Key Engineering Materials Vols. 306-308

Abstract: It is important to estimate the intensity distribution of a magnetic field as a possible magnetic method in an industrial nondestructive evaluation application. A magnetic camera provides the distribution of a quantitative magnetic field with homogeneous lift-off and same spatial resolution. Magnetic flux leakage near a crack on the specimen can be amplified by using a 3- dimensional magnetic fluid, that is to say a magnetic lens. This study introduces the experimental consideration of the effects of magnetic lenses for concentrating of magnetic flux. The experimental results showed that the magnetic fluid has a sufficient lens effect for magnetic camera and effect of improvement in probability of detection.

Abstract: This study evaluates cracks in a ceramic ferrule. When an optical fiber is connected to machinery, a ferrule is used to precisely set the optical axes of an optical fiber. A ferrule is a cylinder of 10.5mm in length and 2.5mm in diameter. The crack size of a ferrule can range from 10.40
m to 32.35
m. An acceptable ferrule must have no cracks. In this paper, we predicted and examined defects by Resonant Ultrasound Spectroscopy (RUS). To verify the data obtained from the experiment, we carried out simulation by explicit finite elements method, and compared the experimental results with simulation results. The average error range is 1~2 percent between the experimental resonant frequency of the acceptable specimens and the simulated FEM resonant frequency. Also, we compared the resonant frequencies of specimens with a crack with those of acceptable specimens. The histogram drawn from these data and the available mode surveyed were used to classify the specimens in the plant as either cracked or non-cracked specimens.

Abstract: Damage detection using changes in global dynamic characteristics has been a hot research topic in recent years. In the present paper, based on natural frequencies and mode shapes, a numerical tudy locating and assessing damage in a circular cylindrical shell is presented. The axial position of the damage can be easily obtained by comparing the theoretical and measured frequency changes due to damage. Additional information of mode shapes is introduced to locate the exact damage position. And the damage extent can be estimated by the first order approximation method. The feasibility and practicality of the damage detection scheme are evaluated for several damage scenarios by locating and sizing damage in free-clamped shells. The results illustrate that the proposed detection scheme can confidently locate the single or multiple positions of damage. It is also observed that damage extent can be estimated with a relatively small error.

Abstract: We consider fracture growth controlled by fluid diffusion into the fracture. If the fluid is accumulated inside the fracture, after some incubation period, it starts growing under the pressure of the accumulated fluid. An important example is given by hydrogen induced cracking. Hydrogen absorbed by a metal is typically dissolved in the lattice in the proton form. Some of the protons reach the surface of a pre-existing or freshly created cracks where they recombinate with electrons and form molecular hydrogen in the crack cavity. Because usually the molecular form of hydrogen is thermodynamically more stable, this process leads to accumulation of gas hydrogen inside the crack. Then, the fracture often takes place even in the absence of any external loading, that is, only under the excessive hydrogen pressure. Another example is given by heating of water saturated rock. If due to the heat diffusion, the water evaporates/boils inside the cracks, the pressure exerted by the vapor can cause the cracks to propagate, increasing the crack sizes by orders of magnitude.

Abstract: This paper discussed about the method used by operator to perform damage tolerance analysis for repaired fuselage skin. Skin damage may be caused by accidental damage or fatigue crack. Some element needed for damage tolerance analysis and the method to develop load spectrum will be discussed briefly. Therefore, this paper will describe a case study of damage tolerance analysis on repaired fuselage skin of narrow body jet aircraft.

Abstract: A series of laboratory investigations concerned about fatigue assessment with acoustic emission method was presented. Fatigue aspects including cumulative fatigue damage, fatigue crack growth and creep-fatigue interaction were considered. As a basic approach, residual strength and acoustic emission characteristics of fatigue damaged materials were considered from the nominal stress-life (S-N) viewpoint. Acquired signal indicated that counts emission quantity can be a good measure of cumulated fatigue damage. In the fatigue crack growth approach, interrelationship between acoustic emission parameter and stress intensity factor was examined with different stress level and crack length. Experimental results were somewhat scattered since sensitive characteristics of acoustic emission method. However, their empirical relation indicated that counts rate correlated with fracture mechanics parameter. Finally, acoustic emission application was extended to the creep-fatigue interaction at elevated temperature. Emission response under each damage mode was compared and characterized. Based on these characteristics, creep-fatigue interaction was evaluated by use of acoustic emission parameter. Overall investigations concluded acoustic emission is very effective tool of fatigue assessment.

Abstract: The goals of the paper are to identify the impact damage and strength reduction behavior of sandwich structure, composed of carbon/epoxy laminates skin and Nomex core with two kinds of thickness (10 and 20mm). For these, low velocity impact tests were conducted using the instrumented impact-testing machine and damages are inspected by SAM. And then, subsequent static tests are conducted under flexural loading to identify the strength reduction behavior of the impacted sandwich structures. The impact damages are mainly delamination in carbon/epoxy skin and their behavior is mostly independent of core thickness. Also, their energy absorbing behavior is identified through calculating the energy absorbed by impact damage. Finally, the strength reduction behavior is evaluated through Caprino’s model, which was proposed on the unidirectional laminates.

Abstract: The laminated composite structures applied to the wing and the speed brake of an aircraft or the turbine blade of a compressor. These structures may be impacted by birds and hails during operation. They may also be impacted by drop of a tool during manufacture or repair. Unlike high velocity impact damage, which can be easily found by the naked eye, the damage due to low velocity impact may be difficult to detect. Damage which is not detected may cause failure of a structure and result in damage propagation. Growth of damage means reduction of stiffness on the structure. So, exact prediction of damage caused by a low velocity impact is very important in order to guard against sudden failure of the structure. In this study, modified delamination failure criterion has suggested in order to predict the failure behavior of a composite plate subjected to low-velocity impact. The criterion includes the assumption which is matrix cracking mode causes delamination failure. Predicted damage using supposed delamination criterion is similar to experiment results.

Abstract: CFRP (Carbon Fiber Reinforced Plastics) of the advanced composite materials as structural materials for vehicles, has a wide application in light weighs structural materials of airplanes, ships and automobiles because of high strength and stiffness. However, there is a design variable to be considered in practical application of the laminate composite material, that is, this material is vulnerable to transverse impact. This paper aims at evaluating the effects of curvature and oblique impact on the penetration characteristics of composite laminate shells. They are stacked to [02/902]S, [03/903]S and [02/903/0]S, [0/902/0]S and their interlaminar number two and four. Those specimens were prepared to varied curvature radius (R=100, 150, 200mm and ∞). They are cured by heating to the appropriate hardening temperature (130oC) by means of a heater at the vacuum bag of the autoclave. Test specimens were prepared with dimensions 100mm×140mm. When the specimen is subjected to transverse impact by a steel ball, the velocity of the steel ball was measured both before and after impact by determining the time for it to pass two ballistic-screen sensors located a known distance apart. In general, the critical penetration energy increased as the curvature increased, interface decreased and slope angle on the impact surface increased.

Abstract: An aluminum or CFRP (Carbon Fiber Reinforced Plastics) tube is representative light-weight materials but its axial collapse mechamism is different from each other. The aluminum tube absorbs energy by stable plastic deformation, while the CFRP tube absorbs energy by unstable brittle failure with higher specific strength and stiffness than those in the aluminum tube. In an attempt to achieve a synergy effect by combining the two members, aluminum/CFRP square tubes were manufactured, which are composed of aluminum tubes wrapped with CFRP outside aluminum square tubes with different fiber orientation angle and thickness of CFRP, and axial collapse tests were performed for the tubes. The crushing behavior and energy absorption capability of the tubes were analyzed and compared with those of the respective aluminum square tubes and CFRP square tubes. Test results showed that the collapse of the aluminum/CFRP square tube complemented unstable brittle failure of the CFRP square tube due to ductile characteristics of the inner aluminum square tube. The collapse modes were categorized into four modes under the influence of the fiber orientation angle and thickness of CFRP. The absorbed energy per unit mass, which is in the light-weight aspect, was higher in the aluminum/CFRP square tube than that in the aluminum square tube or the CFRP square tube alone.