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Tensile Tests ( Part – X)

Crack propagation - compounding stress intensity factors, growth in steels including short cracks, NDT.

The compounding method of estimating stress intensity factors for cracks in complex configurations using solutions from simple configurations.

ESDU 78036 gives equations that enable known solutions for the stress intensity factor in simple configurations to be compounded to give a stress intensity factor for a complex component. The stress intensity factor is affected by the presence of "boundaries"; the surfaces of the component, holes in it, stiffeners attached to it. The method takes account of the boundaries by breaking the component down into a group of ancillary configurations each of which contains the crack and one boundary, and for each of which there is a known solution. Two conditions are considered: with the crack crossing a boundary or not crossing a boundary. Comprehensive practical worked examples illustrate the application of the method.

Keywords: FACTOR, INTENSITY, STRESS

Introduction to the use of linear elastic fracture mechanics in estimating fatigue crack growth rates and residual strength of components.

ESDU 80036 introduces the concepts of linear elastic fracture mechanics and shows how these concepts may be used for analysing the behaviour of cracked structures under fatigue or static loadings. It describes the analytical method, based upon the stress intensity factor, and outlines the equations and limitations of usage. It shows how the stress intensity factor, K, which characterises the stress distribution in the vicinity of the crack tip, can be used to correlate crack propagation rate data and describes static failure (fracture toughness) and residual strength of materials under conditions where gross yielding does not occur. Details of how solutions for K may be obtained are discussed together with the effect of plasticity at the crack tip.

Keywords: CRACK, CRITICAL, CURVE, FACTOR, FATIGUE, FRACTURE, GROWTH, INELASTIC, INTENSITY, LENGTH, MECHANICS, PLASTICITY, PROPAGATION, R, RANGE, RATE, RESIDUAL, RESISTANCE, SIZE, STRENGTH, STRESS, THRESHOLD, TOUGHNESS

Fatigue crack propagation in low and medium strength low alloy steel plate, bar and forgings.

Constant amplitude fatigue crack propagation rate (da/dN) data are provided for a wide range of low and medium strength low alloy steels in the form of plate, bar and forgings. The ultimate tensile strength of the materials is less than 850 MN/sq m and their thicknesses range from 5 to 76 mm. The data have been derived from tests conducted in laboratory air at room temperature and mean da/dN curves are presented in terms of the stress intensity factor range and the 0.2 per cent proof stress of the material. Curves are drawn either side of the mean curve that indicate limits of scatter of 90 per cent of the data. The range of da/dN covered is from 10E-5 m/cycle to 10E-9 m/cycle. The effects of variations in mean stress, specimen size and thickness, and material yield strength are discussed. Details of all the materials and types of test specimen are tabulated and summary da/dN curves that compare the different groups of materials are given. A simple worked example illustrates how the data may be used in practice.

Keywords: BAR, CRACK, FATIGUE, FORGING, GROWTH, PLATE, PROPAGATION, STEEL

Fatigue threshold stress intensity factors and slow crack propagation rates in low and medium strength low alloy steel.

Constant amplitude fatigue crack propagation rate (da/dN) data are provided for a wide range of low and medium strength steels in the form of plate and a forging. The ultimate tensile strength of the materials is less than 850 MN/sq m and their thicknesses range from 5 to 70 mm. The data have been derived from tests conducted in laboratory air at room temperature.

Compiled by Sabita Das