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Got Shear Lock?

Funny…I was at the barber the other day and he kept complaining about his shears locking up…pesky scissors keep jamming on me he said! Well…obviously this is nothing but banal witticism, because that’s surely not what this article is about. I draw your attention to an experience I had regarding a fabric drapeability study that I performed inorder to evaluate fabric compliance over non-developable tool geometry…huh…bear with me. First the basics, when draping a composite fabric (or tape) over a tool, particularly a tool with compound curvatures, it is incumbent upon the designer to check for wrinkling and/or bridging in the fabric; especially near a radius or doubly curved surfaces. Fabric distortions of this type are clearly undesirable, and often yield poor laminate performance in terms of stiffness and lower than expected margins.  

Naturally, for analysis endeavors such as this, I turn to simulation for the answers. Fibersim® was the tool of choice for generating a simulated draping behavior to check for anomalies. Fibersim® manages multiple combinations of material, ply orientations and stacking sequences. It also captures variations in producibility that may yield wrinkling or bridging. Other capabilities include managing splice, dart and seed point locations that control fabric drapeability...but I digress.

Ok…now let’s talk technical. Arguably, the most important variable when grading a composite design for drapeability, is the shear deformation limit-lock angle (or, being a fan of the acronym I like SDLA). Depending on the material type used (i.e., plain weave, eight-harness or tape…etc.) encountering SDLA  during fabric deposition can produce significant problems for technicians. Therefore, its important that an engineer is aware of the hazrads associated with SDLA by first having a basic understanding of SDLA and its influence on structure.

"Basically", SDLA is the maximum allowable angle allowed when, during deposition, the fabric is stretched until an excessive in-plane shearing occurs. This in-plane shearing of the fiber tows, a kinematic behavior known as trellising, is simply a scissoring action between the warp and weft fiber tows resulting in an increase in elastic tensile stress between the two tows during pivoting. Invariably, the cessation of trellising occurs when an intra-ply shear lock event has occurred (meaning: the tows can no longer shear over one another...the fabric has reached a maximimum permissible pivot angle). The telltale sign is when one can notice an out-of-plane fabric distortion (See image below).

Image showing Out-of-Plane Fabric Distortion during In-Plane Shearing

Okayyy…that was a hand full! The important concept to take away here is...recognition of a fabric's shear limit-lock angle and the potential hazards associated with it as it applies to draping, splice and dart locations and process integrity. So, how do we prevent SDLA…well, heck if I know! Actually I have a few ideas. Namely, consider designing the part correctly to begin with by observing the readily available quasi industry-driven guidelines that have already established the do’s and the don’ts (like those “technical” terms) when considering manufacturability. One of the more egregious design-side errors I see is the presence of undersized radii. I can’t for the life me understand why this egregious practice is permitted, the tight radii alone (if a female tool) contributes to most of the bridging issues (or worse yet, flatwise tension failures) that I (unfortunately) continue to see in post-processed parts. Simply put...not good; particularly when a radius-region is subjected to high flatwise tension stresses...now that's trouble! Of course, there are other strategies such as: optimizing seed point locations; manipulation of fiber propagation direction and managing the application of splicing and/or darting to mitigate excessive fabric stretching when negotiating complex geometries...and unfortunately that's a discussion that is beyond the scope of this article (my apologizes).  

In closing, SDLA is an intrinsic property that limits a fabric or tape's shearing capabilities. The SDLA is an invariant property and cannot be reliably determined without testing. Therefore, it is critical to have established shear limit-lock angles prior to commencing any drapeability analyses from which flat-patterns are developed for use in production. If you, or your company, hasn’t considered the impact of a material’s SDLA, then one should seriously consider a comprehensive test program that captures the shear limit-lock angles for all materials. This testing can only serve to improve fabric draping simulations; manufacturability; avoid costly part redesign and hopefully mitigate unexpected in-service part failures.

The topic of fabric shear-locking is extensive. I have only highlighted the basics with the sole intent of creating awareness for both designer and analyst. Further discussion on this topic is planned in future articles. Thanks for your interest in abdmatrix.com.

2018-05-14 05:59:35
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