J resistance curves in aluminum SEN specimens using Moiré interferometry

TheU _y -displacement field obtained by white-light moiré interferometry were used to estimate the approximate far- and near-fieldJ-integral values associated with the subcritical crack growths in fatigue precracked 7075-T6 and blunt notched and fatigue precracked 2024-0 and 5052-H32 aluminum, single-edged notch (SEN) specimens. The initial phases of theJ-resistance curves for the somewhat brittle 7075-T6 and the two ductile 2024-0 and 5052-H32 aluminum SEN specimens are presented. Paper was presented at the 1987 SEM Spring Conference on Experimental Mechanics held in Houston, TX on June 14–19.     

Random-load fatigue tests on a fail-safe structural model

Experiments performed on a ten-member redundant “fail-safe” structural model subjected to randomized load sequences confirm predictions of fatigue life and reliability based on a probabilistic approach. The statistical variation in ultimate strength of 2024-T4 aluminum alloy combined with an exponentially distributed, Markovian, bending load-amplitude sequence with a constant-amplitude S-N relation for single specimens, is utilized in the analysis. Experimental results are presented for the statistical distribution of ultimate bending strength of 2024-T4 aluminum alloy. Constant load-amplitude flat-bending fatigue tests on single specimens and on multimember structures, and variable-amplitude flat-bending tests on fail-safe structures are reported. Life to failure of the weakest member, as well as the remaining “fail-safe” life in the parallel structure, are examined.     

Small surface and corner crack propagation in aluminum and steel alloys

The purpose of the investigation reported here was to examine experimentally the transition behavior of a short crack in detail using surface-crack measurements with a traveling microscope and micromeasurements obtained by scanning-electron microscopy (SEM). The cyclic growth and transition of initial part-through cracks into through the thickness flaws are documented through experiments by using compact-type specimens fabricated from 7075-T6 aluminum, 2024-T3 aluminum, and mild-steel material, with a keyhole notch. Paper was presented at the 1986 SEM Fall Conference on Experimental Mechanics held in Keystone, CO on November 2–5.     

Crack growth in bi-material laminates

Growth rates of fatigue cracks have been measured in laminates fabricated by adhesively bonding layers of 2024-T351 and 7075-T6 aluminum alloys. The laminates had either two or four layers, with equal thicknesses and numbers of layers of each alloy. Fatigue-crack-propagation tests were performed with through-cracks, giving a crack-divider geometry, the results being compared to those for the two alloys tested in monolithic form. Crack-propagation rates in the bi-material laminates were intermediate between those of the monolithic alloys, with the slower growth in 2024-T351 tending to dominate over a portion of the growth-rate range. Fracture toughnesses of the laminates are also discussed.     

The fatigue of aluminum alloys subjected to random loading

This paper describes an expeirmental investigation which was carried out to determine the fatigue life of two aluminum alloys (2024-T3 and 6061-T6). They were subjected to both constant-strain-amplitude sinusoidal and narrow-band random-strain-amplitude fatigue loadings. The fatigue-life values obtained from the narrow-band random testing were compared with theoretical predictions based on Miner's linear accumulation of damage hypothesis. Cantilever-beam-test specimens fabricated from the aluminum alloys were subjected to either a constant-strain-amplitude sinusoidal or a narrow-band random base excitation by means of an electromagnetic vibrations exciter. It was found that the ε-N curves for both alloys could be approximated by three straight-line segments in the low-, intermediate- and high-cycle fatigue-life ranges. Miner's hypothesis was used to predict the narrow-band random fatigue lives of materials with this type of ε-N behavior. These fatigue-life predictions were found to consistently overestimate the acutal fatigue lives by a factor of 2 or 3. However, the shape of the predicted fatigue-life curves and the high-cycle fatigue behavior of both materials were found to be in good agreement with the experimental results.     

Fracture prediction based on plastic stress singularity strength

The objective of this study was to predict the fracture behavior of specimens of various cracked configurations and materials based on information from standard fracture specimens of the same thickness. A three-dimensional elastic-plastic finite-element analysis was used to determine the critical value of plastic stress singularity strength and thus characterize specimen fracture behavior. Fracture instability was predicted for specimens of the following structural materials: 7075-T651 and 2024-T351 Aluminum alloys; Type 304 Stainless Steel. This study was one of many contributed by participants in a fracture-predictive round-robin conducted by ASTM Task Group E24.06.02.     

A stress criterion for crack growth

A stress criterion for crack growth was developed from test results with 7075-T6 aluminum-sheet specimens containing transverse machined cracks. Stress distributions near the crack tip were obtained using strain gages and by reducing the strain data to stresses with the aid of Reuss plasticity theory. These distributions indicated the biaxial nature of stress at the crack tip, the high stress gradients a short distance from the tip, and the variation in stress-concentration factor with crack length. Crack growth was found to occur when the effective stress at the crack root reached the engineering ultimate strength.     

Fatigue crack growth retardation assessment of 2024-T3 and 6061-T6 aluminium specimens

A fatigue crack growth retardation model is developed. It considers a strip plastic zone with material hardening effect which is taken as one of the basic mechanisms controlling fatigue crack growth. Crack growth is treated incrementally and corresponds to the failure of material elements ahead of an existing crack after a certain critical number of low cycle fatigue. Computed curves are correlated to test data obtained from the 2024-T3 and 6061-T6 aluminium specimens. Deviations from test data increase with increasing crack length.     

Fatigue-life prediction by an order statistics treatment of acoustic-emission signals

In fatigue, both the crack-propagation rates and the cumulative acoustic-emission activity are known to be related to the applied stress-intensity range. By considering the energy balance during crack propagation and the relation of strain energy release to the acoustic-emission characteristics, a formal relation between acoustic emission amplitudes and initial fatigue-crack-propagation rates has been derived. Continuous monitoring of acoustic emission during low cycle (tension-tension) fatigue tests has been conducted on aluminum 2024-T3 and 7075-T6 alloys, until fracture. Initial crack sizes and orientations in the fatigue specimens were randomly distributed. Every few hundred cycles, the acoustic signal having the highest peak amplitude was recorded as the extreme acoustic-emission event for the elapsed period. The extreme peak amplitudes, related to extreme crack-propagation rates, were shown, by an order statistics treatment, to be extremally distributed. Statistical, nondeterministic, approach to fatigue considers that only extreme crack-propagation rates are vital to fatigue lives. Knowledge of the distribution function of propagation rates is therefore essential in design for fatigue. Such knowledge can now be obtained in a nondestructive manner, during service in real time, by analyzing the distribution of amplitudes of acoustic-emission signals emitted during cyclic stressing. The statistical treatment enables the prediction of the number of cycles left until failure. Predictions performeda posteriori, based on results gained early in each fatigue test, were in good agreement with actual fatigue lives. The amplitude distribution analysis of the acoustic signals emitted during fatigue tests has been proven to be a feasible nondestructive method for predicting fatigue life.     

Crack-opening displacements and normal strains in centrally notched plates

Experimentally determined crack-opening displacements of stationary and running cracks and of inclined stationary cracks in centrally notched plates are reported in this paper. The moiré-fringe technique was used for the determination of displacement fields in test specimens of magnesium, 7075-T6 and 7178-T6 aluminum alloys. Experimentally determined crack-opening displacements were compared with corresponding re ults based on theoretical models of Westergaard, Dugdale, Craggs and Craggs-Dugdale. In addition, normal-strain fields derived from the moiré-fringe data were compared with static or dynamic strain fields of these theoretical models. The results of this investigation indicate that while the Dugdale crack is a fair model of a stationary crack in ductile materials, the Craggs crack appears to be a better representation of a running crack in the ductile materials investigated.     

Mixed mode fatigue crack growth in test coupons made from 2024-T3 aluminum

In this paper, two different fracture criteria are applied to determine the crack trajectory or angle of crack propagation in test specimens containing inclined cracks emanating from open holes. Also, different crack growth rate models are assumed for each criterion. The maximum principal stress criterion is used with a crack growth-rate equation based on an effective stress intensity factor. The strain energy density criterion is used with a crack growth-rate equation corresponding to an effective strain energy density factor. The crack growth-rate models for each criterion were constructed using unpublished fatigue crack growth data for 2024-T3 aluminum.     

The effects of out-of-phase biaxial-strain cycling on low-cycle fatigue

The effects of out-of-phase or nonsynchronous straining on low-cycle fatigue was investigated. Biaxial strains were imposed on thin-walled tubular 7075-T6 aluminum specimens by tension—compression and torsion. Phase angles of 0 deg, 30 deg, 45 deg, 60 deg and 90 deg were applied between two strains. It was found that out-of-phase cycling has an effect on the failure mode in the low-cycle-fatigue range. An analysis based on the maximum total strain in three-dimensional strain is proposed for treating “out-of-phase” straining conditions in low-cycle fatigue.     

Strain energy density approach to stable crack extension under net section yielding of aircraft fuselage

In this paper, the incremental theory of plasticity is used in conjunction with the strain energy density criterion to determine the stress field in 4-in. wide test specimens containing 3 holes. These specimens, made from 0.04-in. thick sheets of 2024-T3 aluminum, also contained small collinear cracks emanating from the holes. The initial crack sizes varied from 0.15 to 0.26 in. Residual strength tests conducted with these specimens revealed that stable tearing occurred before failure. Analyses were performed to predict the stable crack extension and failure by plastic collapsed. Because of the complexities involved with nonlinear stress analysis combined with subcritical crack extension, the finite element method was used with the grid pattern adjusted for each increment of stable tearing. Reasonable correlation between the experimental data and predicted results was achieved.     

Further studies on the hrr field of a moving crack, an experimental analysis

An improved moire interferometry was used to record simultaneously both the vertical and horizontal displacements associated with stable crack growth in uniaxially loaded 5052H32 aluminum, single-edge-notched specimens. For stable crack growth up to 2 mm, the vertical displacement field showed the dominance of the HRR field. The HRR field was detected in the horizontal displacement only at the initial stage of loading. The far and near field J-integrals were path independent during this incremental crack extension period. These and previous results involving 2024-0 and 2024-T3 aluminum specimens indicate that J-characterization of a crack is not valid for these ductile materials in this specimen configuration.     

Low-cycle fatigue of 7075-T651 aluminum alloy in biaxial bending

The uniaxial and biaxial low-cycle-fatigue life of 7075-T651 aluminum alloy in completely reversed bending is investigated. Uniaxial data is obtained from a cantilever specimen, and round and elliptical simply supported plate specimens are used for nominal strain biaxialities of 1∶1, 1∶075 and 1∶0.50 in the plane of the specimen. Experimental data is correlated on the basis of the total octahedral shearing strain range in which the dependent component strains are calculated using the “effective value” of Poisson's ratio for elastic-plastic loading. Good agreement is obtained oetween the uniaxial and biaxial data when the total octahedral shearing-strain range is plotted against the number of cycles to failure in logarithmic coordinates. Also, it is shown that, in anisotropic materials, the directional uniaxial and biaxial low-cycle properties can be predicted from fatigue properties in any one direction if the anisotropy of fracture ductility is known.     

Composite patch reinforcement of cracked aircraft upper longeron: analysis and specimen simulation

The use of composite patches on cracked portions of metallic aircraft structures is an accepted means of improving fatigue life and attaining high structural efficiency. As more and more advanced composite materials are beng developed, the wider use of the repair technology is anticipated even for the reinforcement of primary aircraft structure. The objective of this work is to illustrate how the composite patch repair technology can be successfully applied to restore the structural integrity of cracked components.The Phosphoric Acid Anodize (PAA) surface treatment on aluminum when applied in conjunction with the AVI13/HV998 adhesive were essential for achieving the appropriate patch bonding strength. Such a process was done without immersing the component into the PAA tank; dismantling the component from the aircraft was not necessary. Boron/epoxy and carbon/epoxy patches were applied at room temperature to the 7075-T6511 cracked specimens and tested under fatigue simulating the load spectrum for the upper longeron attached to the access door of the electronic equipment bay. Considerable improvement in the fatigue life was observed after the repair. Equivalent flight test hours were increased from approximately two thousand hours at which the component fractured completely when not repired to twelve thousand hours when the repair was made with only a small amount of crack growth. A six times increase fatigue life is obtained. The laboratory developed technique has been applied to several in-service aircraft which have now been flown for more than 700 h without detection of crack growth.     

Thermo-mechanical response of tension panels under intense rapid heating

The temperature dependence of the fracture strength of 7075-T6 aluminum alloy was determined under rapid heating (0.02–.50 sec) conditions by exposing thin-section specimens to intense surface irradiation while under constant tensile load. When combined with a numerical thermal analysis and an appropriate limit analysis, these data enabled accurate prediction of the heating time required to produce ductile fracture in spot irradiated tension panels. The transient heat transfer model employed in the computations incorporated material removal due to melting, temperature-dependent material properties, and convection/radiation losses.     

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.     

Development of an apparatus for fatigue testing in vacuum

The development and use of a small, light-weight, remotely controlled fatigue apparatus are described. The particular application was for fatigue tests of 7075-T6 aluminum alloy in vacuums of 8.6×10^?8 to 4×10^?9 Torr. The fatigue life in vacuum was 1.8 times greater than the fatigue life in air based on the upper 95 percent confidence limit for tests in air and the lower 95 percent confidence limit for tests in vacuum.     

In Situ Observation of Adiabatic Shear Band Formation in Aluminum Alloys

We used high-speed X-ray phase contrast imaging and infrared thermal imaging techniques to study the formation processes of adiabatic shear bands in aluminum 7075-T6 and 6061-T6 alloys. A modified compression Kolsky bar setup was used to apply the dynamic loading. A flat hat-shaped specimen design was adopted for generating the shear bands at the designated locations. Experimental results show that 7075-T6 exhibits less ductility and a narrower shear band than 6061-T6. Maximum temperatures of 720 K and 770 K were locally determined within the shear band zones for 7075-T6 and 6061-T6 respectively. This local high temperature zone and the resulting thermal instability were found to relate to the shear band formation in these aluminum alloys.