Fracture toughness of plain concrete from three-point bend specimens

A simple method is proposed for determining the fracture toughness of plain concrete from three-point bend specimens, based on the concept of effective notch depth to account for the non-linear pre-peak load-deflection behaviour. The fracture toughness so determined is shown not to depend on the specimen size. The method improves an earlier version of the effective crack model in several ways. First, it is no longer necessary to linearize the pre-peak non-linearity, thereby eliminating the inaccurate task of establishing the limit of elastic response. Secondly, regression expressions for determining the effective notch depth should be far more accurate because they are based on an analysis of not only the authors’ test data but that of several researchers around the world. Thirdly, these expressions do not depend on the size of the test specimen. It is shown that the predictions of the effective crack model are in good agreement with two other non-linear process zone models, as far as three-point notched beam specimens are concerned.     

Fracture toughness measurement at cryogenic temperatures using chevron notched specimens

Fracture toughness measurement at cryogenic temperatures using chevron notched specimens     

Fracture toughness determination of bearing steel using chevron-notch three point bend specimen

Stress intensity factor formulas and dimensionless compliance formula of chevron-notch three point bend specimen obtained by use of straight-through-crack assumption (STCA) and Bluhm's slice model have been presented. Two stress intensity factor coefficient formulas have been compared with the experimental data of GCr 15 bearing steel. The comparison has shown that the formula obtained using slice model is in better agreement with experimental data. The plane-strain fracture toughness measurements by chevron-notch specimen and by ASTM E399 standard method have been compared and are in agreement. The effect of slot width of chevron notch on the measurements has been studied.     

Characterization of acoustic emission signals from particulate filled thermoset and thermoplastic polymeric coatings in four point bend tests

Four point bend tests were conducted on two previously characterized particulate filled thermoplastic and thermoset polymeric coatings, the acoustic emission (AE) method being utilized to monitor the damage progress during the tests. Different damage mechanisms operating in the polymeric coatings can be recognized by the different amplitude range of the AE signals emitted. AE can be used to assess the relative strength of particle/matrix bonds in particulate filled polymeric coatings.     

Estimation of fracture toughness of steel using chevron notched round bar specimens

A generalized methodology has been outlined in this paper for estimating the minimum normalized stress intensity factor (Y*_min) of chevron notched round bar specimens, subjected to three‐point bend loading. Using such specimens, a series of fracture toughness tests have been carried out for the first time on two steels. The major inferences drawn from this investigation are: (i) reproducible fracture toughness values can be achieved using chevron notched rod specimens of identical configuration and (ii) the estimated magnitudes of fracture toughness obtained by using chevron notched rod specimens are in good agreement with those achieved by using chevron notched rectangular bar specimens of the same material.     

Effects of finite notch width on the fracture of chevron – notched specimens

The chevron notched three-point bend test specimen is often used for measuring the fracture toughness of brittle materials such as ceramics. Specimen sizes are often very restricted when testing advanced materials due to limited volume of material available or high material costs. Since the minimum chevron notch width is limited by the size of the cutting wheels or wire saw used to produce it, as the sample size gets small enough, the notch width becomes large in relation to the sample size. It is shown via finite element analysis that the notch width has an important effect on the stress intensity factors of short cracks. The minimum in the normalized stress intensity factor versus crack length is lost, rendering the usual analysis of the experimental results invalid and contributing greatly to decreased fracture stability of such specimens. Previous analytical and numerical studies do not take into account the width of the chevron notch. Based on the calculations, a guideline to permissible notch widths is introduced. This revised version was published online in July 2006 with corrections to the Cover Date.     

An evaluation of the chevron V-notched bend bar fracture toughness specimen

The chevron V-notched bend bar specimen recently proposed as a practical approach to the fracture toughness testing of brittle materials has been further evaluated. The accuracy of the stress intensity expression has been improved through experimental analysis. The applicability of this specimen for determining the environment induced cracking threshold, K_Iscc was also investigated. Fracture toughness results developed for Pyroceram 9606, 7079-T6 aluminum and the apparent K_Iscc value of AISI 4340 steel exposed to hydrogen sulfide gas show that the chevron V-notched bend bar specimen can yield data essentially identical to that developed with more conventional fatigue precracked specimens. However, results also show that when catastrophic failure of a specimen occurs rather than the expected stable crack growth, the test is invalid. Further work is required to resolve this complication.     

Dynamic analysis of three point bend specimens under impact

A simple one dimensional inertial model is presented for transient response analysis of notched beams under impact, and extracting dynamic initiation toughness values. The model includes the effects of striker mass interactions, and contact deformations of the beam. Displacement time history of the striker mass is applied to the model as forcing function, The model is validated by comparison with the experimental investigation on ductile aluminium 6061 alloy and brittle polymer, PMMA.     

Fracture energy for three point bend tests on single edge notched beams: Proposed evaluation

The research in fracture mechanics of concrete is reviewed in an attempt to reconcile the different, and sometimes contradictory, approaches for evaluation of the fracture energy, G_f. An improved method to measure G_f for beams in three point bending is presented. This method is expected to provide a more reliable characterization of the fracture toughness of concrete.     

Fracture toughness of concrete determined on large specimens

It has often been questioned whether linear elastic fracture mechanics can be applied to describe crack propagation and failure of concrete. An important argument is that most test results are obtained on specimens too small to be representative of a material with a composite structure such as concrete. Large specimens with four different geometries have been prepared and tested. Crack length was increased under controlled conditions to at least 250 mm. It was found that fracture toughness increases initially as a crack propagates, but that a length-independent value is reached asymptotically. Within the range of accuracy, asymptotic values obtained with the four different geometries were the same. It is concluded that failure of large size concrete elements can be predicted realistically on the basis of linear elastic fracture mechanics. For comparatively small specimens, however, an approach which takes total fracture energy into consideration (for instance the fictitious crack model) is more appropriate. It is pointed out that the role of subcritical crack growth on fracture toughness needs further investigation.     

Plastic limit load and its application to the fracture toughness testing for heterogeneous single edge notch tension specimens

The current investigation pursues the confirmation of the applicability of the limit load solutions in determination of the η factors necessary for fracture toughness testing protocols. The procedure begins with the correct calculation of limit load values in welded single edge notch tension (SE(T)) fracture specimens containing centreline cracks. Hence, the η factor is inferred through the principle of potential energy. Additionally, such results are compared with those obtained from finite element analyses, including strain hardening effects available in the literature. SE(T) specimens subject to pin‐loading display that the η factors are insensitive to the configurational effects and hardening properties. On the other hand, in clamped SE(T) specimens, such effects become meaningful, making its usage in fracture toughness experiments questionable. This work provides an alternative methodology to compute fully plastic proportionality coefficients (η) based on limit load solutions for heterogeneous cracked SE(T) specimens. These analyses also consider the limitations and potentialities of such an approach in experimental measurements of ductile crack growth.