5.2.5 Customer-Specific Requirements and Testing

Since a true ‘general-purpose’ adhesive has not yet been developed, the end user must allow time and resources to test candidate adhesives, bonding processes, environmental resistance and finished assembly strengths. Testing and validation are extremely important steps in the adhesive process as one cannot reliably predict the strength of the adhesive bond based solely on information on the TDS as relates to characteristics of the adhesive, substrate and the joint design. To have confidence in the strength of the assembly or joint, tests should be performed. Through testing, the many factors that can affect the strength of the joint can be applied to the assembly or joint, measured and quantified, and subsequently corrected if shortcomings are identified. Sometimes it is impossible or impractical to separate the variables that affect joint strength, but through testing, a quantitative approximation can be achieved.

Standard tests, such as ASTM or ISO tests, are effective for comparing and determining the consistency of materials and processes, as has been covered in Section 4; however, they are less valuable in accurately predicting the strength of specific production joints or assemblies. Ideally, prototype assemblies should be designed and tested for this purpose. The most reliable test is to measure the strength of an actual assembly under actual operating conditions; these tests are often prohibitively expensive or impractical. The next best method is to measure the strength of an actual assembly under simulated operating conditions that do not stress the joint significantly differently from what would normally be stressed in service.¹⁸

The challenge herein is determining which test method one selects. It may be surprising to learn that many engineers develop testing parameters and procedures that they feel most accurately duplicate real-world conditions, rather than standardised testing methods (ASTM, ISO, DIN, etc). These tests may be complex rigs designed for repeated force application or they may be as simple as a drop-from-height test, hammer blow test or crush test. Whatever the method of testing or pass/fail criteria, it is important to have these clearly defined before engaging in adhesive selection, as this will make the adhesive selection process much simpler and lead to a higher probability of success.

18) Edward M. Petrie, Handbook of Adhesives and Sealants, McGraw-Hill, 2007.