Kozo Okita

Fatigue Life Estimation of Adhesively Bonded Lap Joints

This paper describes a method of estimating the fatigue life of adhesively bonded lap joints on the basis of the stress analysis in adhesive layer with finite element method. First, cyclic tensile fatigue tests were conducted for adhesively bonded lap joints with different lap length and adhesive layer thickness. The results were evaluated from the viewpoint of the maximum values of both tensile and shear stress obtained numerically, instead of the apparent stress. Then these standardized fatigue strength were compared with those of adhesively bonded butt joints of a thin wall tube under cyclic tensile and fully reversed torsional load conditions. The results indicate that fatigue strength of lap joints evaluated from the maximum tensile stress of the adhesive layer agrees well with the fatigue strength of adhesively bonded butt joints of thin wall tube under cyclic tensile load condition. It is confirmed that fatigue strength of lap joints can be estimated adequately based on the fatigue strength of the butt joint of thin wall tube and the numerical results for the stress state of adhesive layer.

Improvement of Fatigue Strength of Adhesive Joints through Filler Addition

The effect of filler addition on the fatigue strength of adhesive joints was in vestigated. Both epoxy resin (Epikote 828) and polyamide (Versamid 125 or 115) were adopted as the adhesive components; the filler added in the adhesive was chrysotile- asbestos. The experimental results showed that the fatigue strength is improved by add ing the filler and depends on adhesive layer thickness as well as adhesive composition. Furthermore, the variation trends in strain range of the adhesive layer during the fatigue process were examined for the specimens with/without filler. For the specimen with the filler the strain range rapidly increases just before the fracture, and a fatigue hardening phenomenon is revealed in the fatigue life over 106 cycles.

Friction welding of pure tungsten to oxygen free copper with an intermediate layer

Summary This paper describes an investigation of commercially pure tungsten friction-welded to oxygen free copper with intermediate layers of various metals to improve friction weldability. The intermediate layers used were 20–30 μm thick Ti, Al, Nb, Ni, Fe, and Ag foils. The effects of the intermediate layers on burn-off deformation behaviour and joint performance are examined. The friction welding conditions adopted were: a friction pressure of 50 MPa, forging pressure of 340 MPa, rotational speed of 40 sec−1, and forging time of 6 sec. The friction time was varied in the 1.0–10.0 sec range depending on the type of intermediate layer concerned. The burn-off rates observed during friction welding with all intermediate layers other than Ag are greater than those found without an intermediate layer. Ti, Nb, and Fe intermediate layers most notably lead to a sharp increase in burn-off rate. All intermediate layers other than Ag become finer or thinner during friction welding and are mixed with Cu to form str...

Estimation of rotational degrees‐of‐freedom components of experimental mode shapes using an analytical model

Rotational degrees of freedom (RDOFs) have difficulty with measurement, although structural modifications with beam and plate elements require mode shapes including RDOFs. This paper presents a method to estimate RDOF components as a linear combination of mode shapes of the finite‐element (FE) model of a test structure. Since the estimation accuracy depends on a set of combined modes, an effective technique for selecting modes to be combined is proposed. This technique is based on the modal assurance criterion (MAC) value which indicates the degree of correlation between experimental mode shapes having only translational degrees of freedom and analytical mode shapes of the FE model. To demonstrate the applicability of the proposed technique, a frame model which has simulated experimental mode shapes with added measurement noise is analyzed. It is confirmed that the optimal set of combined modes can be predicted from the MAC values, although the set differs with each mode number to be estimated.

Resistance to fracture of friction welded joints. Friction welded joints of S45C.

In order to determine the requisite value of toughness for friction welded joints on the basis of fracture mechanics, Charpy impact test and fracture toughness test were carried out at various temperatures on the friction welded butt joints composed of carbon steel JIS. S45C and the base metal.The experimental results indicated that the Charpy absorbed energy of the friction welded joints was almost equal to that of the base metal in the temperature range below room temperature, while the absorbed energy of the joints was about a half of those for the base metal above room temperature. The fracture toughness of the friction welded joints increased with an increase in temperature, but the toughness was slightly lower than those of the base metal over the whole temperature range. Particularly, in the temperature range above 185K, the value of the net section stress σnet for fracture of the joint was higher than the 0.2% proof stress of the base metal.When the transition temperature for the Charpy absorbed energy was compared with that for the fracture toughness, the former was much higher than the latter. This suggests that the fracture toughness for the joints is high enough, even when the Charpy absorbed energy is low. Furthermore, the usable critical stress for the friction joints with circumferential or penny-shaped cracks was estimated for various kinds of specimen size and temperature based on the experimental results of fracture toughness.

Fatigue strength and stress distribution of adhesive bonded lap joints with fillet.

In adhesive bonded lap joints, stress in the adhesive layer is concentrated at the lap end because of difference in elastic moduli of constituents and abrupt thickness change. Hence, in order to improve the joint stength by reducing the stress concentration in the adhesive layer, it is practiced in industry to form it with a fillet of adhesive spew or to round the corners of the adherend. However, there are few studies on the fatigue strength charactristics of these improved adhesive joints.In order to investigate the relation between the fatigue stength of these joints and the local stress distribution near the lap end, a series of fatigue tests were conducted on the adhesive bonded lap joints with an adhesive fillet plus a rounded adherend, and the local stress near the adherend corner was analysed by FEM. Furthermore, the fatigue crack propagation behavior was observed with an optical microscope.As a result, It is confirmed that the fatigue strength of lap joints with different corner radii can be standardized by the maximum value of maximum shear stress near the lap end.