A compact C-shape specimen for fracture toughness testing

A compact form of C-shape fracture toughness test specimen is investigated.The specimen can be taken from thick-walled cylinders with greater material economy than the proposed ASTM C-shape, but is more difficult to manufacture. The $\text{K-}\text{calibration}$ is identical with the ASTM compact tension specimen.     

Warm pre-stress based pre-cracking criteria for fracture toughness testing

There are presently a magnitude of different fracture toughness testing standards that have different criteria for fatigue pre-cracking specimens prior to testing. The reason for the criteria is that too high pre-fatigue load may influence the subsequently measured fracture toughness value. The criteria have to a large extent been developed specifically for each standard in question and this has lead to the considerable variability in the criteria. The basic reason for the pre-fatigue having an effect on the fracture toughness is the warm pre-stress (WPS) effect. Here, existing data relating to pre-fatigue load levels are examined with the help of a newly developed simple WPS correction and a criteria and correction procedure for too high pre-fatigue loads are proposed. The new criteria focuses on brittle fracture, but is equally applicable for ductile fracture, thus enabling a unification of pre-fatigue criteria in different fracture toughness testing standards.     

Effect of initial notch orientation on fracture toughness in fail-safe steel

A 0.4C–2Si–1Cr–1Mo steel bar with an ultrafine-elongated grain (UFEG) structures was produced by multi-pass warm caliber rolling. The test sample was machined from the rolled bar with 0°, 45°, and 90° rotation along the rolling direction, and a static three-point bending test was conducted at ambient temperature. The toughness anisotropy on the steel with UFEG structures were studied, including the crack propagation on the basis of the microstructural features. The strength and toughness decreased with an increase in the rotation angle along the rolling direction. The toughness decreased drastically, compared to the strength. The notch orientation dependence on toughness is due to differences in the spatial distribution of weak sites such as {100} cleavage planes and boundaries of elongated grains. For the toughness design in ultrafine-grained materials, it is essential to understand the spatial distribution of these weak sites as well as the grain size.     

Determination of fatigue fracture toughness by conventional rotary bending specimen

Determination of fatigue fracture toughness,K _fc, is made by rotary bending specimen considering partial contact of fatigue cracked surfaces in the compression side of the beam specimen. It is shown thatK _fc is a material constant independent of the nominal stress at the notch section, the specimen geometry, and the shape of the final fracture area.

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Résumé On détermine la ténacité à la rupture par fatigueK _fc à l'aide d'éprouvetes soumises à flexion rotative, en considérant le contact partiel entre les surfaces fissurées par fatigue, au cours de la phase de compression. On montre queK _fc est une constante du matériau, indépendante de la tension nominale au droit de la section entaillée, de la géométrie de l'éprouvette, et de la forme de la surface finale de rupture.

     

A cut-ply specimen for the mixed-mode fracture toughness and fatigue characterisation of FRPs

The characterisation of mixed-mode fracture toughness and fatigue delamination growth in fibre-reinforced composites is crucial for assessing the integrity of structural elements in service. An asymmetric cut-ply coupon (ACP) loaded in four-point bending is here proposed to carry out the aforementioned characterisations. Analytical expressions of the energy release rate and mode-mixity for the ACP are derived and validated by means of finite element analysis. A fracture toughness and fatigue characterisation of the carbon/epoxy material IM7/8552 is carried out via ACP specimens. It is proved that the material data obtained from ACP specimens match those generated using ASTM standard mixed-mode bending (MMB) coupons. The main reason for the introduction of the ACP test resides in its applicability to characterisation scenarios where measuring the delamination length with optical means, as required for MMB coupons, is difficult. Such scenarios include the investigation of static and fatigue delamination growth at low and high temperatures, which requires the usage of environmental chambers. This poses significant constraints in terms of volume available for the test rigs, and, most importantly, limitations on visual access to observe delamination propagation. However, the manufacturing of ACP coupons is more complex than for MMB specimens and the testing requires several additional precautions that are here discussed in detail.     

The effect of friction on pin jointed single edge notch fracture toughness test specimens

A theoretical estimate is made of the effect of pin friction on values of the stress intensity factor for the ASTM type single edge notch fracture toughness test specimen. It is shown that for a coefficient of friction of 0.2 between pins and specimen the stress intensity factor is reduced by approximately 10 per cent. It is therefore concluded that precautions must be taken to keep friction to a minimum when measuring fracture toughness using this type of specimen.

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Zusammenfassung Eine theoretische Schätzung des Einflusses der Stiftreibung auf die Werte der Spannungsintensitätsfaktor für die Bruchduktilitätsprobe des Types ASTM mit einzelner Randkerbe ist gemacht geworden. Erwiesen ist, dass wenn die Reibungsfaktor zwischen Stiften and Probe 0,2 beträgt, so wind die Spannungsintensitätsfaktor um etwa 10 Prozent vermindert. Hieraus lässt sich folgern, class man Vorsichtsmassregeln treffen muss, um die Reibung bei einem Mindestwert.zu halten, wenn man die Bruchduktilität bei Anwendung dieser Probentyp misst.

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Résumé L'influence de la friction de la cheville sur les valeurs du facteur d'intensité de contrainte pour l'éprouvette du type ASTM de ductilité à la cassure a entaille unique de bord a été évaluée théorequement. Il est démontré que pour un coéfficient de friction de 0,2 entre chevilles et éprouvette, le facteur d'intensité de contrainte s'abaisse d'approximativement 10 pour cent. 11 est donc estimé qu'en mesurant la ductilité à la cassure au moyen de ce type d'éprouvette il faut prendre de precautions pour maintenir la friction à une valeur minimale.

     

An insight into mode II fracture toughness testing using SCB specimen

The effect of friction forces between the test specimen and its bottom supports on the mode II fracture toughness values obtained using the semicircular bend (SCB) specimen is investigated. First, a number of experiments were conducted on SCB specimen in order to determine the mode II fracture toughness of polymethyl methacrylate (PMMA) according to the conventional approaches available in the literature. Three different types of supports that have been frequently employed by researchers in recent years were used to evaluate the effect of support type on the fracture loads. It was found that the friction forces between the supports and the SCB specimen have a significant effect on the value of mode II fracture toughness measured using the SCB samples. Then, the specimen was simulated using finite element method for more detailed investigation on the near crack tip stress field evolution when friction forces increase between the supports and the SCB specimen. The finite element results confirmed that the type of support affects not only the stress intensity factors K_I and K_II but also the T‐stress. The experimental and numerical results showed that the use of the crack tip parameters available in literature for frictionless contact between the supports and the SCB specimen can result in significant errors when the mode II experiments are performed by using the fixed or roller‐in‐grove types of supports.     

The effect of fatigue pre‐cracking forces on fracture toughness

Testing procedures for the determination of the fracture toughness of a material by monotonic loading of fatigue pre‐cracked specimens are well established in standards such as BS 7448, BS EN ISO 15653, ISO 12135, ASTM E1820 and ASTM E1921. However, a review of these standards indicates a wide range of permitted fatigue pre‐cracking forces, whilst the underlying assumption in each standard is that the pre‐cracking conditions do not affect the fracture toughness determined. In order to establish the influence of different fatigue pre‐cracking forces on the fracture toughness, tests were carried out on specimens from an API 5L X70 pipeline steel. Single‐edge notch bend specimens of Bx2B geometry were notched through thickness and tested at temperatures of +20 °C, ?80 °C and ?140 °C to show the fracture behaviour in different regions of the fracture toughness ductile‐to‐brittle transition curve. Fatigue pre‐cracking was conducted on a high‐frequency resonance fatigue test machine over a range of pre‐cracking forces permissible within the various standards and beyond. The results showed that an excessively high pre‐cracking force can result in a significant overestimation of the value of fracture toughness for material exhibiting brittle behaviour, whilst very low fatigue pre‐cracking forces appeared to result in an increase in scatter of fracture toughness. A review of standards indicated that there was a possibility to misinterpret the intention of the ISO 12135 standard and potentially use excessively high pre‐cracking forces. Suggested clarifications to this standard have therefore been proposed to avoid the risk of overestimating fracture toughness.     

Quasi-static fracture toughness testing of a single CT specimen at two test conditions

Abstract

It is proposed that a single CT specimen can be used for determining J ₀.₂ at two testing conditions, provided it can be ensured that the crack tip plastic zones for the two tests do not interfere. This is achieved by extending the crack at the end of the first fracture test by fatigue cycling at ambient temperature to obtain the starting crack for the second test. This method has been validated by testing thermally aged CT specimens of modified 9Cr - 1Mo steel at 653 K and 803 K. The J-Δa values obtained by a multispecimen method at a specific temperature were on a single curve irrespective of whether the data were generated from the first test or second test on that sample. Also, the J-Δa curves obtained using a single specimen normalisation method from data on first and second tests were within the expected specimen to specimen variation.     

The effect of crack front curvature and side-grooving on three point bend specimen fracture toughness measurements

Finite element simulations of the three point bend fracture toughness specimen have been performed to investigate the effect of crack front curvature and side-grooving. Even modest crack front curvature moves the position of maximum energy release rate from the center towards the free surfaces of the specimen. A 30 percent difference between the maximum and minimum crack length can double the maximum energy release rate compared to that calculated for a straight crack of the same average length. A correction curve has been derived from which the curved crack energy release rate can be obtained using two dimensional solutions. Deep side-grooving substantially increases the energy release rate at the root of the groove, but for groove depths no more than 30 percent of the section, an energy release rate can be estimated from the two dimensional ungrooved solution scaled by the ratio of ungrooved to grooved thicknesses.