonic tension, bending fatigue and tensile creep fracture tests were carried out at room and elevated temperatures, using three oriented specimens extracted from a Cr-Mo-V steel turbine rotor. In the case of monotonic tension, although the effects of temperature on tensile properties and fractography was prominent, the effects of specimen orientation on those were quite small at room and 839K temperatures. The fatigue limit of L-R oriented specimen was about 380MPa, and 25% higher than those of C-R and C-L oriented specimens. The scattering of fatigue life of C-R oriented specimen at the near fatigue limit stress condition was clearly larger than those of the other oriented specimens. Under bending fatigue, markedly different fractographic patterns were observed at the crack initiation site for all the three oriented specimens. It was found that the orientation of two kinds of inclusions, MnS and Al2O3 particles at the fatigue crack initiation site, influences the fractographic mode strongly. The creep result of L oriented specimen showed the longest life, as compared with those of the other oriented specimens, under nearly the same stress condition. The effect of specimen orientation on the fractographic patterns of creep was quite small.

塩水中におけるＴｉ‐６ＡＩ‐４Ｖ合金の微小疲労き裂進展特性

Small fatigue crack growth tests in 3%NaCl solution were conducted on a solution-treated and aged material and two kinds of annealed materials of Ti-6Al-4V alloy. The crack growth rate in 3%NaCl solution was enhanced above a certain value of δK compared with that in air. The difference in crack growth rate between small and long cracks was not observed for all the microstructures studied, and so no specific chemical behavior appeared in short cracks. The effect of specimen orientation on crack growth was not found in air, while in 3%NaCl solution the crack growth rate in T-orientation for which load was applied normal to the rolling direction was faster than that in L-orientation. From fractographic observations, cleavage fracture was observed remarkably with increasing crack growth rate.

FATIGUE CRACK GROWTH BEHAVIOUR IN MOLTEN‐METAL PROCESSED Sic PARTICLE‐REINFORCED ALUMINIUM ALLOYS

AbstractFatigue crack initiation and subsequent short crack growth behaviour of 2014‐5wt%SiC aluminium alloy composites has been examined in 4‐point bend loading using smooth bar specimens. The growth rates of long fatigue cracks have also been measured at different stress ratios using pre‐cracked specimens. The distributions of Sic particles and of coarse constituent particles in the matrix (which arise as a result of the molten‐metal processing and relatively slow cooling rate) have been investigated. Preferential crack initiation sites were found to be Sic‐matrix interfaces, Sic particles associated with constituent particles and the coarse constituent particles themselves. For microstructurally short cracks the dispersed SiC particles also act as temporary crack arresters. In the long crack growth tests, higher fatigue crack growth rates were obtained than for monolithic alloys. This effect is attributed to the contribution of void formation, due to the decohesion of Sic particles, to the fatigue crack growth process in the composite. Above crack depths of about 200 μm “short” crack growth rates were in good agreement with the long crack data, showing a Paris exponent, m= 4 in both cases. For the long crack and short crack growth tests little effect of specimen orientation and grain size was observed on fatigue crack growth rates, but, specimen orientation affected the toughness. No effect of stress ratio in the range R=0.2‐0.5 was seen for long crack data in the Paris region.

High Resolution Electron Microscopy of Twist Grain Boundaries

Preliminary experimental high resolution micrographs have been obtained for a Au thin film containing a [001], Σ=5 (θ=36.5 °) twist boundary. Other twist boundary cases such as the [001], Σ=13 (θ= 22.6°) and [111] low angle (˜3°) have also been fabricated. In order to assess whether the atomistic boundary structure is being imaged, extensive computer simulations of the Σ=5 bicrystal images have also been undertaken. Structurally, several contributions to the scattering must be considered which include: both upper and lower surface layers, bulk contributions above and below the interface and the relaxed interfacial structure at the composite film midplane. The effect of specimen orientation on image features is also evaluated for dynamical diffraction conditions. Likewise, the types of image features which arise by including bulk lattice reflections or alternately including only the reflections arising from the interface and surfaces will be discussed. Several schemes for analyzing twist boundary structures flat-on or near flat-on will be presented for a wide range of microscope conditions for a 400 kV microscope.

Electron diffraction patterns of chrysotile: effect of specimen orientation

Electron diffraction patterns of chrysotile asbestos fibrils should have the 2mm symmetry of a rotation photograph because the layers in the structure are curled as cylinders. The way in which the fibril orientation affects the diffraction patterns is considered theoretically. The departures from ideal symmetry attributable to specimen orientation are noted: they affect mainly the h00 reflections close to the fibre axis. Actual diffraction patterns show a consistent difference in the separation of the h0l-h0\bar l pairs on the upper levels, and \bar h0l-\bar h0\bar l pairs on the lower. The experimental conclusion [Yada (1979). Can. Mineral. 17, 679-691] that this difference is not an effect of specimen orientation is confirmed theoretically but its cause remains obscure.

The fatigue crack growth of a ship steel in seawater under spectrum loading

Fatigue crack growth of ABS EH36 steel under spectrum loading intended to simulate sea loading of offshore structures in the North Sea was studied using fracture mechanics. A digital simulation technique was used to generate samples of load/time histories from a power spectrum characteristic of the North Sea environment. In constant load-amplitude tests, the effects of specimen orientation and stress ratio on fatigue crack growth rates were found to be negligible in the range 2 × 10 −5 to 10 −3 mm/cycle. Fatigue crack growth rates in a 3.5% NaCl solution were two to five times faster than those observed in air in the stress intensity range 25 to 60 MPa √m. The average fatigue crack growth rates under spectrum loading and constant-amplitude loading were in excellent agreement when the fatigue crack growth rate was plotted as a function of the appropriately defined equivalent stress intensity range. This procedure is equivalent to applying Miner's summation rule in fatigue life calculations.

Effect of Specimen Orientation on Elastic—Plastic Fracture Resistance for A533B-1 Steel

Effect of Specimen Orientation on Elastic—Plastic Fracture Resistance for A533B-1 Steel

Computer experiments on the imaging of the [111] split crowdion interstitial in tungsten by transmission electron microscopy

Computer simulated dark-field electron micrographs at atomic resolutions have been generated by calculating the diffuse elastic scattering distribution of short range order objects with the important point being that images are formed from regions of reciprocal space that do not contain Bragg reflections of the perfect crystal. Interpretation of these images in terms of atom positions and atom correlations becomes straightforward and it is a simple matter to distinguish between real structural information and image artifacts produced by the phase contrast mechanism in the electron optical imaging process. In this paper images were calculated under a variety of microscope conditions for a [111] split crowdion interstitial in tungsten which included up to 182 atoms of the surrounding strain field. The effect of specimen orientation, microscope objective lens defocus and the contribution of atoms lying in different shells around the defect have been considered. To aid in image interpretation accompanying diffraction patterns have been computed for different specimen orientations which show either the perfect crystal Bragg diffraction pattern or the diffuse scattering distribution produced by the crowdion defect.

軟鋼溶接部のNRL式落重試験

Following the previous work on the Charpy impact test on multi-run mild steel welds, the results of the NRL drop weight test on the same welds are reported in this paper. Much has been known about the NRL drop weight test on the base plate, but very little attention has been directed to the welds. Therefore, in this study, not only to test the specimens taken from the welds in different orientations, but also to clarify the significance of the NRL drop weight test, the experiment was carried out to record the load-time curve during impact as in the Charpy test.No difference in NDT was observed between the base plate and the welds. The effects of specimen orientation and welded structures that were observed in the Charpy test did not appear in the NRL drop weight test. Furthermore, evaluation of the fracture toughness or stress intensity parameter, KIC, was made by recording the load-time curve: and it was found that NDT corresponded to the temperature at which the yield point fracture occurred.

Scattering Factors and Other Properties of Low-Energy Electron Diffraction

Improved low-energy electron diffraction equipment which yields better resolution and a proper background is described. The equipment is sufficiently flexible for measurements of total elastic and inelastic backscattering, and this is illustrated with several examples. Elastic and inelastic scattering factors are considered and illustrated by experimental data. Since they are complex, complicated, and only semiquantitatively understood, precise surface structure analysis is generally not possible at the current stage of development of this technique, though it now provides a very powerful method for characterizing surface structures. Various approximate methods for interpreting diffraction data are outlined. A preferred approach to a dynamic theory is also discussed. Other physical properties such as Fresnel zone corrections, Lorentz factors, effects of contact potential, properties of diffraction by two-dimensional gratings, properties of averages, specimen orientation effects, effects of steps, and thermal disorder are considered and illustrated. The analytical results for x-ray and high-energy electron diffraction are sueful but sometimes not directly applicable.