How to make SN fatigue material properties dependent on flow stress values in Ansys nCode DesignLife

Javier Vique

Assuming that my fatigue material properties are calculated by some relationships with UTS (ultimate tensile strength), I would like to update UTS based on nodal stress values given by a text file and then calculate fatigue damage based on updated fatigue material properties.

Javier Vique

Ansys nCode DesignLife has the custom SN method which allows users to calculate damage based on a specific criteria or fatigue material properties as required. Following the Worked Example "Using Python to Define a Custom SN Method" available in the user manual, the below script implements the methodology.

import math

f = open("customtest.out", "w")



def Initialise(SNMethodState):

	f.write("In Initialise\n")


	# Get algorithm properties

	methodProps = SNMethodState.GetMethodProperties()

	FlowStressFile = methodProps['FlowStressFile']

	f.write("File with flow stress is " + str(FlowStressFile) + "\n")


	flow_dict = {}

	with open(FlowStressFile, "r") as file:

		for line in file:

			parts = line.strip().split(',')

			key = int(parts[0])

			value = float(parts[1])

			flow_dict[key] = value



	# Save algorithm props to method state

	SNMethodState.AppendVariable(flow_dict)	


	f.flush()

	return ""



def Reset(SNMethodState):

	return ""



def CalcCycleDamage(SNMethodState):


	# Get the Element and the Layer Ids of the current entity

	EngnProps = SNMethodState.GetEngineProperties()

	NodeId = EngnProps['NodeId']

	flow_dict = SNMethodState.GetVariable(0)


	# Evaluate fatigue damage


	minStress = SNMethodState.GetCycleMinStress()

	maxStress = SNMethodState.GetCycleMaxStress()

	# Calculate Range and Mean

	RangeStress = maxStress - minStress

	MeanStress = 0.5 * (maxStress + minStress)

	f.write("Range stress is " + str(RangeStress) + "\n")

	f.write("Mean Stress is " + str(MeanStress) + "\n")


	# Extract flow stress for node id

	flow_stress = flow_dict[NodeId]

	f.write("Flow Stress is " + str(flow_stress) + "\n")


	# Damage calculation

	coeff =(150./float(flow_stress))**2.5

	new_UTS = flow_stress*(1.+2*coeff)

	#

	# Goodman Mean Stress Correction

	#

	RangeStress = RangeStress/(1 - (MeanStress/new_UTS))

	f.write("Range after mean stress correction " + str(RangeStress) + "\n")

	S1 = 0.9*new_UTS

	S2 = 0.357*new_UTS

	b1 = (math.log10(S2) - math.log10(S1))/3

	SRI1 = 2*S2/math.pow(10,6*b1)

	if RangeStress*0.5 < S2:

		damage = 0.0

	else:

		Life = math.pow(10,(math.log10(RangeStress)-math.log10(SRI1))/b1) 

		damage = 1/Life


	SNMethodState.SetCycleDamage(damage)

	SNMethodState.SetCycleEffectiveStressAmplitude(RangeStress*0.5)


	f.write("Damage is " + str(damage) + "\n")


	return ""