01. Definition of bonding strength
      Bonding strength refers to the stress required to cause damage to the interface between the adhesive in the adhesive part and the adherend or its vicinity under the action of external force. It is one of the important indicators for evaluating the quality of the adhesive. Its size not only depends on the adhesive force, the mechanical properties of the adhesive, the nature of the adherend, and the bonding process, but also depends on the joint form, stress conditions (type, size, direction, frequency), environmental factors (temperature, humidity, Pressure, medium) are related to test conditions, experimental technology, etc.

02. Classification of bonding strength
(1) Shear strength
      Shear strength refers to the shear force that the unit bonding surface can withstand when the bonded part is damaged, and its unit is expressed in megapascals (MPa). Shear strength is divided into tensile shear, compression shear, torsional shear and bending shear strength according to the force mode during testing.
      Adhesives with different properties have different shear strengths. In general, tough adhesives have greater shear strength than flexible adhesives. A large number of tests have shown that the thinner the thickness of the adhesive layer, the higher the shear strength.
(2) Tensile strength
      Tensile strength, also known as uniform tear-off strength and positive tensile strength, refers to the tensile force endured by unit area when the adhesive is damaged by stress, and the unit is expressed in megapascals (MPa). Because tensile stress is much more uniform than shear stress, the tensile strength of general adhesives is much higher than the shear strength.
      In actual measurement, under the action of external force, the deformation of the specimen is larger than that of the adherend due to the adhesive, and the non-axiality of the external force is likely to cause shearing and lateral compression. Therefore, it will break when it is broken. Simultaneous fractures may occur. If the length of the sample can be increased and the bonding area can be reduced, the impact of peeling during tearing can be reduced and the stress distribution can be more uniform.
(3) Peel strength
      Peel strength is the maximum load that the unit width can bear when the bonded parts are separated under specified peeling conditions, and its unit is expressed in kN/m. There are various forms of peeling, which can generally be divided into L-shaped peeling, U-shaped peeling, T-shaped peeling and curved surface peeling.
      As the peeling angle changes, the peeling pattern also changes. When the peeling angle is less than or equal to 90°, it is L-shaped peeling, and when it is greater than 90° or equal to 180°, it is U-shaped peeling.
(4) Uneven tear-off strength
      Uneven tear-off strength indicates the maximum load that an adhesive joint can withstand when subjected to uneven tear-off force. Because the load is mostly concentrated on two edges or one edge of the adhesive layer, it is the force per unit length rather than per unit area. , the unit is kN/m2.
(5) Impact strength
      Impact strength refers to the maximum work consumed per unit bonding area when the bonded part is damaged due to impact load, the unit is kJ/m2. The size of the impact strength is affected by the toughness of the adhesive, thickness of the adhesive layer, type of adherend, specimen size, impact angle, environmental humidity, test temperature, etc. The tougher the adhesive, the higher the impact strength.
(6) Durable strength
      The lasting strength is the maximum load that the unit bonding area can withstand after the bonded part is subjected to static load for a long time. The unit is expressed in megapascals (MPa). The endurance strength is affected by the loading stress and test temperature. As the loading stress and temperature increase, the endurance strength decreases.
(7) Fatigue strength
      Fatigue strength refers to the maximum stress that can be applied to a bonded joint repeatedly for a specified number of times without causing damage. Generally, the fatigue strength at 10 times is called the fatigue strength limit. Generally speaking, adhesives with high shear strength always have low peeling, bending, and impact strengths; while adhesives with high peeling strength have high impact and bending strengths. Different types of adhesives have very different strength properties.