A Novel Methodology for Calculating Crack Opening Stress under Tension-Compression Cyclic Load

Crack closure model has been used in several applications on the prediction of fatigue crack growth life, with expression of crack opening stress often serving as milestones. A typical difficulty in calculating the crack opening stress is the phenomenon of crack closure caused by the compressive load effect. Compressive load effect, resulting in the change of residual stress status at the unloading stage and the decrease of crack opening stress, is a long-term challenge for predicting fatigue crack growth life. We propose the expression of crack opening stress to predict fatigue crack growth life based on the analysis of compact tensile specimen with elastoplastic element method. It combines the characteristics of material and load to deal with the phenomenon of crack closure and uses stress ratio and normalized maximum applied load variable to construct the expression of crack opening stress. In the study of tensile-compression fatigue crack growth experiments, the proposed expression is proved to improve, by comparative analysis, the predictive ability on the whole range of experiment data. The novel expression is accurate and simple. Consequently, it is conducive to calculate the crack opening stress under tension-compression load.

Application of the Modified Ritchie–Knott–Rice Criterion to Predict Fracture of Miniaturized Charpy Type SE(B) Specimens From Full Sized Specimens

This paper introduces our experience of using miniaturized Charpy type SE(B) specimen in obtaining fracture toughness Jc of a material in the ductile to brittle transition temperature (DBTT) region. 0.55% carbon steel JIS S55C, whose tensile to yield stress ratio σTS/σYS was equal to 1.8 was chosen as a material to simulate a degraded (embrittled) material in the DBTT region. Focus was placed on whether the modified Ritchie–Knott–Rice (RKR) failure criterion which predicts the onset of cleavage fracture when the crack opening stress measured at 4 times the crack-tip opening displacement σ22d exceeds this σ22c can be applied to miniaturized specimen that does not meet the ASTM E1921 requirement of M = (W-a)σYS/Jc ≥ 30. In concrete, the points 1) whether it could predict whether cleavage fracture will occur or not and 2) whether the σ22c in the case cleavage occurs is identical with that of full sized specimens, were focused. Charpy type SE(B) specimens of Width W × thickness B of 2 × 2 mm, 3 × 3 mm and 10 × 10 mm, whose M were predicted to be smaller than 30, were chosen as miniaturized specimens and 25 × 25 mm were chosen as full sized specimen. Results showed that the modified RKR criterion could appropriately predict cleavage fracture naturally for 25 × 25 mm specimens and 10 × 10 mm specimens, though M < 30 for this type specimen. The modified RKR criterion could also predict that cleavage fracture will not occur for 2 × 2 mm specimens. σ22c obtained from 25 × 25 and 10 × 10 mm specimens showed small difference, indicating that Jc obtained from 10 × 10 mm specimens can be used to transfer Jc obtained by 25 × 25 mm specimens.

Applicability of a Deterministic Approach to Transfer the Minimum Fracture Toughness Between Different Temperatures in the DBTT Region

In this paper, we demonstrate that a deterministic approach requiring only tensile test data for different temperatures has a possibility to predict the minimum fracture toughness for these temperatures. The material is assumed to be in the Ductile-to-Brittle-Transition Temperature (DBTT) region. The approach was based on one of the authors’ finding that the critical stress σ22c of the modified Ritchie-Knott-Rice criterion is correlated with the minimum fracture toughness and shows very small scatter and is specimen configuration independent. The criterion predicts onset of cleavage fracture of a material in the DBTT transition temperature region, when the crack-opening stress σ22 measured at a distance from the crack-tip equal to four times the crack-tip opening displacement δt exceeds a critical value σ22c. The proposed approach is expected to overcome some inconveniences which recent studies have reported to the Master Curve parameters vary with size and temperature.

Applicability of the Modified Ritchie-Knott-Rice Failure Criterion to Examine the Feasibility of Miniaturized Charpy Type SE(B) Specimens

This paper examined whether the modified Ritchie-Knott-Rice (RKR) failure criterion can be applied to examine the feasibility of miniaturized Charpy type SE(B) specimens of thickness-to-width ratio B/W=1. The modified RKR failure criterion considered in this paper is the (4δt,σ22c) criterion which predicts the onset of cleavage fracture when the midplane crack-opening stress measured at a distance equal to four times the crack-tip opening displacement, denoted as σ22d, exceeds a critical stress σ22c. Specimens with B values of 25, 10, 3, and 2 mm (denoted as 25t, 10t, 3t, and 2t specimens, resp.) manufactured with 0.55% carbon steel were tested at 20°C. The results showed that the modified RKR criterion could appropriately predict the occurrence of cleavage fracture accompanied by negligibly small stable crack extension (denoted as KJc fracture) naturally for the 25t and 10t specimens. The modified RKR criterion could also predict that KJc fracture does not occur for the 2t specimen. The σ22c obtained from specimens for the 25t and 10t specimens exhibited only a small difference, indicating that the Jc obtained from the 10t specimens can be used to predict the Jc that will be obtained with the 25t specimens.