The influence of specimen geometry on crack tip plasticity

The effect of panel width and thickness on the crack tip plasticity of center-cracked fracture panels of 7075-T6 aluminum alloy was examined. The plastic zone formation was continually monitored photographically during the fracture test and the zone size and shape determined by an image distortion technique. The influence of specimen width and thickness was examined to determine its effect on the plastic zone behavior and the mode (stable tear or pop-in) of initial crack extension. Plastic zone models were then compared with the characteristic plastic zone size parameters measured experimentally and the models evaluated. From these results, the effect of panel thickness and width on the fundamental characteristics of crack tip plasticity and initial crack extension was assessed. The importance and implications of these findings on methods of evaluating material toughness are examined. In addition, the applicability of these zone size models in linear elastic fracture mechanics applications are discussed.     

The green's function for a slant edge crack

The Green's functions are determined for plane edge cracks which meet the free surface at an arbitrary angle. Modes I and II stress intensification factors are found for both normal and shear loading of the crack, since coupling is found to occur between each type of loading and the two possible modes of crack-tip response.     

Correction and extension of Broberg's results on brittle crack propagation

The solution to the problem posed by Broberg is deduced as a special case of the problem of a line crack with ends moving at different speeds. This latter problem is solved using the similarity method recently developed by the author. The generality of the method allows for a unified treatment and extension of several problems which have been considered individually by other authors. The similarity method is used in conjunction with the techniques introduced by Muskhelishvili for static elasticity and is proposed as an alternative to the Wiener-Hopf method.     

Rapid crack propagation and arrest in the dcb specimen—an improved numerical analysis

A two-dimensional linear elastodynamic analysis of crack initiation, fast crack propagation and crack arrest in the DCB specimen is presented. The analysis is performed using the previously developed SMF2D code in its generation mode. The experimentally measured crack tip motion, as well as the specimen's geometry and its material characteristics, serve as input to the simulation. The dynamic stress intensity factor, the dynamic energy release rate and the various distributions of energies are subsequently evaluated. The numerical results are found to be in very good agreement with analytical and experimental findings.     

The elastic-plastic mechanics of crack extension

This paper briefly reviews progress in the elastic plastic analysis of crack extension. Analytical results for plane strain and plane stress deformation fields are noted, and elastic-plastic fracture instability as well as transitional behavior and combined rate and thermal effects are discussed.

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Zusammenfassung In dieser Abhandlung wind kurz der Erfolg von elastisch-plastischen Analysen bei Rissausdehnung. Es wurden analytische Ergehnisse für planierte Anspannungsverformungsfelder und planierte Druckverformungsfelder festgestellt. Ausserdem wurden elastisch-plastische Frakturunbeständigkeit, Übergangsverhalten und Mass- und Thermaleffekte diskutiert.

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Résumé Le mémoire décrit brièvement les progrès réalisés dans l'analyse élasto-plastique de l'extension d'une fissure.Les résultats analytiques pour les champs de déformation en état plan de tension et en état plan de déformation sont discutés. On étudie également le comportement de transition ainsi que les effets combinés de la vitesse et de la température.

     

Materials for the 80's and 90's

The factors governing the application of new materials and manufacturing process are reviewed and the criteria for successful exploitation are described. It is shown that developments should be targetted on specific components but there should preferably be several of these in different industries to reduce the risk and maximise the benefits of success. Composite materials — combinationsof two or more material types are believed to have the best future provided that the necessary production and quality control techniques can be developed.     

The fracture mechanics of finite crack extension

This paper describes a modification to the traditional Griffith energy balance as used in linear elastic fracture mechanics (LEFM). The modification involves using a finite amount of crack extension (Δa) instead of an infinitesimal extension (da) when calculating the energy release rate. We propose to call this method finite fracture mechanics (FFM). This leads to a change in the Griffith equation for brittle fracture, introducing a new term Δa/2: we denote this length as L and assume that it is a material constant. This modification is extremely useful because it allows LEFM to be used to make predictions in two situations in which it is normally invalid: short cracks and notches. It is shown that accurate predictions can be made of both brittle fracture and fatigue behaviour for short cracks and notches in a range of different materials. The value of L can be expressed as a function of two other material constants: the fracture toughness K_c (or threshold ΔK_th in the case of fatigue) and an inherent strength parameter σ₀. For the particular cases of fatigue-limit prediction in metals and brittle fracture in ceramics, it is shown that σ₀ coincides directly with the ultimate tensile strength (or, in fatigue, the fatigue limit), as measured on plain, unnotched specimens. For brittle fracture in polymers and metals, in which larger amounts of plasticity precede fracture, the approach can still be used but σ₀ takes on a different value, higher than the plain-specimen strength, which can be found from experimental data. Predictions can be made very easily for any problem in which the stress intensity factor, K is known as a function of crack length. Furthermore, it is shown that the predictions of this method, FFM, are similar to those of a method known as the line method (LM) in which failure is predicted based on the average stress along a line drawn ahead of the crack or notch.     

Publisher's note

Sea ice pressure ridge sail heights and the dimensions of ice blocks that comprised the sails were measured for 30 ridges in April, 1980. The ridges were located from 30 to 200 km offshore in the Prudhoe Bay, Alaska region. Sail height was found to be a function of the thickness of the ice in the ridge. A reasonable relationship shows that height is dependent on the square root of block thickness. The data also verify that the ratio of sail height to ice block thickness is much larger for ridges composed of thin ice than for those composed of thick ice. Ridge width and cross-sectional area are also found to be related to block thickness. For the largest blocks in the ridge, block surface area is related to thickness squared, as would be expected from plate deflection theory. Examination of lateral variation of height and width along the ridge shows an expected large variation, particularly with height. No geographic variations of individual pressure ridge morphology were discernible from these data, but a laser profile taken during the study period shows that the mean heights and numbers of ridges decrease as the distance from the coast increases.     

The influence of cell geometry on the elasticity of softwood

The cellular microstructure of softwoods such as spruce may be approximated as an irregular two-dimensional honeycomb. The nine macroscopic elastic constants of the wood, regarded as an orthotropic continuum, are governed by the geometric configuration of this honeycomb, together with the intrinsic material properties of the cell walls. Simple modelling is developed to allow all nine of these constants to be estimated from detailed microscopic measurements of the cell geometry, using assumed values for the cell-wall properties. Account is taken both of the cell-to-cell variations in growth and of the larger-scale modulation of cell properties in the annual growth rings. Results based on study of four samples of Norway spruce show very encouraging agreement with published measurements, and allow the relative importance of various effects to be assessed quantitatively.     

The crack tip opening angle (CTOA) of the plane stress moving crack

Recently, Crack Tip Opening Angle (CTOA) was proposed by C.F. Shih et al. to describe the instability criterion of ductile crack propagation during plane strain (flat crack) conditions, and was derived by J. R. Rice analytically by means of the slip line field theory and the incremental theory of plasticity. CTOA appears to be applicable in (some or most) cases, but does not accurately describe the plane stress growing crack (slant crack).Unstable ductile crack propagation of the plane stress crack is widely studied for the safe design of highly pressurized gas pipelines. The impact absorption energy of the Charpy test is well correlated to the fracture arresting properties of the structures, but the mechanics of the fracture are not yet well established.In this paper, CTOA of the plane stress growing crack is derived from the plane stress plasticity of perfectly plastic materials by Sokolovsky's approach. Our proposed modification of CTOA expressed as follows: $\text{CTOA}\text{= (α/δ}{}_0\text{)(}\text{d}\text{J/}\text{d}\text{l) + β(δ}{}_0\text{/E)}\text{ln}\text{(eR/r)}$ where $\text{β = 1.40}$ under the plane stress conditions.CTOA in the Dugdale model is also defined and compared with the results of laboratory test. The results show that $\text{α = 0.5}$, and $\text{β = 1.27}$ for plane stress crack growth. These analyses give similar results to those obtained by Rice et al. for CTOA under plane strain conditions, that is, $\text{α = 0.65}$ from the experimental results and $\text{β = 5.08}$ from the slip line theory.The CTOA obtained for plane stress ductile crack growth is applied to the wide plate tensile crack growth test. The results of the present analysis coincide well with those of the plane stress finite element method (FEM) computed by T. Kanazawa et al. The phenomena of plane stress ductile crack propagation are also explained by the CTOA criterion under plane stress conditions.     

Non-symmetric extension of a crack

Summary The dynamic in-plane problem of the non-symmetric extension of a crack in an infinite, isotropic elastic medium under normal stress is analyzed. Following Cherepanov [8], Cherepanov and Afanas'ev [9] the general solution of the problem is derived in terms of an analytic function of complex variable. The results include the expressions for the stress intensity factors at the crack tips and the rate of energy flux into the cxtending crack edges. For a particular case, numerical calculations for the stress intensity factor and the energy flux rate are carried out.     

The influence of structural dimensions on crack arrest

An energy balance has been used for the prediction of toughness values required to ensure arrest in double tension crack arrest tests. The analysis required calculation of the total energy that depended on the force applied to the main plate and the effective length of the structure including the test frame. Theoretical calculations indicated that the energy release rate at crack arrest was governed by both the width and the length of the structure

     

CAMAC and LAN's

We are implementing an Ethernet interface for our intelligent CAMAC crate controller, CANDI 2, to connect it to the Ethernet Network which will tie together all of the Frascati National Laboratories computing facilities. We feel that Local Area Networks can add a lot of power and flexibility to a multicrate CAMAC installation.     

On Barenblatt's problem

The problem of flow in media with double porosity (e.g. fissured rocks) is considered by utilizing the author's concept of “multiporosity” and employing the apparatus of continuum theory of mixtures. Relevant results obtained by Barenblatt et al. within the Russian literature are shown to be contained in the present theory.