Characteristics of the double-torsion test to determine the R-curve of ceramic materials

Albuquerque, Maria da Consolação Fonseca de; Rodrigues, J. A.

doi:10.1590/s1516-14392006000400004

Cited by 3 publications

(10 citation statements)

References 32 publications

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“…In the double-torsion test, in both the material studied here and a material studied by Albuquerque & Rodrigues 36 , catastrophic propagation occurred after a certain length of stable propagation, as shown in Figure 5a, for example. The crack length at which catastrophic break of the specimen occurs was defined as the critical crack length, a c .…”

Section: Critical Crack Lengthmentioning

confidence: 82%

“…Therefore, only the curves with notch lengths with a 0 /L of 0.1 and 0.2 will be shown here. As mentioned in a previous paper by those authors 36 , again, for this material, the use of short notch lengths is also recommended. In that work, the lowest a 0 /L ratios presented the best results for the R-curve analysis, although, according to Shetty & Virkar 31 , the lowest a 0 /L ratio would be 0.2 for the 1:31.25:75 for the proportion of dimensions of the test specimen and, according to Pletka et al 17 , a 0 /L should be higher than 0.18 and lower than 0.78.…”

Section: Validity Range Of the Notch Lengthmentioning

confidence: 89%

“…Details of this test are given by Albuquerque & Rodrigues 36 . The equations associated with the double-torsion test are based on the assumption that the test specimen is a thin plate 10 .…”

Section: Methodsmentioning

confidence: 99%

“…Equation 5 presents R by the Linear Elastic Fracture Mechanics (LEFM) method, which establishes that R=K I 2 /E 40 . The value of a i , considered the instantaneous length of the crack, was estimated from the compliance calibration curve for the material under study 32,36 , and is written as:…”

Section: Methodsmentioning

confidence: 99%

“…The value of R was obtained by the arithmetic average of the R values in the stable propagation portion of the crack, i.e., the values of R for which the values of a begin to increase (Figures 5b, 5d, 6b and 6d). The equations used to calculate the values of R and γ wof are given in Albuquerque & Rodrigues 36 and Albuquerque 32 . Table 5 shows the values of R and 2⋅γ wof obtained in this work.…”

Section: Double-torsion Testmentioning

confidence: 99%

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Study of the dimensions of double-torsion test specimens

Albuquerque

Rodrigues

2008

Mat. Res.

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Double-torsion tests were carried out to ascertain whether a thin test specimen would affect the results of this test. The tests involved two test specimen sizes, one with a thickness five-fold greater than the particle size of the material and the other with a three-fold greater thickness. The width of both test specimens was ten times that of their thickness and their length was double their width. The material utilized was an alumina-based refractory castable without cement. It is important to ascertain the influence of the thickness of double-torsion test specimens when materials with a coarse microstructure are involved. Otherwise, test specimens would have to be very large, rendering the experimental procedure difficult, as in the case of the size of sintering furnaces and the size of accessories for the test. This study shows that test specimens with lower thicknesses are representative when one analyzes the calibration curve of the compliance, the length of the crack at which the break occurred (critical crack length), and the shapes of the R-curve and of the load (P) vs. displacement curves. However, the analysis of the mean values of R (R), obtained by the arithmetic average of the R values in the section of stable crack propagation and the total fracture energy (γ wof ), showed that these values are dependent on the size of the test specimens, indicating that this subject deserves further investigation.

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Section: Critical Crack Lengthmentioning

confidence: 82%

Section: Validity Range Of the Notch Lengthmentioning

confidence: 89%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Double-torsion Testmentioning

confidence: 99%

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Study of the dimensions of double-torsion test specimens

Albuquerque

Rodrigues

2008

Mat. Res.

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An Evaluation of the Double Torsion Technique

2011

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Double Torsion (DT) is a powerful testing technique for fracture mechanics characterisation of brittle materials as, in principle, it provides a crack length independent test configuration. However, several corrections have been proposed to address variations of experimental results reported from various laboratories. These correction factors address the validity of the DT configuration and its crack length independent stress intensity. Never the less, there seems to be no consensus in literature on the various corrections and the reason of reported variations. This paper presents firstly a critical review of the DT technique, followed by proposed corrections through an experimental analysis using the proposed corrections, a Finite Element model of the geometry and the use of Digital Image Correlation to measure out-of-plane surface deformations. It focuses on the validity of the constant stress intensity regime and the independence of crack length in a critical evaluation using Polymethylmethacrylate test specimens. Assessment of three un-grooved specimen geometry configurations demonstrated the apparent regime of approximately constant stress intensity, although a small but clear dependence of the stress intensity on crack length was observed in all specimen configurations. This dependence is attributable to significant load-point deflections and out-of plane deformations that are not accounted for in the DT analysis. Revisions of the proposed analysis methodologies show that a crack length independent specimen geometry can be achieved, however at the cost of less accurate data. Reliable and accurate data can be achieved with a DT testing configuration using an optimum specimen configuration.

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Crack Growth Resistance of Carbide Reinforced Composite Ceramics Based on Alumina

Szutkowska¹,

Boniecki

2009

KEM

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The relationship of KR versus crack length c (R curve) for Al2O3-30wt.% Ti(C,N).and for comparison alumina ceramics has been examined. The R-curve has been evaluated using pronounced long-crack formed during the three point bending (3PB) of the double edge notched beam. A combination of in situ microscopic crack growth observation and mechanical testing enabled measurement of crack growth resistance curves. The special device consisting of light microscope coupled with CCD camera, was fitted to Zwick 1446 testing machine. These observations reveal the existence of flat R-curve for Al2O3-30wt.% Ti(CN) and increasing R-curve for pure alumina. A study of slow-crack-growth (SCG) in tested materials was carried. The load-relaxation technique was used for observation at slow-crack-growth. The crack length was evaluated by linear-elastic analysis from the compliance of single-edge-notched specimen in three-point bending test. Parameters of stable crack growth n and logA, work-of fracture (WOF), stress intensity factor at the moment of crack initiation KI0 and maximum values of stress intensity factor KImax were determined. Mechanism of grain bridging responsible for occurrence of R-curve was observed by SEM and TEM.

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Characteristics of the double-torsion test to determine the R-curve of ceramic materials

Cited by 3 publications

References 32 publications

Study of the dimensions of double-torsion test specimens

Study of the dimensions of double-torsion test specimens

An Evaluation of the Double Torsion Technique

Crack Growth Resistance of Carbide Reinforced Composite Ceramics Based on Alumina

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