Lixun Cai

IMPROVED NORMALIZATION METHOD FOR DUCTILE FRACTURE TOUGHNESS DETERMINATION BASED ON DIMENSIONLESS LOAD SEPARATION PRINCIPLE

This study successfully deals with the inhomogeneous dimension problem of load separation assumption, which is the theoretical basis of the normalization method. According to the dimensionless load separation principle, the normalization method has been improved by introducing a forcible blunting correction. With the improved normalization method, the J-resistance curves of five different metallic materials of CT and SEB specimens are estimated. The forcible blunting correction of initial crack size plays an important role in the J-resistance curve estimation, which is closely related to the strain hardening level of material. The higher level of strain hardening leads to a greater difference in JQ determined by different slopes of the blunting line. If the blunting line coefficient recommended by ASTM E1820-11 is used in the improved normalization method, it will lead to greater fracture resistance than that processed by the blunting line coefficient recommended by ISO 12135-2002. Therefore, the influence of the blunting line on the determination of JQ must be taken into full account in the fracture toughness assessment of metallic materials.

Ratcheting Behavior of T225NG Titanium Alloy under Uniaxial Cyclic Stressing: Experiments and Modeling

Abstract In order to investigate the ratcheting behavior of T225NG alloy, a series of ratcheting tests under uniaxial long-cyclic stressing were performed. The results show that the ratcheting strain of this alloy can get into shakedown after tens (or hundreds) of thousand cycles. After the ratcheting strain is saturated under the condition that stress amplitude is half of peak stress, it will bring about subsequent fatigue failure, and relationship between fatigue life and one of peak stress and saturated ratcheting (SR) strain meets power law. As the alloy is under stress jiggling with stress amplitude that is 1%–2.5% of peak stress, the ratcheting strain still become remarkable and goes into shakedown after several hundreds of thousand cycles but there exists little accessional strain caused by creep effect. It is notable that, when the peak stress is 85%–100% of yield stress, the long cyclic stressing will lead SR strain to be from 1.4% to 2.5% even if the initial ratio of ratcheting strain is zero. Based on ratcheting threshold property of peak stress and monotonicity of relationship between the peak stress and SR strain, a saturated ratcheting model (SRM) is developed to predict SR strain and to estimate saturated creep strain also. In addition, the classes of ratcheting evolutions of metals are discussed.

Estimation ofJ-resistance Curves for CT Specimen Based on Unloading Compliance Method and CMOD Data

The unloading compliance method is one of the most widely used methods in laboratory testing for J-resistance estimation. A specimen configuration including compact tension (CT) is recommended by almost all the standard test procedures for evaluating fracture toughness, such as ASTM E1820-11, ISO 12135-2002, and GB/T 21143-2007. In the unloading compliance method, key techniques including instantaneous crack size measurement, correction of specimen rotation, and J-integral computation can be used for a CT specimen when using load line displacement (LLD) but not when using crack mouth opening displacement (CMOD) from those standard test procedures. However, a CT specimen with CMOD measurement is also used for J-resistance estimation under certain severe environments because of the convenience of measuring displacement. This study proposes a simplified linear formula for crack size calculation for a CT specimen with CMOD data, discusses the rotation correction in detail based on the equivalence of work, and develops reasonable incremental expressions for J-integral computation and a CMOD-to-LLD conversion equation. By measuring the LLD and CMOD data of a CT specimen simultaneously, we carried out experiments on J-resistance curve estimation for four different types of materials. According to the developed procedure of the unloading compliance method in this study, J-resistance curves for all materials determined from LLD and CMOD data of the CT specimen are clearly exhibited.

Experimental Estimation of J Resistance Curves From Load-CMOD Record for FFCT Specimens

Fracture toughness of ductile materials is often characterized by J resistance curves. It is generally required to adopt load line crack mouth opening displacement (CMOD) records in load line compact tension (LLCT) specimens to calculate the J integral according to ASTM E1820-13e1. However, in some cases, for example in nuclear engineering, due to material size limitations, a standard test specimen size is not achievable, and attaching or mounting the displacement transducer to measure the load line CMOD (LLD) of a smaller compact tension specimen is very challenging, and an alternative approach is required. The present study is aimed at evaluating the use of front face CMOD (FFD) records in front face compact tension (FFCT) specimens to estimate J resistance curves. Results are compared with those using LLD-based J calculations for P92 steel. It is shown that J resistance curves determined from the two approaches are in good agreement.