DURABILITY OF ADHESIVELY BONDED JOINTS FOR AIRCRAFT STRUCTURES

Although significant gains have been made in recent years towards understanding the science of adhesively bonded metallic and composite joints, a better understanding of the role of processing parameters and material characteristics on the long-term durability on bonded structures is desired. One such need involves improvements to a commonly used test method for assessing bond durability, the ASTM D3762 metal wedge crack durability test. While the test is considered to be a useful method for investigating bond durability, the existing standard provides little guidance regarding specifics on test conditions and requirements that constitute an acceptable metal bonded joint. Thus the objective of this research investigation is to revisit and revise the ASTM D 3762 metal wedge crack durability test. Following a review of the literature and discussions with identified stakeholders, a listing of potential issues with the current wedge test method was prepared. Several aspects of the ASTM D3762 wedge test were identified for experimental investigation, including methods of specimen manufacturing, testing procedures, accounting for the failure mode produced (cohesion vs. adhesion), environmental conditions during testing, and the need for an improved acceptance criterion. Those aspects associated with specimen manufacturing and the initial test procedure have been investigated first. Two issues associated with the wedge specimen manufacturing that were investigated are controlling the bondline thickness and proper machining of the specimens from the test panel. Additionally, three issues associated with the initial testing procedures were also investigated concurrently: the method of wedge insertion, measurement of the initial crack length; and the specimen orientation during testing. Testing was performed using 2024-T3 aluminum specimens bonded using AF 163-2K adhesive. Expected benefits to aviation include an improved adhesive bond durability test method for use in assessing the reliability of adhesively bonded aircraft structures as well as an FAA Technical Center report to provide additional guidance for aviation industry users. INTRODUCTION Significant gains have been made in recent years towards the understanding of the science of adhesively bonded metallic and composite joints. However, a better understanding of the role of processing parameters and material characteristics on the short-term quality and subsequent long-term durability on bonded structures is desired. One such need involves a commonly used test method for assessing bond durability, the ASTM D3762 metal wedge crack durability test. In this test, a double cantilever beam specimen is loaded by forcing a wedge between the beams (rather than pulling them apart in a tensile testing machine) as shown in Figure 1. The wedge is retained in the specimen, and the assembly is placed into a test environment, typically an aqueous environment at elevated temperature. Further crack growth is measured by inspection following a prescribed time period. While the test is considered to be a useful method for investigating bond durability, the existing standard provides little guidance regarding specifics on test conditions and requirements that constitute an acceptable metal bonded joint. Of particular concern is the reduction in strength of the bonded metal joint over time due to moisture. Moisture absorption of the adhesive can lead to a reduction in bond strength through hydration. Figure 1: ASTM D3762 wedge test for assessing bond durability. While the existing metal wedge crack durability test, ASTM D3762, is considered to be suitable for assessing bond durability, the acceptance criteria stated in the standard is not as specific as desired. Thus a need exists to assess candidate acceptance criteria and revise the existing test standard such that it provides specific guidance on how to determine acceptance using the wedge crack durability test. The objective of this research project is to develop and implement improvements in the ASTM D3762 wedge test. Following a review of the literature and discussions with identified stakeholders, a listing of potential issues with the current wedge test method was prepared. Several aspects of the ASTM D3762 wedge test were subsequently identified for experimental investigation, including methods of specimen manufacturing, testing procedures, accounting for the failure mode produced (cohesion vs. adhesion), environmental conditions during testing, and the need for an improved acceptance criterion. FINDINGS FROM THE LITERATURE The first task of this research investigation was to review published studies that had been performed using the ASTM D3762 wedge test. It was found that pertinent topics from the literature could be grouped into three main categories. These topics, which will be discussed below, are: bond degradation, test applications, and test variations.