Fatigue Life Prediction for Overlap Friction Stir Linear Welds of Magnesium Alloys

This work was undertaken to analyze the stress/strain state at the critical sites in friction stir welded specimens and, further, to assess the fatigue strength of friction stir welded specimens with conventional fatigue life prediction approaches. Elastoplastic and elastic finite-element stress/strain analyses were carried out for friction-stir-linear-welded (FSLW) specimens made of magnesium alloys. The calculated stress/strain at the periphery of the weld nugget was used to evaluate the fatigue life with local life prediction approaches. First, elastoplastic finite-element models were built according to experimental specimen profiles. Fatigue life prediction was conducted with Morrow's modified Manson–Coffin (MC) damage equation and the Smith–Watson–Topper (SWT) damage equation, respectively, for different specimens under different loading cases. Life prediction results showed that both equations can to some extent give reasonable results, especially within a low-cycle fatigue life regime, with the SWT damage equation giving more conservative results. As for high-cycle life, predicted results were much longer and scattered for both methods. Shell element elastic models were then used to calculate the structural stress at the periphery of the weld nuggets. The correlation between structural stress amplitude and experimental life showed the appropriateness of the structural stress fatigue evaluation for friction stir welds. The effect of the notches at the periphery of the faying surface on life prediction was further discussed.

A Theory for Mathematical Framework of Fatigue Damage on S-N Plane

A theory for mathematical framework is developed with axioms for fatigue damage, quantification and relativity concept, boundary conditions, and compatibility, allowing us to evaluate the validity of candidate damage equations/quantities on S-N plane for prediction of fatigue life. Manifestation points for accumulated damage were defined for boundary conditions by differentiating between damage accumulated before failure, and failure caused by damage at quantised fatigue loading cycles. A selected damage equation leading to a theoretical S-N curve was validated as an example.

Structural Damage Detection Based on Ridge Estimation Method

In order to identify structural damage locations and extent, a method based on ridge estimation and modal strain energy is presented in this paper. First, structural modal strain energy is given and a modal strain energy sensitivity damage equation is obtained. Then, considering the TikhonovTT regularization theoryTT, a ridge estimation method is proposed to solve the damage equation and ridge parameter of the method is optimized. Simulation results demonstrate that the proposed damage detection method based on ridge estimation and modal strain energy can identify structural damage locations and extent with good accuracy.

Preliminary Review of the Influence of Cavitation Behavior in Creep Damage Constitutive Equations

This review paper mainly consists of from two aspects: (a) the evolution of the cavitation damage equation from Dyson to current application in high Cr steels by traditional techniques; (b) quantitation analyses of cavitation behavior in brass, copper, dual phase steel from X-ray microtomtograph. Though there is a lack of experimental data for high Cr steels by X-ray microtomography currently, but (b) has provided reference value for studying creep cavitation behavior in high Cr steels. This paper will be the fundamental of development new creep damage constitutive equation through quantitation analyses of X-ray tomography.

Research on a New Secondary Creep Model and Creep Damage Evolution for P92 Steel

In the traditional Norton-Bailey model, the stress exponent is a constant value when the temperature keeps constant, But for some materials, this situation can’t be suitable. Based on the analysis of the experimental data, a secondary creep constitutive model which can be used in the stress exponent changing situation has been proposed. By introducing Kachanov-Rabotnov damage equation, the modified creep model has been established for P92 steel at 610°C and 670°C, which can describe the second and tertiary stage. And the method to determine creep parameters of tertiary stage has been derived. The new model was embedded into ANSYS interface program, and used for calculating the creep life of P92 steel. The results show that the model is in agreement with the experimental data.

A Simplified Concrete Damage Constitutive Model

In view of the limitation of seismic damage model for concrete structures with recent methods, based on Drucker-Prager constitutive model, with the concept of damage surface, Mazars concrete damage equation obtained by uniaxial test was extended to the triaxial state. Material stiffness damage degradation impact on the yield process of concrete yield surface was considered. Given Druker-Prager constitutive model adapted to the low friction angle soft clay type, the yield parameters threshold of yield surface is low that does not meet the concrete reality, draw double-shear strength theory, the strain threshold of double shear strain yield equation has been adopted, Modified damage threshold of uniaxial test strain damage equation. This paper developed a simplified concrete damage constitutive model in FLAC3D environment. The model is simple and easy to practical engineering applications, and the model has a certain accuracy. Try to explore a new constitutive model of concrete.

A Constitutive Model of Anchored Jointed Rock Mass and its Application

The jointed rock mass distributed in the nature widely and its mechanical characteristic influenced the stability of the rock engineering badly. The cracks propagated and coalesced each other and macroscopic failure happened. Bolts were a kind of effective reinforcement instrument and they could prevent the cracks from propagating. However, the anchoring mechanism of bolts was not realized clearly and their reinforcement could not be reflected effectively in the numerical simulation yet. Based on the damage mechanics, a constitutive relation and damage equation of anchored jointed rock mass were presented in this paper. With a project application, the model was proved to be feasible one.

Creep-Fatigue Evaluation Based on the Overstress

There is summarized a series of creep-fatigue data on a normalized and tempered 2 1/4Cr-1Mo steel obtained at 550°C in a very high vacuum environment under a wide variety of a strain-time program. A special feature of a stress-strain response of the steel is described to give the overstress and the internal back stress. A damage equation is developed to predict a creep-fatigue behavior which is free from the environmental effect.