Effect of loading paths on hydroforming tubular square components

The influence of loading path on tube hydroforming process is discussed in this paper with finite-element simulation. Four different loading paths are utilized in simulating the forming process of square tubular component with hydroforming and the result of different loading path is presented. Among the result, the thickness distribution of bilinear loading path is the most uniform one. It shows that the increase of punch displacement in the stage of high pressure is beneficial to the forming of component for optimized stress condition.     

Prediction of reliability analysis of composite tubular structure under hygro-thermo-mechanical loading

The present paper focuses on reliability prediction of composite structure under hygro-thermo-mechanical loading, conditioned by Tsai-Wu failure criterion, where the Monte–Carlo method is used to estimate the failure probability(Pf). This model was developed in two steps: first, the development of a deterministic model, based on an analytical and numerical approach, and then, a probabilistic computation. Using the hoop stress for each ply, a sensitivity analysis was performed for random design variables, such as materials properties, geometry, manufacturing, and loading, on composite cylindrical structure reliability. The probabilistic results show the very high increase of failure probability when all parameters are considered.     

加载条件对内高压成形管件尺寸精度的影响

为了研究内高压成形管件横截面尺寸变化规律,获得管件横截面尺寸精度的调控方法,采用内高压成形实验研究了内压和合模力加载条件对低碳钢变径管直径尺寸精度的影响规律。结果表明:随着内压从60 MPa增加到210 MPa,变径管直径逐渐增加,卸压出模后管件发生0.045%～0.075%的回弹,当内压为150 MPa时,获得的管件直径尺寸精度最高。随着合模力增加,变径管水平方向的直径尺寸逐渐增大,竖直方向的直径尺寸逐渐减小,横截面不圆度增大,导致尺寸精度降低。因此,在内高压成形中,可通过增加模具尺寸或采用可变合模力加载以降低合模力对管件横截面尺寸精度的影响,也可通过控制内压使模具弹性变形量恰好等于管件回弹量,从而使管件最终直径尺寸等于设计值,以保证管件的横截面尺寸精度。     

Fatigue strength of tubular carbon fibre composites under bending/torsion loading

Carbon fibre reinforced polymer composites have been increasingly used on structures frequently subjected to biaxial fatigue loadings. This paper studies the fatigue behaviour of tubular carbon fibre composites under in phase biaxial bending/torsion dynamic loadings. Particularly, it was analysed both the torsion stress and mean stress effects on the fatigue strength and failure mechanisms. Fatigue strength decreases significantly with increased torsional/bending stresses ratio, while the damage becomes faster. For the cases in which a torsion loading was applied the effect of the mean stress on the fatigue strength seems to be well fitted by using a quadratic equation.     

Fatigue behaviour of tubular AlMgSi welded specimens subjected to bending–torsion loading

This paper is concerned with an experimental and numerical study of the fatigue behaviour of tubular AlMgSi welded specimens subjected to biaxial loading. In‐phase torsion–bending fatigue tests under constant amplitude loading were performed in a standard servo‐hydraulic machine with a suitable gripping system. Some tests in pure rotating bending with and without steady torsion were also performed. The influence of stress ratio R and bending–torsion stress ratio were analysed. Correlation of the fatigue lives was done using the distortion energy hypothesis (DEH), based on the local stresses and strains. The applicability of the local strain approach method to the prediction of the fatigue life of the welded tubular specimens was also investigated. Static torsion has only a slight detrimental influence on fatigue strength. The DEH (von Mises criterion) based on local stresses in the weld toes was shown to satisfactorily correlate fatigue lives for in‐phase multiaxial stress–strain states. The stress–strain field intensity predictions were shown to have less scatter and are in better agreement with the experimental results than the equivalent strain energy density approach.     

Kinetics of crack propagation in PMMA samples under cyclic tensile loading

The kinetics of crack propagation has been investigated experimentally and theoretically for PMMA specimens under low frequency zero-tension cyclic loading. On the basis of the energy balance equation taking into account the dependence of energy dissipation at the crack tip plastic zone on the crack growth rate an analytic relation was obtained describing the propagation of a central crack in infinite plates in the fatigue mode. It is shown that for uniaxial tension the growth of a fatigue crack is determined by characteristic parameters having the dimensionality of the length and number of cycles. The results of the calculation of crack propagation kinetics for a specimen with an edge crack taking into account the finite value of the specimen's width are compared to the experimental dependences of the length of a fatigue crack on the number of loading cycles from the start of cycling up to the failure of the specimen when the load range is constant. A reasonable agreement between the experimental and calculated data is observed. The size of mirror areas measured experimentally and calculated on the basis of a previously established “rate criterion” are in good agreement.     

Fatigue strength of tubular structural elements under bending oscillations. Report 2. Fatigue strength of tubular structural members under programmed loading

Programs are presented for block loading with determined and random alternation of stress amplitude simulating the service loading spectrum. The results of fatigue tests of straight and bent tubular structural members are presented. It is concluded that low fatigue strength of bent tubular structural members is caused by the unfavorable technological effects of bending these pipes and by warping of the cross section during testing.Translated from Problemy Prochnosti, No. 4, pp. 89–93, April, 1994.     

An inverse method for material parameters determination of titanium samples under tensile loading

Different approaches used for the simulation of woven reinforcement forming are investigated. Especially several methods based on finite element approximation are presented. Some are based on continuous modelling, while others, called discrete or mesoscopic approaches, model the components of the fabric. A semi discrete finite element made of woven unit cells under biaxial tension and in-plane shear is detailed. In continuous approaches, the difficulty lies in the necessity to take the strong specificity of the fibrous material into account. The yarn directions must be strictly followed during the large strains of the fabric. This is the main goal of the non-orthogonal model and of the hypoelastic constitutive model based on the yarn rotation presented in this paper. In the case of discrete and semi-discrete approaches the directions of the yarns are “naturally” followed because the yarns are modeled. Explicitly, however, modeling each component at the mesoscopic scale can lead to high numerical cost.     

Assessment of notched tubular ferritic oxide-dispersion-strengthened components subjected to multiaxial creep loading at 1100°C

Notched tubular components of a ferritic oxide-dispersion-strengthened material, MA 956, were subjected to multiaxial creep loading at 1100°C by applying a constant internal pressure. The components proved to be extremely insensitive to circumferential notches of up to 80 percent of the wall thickness due to the high axial creep rupture strength of this material. However, the components were sensitive to both externally and internally axially aligned notches, and displayed similar stress rupture behaviour but consistently longer rupture lives than plane components at the same ligament stress level. Failure was found to be due to strain controlled cavitation in the ligament rather than as a consequence of creep crack growth from the notch. A direct current/potential drop method was shown to provide a reasonable indication of the development of cavitation in these tests. It is shown that the low ductility failure of notched MA 956 components is best described by a creep fracture mechanism rather than by fracture mechanics.     

Precipitation of mobile copper from Cu2−x Se samples under impact loading

It has been found that the rate of copper precipitation from samples of the superionic conductor copper selenide, Cu_2−x Se, exposed to impact loading is substantially higher than that under static pressure. This effect is attributed to the action of excess pressures and temperatures at crystal grain boundaries during the plastic deformation of the samples under impact. Pis’ma Zh. Tekh. Fiz. 23, 65–69 (March 12, 1997)     

The Peak Stress Method applied to fatigue assessments of steel tubular welded joints subject to mode-I loading

The paper presents the practical aspects involved in structural design of tubular joints mainly employed in roller coasters manufacturing. Different design standards, commonly adopted in fatigue assessments, are considered and compared. Then some constant amplitude fatigue test results concerning typical tubular welded joints are presented. Finally the experimental results and additional test data taken from the literature are compared with the theoretical estimations based on the Peak Stress Method, which has been recently conceived and proposed as a simplified, application-oriented version of the well known Notch-Stress Intensity Factor approach. The results indicate that the Peak Stress Method is suitable to estimate the fatigue life up to crack initiation of tubular welded joints subject to mode-I loading.