Distributions of stress and plastic strain in circumferentially notched tension specimens

Distributions of stress and plastic strain at the minimum diameter in circumferentially notched bars with three different notch profiles are deduced from observation of the displacements of bands within the material and a modified Bridgman analysis. The strain distributions were found to be non-linear; the degree of non-linearity being dependent on the degree of straining and strongly dependent upon notch geometry, which cannot be described only by the ratio of the minimum radius to the profile radius.

For two of the three profiles studied the longitudinal stress distributions were found to be nearly uniform across the minimum diameter whilst the third was found to be extremely non-uniform. Qualitative confirmation of the strain distribution results were obtained by use of transversely orientated specimens.     

Correlation of acoustic emission with stress intensity factor and plastic zone size for notched tensile specimens of AISI type 304 stainless steel

Abstract

Correlation of acoustic emission (AE) total counts N with stress intensity factor K and plastic zone size r_p has been examined for data generated during tensile deformation of notched specimens of nuclear and commercial grade AISI type 304 stainless steel. The variation of total AE counts with stress intensity factor in log-log scale has indicated that N is related to K as N=AK^m where A and m are constants but the magnitudes of A and m are different at low and high K regimes. Both the steels indicate higher values of m up to macroyielding than those obtained from analysis of AE data between macroyielding and the stress corresponding to K _max values in the experiments. The magnitudes of m were found to be higher for the commercial grade steel than that for the nuclear grade one and dependent on thickness. Analysis of AE data has also indicated that the relationship between N and r _p can be expressed by the equation N=αr^β_p. The value of the constant β was experimentally found to be 1.3 via microhardness measurements. This value is higher than the theoretically assumed value of β~ 1in the literature. The value of m in the equation N=AKm can be obtained from the value of the exponent β of the relationship N=α r^β as m=2β and such values of m can be correlated to the directly estimated values of the exponent in the N-K relationship. The amounts of strain induced α' martensite formed at notch tips in the two steels were found to increase with increasing applied stress below the nominal yield strength of unnotched specimens.     

Fracture resistance of notched specimens

This paper gives a theoretical approach to the general problem of the static fracture resistance in the presence of stress concentrations. First of all, an approaching calculation model is proposed for elastic and plastic stress in the reduced section of a circumferentially notched cylindrical bar. The introduction of a normal stress criterion or of an axial strain fracture criterion on the elastic plastic interface makes it possible to explain the experimental results covering the fracture of various steels above a certain sharpness.     

Fatigue-creep and plastic collapse of notched bars

The paper describes a study of the application of a new assessment method, based on the linear matching method (LMM), to the creep fatigue of notched bars of Udimet 720Li at 650 °C. This high strength nickel based alloy is taken as typical of alloys used in high temperature gas turbine applications. The primary purpose of the study is to see whether it is possible to predict the failure modes for such alloys in terms of standard materials data by the evaluation of a sequence of simplified calculations corresponding to the steady state cyclic stress history. These calculations involve the evaluation of limit load, shakedown limit and ratchet limit for perfect plasticity, rapid cycle creep solutions and the evaluation of an elastic follow‐up factor. Within the limitations of the tests, the correlation between predicted failure mode and observed mode is very good and the calculations clearly show up the differences between the two types of notched bars discussed. This implies that LM methods are well suited to the evaluation of failure modes in materials of this type.     

The fracture behaviour of notched specimens of polymethylmethacrylate

The fracture behaviour of notched specimens of polymethylmethacrylate has been examined for a wide range of geometries in Charpy impact tests, and in tensile and slow bend fracture tests. It was found that the failure of the very sharply notched specimens was consistent with linear elastic fracture mechanics and defined a constant fracture toughness K _IC for a constant notch tip radius, whereas the blunt notched specimens failed at a constant critical stress at the root of the notch.     

Variable amplitude crack growth in notched specimens

Predictions of variable amplitude fatigue crack growth in notched tensile specimens at an elevated temperature and relative high loads were conducted. The predictions were based on constant amplitude fatigue data and mean crack growth rates were calculated. Crack closure levels were calculated by comparison of crack closure free experimental data obtained with the so called constant K_I,max-method and nominal data. Also, numerical simulations of the closure level throughout the specimens were performed. Although experimental data showed rather large a scatter it was concluded that variable amplitude crack growth rates can be predicted with reasonable agreement using constant amplitude fatigue data. Also, crack growth rates cannot be predicted by LEFM in the inelastic notch affected zone, even if absence of crack closure is assumed.     

Fracture behavior of notched specimens of TiAl alloys

Fracture behavior of a two-phase TiAl alloy was investigated using notched specimens. Fracture surfaces and metallographic sections of surviving notch in double notched specimens are observed. The fracture process of notched specimens of TiAl alloys was described as that several inter-lamellar cracks initiate and extend directly from the notch root and propagate preferentially along the interfaces between lamellae and stop at various obstacles. With increasing applied load, cracks connect with each other and propagate further by translamellar cracks. The toughening mechanisms, which make the main crack difficult to propagate or cause it to be stopped, could be reducing the driving force for crack propagation. The higher toughness of near fully lamellar microstructure than that of finer duplex microstructure is attributed to the path of crack propagation. On the fracture surfaces of the finer duplex microstructure, more low-energy-spending interlamellar fracture facets are observed, which means that it is easier for crack to bypass a fine duplex lamellar grain with lamellae perpendicular to the main crack and to take a interlamellar path.     

Results of elastic analysis of bluntly notched compact specimens

Stress concentration factors are given for a number of variations in geometry. These are compared to estimates based on fracture mechanics, and a method is presented for correcting the inaccuracies of the latter, so that improved estimates may be made. For one particular specimen geometry, stress intensities are also presented for cracks of various lengths at the end of the blunt notch.     

The impact fracture behaviour of notched specimens of polycarbonate

The impact fracture behaviour of notched specimens of polycarbonate has been studied for a range of notch tip radii. For razor-notched specimens a simple fracture toughness analysis is appropriate, as shown by previous workers. Very blunt notches also give constant fracture toughness values, but at a much higher level, corresponding to a different mode of failure. For intermediate notch tip radii the situation is much more complex, and comparison of results for two molecular weight grades shows that the behaviour is molecular weight-dependent. Analysis of these results has been discussed either in terms of a combination of plane strain and plane stress fracture modes, or in terms of a critical stress at the root of the notch, which appears to be appropriate in certain cases.     

Hydrogen embrittlement fracture of notched amorphous Ni alloy specimens

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.