A Fundamental Study on Static Strength Improvement of CFRP Bolted Joints by Increasing Friction Force

Abstract

The bolted joint is a common assembling method for carbon fiber reinforced plastic (CFRP) members. However, cracks or plastic deformation can occur around bolt holes of CFRP members even when employing low fastening forces. As a result, CFRP failure occurs around bolt holes because of bearing forces, and the strength of a CFRP joint is reduced. To address this problem, we change the configurations of washers and insert a thin sheet having a high friction coefficient between CFRP members. In friction coefficient measurements, the insertion of sandpaper was found to be the most suitable method for increasing friction forces at the interface between CFRP members. Three-dimensional finite element method analyses were conducted to investigate the effect of washer configurations in terms of the maximum allowable fastening force and the joint strength. From these analyses, the cone washer was found to be the most effective design for increasing the joint strength. In the full model analyses, the failure strengths of proposed CFRP bolted joints were higher than those of normal CFRP bolted joints. The verification tests for the proposed joint were conducted according to ASTM D5961. It was observed that the load at failure for the proposed bolted joint was about 45 % greater than that for the conventional joint.

Study of high-strength CFRP bolted joints with failure-monitoring cone washers

“…Sandpaper has a high friction coefficient that does not depend on the normal stress acting on the contact area. The measured friction coefficient between untreated CFRP plates was 0.36 [2], and that between CFRP plates with the high-friction sheet inserted was 0.68 [2].…”

“…To realize high-strength CFRP bolted joints, we used a high-friction sheet to increase the friction coefficient of the CFRP contact surface [2]. The sheet comprised two sheets of sandpaper (#240), which were bonded with adhesive and inserted between CFRP specimens.…”

“…1 to realize high-strength CFRP bolted joints. The configuration of the washer was determined to maximize allowable boltfastening forces with lowest weight gain using the FEM [2].…”

“…We have proposed two approaches for increasing friction forces [2]. The first approach is increasing friction coefficients of CFRP contact surfaces and the other is increasing bolt-fastening forces without damaging the CFRP.…”

Tensile properties of CFRP manufactured by resin transfer molding considering stacking sequences at various strain rates

“…Before the grip tab attachment, grinding process is carried out with 1200 grit sandpaper which has smooth enough surface to prevent damage on the specimen surface from the grinding process in order to achieve a strong enough adhesive force. 24 Two different types of adhesives are attempted in order to figure out their performance: (a) DEVCON 2 ton as general epoxy which has the 30-min curing time and the maximum shear strength of 17 MPa and (b) 3M DP460 as strong epoxy which has the 24-h curing time and the maximum shear strength of 34 MPa. In this section, tensile tests are conducted for selection of a grip tab material and adhesive in specific conditions of a 0° unidirectional specimen for its high failure strength with the thickness of 2.0 mm.…”