Residual strength

Notch effect. There may be fibre and matrix damage through the thickness, causing a local loss of stiffness. This has the same effect as an insertion or hole that raises a stress concentration, but probably less than 3 as in the case of a circular hole. 

Local buckling. The shear-driven delaminations can reduce the buckling stress of sublaminates by an order of 4, so that local buckling can release enough energy to propagate the buckling envelope in an unstable fashion. 

Global buckling. Can occur in the absence of delamination just due to the local reduction in flexural stiffness. 

The behaviour of structural elements, like stiffened compression panels, can differ from these simple coupon tests. For example, the buckling modes along a panel will have crests and node lines, spaced at half-wavelengths of order of the stiffener pitch. 

Notch effect Local buckling Global buckling Global/local Buckling 

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The University developed a method of determination of the material residual strength, based on measurement of the change of phase velocity of ultrasonic waves, as well as an ultrasonic flaw detector-tomograph with multi-element transducers of the type of phased acoustic array. It enables control of the internal structure of materials and products of up to 300 mm thickness, with the resolution of up to 0.5 mm. In the same university, work on NDT is also carried out in the welding and electro-acoustic departments. 

An important aspect of micromechanical evolution under conditions of shock-wave compression is the influence of shock-wave amplitude and pulse duration on residual strength. These effects are usually determined by shock-recovery experiments, a subject treated elsewhere in this book. Nevertheless, there are aspects of this subject that fit naturally into concepts associated with micromechanical constitutive behavior as discussed in this chapter. A brief discussion of shock-amplitude and pulse-duration hardening is presented here. 

It is no surprise that increased shock amplitude should result in increased residual strength or material hardness. Increasing shock amplitude results in... 

Appleton and Waddington [40] present experimental evidence that pulse duration also affects residual strength in OFHC copper. Samples shock loaded to 5 GPa for 1.2 ps pulse duration exhibit poorly developed dislocation cell structure with easily resolvable individual dislocations. When the pulse duration is increased to 2.2 ps (still at 5 GPa peak stress) recovered samples show an increase in Vickers hardness [41] and postshock electron micrographs show a well-developed cell structure more like samples shock loaded to 10 GPa (1.2 ps). In the following paragraphs we give several additional examples of how pulse duration affects material hardness. 

Sun, C., Gent, A.N., and Marteny, P., Effect of fatigue step loading sequence on residual strength. Tire Sci. Tech., 28, 196, 2000. 

Mai Y.W. (1988). Controlled inlerfacial bonding on the residual strength of fatigue-damaged carbon fiber-epoxy composites. J. Mater. Sci. Lett. 7, 581-582. 

Fig. 36. Dependence of weight gain during oxidation (1500 °C 1000 h) and residual strength after oxidation of HPSN with different Si02/Y203 ratios [446]...

Fig. 39. Weight loss and residual strength of // Si3N4 ceramics with 3.2 (1), 5.0 (2) and 7.3 (3) vol% Y2O3/AI2O3 additives in 1 N HC1 at 60 °C as function of time...

Fig. 42. Weight loss and residual strength of Si3N4 ceramics under hydrothermal conditions at different temperatures (corrosion time 200 h)...

Material Tensile strength, a (MPa) Max. fatigue stress, amax (MPa) max Fatigue cycles Residual strength (MPa)... 

All the above nanocomposites showed improvements of thermal resistance by h-BN addition. In particular, Si3N4/BN has aA /c (the temperature difference above which the residual strength decreases suddenly) as high as 1500°C, 50% higher than that of Si3N4 monolithic ceramics. 

Finally, the mode of application of different forms of Palmgren-Miner s rule modifications or residual strength approaches is linked also to the counting algorithm involved, see also the next chapter. 

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