Chevron-notched Short Bar Research Articles

Optimization of chevron-notched short bar specimen configuration for determination of the fracture toughness and crack growth rate of aluminum alloys

The goal of the study is optimization of the fracture toughness determination procedure on the samples with a chevron notch. Well-known principles of fracture mechanics are used to determine the geometry of a specimen with a chevron notch, which ensures the independence of the stress intensity factor from the crack length in the range sufficient for its practical application. A requirement to the shape of the dependence «compliance – crack length» is formulated for a chevron-notched specimen. The choice of the cross-sectional shape of the specimen with a chevron notch, the geometric dimensions of the specimen, and the drawing of the gripping device are justified. The formula for calculation of the fracture toughness from the maximum test load and thickness of the specimen is derived and experimentally validated. The results of the fracture toughness tests of the samples with a chevron notch and standard samples are compared for a number of aluminum alloys. The data on the scale factor in the range of a sample thicknesses of 12.5, 25, and 50 mm and variation coefficient obtained upon the fracture toughness determination revealed the absence of the significant effect of the scale factor, 6% variation under the normal law of the fracture toughness distribution in the tests of chevron-notched specimens. The results on the fracture toughness distribution in the height of the thick plates of alloys 1163T1 and V95pchT2 are presented. The temperature dependence of the fracture toughness of the plate (alloy 1201T1) are constructed for chevron-notched specimens within a temperature range of –196 – +200°C. The features of using chevron-notched specimens for determination of the crack growth rate attributed to independence of the stress intensity factor at the crack tip on crack length are considered in a rather wide range. The method of the crack growth rate determination is described and the results of assessing d a /d N – Δ K for an aluminum alloy plate 1163T are presented. The simplicity and reliability of the methods for determination of the fracture toughness and fracture growth rate (fatigue, creep, corrosion) on chevron-notched specimen of the proposed geometry allow us to recommend them for control tests and research in various test conditions.

Environment-assisted failure of structural tendons for construction and building applications

The work focuses on the environment-assisted crack growth conditions and addresses the threshold stress intensity factors of three commercial high-strength steel bars widely used in construction and building tensile applications. To quantify the sensibility to stress corrosion cracking, chevron-notched short bar specimens were simultaneously subjected to tension tests and hydrogen embrittlement in both artificial seawater and ammonium thiocyanate solution. The latter was used to enhance the stress corrosion damage and to reveal the fracture morphologies that characterize the rupture of the bars. Thus, new insights regarding the lifetime of the bars in seawater environment are brought into attention.

Environmental effects on large diameter high-strength rods for structural applications

Abstract This work aims assessing the stress corrosion cracking behaviour in distinct aggressive environments of a commercial class of high-strength low alloy steel bars of randomly oriented pearlite structure, intensively used in tensioning systems of structural engineering. The research is based on tensile testing of un-notched and circumferentially notched cylindrical specimens that are simultaneously subjected to slow strain rate loading and environmental damage. To vary the damage intensity, seawater media of distinct aggressiveness, and ammonium thiocyanate solution are employed. In addition, fracture tests of chevron-notched short bar specimens, fatigue pre-cracked perpendicularly to the bars axis, provided the experimental data to benchmark the bars toughness. The results indicate that damage inducing failure consists of pearlite interlamellar decohesion as result of hydrogen-induced local embrittlement of the steel. This activates a continuous stable cracking of sickle shape until brittle rupture of the remnant undamaged ligament is finally triggered.

New testing method for assessing the cracking sensibility of stressed tendon rods in aggressive environments

Abstract The paper presents a new testing method for assessing the cracking sensibility in aggressive environments of tendon rods for prestressed concrete structures, on the basis of Fracture Mechanics concepts. First, it approaches the fundamentals of the designed test in the context of existing fracture specimens, regarding the geometrical limitations introduced by the environmental assisted cracking of the rods, perpendicularly to the load direction. The analysis showed that fatigue precracked chevron-notched short bar specimen (SBS) is providing the largest measurement range of stress intensity factor to be explored in stress-corrosion cracking (SCC) tests. Then, the equation relating the elastic stiffness of SBSs to crack size was experimentally validated for specimens with true cracks produced by fatigue. SCC verification tests were made with an innovative horizontal loading device and the crack mouth opening displacement (CMOD) was acquired and numerically analyzed with a video digital image correlation system. The tests showed that SCC is fully governed by the small scale-yielding regime at the crack tip. Hence, the main process parameters as crack extension and environment-assisted stress intensity factor were empirically obtained for each time sequence of the SCC tests. To simplify further SCC tests instrumentation, an empirical correlation was stated between CMOD and crack opening displacement (COD) as measured by a conventional extensometer mounted on the loading grips of the specimen. This correlation was also validated in the SCC tests.

Ａ１−Ｌｉ系合金８０９０厚板の二段時効による破壊じん性の改善

The short-transverse fracture toughness behavior of aluminum-lithium alloy 8090 plate was investigated by using a CNSB (chevron notched short bar) specimen. The fracture toughness KQVM was reduced with a progress of the first-step aging. The double-aged (DA) alloy, which was subjected to the initial underaging (UA2) at 433K followed by the second-step aging at a higher temperature for a short time, exhibited a fairly improved KQVM and a superior strength-toughness balance in comparison with the single-aged alloy, good enough to satisfy the toughness requirement in aircraft design, KQVM/σys≥2.5√mm, where σys is yield strength. The increase in toughness by DA treatment is attributed to the relaxation of intense slips due to dissolution of shearable δ' phase and growth of δ'-PFZ width, leading to the inhibition of intergranular fracture.

２０９０および２０９１アルミニウム合金の破壊靭性に及ぼす二段時効の効果

Effect of double-aging on the short-transverse fracture toughness of Al-Li alloys 2090 and 2091 was investigated by using a CNSB (chevron notched short bar) specimen. The fracture toughness KQVM is reduced with a progress of the firststep aging, but is fairly improved by a double-aging treatment. The double-aged alloy 2091 exhibits a superior strength-toughness balance to that of the single-aging. The increase in KQVM by the double-aging is attributed to a relaxation of intense slip due to dissolution of shearable δ' phase and growth of δ'-PFZ width. For alloy 2090, coasening of grain boundary precipitates concurs with the δ' phase dissolution, which assumedly accelerates microvoid growth at grain boundaries, leading to a lower improved toughness.

A materials failure relation of accelerating creep as empirical description of damage accumulation

A general materials failure relation, $$\ddot \Omega = {\rm A}\dot \Omega ^\alpha$$ , describes accelerating creep of materials with rate coefficients α andA, by relating rates of deformation, $$\dot \Omega$$ , to changes in deformation rate, $$\ddot \Omega$$ (Voight, 1988). Time of failure can be extrapolated from inverse rate versus time data, and α andA may be derived to permit one to calculate the failure time. The method is of value for quantitative hazard assessments. Mechanisms leading to damage accumulation during accelerating creep include creep fracture by stress corrosion and power law lattice deformation. These mechanisms are examined here as phenomenologically related to the materials failure relation. Apparently, both mechanisms favour α≅, where α is the parameter of the materials failure relation controlling the sensitivity to accelerating activity. For pure shear governed by power law creep of powerp, under constant load, α=2.0 andA=p. Stress corrosion is widely described by Charles' equation, relating crack velocity to stress intensity during subcritical crack growth by the stress corrosion indexn. The relationship betweenn and α is given by α=(2n−2)/n. The quantitative, predictive qualities of the general materials failure relationship are illustrated with examples from Mount Toc, Italy, and Mount St. Helens, Washington. Six chevron-notched short bar experiments under constant load serve as examples for accelerating creep fracture in the laboratory. The experiments were conducted on silstones of the Ithaca formation, which show a mean stress corrosion index ofn=78±24. Analysis with regard to the materials failure relation results in a mean α=2.0±0.3, which complies with the established relationship betweenn and α.

Fracture Toughness Measurement of Glass and Ceramic Materials Using Chevron‐Notched Specimens

Fracture toughness of several different materials was measured using chevronnotched short bar and four‐point bend specimens For glass‐ceramic and ceramic samples both specimens gave valid results. Fracture toughness values measured with bend specimens are 5% to 10% higher compared to those of the short bar. Consistent results for glass could be obtained only with short bar specimens. The notch width of the bend specimen affected the stability of crack growth in glass samples. Fracture toughness values measured in the present study are in good agreement with those of the previous studies.

Extended range stress intensity factor expressions for chevron-notched short bar and short rod fracture toughness specimens

Extended range stress intensity factor expressions for chevron-notched short bar and short rod fracture toughness specimens

Determination of fracture toughness of high strength aluminum alloys with chevron notched short rod and short bar specimens

Fracture toughness of high strength aluminum alloys, determined with chevron-notched short bar and short rod specimens is in agreement with results from compact specimens only for K Ic < 30 MNm −3 2 . For higher K ic -values the short rod/short bar specimens exceed the K Ic -values up to 60%. The reason for the high short bar/short rod values is a rising crack growth resistance curve.