A. Berkovits

Study of fatigue crack characteristics by acoustic emission

Abstract Acoustic emission (AE) technique was used to investigate fatigue crack characteristics such as initiation closure and propagation on smooth specimens of Incoloy 901 at room temperature, over the range of stress ratios −1.0 ⩽ R ⩽ 0.2. AE technique was applied to determine when and where a microcrack initiated on the specimens. The threshold stress intensity ranges were determined by combining AE tests and microscopic examination for crack initiation points. The AE technique revealed the crack closure process in smooth specimens, and crack-opening/closing stress was easily determined from the AE signature. Several published formulae for crack-opening stress were examined. During crack propagation, AE count rate and total counts, associated with the crack growth state, were stress- and lifetime-dependent. The results showed a nonlinear relation between the AE count rate and the crack propagation rate.

Mean stress models for low-cycle fatigue of a nickel-base superalloy

The aim of this study was to relate observed low-cycle fatigue response under asymmetric loading to mean stress effects on the cyclic behaviour and fatigue life of nickel-base Incoloy 901 superalloy at room temperature. Based on experimental data, a mean stress relaxation model was proposed, which described the dependence of mean stress on the total strain range and on cyclic plasticity. It was found that cyclic softening and mean stress relaxation began when plastic strain started to develop, and increased in importance with increasing strain range. Both depended on the plastic strain range rather than on monotonic plastic deformation. For any given strain ratio there was a value of strain range corresponding to peak mean stress sustained. Beyond this strain range, mean stress relaxation began. A mean stress strength constant representing the apex on the σm axis of the Goodman diagram was introduced in order to modify the Goodman criterion, resulting in a modified strain-life relation. Both the mean stress-relaxation model and the strain-life model were evaluated against LCF test data.

Buckling of unstiffened conical shells under combined loading

An experimental program was conducted in order to determine the family of interaction curves for the buckling of unstiffened conical shells under combined axial compression, torsion, and external or internal pressure. Careful experimental technique permitted many repeated buckling tests on the same aluminumalloy shell without noticeable damage and yielded reliable interaction curves. Results of combined-loading tests are presented and compared with linear theory. Test results show that the interaction curve for compression-torsion-pressure loading is defined by superposition of compression-pressure, torsion-pressure and compression-torsion behavior.

High-temperature low-cycle fatigue of a nickel-based MAR-M200 + Hf alloy in Ar and Ar + 20% O2 environment

A systematic study was made on the environmental influences of inert gas and oxygen on the low-cycle fatigue failure of MAR-M200 + Hf unidirectionally solidified nickel-based alloy at 975 ‡C. The cyclic load was constant and comprised creep tension and plastic compression. Under an inert argon atmosphere, a pure transgranular fracture free of environmental influences was obtained. Under an Ar + 20% O2 atmosphere the fracture was intergranular along a path through the interdendritic microsegregation zone. In the area close to the propagating crack tip, a uniphase layer was detected, resulting from a process of internal oxidation which developed a preferred oxide-type bonding with the elements making up the principle precipitating phase, γ′, namely titanium and aluminium. The internal oxidation led to changes in the boundary layer between the matrix and γ′ phase, as a result of which the original coherence between them was lost. In order to reduce the surface energy between the precipitate and the matrix, a process of growth and coarsening of γ′ phase took place in these conditions. The internal oxidation and the formation of the uniphase layer increased the brittleness over a comparatively wide area adjacent to the crack tip. As a result, crack branching and blunting took place during propagation in the oxygen-containing atmosphere. The branching and the blunting were instrumental in lengthening the active life of the alloy in the oxygen-containing atmosphere compared with that in the inert atmosphere.

Further Modification of Bolotin Method in Vibration Analysis of Rectangular Plates

A further modification of the Bolotin method for the determination of the natural frequencies and mode shapes of isotropic and orthotropic rectangular plates with various types of boundary conditions is given. Unlike the Bolotin method (BM) or the modified Bolotin method (MBM), the present approach does not postulate the formula for the eigenfrequency, but rather is based on the condition that the frequency obtained from the governing differential equations has to be equal to that yielded by Rayleighs method. This modification is shown to be more straightforward and faster in computation, and the mode shapes derived are valid on a larger portion of the plate. Furthermore, the proposed modification easily provides a solution for boundary conditions for which the BM and MBM cannot provide a solution. Problems with two different sets of boundary conditions were solved in this study: a rectangular orthotropic plate with all edges clamped and rectangular isotropic and orthotropic plates clamped along one pair of opposite edges and free along the other pair. The results obtained for the first set compared favorably with those yielded by the MBM and Rayleigh methods, whereas in the second case the BM and MBM failed to predict the beam-like modes of vibration, while the present modification treats the problem satisfactorily.

Buckling of an elastic column containing a fatigue crack

Results of an experimental program conducted in order to study the residual buckling strength of an elastic column containing a fatigue crack are presented in this paper. Tests were performed on bars of rectangular cross section, made from 2024-T351 aluminum-alloy plate and containing a fatigue crack at the midsection. Results obtained show a reduction in buckling strength of 8 percent under highed-highed end conditions, in apparent tentative accord with data obtained from columns containing machined notches. Although it is small, the reduction in buckling strength is thought to be significant for design and requires further investigation.

An analytical master curve for Goodman diagram data

Abstract Estimation of the remaining safe life of structural parts which are not easily inspectable continues to be a problem. Even when load histories are available, laborious interpolation of Goodman diagram data is required in order to determine the remaining fatigue life of such parts. An analytical formulation of Goodman diagram data would expedite the life check. It is shown in this paper that, for many engineering materials at room temperature, the entire range of Goodman diagram data collapses on to a single master curve when presented as the ratio of lifetime with mean stress to lifetime at R = −1 for a given stress amplitude, as a function of a non-dimensional load parameter consisting of stress amplitude, mean stress, and material strength. The master curve is conveniently expressed in terms of two easily determined Weibull constants. Stress-concentration factor influences the value of the constants, as does the strain-rate sensitivity of some materials. By use of the master curve formula in an algorithm together with the Manson-Coffin life relation and Miner cumulative damage rule, computed fatigue lives lay within a factor of 2 of results obtained in tests under aircraft spectrum loads.

Estimation of loads causing fatigue failures in accident investigations

Abstract A procedure was developed by which the relationship between the load levels which led to fatigue failure of a part, and the crack depth and crack growth rate observed on the fracture surface, can be determined by combining basic materials data obtained from a compact-tension (CT) fatigue test with a finite element analysis of the part in question. The procedure, which permits the estimation of the load levels in the structure at any given crack depth and growth rate, is described, as well as the construction of the three-parameter plot which facilitates this determination. Two case studies are presented and discussed. The first concerns a cracked aluminum-alloy anchor fitting for a speed-brake actuator on the wing of an A-4 fighter aircraft. CT fatigue data showed that service loads experienced by the part were not excessive. However, deformation imbalance, perhaps due to wear in the actuator attachment bearing, may have been sufficient to cause an initiating flow in the flange, eventually resulting in a crack. The second case is a steel torsion-bar from a Subaru 600 automobile. Although the bar had been loaded in torsion, it developed the usual mode I fatigue crack which led to failure. Here material fatigue data led to the conclusion that the fatigue crack, which began at a mark probably left by the impact of a piece of gravel, could well have caused failure under normal loads between two consecutive inspection times.

Variation of the cyclic strain-hardening exponent in advanced aluminium alloys

Abstract The cyclic strain-hardening exponents for five fatigue-resistant aluminium alloys were determined throughout the fatigue life to study the degree of cyclic stability of these alloys. Data were compared with results for 2024-T4 aluminum and for two high-pressure steels. The strain-hardening exponent increased logarithmically in all cases except 2024-T4, although the increase was small and did not exceed 33% over the fatigue life. 7475-T351 aluminium alloy was found to be entirely stable, and 7075-T7351 almost so. These were followed in order of rising sensitivity by 2014-T6, 7050-T73651, and 2124-T851 aluminium alloys, and 28NiCrMo7.4 and 30CrNiMo8 steels. 2024-T4 aluminum alloy demonstrated a strong decrease in strain-hardening exponent with fatigue life.

Acoustic emission during the tensile deformation of Incoloy 901 superalloy

Observations have been made on the acoustic emission (AE) response related to the deformation-damage mechanisms during tensile tests of a common engine material, Incoloy 901 superalloy. Results show that dislocation motion, twinning and inclusion fracture cooperated to generate acoustic emission during tensile deformation of Incoloy 901. Based on AE recorded results and microstructural examination, a dislocation-saturation model was developed to describe AE activity during elastic and plastic deformation, and to distinguish between the AE response in the yield region and in the work-hardening region. Furthermore, the effects of strain rate and loading methods on AE outputs were examined. The dependence of acoustic emission on dislocation motion and saturation, deformation twinning, and decohesion and fracture of inclusions and secondary particles are discussed.