Fatigue endurance

Cumulative Damage. Pressure vessels may be subjected to a variety of stress cycles during service some of these cycles have ampHtudes below the fatigue (endurance) limit of the material and some have ampHtudes various amounts above it. The simplest and most commonly used method for evaluating the cumulative effect of these various cycles is a linear damage relationship in which it is assumed that, if cycles would produce failure at a... 

Properties desired in cable insulation and flexible circuit substrate materials include mechanical flexibiUty, fatigue endurance, and resistance to chemicals, water absorption, and abrasion. Both thermoplasts and thermosets are used as cable-insulating materials. Thermoplastic materials possess excellent electrical characteristics and are available at relatively low cost. 

Although in many respects acetal resins are similar to the nylons, they may be considered to be superior to them in their fatigue endurance, creep resistanee, stiffness and water resistance. The nylons (except under dry conditions) are superior in impact toughness and abrasion resistance. 

The data presented in Figure 19.7 were obtained on a Sonntag-Universal machine which flexes a beam in tension and compression. Whereas the acetal resin was subjected to stresses at 1800 cycles per minute at 75°F and at 100% RH, the nylons were cycled at only 1200 cycles per minute and had a moisture content of 2.5%. The polyethylene sample was also flexed at 1200 cycles per minute. Whilst the moisture content has not been found to be a significant factor it has been observed that the geometry of the test piece and, in particular, the presence of notches has a profound effect on the fatigue endurance limit. 

Example 2.22 A certain grade of acrylic has a Kc value of 1.6 MN and the fatigue crack growth data as shown in Fig. 2.77. If a moulding in this material is subjected to a stress cycle which varies from 0 to 15 MN/m, estimate the maximum internal flaw size which can be tolerated if the fatigue endurance is to be at least 1(P cycles. 

There is no general rule as to whether or not glass reinforcement enhances the fatigue behaviour of the base material. In some cases the matrix exhibits longer fatigue endurances than the reinforced material whereas in other cases the converse is true. In most cases the fatigue endurance of grp is reduced by the presence of moisture. 

Dauer-haftigkeit, /, durability, permanence, -haltbarkeit, /. Mech.) fatigue endurance, -hefe, /. permanent yeast, zymin. -leistung, /. continuous output, -magnet, m. permanent magnet, -milch,/, sterilized milk condensed milk dried milk. 

Fig. 8.71 Corrosion fatigue endurance limits for various alloys in aerated salt solutions or...

Fatigue data are normally presented as a plot of the stress (S) versus the number of cycles (N) that cause failure at that stress the data plotted defined as an S-N curve (Fig. 2-43). The use of an S-N curve is used to establish a fatigue endurance limit strength. The curve asymptotically approaches a parallel to the abscissa, thus indicating the endurance limit as the value that will produce failure. Below this limit the material is less susceptible to fatigue failure. 

Fig. 2-43 S-N curve establishes fatigue endurance limit strength.

Two conclusions can be drawn from an inspection of the S-N curve (1) the higher the applied material stress or strain, the fewer cycles the specimen can survive and (2) the curve gradually approaches a stress or strain level called the fatigue endurance limit below which the material is much less susceptible to fatigue failure. Different materials may... 

These characteristics can be further enhanced and their applications widened by fillers, additives, and reinforcements. Compounding properly will yield an almost limitless combination of an increased loadcarrying capacity, a reduced coefficient of friction, improved wear resistance, higher mechanical strengths, improved thermal properties, greater fatigue endurance and creep resistance, excellent dimensional stability and reproducibility, and the like. 

Consequently, the mechanical properties normally are characterized by lower strength, lower ductility, lower impact strength, and reduced fatigue endurance... 

Polysulfones are appreciated for their good mechanical and electrical properties, rigidity, good creep behaviour, fatigue endurance, fair shrinkage and moisture uptake, broad range of service temperatures (-100 up to +150/200°C), optical and microwave transparency, fire resistance, suitability for food contact and sterilization. 

Polyamide-imides are appreciated for good mechanical and electrical properties high service temperatures (up to 220°C with possible long service times at 260°C) rigidity good creep behaviour fatigue endurance low shrinkage and moisture uptake inherent flame retardancy chemical resistance usability down to -196°C. 

TPU applications are in soling materials, ski boots, industrial belting, and in wire cable needing good tear strength, abrasion resistance and flex fatigue endurance. 

TPEE or COPE applications are in the automotive sector for blow-moulded boots and bellows, wires and cables, industrial hoses needing a higher rigidity, good heat resistance, good fatigue endurance and tear strength. 

TPS are appreciated for their broad range of hardnesses, down to very low values (25 Shore A for SBS) low price fatigue endurance to repeated flexions and impacts abrasion resistance... 

Good thermo-mechanical and electrical properties rigidity impact strength fatigue endurance heat behaviour with continuous use temperatures from -196°C up to +220°C tribological properties for suitable grades. 

RESINS (Acetal). These are thermoplastic resins, obtainable both as homopolymers and copolymers, and produced principally from formaldehyde or formaldehyde derivative. Acetal resins have the highest fatigue endurance of commercial thermoplastics. A variety of ionic initiators, such as tertiary amines and quaternary ammonium salts, are used to effect polymerization of formaldehyde. Chain transfer, shown by the following reactions, controls the molecular weight of resulting resins ... 

---