Yukio Sanomura

Unambiguous Determination of 3D Fiber Orientation Distribution in Thermoplastic Composites Using SAM Image of Elliptical Mark and Interference Fringe

The in-plane angle determined using an optical reflection microscope with an image analyzer is ambiguous, because the same elliptical image would be obtained for a fiber. A method of determining the three-dimensional (3D) fiber orientation distribution using the scanning acoustic microscopy (SAM) image of an elliptical mark and interference fringe is proposed. The interference fringe appearing in the composite in the direction of a fiber is observed by SAM. One can easily determine the in-plane angle without ambiguity by adopting this information of interference fringes. It is shown that this technique is effective for the determination of the 3D fiber orientation distribution in plaque and cylindrical composite parts.

Viscoplastic Constitutive Equation of High-Density Polyethylene

A viscoplastic constitutive equation based on the kinematic hardening creep theory of Malinin-Khadjinsky and the nonlinear kinematic hardening rule of Armstrong-Frederick is formulated to describe the inelastic behavior of high-density polyethylene under various loading. The gentle progress of back stress by the introduction of loading surface in the viscoplastic strain space and smaller material constant under unloading can be expressed. Material constants are identified by various stress-strain curves under compression at constant strain rate and creep curves under compression at constant stress. The viscoplastic model can describe stress-strain curve under compression with change in strain rate and shear stress-strain curve including unloading. The model can qualitatively describe stress-strain curves under compression with changed strain rate including unloading, but it is quantitatively insufficient.

Finite Element Analysis of V-Bending of Polypropylene Using Hydrostatic-Pressure-Dependent Plastic Constitutive Equation *

Finite element analysis of V-bending process of polypropylene was performed using hydrostatic-dependent elastic-plastic constitutive equations proposed by the present authors. Kinematic and isotropic hardening rule was employed for the plastic constitutive equations. The kinematic hardening rule was more suitable for the expression of the stress reversal in uniaxial stress - strain relation than the isotropic hardening. For the result of the finite element analysis of V-bending, the kinematic hardening rule was able to predict the experimental behavior of springback more properly than the isotropic hardening. Moreover, the effects of hydrostatic pressure-dependence were revealed by examining the calculated distribution of bending plastic strain, bending stress and the width of the bent specimen.

Simulation of Inelastic Deformation of Polyethylene in Multiaxial State of Stress by Viscoelastic Constitutive Equtaion

Polymers show significant strain recovery after reversal of the loading direction, and conventional constitutive equations can not describe inelastic deformations including the strain recovery properly. Then the present authors have proposed a viscoelastic constitutive equation for polyethylene in order to describe the inelastic deformations. In the formulation, a total strain was assumed to be the sum of an elastic strain and a viscous strain. The elastic strain was subjected to the Hooke’s law, and the viscous strain was derived from the kinematic hardening creep theory of Malinin and Khadjinsky, which was combined with the nonlinear kinematic hardening rule of Armstrong and Frederick. In order to describe the strain recovery, a loading surface was defined in a viscoelastic strain space, and a new parameter was defined by using the loading surface. Then the nonlinear kinematic hardening rule was modified by using the parameter. Inelastic deformations in a uniaxial state of stress were simulated by using the constitutive equation and the validity of the formulation and the modification was verified by comparing the simulations with experimental results of polyethylene. Then inelastic deformations under typical cyclic loadings in the uniaxial state of stress were predicted, and features of the deformations were discussed.

Evolution Equation of Creep Damage Under Stress Variation

Design and assessment of structural components at elevated temperature are very significant for ensuring the safety. Lear damage accumulation (summation of creep time fraction) is widely used to predict creep rupture time under stress and temperature variation. Life prediction of creep under stress variation by creep damage mechanics of Kachanov-Rabotnov concides with that of linear damage accumulation model. However, creep rupture time under stress variation is essentially the nonlinear problem. A modified evolution equation of creep damage by Kachanob-Rabotnov is formulated by memory effect of the previous stress. The evolution equation of creep damage consists of tow terms as follows:(1) the power damage law of Kachanov-Rabotnov, (2) acceleration anad deceleration by memory effect of the previous stress. The memory effect of the previous stress gradually disappears under the present stress. Additional internal state variable describing this effect is defined and the evolution equation is formulated in order to approach the present stress value from previous stress. The evolution equation of creep damage can be extended to the multiaxial state of stress with isotropic creep damage (scalar) and anisotropic creep damage (2nd symmetric tensor) proposed by Murakami and Ohno. Creep constitutive equation (McVetty type) for damaged material is formulated by the conventional creep damage theory. The model is lacking in the representation of the transient creep after increased stress. Micrographs of specimen ruptured under constant stress and stress variation are observed and creep damage mechanisms are discussed. After material constants are identified by describing creep curves at constant uniaxial stress, the validity of the proposed theory is discussed by the theoretical predictions with the corresponding experiments on tough pitch copper under nonsteady uniaxial stress at 250°C. The prediction of creep rupture time by the present theory is fairly good with the experiment results under stress variation.

Formulation of cross hardening in creep and its effect on creep damage process of copper.

The present paper is concerned with a more elaborate modelling of creep and creep damage in polycrystalline metals and the experimental evaluation of the proposed model. By ascribing the reduction of creep rates caused by the principal stress rotation (i.e., cross hardening) to the intersection mechanism of dislocation on active slip planes in crystal grains, a constitutive equation of creep is first formulated. Then, in view of the metallurgical observations on the nucleation and the growth of grain boundary cavities in creep damage process, an evolution equation of anisotropic creep damage is expressed as a function of the stress, the damage tensor and the creep rates of the material. Finally, the validity of the proposed theory is discussed by performing systematic creep damage tests of thin-walled copper tubes under non-steady multiaxial states of stress at 250°C.