Applied Mechanics and Materials Vol. 750

Abstract: The purpose of this paper is to present a new approach to finding a risk-informed safety factor for the “fail-safe” design of a high-consequence engineering system. The new approach is based on the assumption of a 99.99 % confidence level and a 99.99 % coverage, and the application of the classical theory of tolerance limits, error propagation, and a method of statistical model parameter estimation known as the bootstrap method. To illustrate this new approach, we first apply the methodology to the UTS data of six materials ranging from glass, ceramics, to a high-strength steel at both 20 C and 600 C, and then to the fatigue life estimation of a BK-7 glass using two available additional sets of laboratory test data. Significance and limitations of our new approach to the “fail-safe” UTS design and fatigue life prediction of an aging PVP or aircraft are presented and discussed.

Abstract: In this paper, the finite element method (FEM) based on GTN model was used to investigate the in-plane/out-of-plane constraint and strength mismatch effects on local fracture resistance of A508/Alloy52Mb bimetallic joint. The J-resistance curves, crack growth paths and local stress-stain distributions in front of crack tips were calculated for cracks with different constraints and strength mismatches. The results show that the local fracture resistance of the interface crack in this joint is sensitive to constraint and strength mismatch effects. With increasing in-plane constraint (crack depth a/W), out-of-plane constraint (specimen thickness B) and strength mismatch degree, the plastic strain and stress triaxiality around crack tip increase, and the corresponding crack growth resistance decreases. The crack with strength mismatch factor M=1 displays a markedly higher crack growth resistance than the other cracks with M>1 and M<1. It also has been found that there is an interaction between in-plane/out-of-plane constraint and strength mismatch for the bimetallic joint. With increasing in-plane/out-of-plane constraint, strength mismatch effect on fracture toughness becomes weaken. For accurate and reliable safety design and failure assessment of the bimetallic joint structures, the effects of constraint and strength mismatch on local fracture resistance need to be considered.

Abstract: The creep crack growth rate in Cr-Mo-V steel has been numerically predicted for specimens with different constraints for a wide range of C* by using stress dependent creep model and ductility, and the simulated da/dt-C* curves were compared and analyzed with experimental data. The results show that the simulated da/dt-C* curves agree well with experimental data. At low and transition C* regions, the crack-tip constraint has obvious effect on CCG rates, while at high C* region it almost has no effect. With increasing constraint, the CCG rates and transition region size on da/dt-C* curves increase due to higher stress traxiality ahead of crack tip and stress-regime dependent creep ductility. If the extrapolation CCG rate data of standard high constraint CT specimen from high C* region (above the turning point 2) or from transition C* region are used in life assessments of the components with various constraints at low C* region (below the turning point 1), the non-conservative or excessive conservative results may be produced. Therefore, the CCG rate data for considering constraint effect should be obtained for a wide range of C* by long-term laboratory tests or numerical predictions using the stress dependent creep model and ductility.

Abstract: Strain controlled uniaxial low cycle fatigue (LCF) tests of P92 steel were conducted at strain amplitudes of 0.4%, 0.6% and 0.8% in fully reversed manner with strain rate of 1.0×10^-3s^-1 at high temperature of 650 °C. Cyclic softening behavior was studied and time-independent cyclic plasticity model was used to represent the cyclic mechanical behavior of this steel. Material parameters were determined step by step at higher strain amplitude of 0.8%, experimental data with lower strain amplitude were used to validate the extrapolation of the model. Comparison of the simulated and experimental results shows that the proposed model can give a reasonable prediction of stress-strain hysteresis loop for P92 steel at high temperature.

Abstract: Edge cracking is a commonly observed phenomenon in cold rolling process, but researchers appear to be far from fully understanding its failure mechanism due to the complex stress conditions of steel strip under the rolling condition. In this research, the shear modified GTN damage model coupled with Nahshon-Hutchinson shear damage mechanism was applied to investigate the damage and fracture behavior of steel strip in cold rolling. The results show that the shear modified GTN damage model is competent to predict the damage and fracture behavior of steel strip in cold rolling. By comparison to the cold rolling experiment, it presents that the prediction of edge crack occurrence of the shear modified GTN damage model is more accurate than that of the original GTN damage model.

Abstract: Material damage state is described by a restraining stress zone. Then, a macro/micro trans- scale fatigue crack growth model is established. The proposed model is used to simulate the whole fatigue process from a micro-defect to the final macroscopic fracture. Numerical calculations are performed. The fatigue test data for the LC4 aluminum alloy plate specimens are accurately re-produced by the present model. Furthermore, when the microscopic effects are taken into account, the scatter feature of the fatigue test data can also be reflected.

Abstract: Small punch test (SPT) is a miniature sample test technique which can evaluate in-service material properties with an almost non-destructive method. Since the deformation behavior of the small punch specimen is complicated, finite element simulation embedded with Gurson-Tvergaard (GTN) model is adopted to simulate elastic-plastic behaviour until fracture. Choosing the proper GTN parameters is crucial for the small punch simulation, which directly influence the precision of load-displacement curve obtained from simulation. In this paper, load-displacement curve is divided into five stages and the parameters identification process is done by adjusting the simulation curve with experimental load-displacement curve in different stages which controlled by separately parameters. The results show that the parameters determined based on this criterion are not unique. In order to validate the reliability of this method, specimen’s minimum thickness of cross-section after fracture was introduced as an extra criterion which turned out to be feasible. Load-displacement curves cannot serve as the only criterion to verify the GTN parameters.

Abstract: Under pseudo-periodic strain, the thermo-effect plays an important role in the fatigue fracture of material. How to construct the related mechanics model is a practical issue. For simple bending of iron wire under pseudo-periodic, based on local rotation concept, the deformation is described by the local rotation angle. Then, the related motion equations are used to obtain the theoretical form of life-strain rate relation. The results show that: (1) The local rotation angle rate can represent thermo-effects well; (2) The fatigue fracture is nucleated at the position of maximum local rotation angle; (3) The intrinsic mechanism of fatigue fracture of the material is the local rotation.

Abstract: According to the all set theory, a fuzzy-random creep fracture model was presented in this work. To deal with the function, the following steps were taken. First, the steady state creep coefficient (A) and steady state creep exponent (n) were considered in the fuzzy-random variables, then the C*-integral was considered in a fuzzy-random variable. Finally, with the interval analysis, the result of the fuzzy-random creep fracture model was given.