I am not new to theoretical information pertaining to Fatigue & Damage Tolerance but kinda a newbie when it comes to practical application of it. Even my theoretical exposure is just above a year and thus I don't have the exposure of even a moderately experienced F&DT practicing engineer.
With my prior employer, I have done a couple of F&DT projects of typical cases i.e. stiffened panel of a fuselage and Wing bottom skins subjected to only one loading condition i.e. in case of fuselage skin, it was just internal pressurization & with Wing, it was as limit lift load condition.
In both cases, a crack was initiated on both sides of rivet/fastener hole of a center bay with the stiffener failed to demonstrate fracture arrest, perform a Fatigue crack growth & eventually establish inspection intervals.
I used FEM to calculate SIF (K) variation vs crack length and plot the residual strength diagram manually (Excel). We had a spectrum, in which the fluctuations were already cycle counted and broken down in to a series of constant amplitude cycles. So, I just used Excel (no retardation was accounted however Crack closure i.e. (Delta K)effective was used) to determine Fatigue Crack Growth rates.
Since I used FEM result i.e. max tensile stress levels at limit load, I was able to scale down the peak stresses or K linearly down to the cycle Ratio (Smin & Smax). Also, I don't know which original VA spectrum was used to arrive at the cycle counted one which was handed over to me.
At my spare time outside work, I want to learn the usage of a F&DT software to do DT analysis. I found a free version of AFGROW and I think it will do to start with. I do a lot of static repairs and I want to try to perform a DT analysis to some of the simpler repairs to get some familiarity of the software & procedure.
I've been going through the user manual of Afgrow and I have a few questions.
1. Spectrum: Most of our repairs are with in the fuselage. So which spectrum should I use to get an accurate VA loading representation. I understand TWIST is used mainly for Wing bottom skins and some argue that it is applicable to top portion of fuselage since they see similar loads (of course in fuselage internal pressurization hoop & longitudinal tensile stresses need to be added as well). Most of our repairs go on the cargo pit of the transport aircraft and thus the bottom portion of the fuselage. Any suggestions on what spectrum I can use and how to get that spectrum?
2. Load Level input to AFGROW: I am a little confused about this part as well. In the user manual, the Tensile loads/stresses are entered as ratio to reference stress. What is this reference stress? For ex: At a hole in a plate, if I calculate the stress due to pin loading along with stress due to bypass load at limit load, how do I enter that in to corresponding AFGROW model?
I have a person at my current employer who I can ask a few questions, but usually he is very busy and since this is not an employer sponsored exercise, I would want to limit my interaction with him only to items which are really mind boggling.
I am hoping to get a discussion going on here with the experienced good folks on this forum.
Thanks & regards,