Stress-cycle diagram

Stress cycle diagrams (Woehler diagrams) remain the most commoniy used means for evaluating long-term cyclic, i.e., fatigue, behavior of fiber composite piastics. With this method, totai specimen faiiure, i. e., fracture, is equated with damage = 1, Fig. 1.65 [148]. 

Their stress-cycle diagram consists of the straight line portion only, representing the finite life region FL (curve 3 in Fig. 7.12). 

Figure 7.12 Schematic stress-cycle diagrams of different materials.

As an example, for room-temperature applications most metals can be considered to be truly elastic. When stresses beyond the yield point are permitted in the design, permanent deformation is considered to be a function only of applied load and can be determined directly from the stress-strain diagram. The behavior of most plastics is much more dependent on the time of application of the load, the past history of loading, the current and past temperature cycles, and the environmental conditions. Ignorance of these conditions has resulted in the appearance on the market of plastic products that were improperly designed. Fortunately, product performance has been greatly improved as the amount of technical information on the mechanical properties of plastics has increased in the past half century. More importantly, designers have become more familiar with the behavior of plastics rather than... 

Notable features were the pronounced hysteresis, unrecovered strain and Mullins effect (whereby re-loading follows a stress-strain path closer to the unloading path than the original loading path). From curves such as these we calculated several quantifiers of the inelasticity. Consider the first cycle for material PU1, shown above. It defines three zones A, B and C in the stress-strain diagram. 

Fig. 23. Stress-strain diagram for a loading (1), unloading (2), and reloading (3) cycle of a PTMT film...

F%. 48a-c. Stress-strain diagrams of the loadin unloading cycles of the polyester urethanes a, b and c iiKasured at 348 K... 

The close relation between the composition and the mechanical properties of these polymers is reflected in the stress-strain diagrams measured at 300 K and 348 K (Figs. 47 and 48). Hence, at ambient temperature for the spedfied experimental conditions a distinct increase of initial modulus (11. 45 and 1 MNm ), stress-hysteresis (ratio of area bounded by a strain cycle to the total area underneath the elongation curve 60,80 and 90 %) and extension set (30,65 and 100 %) can be obsened with increasing hard segment content of polyester urethane (a) to (c). 

By subjecting a single lap shear specimen to dynamic cyclic load, the fatigue behaviour of adhesive bonds is determined. The test values are plotted in the stress-cycle (Woehler) diagram where the number of load cycles is represented as a function of the component load. An example is shown in Fig. 23. 

Figure 2-47. Stress-strain diagram of an elongation-recovery cycle of the reference elastomer at 27 "C.

Fig. 7.5. Stress-strain diagram of a volume element during microcrack formation. During the loading and unloading cycle, energy is dissipated, increasing the fracture toughness...

Draw a stress-strain diagram for a complete cycle ... 

Figure 6-75. Break curve (as) and curves for damping extremes yam) on stress versus number of cycles diagram.

Figere 6.5 Schematic stress-strain diagram showing linear elastic deformation for loading and unloading cycles. 

In the discussion on dynamic properties (Section 2.4), it was pointed out that there is an energy loss per cycle of stress and strain proportional to the loss creep compliance J". This is represented by a hysteresis loop on the stress-strain diagram. Figure 4.7. 

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