Flexural Modulus of Composite Deck Boards

Flexural strength of the material or the profile is basically, in a simplified description, a force applied per square inch at the sample failure, normalized with the support span and the specimen cross-sectional configuration. In the same manner, flexural modulus is a force applied per square inch and causing a certain deflection of the sample, normalized with the support span and the specimen cross-sectional configuration. 

The principal readout of a flex strength test is a load at failure, which linearly depends on a support span, that is, on a support span in the first power. An increase of the support span leads to a proportional decrease of the load at failure for the same specimen. 

The principal readout of a flex modulus is a load causing the given deflection, or, alternatively, a deflection caused by the given load. Both of them are much more sensitive to a support span compared to that of a load at failure in a flex strength test. The load/deflection ratio depends on a support span in the third power. 

For wood-cellulose fiber composite materials, flex strength is measured typically in thousands of psi (pounds per square inch), whereas flex modulus is measured in hundreds of thousands of psi. 

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TABLE 7.32 Effect of support span on apparent flexural modulus of composite deck boards... 

A comparison of Tables 7.33 and 7.34 (and Table 7.37 below) allows us to outline some patterns regarding flexural modulus of composite deck boards ... 

Effect of Commercial HDPE on Flexural Modulus of Composite Deck Boards... 

In a similar manner, as it was discussed in the preceding section, we can ask— how stiff a WPC deck board can possibly be, if not filled with mineral fillers We know that wood is very stiff, at least in applications WPCs are intended for. As it is shown in Chapter 7, flexural modulus of wood is about 1,500,000 psi. Polymers are much more flexible, and flexural modulus for HDPE is at best at 150,000 psi (Chapter 2). Again, in a very simplified case, for 50% HDPE - 50% wood fiber composites, in which both principal ingredients are ideally mixed and wood fiber is oriented along the flow, that is, longitudinally, the flexural modulus would be equal to a symmetrical superposition of the flexular moduli of the matrix and the fiber, which is about 825,000 psi. 

TABLE 3.13 Flexural strength and modulus of composite 2X6 deck boards, filled with rice hulls (57% by weight) and those filled with rice hulls (32% w/w) and Biodac (29% w/w)... 

TABLE 6.2 The effect of density (specific gravity) of GeoDeck composite deck boards on their flexural modulus. Center point load, support span 14 inches... 

Hence, direct comparisons of flexural strength data published on different composite deck boards should be viewed with the respective reservations. Data on flexural modulus, as it is shown below, are subjected to these factors even much greater. 

However, not aU composite deck boards would satisfy even this low support span requirement. Trex deck board, for example, has flexural modulus equal to 175,000 psi (hsted on Trex technical data sheet). Using Eq. (7.45), one can calculate that at L = 12 in., I = bhVl2 = 0.895 (b = 5-1/2", h = 1-1/4"), load at the deflection of L/180 =... 

Hence, direct comparisons of flexural modulus data published on different composite deck boards should be viewed with the respective reservations. 

Table 7.43 shows a series of flexural modulus values obtained for the same composite deck boards (the same composition and profile) but made from different HDPE (and LLDPE). 

Obviously, flexural modulus values for composite deck boards made with different HDPE materials differ quite appreciably, in the range of 166,000-254,000 psi. 

It is often assnmed that increase of the plastic content in composite deck boards would decrease their flexural modulus. However, it is not always so. Increase in plastic content is often accompanied by changes in other important factors, more directly related to flexnral modnlns. Snch factors are, for example, the filler content... 

TABLE 7.48 Flexural modulus of HDPE- -rice hulls composite deck boards ... 

Calculations show that for a stair tread with a 24-in. span (allowed deflection 0.133"), a composite board should have aEX I value of 548,016 lb X in.. This would be applicable to a hollow GeoDeck Heavy Duty composite deck board (8.1" X 1.55,1 = 1.858 in., E = 374,000 psi, and EX I = 694,892 lb X in. ). For a solid board of a standard dimension (5.5" X 1.25", I = 0.895 in. ), flexural modulus should be at least 776,416 psi, and composite deck boards of such stiffness are not available as yet on the current market (see Table 7.34), except those made of wood. For thin solid board, such as 5.5" X 15/16" (/ = 0.378 in. ), flexural modulus applicable for stair tread with 24" span should be at least 2,054,000 psi, which is much higher than that for typical wood (Table 7.34). 

Let us consider a fence around a section of an animal farm. The fence is constructed as posts connected by composite deck boards. Animals can lean against a board, deflecting it up to a certain extent depending on the force applied, length of the board, and its flexural modulus. 

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