Determination of impact fracture toughness of polyethylene using arc-shaped specimens

This paper describes a methodology for the determination of the impact radial fracture toughness, G_IC, of cylindrical polymeric molded parts using arc-shaped specimens. The proposed methodology is an extension of the ISO/DIS 17281 Standard which states that for brittle behavior, a basically linear relationship exits between the fracture energy, U; and the energy calibration factor, φ. This relationship allows calculating the critical strain energy release rate from the slope of the U vs. φ plot. An expression for the energy calibration factor, φ, for the arc-shaped specimen is proposed in this work, by combining tabulated functions and finite element results. The methodology is applied to test high density polyethylene arc-shaped specimens taken from cylinder walls.     

Determination of fracture toughness parameter of quasi-brittle materials with laboratory-size specimens

An experimental technique based on four-point bend specimens has been developed for indirect determination of the tension-softening curve, which is suggested to be a sizeindependent fracture parameter for quasi-brittle materials. The technique was applied to fibrereinforced mortar. Good agreement between results from this indirect test and those from the direct tension test was obtained.     

Determination of the fracture toughness of inhomogeneous three-layer fusion claddings on compact specimens

We propose a method for the evaluation of the fracture toughness (K _1C ) of multilayer fusion claddings on compact specimens at elevated temperatures (20, 70, and 300°C) with cladding (in the slot) of three layers of materials with gradually increasing crack resistance. As the testing temperature increases, the value of K _1C decreases for harder and stronger fusion claddings and, vise versa, increases for less strong and softer materials. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 3, pp. 121–122, May–June, 2007.     

Estimation of fracture toughness using miniature chevron-notched specimens

An approach for determining fracture toughness of materials using miniature chevron‐notched specimens is reported here for the first time. An outline of the principle and the experimental procedure for the test is depicted using results generated on eutectoid steel. Fracture toughness values of soft annealed and fully annealed eutectoid steels with inter‐lamellar spacing 167 nm and 549 nm are found to be 76 and 56 , respectively. The estimated fracture toughness values from miniature specimens are found to be in good agreement with the ones reported for similar bulk materials.     

Short-rod elastic-plastic fracture toughness test using miniature specimens

The standard ASTM-E399 plane-strain fracture toughness (K _IC) test requires (1) the test specimen dimensions to be greater than a minimum size and, (2) fatigue precracking of the specimen. These criteria render many materials impractical to test. The short-rod elastic-plastic plane-strain fracture toughness test proposed by Barker offers a method of testing not requiring fatigue precracking and furthermore, it appears that test specimens smaller than that stipulated by ASTM can be used to obtain validK _IC values. In this study, the use of a modified miniature short-rod fracture toughness test specimen was investigated. Our miniature short-rod specimen is approximately 7 mm long and 4 mm diameter. These mini specimens are well suited for the purpose of testing biomaterials. The value of the minimum stress intensity factor coefficient (Y _m ^* ) for the mini short-rod specimens was determined experimentally using specimens machined from extruded acrylic rod stock. An elastic-plastic fracture toughness analysis using the mini specimens gave values ofK _IC for extruded acrylic (nominally PMMA) equal to 0.67 ± 0.06 MPa m^1/2. The problem of testing non-flat crack growth resistance curve materials (such as PMMA) using the short-rod fracture toughness test method is discussed. A modification to the test procedure involving the use of aY ^* value corresponding to a short crack length is suggested as a method of overcoming this difficulty.Nomenclature a crack length - a ₀ initial crack length - a ₁ length of the chevron notch on the mini short-rod specimen - a _m critical crack length — crack length atY _m ^* - C specimen compliance - C dimensionless specimen compliance = CED - D mini short-rod specimen diameter - E Young's modulus - K ₁ stress intensity factor - K _1C plane-strain fracture toughness - K _max fracture toughness calculated usingP _max - P load applied to the test specimen during a short-rod fracture toughness test - P _c load applied to the test specimen atY _m ^* - P _max maximum load applied to the specimen during a short-rod fracture toughness test - p plasticity factor - W mini short-rod specimen width - Y ^* stress intensity factor coefficient - Y _m ^* minimum of the stress intensity factor coefficient - dimensionless crack length =a/W - ₀ dimensionless initial crack length = ₀/W - ₁ dimensionless chevron notch length =a ₁/W - _m dimensionless critical crack length =a _m/W     

Prediction of instability in ductile senb fracture toughness specimens

An approximate flow stress model is proposed as a possible explanation of the variability observed for maximum load values of crack opening displacement (COD) and pseudo J integral measurements in small specimens of fully ductile material. The model presented is limited to test pieces which exhibit ductile crack initiation after general yielding and more accurately describes low work hardening materials. However, the predictions of the dependency of such toughness parameters on specimen size are shown to be consistent with documented data.

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Résumé On propose un modèle approché d'écoulement de contraintes pour expliquer les variations observées pour la charge maximum lors de mesures du COD et de la pseudo-intégrale J dans des petites éprouvettes de matériau complètement ductile. Le modéle proposé est limité à des pièces d'essai qui présentent un amorçage de fissure ductile après plastification complète, et décrit plus précisément les matériaux dont l'écrouissage est faible. Cependant, on montre que la dépendance de ces paramètres de ténacité avec la dimension des éprouvettes peut être prédite en conformité avec les données expérimentales.

     

Validity requirements for fracture toughness measurements obtained from small circumferentially notched cylindrical specimens

The authors have earlier proposed a small cylindrical circumferentially notched and fatigue cracked specimen for estimating K_Ic Finite element studies carried out are discussed with specific attention to the comparison between K_Ic thus obtained to that obtained experimentally. The size of the plastic zone is of particular interest in respect of the validity of the results; the F.E. result is compared to that obtained using Von Mises and Irwin approximations. Validity requirements are proposed for this specimen considering general yield of the final ligament, final ligament size, plastic zone size and depth of fatigue crack.     

Determination of dynamic fracture toughness for PMMA

A dynamic FEM (finite element method) and a strain gage method are applied to analyze the dynamic fracture toughness and SIF (stress intensity factor) for PMMA (polymethyl methacrylate). The analyses are carried out for plates with an edge crack subjected to one-point bending in a plane of the plate. A simple procedure that the present author has proposed is applied to the problem of using a triangular element of assumed constant strain on finite element analysis. The numerical simulation by FEM provides values for the applied forces as measured with the strain gages. Also, a crack initiation time is measured with the strain gage mounted around the crack tip. The dynamic fracture toughness is determined by adapting the crack initiation time to the simulation curve of the dynamic SIF calculated by the FEM. In this study, the usefulness of the method to determine the dynamic fracture toughness is investigated by comparing predictions with the experimental results for dynamic stresses and SIFs.     

Determination of the fracture toughness by applying a structural integrity approach to pre-cracked Small Punch Test specimens

The Failure Assessment Diagram (FAD) is a procedure for evaluating the structural integrity of cracked components. The component’s failure conditions (load and crack size) are based on the material fracture properties (K_mat) considering its plastic behavior. In this paper, a new methodology that combines the FAD approach and the load–displacement curve obtained from pre-cracked Small Punch Test (SPT) specimens is presented, in order to estimate the fracture toughness of 15.5PH stainless steel. This research is based on Finite Element Modeling (FEM) of the pre-cracked specimen to determine both the plastic collapse load and the stress intensity factors during the loading process. A set of interrupted tests on pre-cracked SPT specimens are also conducted in order to identify the initiation point of the crack propagation. This set of numerical and experimental values allows the toughness ratio (K_r) and load ratio (L_r), at the instant the cracked specimen fails, to be determined. The only unknown variable in this process is the value of the material toughness, K_mat. The parameters in question are then combined with the FAD of the material generated from the different options available on the ASME-API 579 procedure, thus yielding an estimate for the value K_mat. Finally, to evaluate the accuracy of this methodology, predicted toughness values are compared to those from normalized tests of the material being analyzed.     

The use of pre-cracked charpy specimens to determine dynamic fracture toughness

The use of an instrumented impact hammer on pre-cracked Charpy V specimens has led to an inexpensive method for determining the dynamic fracture toughness, K_ld. Measurements were made on A-533 steel over a range of temperature (−125 to + 75°F) at a loading rate K ≈ 10⁶ksi √in/sec. The data were found to be in excellent agreement with those obtained by other workers on much larger specimens. Analysis of the data leads to a new method for estimating the NDT temperature, which may be of practical value in nuclear reactor surveillance programs.     

On the embrittlement of fracture toughness specimens of two high strength steels

Hydrogen embrittlement of two commercial materials, a 1$\text{1}\text{2}$ Ni-Cr-Mo steel and a 5 Cr-V-Mo steel both quenched and tempered to a nominal ilrength of 1.60 × 10³MN/m² has been studied. Frecture toughness testing was carried out on each steel in three conditions, (a) heat-treated and tested in air, (b) thermally charged with hydrogen and tested in air, and (c) uncharged and tested a an atmosphere of dry hydrogen. The effects of hydrogen pressure and loading rate on embrittlement were examined. The experimental results show that thermal charging had a greater embrittling effect on the 1$\text{1}\text{2}$ Ni-Cr-Mo steel than on the 5 Cr-V-Mo steel although the latter had a higher hydrogen concentration. In a hydrogen environment the 1$\text{1}\text{2}$ Ni-Cr-Mo steel was much more sensitive to embrittlement than the 5 Cr-V-Mo steel.     

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.