Evolution of residual stress field in 6N01 aluminum alloy friction stir welding joint

Based on the characteristics of friction stir welding( FSW) and Coulomb friction work theory,the residual stresses field of FSW joints of 6 N01 aluminum alloy( T5),which was used in high speed train,were calculated by using the ANSYS finite element software. During the FEM calculation,the dual heat source models namely the body heat source and surface heat source were used to explore the evolution law of the welding process to the residual stress field. The method of ultrasonic residual stress detecting was used to investigate the residual stresses field of the 6 N01 aluminum alloy FSW joints. The results show that the steady-state temperature of 6 N01 aluminum alloy during FSW is about 550 ℃,and the temperature mutates at the beginning and at end of welding. The longitudinal residual stress σ_x is the main stress,which fluctuates in the range of-25 to 242 MPa. Moreover,the stress in the range of shaft shoulder is tensile stress that the maximum tensile stress is 242 MPa,and the stress in the outside of shaft shoulder is compressive stress that the maximum compressive stress is 25 MPa. The distribution of the tensile stress in the welding nugget zone( WNZ) is obviously bimodal,and the residual stress on the advancing side is higher than that on the retreating side. With the increasing of the welding speed,the maximum temperature decreased and the maximum residual stress decreased when the pin-wheel speed kept constant. With the increasing of the pin-wheel speed,the maximum temperature of the joint increased and the maximum residual stress increased when the welding speed was constant. The experimental results were in good agreement with the finite element results.

Shakedown Criterion Employing Actual Residual Stress Field and Its Application in Numerical Shakedown Analysis

Construction of the static admissible residual stress field and searching the optimal field are key tasks in the shakedown analysis methods applying the static theorem. These methods always meet dimension obstacles when dealing with complex problems. In this paper, a novel shakedown criterion is proposed employing actual residual stress field based on the static shakedown theorem. The actual residual stress field used here is produced under a specified load path, which is a sequence of proportional loading and unloading from zero to all the vertices of the given load domain. This ensures that the shakedown behavior in the whole load domain can be determined based on the theorem proposed by K6nig. The shakedown criterion is then implemented in numerical shakedown analysis, The actual residual stress fields are calculated by incremental finite element elastic-plastic analysis technique for finite deformation under the specified load path with different load levels. The shakedown behavior and the shakedown limit load are determined according to the proposed criterion. The validation of the criterion is performed by a benchmark shakedown example, which is a square plate with a central hole under biaxial loading. The results are consistent with existing results in the literatures and are validated by full cyclic elastic-plastic finite element analysis. The numerical shakedown analysis applying the proposed criterion avoids processing dimension obstacles and performing full cyclic elastic-plastic analysis under arbitrary load paths which should be accounted for appearing. The effect of material model and geometric changes on shakedown behavior can he considered conveniently.

Nondestructive Testing and Characterization of Residual Stress Field Using an Ultrasonic Method

To address the difficulty in testing and calibrating the stress gradient in the depth direction of mechanical components, a new technology of nondestructive testing and characterization of the residual stress gradient field by ultrasonic method is proposed based on acoustoelasticity theory. By carrying out theoretical analysis, the sensitivity coefficients of different types of ultrasonic are obtained by taking the low carbon steel（12%C） as a research object. By fixing the interval distance between sending and receiving transducers, the mathematical expressions of the change of stress and the variation of time are established. To design one sending-one receiving and oblique incidence ultrasonic detection probes, according to Snell law, the critically refracted longitudinal wave（LCR wave） is excited at a certain depth of the fixed distance of the tested components. Then, the relationship between the depth of LCR wave detection and the center frequency of the probe in Q235 steel is obtained through experimental study. To detect the stress gradient in the depth direction, a stress gradient LCR wave detection model is established, through which the stress gradient formula is derived by the relationship between center frequency and detecting depth. A C-shaped stress specimen of Q235 steel is designed to conduct stress loading tests, and the stress is measured with the five group probes at different center frequencies. The accuracy of ultrasonic testing is verified by X-ray stress analyzer. The stress value of each specific depth is calculated using the stress gradient formula. Accordingly, the ultrasonic characterization of residual stress field is realized. Characterization results show that the stress gradient distribution is consistent with the simulation in ANSYS. The new technology can be widely applied in the detection of the residual stress gradient field caused by mechanical processing, such as welding and shot peening.

Modeling of Temperature and Residual Stress Fields Resulting from Impacting Process of a Molten Ni Particle onto a Flat Substrate

Several effective numerical techniques,based on a finite element analysis,have been developed and computed independently.Results are presented describing the impacting process,and the subsequent temperature and residual stress fields of a molten nickel particle impacting onto a flat substrate.Problems of this type,especially the prediction of the thermal residual stresses,are of major practical interest in thermal spray operations as a pioneering approach.

EFFECT OF TENSILE STRESS AND RESIDUAL STRESS ON THE SPONTANEOUS STRAY FIELD SIGNALS FROM THE SURFACE OF 0.45%C STEEL

In order to explore the quantitative method of metal magnetic memory testing（MMMT） and clarify the relationship between Hp（y）, the normal component of spontaneous stray field, and applied stress or residual stress, the static tensile tests of 0.45%C steel sheet specimens are carried out on a servo hydraulic MTS810 machine. Hp（y） values are measured during the test process by an EMS-2003 metal magnetic memory diagnostic apparatus and a non-magnetic electric control displacement instrument. Residual stresses of some points on the surface of a specimen are measured by a Stress Tech X-Stress 3000 X-ray diffraction instrument. The results show that the same variation rules of Hp（y） value versus applied tensile stress are presented under the different conditions of load-on and load-off. However, the same rule does not exist between the Hp（y） value and residual stress. The variation of Hp（y） value reflects the history of applied tensile stress.

Characterization of EBPVD micro-layer composites and simulation of its internal stress state

Based on the basic operating principal and the technology characteristic of electron beam physical vapor deposition(EBPVD) technique, EBPVD was used to prepare the micro-layer composites. The effect on the substrate preheating temperature was taken into accounts and the finite element analysis package ANSYS was used to simulate the internal stress field and the potential displacement changing tendency. The results show that one of the most important quality factors on the judgment of micro-layer composites is the adhesion between the substrate and the deposition layers as well as among the different deposition layers. Besides the existance of temperature gradient through the thickness of layers, the main reason for the internal stress in micro-layer composites is the mismatch of various properties of the layer and the substrate of different thermal expansions and crystal lattice types. With the increase of substrate preheating temperature, the inter-laminar shear stress also takes on a tendency of increase but the axial residual stress decrease.

SIMPLE SHAKEDOWN OF STRUCTURES UNDER VARIABLE MULTI-LOADINGS

The shakedown analysis of structures under variable multi-loadings is considered, and the corresponding simple shakedown condition is presented in this paper. Distribution of fixed stresses field is given, and the self-equilibrium of fixed stresses field is analyzed. Elastic shakedown and plastic shakedown conditions are presented based on the fixed stresses field. The theorem is convenient to evaluate the shakedown limit of structures under cyclical variable multiloadings through solving positive scalar fields and fixed stresses field factors at a series of dangerous positions of the structure, and tedious computations are avoided. Finally the theorem is applied to a thick-walled cylindrical tube under variable pressure and temperature, and the rolling contact problem. The results are in good agreement with some computational results.

SIMULATON ON WELDING PROCESS OF T - JOINTS

T-joints are widely used in ship manufacturing and other welding structures.The welding residual stress and deformation hare significant influence on failure and service life in welding structures.By us- ing of 3D thermal elasto - plastic FEM, the temperature fields,stress fields and deformation of weld- ing processes on T - joints are analyzed in this paper.The moving heat source is considered in the com- putation. Moreover, weld metal filling and multiple - pass welding are simulated using activate/deac- tivate ability.For avoiding 'locking' in full integration elements of welding computation,the re- duced intergration elements are used in weld area.The nonlinear FEM analysis program MARC is used to complete the computations, as well as its user subroutines that are programmed to develop the special techniques needed in the simulation of welding process.Some different welding procedures are com- pared, and the welding residual stress and deformation are discussed in details.