Onset Of Stable Crack Growth Research Articles

Size-Induced Constraint Effects on Crack Initiation and Propagation Parameters in Ductile Polymers.

Fracture mechanics are of high interest for the engineering design and structural integrity assessment of polymeric materials; however, regarding highly ductile polymers, many open questions still remain in terms of fully understanding deformation and fracture behaviors. For example, the influence of the constraint and specimen size on the fracture behavior of polymeric materials is still not clear. In this study, a polymeric material with an elastic plastic deformation behavior (ABS, acrylonitrile butadiene styrene) is investigated with regard to the influence of constraint and specimen size. Different single-edge notched bending (SENB) specimen sizes with constant geometrical ratios were tested. The material key curve was used to investigate differences in the constraint, where changes for small and large specimen sizes were found. Based on a size-independent crack resistance curve (J–R curve), two apparent initiation parameters (J0.2 and Jbl) were determined, namely, the initiation parameter Jini (based on the crack propagation kinetics curve) and the initiation parameter JI,lim (based on an ESIS TC 4 draft protocol). It was found that J0.2 and Jbl could be used as crack initiation parameters whereby Jini and JI,lim are indicative of the onset of stable crack growth.

Damage tolerant analysis of corner cracks in small nozzle process penetration of the CAREM-25 reactor pressure vessel

This paper presents a damage tolerant approach assessment of corner cracks in small nozzle process penetrations in the pressure vessel of the small modular reactor CAREM-25 subjected to mechanical and thermal shock loadings during certain operation events. Fracture mechanics analyses were performed following ASME XI guidelines considering a representative geometry and specific loading conditions. Stress distributions in the uncracked component were calculated by finite element techniques, while fracture driving forces were estimated using closed form solutions. Maximum allowable corner crack sizes were calculated using linear elastic fracture mechanics and failure assessment diagram (FAD) approaches. The onset of stable crack growth and crack instability were considered as failure conditions. Additionally, fatigue crack growth estimations during the service life were performed integrating a Paris law up to the maximum allowable crack sizes. The results indicate that it seems unlikely that pre-existing cracks reach allowable sizes during the reactor's service life.

Fracture toughness evaluation of NiAl single crystals by microcantilevers—a new continuous J-integral method

Abstract

Prediction of crack initiation and propagation in X70 pipeline steels

The present work deals with the concept of post-initiation softening applied to ductile crack initiation and propagation problems in thick structural components such as pre-cracked pipes and fracture specimens. The methodology includes a set of calibration steps to determine model parameters of crack initiation and propagation under monotonic quasi static loading condition. The phenomenological Modified Mohr-Coulomb (MMC) fracture criterion is used for fracture initiation along with post-initiation softening for crack extension. A comprehensive experimental/numerical program provides access to the load histories of each specimen geometry at the critical point, where fracture is assumed to occur. This first step enables the construction of the limits of ductility of the material in terms of a weighted von Mises equivalent plastic strain, which strongly dependent on stress triaxiality and Lode angle evolution. Post-initiation softening parameters are then calibrated from tearing analysis built upon resistance curves of single edge notch tension (SENT) specimens. The calibrated parameters model along with detailed FE analyses shows that the present fracture criterion accurately predicts the onset of stable crack growth and propagation along the uncracked ligament of low constraint specimens.

Crack growth resistance curves of GLARE 3 5/4 0.3 fiber–metal laminates at low temperature

ABSTRACTThe objective of this work was the evaluation of crack growth resistance curves ( J–R curves) of commercial GLARE® 3 5/4 laminates at −50 °C. The experimental evaluation of these curves was performed on 50‐mm‐wide compact tension specimens through the unloading compliance technique. The tests were based on the ASTM E1820 standard with minor modifications. Additionally, tensile tests were also performed at −50 °C on dogbone specimens according to the ASTM E8M standard. Comparisons between low and room temperature properties indicate that the material preserves both its tensile strength and fracture toughness at −50 °C, although low temperature J–R curves presented smaller slopes than the room temperature ones after the onset of stable crack growth.

Influence of material ductility and crack surface roughness on fracture instability

This paper presents a stability analysis for fractal cracks. First, the Westergaard stress functions are proposed for semi-infinite and finite smooth cracks embedded in the stress fields associated with the corresponding self-affine fractal cracks. These new stress functions satisfy all the required boundary conditions and according to Wnuk and Yavari's (2003 Eng. Fract. Mech. 70 1659–74) embedded crack model they are used to derive the stress and displacement fields generated around a fractal crack. These results are then used in conjunction with the final stretch criterion to study the quasi-static stable crack extension, which in ductile materials precedes the global failure. The material resistance curves are determined by solving certain nonlinear differential equations and then employed in predicting the stress levels at the onset of stable crack growth and at the critical point, where a transition to the catastrophic failure occurs. It is shown that the incorporation of the fractal geometry into the crack model, i.e. accounting for the roughness of the crack surfaces, results in (1) higher threshold levels of the material resistance to crack propagation and (2) higher levels of the critical stresses associated with the onset of catastrophic fracture. While the process of quasi-static stable crack growth (SCG) is viewed as a sequence of local instability states, the terminal instability attained at the end of this process is identified with the global instability. The phenomenon of SCG can be used as an early warning sign in fracture detection and prevention.

Fracture toughness and tensile properties of nano-structured ferritic steel 12YWT

The oxide dispersion strengthened (ODS) steels are being developed and investigated for fission and fusion structural applications in Japan, Europe, and the United States. In this paper, the fracture toughness and tensile properties of an ODS steel with nominal composition Fe–12Cr–2.5W–0.4Ti–0.25Y 2O 3 (designated 12YWT) were investigated and compared to commercial reduced-activation ferritic/martensitic (RAFM) steels. Small, 1.6-mm thick and 3.2-mm wide, 3-point bend specimens were used for fracture toughness characterization of this steel. Specimens were fatigue pre-cracked to initial crack length ( a) to width ( W) ratio of 0.45 and tested quasi-statically in the temperature range from −50 °C to 550 °C. Specimens tested up to 50 °C exhibited elastic–plastic cleavage fracture that was typical for the transition region in ferritic steels. The master curve transition temperature, T 0, for the 12YWT alloy was determined to be 102 °C. Specimens tested at 100 °C and higher exhibited ductile stable crack growth. In these cases, the J-integral at the onset of stable crack growth ( J Ic) was determined from the J– R curves. Their equivalent values in terms of stress intensity factor, K J Ic , were about 93 MPa√m at 100 °C and decreased to 53 MPa√m at 550 °C. This study showed that oxide dispersion strengthening resulted in significant decreases in the toughness properties compared to commercial RAFM steels, although appreciable level of toughness was still retained. Tensile tests were performed at temperatures between room temperature and 800 °C. As expected, this material exhibited very high yield strength, ∼1300 MPa, at room temperature. For comparison, the yield strength of commercial RAFM steels is about 550 MPa. Yield strength of 12YWT decreases as test temperature increases and at 800 °C it is about 323 MPa.

A threshold stress intensity factor at the onset of stable crack extension of Knoop indentation cracks

Observation of the stable growth of indentation cracks in controlled bending tests is an attractive tool to study the R-curve behaviour of ceramic materials. In some crack growth experiments deviations from the ideal behaviour were observed––i.e. the cracks did not grow immediately upon application of the external load, but only after a certain applied stress is exceeded. The existence of such a threshold stress intensity for the onset of stable crack growth can be described by a disbalance of the residual stress intensity at the indentation crack tip and the fracture toughness after completion of the indentation cycle. A fracture mechanical description of this phenomenon is presented. The consequences which arise for the evaluation of crack growth data are explained by means of measured crack extension curves of Knoop indentation cracks in silicon nitride.

A threshold stress intensity factor at the onset of stable crack extension of Knoop indentation cracks

Observation of the stable growth of indentation cracks in controlled bending tests is an attractive tool to study the R-curve behaviour of ceramic materials. In some crack growth experiments deviations from the ideal behaviour were observed––i.e. the cracks did not grow immediately upon application of the external load, but only after a certain applied stress is exceeded. The existence of such a threshold stress intensity for the onset of stable crack growth can be described by a disbalance of the residual stress intensity at the indentation crack tip and the fracture toughness after completion of the indentation cycle. A fracture mechanical description of this phenomenon is presented. The consequences which arise for the evaluation of crack growth data are explained by means of measured crack extension curves of Knoop indentation cracks in silicon nitride.

Stable growth of penny-shaped crack in viscoelastic composite material under time-dependent loading

Considered is the long-term cracking of the three-dimensional fiber-reinforced viscoelastic composite with a plane penny-shaped crack under time-dependent loading. The composite has a hexagonal structure and consists of elastic isotropic fibers and viscoelastic isotropic matrix. The material is modeled by transversally isotropic homogeneous linearly viscoelastic medium with some averaged characteristics. The crack propagation planecoincides with the plane of isotropy. A ring-shaped yield zone in front of the moving crack is modeled as a Dugdale's zone with time-dependent stresses. Crack growth under deformation of the composite occurs by application of a slowly increasing tensile load; it is normal to the plane of crack propagation. A convolution-type time operator describes the viscoelastic properties of the matrix material. Use is made of the Volterra principle and the theory of long-term cracking of viscoelastic bodies. The irrational function of integral operator associated with the viscoelastic crack opening expression is expanded into a continued fraction of operators. The solution is reduced to the nonlinear integral equations of crack growth. Numerical results are obtained for a specific material. Crack growth kinetics is discussed in connection with the onset of stable crack growth and crack border stress intensity factor.

Examination of the onset of stable crack growth under fracture toughness testing of paper

Successive temperature distribution images around the notch tips during fracture toughness testing of paper were obtained by means of an infrared thermography system. Analysis of the images gave the critical times when the temperature significantly rose at the notch tip and when the distance between two maximum temperature spots started to decrease during the testing. Other successive microscopic images around the notch tips showed the relation between crack opening and displacement and the transitional point of the relation. The onset of stable or unstable crack growth as indicated in these critical times and the point agree with each other. For the specimen with a small width, an unstable crack starts to grow at the maximum load point without the stable crack growth period. On the other hand, a stable crack grows before the maximum load point unless the specimen width is small.The period of the stable crack growth increases with an increase in width. Differing from the methods based on thermal images to determine the onset of crack growth, the microscopic method is applicable at a wide range of strain rates and is thus suitable for quasistatic fracture toughness testing.

J-integral Jc and Crack Tip Opening Displacement CTODc as a Material Property Evaluating the Fracture Toughness of Paper

Determination of the onset of stable crack growth is very significant for evaluating the fracture toughness of paper such as J-integral and crack tip opening displacement (CTOD). For the specimen with small width, an unstable crack growth starts at the maximum load point without a stable crack growth and further no stress concentation is observed at the crack tip. Therefore, the concept of J-integral and CTOD are not applicable to the specimen with small width. J-integral at the onset of stable crack growth (Jc) was examined on the double edge notched specimens of machine-glazed paper, comparing with J-integral at the onset of unstable crack growth i.e. at the maximum load point (Jmax). Jmax depends on the specimen width and is considerably larger than Jc. On the other hand, Jc is nearly constant independent of the specimen width and thus is regarded as a material property for evaluating fracture toughness of paper as long as a stable crack grows before the maximum load point. The specimen geometry used in this study fulfills the condition required for the RPM (single specimen) method of J-integral determination. CTOD at the onset of stable crack growth (CTODc) is also essentially constant independent of the specimen width and thus is regarded as a material property for evaluating fracture toughness of paper unless the width is small.

Mixed mode fracture of an HSLA-100 steel

Traditionally, mode I instability criteria have marked the conservative fracture toughness limit. However, some high-toughness, particle strengthened materials have been shown to have a lower resistance to the onset of stable crack growth under mixed-mode loading. By testing both standard compact tension specimens and modified mixed-mode compact tension specimens, the authors have determined the pure mode I plain strain fracture toughness and mixed-mode I/III fracture toughness for an HSLA 100 steel. Limited mixed-mode I/II tests of the type described by Kamat were also performed. The steel was provided by E. Czyryca of the Naval Research Laboratory and tested in the as received condition. Czyryca found {sigma}{sub ys} = 780 MPa, {sigma}{sub uts} = 800 MPa, and a two specimen average J{sub Ic} = 490 kJ/m{sup 2} for specimens tested in the T-L orientation. The steel was austenitized at 900 C for 40 minutes, water quenched then aged at 665 C for one hour than air cooled. The hardenability provided by manganese, molybdenum and nickel resulted in a mixed ferrite-bainite microstructure with fine carbonitride and copper precipitates. Such a microstructure gives high strength and toughness, improved weldability, lower DBTT as well as lower cost.

Crack propagation under mixed mode (I+III) loading : Yates, J.R. and Mohammed, R.A. Prof. 4th Int. Conf. on Biaxial/Multiaxial Fatigue II, St. Germain en Laye, France, 31 May–3 June 1994, pp. 99–106

The blunt notch fracture toughness of four types of carbon-manganese steel (ASTM A516 grade 70) has been determined by J-integral tests on Charpy-V type samples with different values of notch root radius, ρ. J-ρ plots, determined using specimens with a notch depth to width ratio, a/w, equal to 0.5, have shown the existence of a limiting ρ value (ρeff) below which applied J-intergral values at fracture initiation are constant. These ρeff values have been seen to depend only on second-phase particle distribution and not on their volume fraction or on the steel ferritic grain size. The procedure for deriving J-integral values at the onset of stable crack growth from J resistance curves in the case of notches has also been discussed. Experiments with Charpy specimens with a/w = 0.2 do not allow the derivation of meaningful J-ρ plots. In all cases, a ductile fracture criterion based on the constancy of the notch tip strain at rupture initiation has been proved when ρ >ρeff.

Applicability of ultrasonic technique for evaluation of elastic plastic fracture toughness of high manganese steel at low temperatures

A study on the evaluation of elastic-plastic fracture toughness, JIC, by ultrasonic technique is described for a newly developed high manganese steel at low temperature. In order to see the applicability of the ultrasonic technique based on pulse echo method at low temperature, special attention was paid to detect change point of ultrasonic echo due to the onset of stable crack growth. The JIC values are evaluated by the ultrasonic method (which needs single specimen) and by stretched zone method (which needs several specimens) for compact tension (CT) specimens and three-point bend (3PB) specimens. The temperature dependence of these JIC values of theCT specimens by the ultrasonic method show almost the same values to 3PB specimens over the temperature range tested. The JIC values of 3PB specimens by the stretched zone method show slightly higher values than those of theCT specimens at low temperature. The JIC values evaluated by the ultrasonic method give more conservative values than those evaluated by the stretched zone method for bothCT and 3PB specimens.

THE RELATION BETWEEN STRESS INTENSITY FACTOR AND ENERGY RELEASE RATE IN THE PRESENCE OF R‐CURVE BEHAVIOUR

Abstract— Failure of ceramic materials occurs when the stress intensity factor of the most serious crack in a component reaches a critical value KI,C, the fracture toughness of the material. In case of ideal brittle materials the fracture toughness is independent of the crack extension and, consequently, identical with the stress intensity factor KI,Onecessary for the onset of stable crack growth. It is a well‐known fact that failure of several ceramics is influenced by an increasing crack‐growth resistance curve. The effect of increasing crack resistance has consequences on many properties of ceramic materials. In this report the authors discuss some aspects of R‐curve behaviour as represented by stress intensity factors or energies.

CED-based fracture resistance evaluation of stably growing crack by generation phase simulation.

It was shown previously that the CED (crack energy density) or its additional rate evaluated through the experiments of stable crack growth can be a parameter which expresses the fracture resistance, peculiar to fracture mode, of a material from the onset of stable crack growth to the rapid unstable fracture consistently. In this paper, simulation methods for stable crack growth by finite element analyses of the discontinuous model are studied, and the availability of CED above is confirmed through generation phase crack growth simulations corresponding to the experiments. The variation of the J-integral is also evaluated in simulations, and it is shown that the CED is more promising and reasonable than any J-based parameter to express the fracture resistance of a crack.

Investigation of DynamicJId for Alloy Steel Weldments Using the Split Hopkinson Bar

AbstractThe fracture toughness parameter JId, under dynamic loading conditions, was determined for weldments of HY80 and QT28 alloy steels using a split Hopkinson bar technique. JId was obtained for the fusion zone, the heat-affected zone, and the base metal for these steels as a function of temperature in the range of −80 to 25°C and at loading rates of ˙K = 106 MPa√m s−1. Wedge loaded compact tension specimens were loaded in a split Hopkinson bar. The onset of stable crack growth was determined by using a strain gage that was attached ahead of a fatigue crack. The results show that, at −80°C, fracture with limited stable crack growth takes place, while for all other testing conditions considerable ductility precedes fracture of these alloys.

Investigations into the fatigue crack initiation and propagation behaviour in austenitic stainless steel X5 CrNi 18 9 (1.4301)

AbstractControlled load fatigue‐crack growth rate tests were conducted using three point bend type specimens. The austenitic stainless steel X5 CrNi 18 9 (material number 1.4301, similar to AISI 304) was tested. For this high toughness material, the crack initiation and crack propagation is discussed in terms of linear‐elastic analysis even for the highest plasticity regime. The curve for the onset of stable crack growth during fatigue starting from a notch is discussed as well as the stable crack growth up to large plasticity. From the standpoint of load controlled fatigue it is indicated that the time between crack initiation and crack length at the „end of life”︁ is large.

Material characterization by J-R curves of a 20MnMoNi55 forging

J-R curves have been determined for one specific forging of the reactor pressure vessel steel 20MnMoNi55 to characterize its fracture resistance in the upper shelf toughness regime. The multiple specimen unloading (MSU), a direct current potential drop (DCPD), and the single specimen partial unloading compliance (SSPUC) methods have been applied to test different CT-specimen geometries at temperatures between 25 and 300°C. The J–R curves are temperature dependent showing a minimum of slope and absolute values at 200°C. The critical J-values for onset of stable crack growth in the scatter of the results are independent of temperature and geometry: J i ≈ 180 kJ/m 2. The values of the technical initiation toughness J IC according to ASTM E813 7 and according to Loss et al. (1979), 8 the tearing modulus T, and the instability parameter J 50 show corresponding temperature trends as the J-R curves with a minimum at about 200°C. The material data have been used to evaluate the ductile failure initiation and tearing instability of several structural tests. The quality of agreement between calculable predictions and experiments is shown to depend on the input material J-R curve and evaluation of the loading which requires detailed knowledge about the material stress-strain behaviour and the relevant crack tip constraint.