Creep is defined as the time-dependent deformation of a material that is subjected to a constant load, and occurs after the initial elastic deformation produced by the initial loading. Typically, within a bonded joint, it will be the adhesive that will suffer creep deformation, not the adherends. It is therefore important that within structural joints there remains a region of adhesive that is either not stressed or stressed to a very small amount, for example up to 10% of its elastic limit. If this is maintained then failure by creep-rupture is unlikely to occur, provided that the service temperature remains below the glass transition temperature (Tg) of the adhesive.
If creep occurs within a joint, there is typically a delay between initial loading and the onset of creep, shown in figure 1 below as the induction period.
The length of this period is not well defined and dependent on several factors. The main factor is believed to be the time it takes for the stress to redistribute within the length of adhesive. The shear strain distribution for a simple lap joint, with concentrations at the adherend ends, is shown in Figure 2. The peaks will be reduced with time, and the stress within the central region will increase, making the adhesive more susceptible to creep.
Once creep initiates, it will follow a linear relationship with logarithmic time, shown in figure 1. If significant creep occurs during the life of the structure then sufficient bonds may be broken so that the load can no longer be supported. Failure by stress rupture may occur.
The induction period will be affected by moisture uptake, which lowers both the Tg of the adhesive and its modulus, quickening the re-distribution of stress. Elevated service temperature will soften the adhesive and increase the creep rate as it is gets closer to the Tg
Lengthening the overlap will deepen the stress distribution curve and help resist creep by creating areas of low stress at the centre of the overlap.
Standard tests that determine creep resistance
The resistance to creep of any adhesive system can be assessed by either using standard test piece geometries, such as the lap-shear or the T-peel specimens, or the actual component joint.
Some Standard test methods are given below, including the assessment of environmental effects on creep-rupture.
ASTM D1780-99 Standard Practice for Conducting Creep Tests of Metal-to-Metal Adhesives
ASTM D2293-96(2002) Standard Test Method for Creep Properties of Adhesives in Shear by Compression Loading (Metal-to-Metal)
ASTM D2294-96(2002) Standard Test Method for Creep Properties of Adhesives in Shear by Tension Loading (Metal-to-Metal)
ASTM D4680-98 Standard Test Method for Creep and Time to Failure of Adhesives in Static Shear by Compression Loading (Wood-to-Wood)
ASTM D2919-01 Standard Test Method for Determining Durability of Adhesive Joints Stressed in Shear by Tension Loading (lap shear)
ISO 15109:1998 Determination of the time to failure of bonded joints under static loads.
BS5350 part C7:1990 Methods for test for adhesives - Adhesively bonded joints: mechanical tests. Determination of creep and resistance to sustained application of force.