Intensity of Singular Stress Field over the Entire Bond Line Thickness Range Useful for Evaluating the Adhesive Strength for Plate and Cylinder Butt Joints

Our previous research has indicated that the bonded strength can be expressed in terms of the intensity of the singular stress field (ISSF). Since the ISSF is quite useful for evaluating the bonded strength, in this study, the variation of the ISSF is investigated over the entire bondline thickness range of plate and cylinder butt joints. Here, an effective mesh-independent technique combined with a standard FEM approach is used to obtain the ISSFs under arbitrary material combinations. A reference solution of simply bonded plate is used to eliminate FEM error since the exact ISSF is available. This paper clarifies the differences between the fracture behaviors of the bonded plate and cylindrical butt joints

Intensity of singular stress fields (ISSFs) in micro-bond test in comparison with ISSFs in pull-out test

Micro-bond test is often used to investigate fiber/matrix bonding behavior. In this experiment, the average shear stress is generally used as the interface strength without considering the singular stress. Therefore, in this paper, the intensity of singular stress field (ISSF) is newly analyzed at the fiber entry/exit points. The obtained ISSFs at the fiber entry point in micro-bond test are compared to the single fiber pull-out under the same fiber geometry. The results show that care should be taken for the previous micro-bond test geometry since the ISSF varies sensitively depending on the testing geometry. To control the initial fiber/matrix debonding and evaluate the bonding behavior correctly, suitable testing geometries are proposed in micro-bond testing

Convenient Adhesive Strength Evaluation Method in Terms of the Intensity of Singular Stress Field

A convenient evaluation method is proposed for the debonding adhesive strength in terms of the intensity of singular stress field (ISSF) appearing at the end of interface. The same FEM mesh pattern is applied to unknown problems and reference problems. It is found that the ISSF is obtained accurately by focussing on the FEM stress at the adhesive corner. Then, the debonding condition can be expressed as a constant value of critical ISSF. The usefulness of the present solution is verified by comparing with the results of the conventional method

Most suitable evaluation method for adhesive strength to minimize bend effect in lap joints in terms of the intensity of singular stress field

The lap joint testing is designed to investigate the adhesive strength under pure shear loading. However, actually pure shear testing is very difficult to be realized in the experiment because of the bend deformation during testing causing the peeling force appearing at the adhesive region. To reduce the bend effect, this paper focuses on the intensity of singular stress field (ISSF) at the interface end in order to minimize the ISSF for lap joints. The results show that the ISSF decreases with increasing the adherend thickness. The minimum ISSF is obtained when the adherend thickness is large enough with the small deformation angle defined at the interface end. Since the strength of double lap joint (DLJ) is sometimes about two times larger than the strength of single lap joint (SLJ), the equivalent strength condition is discussed by changing adherend thicknesses of DLJ and SLJ. It is found that the strength of SLJ with adherend thickness t1=7 mm is nearly equal to that of double lap joint with t1=1.5 mm prescribed in Japanese Industrial Standard

Development of multi-layered sewer pipe plug — 2nd report: Tensile strength of protective sheet bonded by seams

Since Japanese sewer system is becoming obsolete, it is necessary to reinforce and repair the system without stopping sewer functions. In this study, a multi-layered sewer pipe plug consisting of a protective sheet and inner and outer rubber balls is focused to be installed and removed conveniently at the construction site. In this paper, a suitable testing method and the strength of the protective sheet are discussed experimentally by changing the specimen geometry providing slit and seams. It is found that the slit specimen is most desirable to obtain the standard tensile strength of UHMWPE cloth named Izanas cloth. It is found that the seamed strength is σB=34 MPa, which is about 17% of the standard tensile strength σB0=200 MPa

Development of multi-layered sewer pipe plug — 3rd report: Tensile strength of protective sheet bonded by adhesive

Since the sewer system in Japan is becoming obsolete, it is, therefore necessary to reinforce or repair without stopping sewer functions by applying a suitable water stopping method. In this study, a multi-layered sewer pipe plug consisting of a protective sheet and inner and outer rubber balls is focused since it can be installed and removed at the construction site in a short time conveniently. Four types of adhesively bonded structures are investigated experimentally by changing bonding processes and adhesives. It is found that the main and base adhesive joint is the most suitable since the pressurized adhesive strength σB=60 MPa is 30% of the standard tensile strength of the protective sheet σB0=200 MPa

Development of multi-layered sewer pipe plug — 1st report: Ruptured test and stress analysis of protective sheet

In recent years, the sewer system in Japan is becoming obsolete. It is, therefore, necessary to reinforce or repair without stopping sewer functions by applying a suitable water stopping method. In this study, a multi-layered sewer pipe plug consisting of a protective sheet and inner and outer rubber balls is focused since it can be installed and removed at the construction site in a short time conveniently. This sewer pipe plug has several advantages dealing with various diameters compared to the conventional type. In this study, the rupture test is conducted to improve the water stopping performance. The fractured position of the protective sheet of the sewer pipe plug is investigated experimentally. It is clarified that the maximum stress around the flange portion can be reduced by decreasing the flange inner diameter

Fractographic Identification of Fracture Origin Mainly Controlled by the Intensity of Singular Stress Field (ISSF) in Prismatic Butt Joint with Corner Fillet

In this study, the fracture origin is identified for the prismatic butt joints whose debonding condition can be expressed as a constant value of the ISSF. The ISSF variation is newly analyzed along the interface side by considering the real specimen geometry with chamfer at the corner. The detail fractographic observation shows that most of the fracture starts from the maximum ISSF region at the interface side instead of the interface corner. When the bondline thickness h is larger, the fracture origin can be seen at sub-surface because the stress triaxiality decreases inside of the specimen. The fracture origin under thermal loading can be estimated similarly because of the coincidence the ISSF variations

Relationship between strain rate concentration factor and stress concentration factor

In this study, the strain rate concentration is considered for high speed tensile test, which is now being recognized as a standard testing method. To evaluate the impact strength of engineering materials under high impact speed, Izod and Charpy tests are unsuitable since they cannot control the impact speeds and therefore do not coincide with the real failure of real products. For smooth specimens, the strain rate can be determined from the tensile speed u/t and specimen length l as ̇smooth =u/tl. For notched specimens, however, the strain rate at the notch root enotch should be analyzed accurately. In this study, therefore, the strain rate concentration factor defined as Kte=enotch/esmooth is studied with varying the notch geometry and specimen length. It is found that the strain concentration factor Kte can be estimated from stress concentration factor Kt

Intensity of singular stress field (ISSF) variation as a function of the Young\u27s modulus in single lap adhesive joints

Employing mixed adhesive joints has been proven to be very useful. This type of joint leads to improved performance by increasing strength and decreasing stresses in critical areas of the joint. In the same way, the use of the Intensity of Singular Stress Field (ISSF) has been shown to be suitable for adhesive joint calculation, since the adhesive strength can be controlled by the ISSF at the interface end. Four finite element models have been created by combining two epoxy adhesives with different mechanical properties, and therefore with different Young\u27s moduli. New mixed adhesive joints have been compared with respect to only-one adhesive joints in terms of the ISSF. The results show a clear improvement with one of the configurations of mixed adhesive joints. A significant decrease of 35.64% in the ISSF is obtained compared to the only-one adhesive configuration