Takayuki Kusaka

Rate dependence of mode I fracture behaviour in carbon-fibre/epoxy composite laminates

Abstract The rate dependence of mode I interlaminar fracture behaviour in unidirectional carbon-fibre/epoxy composite laminates has been investigated over a wide range of loading rates from quasi-static (displacement rate, δ = 0.01–500 mm min−1) to impact (δ = 5–20 mm see−1) at room temperature. Impact fracture tests were performed by the WIF (wedge-insert-fracture) method with a SHPB (split Hopkinson pressure bar) system for accurate measurement of impact fracture toughness, while quasi-static fracture tests were performed by the DCB (double-cantilever-beam) method with a screw-driven testing machine. In the present composite laminates, the fracture toughness decreased stepwise with increasing loading rate showing a distinct rate-sensitive transition region and two rate-insensitive regions above and below. As a consequence of this stepwise characteristic, the crack growth behaviour varied with loading rate: in and below this transition region, the crack grew unstably accompanied by high-speed propagation and arrest; but above the transition region, the crack grew stably and continuously. This trend was well explained by a simple model incorporating the rate dependence of fracture toughness and the contribution of kinetic energy in the specimen during unstable crack propagation.

The Development of High Performance Ti-6Al-4V Alloy via a Unique Microstructural Design with Bimodal Grain Size Distribution

The present work deals with the strengthening of Ti-6Al-4V alloy by creating a unique microstructure with bimodal grain size distribution, termed as “harmonic structure.” The Ti-6Al-4V compacts with harmonic structure design were successfully prepared via a powder metallurgy approach consisting of controlled mechanical milling and spark plasma sintering of the pre-alloyed Ti-6Al-4V powders. The microstructural evolution at each stage of processing has been investigated to establish a correlation between the processing conditions and the microstructural evolution. The Ti-6Al-4V compacts with heterogeneous harmonic structure exhibited better mechanical properties as compared to their homogeneous fine/coarse-grained counterparts. An attempt has also been made to explain the deformation mechanism of the harmonic-structured Ti-6Al-4V specimens with the help of the experimental evidences. The superior mechanical properties of the harmonic structure Ti-6Al-4V were found to be related to the peculiar topological distribution of strong fine-grained and ductile coarse-grained regions, which promotes uniform distribution of strain during plastic deformation and results in improved mechanical properties by avoiding the localized plastic deformation in the early stages of deformation.

Experimental characterization of dynamic crack growth behavior in CFRP adhesive interface

The dynamic crack growth behavior of adhesively bonded joints under mode I and mixed mode (I + II) loading were investigated. The split Hopkinson pressure bar (SHPB) apparatus and the digital image correlation (DIC) technique were employed to determine the mode I fracture toughness of the adhesively bonded joints during crack propagation under impact loading. The dynamic crack growth behavior for carbon fiber reinforced plastics (CFRP) adhesively bonded joints under mode I loading was studied using this method. In order to verify the proposed method, the dynamic crack growth behavior of titanium alloy adhesively bonded joints was also studied. Moreover, the crack growth behavior of CFRP adhesively bonded joints under mixed mode loading was studied using the SHPB technique. For the considered CFRP adhesively bonded joints, the fracture toughness decreased under both mode I and mixed mode loading as the loading rate increased. Microscope observation showed that a shift in the crack location occurred in the high loading tests.

Dynamic Compression Test of CFRP Laminates Using SHPB Technique

A novel experimental method was proposed for characterizing the compressive properties of composite materials under impact loading. Split Hopkinson pressure bar system was employed to carry out the dynamic compression tests. The dynamic stress-strain relations could be precisely estimated by the proposed method, where the ramped input, generated by the plastic deformation of a zinc buffer, was effective to reduce the oscillation of the stress field in the specimen. The longitudinal strain of gage area could be estimated from the nominal deformation of gage area, and consequently the failure process could be grasped in detail from the stress-strain relation. The dynamic compressive strength of the material was slightly higher than the static compressive strength. In addition, the validity of the proposed method was confirmed by the computational and experimental results.

A study on introduction of notch into thin-walled polygonal shell member to control plastic buckling behaviour in axial collapse

Focusing upon finding an optimum shape of crash box, which is one of the most important part of an automobile body to absorb impact energy at the time of a collision, for ensuring large energy absorption, the effect of introducing notch into a crash box on controlling plastic buckling behaviour was examined by three-dimensional finite element analyses. The influences of notch shape and arrangement on the deformation mode of a polygonal crash box was numerically examined and an optimum range of the dimensions of notch was quantitatively determined for ensuring stable buckling deformation. As a result, a fundamental scheme to optimise the shape and arrangement of the notches to ensure large energy absorption was proposed, which is applicable to a crash box with optional cross-sectional shape.

Dynamic Interlaminar Fracture Toughness of CFRP Composite Laminates

New experimental techniques were devised for determining dynamic mode I, moode II and mixed mode interlaminar fracture toughness of CFRP laminates. Up to the impact velocity of 10–20m/s, rate effects on interlaminar fracture toughness of each mode were investigated and compared with the quasi static results. Test results show that critical energy release rate decreases with rate in mode II fracture and increases with rate in mixed mode, however difference must be still investigated from the view point of the difference of mode and also the difference of crack initiation or propagation.

Damage localization method for plates based on the time reversal of the mode-converted Lamb waves.

HighlightsDamage localization method by the time‐reversed Lamb waves is proposed for plates.Mode conversion behavior of Lamb waves is considered in the time reversal process.Localization of a notch‐type defect in plates is numerically simulated.Focused spots appear at the damage location by the proposed time reversal method. ABSTRACT A damage localization method based on the time reversal focusing of the mode‐converted scattered Lamb wave is proposed for plate structures with a non‐symmetric defect in the thickness direction. Dual transducers are attached symmetrically on the upper and lower surfaces of the plate to selectively emit and receive the lowest‐order symmetric (S0) and antisymmetric (A0) modes. The localization of damage is achieved by the numerical time‐reversed (TR) simulation of the mode‐converted Lamb wave generated at the defect. To investigate the validity of the proposed method, the signals of the Lamb waves in a plate with a partial‐thickness notch are numerically simulated by the three‐dimensional elastodynamic finite integration technique (EFIT). When the S0 mode is emitted in the damaged plate, not only the S0 mode is scattered but also the A0 mode is generated due to mode conversion at the notch. Similar mode conversion behavior is confirmed when the A0 mode is emitted. The time reversal of the mode‐converted scattered Lamb waves creates focused spots at the damage location without using baseline data for the undamaged plate. The proposed method reduces the magnitude of the artifacts compared to the time reversal of the non‐mode‐converted Lamb wave, and yields the focused spot whose size is associated with the size of the notch and the half wavelength of the time‐reversed wave mode. Furthermore, the proposed method is applied to a plate with a notch‐type defect adjacent to an a priori known through‐thickness hole, demonstrating the damage localization in a relatively complicated structure.

Numerical study of the second harmonic generation of Lamb waves at an imperfect joint of plates

Lamb waves are attracting significant attention in the nondestructive evaluation for plate structures due to their capability to propagate long distances. In this study, the nonlinear interaction of Lamb waves with an imperfect joint of elastic plates is numerically investigated. In particular, the second harmonic generation behavior from the joint is examined by perturbation analysis. The imperfect joint is modeled as a spring-type interface with quadratic nonlinearity, and the perturbation analysis is carried out using the hybrid finite element method (HFEM). For the incidence of the lowest-order symmetric (S0) Lamb mode below the cut-off frequencies of the higher-order modes, the double-frequency S0 mode is generated from the imperfect joint due to the nonlinear interaction. A nonlinear parameter calculated from the amplitude of the second harmonic S0 mode shows a sharp peak at a certain incident frequency. The peak frequency increases with increasing interfacial stiffness, and the double of the peak frequency corresponds to the resonance frequency of the imperfect joint subjected to the S0 mode incidence. This result shows that the resonance of the imperfect joint leads to the amplification of the second harmonic S0 mode.Lamb waves are attracting significant attention in the nondestructive evaluation for plate structures due to their capability to propagate long distances. In this study, the nonlinear interaction of Lamb waves with an imperfect joint of elastic plates is numerically investigated. In particular, the second harmonic generation behavior from the joint is examined by perturbation analysis. The imperfect joint is modeled as a spring-type interface with quadratic nonlinearity, and the perturbation analysis is carried out using the hybrid finite element method (HFEM). For the incidence of the lowest-order symmetric (S0) Lamb mode below the cut-off frequencies of the higher-order modes, the double-frequency S0 mode is generated from the imperfect joint due to the nonlinear interaction. A nonlinear parameter calculated from the amplitude of the second harmonic S0 mode shows a sharp peak at a certain incident frequency. The peak frequency increases with increasing interfacial stiffness, and the double of the peak f...

Experimental study of size effect on quasi-static strengths for short glass fibre-reinforced plastics

The present paper examines size effect on the strength of short glass fibre-reinforced phenolic resin (SGP) composites made by press moulding with different loading modes and specimen shapes. Three- and four-point flexural tests and tension–torsion combined tests were conducted at room temperature in order to evaluate the influence of Vf and loading mode on fracture strength. The obtained uniaxial strength data were analysed using the Weibull statistical theory. The relationship between fracture strength and effective volume was investigated based on the Weibull statistical theory and agreed well with the effective volume theory (EVT), regardless of specimen size, dimensions or loading mode. The experimental results revealed that the tension–torsion multiaxial SGP strength was in agreement with the Tsai–Hill failure criterion. The EVT was also applied to the Tsai–Hill failure criterion to consider the size effect, and the validity of the proposed method was confirmed experimentally.

Effect of Material Composition on Impact Energy Absorbing Capability of Composite Laminates

A novel experimental method was proposed for characterizing the energy absorbing capability of composite materials during the progressive crushing process under impact loading. A split Hopkinson pressure bars system was employed to carry out the progressive crushing tests under impact loading. The stress wave control technique was used to avoid the inhomogeneity of dynamic stress field in the specimen. The progressive crushing behavior was successfully achieved by using a coupon specimen and anti-buckling fixtures. With increasing strain rate, the absorbed energy during the crushing process slightly decreased, whereas the volume of the damaged part clearly increased regardless of material type. Consequently, the energy absorbing capability decreased with increasing loading rate. The effects of material composition, such as fiber type, matrix type and fabric pattern, on energy absorbing capability were also investigated by using the proposed method.