Single Edge Notch Bend Specimens Research Articles

Crack Growth in Hardened Cement Paste

ABSTRACTThe subcritical crack growth parameter (n) has been determined both from the variation in the crack velocity (V) with stress intensity (K) in double torsion specimens, and from the variation in critical stress intensity (Klc) with loading rate (x) in single edge-notch bend specimens. As in previous cases for hardened cement paste (HCP), and other inhomogeneous materials, the value of n determined using double torsion specimens (˜70) was higher than that determined using SENB (˜30).Backscattered electron image examination of cracks in polished sections of HCP has shown that all of the components, both hydration products and anhydrous phases, are involved in determining crack paths although, as expected; calcium silicate hydrate and calcium hydroxide form the majority of the crack path; unnhydrated cement cores are resistant to crack growth and cause the crack to deviate around them. There is some evidence that crack growth occurs by the formation and coalescence of microcracks ahead of the crack tip, with only a limited formation of an actual process zone.

Elastic-plastic fracture mechanics analysis of an SENB specimen

This paper describes the elastic-plastic analysis of a Single Edge Notched Bend (SENB) specimen, a specimen commonly used for the determination of the fracture toughness of a material. First, a two-dimensional convergence study is described to investigate the influence of mesh size on the global response of the specimen and the fracture mechanics parameters J and COD. Then a three-dimensional analysis is compared with experimental results. The three-dimensional computational results show a good agreement with the experiments, except for the load region where the global deformations were dominated by the Lueders zone in the material stress-strain curve.

Displacements and rotational factors in single edge notched bend specimens

Displacements and rotational factors in single edge notched bend specimens

Influence of hydrostatic tension on cleavage fracture of bainitic pressure vessel steel

Experiments have been performed to investigate the influence of hydrostatic tension on the cleavage fracture of steel to ASTM Standard A508 Class II, a bainitic pressure vessel steel. Material given either of two heat treatments was studied, one giving a fully tempered granular bainitic microstructure and the other an as-transformed upper bainitic microstructure. Single-edge notched bend specimens with different notch angles were tested over a range of temperatures to locate the temperature at which fracture was coincident with general yielding. A slip line field theory analysis showed the maximum tensile stress present below the notch at fracture to be largely independent of notch angle and of temperature. It is concluded that the cleavage fracture of bainitic steels, like that of mild steel, is tensile-stress controlled. Thus it may be possible to apply a recently developed micromechanistic model of cleavage fracture to investigate the effects of temperature, strain rate, warm prestressing, strain aging, and irradiation on the fracture toughness of bainitic pressure vessel steels. The micromechanism of cleavage in bainitic steels is discussed in the light of this conclusion. Metallographic evidence supports the view that cleavage fracture is not nucleated by cracked carbide particles in these bainitic microstructures.

ELASTIC?PLASTIC FINITE ELEMENT ANALYSES OF SHORT CRACKS

Abstract— Stress and strain distributions and crack opening displacement characteristics of short cracks have been studied in single edge notch bend and centre cracked panel specimens using elastic–plastic finite element analyses incorporating both a non strain hardening and a power law hardening behaviour. J contour integral solutions to describe stress strain conditions at crack tips for short cracks differ from those for long cracks. The analyses show that (i) short cracks can propagate at stress levels lower than those required for long cracks and (ii) a two-parameter description of crack tip fields is necessary for crack propagation.

The fracture toughness testing of ceramics and acoustic emission

Measurements of the fracture toughness of commercial sintered alumina were monitored using acoustic emission to detect subcritical crack growth. Double-cantilever-beam specimens gave higher values of toughness than single-edge-notched bend (SENB) or double-torsion specimens did, even after correction for slow crack growth. The SENB test with a sharp crack appears to give the most reliable results, requires the easiest specimens to prepare and suffers the least slow crack growth. In view of this, detailed studies of the damage made by sawing notches, of the effects of the a/W ratio and of the cross-head displacement rate were made for SENB specimens. Sharp cracks with small a/W ratios and high cross-head speeds minimize the effects of slow crack growth.

Creep crack growth in variously tempered bainitic and martensitic 0.5r0.5Mo0.25V steel

Creep crack growth tests were performed on single edge notch bend (S.E.N.B.) and tension (S.E.N.T.) specimens of tempered, coarse grained martensitic and bainitic 0.5Cr0.5Mo0.25V steel under vacuum at temperatures of 525 – 565 °C. With certain exceptions, crack growth rates showed a general decrease with increasing tempering time and temperature. Granular bainitic structures had higher resistance to crack growth than martensite of a similar room temperature hardness, which, in turn, cracked marginally slower than mixed upper/lower bainites. A correlation of crack growth rates with linear elastic stress intensity was found to hold for most tests, the stress intensity exponent being 4.5 ± 0.5 (S.E.N.B.) or 3.6 ± 0.4 (S.E.N.T.). Apparent activation energies for crack growth of 368 ± 20 kJ mol −1 (S.E.N.B.) and 430 ± 20 kJ mol −1 (S.E.N.T.) were found.

The use of the C∗ parameter in predicting creep crack propagation rates

The applicability of the C∗ parameter for the prediction of creep crack propagation rates is considered. A new method for estimating C∗ is presented, the results from which show good agreement with those from an existing technique. Experimental results from creep crack growth tests, conducted on a 1 Cr Mo V steel using both compact tension and single edge notch bend specimens, indicate that good correlation with C∗ is obtained once the effects of stress redistribution become negligible. Finally, comparisons are drawn between C∗ and other possible correlating parameters, and the limitations of each approach are discussed.

The effect of microstructure on creep crack growth in notched bend tests on 0.5Cr0.5Mo0.25V steel

Creep crack growth tests under vacuum at 565 °C were conducted on small (10 mm × 10 mm cross-section) single-edge notch bend specimens of a 0.5Cr0.5Mo0.25V steel. The effect of varying the microstructure from bainite, to the ferrite-pearlite state was investigated. Crack lengths and crack-opening displacements were recorded as a function of time and were related to the applied stresses and to the material microstructures. The presence of small amounts of ferrite in an otherwise bainitic structure had a marked effect on crack growth rates. The formation of 15% bainite in an otherwise ferritic structure drastically reduced the ductility. This was attributed, in part, to the lower creep strength of the ferrite-pearlite resulting from a slow cooling rate from the austenitising temperature which allowed heavy carbide precipitation and overaging.

Experimental determination of fracture mechanics stress intensity calibration in four-point bending

A precise experimental method to determine K- calibration factors from compliance measurements is presented. The usefulness of the method is demonstrated on a series of single edge-notch bend specimens loaded in four-point bending with relative notch depths ranging from 0.15 to 0.91. The results are expressed in terms of a convenient dimensionless parameter Z(X) = c 0((N(X)-N 0)/N 0) where N(X) and N 0 are the compliance of the notched and the unnotched specimens respectively, and c 0 is a constant depending only on test and specimen geometry. Comparision of the measured data with published results calculated by boundary collocation procedures shows an excellent agreement. Investigations of specimen compliance as a function of the notch width indicate that the effect of using a machined slot to simulate an atomicatly sharp crack is negligibly small up to 0.25 mm notch width.

On slant (45°) crack growth in fatigue

to be expected for slant crack growth,4 as confirmed by the Ritchie, Smith, and Knott tests. Buckling involves out-of-plane deformations, but these should not be confused with those caused by throughthickness yielding associated with plane-stress conditions at a crack tip. When pronounced, buckling in very thin sheets can reduce strength through an increase in K I due to the inflexion which appears at a crack tip,5 not by the appearance of Mode III .deformations; in this case slant growth is Mode I, by the small-scale argument. Buckling would not normally be expected in 'thin' specimens of the type used by Ritchie, Smith, and Knott, but if it did occur it would obviously cause a particular 45 plane to be favoured. Observation of fracture surfaces shows6 that the transition to slant crack growth takes place by the development of shear lips (Fig. 1), which increase in width until they reach a maximum size or, in 'thin' specimens, they meet, completing the transition. (Tunnelling of the crack in the centre of the specimen while the transition is taking place is more likely to be the result of the complex three-dimensional stress system in the transition region than evidence that square crack growth is faster.) The length of this transition zone of mixed square and slant fracture depends on precise loading conditions.6 The problem of what causes slant growth therefore reduces to finding the conditions for a shear lip to develop. The appearance of shear lips in thick specimens rules out buckling as a cause. Shear lips appear on both sides of a specimen at about the same time, so their appearance must be due to the satisfaction of some condition. In thick specimens shear lips appear randomly on parallel and perpendicular planes (Fig. 1), whereas in thin specimens they usually, but not invariably,6 appear on parallel planes. Material textures which suppress through-thickness yielding also suppress shear lips,' as can changes in work-hardening behaviour.! Slant crack growth only Qccurs in those materials in which the static tensile fracture of a piece of the same material of the same thickness also occurs on a 450 through-the-thickness plane.6 RITCHIE, SMITH, and KNOTT have recently published! some data on the fatigue crack growth properties of mild steel. Loading conditions were such that medium rates of crack growth (5 x 10-6 to 5 X 10-4 mm/cycle) were obtained. This is above the region of the threshold for fatigue-crack growth, but below the region where gross plasticity effects are likely. Data were analysed in terms of !:::.K I, the range of opening mode stress-intensity factor K I, in a fatigue cycle. As would be expected, results f~r thick (13, 15, 20, and 48 mm) singleedge-notched (SEN) specimens tested in pure bending agreed reasonably well with results for 20 mm thick SEN specimens tested in tension. The relatively minor inconsistencies observed were probably rightly attributed to shortcomings in the use of !:::.K I in the analysis. An interesting feature of the results for thin specimens is that for 2 mm SEN bend specimens the crack -growth rates were about half those for thick specimens, with 'square' growth on a 90 plane, whereas for 1 and 2 mm SEN tension specimens the crack -growth rates were about twice those for thick specimens with 'slant' growth on a 45 plane through the specimen thickness. Possible reasons for these differences were discussed in general terms, but some further clarification of the fracture mechanics involved is desirable; various factors involved -are therefore briefly _reviewed and discussed.