Virtual Testing and Its Application in Aerospace Structural Parts

Abstract

In many occasions, metallic parts will undergo plastic deformation either during their service usage or prior to their actual operation in the manufacturing and assembly phases. Under these circumstances, material properties may change considerably and must be accounted for when estimating the residual strength capability of parts. The change in material properties will occur when load has been removed and the work-hardening phenomenon has caused increase in material yield value, reduction in percent elongation, and degradation in fracture allowables. For these reasons, new static and fracture data must be generated if safe-life assessment must be performed on fracture critical parts. Fracture data are currently obtained through the ASTM testing standards, which are costly and labor intense. Difficulties associated with preparing the specimen, precracking the notch, recording and monitoring the data, obtaining the variation of fracture toughness versus part thickness, capturing data in the threshold region, and repeating the test in many cases due to invalid results make the virtual testing technique extremely helpful to overcome the time and cost related to the above-mentioned testing difficulties. Farahmand (Fatigue & Fracture Mechanics of High Risk Parts, Chapman & Hall, 1997, Chapter 5; Fracture Mechanics of Metals, Composites, Welds, and Bolted Joints, Kluwer Academic Publisher 2000, Chapter 6) utilized the extended Griffith theory to obtain a relationship between the applied stress and half a crack length by using the full stress–strain curve for the material under consideration. The extension of this work led to estimation of material fracture toughness and construction of full fatigue crack growth rate curve (Farahmand, J. Fract., 2007, Vol. 75, pp 2144–2155). This technique will be very useful to implement when materials undergo plastic deformation (the work-hardening phenomenon) and fracture allowables are needed to conduct a meaningful life analysis assessment of parts. First, Farahmand’s virtual testing theory is described briefly and, subsequently, the application of this methodology to two cases of aerospace pressurized tanks, exposed to plastic deformation, presented in this chapter.

Appendix

To appreciate the usefulness of the virtual testing technique, numerous fatigue crack growth rate data (da/dN versus ΔK curve) for several aerospace alloys were plotted using the virtual testing approach. In all cases, data generated by this technique were compared with test data extracted from the NASGRO database. Excellent agreement between the test data and virtual testing technique can be seen. The following is a list of a few alloys that have been used in manufacturing aerospace parts. Figures 1.22, 1.23 and 1.24 are 2000, 6000, and 7000 series aluminum alloys. Figures 1.25, 1.26 and 1.27 are unalloyed and binary, ternary, and quaternary titanium alloys. Figures 1.28, 1.29 and 1.30 are magnesium, copper–bronze, and Russian aluminum alloys. Figures 1.31, 1.32, and 1.33 are miscellaneous super-alloys, miscellaneous corrosion- and heat-resistance steel, and Ni alloys. In all cases, the fracture toughness data from the test data were used for the accelerated region to generate the da/dN versus ΔK curve. If the fracture toughness value is not available for the material, the extended Griffith theory can be used to provide the needed Kc value. However, as mentioned previously, the full stress–strain curve is required for the virtual testing analysis.

Fig. 1.22

Fatigue crack growth by virtual testing versus NASGRO database (2000 series aluminums)

Fig. 1.23

Fatigue crack growth by virtual testing versus NASGRO database (6000 series aluminums)

Fig. 1.24

Fatigue crack growth by virtual testing versus NASGRO database (2000 series aluminums)

Fig. 1.25

Fatigue crack growth by virtual testing versus NASGRO database (binary and unalloyed titanium alloys)

Fig. 1.26

Fatigue crack growth by virtual testing versus NASGRO database (ternary titanium alloys)

Fig. 1.27

Fatigue crack growth by virtual testing versus NASGRO database (quaternary titanium alloys)

Fig. 1.28

Fatigue crack growth by virtual testing versus NASGRO database (magnesium alloys)

Fig. 1.29

Fatigue crack growth by virtual testing versus NASGRO database (Russian and magnesium alloys)

Fig. 1.30

Fatigue crack growth by virtual testing versus NASGRO database (Al-Bronze and Russian zinc alloys)

Fig. 1.31

Fatigue crack growth by virtual testing versus NASGRO database (super alloy and corrosion resistance Nitronic alloys)

Fig. 1.32

Fatigue crack growth by virtual testing versus NASGRO database (corrosion resistance alloys)

Fig. 1.33

Fatigue crack growth by virtual testing versus NASGRO database (corrosion resistance and super alloys)