Blunt-notched Specimens Research Articles

Numerical simulation of creep notched bar of P91 steel

Numerous components designed for use at elevated temperatures now exhibit multiaxial stress states as a result of geometric modification and material inhomogeneity. It is necessary to anticipate the creep rupture life of such components when subjected to multiaxial load. In this work finite element analysis has been performed to study the influence of different notches, namely blunt and medium notches on the stress distribution across the notch throat during the creep exposure. Within the FE model, a ductility exhaustion model based on the Cocks and Ashby model was utilized to forecast the creep rupture time of notched bar P91 material. The lower and upper bound of creep ductility are employed in the FE analysis. Different notch specimens have different stress and damage distribution. It is shown that for both types of notches, the von Mises stress is lower than the net stress, indicating the notch strengthening effect. The accumulation of creep damage in the minimum cross-section at each element across the notch throat increases over time. The point at which damage first occurs is closer to the notch root for the medium notch than for the blunt notch. The long-term rupture life predicted for blunt notch specimens appears to be comparable to that of uniaxial specimens. The upper bound creep ductility better predicts the rupture life for medium notches.

Notch Effect on the Fatigue Behavior of a TC21 Titanium Alloy in Very High Cycle Regime

The very high cycle fatigue (VHCF) property of TC21 titanium alloy blunt-notched specimens were investigated by using an ultrasonic fatigue test machine with a frequency of 20 kHz. S–N of blunt-notched specimens illustrated a continuous decrease characteristic with a horizontal line over the 105–109 cycle regimes. However, the fatigue life showed a large scatter for blunt-notched specimens. Blunt-notch significantly reduced the fatigue property in the high cycle and very high cycle regimes compared with that of smooth specimens. The crack initiation modes for blunt-notched specimens in the very high cycle regime can be divided into three types: (i) surface initiation, (ii) subsurface with flat facet, and (iii) subsurface with “facet + fine granular area”. The crack initiation mechanism of blunt-notched specimens is discussed in view of the interaction of notch stress gradient distribution and heterogeneous microstructure. Furthermore, the fatigue limit model based on the theory of critical distance (TCD) was modified for the very high cycle regime, and the scatter of the fatigue property of the blunt-notched specimens were well predicted by using this model.

Load-Separation Method, Spb, Used to Estimate Crack Extension for J-R Curves, Modified for Geometric Variations in Blunt-Notched Remaining Ligament

Abstract The Spb method estimates stable crack growth in fracture mechanics precracked specimens test. It compares the load ratio between a precracked specimen that exhibits stable crack growth and a blunt-notched specimen with apparent constant crack length when tested at constant displacement. When it was applied to very tough material, it was found that blunt-notched remaining ligaments varied by as much as 11.83 %, affecting visibly the results. Therefore, to apply the Spb method to very tough material for which non-standard specimens could be used as coiled tubing, the methodology limits are investigated. In this work, particularly the effect of geometry changes through an incipient crack growth produced on the blunt-notched specimens for very tough materials is analyzed. Hence, a modified Spb method is presented that takes into account the change in geometry of the blunt-notched specimens. Results provided in this paper are quite encouraging.

Effect of cyclic fatigue on the fracture toughness of Polyoxymethylene

Polymers are used in a wide range of applications and their properties are dependent upon the morphological development during processing and the specimen configuration which in turn define the mechanical properties. In this context fatigue and monotonic testing are part of the standard procedure to assess relevant mechanical and material parameters to ensure a better part design. The present work addresses the performance issues of a real component made of Polyoxymethylene (POM) which is subjected to cyclic loads from intermediate levels to high peak values inside a damping mechanism. For this linear elastic fracture mechanics concepts were used to characterize the behavior of a POM homopolymer resin used in this application. Injection molded compact tension specimens, with sharp and blunt notches, were tested under a combination of cyclic and monotonic loads and the fracture surfaces were examined. The critical stress intensity factor obtained by monotonic tests was evaluated as a function of the cycle number, where an increase after the first 1000 cycles followed by a continuous decrease with higher numbers of cycles was observed. A variation of approximately 50% and 70% were obtained along the duration of the tests for the sharp and blunt notch specimens, respectively. In light of the obtained results, a discussion is presented considering the dynamic specimen compliance and the structural features observed on the fracture surfaces in combination with the fracture mechanical response.

切欠の鈍い試験片における切欠引張強さ比とひずみ硬化指数の関係

In order to develop the relationship between the notch strength ratio, defined as the notch tensile to tensile strength ratio, and the strain hardening exponent, the rate of increase in stress with elongation was investigated on the notched specimens associated with strain hardening ability of the materials. The plastic constraint resulting from strain localization due to low rate of strain hardening raised the stress-elongation curves distinctly. Hence, the stress increase rate increased with decreasing strain hardening exponent for the notched specimens with widely different notch acuity. The useful correlations were thus established to estimate the tensile load required to break the blunt notch specimens as well as the sharp notch specimens from strain hardening exponent.

Prediction of Ductile Fracture in Steel Connections Using SMCS Criterion

Conventional fracture mechanics approaches have limited capabilities to accurately predict fracture under conditions of large scale yielding or in complex geometries where there are no significant pre-existing flaws. Such conditions are found, for example, in welded steel moment connections that meet stringent detailing and quality assurance requirements for seismic design that have been imposed in response to damage caused by the Northridge earthquake. The stress modified critical strain (SMCS) criterion provides an alternative approach for modeling ductile crack initiation by relating the fundamental process of void initiation, growth, and coalescence to macroscopic stresses and strains, obtained using detailed finite element models. The material-specific parameters of the SMCS model are calibrated for mild A572 Grade 50 steel using a series of notched tensile tests and fractographic data. Accuracy of the SMCS criterion is demonstrated for specimens with a range of triaxial constraint conditions, including both sharp-crack fracture specimens and blunt-notch specimens, the latter being examples of cases where conventional fracture mechanics approaches would not apply. The SMCS model is then applied in a practical demonstration to relate data from idealized pull-plate tests to the fracture resistance of beam–column moment connections.

鈍くて浅い切欠きをもつＦＲＰ板の静荷重による破壊

The fracture criterion of notched FRP plates based on the concept of stress concentration and the similarity of stress distribution near the notch root is investigated experimentally. An experimental method is presented for examining the fracture strength of notched FRP plates under static loads. The investigation is accomplished by obtaining experimental data on the tension and bending tests of a glass fiber-reinforced polycarbonate plates containing blunt and shallow notches. For sharp-notched specimens, the stress distributions near the notch root are the same in all specimens, for which both the maximum elastic stress and notch-root radius are equal to each other. Therefore, the maximum elastic stress at fracture is determined by the notch-root radius only. On the other hand, for blunt-notched specimens the situation is much more complex. The experimental results were discussed in terms of a combination of the maximum elastic stress, notch-root radius and the width of notch section. Applying the fracture criterion derived here, the experimental results for blunt-notched specimens can be clearly explained.

Understanding the potential and pH dependency of high-strength β-titanium alloy environmental crack initiation

An explanation for the strong dependency of crack initiation of precracked high-strength β-titanium alloys in room-temperature 0.6 M NaCl on applied potential and bulk-solution pH is presented. It is proposed that environment-assisted cracking (EAC) susceptibility in neutral aqueous NaCl results from (1) film rupture due to plastic deformation at actively deformed crack tips, (2) accelerated dissolution of titanium, (3) crack tip acidification by hydrolysis of titanium ions, (4) crack tip potential excursions toward bare metal open-circuit potentials (OCPs) during film rupture due to large ohmic voltages in the crack solution, (5) accelerated crack tip proton or water reduction concurrent with titanium dissolution, (6) bare surface-dominated hydrogen ingress into a fracture process zone, and (7) crack initiation by hydrogen embrittlement. Evidence for each of the above stages of the crack initiation scenario is presented, with emphasis on crack tip electrode kinetics and ohmic voltage calculations which govern process zone-controlled hydrogen uptake. The seven stages are consistent with the strong dependencies of crack initiation and growth in precracked high-strength β-titanium alloys on (1) solution pH, (2) applied potential, and (3) strain rate, and they explain the “apparent” EAC resistance of smooth- and blunt-notch specimens. The latter lack both occluded crack tip geometries to promote acidification and ohmic voltage drops below reversible hydrogen, as well as localization of dynamic plastic strain. Hydrogen uptake is, subsequently, limited.

Determination of ηpl Factor: Influence of Side Grooving

Abstract The objective of this paper is to analyze the influence of side grooving on the ηpl factor in the ASTM Standard Test Method for JIc, a Measure of Fracture Toughness (ASTM E 813-89), for C(T) and SE(B) specimens. The method proposed by Sharobean and Landes to calculate the ηpl factor from blunt-notched specimens was utilized. Standard C(T) and SE(B) specimens were prepared out of a 30-mm-thick, 2.25 Cr-1 Mo. ASTM A-387-Gr22 type steel plate. The crack lengths considered ranged from 55% to 75% of the specimen width W, and three sets of specimens were prepared with 0% B, 25% B and 50% B double side grooving, respectively.

Fracture criteria for hydrogen and temper embrittlement in 9Cr1Mo steel

A reduction in ductility and toughness is observed on thermal ageing of 9Cr1Mo steels in the temperature range 400–650°C. Previous investigations using tensile tests have shown a synergistic effect of hydrogen on such temper embrittlement. To investigate the mechanisms underlying this effect of hydrogen a series of fracture tests have been undertaken on a commercially produced 9Cr1Mo steel using blunt-notched (SENB) specimens tested in four-point-bend. The results were analysed to determine the maximum local tensile stress σ F ∗ and the local stress and strain levels at the point of crack initiation as a function of test temperature, hydrogen content and thermal ageing. Local fracture stress, rather than σ F ∗ , was relatively constant with test temperature, and it is proposed that this parameter, together with local fracture strain, controls the failure mechanism. A dual stress/strain criteria model is proposed, and it is suggested that both hydrogen and temper embrittlement reduce the local fracture stress, indicating an influence on cohesive strength for both mechanisms of embrittlement.

Dependence of ductile fracture toughness of a weld metal on notch root radius and inclusion content

Using different fluxes of high and low basicity, two different welds have been produced with low and high inclusion content. Inclusions have been characterized in 2 and 3 dimensions, from measurements on both polished sections and on fracture surfaces. The fracture toughness is evaluated with J0integral, crack tip opening displacement (CTOD) and stretch zone width measurements. Ductile fracture satisfies the criterion of a critical fracture strain over a characteristic distance. These two critical values can be determined from the results obtained with blunt notch specimens. Characteristic distances are an order of magnitude larger than the inclusion spacing. This is attributed to the need to reach conditions of sustained crack growth for initiation of ductile tearing.

Cleavage fracture in pearlitic eutectoid steel

The effect of microstructure on flow and fracture properties of fully pearlitic steel has been studied by independently varying the prior austenite grain size and the pearlite interlamellar spacing through appropriate heat treatments. The yield strength is independent of the prior austenite grain size but increases as the interlamellar spacing or the temperature decreases. The microstructural dependence can be explained by using a model which assumes that yielding is controlled by dislocation motion in the ferrite lamellae. The critical tensile stress for cleavage fracture is found to be independent of prior austenite grain size, increasing as the interlamellar spacing decreases. The cleavage fracture stress is independent of temperature for fine pearlite but increases as the temperature decreases for coarse pearlite. The associated fracture in blunt notch specimens initiates at inclusions beneath notch surface near the location of maximum tensile stress. From the size of such inclusions, the effective surface energy for cleavage fracture can be directly calculated and is found to be independent of temperature and prior austenite grain size but to increase as the interlamellar spacing decreases, from about 5 to 13 J/m2 for the range of microstructures and temperatures used in this study. Additional measurements of the effective surface energy and further theoretical analyses of the cleavage process are needed.

Instrumented impact testing of polyethersulphone

The impact behaviour of polyethersulphone has been studied using a specially constructed instrumented impact testing machine. This machine is of the pendulum type and the samples are fractured in three-point bend loading. It is shown that accurate force/deformation curves can be obtained, in spite of complications due to flexural vibrations of the test sample. Measurements were made on both sharp-notched and blunt-notched specimens over a range of crack lengths. It was found that the sharp-notched samples could be analysed in terms of fracture toughness, G C, whereas the blunt-notched samples corresponded to a constant critical stress at the root of the notch. The importance of multiple crazes at the crack tip in bluntnotched specimens is emphasized. It is also shown that ageing reduces the fracture toughness, while on the other hand, the critical stress observed in blunt-notched specimens, which has been associated with the craze initiation stress, is not affected by ageing.

A model for the initiation of hydrogen embrittlement cracks at blunt notches during elastic fatigue loading

A model describing the initiation of hydrogen embrittlement cracks during square wave low cycle fatigue loading of blunt-notched specimens of high strength steels is presented. A quantitative analysis of the number of cycles required to initiate a crack is developed which assumes the rate-determining step to be the slow diffusion of hydrogen to the region of high triaxial stress ahead of the notch. The predictions of the analysis are in good qualitative agreement with the limited available experimental data.

The mechanisms of fracture in blunt-notched specimens of a low alloy steel and the effect on failure assessment

Blunt-notched three-point bend bars (55 mm × 10 mm × 10 mm) of a low alloy steel with root radii ϱ in the range 0.51 – 4.07 mm have been tested to failure. As ϱ increased, the mode of failure changed from stress-controlled cleavage to strain-controlled microvoid coalescence. For specimens with ϱ ⩽ 1.02 mm the elastic outer fibre stress σ f at failure increased with ϱ, consistent with stress-controlled mechanisms. For ϱ > 1.02 mm the stress-controlled fracture requirements could not be met and failure became strain controlled. The effect of ϱ on fracture behaviour has been analysed in terms of the dependence of σ f , the plane strain fracture toughness K 1 C and the apparent fracture toughness K A ( ϱ) on 1/K t where K t is the elastic stress concentration factor at the root of the notch. It is concluded that K 1 C may in some circumstances be the failure parameter controlling the initiation of fast fracture from blunt notches. The incorporation of root radius effects in a failure assessment procedure is described, and predicted failure stresses are shown to be in good agreement with observed values.

The Relationship Between Fracture Toughness and Stress Concentration Factors for Several High-Strength Aluminum Alloys

For the case of an elliptical notch in an infinite solid, a relationship between the stress concentration factor and the fracture toughness parameter was examined. Edge-notched specimens from three high-strength aluminum alloys were tensile loaded to failure. The resulting data were analyzed in the light of this relationship. It was indicated that a predicted proportionality between the fracture toughness parameter and the square root of the notch root radius exists. Further examination of the relationship based upon the proportionality showed that fracture occurs at a fixed state of strain within a plastic zone having a size proportional to the original root radius. However, a departure from the predicted behavior was evident with the introduction of plane strain components at the notch root. It was also demonstrated that the use of specimens with intermediate root radii for either the evaluation of a single material or as a basis of comparison between materials can lead to invalid conclusions. The reversion of fracture toughness data from blunt notch specimens to stress concentration factors is shown for one alloy. Due to a constancy in the ratio of the fracture parameter to the nominal stress, the resulting factor lacks sensitivity.