Back-face strain compliance relation for compact specimens for wide range in crack lengths

A back-face strain (BFS) compliance relation has been developed for the standard compact C(T) specimen for a very wide range in crack-length-to-width (a/W) ratios. Both finite-element and boundary-element methods were used to develop the BFS relation for a/W ratios from 0.2 to 0.95. In addition, experimentally determined compliance values on four metallic materials compared well with the new relation over nearly the complete a/W range. The new relation can be used to monitor crack-length-against-cycles using computerized crack-monitoring systems that are currently used with the crack-mouth-opening-displacement (CMOD) gage method, and the relation has been cast in a standard form compatible with existing compliance crack length monitoring systems.     

An improved method of sessile drop for determining the surface tension of liquids

An improved procedure is developed for the processing of images of meridional section of a liquid drop, obtained as a result of realization of the sessile drop method for determining the surface tension of liquid. The procedure provides for the scanning of digital image of droplet, for numerical solution of the Young-Laplace equation, and for the calculation of surface tension, wetting angle, and volume of the drop.     

Measurement of fatigue crack growth in large compact tension specimens using an AC magnetic field method

Fatigue crack growth rate data are required in order to carry out a numerical analysis of the fatigue performance of complex structural components. These data are obtained by measuring crack growth in standard fracture mechanics specimens. A new method for measuring fatigue crack growth in compact tension specimens has been developed. The technique is based on the measurement of the surface magnetic fields produced when passing a high-frequency alternating current through the specimen. Fatigue crack growth data recorded using this method indicated an accuracy of ±0.02 mm when compared with optical measurements. The technique is suitable for computer-controlled operation and could easily be applied to other standard specimen geometries.     

Electrical potential drop method for evaluating crack depth

Analytical and experimental studies are performed on the electrical potential drop method for monitoring the properties of a surface crack. Integral formulas for potential drops for direct currents are derived by applying Green's theorem to the Laplace equation governing potential drop behaviour. Based on these formulas, a simple non-destructive method is proposed for evaluating the depth of a surface half-elliptic crack and for calibrating potential drop behaviour. The validity and applicability of this method are assessed through the comparison of analysis and experiment.     

Effect of crack curvature on stress intensity factors for ASTM standard compact tension specimens

The stress intensity factors (SIF) are calculated using the method of lines for the compact tension specimen in tensile and shear loading for curved crack fronts. For the purely elastic case, it was found that as the crack front curvature increases, the SIF value at the center of the specimen decreases while increasing at the surface. For higher values of crack front curvatures, the maximum value of the SIF occurs at an interior point located adjacent to the surface. A thickness average SIF was computed for parabolically applied shear loading. It is assumed that it reflects the average stress environment near the crack edge. These results were used to assess the requirements of ASTM standards E399-71 and E399-81 on the shape of crack fronts.     

Quantitative characterization of creep constraint induced by crack depths in compact tension specimens

In this paper, the C(t) and C∗ integrals, stress redistribution time t_red and creep crack-tip stress distributions in the CT specimens with various crack depths have been calculated by the finite element method (FEM), and the creep constraint induced by crack depths are quantitatively investigated in detail. The results show that the creep constraint could be characterized by the new constraint parameter R. The constraint effect induced by crack depths at non-steady-state creep is more pronounced than that at steady-state creep. The effects of the crack depths, load levels (C∗) and distances from the crack tips on the creep constraint parameter R are analyzed.     

An automatic algorithm for mixed mode crack growth rate based on drop potential method

A new automatic algorithm for the assessment of mixed mode crack growth rate characteristics is presented based on the concept of an equivalent crack. The residual ligament size approach is introduced to implementation this algorithm for identifying the crack tip position on a curved path with respect to the drop potential signal. The automatic algorithm accounting for the curvilinear crack trajectory and employing an electrical potential difference was calibrated with respect to the optical measurements for the growing crack under cyclic mixed mode loading conditions. The effectiveness of the proposed algorithm is confirmed by fatigue tests performed on ST3 steel compact tension–shear specimens in the full range of mode mixities from pure mode I to pure mode II.     

Detailed three-dimensional analysis of side-grooved compact tension specimens under creep loading

Abstract

The three-dimensional (3D) effect on the stress field near the crack tip under creep loading has been investigated via detailed finite-element analyses on side-grooved compact tension (CT) specimens with various thicknesses. The mechanical properties used in the model are typical of those for a 9%Cr steel at 550°C and a Norton creep exponent of 10 was applied. Plane strain and 3D analyses for the CT specimens without side grooves were also performed and the differences in stress distributions in the three analysis conditions are discussed in relation to the creep J-integral C^*. The value of C^* was found to decrease with increasing thickness and to decrease significantly because of the enhanced constraint effect of the side grooves. With increasing thickness, the stress state in the central region is close to that under plane strain conditions, irrespective of the existence of the side groove. Stress triaxiality, however, is distributed more uniformly along thickness in the side-grooved specimen than in the specimen without side grooves. The C^* values of the side-grooved specimens are similar across the full range of thickness studied: no strong effect of thickness was observed.     

Energy density approach for calculation of three-dimensional inelastic strain and stress at the crack tip in compact tension specimens of polycarbonate

An energy-based method is utilized for calculating elastic-plastic strains and stresses near fatigue crack tip in specimens of Merlon polycarbonate. The stress redistribution caused by the plastic yielding around the crack tip is taken into account so that theoretical crack tip strain is improved. The estimated values of crack tip strain based on an energy density approach are compared with experimental results obtained from an embedded grid moire technique and embedded strain gages. Large-scale yielding seems to dominate near the crack tip. In fact, the measured strain is in agreement with the elastic solution, which means, in reality, only small-scale yielding takes place near the crack tip. The strain in the mid-plane (plane strain) is found to be higher than in the surface plane (plane stress). The experimental and theoretical results are in good agreement.     

An investigation into the effect of thickness on the fracture behaviour of compact tension specimens

An experimental investigation into the fracture behaviour of 1 Cr Mo V compact tension specimens is described. The plane strain fracture toughness of 64.2 MN/m^3/2 is compared with the fracture toughness determined from compact tension specimens of thickness 50 mm, 25 mm, 13 mm, 6 mm, and 3 mm.With the 13 mm and 6 mm thickness compact tension specimens pop-in steps occurred in the load-displacement records associated with the formation of thumbnail shaped cracks. Following the pop-in steps much higher maximum loads were attained prior to complete fast fracture.A three-dimensional elastic finite element analysis of an observed thumbnail shaped crack indicated that the stress intensity factor was less than that of the initial straight crack. Using this result it is shown that the fracture toughness at the maximum load was in agreement with the fracture toughness at the pop-in step of the load-displacement test record and the plane strain fracture toughness.

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Résumé Une méthode expérimentale est décrite pour étudier le comportement à la rupture de l'acier à 1 Cr Mo V à l'aide d'éprouvettes de tension compacte. La ténacité à la rupture en condition d'état plan de déformation de 64,2 MN/m^3/2 a été comparée avec la ténacité à la rupture déterminée à l'aide d'éprouvettes de tension compacte ayant des épaisseurs de 50 mm, 25 mm, 13 mm, 6 mm et 3 mm. Avec les éprouvettes de tension compacte d'épaisseur 13 mm et 6 mm, on observe des pop-ins dans les enregistrements de charge-déplacement, lesquels sont associés à la formation de fissures en coup d'ongle. Après l'apparition des pop-ins, des charges maximum beaucoup plus élevées ont été atteintes avant l'apparition de la rupture instable. Une analyse par élément fini élastique à trois dimensions d'une fissure en forme d'onglet indique que le facteur d'intensité de contrainte était plus faible que celui de la fissure droite initiale. En utilisant ce résultat, on montre que la ténacité à la rupture à la charge maximum était en agrément avec la ténacité à la rupture correspondant au moment du pop-in dans l'enregistrement charge-déplacement et avec la ténacité à la rupture correspondant à l'état plan de déformation.

     

Finite element analysis of crack mouth opening displacement compliance in crack length evaluation for clamped single edge tension specimens

In the unloading compliance method developed for clamped single edge tension (SE(T)) specimens, six crack mouth opening displacement (CMOD)‐based compliance equations (i.e. a/W = f(BCE′)) were proposed for the crack length evaluation without clearly clarifying the corresponding predictive accuracies. In addition, the effective elastic modulus (E_e) that reflects the actual state of stress should also be introduced in the crack length evaluation for SE(T) specimens, because the actual state of stress in the remaining ligament of the test specimen is neither plane stress (E) nor plane strain (E′). In this study, two‐dimensional (2D) plane strain and three‐dimensional (3D) finite element analyses (FEAs) are carried out to investigate predictive accuracies of the six compliance equations. In both 2D and 3D FEA, specimens with a wide range of crack lengths and geometric configurations are included. For a given specimen, the value of E_e that presents the equivalent stress state in the remaining ligament is calculated on the basis of 3D FEA data. A set of formulae for the clamped SE(T) specimen is proposed that allows to evaluate E_e from the corresponding CMOD compliance. This approach is verified using numerical data. The observations of the numerical verification suggest that the use of E_e instead of E or E′ in CMOD‐based compliance equations markedly improves the accuracy of the predicted crack length for clamped SE(T) specimens.     

On the calibration of the direct current potential difference method for the determination of semi-elliptical crack lengths

This paper treats the calibration of the direct-current, potential difference (d.c.p.d.) technique for the measurement of the lengths of semi-elliptical (thumbnail) cracks. Calibrations utilised the beachmarking of steel specimens and a three-dimensional analogue method, which employed an electrolytic tank to simulate a testpiece. The agreement between the two sets of calibrations was good, and the results have been compared with an analytical solution for a semi-circular crack in a half-space.

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Résumé L'étude a trait à l'étalonnage de la technique des différences de potentiel en courant continu pour la mesure des longueurs de fissures semi-circulaires dites en ongle. On procède au calibrage d'échantillons en acier et on recourt à une méthode analogique à trois dimensions, qui utilise, pour simuler une pièce d'essai, le concept d'un récipient rempli d'électrolyte.On constate un bon accord entre les deux méthodes d'étalonnage, et on compare les résultats à ceux d'une solution analytique appliquée à une fissure semi-circulaire située dans un demi-espace.

     

A new variant for measuring the surface tension of liquid metals and alloys by the oscillating drop method

The theoretical background of the oscillating drop technique for measuring surface tension is briefly presented and the different analysis procedures are cited. A new method is described for obtaining oscillation frequencies by fast fourier transformation (FFT) of the pyrometer voltage signals from temperature measurements at the top of the levitated sample. The results on the first experiments on liquid nickel are in a good agreement with the hterature data.Paper presented at the Fourth International Workshop on Subsecond Thermophysics. June 27–29, 1995, Köln, Germany.