Notch bluntness effects on fracture toughness of a modified S690 steel at 150 °C (original) (raw)
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Fracture toughness characterization through notched small punch test specimens
Materials Science and Engineering: A, 2016
In this work a novel methodology for fracture toughness characterization by means of the small punch test (SPT) is presented. Notched specimens are employed and fracture resistance is assessed through a critical value of the notch mouth displacement δ SPT. Finite element simulations and interrupted experiments are used to track the evolution of δ SPT as a function of the punch displacement. The onset of crack propagation is identified by means of a ductile damage model and the outcome is compared to the crack tip opening displacement estimated from conventional tests at crack initiation. The proposed numerical–experimental scheme is examined with two different grades of CrMoV steel and the differences in material toughness captured. Limitations and uncertainties arising from the different damage phenomena observed in the lowest toughness material examined are thoroughly discussed.
Notch Fracture Mechanics Approaches in an Analysis of Notch-like Defects
Ecf19, 2013
The concept of notch fracture mechanics has been developed for describing the notch failure assessment diagram as well as the J-integral for U-and V-notches under Mode I loading and materials obeying a power hardening law. Effects of constraint were incorporated into the basic equations which describe the constraint-dependent fracture toughness and failure assessment diagrams for various structural elements with a crack/notch and various types of loading. It is shown that a crack can be considered as a special case of a notch. The load separation method has been employed to measure the notch fracture toughness c J , using non-standard specimens with notches. Structural integrity assessment of the components damaged by notch-like defects is discussed from viewpoint of the notch failure assessment diagram. Acceptable state of the damaged component with a notch-like defect is determined by introducing safety factors against fracture and plastic collapse in the fracture criterion describing the notch failure assessment diagram.
Influence of notch parameters on fracture behavior of notched component
2014
In the present study, the influence of variation of notch parameters on the notch stress intensity factor KI is studied using CT- specimen made from steel construction. A semi-elliptical notch has been modeled and investigated and is applied to finite elements model. The specimen is subjected to a uniform uniaxial tensile loading at its two ends under perfect elastic-plastic behavior. The volumetric method and the Irwin models are compared using a finite elements method for determined the effective distance, effective stress and relative gradient stress which represent the elements fundamentals of volumetric method. Changing made to notch parameters affect the results of stress intensity factor and the outcomes obtained shows that the increase in size of minor axis reduces the amplitude of elastic-plastic stresses and effective stresses. In lengthy notches, the Irwin model remains constant with very little disturbance of outcomes.
ISRN Materials Science, 2012
This paper applies a methodology that allows the fracture toughness of a given material to be estimated by testing notched fracture toughness specimens and applying the Theory of Critical Distances, which requires the elastic stress field at the notch tip to be determined by finite elements simulation. This methodology, which is not intended to substitute any standardised fracture characterisation procedure, constitutes an alternative in those situations where pre-cracking processes may be too time-consuming, too expensive or, simply, cannot be performed. It comprises testing two notched specimens with different notch radii, defining the corresponding stress fields at fracture by using finite elements analysis, and applying the Theory of Critical Distances in order to calibrate the material’s critical distance and to apply the corresponding apparent fracture toughness formulation. The methodology has been applied to two different materials, PMMA and Al7075-T651, and the results have...
NATO Science for Peace and Security Series C: Environmental Security, 2011
In the present research, T-stress solutions are provided for a U-shaped notch in the case of four specimens: CT, DCB, SENT and Romain Tile (RT). The U-shaped notch is analyzed using the finite element method to determine the stress distribution ahead of the notch tip. In contrast to a crack, it was found that the T-stress is not constant and depends on distance from the notch-tip. To estimate the T-stress in the case of a notch, a novel method, namely, method of line, inspired from the volumetric method approach proposed by Pluvinage has been developed. Thus, the two-parameter approach was adopted for the notch two-parameter fracture mechanics in terms of the notch stress intensity factor Krc and the effective (average) T-stress, Tef. Fracture toughness transferability curve (Krc -Tef) of X52 pipe steels has been established. of theories which extend significantly the range of deformation over which fracture can be applied accurately to predict the load capability of a given structure. In this way, recent numerical and experimental studies have attempted to describe fracture in terms of two or three fracture parameters [1]. One of candidate parameters is the elastic T-term stress . The T-stress is not singular as r ! 0, but is can alter the elastic crack-tip stress state described by the stress intensity factor K I . The T-stress is a function of geometry, loading conditions, and is proportional to the nominal applied stress . Sufficient information about the stress state is available, if the Stress Intensity Factor, SIF and the constant stress term, the T-stress, are know. While SIF solutions are reported in handbooks for many crack geometries and loading cases, T-stress solutions are available only for a small number of test specimens and simple loading cases for instance pure tension and bending.
The effects of testpiece thickness (B) on fibrous fracture and fatigue-crack propagation have been studied using singleedge-notched testpieces of two low-strength steels. Most tests were carried out in four-point bending, but some fatigue tests were carried out in tension or three-point bending. The results for fibrous fracture show that a higher crack-tip displacement (COD) is necessary for crack initiation in a thin testpiece than in a thick. It is shown by using micro hardness measurements that the change in critical displacement is consistent with the difference in the strain distributions about crack tips found in thick and thin testpieces. An analogous effect was observed in fatigue-crack propagation in bending, where lower growth rates were found in thin specimens. The magnitude of the alternating COD should be greater in a thin specimen for a given level of alternating stress intensity, flK, and the lowe:r growth rate is again attributed to the change in crack-tip strain distribution with thickness. Fatigue-crack propagation rates in tension were found to be higher than in bending for thick specimens, and this is explained in terms of the errors arising in the stress-intensity factor for different geometries, when there is a small amount of crack-tip plasticity. In tension it was found also that fatigue cracks grow faster in thin testpieces, and this is considered to occur because this loading geometry allows out-of-plane sliding and an antiplane (KIll) mode of fracture to operate. This was confirmed by fractography.
Transferability of Cleavage Fracture Parameters Between Notched and Cracked Geometries
Fracture of Nano and Engineering Materials and Structures
The present study investigates the temperature and specimen geometry dependence of cleavage fracture micromechanisms of a French pressure vessel steel (A508 Cl.3) over a temperature range which covers the lower shelf up to the DBT fracture toughness range. Notched tensile (NT) specimens with 3 different radii and CT specimens with two different thicknesses (12.5 and 25mm) were used to study the effect of temperature and geometry (constraint) on (i) the nature of the defects involved in the cleavage triggering, (ii) the mechanical parameters (stress and strain) values (iii) the Weibull parameters. The fractographic investigations showed that Manganese sulfide clusters and/or stringers were involved in the cleavage initiation process on the notched geometry at all temperatures from ¢ 130 £ C whereas they were never encountered in the cracked geometries. It is shown that only NT tests with a mean fracture strain lower than 25% have to be considered to make sure that the same nature of defects will be involved in the cleavage initiation. The mean value of the local stress was found to be approximately constant for NT and CT25 specimens over the temperature range [¢ 150 £ C ; ¢ 50 £ C ]. Also a unique set of Weibull parameters was found to describe all the NT tests over this temperature range.
Two-parameter fracture criterion (K ρ,c -T ef,c ) based on notch fracture mechanics
International Journal of Fracture, 2011
A two-parameter fracture criterion has been proposed to predict fracture conditions of notched components. This criterion includes the critical notch stress intensity factor K ρ,c , which represents fracture toughness of a material with a notch of radius ρ, and the effective T -stress. The effective T -stress T e f has been estimated as the average value of the T -stress distribution in the region ahead of the notch tip at the effective distance X e f . These parameters were derived from the volumetric method of notch fracture mechanics. The results of numerical T e f,c -stress estimation are compared to the T e f,c -stress results obtained from experimental analysis. The material failure curve or master curve K ρ,c = f T e f,c has been established as a result of the notched specimen tests. A large T e f,c range was covered from −0.80 σ Y to +0.19 σ Y using SENT, CT, RT (roman tile) and DCB specimens. It was shown that the notch fracture toughness is a linear decreasing func-tion of the T e f,c -stress. The use of the material failure curve to predict fracture conditions was demonstrated on gas pipes with the surface notch.
New Method of Determine Fracture Toughness Characteristics of Small-Size Chevron-Notched Specimens
A new method is proposed to determine fracture toughness of structural materials according to the test data of non-standard small-size chevron-notched specimens. The analytical expressions are obtained being based and derived from the constitutive equations of engineering fracture mechanics to determine the crack-driving force (specific fracture energy) and the stress intensity factor. There are no empirical constants and phenomenological dependencies in the calculations. The method allows us to exclude the periodic unloading of the specimen applied. All necessary calculation parameters are determined according to the experimental data. The method allows us to use the low-power test machines and does not require large amounts of material for the production of specimens, as well as fatigue pre-cracking. The method allows us to certify fracture toughness of the material without restrictions regarding the amount of plastic deformation and in front of the crack tip and in the specimen as a whole. The examples are given to calculate the fracture toughness criteria for a number of structural materials characterized by the ability to plastic deformation and by the Young's modulus value. The compliance of the calculated fracture toughness characteristics with test data of standard methods is observed.
Local approaches for the fracture assessment of notched components the research.PDF
Brittle failure of components weakened by cracks or sharp and blunt V-notches is a topic of active and continuous research. It is attractive for all researchers who face the problem of fracture of materials under different loading conditions and deals with a large number of applications in different engineering fields, not only with the mechanical one. This topic is significant in all the cases where intrinsic defects of the material or geometrical discontinuities give rise to localized stress concentration which, in brittle materials, may generate a crack leading to catastrophic failure or to a shortening of the assessed structural life. Whereas cracks are viewed as unpleasant entities in most engineering materials, U-and V-notches of different acuities are sometimes deliberately introduced in design and manufacturing of structural components. Dealing with brittle failure of notched components and summarising some recent experimental results reported in the literature, the main aim of the present contribution is to present a review of the research work developed by Professor Paolo Lazzarin. The approach based on the volume strain energy density (SED), which has been recently applied to assess the brittle failure of a large number of materials. The main features of the SED approach are outlined in the paper and its peculiarities and advantages accurately underlined. Some examples of applications are reported, as well. The present contribution is based on the author's experience over about 15 years and the contents of his personal library. This work is in honor and memory of Prof. Paolo Lazzarin who suddenly passed away in September 2014.