Design Guidelines

Shear Properties

Shear strength is used as one of the input factors to determine the sandwich laminate thickness. However, laboratory tests of shear properties do not do justice for some of the materials, primarily honeycomb. ASTM and ISO test standards for testing for shear strength specify the sample size; essentially a thin strip of material that is then loaded on a relatively short span and measured. Honeycomb structures are disadvantaged in thin strips, since the cell structure is broken along the edges when the sample coupons are cut from a panel. Furthermore, strength values may often be sited without indication of deflection or elongation. High shear strength may be recorded for low elongations, which may necessitate over-designed skin laminates to assure that small deflections will not induce "Core Shear Failure" and consequently catastrophic structural failure. Conversely, low shear strength may be recorded for high elongations, corresponding to deflections that could not be sustained by other design factors or limitations of the laminates, such that this low shear material would never fail in shear. Full scale structures must be evaluated as a whole, and increasingly large panel performance and shear elongation are a critical consideration. This is particularly true when considering honeycomb sandwich laminates. In early 1960s and 1970s, a lot of boats were built with an early version of PVC linear foam, with comparatively low shear strength. Several of these boats are still in service and have been obviously very successful designs.

Any presumption that shear strength is the key design parameter is simply not true. While many composite professionals may have built airplane models with balsa before learning about fiberglass, it was logical to start with a rigid base and laminate to it. Conversely, to apply high performance laminates on either side of a compliant material is counter-intuitive. But the current generation of young airplane model builders is actually using fiberglass strapping tape over the surfaces of expanded polypropylene foam. Build time is reduced, but most importantly the virtually indestructible nature of the resulting structure has provided a far more satisfying product. Likewise, when designing structures using polypropylene honeycombs, one must remember that there is a significant difference in the value of the stress and strain at yield than there is at shear ultimate. Polypropylene honeycomb can stretch and carry loads without failure after the yield point, so that the value at ultimate shear is still higher than at yield. It must also be reminded that the basis of many design specifications pre-date the common of multi-axial stitched reinforcements, which are generally higher in strength but not as thick, and therefore have given up some flexural stiffness. However, when used with sandwich construction to provide the required cross section for flexural stiffness, multi-axial stitched reinforcements are ideally suited for sandwich construction, achieving further weight reductions compared with previous laminations. Furthermore, since the increased strength of multi-axial stitched reinforcements is achieved at greater strains, Nida-Core Polypropylene Structural Honeycomb is increasingly selected as the most appropriate core material. A thorough designer must therefore consider the most important test for core materials - shear strain in %, or shear elongation after the yield point (ISO 1922) which most accurately determines the degree of toughness for a specific core. It is not important whether one uses the shear yield or shear ultimate value in design, what is important that based on these figures, appropriate safety factors are built in. For polypropylene honeycomb one can design much higher up the elastic curve because the factor of safety is in the balance of the elastic range of the curve, and then in shear elongation after yield. We are not saying here that successful designs can not be made with cross-linked PVC or balsawood, with inherently low shear elongation factors, simply the shear stress must be in the lower portion of the curve and not too close to the yield. However, even the balance of the elastic range of the curve is seldom sufficient under severe impacts. Primary focus should be stiffness, while at the same time ensuring there is an adequate safety margin to fall back on. If the structure is stiff enough, the stresses are usually low. However, stiffness without damage tolerance is not a desirable criterion. Several different sources have been used to obtain criteria for composites boat construction. Some are adaptations of wood designs with interchangeable single skin fiberglass equivalent. Several criteria is derived from equivalent designs using metallic materials , primarily aluminum. This criteria seems to work well with some older types of core materials but are lacking when it comes to NEW core materials such as polypropylene honeycomb, especially when thinner skins are used. Most design criteria lacks in areas where stresses beyond normal loads are applied.

The primary goal of most Naval Architects is to design a structure with adequate stiffness, resistance to buckling and impact tolerance. All of these criteria are achievable with Nida-Core Structural Honeycomb.

Sandia National Laboratories

In the early 1990's Sandia National Laboratories in Albuquerque, NM conducted a series of tests to determine the best suited material for the construction of their planned blast chamber for laboratory test purposes. Existing chambers were made of steel, were expensive to maintain, and most importantly were hard and time consuming to reload.

Sandia engineered a blast chamber with lightweight sectional construction, using pins to hold adjoining sections together.

A series of tests were conducted to determine the laminate suitability for such a blast chamber. The test fixture consisted of 28" diameter open end steel blast chamber with (3) 1/2" holes for chamber venting and detonation charge wiring installations.

A laminated Nida-Core polypropylene honeycomb panel with Kevlar (KB125X2) and DOW Derakane vinyl ester resin on both sides spherical 28" panel was mechanically attached to the open end of the cylinder by the 5/8" thick aluminum ring and (4) 3/4" thick bolts at 21" centers.

Here is a fine example of an 120' Mega-yacht hull tooling (see image on right). Built by master mold makers Vertorworks in Titusville, FL, fully cored with Nida-Core Structural polypropylene honeycomb (38 mm thickness). Result is increased stiffness reduced resin and fiberglass consumption, excellent long term structural stability.

1. A 5 gram explosive charge of C-4 was set off inside that cylinder.
A sample was removed and cut into 4 quarters.
NO VISIBLE DAMAGE WAS DETECTED

2. A 10 gram explosive charge of C-4(plastic) was set off inside that cylinder, The test specimen was removed and cut into 4 quarters and inspected for damage. Again, NO VISIBLE DAMAGE OR DELAMINATION OCCURRED.

3. A 15 gram explosive charge of C-4 was set off inside that test cylinder, The test specimen was removed and cut into 4 quarters and inspected. 15 grams of explosive C-4 is equivalent to 19.2 grams of TNT Specimen shows no delamination, 5/8" thick aluminum ring is deformed between 2.5 and 3.5 inches, 3/4 " bolts are deformed and must be sawed off.

Preliminary data analysis indicated the following:

1. Incident shock : 112 psi

2. Reflective shock load(multiple) : 220 psi

It was concluded that acoustic transmission showed no delamination damage to the specimen. Same tests with alternative core materials (like balsa wood) showed catastrophic damage to the specimen.

It was concluded, based on this test that Nida-Core was most suited core material for construction of lightweight modular blast chamber for Sandia National Laboratories.