J’s Disability Could Explain all the Test Specimen Size Effects Observed for the Fracture Toughness (Jc) of a Material in the DBTT Region

2015 ◽  
Author(s):  
Toshiyuki Meshii
Keyword(s):  
Fracture Toughness ◽  
Size Effect ◽  
Failure Criterion ◽  
Test Specimen ◽  
Crack Tip ◽  
Specimen Size ◽  
Finite Size Effect ◽  
Specimen Thickness ◽  
Deterministic Approach ◽  
Specimen Type

This paper summarized our recent studies on the test specimen size (TSS) effects on Jc of a material in the ductile-to-brittle transition temperature (DBTT) region. The validity of the deterministic approach to transfer the fracture toughness Jc obtained with different thickness specimens is demonstrated in these works. Based on the detailed finite element analysis results, it was found that the crack-tip stresses were different at the identical J in the test specimen thickness (TST) effect on Jc observed with both the non-proportional and proportional specimens. And adjusting loads to make the stress level equivalent showed increment in J that was equivalent to the Jc difference due to TST effect on Jc. This was similar with the past result obtained for the planar size effect on Jc (the difference in Jc due to the planar specimen configuration including crack length difference for the same specimen type or the specimen type difference). Thus, it was concluded that all of the TSS effects on Jc could be explained as due to J’s disability to characterize the crack-tip stress field accurately, or in a more general explanation, due to the finite size effect. In addition, the (4δt, σ22c) failure criterion (Dodds et al., 1991) was verified to transfer Jcs obtained for different specimen thicknesses and planar configurations. The critical value σ22c varied for only a few percent. The fact that these critical values were always reached at the specimen mid-plane and the fact that cleavage always initiated at the specimen mid-plane supported the validity of the deterministic approach. Because the (4δt, σ22c) failure criterion requires only “single” set of test data for Jc transfer and because σ22c shows only a few percent scatter, it seems to have a possibility to replace what Weibull stress is expected to do.

Ductile Crack Growth Resistance and Rotation Behavior of Miniature C(T) Specimen

2020 ◽  
Author(s):  
Tomoki Shinko ◽  
Masato Yamamoto
Keyword(s):  
Fracture Toughness ◽  
Size Effect ◽  
Crack Growth ◽  
Crack Extension ◽  
Crack Tip ◽  
Crack Growth Resistance ◽  
Specimen Size ◽  
Ductile Crack ◽  
Rotation Center ◽  
Ductile Crack Growth

Abstract A utilization of a miniature compact tension (Mini-C(T)) specimen is expected to enable effective use of limited remaining surveillance specimens for the structural integrity assessment of a Reactor Pressure Vessel (RPV). For developing a direct fracture toughness evaluation method using Mini-C(T) specimen in the upper-shelf temperature range as well as ductile-brittle transition temperature range, this study is aimed to experimentally characterize the Mini-C(T) specimen’s size effect on ductile crack growth resistance and interpolate its mechanism. Mini-C(T) specimen and 0.5T-C(T) specimen were prepared from a Japanese RPV steel SQV2A, and the ductile crack growth tests were conducted on them at room temperature. As a result, the crack growth resistance of Mini-C(T) and 0.5T-C(T) specimens are comparable if the crack extension Δa is less than 0.5 mm. On the other hand, if Δa exceeds 0.5 mm, the crack growth resistance of Mini-C(T) specimen becomes lower than that of 0.5T-C(T) specimen. The measurements of stretch zone width and depth support the fact that the fracture toughness for ductile crack initiation of Mini-C(T) specimen is lower than that of 0.5T-C(T) specimen. From the rotational (crack mouth opening) deformation of Mini-C(T) specimen was measured by simultaneously measuring load-line and front face displacements. The distance between the crack tip and the rotation center of Mini-C(T) specimen is smaller than that of 0.5T-C(T) specimen during the test. Furthermore, The plastic zone in front of the crack tip reaches the rotation center up to the crack extension of Δa = 0.3 mm on Mini-C(T) specimen, indicating that the mechanism of the specimen size effect of Mini-C(T) specimen is likely a plastic constraint due to the influence of the rotation center locating near the crack tip. This suggests that the specimen size effect of Mini-C(T) specimen on ductile crack growth resistance is expected to be corrected by considering an effect of the plastic constraint.

Framework to Correlate Test Specimen Thickness Effect on Fracture Toughness With T33-Stress

2011 ◽  
Author(s):  
Toshiyuki Meshii ◽  
Tomohiro Tanaka
Keyword(s):  
Fracture Toughness ◽  
Transition Temperature ◽  
Temperature Region ◽  
Test Specimen ◽  
Crack Tip ◽  
Thickness Effect ◽  
Specimen Thickness ◽  
Out Of Plane ◽  
Elastic Crack ◽  
Crack Tip Constraint

This paper considered the test specimen thickness effect on the fracture toughness of a material Jc, in the transition temperature region, for CT and 3PB specimen. Framework to correlate test specimen thickness effect on fracture toughness with T33-stress, which is the out-of-plane elastic crack tip constraint parameter, was proposed. The results seemed to indicate a possibility of improving the existing methods to correlate the fracture toughness obtained by test specimen with the toughness of actual cracks found in the structure, in use of T33–stress.

The Effect of Specimen Size for the P91 Steel at Elevated and High Temperatures

2017 ◽  
Author(s):  
Ludek Stratil ◽  
Filip Siska ◽  
Hynek Hadraba ◽  
Stanislava Fintova ◽  
Tomas Mrna ◽  
...  
Keyword(s):  
Fracture Toughness ◽  
Size Effect ◽  
Temperature Dependence ◽  
Pressure Vessels ◽  
Specimen Size ◽  
Dynamic Strain ◽  
Dynamic Strain Ageing ◽  
P91 Steel ◽  
Piping Systems ◽  
Curve Determination

The contribution aims to evaluate fracture toughness of the P91 steel in the ductile regime. This steel is broadly used for applications for pressure vessels and piping systems. The J–R curves were obtained using 1T, 0.5T and 0.25T CT specimens at 23 °C and using 0.5T and 0.25T CT specimens up to 600 °C. The energy normalization method for the J–R curve determination according to the ASTM E1820 was used. The resistance to crack propagation shows temperature dependence and the dynamic strain ageing effect with minimum values at 400 °C. Both specimen sizes 0.5T and 0.25T give a similar trend of the temperature dependence of fracture toughness. However, the size effect is observed as fracture toughness decreases with the specimen size. The results obtained are compared with the results of other authors pointing the specimen size effect and the temperature dependence of the steel.

Prediction of Lower Bound Fracture Toughness in the Transition Temperature Region by T33-Stress

2012 ◽  
Author(s):  
Kai Lu ◽  
Toshiyuki Meshii
Keyword(s):  
Fracture Toughness ◽  
Transition Temperature ◽  
Lower Bound ◽  
Temperature Region ◽  
Failure Criterion ◽  
Large Strain ◽  
Specimen Thickness ◽  
Fracture Toughness Test ◽  
Out Of Plane ◽  
Planar Failure

It is well known that the fracture toughness Jc in the ductile-to-brittle transition temperature region depends highly on the specimen thickness (hereafter, TST). The TST effect on Jc, which Wallin [1] described as Jc (∝ KJc2) ∝ B(-1/2) (Jc was calculated from the equations outlined in ASTM E1820 [2], KJc was derived from Jc as KJc = (Jc·E′)1/2; E′ = E/(1−ν2), B: TST), has been reproduced by Anderson et al. [3] based on the weakest link model. However, as Anderson et al. [3] themselves admit, Jc does not decrease indefinitely with B. On the other hand, Meshii et al. [4–6] tried to explain this TST effect on Jc as a mechanical issue. They obtained the same relationship, Jc ∝ B(-1/2) from the fracture toughness test for a non-standard CT and 3PB specimen (non-standard on the point that planar configuration was identical and thickness to width ratio B/W was varied from 0.25 to 0.5) and the stress intensity factor (SIF) corresponding to fracture load Pc denoted as Kc (Kc was calculated from the equations outlined in ASTM E399 [7]), was almost constant for TST. They also reproduced the experimental tendency by large strain FEA under the assumption based on their experimental observation that Kc was independent of TST. In addition, they expressed the TST effect on Jc by correlating Jc with the out-of-plane elastic T-stress T33. We thought that if TST effect on Jc is a mechanical issue, the lower bound Jc for TST could be predicted by FEA under some assumption such as Kc = constant for TST, and the TST corresponding to the lower bound Jc could be predicted by T33. However, before proceeding to this prediction, we thought we have to understand the candidate assumption for prediction more deeply, i.e, understand why Kc was constant for TST. Thus in this work, we attempted to explain the reason why Kc was constant for TST. Our idea was to apply the well-known “planar” failure criterion to our out-of-plane TST issue. After demonstrating our idea was valid, the lower bound Jc of carbon S55C for non-standard 3PB specimen was predicted based on this planar failure criterion and the large strain elastic-plastic FEA. The results showed that Jc showed a lower bound for specimen of B/W ≥ 1.5. In addition, it was shown that this threshold B/W could be estimated by the elastic out-of-plane T33.

Asymptotic Analysis of Size Dependent Fracture Strength Using Boundary Effect Concept

2008 ◽  
Vol 41-42 ◽  
pp. 125-133
Author(s):  
Kai Duan ◽  
Xiao Zhi Hu
Keyword(s):  
Experimental Data ◽  
Fracture Toughness ◽  
Asymptotic Analysis ◽  
Size Effect ◽  
Boundary Effect ◽  
Damage Zone ◽  
Crack Tip ◽  
Characteristic Size ◽  
Asymptotic Model ◽  
Size Dependent

In this paper, the extensively-reported “size effect” phenomena in fracture mechanics tests are explained using the boundary effect concept. It is pointed out that the widely-observed size effect in fracture, including the dependence of the fracture energy on ligament, strength and fracture toughness on crack and/or ligament and the strength of geometrically similar specimens on characteristic size, is in fact, due to the boundary influence on the crack tip damage zone. Furthermore, the recently-developed asymptotic model is used to demonstrate that the dependence of strength on crack and ligament lengths as well as on the characteristic size of geometrically similar specimens is a result of the dominance of the distance of the crack tip to specimen boundaries on the specimen failure mode. To verify further the boundary effect concept, the asymptotic model is also applied to two sets of selected experimental data available in the literature, and the implications are discussed.

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