Key Engineering Materials Vols. 326-328

Abstract: In this study, a new type of surface carburizing process was introduced using superplastic duplex stainless steel (DSS). The superplastic DSS was carburized at temperatures ranging from 1123 K to 1223 K for various durations. Initial pressures of 25 MPa, 49 MPa and 74 MPa were applied to give the superplastic deformation effect on the carburized specimens. SEM studies revealed a thick, uniform, smooth and dense hard carbon layer was formed on the surface of the superplastic DSS. By using metallographic technique and SEM, the resulting case depth of carbon layer was between 15 /m to 76 /m. The kinetics of this process in terms of carbon diffusion and its variation with processing time and temperature was determined using Arrhenius equation. Activation energy (Q) was determined as 152 kJ/mol.

Abstract: Functionally Graded Materials (FGMs) are composite materials that have continuous material variation along with geometry. This paper introduces a method for FEA-based design and layered manufacturing (LM) of FGMs. An FGM solid model is first created by referring to the libraries of primary materials and composition functions. The model is then discretized into an object model onto which appropriate material properties are mapped. Next, the object model is adaptively meshed and converted into an FE model. FEA using ANSYS is finally performed to estimate stress levels. This FEA-based design cycle is repeated until a satisfactory solution is obtained. The object model is then fed to the fabrication system where a process planning is performed to create instructions for LM machines. As a laser-based LM method, Direct Metal Deposition (DMD) at the University of Michigan is briefly described. A specific example (FGM pressure vessel) is shown to illustrate the entire FEA-based design and DMD fabrication cycle.

Abstract: Tensile tests of two-dimensionally braided C/SiC composites and three-dimensionally braided C/SiC composites had been carried out at room temperature. Some specimens had been unloaded during experiments. Acoustic Emission signals also had been collected during experiments. The following conclusions were arrived. The stress-strain curves of these two materials were of nonlinear characters, and there were no obvious linear segments on those curves. Failure characters of these two materials were different: There appeared ply pull-out for 2D braided C/SiC specimens and there appeared zigzag shape for 3D braided C/SiC specimens. Stress-strain curves of loading-unloading tests and Acoustic Emission signals of those two materials showed damage evaluation during tests. There were different AE counts and AE energy characters between two materials.

Abstract: Air-coupled ultrasound is a non-contact technique and has obvious advantages over water-coupled experimentation. Especially, wood materials are very sensitive to water and inspection without any coupling medium of a liquid is really needed to wood materials due to the permeation of coupling medium such as water. In this work, it is desirable to perform contact-less nondestructive evaluation to assess wood material homogeneity. A wood material was nondestructively characterized with non-contact and contact modes to measure ultrasonic velocity using automated data acquisition software. We have utilized a proposed peak-delay measurement method. Also through transmission mode was performed because of the main limitation for aircoupled transducers, which is the acoustic impedance mismatch between most materials and air. The variation of ultrasonic velocity was found to be consistent to some degree with those in infiltrated area for air-coupled and conventional scan images. Through-transmission scans for aircoupled and conventional UT were used based on both amplitude and time-of-flight images. Finally, results using a peak-delay measurement method well somewhat corresponded to ultrasonic velocities of the pulse overlap method.

Abstract: It is a well known fact that the fundamental causes of most failures in composite structures are in the forms of incipient damages such as delaminations and cracks which usually remain undetected until they grow to levels large enough to cause failure. In this study, unidirectional carbon/epoxy composite plates with known defects are investigated. The known defects are generated by impacting the composite specimens simulating external collision. A pair of transmitter and receiver was used for generation of Lamb waves and reception of signals. The received signals were monitored by scanning the receiver toward internal defect or by scanning both transmitter and receiver with confined distance over the surface of the composite plates which have known defects lie beneath them. The proper selection of incident angle and frequency are also considered. The characteristics of received signals such as amplitude, energy and wave form are analyzed. The acquired information is used to locate and to measure the size of the impact damage. The suggested method is very effective if the internal damage is presented closed to surface of the plate where the conventional pulse-echo method has problems. The proposed technique can be used widely for the real time and online monitoring of composite structures.

Abstract: The embedded structural health monitoring system is envisioned to be an important factor of future structural systems. One of the many attractions of in situ health monitoring system is its capability to inspect the structural systems in less intrusive way over many other visual inspections which require disassembly of built up structures when some indications have appeared that damages have occurred in the structural systems The vacuum assisted resin transfer molding (VARTM) process is used to fabricate woven-glass/phenol composite specimens which have the PZT sensor array embedded in them. The embedded piezoceramic (PZT) sensors are used as both transmitters and receivers. A damage identification approach is developed for a woven-glass/phenol composite laminates with known localized defects. Propagation of the Lamb waves in laminates and their interactions with the defects are examined. Lamb waves are generated by the high power ultrasonic analyzer. A real time active diagnosis system is therefore established. The results obtained show that satisfactory detection accuracy could be achieved by proposed method.

Abstract: A performance evaluation of plate-type piezoelectric composite actuators (PCA) having different lay-up sequences was experimentally carried out at simply supported and fixed-free boundary conditions. The actuating displacement of the manufactured PCAs was measured using a non-contact laser displacement measurement system. It was shown that the actuating displacement with increasing applied electric field at a drive frequency of 1 Hz increased nonlinearly at the simply supported boundary condition whereas it almost linearly increased at the fixed-free boundary condition. In contrast, the actuating displacement of the PCAs depended on the applied electric fields in a drive frequency range from 1 Hz to 10 Hz. However, the displacement behavior of PCAs varied significantly at a higher range of drive frequency, i.e., beyond 15 Hz, due to the occurrence of resonance. On the basis of these experimental results, the bending characteristics of PCAs in relation to applied electric field, drive frequency, and boundary conditions were elucidated.

Abstract: The bridge deck is the most vulnerable element in the bridge system because it is exposed to direct actions of wheel loading, chemical attacks, and temperature and moisture effects including freezing and thawing, shrinkage, humidity, etc. In 1980’s, several countries, such as USA, Japan, and Canada, already realized that the service life of the deck is critical for that of the whole bridge, and the research was initiated to develop new material and structural system to improve deck behavior. In recent years, it has been encouraged to develop more durable, easily constructible, and more cost effective bridge deck than the current one in Korea. In this study, a concise state-ofthe- art survey of the experimental investigations on the FRP-concrete composite bridge deck under developing in Korea is presented.

Abstract: In order to study the strength and deformation properties of rock joint under different shear velocities, normal stresses and undulation angles, series testes are conducted by a RMT-150C testing machines with artificial concrete rock joint samples in the present paper. Base on the experimental results, it can be found that the peak shear strengths decrease with the increment of shear velocity, and the decreasing rates tend to decrease with the increasing shear velocity. The shear strength of rock joints increase with the increasing undulation angles, and linearly increase with the increment of normal stress. It is also indicated that the shear stiffness increase with the increasing normal stress, undulation angle as well as the shear velocity with a decreasing tendency.

Abstract: Five methods are used to measure values of circumferential Young’s modulus and three methods are used to obtain shear modulus using the closed and open ring specimens. Based on the assumption that the technique using a strain gage must yield the correct value of Young’s modulus, the technique using a closed ring specimen gives more consistent results than the technique using an open ring specimen. For the simplicity of measurement, the technique using a closed ring specimen under compression can be used as long as the thickness of the used specimen remains thin enough to neglect the effects of shear deformation. To measure the shear modulus, G12, both techniques using the open and closed ring seems to have the same trend as a function of thickness. As expected from the loading scheme, the technique using an open ring gives more consistent results for the shear modulus.