Disk bend test

Micromechanical failure criterion for FGM architecture studied via disk-bend testing of ZrOj/Ni composites... 

In what follows, we first describe an experiment done by "disk-bend" testing. This method of testing is advantageous to the matter concerned because it allows us to make an equibiaxial plane-stress state of loading to be met in individual "layers" constituting an FGM. However, its application to materials exhibiting inelastic deformation has not been established yet. Under the circumstances, we devise a new method for the analysis of test data. 

DISK-BEND TESTING 2.1. Specimens and test method... 

As shown above, the ZrO/Ni composites examined by disk-bend testing are found to deform in a nonlinear manner, so that composition-dependent fracture strengths cannot be obtained directly from the stress-strain diagram in Fig. 3. Under the circumstances, we now make a micromechanical analysis to estimate actual stresses to be developed by plastic deformation of the ductile constituent on the basis of an established "mean-field" model [12]. In the following, the macrostress a) is related to the microstresses and (o) such... 

Through disk-bend testing on a series of ZrOj/Ni composite specimens fabricated by powder processing, we have examined the fracture behavior of ceramic/metal composites under an equibiaxial plane-stress loading, and derived, by making a micromechanical analysis of elastoplastic stress states, a brittle phase-controlled fracture criterion of the form, ( )max const., in terms of the equivalent normal stress a. This criterion is conceptually simple and quite useful particularly for our micromechanics-based approach to the FGM architecture. 

Manahan M.P., A.S. Argon, and O.K. Harling. 1981. The developnaent of a miniaturized disk bend test for the determination of postirradiation mechanical properties. Nud Mater 104 1545-1550. 

Similar to the ceramic-ceramic joints, the constitutive response of the base zirconia, cast iron, and braze metal were determined in order to predict joint response based on stress-strain properties of the constituent materials. Tensile tests were performed on cast iron and Incusil-ABA, while bend tests were performed on the base zirconia. The temperature of testing ranged from room temperature to 400 C, the upper temperature limit for this type of joint. The strength and toughness of joints were evaluated using unnotched and notched bend bars and disk specimens. 

Prior to bend testing, the specimens were notched using a diamond cutting disk such that initial cracks were formed with relative length Xq = 1/8, 1/4, 3/8 and 1/2 from section depth. 

In these tests, we assume that the contact stiffness of the elastomeric substrate is not substantially affected by the addition of the thin glassy or semicrystalline polymer layers. These layers deform by bending, and the load needed to bend the layers into conformity with the elastomer surface can be estimated from a simple plate deformation calculation. The deflection of the film can be modeled by the central loading of a disk [16], and comparing the stiffness of the disk to the contact stiffness of the elastomer substrate. For a circular plate that is loaded at the center and supported (but not clamped) at a radius of the stiffness is dependent on the modulus of the film, f, the thickness of the film, h, and its Poisson ratio v [Eq. (4)]. 

The earthquake engineering experimental facility at Northeastern University provides large-scale dynamic testing of soil samples under uniform cyclic or earthquake motions. It includes a shaking table (Fig. 21.2), a data acquisition card (NI-DAQ) and software (LabVIEW), a set of instmments including accelerometers (Crossbow, Ig, 2g, 5g, Fig. 21.3a), linear variable displacement transducers (LVDT, RDP DCTH400AG, Fig. 21.3b), miniature pore pressure transducers (GE Druck PDCR 81, Fig. 21.3c), and multiple bender element and bending disk measurement equipment for S and P wave measurements (Fig. 21.3c-e). 

FIGURE 9.1.11 Normalized strength of a monolithic SiC and a whisker-reinforced SiC after pin-on-disk test (four-point bending strength after pin-on-disk test was normalized by the initial four-point bending strength). 

---