Yuuta Aono

Damage mechanism of hybrid composites with nonwoven carbon tissue subjected to quasi-static indentation loads

Damage mechanisms of hybrid composites with nonwoven carbon tissue (NWCT) are investigated under a quasi-static indentation loading condition with a steel ball as an indentor. The damage level corresponds to an initial stage of damage due to an indentation loading, which leads to a strength degradation of the composite. The hybrid composites consist of the CFRP prepreg and the NWCT prepreg, which are made byinterleaving the NWCT prepreg into the CFRP layer interfaces. Four kinds of cross-plylaminates are studied; i.e., CFRP laminates ([05/905/05] and [03/903/03/903/03]) and hybrid laminates ([05//905//05] and [03//903//03//903//03]). Here, the symbol “//” means that the NWCT prepreg is located at an interface between CFRP layers. The hybrid effect is examined by investigating the delamination area and the absorption energyof two cross-plylaminates. As it has been recognized that the quasi-static indentation damage agrees well with the low velocity impact test results, the present static test results can then be used to understand the damage mechanism of low-energyimpact. The damaged parts of the specimens were observed bythe deplytechnique and the cut method which used a microscope. The NWCT interleaving’s effect was clarified based on the observation of the damage for the hybrid specimen’s cross-sections after indentation testing. The delamination area of the hybrid composites was reduced to about a half of the CFRP’s byindentation loading. It was clarified that the Mode II interlaminar fracture is predominant in the delamination bythe impact.

Method for assessing applicability of an artificial flaw as a small initial crack for fatigue limit evaluation and its application to a drill hole and an FIB processed sharp notch in annealed 0.45% carbon steel

In this study, we propose a method for assessing the applicability of an artificial flaw as a small initial crack for fatigue limit evaluation. The proposed method is applied to drill holes and sharp notches introduced using a focused ion beam (FIB) technique in annealed 0.45% carbon steel. It is found that under rotating bending fatigue, an FIB notch can be used as a small initial crack for fatigue limit evaluation, whereas a drill hole cannot, for √area of ∼50 μm. Here, √area is the square root of the area obtained by projecting the defect onto a plane perpendicular to the load axial direction. The results indicate that an FIB notch can be used as a small initial crack for fatigue limit evaluation in a greater number of materials than those in which a drill hole can be used.

Fatigue Characteristics of Vulcanized Natural Rubber for Automotive Engine Mounting

In order to investigate fatigue characteristics of vulcanized natural rubber (NR), fatigue tests are carried out under various stress ratios R (R = minimum stress / maximum stress). It was considered that the fatigue cracks were initiated from flaws in very early stage of total life. The fatigue damage process was almost the fatigue crack propagation process and it is independent of R. The crack growth rate was proportional to the crack length to about the first power, when the crack length was defined as the length of the direction perpendicular to the loading direction. Miner’s rule was examined to observe the fatigue crack behavior and checked by using two-step loading fatigue tests experimentally. It seems Miner’s rule has a possibility to predict fatigue lives.

Residual Compressive Failure Characteristics of Hybrid Composites with Nonwoven Carbon Tissue after Indentation Damage

The purpose of this study is to investigate the relationship between the low-energy impact damages under static indentation loadings and the residual compressive static and fatigue strengths after damage in hybrid composites with nonwoven carbon tissue (NWCT). The hybrid laminates are made by interleaving the NWCT layer between the CFRP layer interfaces. The indentation damages were discussed by investigating the delamination areas and the indentation energies of four kinds of angle-ply laminates ([455/ 455/455] and [453/ 453/453/ 453/453] CFRP laminates, and [455// 455//455] and [453// 453//453// 453//453] hybrid laminates; the symbol “//” means that the NWCT layer is located at an interface between the CFRP layers. The delamination areas of the hybrid laminates were reduced to about half of the CFRP’s by the indentation loading (Lee, S.H., Aono, Y., Noguchi, H. and Cheong, S.K. (2003). Damage Mechanism of Hybrid Composites with Non-Woven Carbon Tissue Subjected to Quasi-Static Indentation Loads, Journal of Composite Materials, 37: 333–349). After the indentation damage, compressive static and fatigue tests are carried out using an anti-buckling guide for the angle-ply laminates. The failure modes of the angle-ply CFRP and hybrid specimens are caused by global buckling under the compressive static loading. However, the failure modes of the angle-ply CFRP and hybrid specimens at Nf <10 6cycles are caused by shearing the damaged part under the compression–compression fatigue loading. Compared with CFRP laminates, compressive fatigue lives of the hybrid laminates were significantly extended in all the stress ranges. The NWCT interleaving effect is examined based on the observations of the damage in the hybrid specimens after the indentation damage. Also, the compressive static and fatigue failure processes are discussed from the observation of the fracture surfaces of damaged parts and failed appearance of specimens when subjected to the compressive loadings after the indentation damage.

Fatigue characteristics of vulcanized natural rubber for antivibration (2nd report, A study on the fatigue life prediction under uniaxial variable loading)

In order to investigate fatigue characteristics of vulcanized natural rubber (NR), fatigue tests are carried out under various stress ratios R and variable loading. In this paper, fatigue crack propagation life is predicted depending on fatigue crack propagation behavior, because the fatigue life is almost the fatigue crack propagation life. Two essential assumptions for the application of Miners rule are satisfied. It is considered that Miners rule can predict the fatigue life safely under two-step loading, because the fatigue damage location is depending on R. Under complex variable loading, it is considered that the crack growth rate under 0<R<1 condition is accelerated because of disappearance of crystallization effect. So, it is necessary to make up the modified constant life diagram. The fatigue crack propagation life can be predicted using the modified constant life diagram and Miners rule.

Prediction of the Fatigue Limit of Prestrained Carbon Steel Under Tensile Mean Stress

The effect of tensile mean stress on the fatigue limit of carbon steel was investigated. Annealed 0.1 % and 0.5 % carbon steel specimens were subjected to monotonic tension to produce prestrain. Fatigue tests were performed using a servohydraulic fatigue test machine. The experimental results showed that tensile mean stress degraded the fatigue limit. However, the tensile mean stress did not affect the decrease in the surface hardness as a result of fatigue loading for the specimen without a defect. In the case of the specimen with a defect, the decrease in the hardness was small and did not depend on the mean stress or stress amplitude. Formulae for predicting the fatigue limit of prestrained carbon steel specimens under tensile mean stress were proposed. The results calculated using the formulae showed good agreement with the experimental results.

Effect of Prestrain on Fatigue Crack Growth of Age-Hardened Al 6061-T6

The effect of prestrain on fatigue crack growth characteristic (FCGC) of age-hardened Al 6061-T6 was investigated. Al 6061-T6 specimens were subjected to tension to produce plastic strain. Extruded bars were prepared to investigate FCGC. Specimens with an artificial hole from the bar were used to observe Mode I fatigue crack. On the other hand, coarse-grained plain specimens were used and observed for Mode II fatigue crack. Fatigue tests were performed under a rotating bending load (R=−1). The experimental results showed the Mode I fatigue crack growth rates decelerated a little due to prestrain. However, in the case of Mode II, fatigue crack growth rates of prestrain specimens accelerated about ten times due to prestrain. The Mode II fatigue crack easily propagates on the slip bands, because precipitation particles on the slip bands are sheared by prestrain. The result means the influence of prestrain on Mode II fatigue should be taken into account for damage-tolerant design.

New Stress Analysis of Plate Structure by Boundary Integral Method.

A boundary integral method is newly applied to a plate structure, where the structure is divided to some plates and each plate is dealed with a thin plate under in-plane loading and bending loading. Although each plate is like a plate-bending element of FEM, it should not be divided into small elements any more by using BFM and the calculation is more effective than that by FEM. This method is a approximate method, (because it is based on Kirchhoffs hypothesis). Therefore the stress state of the bonded part of the plates is not exactly satisfied. However adequate solution can be obtained with very little input data.