Engineering topics

Swain, M. H.

Publications and source records attributed to Swain, M. H..

13 records

NASA NTRS1998

Small Crack Growth and Fatigue Life Predictions for High-Strength Aluminium Alloys: Experimental and Fracture Mechanics Analysis - Part 1

The small crack effect was investigated in two high-strength aluminium alloys: 7075-T6 bare and LC9cs clad alloy. Both experimental and analytical investigations were conducted to study crack initiation and growth of small cracks. In the experimental program, fatigue tests, small crack and large crack tests A,ere conducted under constant amplitude and Mini-TWIST spectrum loading conditions. A pronounced small crack effect was observed in both materials, especially for the negative stress ratios. For all loading conditions, most of the fatigue life of the SENT specimens was shown to be crack propagation from initial material defects or from the cladding layer. In the analysis program, three-dimensional finite element and A weight function methods were used to determine stress intensity factors and to develop SIF equations for surface and corner cracks at the notch in the SENT specimens. A plastisity-induced crack-closure model was used to correlate small and large crack data, and to make fatigue life predictions, Predicted crack-growth rates and fatigue lives agreed well with experiments. A total fatigue life prediction method for the aluminum alloys was developed and demonstrated using the crack-closure model.

Wu, X. R.↗

NASA NTRS1997

Fatigue-Life Prediction Methodology Using Small-Crack Theory

This paper reviews the capabilities of a plasticity-induced crack-closure model to predict fatigue lives of metallic materials using 'small-crack theory' for various materials and loading conditions. Crack-tip constraint factors, to account for three-dimensional state-of-stress effects, were selected to correlate large-crack growth rate data as a function of the effective-stress-intensity factor range (delta K(eff)) under constant-amplitude loading. Some modifications to the delta k(eff)-rate relations were needed in the near-threshold regime to fit measured small-crack growth rate behavior and fatigue endurance limits. The model was then used to calculate small- and large-crack growth rates, and to predict total fatigue lives, for notched and un-notched specimens made of two aluminum alloys and a steel under constant-amplitude and spectrum loading. Fatigue lives were calculated using the crack-growth relations and microstructural features like those that initiated cracks for the aluminum alloys and steel for edge-notched specimens. An equivalent-initial-flaw-size concept was used to calculate fatigue lives in other cases. Results from the tests and analyses agreed well.

Newmann, James C., Jr.↗

NASA NTRS1992

Fatigue mechanics - An assessment of a unified approach to life prediction

Consideration is given to the development of a total-life prediction methodology for aerospace structures based solely on crack propagation from a microstructural defect at stress concentrations. Crack-growth lives were calculated for a given loading condition by integrating the crack-growth-rate-against-delta K relationships for crack growth from a microstructural defect size to failure. Both small- and large-crack growth rate data were used. The assessment was based on data on 2024-T3 aluminum alloy, 2090-T8E41 aluminum-lithium alloy, annealed Ti-6Al-4V titanium alloy, and high-strength 4340 steel under either constant-amplitude or spectrum loading. Good agreement was found between fatigue lives measured on notched specimens with those computed from the total-life analysis.

Newman, J. C., Jr.↗

NASA NTRS1992

Small-crack growth behavior in high-strength aluminum alloys - A NASA/CAE cooperative program

NASA and the Chinese Aeronautics Establishment have participated in a Fatigue and Fracture Mechanics Cooperative Program giving attention to small-crack effects in high-strength Al alloys. Experimental and analytical investigations studied crack initiation and growth behavior of small cracks in both bare 7075-T6 and clad LC9cs Al alloys, with a view to the improvement of their fracture-mechanics analyses in the cases of surface crack and corner crack configurations. It is also expected that superior life-prediction methods will be developed in order to correlate and predict small crack and large crack growth with various load histories.

Newman, J. C., Jr.↗

NASA NTRS1990

The growth of short cracks in 4340 steel and aluminum-lithium 2090

The objectives were to investigate crack initiation characteristics and short crack growth behavior for Al-Li 2090 and for 4340 steel and to evaluate the ability of a closure-based crack-growth model to predict fatigue crack growth rates and total fatigue lives for the steel. Single-edge-notched tension specimens of each alloy were used to obtain the short crack growth rate information via an acetate replica technique. In addition to constant amplitude loading, tests on the steel were conducted using the Felix/28 variable amplitude spectrum (a shortened form of a standard loading sequence for fixed or semi-rigid helicopter rotors). The short crack growth rates were compared to those for long cracks grown under similar loading conditions. Metallurgical features associated with crack initiation are discussed. For Al-Li 2090 under R = -1 loading, the short cracks grew well below the long crack threshold and grew at acute angles to the loading axis. For 4340 steel under constant amplitude loading at R = 0.5 and 0 and for the Felix/28 spectrum loading, short-crack growth rates agreed well with long-crack growth rates, even near the long-crack threshold. A slight short-crack effect, growth below the long-crack threshold, was observed at R = -1. Fatigue lives were found to depend on the size and type of initiation site, especially for the Felix/28 loading sequence. A semi-empirical crack-growth model incorporating crack-closure effects was used to predict crack growth rates and total fatigue lives of notched 4340 steel specimens. An initial defect size and shape typical of those identified in this steel was assumed for the life predictions. For all loading conditions, reasonable agreement was found between measured and predicted values for both crack growth rates and fatigue lives.

Swain, M. H.↗

NASA NTRS1990

Fatigue crack initiation and small crack growth in several airframe alloys

The growth of naturally-initiated small cracks under a variety of constant amplitude and variable amplitude load sequences is examined for several airframe materials: the conventional aluminum alloys, 2024-T3 and 7075-T6, the aluminum-lithium alloy, 2090-T8E41, and 4340 steel. Loading conditions investigated include constant amplitude loading at R = 0.5, 0, -1 and -2 and the variable amplitude sequences FALSTAFF, Mini-TWIST and FELIX/28. Crack growth was measured at the root of semicircular edge notches using acetate replicas. Crack growth rates are compared on a stress intensity factor basis, to those for large cracks to evaluate the extent of the small crack effect in each alloy. In addition, the various alloys are compared on a crack initiation and crack growth morphology basis.

Swain, M. H.↗

NASA NTRS1990

Fatigue crack initiation and small crack growth in several airframe alloys

The growth of naturally-initiated small cracks under a variety of constant amplitude and variable amplitude load sequences is examined for several airframe materials: the conventional aluminum alloys, 2024-T3 and 7075-T6, the aluminum-lithium alloy, 2090-T8E41 and 4340 steel. Loading conditions investigated include constant amplitude loading at R = 0.5, 0, -1 and -2 and the variable amplitude sequences FALSTAFF, Mini-TWIST, and FELIX/28. Crack growth was measured at the root of semicircular edge notches using acetate replicas. Crack growth rates are compared on a stress intensity factor basis, to those for large cracks to evaluate the extent of the small crack effect in each alloy. In addition, the various alloys are compared on a crack initiation and crack growth morphology basis.

Swain, M. H.↗

NASA NTRS1988

Preliminary fatigue tests on short-crack specimens

The objective of the preliminary fatigue tests was to determine the stress levels to be used by the participants in the Cooperative Test Program. As previously mentioned, the United States Air Force Wright-Patterson Aeronautical Laboratory machined about 700 single-edge-notched tension (SENT) specimens from material supplied by the NASA Langley Research Center. In addition, NASA Langley machined about 50 SENT specimens from the same material. Comparison of fatigue tests conducted on the two sets of specimens indicated a large difference in fatigue behavior under constant-amplitude loading. A recovery program was initiated to resolve this difference before the specimens were distributed to participants. The following sections describe the recovery program and the preliminary fatigue tests and analyses conducted to determine stress levels for constant-amplitude, FALSTAFF, and Gaussian loading.

Newman, J. C., Jr.↗

NASA NTRS1988

The growth of small cracks in 4340 steel

A semiempirical crack-growth model incorporating crack-closure effects has been used to predict crack growth rates and total fatigue life in notched specimens of 4340 steel having an initial defect typical of those previously noted in this steel upon subjection to both constant-amplitude and spectrum loadings. Model predictions for fatigue life are found to be in reasonable agreement with experiments and, in particular, the model was able to accurately predict total fatigue life for specimens subjected to the 'Felix/28' spectrum loading sequence on the sole basis of constant-amplitude test test results.

Swain, M. H.↗

NASA NTRS1988

Predicting the growth of small and large cracks using a crack-closure model

An evaluation is conducted of the ability of a crack-closure model to predict the fatigue life of notched specimens under spectrum loading, for the cases of crack initiation and growth along the bore of a semicircular notch and the growth of large cracks in 2024-T3 Al alloy sheets under constant-amplitude loading. These cases are used to establish an initial defect size and relationship between the stress-intensity factor range and crack-growth rate. Experimental and calculation results for small-crack growth rates are in good agreement and exhibited a stress level effect at equal values of stress-intensity factor range.

Newman, J. C., Jr.↗

NASA NTRS1986

An assessment of the small-crack effect for 2024-T3 aluminum alloy

Data on small-crack behavior were obtained for a single-edge-notched tensile specimen made of 2024-T3 Al alloy and used to evaluate the capability of a semiempirical crack-growth and closure model to predict the fatigue life of notched specimens. Fatigue tests were conducted under either constant-amplitude loading (with stress ratios of 0.5, 0, -1, and -2) or spectrum loading, using a replication technique to record growth. It was found that small cracks exhibited the 'small-crack' effect in that they grew faster than large cracks when subjected to the same stress intensity factor range. Experimental small-crack growth rates agreed well with the model predictions. For making predictions of fatigue life, an initial surface defect void size of 3 x 12 x 0.4 microns was used in all calculations; predicted fatigue lives agreed well with experimentally determined values obtained in all tests. The crack-closure model indicated that the 'small-crack' effect on fatigue life was greatest in tests involving significant compressive loads.

Newman, J. C., Jr.↗

NASA NTRS1984

On the use of marker loads and replicas for measuring growth rates for small cracks

The initiation and growth of small cracks (5-500 microns m) from edge notches in 2024-T3 aluminum alloy sheets were studied under constant amplitude loading. Two methods were used to measure crack shape and size. In the first method, striation marker bands were periodically formed along the crack front by interrupting the constant amplitude loading by either an elevated R-ratio load sequence, or by an overload sequence. In the second method, the surface crack length was monitored by taking surface replicas at regular intervals. The marker band techniques did not provide reliable crack length and crack shape information for cracks smaller than 2 mm. The replica technique provided accurate information for surface crack length at all crack lengths, and fracture tests on specimens with small cracks provided crack-shape information. Crack growth rates were plotted against the stress intensity factor ranges. The results exhibited the small crack effect, in that the small cracks grew faster than large cracks at the same stress-intensity factor range. A crack closure model was also used to analyze the growth of small cracks from small (inclusion) defects at the notch surface.

Swain, M. H.↗

NASA NTRS1984

On the use of marker loads and replicas for measuring growth rates for small cracks

The initiation and growth of small cracks (5-500 microns) from edge notches in 2024-T3 aluminum alloy sheets were studied under constant-amplitude loading. Two methods were used to measure crack shape and size. In the first method, striation marker bands were periodically formed along the crack front by interrupting the constant-amplitude loading by either an elevated R-ratio load sequence, or by an overload sequence. In each case the marker loading was selected so as to have minimal influence on the growth rate under the primary loading. In the second method, the surface crack length was monitored by taking surface replicas at regular intervals. The marker band techniques did not provide reliable crack length and crack shape information for cracks smaller than 2 mm. The replica technique provided accurate information for surface crack length at all crack lengths, and fracture tests on specimens with small cracks provided crack-shape information.

Swain, M. H.↗