Single Specimen Method Research Articles

A new alternative method to evaluate the J-integral in the case of elastomers

This study concerns the fracture of rubbers. The objective is to verify the validity of the J-integral expression we proposed for such materials (Ait Hocine et al., 2002). In this expression, the parameter J is represented as a multiplicative form of a geometrical η* (or η) factor and elastic work per unit area as it has been already done in the Linear Elastic Fracture Mechanics (LEFM) (Turner, 1973). The whole problem rests in the fact that the factor η* (or η) is unknown whereas in LEFM, expressions of ηel factors are available for several kinds of specimen geometry. Thus, in this work, an experimental and a numerical analysis have been achieved and we have shown, for both the studied materials and the considered specimens, that these unknown factors could be replaced by those issued from the LEFM or from the work of Kim and Joe (1989) on rubber-like materials. The results obtained confirm the validity of our model representing a single specimen method of an experimental evaluation of the elastomers fracture energy.

Experimental study on the material dynamic fracture properties by Instrumented Charpy Impact test with single specimen method

Experimental study on the material dynamic fracture properties by Instrumented Charpy Impact test with single specimen method

Experimental study on the material dynamic fracture properties by Instrumented Charpy Impact test with single specimen method

With the determination of load-time curve recorded by Amsler/Roell RKP 450 Instrumented Charpy Impact test and based on the Newton's Second Law, Impact character of a single standard V-notch specimen of X70 pipeline steel under the low temperature -70°C was investigated by studying the impact energy distribution. It was revealed that maximum load point (F m point) was not exact the dynamic crack initiation, which was detected somewhere prior and very close to F m point by using Compliance Changing Rate method. This fact was also confirmed by Dynamic CTOD method. That is to say, Impact energy related to the F m point (i.e. E m ) consists not only the crack initiation energy E i , but a small part of crack extension energy as well. Ratio of E i /E m was found to be 0.90 just applicable to the material used here. Dynamic fracture toughness J 1d was then estimated by modified Rice equation. Crack extension behavior and dynamic crack growth resistance curve (J-Δa) during stable crack propagation period was carefully analyzed by Key Curve method. Finally. methods for evaluating tearing module T mat and CTOD curve under the impact test were also briefly introduced in the paper.

Application of Acoustic Emission for Measuring Crack Initiation Toughness in Instrumented Charpy Impact Testing

AbstractAcoustic Emission (AE) has been used to detect onset of ductile tearing in instrumented Charpy impact tests to enable determination of dynamic ductile tearing initiation toughness using a single specimen method. An AE based method was developed whereby a conventional instrumented impact tester is instrumented with a commercial broadband acoustic emission sensor mounted on the anvil. The results obtained using the new AE based method were compared to the results obtained by other researchers using specially developed AE sensors located inside the tup. Tests using both AE and direct measurement of the load-line displacement during the instrumented Charpy testing were conducted simultaneously on a conventional ship grade steel. V-notched and fatigue precracked test pieces were tested in the temperature range +23°C to −80°C.The results obtained with AE monitoring were compared to those obtained using the optical Direct Displacement Interferometric Strip (DDIS) method developed by the authors. Good correlation was found between the initiation fracture toughness determined at 0.2 mm blunting and crack extension, J0.2, by the multispecimen Ductile-Brittle Transition (DBT) method and those derived from single specimens applying AE and the new interferometric method to detect ductile fracture initiation. Fast Fourier Transformation (FFT) was applied to analyze the AE signals. The results showed that the different phases of AE and fracture modes could be distinguished based on characteristic frequency and time variance.

Experimental and numerical analyses of indentation in single piece and split type specimens

Indentation tests using a spherical indenter for split-type two half specimens, or the bonded-interface specimens, have been proposed as a means to evaluate the deformation behaviour or the internal damage of materials during indenting the conventional single piece specimen. The purpose of this study is to assess the validity of the split type technique and to better understand the mechanics during the indentation of the split type specimens with respect to the effect of the interface condition between the two split type half blocks. Indentation tests of Al 6065-T5 alloy specimens are chosen to study the whole elasto-plastic stress-strain behaviour. The finite element method was also employed to investigate the deformation behaviour of the two (single piece and split type) test systems loaded with a spherical indenter. The simulated geometry of the specimens and load-displacement curves during indentations are compared with the experimental data of the Al alloy specimens. The similarity and difference in deformation behaviour and stress distribution between the single piece specimen method and the split type specimen method are investigated. The effect of the friction between the two half blocks in the split type specimen test is discussed.

Fracture problems of rubbers: J -integral estimation based upon η factors and an investigation on the strain energy density distribution as a local criterion

This paper dealing with the fracture of rubber-like materials is a continuation of our previous works (Ait Hocine et al., 1996, 1998). Single specimen methods for measuring the J-integral and, so, its critical value Jc (fracture surface energy) are investigated combining experimental data and finite elements analysis. It shown that the formula of J established by Rivlin et al. in the case of the SENT geometry can be extended to the DENT specimen containing small crack lengths (a/w≤0.3). Moreover, considering DENT and pure-shear (PS) specimens, alternative expressions of this parameter, based on the η factor, are proposed and a good agreement is obtained between the numerical calculations and the experimental data whenever available. The critical values of J are found constant for the two tested materials. Finally, it's pointed out that, at the onset of crack growth, the strain energy density along the crack axis is independent on both the crack length and the specimen geometry.

Experimental and numerical investigation on single specimen methods of determination of J in rubber materials

In this paper, we have verified the validity of some formulations allowing the determination of the fracture surface energy in the case of rubber-like materials. The J-integral is chosen as a fracture characterizing parameter which is experimentally determined by considering a multiplying form that; numerically evaluated using a finite element method. The numerical results are compared to the experimental data and a good agreement has been pointed out for the deeply cracked specimen (a/w≥0.5). Below this limit, a significant divergence is observed which is attributed to a lack of accuracy of the experimental data processing.

The studies on fracture toughness of nuclear pressure vessel steels by using a small round compact tension specimen and a comparison with a Charpy-size specimen

This paper presents a single-specimen method of evaluating the fracture toughness of nuclear pressure vessel steels by using a small round compact tension (RCT) specimen with deep side-grooves. The method has been used to estimate the fracture toughness parameters of the nuclear pressure vessel steels A508CL3-A, A508CL3-B and A508CL3-A′ produced by two plants. A comparison of the measured fracture toughness results has been done among the small RCT specimen, the Charpy-size specimen and the large specimen, which GB2038 and ASTM E813 require. The research results indicate that the fracture toughness values obtained from the RCT specimens with the double-size deep side-grooves have little difference when compared with the valid fracture toughness results. Finally, this paper discusses the possibility of using the RCT specimens in nuclear pressure vessel steel embrittlement surveillance.

A new technique for determining fracture toughness KIC and its confidence with single DCB specimen

An experimental technique for determining fracture toughness K IC and its confidence has been suggested. The Kanninen's solutions of the stress intensity factor K and the displacement Δ at loading points for the double cantilever beam (DCB) specimen has been simplified analytically. According to the simplified expressions of K( a) and Δ( a), a series of fracture toughness values K IC (or K R) with different crack length a can be calculated from the load-displacement ( P− Δ) curve obtained experimentally. The new method, which may be particularly useful to the plate or pipe-type materials with longitudinal crack, has the following major advantages: (i) only one DCB specimen is required to determine both the mean and confidence of K IC or the K resistance curve of the material with a certain thickness; (ii) it is not necessary to define the critical condition of load; (iii) the measuring for crack length may be avoided. The experimental results on the specimens made of two steels of 40Cr and 38NiCrMo4 at various temperatures show that by comparison with the standard method, this single DCB specimen method is more convenient and reliable for practice.

Comparative studies of several methods to determine the dynamic fracture toughness of a nuclear pressure vessel steel A508 CL3 with Charpy-size specimen

In this investigation, five estimation methods have been adopted to estimate the dynamic fracture toughness of a nuclear pressure vessel steel A508 CL3 by using pre-cracked Charpy-size specimens on an instrumented impact test machine. Furthermore, the merits and the demerits of the five methods have also been compared. The experimental results indicate that the maximum load energy method based on the curve of load versus load-point displacement overestimates the dynamic fracture toughness J Id , especially above room temperature. The method of compliance changing rate underestimates the dynamic fracture toughness. The method of measuring the critical stretch zone width (SZW c ) at the crack tip by means of SEM fractography and then converting the SZW c into J Id has a relatively large error. In addition, it is expensive and difficult to measure the SZW c . The method of energy revised at the maximum load may be considered a better single-specimen method for determining the dynamic fracture toughness. Furthermore, the results indicate that although the dynamic resistance curve method can exactly estimate the dynamic fracture toughness of the material, this method needs several specimens. Moreover, the test procedure is complicated. Thus, it is not suitable for nuclear reactor pressure vessel embrittlement surveillance.

The reasonable design of Charpy-size specimen for fracture toughness test in nuclear surveillance

Use of a precracked and side-grooved Charpy-size specimen is an economical and convenient single-specimen method of evaluating the elastic-plastic fracture toughness of nuclear pressure vessel steels. This paper has studied the influence of side-groove depth on fracture toughness and stable increment of crack on several pressure vessel steels in detail. Test results are compared with those of large-size specimens which the National Standard of China (i.e. GB 2038) requires. The research results indicate that using a precracked Charpy-size specimen with side-groove depth 30% and adopting the energy before the maximum load of a three-point bending test curve, the elastic-plastic fracture toughness of materials can be evaluated conservatively when the crack begins to propagate, and the reasonable design of a Charpy-size specimen for fracture toughness tests in nuclear surveillance has been established. At the same time, the constraint effect and thickening action of the side-groove on a Charpy-size specimen have also been discussed, and theoretically explained.

Short fatigue crack growth and J-R curve behavior of particulate Al 2O 3-reinforced Al(2014) alloy

The characteristics of short fatigue crack growth from a sharp notch and the behavior of the J-R curve of 10% Al 2O 3 particulate-reinforced Al(2014) alloy were studied through three-point bend tests. A new single-specimen method for determining the J-R curve was used which employs a cyclic-monotonic alternate loading procedure. The results obtained using this method were similar to those obtained from conventional testing. By considering the results of optical observations, scanning electron microscope fractography and microhardness measurements, it was found that there are several regimes of crack growth behavior, and each regime shows distinct crack-extension mechanisms.

Fracture Toughness of Aged Stainless Steel Primary Piping Welds Evaluated by Multiple and Single-Specimen Methods

Fracture toughness testing was conducted on compact tension specimens cut from the Fast Flux Test Facility primary piping materials of 16-8-2 and 308 stainless steel welds aged at 427 and 482°C for 20,000 and 50,000 hr. The ductile fracture behavior of the materials was characterized at 205, 427, and 482°C using multiple and electric-potential single-specimen techniques. Electric-potential data were used to caculate crack extensions via an electric-potential calibration equation for the construction of J-R curves. Results demonstrate that the critical fracture-toughness values are in good agreement with those from the multiple-specimen method. Results showed that 20,000-hr aging caused more than 35 percent degradation in fracture resistance, and 50,000-hr aging resulted in a slight increase in Jc for 16-8-2 stainless steel welds. It was found that the fracture toughness levels of the primary piping after long-term aging were high and adequate at the aging temperatures and that fuel handling temperature, nonductile fracture was not expected to occur in these materials.

A simplified determination of theJ-integral for paper

A novel, simplified, method for evaluating theJ-integral for paper sheets has been developed. The inaccuracy of the single-specimen method (proposed by Riceet al. [8]), when applied to the fracture toughness testing of paper, was found to result from an erroneous assumption about the relationship between the load and the displacement due to plasticity. A new method was developed theoretically; it is based on a modified assumption about this relationship. Using this method, theJ-integral can be evaluated with a single load-displacement curve together with a new parameter that must be separately evaluated using at least one additional specimen. Although the new method requires a minimum of two notched specimens, it provides more flexibility than previous modifications of the single-specimen method. The experimental study indicated that the present method yields values ofJc (the critical value of theJ-integral) which agree very closely with those obtained using the cumbersome multiple-specimen method, and is therefore more suitable for paper testing than other methods presently available.

Laboratory methods for the diagnosis and confirmation of systemic mycoses.

The concomitant use of culture and nonculture techniques for the diagnosis and confirmation of systemic mycotic infections is recommended and assessed. A diagnosis based upon a single specimen and method, especially when the results are negative, is not always conclusive. In many situations, a single specimen and test method cannot be relied upon for the diagnosis of a fungal infection. The testing of multiple or serial specimens with a battery of procedures increases the chances for establishing a rapid and definitive diagnosis. Positive cultures generally provide unequivocal evidence of infection. In the absence of such data, results obtained by the use of nonculture techniques such as direct examination, conventional and immunohistologic staining, exoantigen identification, DNA hybridization, immunodiffusion, complement fixation, enzyme immunoassay, and radioimmunoassay can allow detection and identification of the etiologic fungus or provide immunologic evidence of a specific fungal infection.

A simple method for measuring [formula omitted] of ductile metals

The whole tensile diagram may be divided into two parts, the loading curve and the fracture curve. The author finds, for the first time, on a fracture curve which differs from the traditional one, a critical point in order to calculate K IC of a ductile metal. It is a single specimen method and there is no artificial precracking. The testing cost and the time needed are much less than those of other methods. It is probably the simplest method for determining the K IC of a ductile metal. The error is better than 5%.

Fracture toughness of liner board evaluated by theJ-integral

The J-integral approach to characterizing the fracture toughness of liner board is investigated. Three methods, namely the single-specimen, multiple-specimen and Liebowitz non-linear energy method are evaluated in terms of accuracy and convenience in the fracture toughness determination. The multiple-specimen and Liebowitz methods yield consistent fracture toughness values that are independent of crack length, while the single-specimen method yields toughness values that depend on crack length. In terms of convenience the Liebowitz methods is superior to the multiple-specimen method since it requires fewer specimens and employs a data reduction methodology that is easily implemented.

Temperature dependence of fracture toughness ( J– R-curves) of a modified type 316L austenitic stainless steel

The influence of temperature on the fracture toughness characteristics of a modified type 316L stainless steel has been investigated in the temperature range 25–550°C. Both multi-specimen and single specimen methods (using a direct current potential drop technique with an experimental calibration) were employed. The single specimen method of evaluating J2 R ̄ - curves shows excellent agreement with the multi-specimen method. For the material used in this study the ASTM recommendation of the validity criteria and evaluation of the crack initiation fracture toughness were not found applicable. The initiation fracture toughness J c which is estimated as the fracture toughness at 0·2 mm crack growth beyond crack initiation is found to decrease with increase in testing temperature. This behaviour could be attributed to dynamic strain ageing effects which are known to occur in austenitic stainless steels in this temperature range. The crack growth resistance dJ da is found to be independent of temperature up to 375°C and decreases by about 50% at 550°C.

Fracture toughness for the specimens with shallow cracks

The relationship between the fracture toughness J i and the crack length/specimen width ratio ( a W ), i.e. J i-( a W ) curve, was investigated by a single specimen method, where the ratio ( a W ) was varied from 0.05 to 0.5 keeping W constant. The fracture initiation was determined by a clip-gauge displacement vs load-point displacement curve method. The J i-( a W ) curve showed that with increasing ratio ( a W ) the J i went up rapidly and then down after it reached the fracture toughness peak (FTP), at last it tended to a constant which was the fracture toughness of the materials in the way of a standard test. The J i values at FTP was about twice as large as that with deep crack specimens. A parameter ( a W ) 0.10 , at which J i was 10% higher than that at a W = 0.5 , was defined to distinguish the shallow and deep cracks. Three regions were divided in the J i-( a W ) curve in order to evaluate the significance of the crack length.

Single specimen test method for determining fracture energy ( [formula omitted]) of highly deformable materials

A single specimen fracture-test method for determining the fracture energy of highly deformable materials is presented. This test method is based on the locus method, which determines the critical J- integral value ( J c ) by partitioning the essential energy for crack propagation from the total absorbed energy. The locus method uses the locus of crack initiation points on the load-displacement record of specimens which differ only in initial crack size. With a proper specimen shape and loading condition, the locus line is predictable. When the locus line is predictable J c can be evaluated from a single specimen. Experimental results show that the fracture energy determined by the single specimen method agrees well with those by the tearing energy approach and multiple specimen locus method.