The use of contact angles to measure surface quality has been widely adopted throughout many industries and applications. Your organization, too, has recognized that contact angle measurements provide consistent and objective results in determining whether a surface has been properly prepared for adhesive bonding.
Determining what locations to measure on your surface as well as inspection frequency are also important considerations in the development of your surface inspection processes.
Measurements occur at specific points on a surface. Typically, you are interrogating a few square millimeters when conducting a contact angle test. When adhesion fails, the failure occurs over a large area — sometimes extending inches, feet, or yards. Sampling methodologies need to be developed that enable you to extrapolate adhesion performance, measured in millimeters, to anticipated performance measured across much larger areas.
To ensure that statistical interpretation of contact angle measurements will produce reliable results, it is necessary to use a common terminology so that everybody involved in the process has a conceptual understanding of how to deploy uniform processes to measure surfaces for adhesion.
A “Bond Unit” represents a portion of a surface that has been prepared consistently at every point in the processing stage. Bond Units are not defined by physical dimensions. A Bond Unit might be one square inch, 20 square feet, or any other shape and perimeter combination. It represents a discrete surface area defined by uniform processing occurring on a specific material surface.
To learn more about using contact angles to inspect incoming materials and monitoring your cleaning processes, download our free eBook: Predictable Adhesion in Manufacturing Through Process Verification
A Bond Unit has been prepared during a consecutive, uninterrupted time period. It has been prepared by one person or team. The same cleaning process was used consistently throughout the surface preparation process – including the same detergents, wipes, or plasma treatment. Any deviation from this definition results in a distinct, separate Bond Unit being prepared. Triggers for transitioning from one Bond Unit to a new Bond Unit include:
Aircraft need to be 100% reliable in their physical structure and their mechanical and electrical systems. Aircraft manufacturers have an extraordinarily stringent set of specifications to follow because lives are literally at stake if failure occurs. Defense aircraft are subjected to even more operational stress than commercial aircraft. Manufacturing military aircraft requires developing intelligent production systems that drive 100% reliability.
Adhesive bonding of primary composite structures on aircraft can provide substantial cost savings by significantly reducing part and fastener counts. Adhesive bonding also saves on time and labor during airframe production.
Defense Advanced Research Projects Agency’s (DARPA’s) Open Manufacturing (OM) program is chartered with reducing the cost and increasing the production rates of high-performance parts for defense systems. The Transition Reliable Unitized Structure (TRUST) initiative, which was led by Lockheed Martin, was part of the OM program.
The TRUST program focused on developing standardized, repeatable production processes that maintain rigorous process control throughout the production cycle of critical parts.
The concept of “Bond Unit” arose from this program. Developing a process that would increase organizational confidence that adhesive bonding was sufficient for aircraft manufacturing, without the need to deploy redundant fastening systems (like mechanical fasteners in conjunction with bonding), compelled BTG Labs to create clear terminology that could accurately convey production techniques that were necessary to ensure proper surface adhesion preparation.
BTG Labs had a leadership role in developing surface adhesion standards for the TRUST program. Knowing that reliable bonds result when surfaces are properly prepared for adhesion, BTG Labs helped create the “Bond Unit” terminology to help accelerate adoption of consistent surface preparation methodologies. The Bond Unit concept helps manufacturers answer the questions of where, when, and how often to take measurements to ensure adhesive bonding will be successful. Additional information regarding DARPA’s TRUST program can be found here.
Surface measurements needed to be taken to prepare for adhesive bonding across all portions of an aircraft – including during ongoing production of the aircraft. The Composites Affordability Initiative (CAI) program concluded that a mobile testing methodology was needed that could provide rapid results on the factory floor to ensure that material surfaces were ready to be bonded.
Recognizing this technology gap and working under Air Force Research Laboratory (AFRL) support, BTG Labs took the initiative to create an innovative instrument that used water contact angles to provide accurate, on-the-fly surface measurements. This initial prototype, used extensively in the TRUST program, has grown into the Surface Analyst™ 5001 product line that continues to be the industry leader for providing rapid, mobile surface analytic results.
Simple comparative statistics show that contact angle measurements are normally distributed. This means we can characterize the contact angles of a Bond Unit by two parameters:
Once we know the mean and standard deviation of the contact angles expected for a given surface and surface preparation method, we can calculate the number of measurements required to determine whether a similar surface has been adequately prepared for adhesion.
The normal distribution of contact angle measurements drives confidence in using the Bond Unit as a sampling technique. We will delve deeper into how leveraging statistical analysis increases confidence and drives certainty in the production process in upcoming blog posts.
Using the Bond Unit approach is best suited for organizations that have standardized procurement and production processes that focus on repeatable process controls.
Although it is important to create surface cleanliness standards for your suppliers, Bond Units are not an effective quality control concept to use for incoming raw materials. Raw materials from vendors have an unknown processing past. As you are not involved in the production of your incoming raw materials, it is impossible for you to know where each Bond Unit begins and ends when you are accepting raw material shipments. However, using handheld and rapid contact angle devices that provide objective and quantifiable measurements of the surface properties is the optimal way to assess whether your incoming parts are “clean”.
Instituting the Bond Unit concept is particularly useful for organizations that have large, critical components which require careful control of surface composition. Examples of this include the manufacturing of aircraft, automobile, appliances, and others.
“Bond Units” are not intended as a one-size-fits-all solution for every adhesion need. There are some instances where a single measurement of surface readiness may be sufficient to confirm a surface is properly prepared for adhesion. This is typically the case for small components, like bonded aircraft fasteners.
You can gain confidence relying on a relatively few contact angle measurements taken on large surfaces by adopting the Bond Unit concept. Utilizing a Bond Unit recognizes that when there is a problem with bonding large areas, the failure typically occurs over a much broader area than a typical point on the surface.
Incorporating Bond Unit parameters into your product specifications will drive uniformity in the adhesion process. It will help you ensure surface readiness and uniformity at each Critical Control Point (CCP) within your production process.
To learn more about using contact angles as an easy, quantitative technique for incoming material inspections and designing and monitoring your cleaning process, download our free eBook exploring common manufacturing CCPs that need to be protected through contact angle measurements: Predictable Adhesion in Manufacturing Through Process Verification.