Masaharu Iwamoto

Wavelet transform of acoustic emission signals in failure of model composites

Abstract The fundamental characteristics of acoustic emission (AE) signals, such as the attenuation, and frequency dependency of AE signals, were investigated and the fracture process of the single fiber composite (s.f.c.) was examined. As a result, the frequencies of AE signals were almost unchanged, while the amplitudes attenuated greatly with the increment of the propagation length. This proved that the frequency analysis is an effective way in processing AE signals of composite materials. In the fracture process of the s.f.c., the number of AE events was in a good agreement with the number of fiber breakages, and the sources of AE signals were the failure modes at fiber breakages. Using the proposed time–frequency method of wavelet transform (WT) to process AE signals, the microfailure modes at a fiber breakage and the microfracture mechanism, such as the sequence of each failure mode and their interaction, were made clearer. These indicated that both processing methods of AE signals, fast-Fourier transform and WT, were powerful for identifying the microfailure modes and for elucidating the microfracture mechanisms in composite materials.

Intralaminar fracture mechanism in unidirectional CFRP composites: Part I: Intralaminar toughness and AE characteristics

Abstract Three types of representative carbon fiber reinforced unidirectional composite materials were used and their intralaminar fracture behavior was investigated using the double-cantilever beam specimen with a simultaneous acoustic emission measurement. The intralaminar fracture toughness was evaluated by both the compliance method and energy area method. As a result, it was found that the intralaminar fracture toughness without bridging fibers had a constant value during crack propagation but it increased greatly when bridging fibers were present. The effect of bridging fibers on the intralaminar fracture toughness was estimated quantitatively by cutting the bridging fibers. Distinct differences in load–displacement curves, compliance, crack propagating behavior and acoustic emission signal characteristics between these three types of unidirectional composite materials were observed. It was also found that bridging fiber failure generated relatively large power spectra and contributed to the peak frequencies of 600–700 kHz in the spectrum analysis of acoustic emission (AE) signals. This suggested that the bridging fibers were also an important source of AE signals. Furthermore, a linear relationship between crack length and normalized cumulative AE event count rate was obtained.

Energy-absorption properties of braided composite rods

Braiding has been attracting a great deal of attention as a technique for manufacturing preforms for composite materials. It has been developed as an integrated braiding technique which permits the fabrication of near-net-shaped preforms for 3-dimensional composite materials with reinforcing fibers oriented continuously. A braided rod is a simple unit of a 3-dimensional near-net-shaped composite. From this point of view, crushing tests have been performed in order to evaluate the braided rods. The specific energy absorption (Es) values of this material were higher than that of glass-cloth/epoxy tube. In this paper, we have tried to apply a flexible interphase concept to braided rods in order to obtain superior energy absorption properties. Braided composite rods with a flexible interphase also showed the characteristic crushing mode called progressive crushing. Consequently, the specific energy absorption of the rod with a flexible interphase was 30% higher than that of a composite without a flexible interphase. The fracture mechanisms of both materials are discussed with reference to microscope observations of crushed samples.

Conductivity stability of carbon nanofiber/unsaturated polyester nanocomposites

Carbon nanofiber (CNF)/unsaturated polyester resin (UPR) was prepared by a solvent evaporation method, and the temperature dependency of electrical conductivity was investigated. The CNF/UPR composites had quite a low percolation threshold due to CNF having a larger aspect ratio and being well dispersed in the UPR matrix. The positive temperature coefficient (PTC) was found in the CNF/UPR composites and it showed stronger effect around the percolation threshold. The electrical resistance of the CNF/UPR composites decreased and had lower temperature dependency with increasing numbers of thermal cycles.

Creep and Cyclic Mechanical Properties of Composites Based on Shape Memory Polymer

Recently, shape memory polymer (SMP) as one of the functional materials has received much attention and its mechanical properties have been investigated. Shape memory polymer of polyurethane series has a glass transition temperature (Tg) around room temperature. According to the large change in modulus of elasticity above and below Tg, the SMP material has excellent shape memory effect. In this study, the glass fiber reinforced shape memory polymer was developed for the improvement of the mechanical weakness of SMP bulk and for wider applications in the fields of industry, medical treatment, welfare and daily life. The specimens with different fiber weight fractions were fabricated and their creep and cyclic behavior were investigated experimentally. It is confirmed that the fiber reinforcement fibers could be used to improve creep properties, such as the rupture strain and rupture time, for the SMP composites. In addition, the resistance to cyclic loading in the developed composites with SMP was clearly improved due to reinforcement fiber. K e y w o r d s : Shape memory polymer, Composite, Creep property, Cyclic behavior, Fiber weight fraction

Intralaminar fracture mechanism in unidirectional CFRP composites — part II: analysis

Abstract Three kinds of representative carbon fiber reinforced unidirectional composite materials are used, and their intralaminar fracture behavior is investigated by using the double-cantilever beam (DCB) specimen with a simultaneous acoustic emission measuring. In Part I, the experimental results on the crack propagation, the bridging fibers, the intralaminar fracture toughness acoustic emission characteristics and microscope observations were obtained. Here, we use a bridging fiber model to analyze the debonding force acting on a bridging fiber and try to estimate the number of bridging fibers during the crack propagating process. At the same time, the intralaminar fracture toughness is calculated by both the adhesive force model and the finite element analysis. As a result, it is found that the intralaminar fracture toughness without the bridging fibers will have a constant value during the crack propagation, but it increases greatly when bridging fibers exist. It is clear that the bridging fibers play an important role in the intralaminar fracture toughness. The debonding forces acting on the bridging fibers and the number of bridging fibers are obtained. Furthermore, the quantitative estimation of the increment of the intralaminar fracture toughness contributed by bridging fibers is made according to the adhesive force model and it is comparable with the results obtained by the finite element analysis.

Mechanical Properties of Flat Braided Composites With a Circular Hole

In this study, fracture behavior and strength in the flat braided bar with a circular hole were investigated by static and fatigue test. Two types of specimen were prepared. They are a braided flat bar with an integrally formed braided hole and a braided flat bar with a machined hole. Moreover, we also examined a specimen that had a metal pin inserted at the circular hole. This specimen was subjected to a static tensile test. The results of the tensile tests indicate that the strength of the flat bar with a braided hole was larger than that of the one with the machined hole. Furthermore, from the results of the fatigue tests, the flat bar with the braided hole showed higher fatigue property than that of the one with the machined hole.

Damage evaluation of sheet moulding compound composite by acoustic emission

Abstract The acoustic emission technique is applied to detect and assess damage in notched specimens made of short fiber-reinforced sheet moulding compound (SMC) composite. Acoustic emission (AE) counts and energy are identified with damage growth ahead of the notch tip. Stress intensification over a local region can be measured by the factor K 1 , while unstable growth is assumed to correspond with the critical condition of an inner region. The former is referred to as the primary-damage zone and the latter as the secondary-damage zone. AE data, correlated with the sizes of the damage zones and intensities of the notch tip stress field, are presented.

Shear Buckling Analysis of Angle-Ply Laminates with Higher- Order Shear Deformation and pb-2 Ritz Functions

In the present paper, a total potential energy approach has been employed to study the buckling behavior of symmetric and asymmetric angle-ply laminates subjected to shear loading. The pb-2 Ritz functions, which can represent an arbitrary edge condition, were introduced, and at the same time a higher-order out-of-plane shear deformation was taken into account. Thus, the proposed analysis is of wider use in bucking analysis of laminated composite plates. As a result, it is confirmed that the proposed analysis was an effective one to clarify shear buckling behavior of both symmetric and asymmetric laminates. Higher precision for buckling loads was obtained due to the consideration of the higher-order shear deformation theory. Furthermore, the buckling loads and buckling shape were discussed by changing the parameters, such as boundary conditions, material anisotropy, plate thickness, stacking sequence, load direction and so on. The 3D illustrations of buckling modes obtained indicated a useful prediction of the buckling characteristics.

Ring-opening addition reaction of cyclopropanol derivatives with carbenes

Abstract Chloro(phenyl)carbene and diphenylcarbene reacted with cyclopropanone hemiacetals, cyclopropanols, and cyclopropanone cyanohydrins to give 4-phenylbutanoic acid derivatives. The mechanism was explained in terms of a homolytic process that the O-H group of cyclopropanols reacted as a formal H donor with carbenes.