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The Shear-Lag Model...

Make that overlap length as long as possible…the longer the better I always say…we don’t want the bonded joint to fail…says the confident designer with an unfettered surety…hmmm...well, not quite.

Longer is not necessarily better in this case. So, for those of you interested in approximating the performance of an adhesively bonded joint “quickly” while adhering to a few basic engineering principles that will establish optimal joint parameters, then perhaps you may want to take a look at one of the simplest closed-form methods available. In 1938, the Volkersen’s Method, also known as the “shear-lag model”, was first used for mechanical joints with fasteners. Invariably, this method was used to assess failure in adhesively bonded lap joints. Of course, the method is not without some underlying assumptions which limit its use, but I will be address those later.

Volkersen shear-lag shear stress bonded joint adhesive

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The Free Edge Effect

The free edge effect is an inter-galactic phenomenon found on the outer reaches of the cosmos that leads to a parallel…well…no, not exactly. What I’m referring to is a region within a laminate that produces a fully three-dimensional stress field at the free edge, then decays quite rapidly to a two-dimensional stress field as the distance from the free edge increases. The three-dimensional stress field is responsible for delamination at a free edge, and is commonly referred to as inter-laminar stresses. A composite analysis is typically confined to classical lamination theory. Unfortunately, this assumes that all out-of-plane stresses are zero, making the determination of transverse stresses impossible. Consequently, this assumption is not valid when one is interested in calculating out-of-plane stresses in regions near or at a free edge.

stress Poisson notch inter-laminar free edge cutout

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Quasi-Isotropy | Wait...what is that?

### Quasi-Isotropy…wait…what is that?

What is it you ask…well, this is one of my favorite meals. A large quasi-isotropy with a side of fries and coke. LOL…but of course is isn’t…actually, this is a laminate that when constructed correctly emulates a metallic material that follows the isotropic relationship defined as: E_{x}= E_{y} = E_{θ. }Quantitatively this can be described using a *general **rule **for a **quasi-isotropic** layup. Simply **apply the following equation** that defines the angle between the plies for a symmetric laminate having an identical number of plies at each orientation**:*

**Equation: π/n → n≥3**

stiffness quasi matrix laminate isotropy composite