Short Bar Specimens 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.

Fracture toughness of high-strength steel bars

Brittle fractures caused by accidental or environmental damage of high-strength steel bars are occasionally reported in civil engineering. To analyse such ruptures, knowledge of bar plane strain fracture toughness is needed. Unfortunately, plane strain fracture toughness cannot be measured as bar diameters of commercial bars are not compatible with the thickness requirements of standardized fracture specimens according ASTM E399. The ASTM E1304 is less restrictive by using short bar specimens (SBS) but these can be ineffective with very brittle steels. This situation can be overcome, as shown in this paper, by modifying the SBS specimen.A second aim of this paper is to provide several examples of how to analyse brittle fractures of commercial high-strength steel bars considering knowledge of its plane strain fracture toughness. Direct and indirect analyses have been carried out.

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.

Fracture toughness of microsphere Al 2O 3–Al particulate metal matrix composites

The fracture toughness and behaviour of COMRAL-85 TM, a 6061 aluminium–magnesium–silicon alloy reinforced with 20 vol% Al 2O 3-based polycrystalline ceramic microspheres, and manufactured by a liquid metallurgy route, have been investigated. Fracture toughness tests were performed using short rod and short bar (chevron-notch) specimens machined from extruded 19 mm diameter rod, heat treated to the T6 condition. The fracture toughness in the R–L orientation was found to be lower than in the C–R or L–R orientations owing to the presence of particle-free bands in the extrusion direction. Short rod tests were also conducted for the R–L orientation on six powder metallurgy composites with particle volume fractions ranging between 5% and 30%. It was found that the fracture toughness decreased progressively with particle volume fraction, but at a decreasing rate. A detailed examination of the fracture behaviour was made for both the liquid metallurgy and powder metallurgy processed composites.

Engineering Properties and Performance of Dental Crowns

Dental crowns are used to replace damaged natural crowns of teeth and are fixed to prepared teeth with luting cements, which should provide an adhesive bond to the tooth structure giving reliable retention and minimal microleakage. Mechanical testing of crowns in vitro gives failure load distributions that are well described by Weibull models, comparing probabilities of survival and reliability. Fatigue testing of crowns is time consuming, but regression analysis to interpolate functions through data points quoting probability limits or applying Weibull analysis is achievable. A complementary approach is to conduct materials tests with appropriate interfacial geometries. Luting cements are used in thin layers of 40-150 microm. Contraction during polymerization is restrained by adhesion to substrates, allowing little relaxation of stresses. Conventional and resin-modified glass ionomer cements create thin zones of interaction with dentine and fail cohesively. The chevron notch short rod technique has been used to measure fracture toughness and rank cements. A development of this method, using chevron notch short bar specimens, permitted fracture toughness to be determined for luting cement--dentine substrate interfaces. Representative fracture experiments need to be developed to apply mixed mode conditions. The basic challenge to predict long-term performance from short-term laboratory tests remains.

Structure, fracture toughness, and fatigue of two aluminium matrix compositesproduced by vertical squeeze casting technique

Composite materials produced from ceramic reinforcement of aluminium alloys have some properties that are better (higher modulus and strength, lower thermal expansion coefficient and density, and good creep and wear resistance) than those of the conventional monolithic aluminium alloys. However, they have a poor fracture toughness. The aim of the present work was to characterise the structure and mechanical properties of two different aluminium matrix composites (AS9C1G/20%(Al2O3-SiO2) and 2014/20%(Al2O3-SiO2)) manufactured using the vertical squeeze casting technique. Tensile, plane strain fracture toughness, and fatigue crack growth rate tests were carried out. In particular, the influence of specimen geometry on the toughness tests was examined. It was found that chevron notched short bar specimens gave toughness values ∼ 40% higher than other types of specimens. Fatigue crack growth rate data were interpolated using some semiempirical models. An accurate metallographic investigation of both the structures and the fatigue fracture surfaces was carried out using optical microscopy and energy dispersive spectroscopy with SEM.

Structure, fracture toughness, and fatigue of two aluminium matrix compositesproduced by vertical squeeze casting technique

Composite materials produced from ceramic reinforcement of aluminium alloys have some properties that are better (higher modulus and strength, lower thermal expansion coefficient and density, and good creep and wear resistance) than those of the conventional monolithic aluminium alloys. However, they have a poor fracture toughness. The aim of the present work was to characterise the structure and mechanical properties of two different aluminium matrix composites (AS9C1G/20%(Al2O3-SiO2) and 2014/20%(Al2O3-SiO2)) manufactured using the vertical squeeze casting technique. Tensile, plane strain fracture toughness, and fatigue crack growth rate tests were carried out. In particular, the influence of specimen geometry on the toughness tests was examined. It was found that chevron notched short bar specimens gave toughness values ∼ 40% higher than other types of specimens. Fatigue crack growth rate data were interpolated using some semiempirical models. An accurate metallographic investigation of both the structures and the fatigue fracture surfaces was carried out using optical microscopy and energy dispersive spectroscopy with SEM.

R-curve and subcritical crack growth behavior at elevated temperatures in coarse grain alumina

The effect of temperature and subcritical crack growth on the R-curve of a 99.5% purity coarse grained alumina was studied using chevron-notched, short-bar specimens. Constant loading rate tests were used to measure toughness as a function of crack length and static load tests were used to measure subcritical crack growth as a function of time. It was found that the intrinsic toughness decreased monotonically from 20 to 1200°C while the bridging contribution to the R-curve remained relatively constant over this temperature range. The constant load tests at 700, 1000 and 1200°C showed that substantial subcritical crack growth occurs under static loading. Using the subcritical crack growth parameters derived from these static load tests, the effect of loading rate on R-curve measurement was predicted and compared to experiment. The predicted decrease in the magnitude of the R-curve with a decrease in over five orders of magnitude in loading rate was relatively small compared to experimental scatter.

Study of fracture and erosive wear of plasma sprayed coatings

Double cantilever beam (DCB) and short bar (SB) specimens were used to determine the critical strain energy release rate (GIC) of plasma-sprayed coatings ofZrO2-base ceramic and WC-Co coatings. Erosion rates were measured for various erosive conditions. By comparing theGIc data and erosion rate (Ev) data with X- ray diffraction and fractographic analysis, fracture and erosion mechanisms of plasma-sprayed coatings were proposed. Based on the erosion models for brittle materials, a proportional relationship between the erosion rate,Ev, andGIC was derived as follows: $$E_v \propto \left( {\frac{1}{H}} \right)^{{1 \mathord{\left/ {\vphantom {1 4}} \right. \kern-\nulldelimiterspace} 4}} \times \left( {G_{IC} } \right)^{ - {2 \mathord{\left/ {\vphantom {2 3}} \right. \kern-\nulldelimiterspace} 3}} $$ (1) whereH is hardness. An increase inGIC leads to a decrease inEv. The exponent value of -2/3 ofGIC was confirmed by the experimental data.

Measurement of the critical strain energy release rate of plasma-sprayed coatings

Based on the compliance calibration method of linear elastic fracture mechanics, three kinds of specimens, i.e. single-edge-notched single-edge-notched bend specimen, double-cantilever-beam (DCB) specimen and short-bar (SB) specimen, have been used to determine the critical strain energy release rate ( G IC) of plasma-sprayed coatings of ceramic ZrO 2 and cermet WC-Co series. By comparing the results and procedures, it was found that DCB and SB methods or a combination of the two, can properly evaluate the value of fracture toughness, optimize the spraying technique and direct the design in plasma-sprayed coatings.

New Formulae for Determining Fracture Toughness of a Wide Range of Different Materials with Chevron-Notched Short-Rod and Short-Bar Specimens

Abstract New formulae are presented to re-evaluate plain strain fracture toughness of brittle and ductile materials from previous works on chevron-notched specimens. The fracture toughness KIm from the new formulae agree with values of KIC from ASTM specimens. The KIm measurements are successful in tests of tough aluminum when the fracture toughness exceeds 40 MPa · m1 2 and steel 20G with the plasticity factor p approach 1. It is found that the valid specimen size could be smaller than 1.25(KIC/σS)2 for aluminum alloys, and even less than 0.1(KIC/σS) for steel 20G when the KIm formulae are used.

Formula omitted] Determination using a chevron notched compact tension specimen

The stress intensity factor of a standard compact tension specimen modified with a 37° chevron notch similar to those employed in short rod and short bar specimens has been evaluated using the boundary integral equation method as well as an experimental compliance test method. The minimum normalized stress intensity factor obtained was found to be in agreement within 10%. The plane strain fracture toughnesses, K IC , of three tool steel materials, viz. Assab XW-41, Assab DF2 and Rochling RCC Extra steel, have been determined at various tempering temperatures from as-quenched to 600°C using the modified compact tension specimens and also by the ASTM-E399 standard testing method. At tempering temperatures below 500°C close agreement, generally within less than 10%, has been observed in each case. As the ductility of the materials increased at higher tempering temperature, however, overestimation was observed.

Inelastic deformation of bioactive glass-ceramics

The fracture of bioactive glass-ceramics was determined in kerosene and distilled water using short bar specimens with chevron notches. By using the stable crack growth technique, an inelastic phenomenon was observed on the diagram of load versus load-point displacement. From these results, it was suggested that the inelastic behavior of bioactive glass-ceramics was produced by the plastic deformation on the grain boundary with the glassy phase.

Effects of microstructure on the R-curve behaviour of sintered silicon nitride

The R-curve behaviour of sintered silicon nitride was investigated by using short bar specimens. The samples were fabricated by sintering at 1700° C for 2 or 8 h in a nitrogen atmosphere, varying the initialα contents of the silicon nitride powder. The R-curve was evaluated with a loading and unloading technique to calculateK R during stable fracture. Steep R-curves were observed in the specimens made from the high initialα content powder. Thus, the grainbridging effect behind the crack seems to contribute to the R-curve, because a steep R-curve corresponds to fibrous texture development. ApparentK c values with an assumption of linear elastic fracture mechanics for the specimens undergoing 8h sintering, are greater than those undergoing 2 h sintering. These results can be attributed to microcracking.

Inelastic deformation and structure of borate glasses

The work of fracture for soda borate glasses was measured in kerosene using short-bar specimens with chevron notch. In unloading-loading cycles during stable crack growth, an inelastic phenomenon was observed on the diagram of load versus load-point displacement for B 2O 3-rich glass. From these results, it is suggested that the inelastic behavior is caused by an increase of the plastic deformation with increasing the boroxol rings.

Calculation of low-cycle fatigue under nonproportional loading conditions

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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.

Geometry effects on fracture toughness of ceramics with chevron notched specimens.

Plane strain fracture toughness (K/ ) measurements were made on ceramics (graphite, engineering ceramics) using short bar and bend bar chevron notched specimens previously calculated by optimum specimen geometry, to enable precracking, by means of 3-dimensional finite element and Bluhm's slice model analysis. Acoustic emission (AE) signals were also measured for detecting precracking and successive crack growth. AE signals due to precracking and crack growth were obtained for all short bar and bend bar specimens of graphite ; and deviations of KIC values were relatively small with variations in chevron notch angle and the width of the slot from 0.2 mm to 0.5 mm. In most engineering ceramics, unstable fracture immediately occurred after popin. About 20% higher KIC values were obtained in comparison to another testing device (fractometer-Terratek Co.), except for one commercial hot-pressed Si3N4. It means that the width of the slot should be less than 0.2 mm in highly brittle solids, and confirmation of precracking is actually important to evaluate KIC value.