Numerical Modeling of Ductile Tearing Effects on Cleavage Fracture Toughness

This work examines the fracture behavior of ferritic steel welds in the transition temperature regime, where failure can occur either by ductile tearing or cleavage fracture. A computational and probabilistic-based mechanistic approach to cleavage fracture and ductile crack growth is adopted to model the fracture processes. The softening effect of ductile damage close to the crack tip is described by a Gurson-type material model. A statistical approach linked to both the Weibull stress and the initial void volume fraction is employed to determine the probability of cleavage fracture and the coupling between both fracture mechanisms. Finite element results are relied upon to interpret experimental fracture toughness data for the welds and to examine the effects of near crack tip damage and crack growth on the cleavage failure probabilities and cleavage and ductile fracture toughness distributions. The scatter in the weld experimental fracture toughness data is well reproduced by the proposed cleavage and ductile tearing models.

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Cleavage fracture toughness for 3Cr–Ni–Mo–V reactor pressure vessel steel: theoretical prediction and experimental investigation

International Journal of Pressure Vessels and Piping ◽

10.1016/s0308-0161(01)00057-6 ◽

2001 ◽

Vol 78 (6) ◽

pp. 429-441 ◽

Cited By ~ 3

Author(s):

B.Z. Margolin ◽

V.A. Shvetsova ◽

A.G. Gulenko ◽

A.V. Ilyin

Keyword(s):

Fracture Toughness ◽

Experimental Investigation ◽

Theoretical Prediction ◽

Cleavage Fracture ◽

Pressure Vessel ◽

Reactor Pressure Vessel ◽

Pressure Vessel Steel ◽

Vessel Steel ◽

Reactor Pressure Vessel Steel ◽

Reactor Pressure

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Determination of Crack-Initiation in Fracture Toughness Testing Using an Experimental Key-Curve Methodology

10.1115/pvp2021-61812 ◽

2021 ◽

Author(s):

S. Pothana ◽

G. Wilkowski ◽

S. Kalyanam ◽

J. K. Hong ◽

C. J. Sallaberry

Keyword(s):

Fracture Toughness ◽

Crack Initiation ◽

Crack Growth ◽

Power Law ◽

Direct Current ◽

Electric Potential ◽

Crack Mouth Opening Displacement ◽

Ductile Tearing ◽

Curve Method ◽

Unloading Compliance

Abstract A new approach was implemented to confirm the start of ductile tearing relative to assessments by other methods such as direct-current Electric Potential (d-c EP) method in coupon specimens. This approach was developed on the Key-Curve methodology by Ernst/Joyce and is similar to the ASTM E-1820 Load Normalization procedure used to determine J-R curves directly from load versus Load-Line Displacement (LLD) record of the test specimen. It is consistent with Deformation Plasticity relationships for fully plastic behavior. Using this Experimental Key-Curve method, crack initiation can be determined directly from load versus LLD data or load versus Crack-Mouth Opening Displacement (CMOD) obtained from a fracture test without the need for additional instrumentation required for crack initiation detection. It is based on the fact that plastic deformation of homogeneous metals at the crack tip follows a power-law function until the crack tearing initiates. Crack tearing initiation is determined at the point where the power-law fit to the load versus plastic part of CMOD or LLD curve deviates from the total experimental load versus plastic-CMOD or LLD curve. The procedure for fitting of the data requires some care to be exercised such that the fitted data is beyond the elastic region and early small-scale plastic region of the Load-CMOD or Load-LLD curve but include data before crack initiation. An iterative regression analysis was done to achieve this, which is shown in this paper. The iterative fitting in this region typically results with a coefficient of determination (R2) values that are greater than 0.990. This method can be either used in conjunction with other methods such as direct-current Electric Potential (d-c EP) or unloading-compliance methods as a secondary (or primary) confirmation of crack tearing initiation (and even for crack growth); or can be used alone when other methods cannot be used. Furthermore, when using instrumentation methods for determining crack-initiation such as d-c EP method in a fracture toughness test, it is good to have a secondary confirmation of the initiation point in case of instrumentation malfunction or high signal to noise ratio in the measured d-c EP signals. In addition, the Experimental Key-Curve procedure provides relatively smooth data for the fitting procedure, while unloading-compliance data when used to get small crack growth values frequently has significant variability, which is part of the reason that JIC by ASTM E1820 is determined using an offset with some growth past the very start of ductile tearing. In this work, the Experimental Key-Curve method had been successfully used to determine crack tearing initiation and demonstrate the applicability for different fracture toughness specimen geometries such as SEN(T), and C(T) specimens. In all the cases analyzed, the Experimental Key-Curve method gave consistent results that were in good agreement with other crack tearing initiation measuring method such as d-c EP but seemed to result in less scatter.

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A Weibull Stress Approach Incorporating the Coupling Effect of Constraint and Plastic Strain in Cleavage Fracture Toughness Predictions

Volume 6B: Materials and Fabrication ◽

10.1115/pvp2014-28062 ◽

2014 ◽

Cited By ~ 6

Author(s):

Claudio Ruggieri ◽

Robert H. Dodds

Keyword(s):

Fracture Toughness ◽

Plastic Strain ◽

Cleavage Fracture ◽

Coupling Effect ◽

Crack Size ◽

Parameter Analysis ◽

Pressure Vessel Steel ◽

Fracture Parameter ◽

Vessel Steel ◽

Weibull Stress

This work describes a micromechanics methodology based upon a local failure criterion incorporating the strong effects of plastic strain on cleavage fracture coupled with statistics of microcracks. A central objective is to gain some understanding on the role of plastic strain on cleavage fracture by means of a probabilistic fracture parameter and how it contributes to the cleavage failure probability. A parameter analysis is conducted to assess the general effects of plastic strain on fracture toughness correlations for conventional SE(B) specimens with varying crack size over specimen width ratios. Another objetive is to evaluate the effectiveness of the modified Weibull stress (σ̃w) model to correct effects of constraint loss in PCVN specimens which serve to determine the indexing temperature, T0, based on the Master Curve methodology. Fracture toughness testing conducted on an A285 Grade C pressure vessel steel provides the cleavage fracture resistance (Jc) data needed to estimate T0. Very detailed non-linear finite element analyses for 3-D models of plane-sided SE(B) and PCVN specimens provide the evolution of near-tip stress field with increased macroscopic load (in terms of the J-integral) to define the relationship between σ̃w and J. For the tested material, the Weibull stress methodology yields estimates for the reference temperature, T0, from small fracture specimens which are in good agreement with the corresponding estimates derived from testing of much larger crack configurations.

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