Damage tolerance analysis for adhesive bonds 2

[blakmax]

I’d like to start a discussion about applying damage tolerance analysis (DTA) to adhesive bonded structures. I presented a paper on this earlier this year, and I’d welcome feedback. The full paper is at

http://www.adhesionassociates.com/p...f Adhesively Bonded Structures and Joints.doc

Essentially my concerns are that the current approach to DTA is based on use of an artificial defect is inserted into an otherwise effective bond. Provided adequate strength can be demonstrated by analysis or testing, then that defect size is considered to be the tolerable defect size for the life of the component, and hence NDI inspection requirements are set.
Inherent in this approach is that the adhesive surrounding the defect is capable of providing full bond strength. My assertion is that this assumption is only true for large production defects (macro-voids) and therefore that approach is invalid for setting NDI requirements for in-service inspection because under some circumstances, the bond strength of adhesive surrounding the defect is well below the level achieved when the bond was first made.
Adhesive bonding depends on chemical bonds formed at the interface during adhesive cure. The strength of the adhesive depends directly on the integrity of those chemical bonds, and that is determined by the process used for preparing the surface prior to bonding. The most common cause of bond degradation for metal bonding is hydration of the oxides on the surface of the metal and this causes dissociation of the chemical bonds to the adhesive resulting in disbonding at the interface. As hydration progresses along the bond, the bond becomes weaker. Hence the assumption of bond integrity inherent in DTA is no appropriate for bonds which are failing interfacially.
Another case where DTA is inappropriate is for managing porosity (micro-voids) because the bond strength will be lower than for bonds without porosity. A reference in the paper shows a 53% loss of peel strength for this type of defect.
Sorry for the length of the posting. I’d welcome comments.
Regards
Blakmax

 

[Compositepro]

I will carefully review your paper. You have posted some good materials before. I've worked for one of the largest aerospace film adhesive manufacturers and I can say that there there is still much opportunity for technology improvements to prevent void issues.

 

[blakmax]

Thanks CP. IMHO the major cause of macro-voids is poor fit-up while micro-voids (porosity) is usually caused by the release of volatiles during the cure cycle. In some cases this may be residual solvents from the adhesive manufacturing process, moisture absorbed by the adhesive is a far more significant contributing factor. This is a major issue for film adhesives transported long distances. The packaging must be thoroughly sealed to prevent moisture entry and care taken to avoid damaging the packaging. (We had one supplier approach us to request acceptance of a batch noting that the package only had a small tear in the bag. My response was that this was comparable to stating that someone was only a little bit pregnant!)
Care is required during handling storage and use. One of the references in my paper outlines best practice in prevention of micro-voiding.
Regards
Blakmax

 

[rb1957]

i too will (in the fullness of time) read your paper.

bonded structures are already analyzed with DT, i'm thinking skin/stringer panels; but your're more concerned about the bond itself.

one issue you're goign to have is that i think it is process dependent ... good process = good bond. another, and i'm not that familiar with the subject matter, is that bonds tend to be there or not (and mostly failure initiates from process issues), you could consider a void in the bond but wouldn't it grow so quickly as to be undetectable ?

 

[blakmax]

Thanks rb1957.

To be precise, there are two issues. The first is the bond itself (because of porosity) but more important is the interface between the adhessive and the structure where hydration and strength loss may occur.
You are somewhat correct in that good processes produce good bonds and bad processes do not. However, the crux of the issue is that mediocre processes can produce excellent short term strength (enough to pass certification) but are susceptible to degradation by mechanisms such as hydration in later service. One of my major concerns is that current FARs require only demonstration of static strength, damage tolerance and fatigue resistance. The only requirement on processes is in 2x.605 where the process used must produce a "sound" structure. There is no requirement to demonstrate long term bond durability, apart from indirect references in Advisory Circulars and policy statements.

The suggestion that bonds are either there or not there is really dependent on a number of factors. Hydration occurs from the edges of a bond and follows Fick's law of diffusion as the moisture diffuses along the bondline. If the bond overlap length is long, then there may be sufficient residual bond to enable loads to be sustained (the bond is "there") and defects may be detected before failure occurs. If the overlap length is short, then diffusion from both ends may result in a catastrophic bond failure (the bond is "not there") before a defect can be detected. This is in the paper.

Another factor is that voids do not occur in service. They are production artifacts and in most cases do not propagate in service unless the DTA was really screwed up. Disbonds which occur in service are either mixed-mode or adhesion failures, which are again attributable to production processes and their ability to resist hydration. One irrefutable fact is that adhesion and mixed-mode failures occur at much lower strengths than full-strength cohesion failures, so DTA based on assuming that the local adhesive is strong, is unsupportable.

Regards

Blakmax

 

[epeus]

Certainly a interesting read, i do not have any substantial to add, however you have a few "[Error: Reference source not found]" throughout the copy posted.
I guess the main thing I took away from it was that unless environmental control (particularly moisture) was implemented throughout the process then micro-voiding is a real possibility, thus your issues of invalid DTA whilst maintaining short term strength for certification.

I will have another read when more time permits to gain a better understanding.

 

[blakmax]

Thanks epeus

I note the reference issues and will attach a copy here. I have checked the file and confirm that there are no cross-reference errors in my copy. These messages are caused by use of Word's cross referencing facility. Bill Gates has a lot to answer for.

The issue of micro-voiding can be minimised by humidity control. For this type of defect, the risk is fatigue of the residual bond adhesive in later service.

The second issue of adhesion and mixed mode failure is a lot more difficult to manage. It requires validation of bond durability prior to even certification testing. If hydration is prevented, bond degradation does not occur and bond strength is maintained. If this step is not undertaken, then DTA is inappropriate for management of such bonded structures.

Regards

Blakmax

 

[blakmax]

Any feeback on this issue?

 

[Hansmeister]

While I did not read your excellent paper in excruciating detail, it appears to be very concise and detailed. Your focus is on establishing methodology for establishing a high quality bond initially, and then observing bond degradation due to hydration. Other factors that you have pointed out contribute to the degradation of bond strength.

In composite adherends, I don't think that same hydration degradation process occurs, but you can correct me on that based on your data.

In your work, I would also like to see the fatigue degradation of the adhesive itself get folded into your discussion. Very little adhesive fatigue data exists in the literature (that I'm aware of).

 

[blakmax]

Thanks for the feedback Hansmeister. You are correct about the issue of hydration being of less importance for composite adherends, although moisture may or may not play a role in disbonds from composites.

The issue of fatigue of adhesive bonds is interesting. In reality, for metallic adherends if the adhesive bond is designed to simple rules then adhesive fatigue should never occur. It should always be possible to design the joint such that the adherends fail first either by static loading or fatigue.

The use of short overlap adherend tests such as ASTM D1002 give a totally false "measure" of fatigue performance in exactly the same way that they give a false measure of bond strength for static loading. The short overlap causes the adhesive to be the critical element in the joint so it fails first, and the failure is strongly influenced by plastic deformation of the adherends. In nearly forty years of experience I have only seen an incredibly small number of bond failures where the cause was actually fatigue of the adhesive and in each case, it was a design issue.

Regards

Max

 

[kellnerp]

Still quite a few "[Error: Reference source not found]" after page 9.

TOP
CSWP, BSSE

http://www.engtran.com

http://www.niswug.org

http://www.linkedin.com/in/engineeringtransport

Phenom IIx6 1100T = 8GB = FX1400 = XP64SP2 = SW2009SP3
"Node news is good news."

 

[Compositepro]

I've read your paper and pretty much agree with all of your points. You do overlook a couple of factors besides moisture that cause voids in bond-lines. Air entrapment is actually the primary cause. Moisture and other volatile materials will drastically expand air bubbles during cure and, therefore, correlate very well with voids. However, if there were no air bubbles to begin with, moisture in the adhesive will not cause voids in most cure cycles.

Air bubbles are present for a number of reasons. The main cause is surface tack of the adhesive, sealing air in the surface texture of the adhesive film and the adherend. Interestingly, moisture and volatiles will also increase tack and, thus, air-entrapment. On a number of occasions I've been involved in efforts to "improve" tack on adhesive films and prepregs due to a customer request. Suddenly, voids became a serious problem.

Most adhesive films are supplied with an embossed poly-film. As a result, this pattern is embossed in the adhesive film. There are many different patterns available. Some will lead to large amounts of air entrapment and others will create grooves in the adhesive surface that facilitate air removal.

Air bubbles will also be entrained when mixing viscous adhesives under less than perfect vacuum (often no vacuum may be used).

Coating the adhesive film can also entrain bubbles, as will embedding a carrier scrim into the adhesive.

Flow in the bond-line during cure will lead to coalescence of small bubbles into large voids. Common effect when bonding flat sheets or plates with a vacuum bag is that the bond line gets thin at the edges due to resin bleed. This causes the sheets to bow slightly which actually makes the bond-line in the center of the plate thicker than the adhesive film. During cure, resin will flow toward the edges due to bleed, and to the center due to the pillowing effect. Thus, large voids can form at the center of large area bonds.

 

[blakmax]

Thanks Compositepro.

Trapped air often results in large (macro) voids, which damage tolerance analysis can actually manage quite well. If there is enough moisture evolved during the cure cycle the evolved material may coalesce to form macro-voids which again may be managed by DTA. However, in my expereince and as shown in the figures in the paper, moisture evolved during the cure cycle in film adhesives often presents as small voids trapped within the weave of the carrier cloth. Althiugh the size of each void may be small, the aggregate loss of bond area may significantly reduce the bond strength significantly. It is this type of defect which is NOT managed by DTA, because the entire DTA methodology assumes defects are MACRO-voids.

While the micro-void issue is of interest, my greatest concern is the degradation of bond interfaces in later service which results in disbonds at the interface. This phenomenum results in a significant loss of bond strength. However, current DTA methodologies either assume or imply that the bond adjacent to a disbond maintains full bond strength and this is just not true with the loss of strength in the effected zone around the disbond depending on the level of moisture diffusion around the defect.

While there is a history of managing adhesive bonded structures by in-service NDI the issue of most concern to me is that the same methodology is often applied to bonded joints with very small overlap lengths, (such as helicopter main rotor blades). In such cases the effected zone may involve the whole bond with a consequent loss of joint strength, and the loss of strength may become critical BEFORE any disbond can be detected.

Kellnerp, I am at a loss to know how to get the paper to you. At my end, the papers are fine before I post the links. Maybe try downloading the original direct from my web site. Just Google Adhesion Associates. (Not allowed to post email addresses or web pages on this forum.)

Regards

Blakmax

 

[kellnerp]

Thanks. I did some googling after post and found a lot more information and your paper.

I found some of the articles on passivation of aluminum to prevent hydrolysis interesting.

TOP
CSWP, BSSE

http://www.engtran.com

http://www.niswug.org

http://www.linkedin.com/in/engineeringtransport

Phenom IIx6 1100T = 8GB = FX1400 = XP64SP2 = SW2009SP3
"Node news is good news."

 

[blakmax]

Kellnerp

Be very careful about selecting processes which "passivate" bonding surfaces. Structural adhesive bonds rely strongly on the formation of chemical bonds at the interface to geenrate strength, and these are the same bonds which are degraded by hydration of the surface oxides on metals. Importantly if the surface is "passive" it may not be sufficiently reactive to form the chemical bonds neccessary for bond strength.

I would not rely on salesman speak to select my processes, and I would always test for new processes or adhesive-adherend combinations. However, do not rely on strength tests because these will only give a snapshot of the strength at a given time and "moisture conditioning" does not usually allow sufficient time for hydration to occur. The best test is the wedge test ASTM D3762, but be sure that the acceptance criteria are as stated in DOT/FAA/AR – TN06/57, May 2007. That document is also on my web site.

Regards

Blakmax