Some ambiguities in the experimental determination of the parameters of fracture mechanics

We consider some examples of the ambiguous assessment of crack growth in structural steels in the process of stress corrosion cracking under cyclic loading. We analyze the nature of these ambiguities, suggest the methods for their elimination, and discuss how to take them into account when using the results of laboratory investigations in the engineering practice. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 4, pp. 49–58, July–August, 1996.     

The micro-statistical fracture mechanics approach to dynamic fracture problems

Continuum fracture mechanics concepts should be applied to solve dynamic fracture problems wherever a continuum approach can provide sufficient answers. Many dynamic fracture problems, however, involve multiple cracks or voids and are not well-suited to the relatively simple continuum approach. This paper describes a statistical fracture mechanics concept on a microscopi scale and illustrates its use for the case of shock-wave-induced ductile spall fracture. The paper further shows how micro-statistical fracture mechanics (MSFM) merges with continuum fracture mechanics by treating a macrocrack propagating in a DCB specimen using MSFM and the MSFM parameters deduced from the spall work.Thus, the two approaches are consistent. Although significantly more complicated, the MSFM approach promises to be helpful in improving our understanding of the influences of microstructure on toughness and in extending continuum approaches to more ductile materials and to smaller specimens.

---

Résumé Les concepts de fracture mécanique d'un continuum devraient être appliqués à la solution des problèmes de fracture de rupture dynamique dès lors qu'une approche par continuum peut fournir des réponses suffisantes. Cependant, plusieurs problèmes de rupture dynamique comportent des fissures ou des cavités multiples et ne sont pas bien adaptés à une approche par continuum relativement simple. Le mémoire décrit un concept de mécanique de la rupture statistique à une échelle microscopique et illustre son utilisation au cas d'une rupture par décollement ductile sous l'effet d'une onde de choc. On montre en outre qu'une mécanique de rupture micro-statistique peut être envisagée dans le mécanisme de rupture d'un continuum en traitant la propagation d'une macro-fissure dans une éprouvette Cantilever et en utilisant ce concept et les paramètres qui y sont liés, déduits du travail relatif à l'écaillage.Ainsi, les deux approches se révèlent applicables. Bien que considérablement compliquées, l'approche de mécanique de rupture micro-statistique se révèle prometteuse pour améliorer notre compréhension des influences de la microstructure sur la ténacité et pour une extension de l'approche du continuum de milieu continu à des matériaux plus ductiles ou à des éprouvettes plus petites.

     

Application of infrared spectroscopy in fracture mechanics problems

Presented at First All-Union Conference on Fracture Mechanics of Materials, Lvov, October 20–22, 1987.     

Recent developments of experimental fracture mechanics in greece

Recent developments associated with the application of the method of caustics to fracture mechanics problems in Greece are reviewed. Results from a variety of these studies including propagating cracks, cracks in composite bodies, bending of cracked plates and bodies with rigid inclusions, are summarized preforially.     

Some considerations on dynamic biem—II: Applications to two-dimensional fracture mechanics problems

Part I of this report, published in a detached part, the theoretical and numerical fundamentals of BIEM techniques in dynamic were presented.In Part II, numerical results for input problems obtained by BIEM, through a dynamic program of general applications, are shown.     

Some contribution to process zone fracture mechanics

The mechanism of the ductile fracture is studied theoretically for the Al Alloy 7075-T6 specimens. A model for the interaction of a crack tip with a void nearby is analyzed by using the Modified Gurson's Model. Taking fracture criterion into consideration, the analysis of a crack propagation is carried out and besides the distribution of the equivalent plastic strain, the void volume fraction f and the localization are obtained. Microcracks nucleate on the ligament between crack and void, and grow and coalesce each other, and at last the main crack thus formed coalesces with the void and the coalescence of the crack and void is completed. And these phenomenon occurs in the localized region.The initiation of the microcrack of 7075 occurs at small J and the microcrack penetration between crack and void occurs at larger J, and the propagation does not occur smoothly. These results coincide with the results of the experiments by FRASTA (FRActure Surface Topographic Analysis) and Fractography.     

Some fracture mechanics properties in high-velocity impact-damaged aluminium plates

The paper describes the results of an experimental programme aimed at the evaluation of the fracture mechanics properties of high velocity, about 6  km  s⁻¹ , impact-damaged aluminium plates, 2219-T851. The energy of the impacts was the ballistic limit, i.e. large bumps were produced but the plates were not perforated. Test were carried out to evaluate the residual stress field in the impact-damaged area, the static resistance of the plates in the presence of a through crack and the crack propagation rate of a through crack located at the centre of the impact-damaged area. The results of the tests clearly show that the mechanical properties of the material remain practically unchanged after the high-velocity impact. Furthermore, by taking into account the geometry of the bumps, the mean crack propagation rate of a defect can be predicted quite accurately.     

Some recent fatigue and fracture mechanics research in BIAM

The paper presents a brief review of some of the major research activities on fatigue and fracture mechanics in recent years at the Beijing Institute of Aeronautical Materials. Attention is mainly given to the studies on weight function methods for analyses of two- and three-dimensional crack problems, fatigue crack growth under variable amplitude loading, small crack effects and a fracture-mechanics-based total fatigue life prediction method.Abbreviations 2(3)D two- (three-) dimensional - BIAM Institute of Aeronautical Materials, Beijing - CCT center cracked tension - COD crack opening displacement - CTOD crack tip opening displacement - FEM finite element method - LEFM linear elastic fracture mechanics - SENT single edge notched tension - SIF stress intensity factor - WFM weight function method     

Some inherently unreliable practices in present day fracture mechanics

A number of current practices in fracture mechanics which use quantities near a crack tip to make conclusions about response at the crack tip itself are examined. Specifically these include: stress and displacement matching to estimate stress intensity factors, monitoring local stress and strain values to predict fracture, and both crack opening angle and crack opening displacement as fracture criteria. By means of a pair of counter applications, all of these procedures are demonstrated to have the potential of leading to completely incorrect conclusions. An understanding of what causes this inadequate performance then indicates that such procedures may be unreliable in general and prompts suggestions as to alternatives.

---

Résumé On examine diverses pratiques courantes en Mécanique de Rupture qui font usage de grandeurs paramétriques au voisinage de l'extrémité d'une fissure pour tirer des conclusions sur ce qui se passe à cette extrémité même. Ces grandeurs sont notamment: les valeurs de la contrainte et du déplacement correspondant à un facteur d'intensité de contrainte estimé, l'enregistrement des contraintes et déplacements locaux en vue de prédire la rupture, et l'angle et le déplacement d'ouverture d'une fissure en tant que critères de rupture.En considérant une paire d'applications divergentes, on démontre que toutes ces procédures risquent d'entraîner des conclusions complètement incorrectes.Une analyse des causes de ces inadéquations montre que ces procédures peuvent se révéler peu fiables en général, et conduit à des suggestions de procédures alternatives.