Performance Behavior

All structures under load will deflect in some manner and the resultant moment will create bending and shear stresses. Therefore, it is important that the load analysis be executed correcdy. There are several major considerations that must be addressed before any mathematical analysis can be performed. The following conditions direcdy affect the design stresses that will be obtained by computations.

Moisture Effect

Nylon, the resin base for the first series of STX composites, is a hygroscopic material. Therefore, STX materials exhibit the normal property and dimensional modifications characteristic of nylon as they absorb moisture. Dimensional changes in improperly designed rigidly affixed parts can develop excessive stresses. (Note that the effects of temperature are similar).

Long Term Vs. Short Term Loading

This is a very important consideration since all materials will exhibit creep or relaxation deformation: with plastic materials, it occurs at lower temperatures and stress levels. Hookean stresses, which are instantaneous stresses, are shown in Fig. 4.60 for reference levels only, or for very short-term loadings. However, it is the flexural creep characteristics that govern the usable design stresses, especially when dealing with continuous loading such as is encountered in beams and spring members. The specific curves representing percentage strain vs. time at a given stress of 7,000 psi, is shown in Fig. 4.61.

Stress-strain relations

Stress-strain relations

Strain (in./in.)
- 1 Example of temperature effect on flexural properties

The effectiveness of gussets is very easily analyzed considering that the deflection of any beam is a function of the unsupported length (L) at the third power. By incorporating a gusset at the midpoint of the span, the unsupported length is reduced by half, therefore, the deflection is now decreased by a factor of 8 [(L/2)2]. These simple examples show how both stress and deflection can easily be minimized by taking advantage of the material manufacturing characteristics that allow the forming of ribs and gussets during the molding cycle. Although beam problems were discussed (they are the most commonly used structural members), the same reasoning can be applied to other structural members such as plates or shell sections. Flanges of covers for example, can be handled in that way.

Stress Concentration

The addition of ribs and gussets while reducing deflection can also increase the bending stress if care is not taken in the design stage. Since the ribs or gussets are always in a plane perpendicular to the original surface, the intersection of the planes must comprise a radius to avoid stress concentration. Consequently, it is suggested that a minimum radius of Vs in. be incorporated in all designs. With specific reference to ribbing, the shape and size of the ribs must be carefully analyzed to prevent an increase in the bending stress (Machine Design books review this subject).

Coefficient of Expansion

Since the coefficient of expansion of metals and plasties is different, stresses and deformation can result. A given plastic beam clamped to a steel support at both ends by fasteners, will elongate about three times more than the steel support: a steel = 6.5 x 106 in/in/°F, a STX = 16.2 x 106 in/in/°F. If the plastic beam is not allowed to slide under the fasteners then end stresses will occur. These stresses can be computed simply as follows:

where L = distance between bolts = strain, E = flexural modulus, p = plastic, s= steel, and S = expansion stress

Since this stress is directed in the plane parallel to the beam axis, the beam becomes a column fixed at both ends. Applying the standard Euler equation, the critical buckling load (Pcr) and corresponding stress can now be computed (Scr =Pcr /A). The Euler equation comprises several parameters: E, L, I. Of these three, E is fixed, and L (distance between bolts) is usually fixed, (in most cases more bolts means additional costs). I is therefore the only possible variable. Adding one or more ribs will change I and substantially reduce the induced expansion load so that the beam will remain flat and free of distortion since the expansion stress is now much smaller than the critical stress.

Bolt Torque Effect

Since there be STX and other plastic components that are bolted to other components, it is important to consider the effects of the bolt torque, namely the compressive and shear stresses under the head of the bolt. To retain the maximum torque, the following parameters must be considered: (1) STX material, when loaded will tend to relax (long term effects) and (2) based on the fastener requirements, there is a minimum torque suggested to stretch the bolt (bolt preload).

When the torque drops below this minimum value, the fastener tends to loosen. If the STX part is a vessel for containment of fluids, leakage could result at the flanges even with a gasket. The load due to the torque is computed by

(Euler equation)

where T = torque

This load (F) is then used to calculate the compressive and shear stress as shown by the following equations respectively:

net A washer

nx tx 0.0. washer

where t = thickness of material; and A = 0.785 (O.D.2 - (ID.2)

These equations show that the use of a flat washer to distribute the load and reduce the respective stresses is recommended (permitted Sc design = 7 000 psi, and permitted Ss design = 6 000 psi). To illustrate the torque and relaxation relationship, see Fig. 4.64. The ordinate represents the ratio of original stress to the actual or time dependent stress and the abscissa is the time cycle. The relaxation curve shows that there is no difference in relaxation between stress levels of 5 000 psi and 1 5 000 psi and that most of the relaxation takes place during the first twenty hours.

Incorporating a gasket between the two surfaces changes the shape of these curves depending upon the thickness and hardness of the gasket material. The analysis is now further complicated and must be handled by the theory of beams on elastic foundations to obtain moment distribution and deflection. Good results can be obtained by using an elastomer such as RTV as gasketing material. The cffccts of gasket behavior are shown in Fig. 4.65.

Figure 4.64 Compressive relaxation at 73°F (0.2655 in2 compressed area)

'nlllal Stress - 5000 — 15.000 psi copmr~8T3~ïïo7e7»T«~nT,~rsrcT"ro~o~pTn---------

Tlmo (Hours)

" ; Compressive relaxation at 73F between STX without and with gasket

" ; Compressive relaxation at 73F between STX without and with gasket


Impact Barrier

The glass mat at the core of the STX sheet was shown to impart some very unusual impact properties to the material. The testing performed in the laboratory (Gardner drop test) and the field results from production components have shown that, in contrast to glass reinforced plastics, the failure mode is elastic rather than britde. The resultant damage is a hole with litde or no crack propagation around the impact area whereas the other reinforced materials will shatter under similar conditions. The STX failure mode applies not only at room temperature, but is equally valid at —4:0oF. STX impact resistance increases with the thickness of the material, i.e., lA in. thick plate will resist much higher impact loads than Vfc in. thick plate.

However, there is a better solution that result both in lower material costs and lower weight. By keeping the impact area to a minimum thickness and adding ribs, strength is dramatically improved. As an example of this approach, a 0.10 in. thick plate was pierced when impacted by a xh in. steel ball (1.85 x 10-2 lb) travelling at 164 mph (198 in-lb of K.E.). By adding small ribs, 0.10 in. wide x 0.10 in. high spaced 0.5 in. apart, (the plate thickness remained at 0.10 in.), the steel ball had to travel at 272 mph (552 in-lb of K.E.) to duplicate the break resulting in a substantial improvement.

Vehicle Oil Pan

The oil pan is a critical component in auto and truck applications because any failure would be catastrophic. It must withstand high temperature and impact, keep the bolt torque intact for good sealing, resist vibration and abuse, especially during assembly, and in many instances, functions as a structural member. The pan is divided into basic elements and analyzed as follows: (1) sides are considered to be flat plates uniformly loaded with rigid supports. (2) flanges are analyzed as beams on elastic foundations because of the gasket/metal interfacing, and (3) bottom section is considered as a plate but needs to withstand high impact loads (dynamic loading).

After the bolt torque is established, the washer size must be determined by computation based upon compressive and shear stresses and flange deflection. The flange thickness will also be determined from calculations. After it has been established that these values are safe, the remaining sections such as the sides and bottom can now be designed for thickness and shape based on the values of the static and/or dynamic loads (impact) supplied as part of the input data. If some elements show either high stress or excessive deflection (1% elongation in the elastic range) ribs or gussets can be added and/or the wall thickness increased in selective areas only. History has shown that theoretical analysis yields a design very close to the final manufactured product and meets intended performance requirements dictated by laboratory as well as field testing.


There will be cases where attachments become necessary. One of the most frequent types is threaded parts. In most instances, these fasteners are used repeatedly to attach or remove components. To ensure that the reliability and the life of the frequently used joint are maximized, a threaded insert is used rather than threading the plastic material. In this manner, the wall thickness of the insert and plastic part is reduced, or, if a boss is used, it is smaller in diameter and shorter in height compared to its plastic counterpart. Since £mctai is much larger than £piastic a higher torque can be applied (shear stress is also much larger).

Extremely close toleranccs such as might be necessary for a seal or a bearing may not be within the capabilities of the STX molding process. Machining is not recommended because it would break the nylon surface and expose the glass fibers that then act as wicks for fluid. An aluminum insert that has been finish machined is used as a substitute. Since the ratios of the moduli are quite large (about 20:1 for steel and 10:1 for aluminum), there will be no deformation of the metal insert and only the plastic will be highly stressed. It is most important that these stresses are calculated so that the boss does not split during assembly or that the metallic ring does not become loose because of long term relaxation effects.

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