Hello,
Two things that I have found beneficial to life cycle testing include conditioning environment and scale of sample/specimens.
With regard to environment, it is not possible to recreate the exact conditions that a structure will experience. However, exposing samples to different extremes or boundaries of the intended application will highlight sensitivities and hint at strategies to improve. For example, condition samples hot/wet, hot/dry in air, hot/dry in vacuum, hot/dry in rarified gas. Measure sample weight as a function of time. Condition both cored laminates as well as the skins, adhesive, and core individually. Cross section the samples periodically to inspect the progression of degradation in from the surfaces and edges.
Epoxies are quite stable at elevated temperatures under vacuum, but as the concentration of moisture, oxygen, and or other reactive materials increases, the rate of mass loss also increases dramatically. The benefits of even a very thin sacrificial coating are impressive.
Often degradation is primarily due to environmental conditions which in turn are a function of diffusion rates, permeability, and surface area. Samples need to be several times bigger than the test specimens to control and characterize edge effects. For my research, at least 2 inches of sample length (distance to the nearest edge) for every week (168hrs) at 350F (177C) to observe the "wavefront" of oxidation.
I look forward to reviewing the papers.
Sincerely,
Jed
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Jed Brich
Research Engineer
Janicki Industries
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Original Message:
Sent: 03-30-2018 12:14
From: Sandesh Hegde
Subject: Thermal-Cycle Endurance of Bonded Joints
Matthew, John
Thank you very much for the information. To be honest, so far we had been working only on the causes for microcrack formation and its effect on the mechanical strength, I am glad you brought up various approaches to address thermal cycling-related issues, in the sandwich structures.
Some information regarding Flatwise tensile test are below
1) Yes, as John pointed out, this is the best test to address issues between faceseet and core.
2) The adhesive we used, to bond the test specimen to the loading block of the test fixture is LOCTITE EA 9392 AERO.
3) Before applying adhesive the sample's facesheet surface needs to be lightly sanded (we used 320 grit size sandpaper).
4) Make sure the sandwich coupons have laminates that are properly cut/damage free/laminates properly aligned top to bottom
5) Test setup as per ASTM C297 is key; proper time/rate of speed, Coupon size (with respect to cell size etc) and the test setup with orthogonal pins will eliminate the most variation in the results.
Good Day!
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Sandesh Hegde
[JobTitle]
Montréal
Original Message:
Sent: 03-22-2018 15:11
From: Matthew Nahan
Subject: Thermal-Cycle Endurance of Bonded Joints
John,
Thanks for your generous and thoughtful response.
We are not planning to fly anything with the disbonds we're experiencing.
In my current bonded joint application, our joints cracked but did not technically fail (i.e. still carried required load). After less than 40 thermal cycles, we lost maybe 25% of our bond area. Given a lack of testing of composite bonded joints under extended thermal-cycle conditions, I lack your confidence that this is not a low-cycle fatigue issue. I agree that microcracking is a significant factor at play, maybe for the problem of core-to-face sheet disband as well.
I eagerly await your paper.
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Matthew Nahan
Sr. Mechanical Engineer
Lockheed Martin Space Systems Co
Decorah IA
Original Message:
Sent: 03-19-2018 13:03
From: John Troll
Subject: Thermal-Cycle Endurance of Bonded Joints
Matthew, Sandesh,
There are a few different useful points I can add to your discussion. I'd be happy to discuss things with you more, of course.
1. There are not always serious fatigue concerns in space structures (e.g., composite bonded joints) because the number of cycles is typically low for a static structure. Note that I am not talking about actuators, motors, cryo-coolers, etc.
2. Once you've survived one cycle, I think you would be more concerned with microcracking and any loss of strength, dimensional stability, or stiffness. For my work with very cold structures I thought this was helpful: "Timmerman J F, Matthew S T. Matrix and fiber influence on the cryogenic microcracking of carbon fiber/epoxy composites[J]. Composites: Part A, 2002, 33(3): 323-329" One key finding here was that a "handful" of cycles was enough (and often we are required to have at least 7 or 8 cycles on spaceflight hardware)
3. I'm surprised you are able to use a partially debonded item, though perhaps that disbond did not occur in development. Perhaps you are talking about flight articles that already exist.
4. I'm also giving a paper at SAMPE and it centers around recovering from a debond between facesheet and core and how to improve the strength during an extreme thermal cycle down to -396 F (-238C). I hope you can make it, it's called "
Development of a Solar Array substrate for the Europa environment".
5. I do believe FWT is your best test method for an issue between facesheet and core.
6. Toughness of your film adhesive can make a major difference.
7. Consider your surface preparation and depth of abrasion. We found less resin to be better than more resin (we abraded until some "black" was present, from the carbon fiber).
8. Recall that composites are not homogeneous and are composites of two different materials, each with unique allowables. A debond would typically be film-adhesive or matrix-driven. You should work with failure criteria which can handle matrix-driven failures (We adapted SIFT, or Onset Theory, to our application and I am very much a believer in this criteria for this kind of situation). This will inform you about the temperature limits of your system and will show you just how much your design window shrinks if you are working at extreme temperature limits.
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John Troll
Senior Mechanical Engineer
Genesis Engineering Solutions, Inc.
Lanham, MD
Original Message:
Sent: 03-12-2018 15:33
From: Matthew Nahan
Subject: Thermal-Cycle Endurance of Bonded Joints
Does anyone know of work toward thermal fatigue of composite bonded joints? Here 's some background to my question. ... I've done my literature search and this subject is a void. It's standard practice to subject space flight hardware to a mere 10 to 20 thermal cycles during component qualification tests. It is not uncommon that these tests reveal partial bond failure, particularly where a metal fitting is bonded to composite, with failure in the composite. It would be costly to require all such qualification tests to include the thousands of cycles representative of end-of-life conditions. We also do not analyze to end-of-life conditions because: 1) parts are typically not flight critical, and 2) analysis method does not exist. As its stands, if a part experiences disbond during the few cycles tested, then the part is either redesigned to pass that test, or if the disbond is partial and not catastrophic, then the component can be subject to a full end-of-life number of cycles in hope of showing that it does not worsen. All that analysis can do is help to guide the design toward passing a single thermal-cycle.
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Matthew Nahan
[JobTitle]
Lockheed Martin Space Systems Co
Decorah IA
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