Aging refers to the change in the properties of materials or bonded joints over time. Aging is caused by external environments or the material's own processing. The influencing factors include mechanical stress, temperature, radiation, and media (such as water). Some substances contained in the adhesive itself may also lead to changes in the performance of the bonded joint. Besides being affected by various factors, the location of aging in the bonded joint can also vary. Aging may occur in the cohesive layer, the adhesive layer, or the base material. Aging is caused by the combined action of several factors, but these influencing factors are difficult to measure and predict thoroughly.

Mechanical Stress
Bonded joints may be subjected to the following types of mechanical stress: A. Peel stress, B. Tensile stress, C. Shear stress, D. Torsional stress.
Peel stress causes a high linear force at the bonded joint, which is far beyond the strength of most adhesives. When designing the structure of a bonded joint, it is necessary to ensure that the bonded joint avoids bearing such peel stress.
When subjected to tensile load, the acting force is perpendicular to the bonding surface. The force is transmitted through the entire bonding surface. In principle, there is no problem with the bonded joint bearing tensile stress. Tensile stress is generally only encountered in butt joints. Butt joints usually have a small bonding area, and the bonding area can only be increased by changing the thickness of the base material. Since the strength of the base material usually exceeds the tensile strength of the adhesive, it is impossible to achieve a design that fully utilizes the strength of the base material.
The best type of stress for a bonded joint to bear is shear stress because this stress is parallel to the bonding surface. Shear stress exists in the lap joints of bonding, which usually have a sufficiently large bonding area. If the existing bonding area cannot bear enough load, it is relatively easy to increase the load that the bonding can bear by increasing the bonding area.
Torsional stress is similar to shear stress, and bonded joints are not sensitive to compressive stress. Over time, any type of mechanical stress can cause aging of the bonded joint, such as the possible occurrence of permanent deformation (creep) or the formation of cracks. Mechanical stress may also exacerbate the effects of other aging factors (for example, joint damage caused by water vapor).