Taketoshi Nojima

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.

Dynamic Strength of Ti-Alloys and Al-Alloys

Continuous compression tests and incremental strain rate change tests of four kinds of titanium alloys and four kinds of aluminium alloys are carried out within the strain rate range of 10–4 ∿103/s and the temperature range of -195°∿50°C. The experimental results reveal the effects of the strain rate and the temperature on the flow stress at 5% strain. The strain rate sensitivity obtained by the continuous tests or the incremental tests is also revealed. The thermal and the athermal components of stress are presumed and the relation between the activation volume and the thermal component of stress is discussed.

Manufacture of Arbitrary Cross-Section Composite Honeycomb Cores Based on Origami Techniques

In recent years, as space structures have become large and require higher accuracy, composite honeycombs, which can reduce weight and have low thermal expansion, are in increasing demand. As observed in the design of antenna reflectors and rocket bodies, both flat and 3D-shaped cores are used in this field. However, these special honeycombs have high manufacturing costs and limited applications. This study illustrates a new strategy to fabricate arbitrary cross-section honeycombs with applications of advanced composite materials. These types of honeycombs are usually manufactured from normal flat honeycombs by curving or carving, but the proposed method enables us to construct objective shaped honeycombs directly. The authors first introduce the concept of the kirigami honeycomb, which is made from single flat sheets and has periodical slits resembling origami. In previous studies, honeycombs having various shapes were made using this method, and were realized by only changing folding line diagrams (FLDs). In this study, these 3D kirigami honeycombs are generalized by numerical parameters and fabricated using a newly proposed FLD design method, which enables us to draw the FLD of arbitrary cross-section honeycombs. Next, the authors describe a method of applying this technique to advanced composite materials. Applying the partially soft composite techniques, folding lines are materialized by silicon rubber hinges on carbon fiber reinforced plastic. Complex FLD patterns are then printed using masks on carbon fabrics. Finally, these foldable composites that are cured in corrugated shapes in autoclaves are folded into honeycomb shapes, and some typical samples are shown with their FLDs.Copyright

Application of Conformal Maps to Origami-Based Structures: New Method to Design Deployable Circular Membranes

This paper presents a new method using conformal transformation to design crease patterns of circular membranes that can be wrapped up compactly. This method is focused on the advantages of origami that are packaged compactly and deployable at will, and enables to design complex deployable structures systematically and efficiently from simple structures, controlling angles among fold lines. Various deployable circular membranes are successfully produced by the method. They are wrapped up around the center of membranes and form structures such as regular polygons, rectangles, diamond shapes, etc. Circular membranes with zigzag fold lines to radial direction are also demonstrated. They are deployable along radial direction of membranes. The proposed method is flexible to generate zigzag fold lines, compared with the method by mirror image, since zigzag fold lines can be designed close to the center of membranes without geometrical constraints. For industrial application, models made of a plastic film, closing and a stainless steel plate are also demonstrated.Copyright

A Consideration on Common Sense in Proof Testing of Ceramic Materials. Analytical Discussion on Truth or Falsehood in Common Sense.

It has been accepted not only that effective proof testing of ceramic materials demands rapid loading and unloading to prevent strength degradation but also that especially rapid unloading brings truncated strength of σp (the stress just before very rapid unloading). Such common sense in proof testing as above has been rechecked with analytical considerations. The analysis reveals that as the loading and unloading rate increases, weaker samples tend to survive easily. As a consequence, the truncated strength is not characterized analytically, and Weibull curve will not have asymptotic feature. The present analytical results come to the conclusion that the common sense in proof testing will not be accepted from the physical point of view.

A New Punching Method in Intractable AFRP Laminated Composite by Press-Working.

The purpose of the present research is to develop a simple and convenient method of punching in intractable AFRP (aramid fiber reinforced plastics) laminated composites. An aramid plate is tightly restrained around the punched region as well as the bottom of the sample, and subsequently it is punched by a specially designed hollow punch. The present improved technique of press working has led to an excellent processing of intractable AFRP composites, which has never been achieved by any other technology such as water jet cutting or vibration press.

Fracture Toughness Behaviours of Rate Sensitive Materials Under Stable Crack Extension.

By assuming that fracture toughness (crack resistance force R) is expressed by constitutive relation R=R1(a)·(a^./a^.*)m (a and a^., crack length and crack velocity, respectively m, rate sensitivity parameter), the fracture behaviour under stable crack extension in a DCB specimen is analyzed for rate-sensitive materials. Analytical results show that an initial crack begins to extend before maximum load (Pmax), and the maximum value of R appears after Pmax. Alternate stable/unstable crack extension is obtained by introducing periodic disorder in R. The analytical work explains well the serrated P-δ curve which is often observed in delamination tests of laminated carbon composites.

A Study of Fracture Behaviours in Ceramic Plates by Impact Load and Its Application to a Development of Hole-Punching Technique

Sphere impact tests and static indentation tests were carried out in various kinds of boundary conditions to characterize fracture behaviours by point loading in brittle materials (a soda lime glass and an alumina) and also to investigate a possibility of hole punching in these plates. It was quite difficult to form a hole in these plates without serious radial cracks when the plates were not satisfactorily clamped. By precompressing the plates, punching works were performed in the glass and some engineering ceramics, and it has become clear that these plates can be well punched. The developed punching technique realized a wide range of punching from normal to quite small size holes less than 1mm as well as punching many holes by a single operation.

Pseudo Stable Phenomenon of Crack Extension in Three Point Bending Test for Soda-Lime Glass.

Recently, a concept of pseudo stable phenomenon (PSP) was proposed by theoretical considerations.The PSP is a phenomenon that crack extension occurs before the load reaches the maximum, Pmax, and it appears in conventional fracture toughness tests for crack-rate sensitive, perfectly brittle materials. The crack extension due to PSP depends on the initial crack length, a0, the testing machine compliance, Λ*, and the crack-rate sensitivity, m*, of crack resistance force. In the theoretical considerations, a method of numerical simulation was proposed by Nojima and Nakai.In the present paper, a series of three point bending tests was carried out on soda-lime glass to examine the PSP experimentally. For the tests, a testing equipment with large machine compliance and specimens with different initial crack lengths were used. The experimental data were compared with the results of numerical simulation. It was found by both experiments and numerical simulations that the amount of crack extension due to PSP becomes maximum when x0≈0.16 (x0=a0/W, W: specimen width) and that the fracture toughness obtained from Pmax, and x0 is always understimated.

Impact. Slow Impact Punching in Brittle Materials.

In order to develop an impact punching technique in brittle materials, slow impact punching tests were carried out in a soda lime glass and some engineering ceramics by using a specially designed clamping device made of a frame and a compact powerful jack. Punching works were performed by using 3 types of punches; the works by conical and spherical punches are successfully carried out without serious radial cracks around the punched hole, while by flat punch, not only the radial cracks are often produced in thick plates, but the load to form a hole is quite large. Therefore, the conical and spherical punches are adopted to investigate a possible range of punching. Although hole formation load depends upon the top radius R of the punch in thick plate, the effect of R is quite small in thin plate. This trend is explained very well by the formation of a Hertzian ring crack and its growth.The load to form a hole is very high in impact punching and the rate effect clearly appeared, but no characteristic feature of impact effect has been found in the observation of punched plate.