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Tensile Tests ( Part – IX)

Fatigue crack propagation rates and threshold stress intensity factors in high alloy and corrosion resistant (stainless) steel.

ESDU 84003 provides constant amplitude crack growth rate data for a wide range of common specifications of high alloy and stainless steels in the form of plate, sheet, bar, castings or forgings. The data were derived from results of tests, drawn from many sources in the literature, made at room temperature in laboratory air and are presented as curves plotted against stress intensity factor range down to threshold values for various values of stress ratio. The chemical composition of the steels and mechanical properties of the test specimens are tabulated, and summary curves show the effects of steel composition on crack growth and of stress ratio on threshold stress intensity factor range. The effect of crack closure in creating an effective stress intensity factor is discussed and another summary graph illustrates the point by showing the variation of threshold stress intensity factor range with maximum stress intensity factor. Other matters discussed are the effects of specimen size, environment, heat treatment, steel composition and stress ratio. Practical worked examples illustrate the use of the data.

Keywords: BAR, CASTING, CRACK, FACTOR, FATIGUE, FORGING, GROWTH, INTENSITY, PLATE, PROPAGATION, RANGE, SHEET, STEEL, STRESS, THRESHOLD

Fatigue crack propagation rates and threshold stress intensity factors in high strength low alloy steel plate, bar and forgings.

ESDU 85019 gives graphs showing the rate of increase in crack length per loading cycle plotted against stress intensity factor range for a large group of steels. The curves were obtained by correlating results taken from the literature for tests on steels of tensile strength from 850 to 1970 MN/sq m (123000 to 286000 lbf/sq in). Specimens were tested under constant amplitude loading at room temperature in laboratory air. Included are details of the test specimens, their loading (mainly tensile axial), and the exact composition of the steels. The influence of fracture toughness, mean stress, and material proof stress can be seen in the data and is discussed. Summary curves are included that show the effect of stress ratio and material proof stress on crack growth rates. Two practical worked examples show how the data may be used to establish the life of a component or whether an existing crack will grow.

Keywords: BAR, CRACK, FACTOR, FATIGUE, FORGING, GROWTH, INTENSITY, MEAN, PROPAGATION, RANGE, STEEL, STRESS, THRESHOLD

Fatigue propagation behaviour of short cracks (1-2 mm) in steels.

ESDU 93033 presents data that compare the behaviour of long and short cracks which are longer than the typical microstructural dimension but are not substantially larger than the radius of the plastic zone at the crack tip. Experimental data extracted from the literature for crack growth rate under constant amplitude loading are presented graphically against stress intensity factor range for both long and short cracks at various stress ratios in air for AISI 4340, HY 130, AISI 1045, SAE 0030 (UNS J 03010) and AISI 1020. In addition, there are data obtained in vacuum for E460, and in salt solution and sea water for HY 130. Data in air for E460, A508, QIN and 304 are for only one stress ratio (but include the effect of sea water for QIN). Data for AISI 316 are for short cracks only in air at two stress ratios and for EN5 are for short and long cracks in sea water and one stress ratio only. No consistent relationship between yield strength and either short or long crack growth behaviour was found, but where a short crack effect was evident the threshold value of stress intensity factor range was lower than for long cracks, and the crack growth rate at low values of stress intensity factor range was greater.

Keywords: CRACK, ENVIRONMENT, FATIGUE, GROWTH, MEAN, PROPAGATION, SHORT, STEEL, STRESS

Compiled by Sabita Das