A. Campagnolo

Brittle Failure of Graphite Weakened by V-Notches: A Review of Some Recent Results Under Different Loading Modes

The present paper summarizes some recent experimental, theoretical and numerical results on brittle fracture of isostatic polycrystalline graphite. The analyses have been carried out on V-notched samples under mixed mode (I+II), torsion and compression loading, considering various combinations of the notch tip radius, opening angle and notch tilt angle. The static strength of the considered specimens is assessed through an approach based on the strain energy density averaged over a control volume. The center of the control volume is located on the notch edge, where the principal stress reaches its maximum value. The correct orientation is obtained by a rigid rotation of the crescent-shaped volume while the size depends on the fracture toughness and the ultimate strength of the material. This methodology has been already used in the literature to analyze U- and V-shaped notches subject to mode I loading with very good results and advantages with respect to classic approaches. The results reported in this new work show, also under mixed mode loading conditions, a good agreement between experimental data and theoretical predictions.

Tensile fracture analysis of V-notches with end holes by means of the local energy

This paper deals with investigating brittle fracture in V-notches with end holes under mode I loading. Thirty-six fracture test results, reported most recently in the literature on a new notched disk-type specimen, namely the Brazilian disk containing central VO-shaped notch made of polymethyl-metacrylate, were theoretically predicted by means of the well-known brittle fracture criterion, namely the strain energy density over a critical control volume which embraces the notch edge. A very good agreement was shown to exist between the experimental and theoretical results.

Brittle Fracture of Rounded V-Notches in Isostatic Graphite under Static Multiaxial Loading

While a large bulk of experimental results from cracked specimens of polycrystalline graphite under pure modes of loading, in particular under mode I loading, can be found in the literature, only a very limited number of tests have been carried out on notches. At the best of the author knowledge dealing with the specific case of V-notches under mixed mode loading (tension + torsion) no results can be found in the literature. With the aim to fill this lack, the problem of mixed mode (I + III) brittle fracture of polycrystalline graphite is investigated systematically here for the first time. The present study considers cylindrical specimens weakened by circumferential notches characterized by different acuities. A new complete set of experimental data is provided considering different geometrical configurations by varying the notch opening angle and the notch tip radius. The multiaxial static tests have been performed considering different values of the mode mixity ratio (i.e. the ratio between the nominal stress due to tension and that due to torsion loading). A criterion based on the local strain energy density previously applied by the same authors only to pure modes of loading is extended here to the case of tension and torsion loadings applied in combination. The proposed criterion allows a sound assessment of the fracture loads.

Mode II Brittle Fracture Assessment of Key-Hole Notches by Means of the Local Energy

Brittle fracture of key-hole notches under pure mode II loading is investigated both experimentally and theoretically. In the experimental part, a new version of the well-known Brazilian disk specimen containing a central dumbbell-shaped slit with two key-ends, called the Key-BD specimen, made of polymethyl-methacrylate (PMMA), is utilized to perform fracture experiments at room temperature for different notch lengths and various notch radii. In the theoretical part, the experimentally recorded fracture loads are predicted by means of the strain energy density (SED) averaged over a specified control volume, which embraces the notch edge. It is found that the SED criterion can successfully predict the test results.

Three-dimensional effects at the tip of rounded notches subjected to mode-I loading under cyclic plasticity

In this work, a comprehensive theoretical and numerical study on the cyclic elastic–plastic notch stress and strain distributions is carried out. In more detail, the incremental cyclic plasticity theory, already proposed by other authors to determine the actual stress and strain state arising in two-dimensional or axisymmetric notched components, is extended to the study of three-dimensional effects at the tip of rounded notches in plates of finite thickness. The analytical frame is initially validated considering a number of plane problems and later modified to consider three-dimensional effects. Different notch geometries are investigated, such as plane specimens with finite thickness weakened by circular holes, U-notches and rounded V-notches subjected to cyclic mode-I loading. Theoretical results based on the incremental cyclic plasticity theory are compared with time-consuming elastic–plastic finite element analyses carried out with a commercial finite element code showing a very satisfactory agreement. Finally, a link between the averaged strain energy density criterion and the area subtended by the hysteresis loops tied to the different stress and strain components acting at the notch tip has been investigated.

Elastic-plastic fracture analysis of notched Al 7075-T6 plates by means of the local energy combined with the equivalent material concept

The main goal of the present research is to analyze tensile fracture in Al 7075-T6 thin plates weakened by blunt V-notches. For this purpose, first, 27 fracture tests are carried out on rectangular plates containing a central rhombic hole with two blunt V-shaped corners horizontally located. The experimental observations indicated that a plastic region initiates from the notch tip and grows as the tensile load monotonically increases, and finally, fracture happens suddenly with a significant opening of the notch tip. By showing significant plastic deformations around the notch tip and also inclined fracture planes, the specimens after fracture confirm well the ductile rupture in V-notched Al 7075-T6 plates. As the main experimental result, the load-carrying capacity of the notched plates corresponding to the onset of crack initiation from the notch tip is recorded. To theoretically predict the experimental results, the equivalent material concept is utilized together with the well-known brittle fracture criterion, namely the averaged strain energy density criterion. Without requiring elastic-plastic finite element analysis, it is shown that the combination of the averaged strain energy density and equivalent material concept is successful in predicting the load-carrying capacity of the V-notched Al 7075-T6 plates that fail by moderate-scale yielding regime.

Fracture tests under mixed mode I + III loading: An assessment based on the local energy

In this paper, some recent data exploring mixed mode I + III fracture of cracked polymethyl methacrylate specimens are re-analysed in terms of the local energy. The data have been obtained by using a new loading fixture proposed for exploring mixed mode I + III fracture experimentally. While a large bulk of experimental results from cracked specimens of polymethyl methacrylate under pure modes of loading, in particular under mode I loading, can be found in the literature, only very few tests have been carried out considering multiaxial static loadings, and for this reason, only limited studies have been performed on possible criteria for the fracture assessment under mixed mode I + III conditions. By taking advantage of the recent set of data on polymethyl methacrylate tested at different values of the mode mixity ratio, a criterion based on the local strain energy density previously applied by the same authors only to pure modes of loading is successfully extended here to the case of tension and torsion loadings applied in combination. Good agreement is shown to exist between the theoretical estimates and the experimental data.

A successful combination of the equivalent material concept and the averaged strain energy density criterion for predicting crack initiation from blunt V-notches in ductile aluminum plates under mixed mode loading

Crack initiation from blunt V-notch borders in ductile A16061-T6 plates is investigated experimentally and theoretically under mixed mode I/II loading. Experimental observations with naked eye during loading indicated large plastic deformations around the notch tip at the onset of crack initiation, demonstrating large-scale yielding failure regime for the aluminum plates. To theoretically predict the experimentally obtained value of the maximum load that each plate could sustain, i.e. the load-carrying capacity, without performing elastic-plastic failure analyses, the equivalent material concept (EMC) is combined with a well-known brittle fracture criterion, namely the averaged strain energy density (ASED) criterion. It is shown that the combined EMC-ASED criterion could successfully predict the experimental results for various V-notch angles and radii.

Tensile Fracture Analysis of Key-Hole Notches by Means of the Strain Energy Density

The aim of the present research is twofold. Firstly, to provide a new set of experimental results regarding tensile brittle fracture in key-hole notches and secondly, to check the suitability of the local strain energy density (SED) criterion in predicting the test results. For this purpose, 21 fracture tests were conducted at room temperature on a new version of the well-known Brazilian disk specimen containing a central dumbbell-shaped slit with two key-ends (Key-BD specimen) made of PMMA. The experimentally obtained fracture loads were theoretically predicted for different notch radii by means of the local SED averaged over a specified control volume which embraces the notch edge. It was shown that the experimental results could be well predicted by means of the SED criterion.

Large-Scale Yielding Failure Prediction of Notched Ductile Plates by Means of the Linear Elastic Notch Fracture Mechanics

The main goal of this research is to propose a failure criterion based on the linear elastic notch fracture mechanics (LENFM) for predicting tensile crack initiation from a blunt V-notch, encountering large plasticity at the notch vicinity. First, some most recently published experimental results on tensile failure of V-notched ductile aluminum plates are briefly described. Then, with the aim to avoid complex and time-consuming elastic-plastic analyses, the equivalent material concept (EMC) is employed together with a LENFM-based fracture criterion, namely the averaged strain energy density (ASED) criterion, for predicting the load-carrying capacity of the V-notched aluminum plates. A very good agreement is shown to exist between the experimental results and theoretical predictions of the EMC-ASED criterion.