Two key parameters that needed to be calculated in order to plot the Failure Assessment Diagram (FAD) are Fracture Toughness ratio, Kr, and Load Ratio, Lr. A structure is considered as safe if the fracture toughness, KI is lesser than its critical value KIc, while, the structure will fail if the KI to KIc ratio is equal to 1. Lr determines how the failure will behave. There are two types of failure of materials; plastic collapse and brittle fracture. Plastic collapse is a slow mechanism where the materials started to permanently change. Plastic collapse provides greater load carrying capacity. Failure due to plastic collapse allows a margin of safety as it developed over a long period of time. Therefore, failure can be detected early and necessary arrangement can be made to avoid accidents.
While, brittle fracture happens in a fast manner and is considered as most dangerous failure as there is no room for a margin of safety. Due to low ductility of the material, evidence of distortion may not present until the material fails. Figure 2 below illustrates the significance of the FAD diagram. If a plot of Kr and Lr fall within the upper-left side of the graph, the material is predicted to approaching brittle fracture, while the material is said to have plastic collapse if a plot fall within the bottom part of the graph.
Figure 2: Failure Assessment Diagram significance (Ragupathy et al., 2010).
As for the question, the Kr and Lr plot fall within the safe region of FAD. It means that the vessel can still be operated within the minimum working pressure of 30 MPa or lesser to maintain the integrity of the vessel. However, it is mentioned that the vessel should be able to withstand twice its working pressure can be misleading. It is because, although the plot fall within the safe area, increasing the vessel’s working pressure to twice of current working pressure (30 MPa) may increase the value of Kr and therefore could cause the system to approach brittle fracture which is undesirable. Therefore, it is not recommended for the pressure vessel to stay in service without any modification to the process as evidence of distortion in brittle fracture mechanism might not able to be detected until it is too late.
Question 2: Corrosion Rate
Polarisation Curve for Carbon Steel Pressure Vessel
The slope of the polarisation curve, ∆y/∆x = ∆I/∆V = 0.5143 µA/mV. However, note that Ohm’s Law is given as R=V/I.
Therefore, the polarisation resistance, Rp= 1/slope
= 1.944 mV/µA
Icorr = B/Rp , where B = BaBc/2.3(Ba+Bc), given Ba = 60mV/decade , Bc = 120 mV/decade
= (60 x 120)/2.3(60+120)
Thus, Icorr = B/Rp = 17.39/1.944
= 8.945 µA/cm2 (assumed that 1 cm2 of sample is used to obtain data)
The final wall thickness over 20 years of usage can be calculated using Faraday’s Law:
m/s = Mit/zF, and m = ρsd, where
m/s = total weight loss, centimetres per unit...