Kazuhiro Kusukawa

Low-cycle fatigue behavior of commercially pure titanium

Abstract Low-cycle fatigue tests have been carried out on annealed commercially pure titanium under strain-controlled conditions. The relation between a plastic strain range and a number of cycles to failure obeyed Manson—Coffins rule. The initiation of fatigue cracks was observed successively with the aid of a replication technique and microstructural deformation was measured on the surface of a specimen. Results show that fatigue crack initiation is intergranular at strain ranges larger than 1%. On the contrary, slip band cracks appear in lower strain ranges. At specified grain boundaries, microstructural deformation concentrates at or near grain boundaries and irreversible steps between grains form gradually and this leads to intergranular cracking. The above fatigue behavior of pure titanium is mainly due to fewer slip systems (only three) than other metals with bcc or fcc crystal structures.

PIEZOELECTRIC PROPERTIES OF PZT FILMS PREPARED BY HYDROTHERMAL METHOD

Lead zirconate titanate (PZT) thin films of 5 μm thick were produced by a hydrothermal method on pure titanium substrates. ZrOCl2-8H2O, Pb(NO3)2 and TiO2 were used as precursors and KOH as a promoter. The hydrothermal synthesis of PZT includes nucleation and crystal growth processes at 120°C or 140°C. The crystallization states were investigated by using scanning electron microscopy and X-Ray diffraction. Piezoelectric properties were evaluated from unimorph cantilever type actuators made of the films. The relationships between the deflection of the actuator due to piezoelectric transverse effect and applied electric field in the direction of thickness of the films showed good linearity. The output voltage from the films under cyclic compressive loading increased with increasing loading frequency, and is saturated at 10 Hz. The PZT films produced by the present methods are satisfactory as a smart material, and are better than the films produced using TiCl4 as Ti precursor.

Characteristics of BNT Films Synthesized by a Hydrothermal Method

Thin films of lead-free piezoelectric ceramics (Bi1/2Na1/2)TiO3 (abbreviated as BNT) were prepared on pure titanium substrates by a hydrothermal method. Several properties of BNT films synthesized in various Bi3+ and Ti4+ concentrations of starting materials were investigated using SEM, EDX, XRD and other instruments. Moreover, the effects of ion concentrations of starting materials on permittivity and piezoelectric effect of BNT films were discussed. The Bi2O3 crystals were more deposited on the surface of films with the increase of the concentration of Bi3+. The relationship between the deflection and applied electric field was measured on unimorph cantilever type actuators made from three samples which had different XRD patterns. The results showed that the piezoelectric effect of BNT films was dependent on the crystallization level of BNT.

Effect of Degree of Cure on Damage Development in FRP

We have developed a fiber-optic sensing system for measurement of degree of cure (DOC) of FRP laminates. By applying this system to monitoring of cure process of FRP laminates, molding time cam be optimized. However, it is considered that some part of the composite has incomplete DOC due to non-uniformity of molding temperature when the optimized molding process is conducted. Therefore, the effect of DOC on quality of FRP must be clear to manufacture high quality FRP using the optimized molding process. In the present paper, GFRP cross-ply laminates were manufactured with arbitrary DOC. Tensile tests to investigate the effect of DOC on the damage development behavior of the cross-ply laminates were conducted. DOC of the specimen was monitored by the fiber-optic sensing system developed by our laboratory. Matrix cracks were observed during the tensile tests. From the experimental results, it was found that the lower DOC specimen showed the lower crack initiation stress and the slower development rate of transverse crack in 90° layer. In addition, it appeared that many splitting cracks were generated in 0° layers when the DOC was less than 100 % due to the low toughness of resin. From these results, it was found that DOC governs damage development behavior of FRP laminates strongly.

ELECTRIC-FIELD-INDUCED CRACK GROWTH AT ELECTRODE EDGE IN PIEZOELECTRIC CERAMICS

Small crack growth behavior in poled lead zirconate titanate was examined under cyclic electric loading. A crack located at the edge of a partial electrode grew along the electrode boundary during the loading. The crack growth rate decreased with increasing crack length until a non-propagating crack was reached. The growth rate and crack length of the non-propagating crack were affected by the amplitude and bias levels of the electric loading. In the case of high-amplitude loading or negative-biased loading, the crack growth rate varied considerably because of domain switching. Finite element analysis of a three-dimensional permeable crack showed that the mode III stress intensity factor range is independent of crack length, but it decreases as a result of the frictional force of a positive electric field. Fracture surface observations showed that intergranular cracking is dominant near the tip of the non-propagating crack.

Electric Loading-Induced Cracking Behavior at Electrode Edges in PZT Ceramics

Fatigue damage behavior under repeated electric loading was investigated on two kinds of PZT ceramics with discontinuous electrodes. Intergranular cracking was observed at the electrode edge in soft PZT under electric fields greater than ±400 V/mm. However, under the same loading conditions, no damage was observed in hard PZT. When cracking occurred, permittivity of specimens decreased with the number of cycles corresponding to the amount of mechanical damage. FEM analyses of the electroelastic field of the specimens showed that cracking due to cyclic electric loading was related to 180˚ domain switching caused by concentrated electroelastic field.

Effect of Molding Conditions on Process-Induced Deformation of Asymmetric FRP Laminates

It is well known that process-induced deformation of laminates affected dimensional accuracy of FRP components strongly. It is very important for manufacturing of FRP laminates to investigate cure shrinkage and thermal strain during molding process. In the present paper, in order to investigate effect of molding conditions on process-induced deformation of asymmetric GFRP laminates, several molding patterns of temperature and pressure and two types of interfacial conditions between FRP and a mold were employed for manufacturing of the laminates by a hot press molding method. Single-hold pattern or double-hold patterns of temperature were applied to the molding process. The highest molding temperature was 140 °C and the first hold temperature of double-hold patterns were 90 °C, 110 °C and 120 °C. Fiber optic strain sensors and refractive index sensors were used for measurement of strain and degree of cure. It was confirmed that prepreg was cured perfectly at the first holding step. Curvature of the specimen were measured after demolding and drying processes. From the experimental results, it was found that the first holding temperature of double-hold patterns did not affect the curvature of warping deflection after demolding. The results of strain measurements suggested that the strain during cure process was governed mainly by friction between laminates and deformation of molding plates. Moreover, it was found that Interfacial constraint between the laminates and molding plates strongly affected process-induced deformation of asymmetric GFRP laminates.

Molding Strain of Glass Fibers of Model GFRP

In order to evaluate strain induced by shrinkage of resin during cure process, model specimen of glass optical fiber-reinforced polymers were manufactured. Single FBG sensor was embedded in the specimen for measuring molding process-induced strain. From the experimental results, it was found that cure shrinkage strain as well as thermal strain should be considered for evaluating residual stress in reinforcing fibers. An viscoelastic FEM analysis was conducted to evaluate the molding strain of reinforcements. Mechanical and thermal properties of resin as functions of degree of cure were measured and used for the simulation. It appeared that the calculated results agreed well with experimental results. Therefore, it was found that viscoelastic property, thermal expansion and curing shrinkage of resin play significant roles for cure process-induced strain of reinforcing fibers of FRP. The ratio of cure process-induced strain by curing shrinkage of the resin to whole molding process-induced strain was 17.7 %. This fact indicates that the effect of curing shrinkage of the resin should be taken into account for evaluation of molding process-induced strain of the reinforcements.

Adhesion Assessment of BNT Films on Titanium Substrates Using a Scratch Test

Lead-free piezoelectric (Bi1/2Na1/2)TiO3 (BNT) films were deposited on 0.2 mm thick pure titanium (Ti) substrates by a hydrothermal method. Scratch tests were performed to quantitatively assess the adhesion strength between BNT films and Ti substrates. Ti substrates were pretreated by chemical polish and mechanical polish respectively prior to BNT film deposition with a view of investigating the effects of substrate surface pretreatments on the adhesion of BNT film. In the scratch test, the behaviour of BNT film exfoliation was investigated by SEM observation. The scratch test results revealed that the chemical polish pretreatment effectively improved the adhesion of BNT films.

EVALUATION OF INTERFACIAL STRENGTH OF BNT FILMS HYDROTHERMALLY DEPOSITED ON TITANIUM SUBSTRATES

Lead-free piezoelectric (Bi1/2Na1/2)TiO3 (abbreviated as BNT) films were deposited on 0.2 mm thick pure titanium(Ti) substrates by a hydrothermal method. Scratch tests and Vickers indentation tests were performed to quantitatively assess the adhesion strength between BNT films and Ti substrates. Some of Ti substrates were pretreated by chemical polish and mechanical polish respectively prior to BNT film deposition with a view of investigating the effects of substrate surface pretreatments on the adhesion of BNT films. In the scratch test, the critical force was determined from the variations of the tangential force and the acoustic emission (AE) signals with the normal force. The scratch test results revealed that the chemical polish pretreatment effectively improved the adhesion of BNT films. In addition, the critical substrate strain inducing the adhesion failure of BNT films has been investigated by the Vickers indentation test combined with finite element analysis (FEM).