Numerical Study of Tube Hydroforming Process Using Conical Dies

In this paper the effect of die angle, fluid pressure and axial force on loading paths were studied. In order to reduce the cost and time for the experimental work, ANSYS program is used for implementing the Finite Element Method (FEM), to get optimized loading paths to form a tube using double – cones shape die. Three double die angles θ (116˚ 126˚, 136˚), with three different values of tube outer diametres (40, 45, 50) mm were used. The tube length L_o and thickness t_o for all samples were 80 mm and 2 mm respectively. The most important results and conclusions that have been reached that had the highest wall thinning percentage of 26.8% with less corner filling is at tube diameter 40 mm and cone angle of (116^°) at forming pressure of 43 MPa with axial feeding 10 mm. However, the lowest wall thinning percentage was 6.9% with best corner filling at diameter 50 mm and cone were angle of (136^°) and forming pressure of 30 MPa with axial feeding 4.5 mm. Two wrinkles constituted during the initial stages of forming the tube with initial diameter of 40 mm where the ratio d⁄(t=20) (thick-walled tubes) for all die angles, while only one wrinkle is formed at the center for tubes diameter 45 and 50 mm (thin-walled tubes) . The difference in the location and number of wrinkles at the first stage of formation depends on the loading paths that has been chosen for each process, which was at the diameter 45 and 50 mm towards thin-wall cylinder deformation mode was uniaxial tension. The maximum wall thinning percentage was at the bulge apex for tube diameter 40 mm. But, the maximum wall thinning for tubes of diameters 45 and 50 mm was found at the two sides of the bulge apex .

Tube Expansion by Single Point Incremental Forming: An Experimental and Numerical Investigation

In this paper, we revisit the formability of tube expansion by single point incremental forming to account for the material strain hardening and the non-proportional loading paths that were not taken into consideration in a previously published analytical model of the process built upon a rigid perfectly plastic material. The objective is to provide a new insight on the reason why the critical strains at failure of tube expansion by single point incremental forming are far superior to those of conventional tube expansion by rigid tapered conical punches. For this purpose, we replaced the stress triaxiality ratio that is responsible for the accumulation of damage and cracking by tension in monotonic, proportional loading paths, by integral forms of the stress triaxiality ratio that are more adequate for the non-proportional paths resulting from the loading and unloading cycles of incremental tube expansion. Experimental and numerical simulation results plotted in the effective strain vs. stress triaxiality space confirm the validity of the new damage accumulation approach for handling the non-proportional loading paths that oscillate cyclically from shearing to biaxial stretching, as the single point hemispherical tool approaches, contacts and moves away from a specific location of the incrementally expanded tube surface

Local Buckling Characteristics of Stainless-Steel Polypropylene Deep-Sea Sandwich Pipe under Axial Tension and External Pressure

In this paper, the effects of different loading paths of axial tension and external pressure on the collapse pressure of sandwich tubes are studied by experiments and finite element models. The difference of the two loading paths is investigated. Eight experiments were carried out to study the influence of different loading paths on pipeline collapse pressure under the same geometric and material parameters. Parameterization studies have been carried out, and the results are in good agreement with the experimental results. The test and finite element results show that the loading path of external pressure first and then the axial tension (P→T) is more dangerous; the collapse pressure of the sandwich pipe is smaller than the other. Through parametric analysis, the influence of the axial tension and the diameter-to-thickness ratio of the inner and outer pipe on the collapse pressure under different loading paths are studied.

Deformation and Instability Properties of Cemented Gangue Backfill Column under Step-By-Step Load in Constructional Backfill Mining

Constructional backfill mining with cemented gangue backfill column can solve the environmental issues caused by mining activities and the accumulation of waste gangue at a low cost. To study the deformation and instability properties of cemented gangue backfill columns during the advancement of coal mining face, five step-by-step loading paths were adapted to mimic the different loading processes of the roof. The lateral deformation at different heights and axial deformation of the sample were monitored. The results show that the deformation and instability of the backfill column have the properties of loading paths and are affected by the step-by-step loading path. When stress-strength ratio (SSR) is less than 0.6, the lateral of backfill column shrinks during the creeping process. In high-stress levels, lateral creep strain develops faster than axial creep strain. The backfill column has characteristics of axial creep hardening and lateral creep softening during the step-by-step loading process. The instantaneous deformation modulus and instantaneous Poisson’s ratio show an upward trend. The bearing capacity of backfill column under the step-by-step load is related to loading paths and is no less than uniaxial compressive strength. The non-uniformity of the lateral deformation of backfill column leads to excessive localized deformation that mainly occurs in the middle, causing the overall instability. The development of cracks of backfill column under step-by-step load could be divided into 4 stages according to SSR. Under different step-by-step loading paths, the axial creep strain rate is nearly a constant before entering the accelerated creep stage. A nonlinear creep constitutive model with a creep strain rate trigger was proposed to depict the development of axial strain under step-by-step load. This research will provide a scientific reference for the design of the advancing distance and cycle for the hydraulic support, and reinforcement of the backfill column.

Development of a kinematic hardening rule to assess ratcheting response of materials under various multiaxial loading spectra

The present thesis develops an Armstrong-Frederick (A-F) type coupled kinematic hardening rule to assess ratcheting response of steel alloys under various multiaxial loading paths. The hardening rule is constructed on the basis of the recently proposed Ahmadzadeh-Varvani (AV) hardening rule to further evaluate the ratcheting response of materials under multiaxial loading spectra. The modified model offers a simple framework with limited number of terms and coefficients in the dynamic recovery portion of the model. The dynamic recovery further holds inner product of plastic strain increment p dand backstress unit vector a a with different directions under multiaxial stress cycles enables the model to track different directions. Term 1/ 2 n. a a taking positive values less than unity for multiaxial loading conditions is to control the accumulation rate of ratcheting strain and to prevent the modified model to experience plastic shakedown over stress cycles in stage II. Term(2 n. a a ) taking the values between 1 and 3 under multiaxial loading, magnifies the effect of coefficient γ2 to take into account the nonproportionality effect of various loading paths and further to shift down the predicted ratcheting strain over the stress cycles. The predicted ratcheting curves by the modified rule were compared with those predicted based on earlier developed hardening rules of Ohno-Wang (O-W), Jiang-Sehitoglu (J-S), McDowell, and Chen-Jiao-Kim (C-J-K) holding relatively complex framework and more number of coefficients. The O-W, the J-S, McDowell and C-J-K models mainly deviated from the experimental ratcheting strain of steel alloys for various multiaxial loading histories, while the predicted curves of the modified model closely agreed with experimental data of steel samples over ratcheting stages. The predicted ratcheting curves based on the modified model closely agreed with experimental data of steel samples under various multiaxial step-loading histories. The modified model was also found capable of predicting ratcheting in the opposite direction as the tensile axial mean stress dropped in magnitude. The O-W, J-S, McDowell and C-J-K models holding more backstress components and coefficients require longer Central Processing Unit (CPU) time. While time required for ratcheting assessment using the modified hardening rule was found to be twice shorter due to its simpler framework and limited number of coefficients.