Other Factors Affecting Fatigue

Surface Condition (Finish and Treatment)

Surface condition is an extremely important factor influencing fatigue strength, as fatigue failures nucleate at the surface. Surface finish and treatment factors are considered to correct the fatigue analysis results.

Surface finish correction factor C f i n i s h is used to characterize the roughness of the surface. It is presented on diagrams that categorize finish by means of qualitative terms such as polished, machined or forged. 1
Figure 1. Surface Finish Correction Factor for Steels


Surface treatment can improve the fatigue strength of components. NITRIDED, SHOT-PEENED, and COLD-ROLLED are considered for surface treatment correction. It is also possible to input a value to specify the surface treatment factor C t r e a t MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4qamaaBa aaleaacaWG0bGaamOCaiaadwgacaWGHbGaamiDaaqabaaaaa@3BA3@ .

In general cases, the total correction factor is C s u r = C t r e a t · C f i n i s h MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4qamaaBa aaleaacaWGZbGaamyDaiaadkhaaeqaaOGaeyypa0Jaam4qamaaBaaa leaacaWG0bGaamOCaiaadwgacaWGHbGaamiDaaqabaGccaaMe8UaeS 4JPFMaaGzaVlaaysW7caWGdbWaaSbaaSqaaiaadAgacaWGPbGaamOB aiaadMgacaWGZbGaamiAaaqabaaaaa@4E41@

If treatment type is NITRIDED, then the total correction is C s u r = 2.0 · C f i n i s h ( C t r e a t = 2.0 ) MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4qamaaBa aaleaacaWGZbGaamyDaiaadkhaaeqaaOGaeyypa0JaaGOmaiaac6ca caaIWaGaaGjbVlabl+y6NjaaygW7caaMe8Uaam4qamaaBaaaleaaca WGMbGaamyAaiaad6gacaWGPbGaam4CaiaadIgaaeqaaOWaaeWaaeaa caWGdbWaaSbaaSqaaiaadshacaWGYbGaamyzaiaadggacaWG0baabe aakiabg2da9iaaikdacaGGUaGaaGimaaGaayjkaiaawMcaaaaa@552A@ .

If treatment type is SHOT-PEENED or COLD-ROLLED, then the total correction is C s u r MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4qamaaBa aaleaacaWGZbGaamyDaiaadkhaaeqaaaaa@39D3@ = 1.0. It means you will ignore the effect of surface finish.

The fatigue endurance limit FL will be modified by C s u r MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4qamaaBa aaleaacaWGZbGaamyDaiaadkhaaeqaaaaa@39D3@ as: F L ' = F L * C s u r MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOraiaadY eacaGGNaGaeyypa0JaamOraiaadYeacaGGQaGaam4qamaaBaaaleaa caWGZbGaamyDaiaadkhaaeqaaaaa@3F6A@ . For two segment S-N curve, the stress at the transition point is also modified by multiplying by C s u r MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4qamaaBa aaleaacaWGZbGaamyDaiaadkhaaeqaaaaa@39D3@ .

Surface conditions can be defined in the Assign Material dialog, where you assign them to each part.

Fatigue Strength Reduction Factor

In addition to the factors mentioned above, there are various other factors that could affect the fatigue strength of a structure, that is, notch effect, size effect, loading type. Fatigue strength reduction factor K f MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4samaaBa aaleaacaWGMbaabeaaaaa@37DD@ is introduced to account for the combined effect of all such corrections. The fatigue endurance limit FL will be modified by K f MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4samaaBa aaleaacaWGMbaabeaaaaa@37DD@ as: F L ' = F L / K f MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOraiaadY eacaGGNaGaeyypa0JaamOraiaadYeacaGGVaGaam4samaaBaaaleaa caWGMbaabeaaaaa@3D79@

The fatigue strength reduction factor may be defined in the Assign Material dialog and is assigned to parts or sets.

If both C s u r MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4qamaaBa aaleaacaWGZbGaamyDaiaadkhaaeqaaaaa@39D3@ and K f MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4samaaBa aaleaacaWGMbaabeaaaaa@37DD@ are specified, the fatigue endurance limit FL will be modified as: F L ' = F L · C s u r / K f MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOraiaadY eacaGGNaGaeyypa0JaamOraiaadYeacaaMe8UaeS4JPFMaaGjbVlaa doeadaWgaaWcbaGaam4CaiaadwhacaWGYbaabeaakiaac+cacaWGlb WaaSbaaSqaaiaadAgaaeqaaaaa@46EA@

C s u r MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4qamaaBa aaleaacaWGZbGaamyDaiaadkhaaeqaaaaa@39D3@ and K f MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4samaaBa aaleaacaWGMbaabeaaaaa@37DD@ have similar influences on the E-N formula through its elastic part as on the S-N formula. In the elastic part of the E-N formula, a nominal fatigue endurance limit FL is calculated internally from the reversal limit of endurance Nc. FL will be corrected if C s u r MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4qamaaBa aaleaacaWGZbGaamyDaiaadkhaaeqaaaaa@39D3@ and K f MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4samaaBa aaleaacaWGMbaabeaaaaa@37DD@ are presented. The elastic part will be modified as well with the updated nominal fatigue limit.

Temperature Influence

The fatigue strength of a material reduces with an increase in temperature. Temperature influence can be accounted by applying the temperature factor Ctemp to modify the fatigue endurance limit FL.

Ctemp can either by assigned directly, or isothermal temperature across the part/element set can be defined to calculate Ctemp as referred by FKM guidelines for elevated temperatures. The temperature defined must be in degree Celsius.

Ctemp at normal temperature = 1

Ctemp at elevated temperature defined as per FKM guidelines for the following materials is highlighted in the table below.

Ctemp user-defined accepts a value between 0 < Ctemp <= 1

Ctemp set to NONE = 1

Type Temp. Condition Ctemp Factor
None**

this is for materials other than the ones below

- = 1
Fine Grain Structural Steel 60℃ < T < 500℃ =1 - [10-3 x (T/℃)]
Other Steels (other than stainless steel)** 100℃ < T < 500℃ =1 - [1.4*10-3 x (T/℃-100)]
GS (Cast steel and heat treatable cast steel) 100℃ < T < 500℃ =1 - [1.2*10-3 x (T/℃-100)]
GJS (Nodular Cast Iron)

GJM (Malleable Cast Iron)
GJL (Cast iron with lamellar graphite)

100℃ < T < 500℃ =1 - aT,D x (10-3 * T/℃)2
Aluminum materials 50℃ < T < 200℃ =1 - [1.2*10-3 x (T/℃-50)]
Material Group GJS GJM GJL
aT,D 1.6 1.3 1.0

If both Ctemp and Kf are specified, the fatigue endurance limit FL will be modified as: FL' = FL ⋅ Ctemp / Kf

Scatter in Fatigue Material Data

The S-N and E-N curves (and other fatigue properties) of a material is obtained from experiment; through fully reversed rotating bending tests. Due to the large amount of scatter that usually accompanies test results, statistical characterization of the data should also be provided (certainty of survival is used to estimate the worst mean log(N) according to the standard deviation of the curve and a higher reliability level requires a larger certainty of survival).
Figure 2. S-N Curve with Scatter Data


To understand these parameters, let us consider the S-N curve as an example. When S-N testing data is presented in a log-log plot of alternating nominal stress amplitude Sa or range SR versus cycles to failure N, the relationship between S and N can be described by straight line segments. Normally, a one or two segment idealization is used.
Figure 3. One Segment S-N Curve in log-log Scale


Consider the situation where S-N scatter leads to variations in the possible S-N curves for the same material and same sample specimen. Due to natural variations, the results for full reversed rotating bending tests typically lead to variations in data points for both Stress Range (S) and Life (N). Looking at the Log scale, there will be variations in Log(S) and Log(N). Specifically, looking at the variation in life for the same Stress Range applied, you may see a set of data points which look like this.
S 2000.0 2000.0 2000.0 2000.0 2000.0 2000.0
Log (S) 3.3 3.3 3.3 3.3 3.3 3.3
Log (N) 3.9 3.7 3.75 3.79 3.87 3.9
As with many processes, the distribution of Log(N) is assumed to be a Normal Distribution. There is a full population of possible values of log(N) for a particular value of log(S). The mean of this full population set is the true population mean and is unknown. Therefore, statistically estimate the worst true population mean of log(N) based on the input sample mean SN curve in Materials and Standard Error in the Material DB and My Material tabs of the sample. The SN material data input in the Material DB and My Material tabs is based on the mean of the normal distribution of the scatter in the particular user sample used to generate the data.
Figure 4. Probability Function of the Log(N) Normal Distribution for S-N Scatter. of a particular user-defined sample data


The experimental scatter exists in both Stress Range and Life data. In the Assign Material dialog, the Standard Error of the scatter of log(N) is required as input (SE field for S-N curve). The sample mean is provided by the S-N curve as log ( N i 50 % ) MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaciiBaiaac+ gacaGGNbGaaiikaiaad6eadaqhaaWcbaGaamyAaaqaaiaaiwdacaaI WaGaaiyjaaaakiaacMcaaaa@3E3A@ , whereas, the standard error is input via the SE field in the Assign Material dialog.

If the specified S-N curve is directly utilized, without any perturbation, the sample mean is directly used, leading to a certainty of survival of 50%. This implies that OptiStruct does not perturb the sample mean provided in the Assign Material dialog. Since a value of 50% survival certainty may not be sufficient for all applications, HyperLife can internally perturb the S-N material data to the required certainty of survival defined by you. To accomplish this, the following data is required.
  1. Standard Error of log(N) normal distribution (SE in Assign Material).
  2. Certainty of Survival required for this analysis (Certainty of Survival in the Fatigue Module context).

A normal distribution or gaussian distribution is a probability density function which implies that the total area under the curve is always equal to 1.0.

The user-defined SN curve data is assumed as a normal distribution, which is typically characterized by the following Probability Density Function:

P( x s )= 1 2π σ s 2 e ( x s μ s ) 2 2 σ s 2 MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiuaiaacI cacaWG4bWaaSbaaSqaaiaadohaaeqaaOGaaiykaiabg2da9maalaaa baGaaGymaaqaamaakaaabaGaaGOmaiabec8aWjabeo8aZnaaBaaale aacaWGZbaabeaakmaaCaaaleqabaGaaGOmaaaaaeqaaaaakiaadwga daahaaWcbeqaaiabgkHiTmaalaaabaWaaeWaaeaacaWG4bWaaSbaaW qaaiaadohaaeqaaSGaeyOeI0IaeqiVd02aaSbaaWqaaiaadohaaeqa aaWccaGLOaGaayzkaaWaaWbaaWqabeaacaaIYaaaaaWcbaGaaGOmai abeo8aZnaaDaaameaacaWGZbaabaGaaGOmaaaaaaaaaaaa@5180@

Where,
x s MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiEamaaBa aaleaacaWGZbaabeaaaaa@3818@
The data value ( log ( N i ) MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaciiBaiaac+ gacaGGNbGaaiikaiaad6eadaWgaaWcbaGaamyAaaqabaGccaGGPaaa aa@3C17@ ) in the sample.
μ s MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeqiVd02aaS baaSqaaiaadohaaeqaaaaa@38D1@
The sample mean log ( N i s m ) MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbiqaaWmdciGGSb Gaai4BaiaacEgacaGGOaGaamOtamaaDaaaleaacaWGPbaabaGaam4C aiaad2gaaaGccaGGPaaaaa@3EDD@ .
σ s MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeq4Wdm3aaS baaSqaaiaadohaaeqaaaaa@38DE@
The standard deviation of the sample (which is unknown, as you input only Standard Error (SE) in the Assign Material dialog).

The above distribution is the distribution of the user-defined sample, and not the full population space. Since the true population mean is unknown, the estimated range of the true population mean from the sample mean and the sample SE and subsequently use the user-defined Certainty of Survival to perturb the sample mean.

Standard Error is the standard deviation of the normal distribution created by all the sample means of samples drawn from the full population. From a single sample distribution data, the Standard Error is typically estimated as S E = ( σ s n s ) MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4uaiaadw eacqGH9aqpdaqadaqaceaacfWaaSWaaSqaaiabeo8aZnaaBaaameaa caWGZbaabeaaaSqaamaakaaabaGaamOBamaaBaaameaacaWGZbaabe aaaeqaaaaaaOGaayjkaiaawMcaaaaa@3FB0@ , where σ s MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeq4Wdm3aaS baaSqaaiaadohaaeqaaaaa@38DE@ is the standard deviation of the sample, and n s MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOBamaaBa aaleaacaWGZbaabeaaaaa@380E@ is the number of data values in the sample. The mean of this distribution of all the sample means is actually the same as the true population mean. The certainty of survival is applied on this distribution of all the sample means.

The general practice is to convert a normal distribution function into a standard normal distribution curve (which is a normal distribution with mean=0.0 and standard error=1.0). This allows us to directly use the certainty of survival values via Z-tables.
Note: The certainty of survival is equal to the area of the curve under a probability density function between the required sample points of interest. It is possible to calculate the area of the normal distribution curve directly (without transformation to standard normal curve), however, this is computationally intensive compared to a standard lookup Z-table. Therefore, the generally utilized procedure is to first convert the current normal distribution to a standard normal distribution and then use Z-tables to parameterize the input survival certainty.

For the normal distribution of all the sample means, the mean of this distribution is the same as the true population mean μ , the range of which is what you want to estimate.

Statistically, you can estimate the range of true population mean as:

log( N i sm )z*SEμlog( N i sm )+z*SE MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaciiBaiaac+ gacaGGNbGaaiikaiaad6eadaqhaaWcbaGaamyAaaqaaiaadohacaWG TbaaaOGaaiykaiabgkHiTiaadQhacaGGQaGaam4uaiaadweacqGHKj YOcqaH8oqBcqGHKjYOciGGSbGaai4BaiaacEgacaGGOaGaamOtamaa DaaaleaacaWGPbaabaGaam4Caiaad2gaaaGccaGGPaGaey4kaSIaam OEaiaacQcacaWGtbGaamyraaaa@539A@

That is,

log( N i sm )z*SElog( N i m )log( N i sm )+z*SE MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaciiBaiaac+ gacaGGNbGaaiikaiaad6eadaqhaaWcbaGaamyAaaqaaiaadohacaWG TbaaaOGaaiykaiabgkHiTiaadQhacaGGQaGaam4uaiaadweacqGHKj YOciGGSbGaai4BaiaacEgacaGGOaGaamOtamaaDaaaleaacaWGPbaa baGaamyBaaaakiaacMcacqGHKjYOciGGSbGaai4BaiaacEgacaGGOa GaamOtamaaDaaaleaacaWGPbaabaGaam4Caiaad2gaaaGccaGGPaGa ey4kaSIaamOEaiaacQcacaWGtbGaamyraaaa@58F7@

Since the value on the left hand side is more conservative, use the following equation to perturb the SN curve:

log( N i m )=log( N i sm )z*SE MathType@MTEF@5@5@+= feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaciiBaiaac+ gacaGGNbGaaiikaiaad6eadaqhaaWcbaGaamyAaaqaaiaad2gaaaGc caGGPaGaeyypa0JaciiBaiaac+gacaGGNbGaaiikaiaad6eadaqhaa WcbaGaamyAaaqaaiaadohacaWGTbaaaOGaaiykaiabgkHiTiaadQha caGGQaGaam4uaiaadweaaaa@4A56@

Where,
log( N i m ) MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaciiBaiaac+ gacaGGNbGaaiikaiaad6eadaqhaaWcbaGaamyAaaqaaiaad2gaaaGc caGGPaaaaa@3D0A@
Perturbed value
log( N i sm ) MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaciiBaiaac+ gacaGGNbGaaiikaiaad6eadaqhaaWcbaGaamyAaaqaaiaadohacaWG TbaaaOGaaiykaaaa@3E02@
User-defined sample mean (SN curve on Materials)
SE MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4uaiaadw eaaaa@3799@
Standard error (SE on Materials)
The value of z MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOEaaaa@36F6@ is procured from the standard normal distribution Z-tables based on the input value of the certainty of survival. Some typical values of Z for the corresponding certainty of survival values are:
Z-Values (Calculated)
Certainty of Survival (Input)
0.0
50.0
0.5
69.0
1.0
84.0
1.5
93.0
2.0
97.7
3.0
99.9

Based on the above example (S-N), you can see how the S-N curve is modified to the required certainty of survival and standard error input. This technique allows you to handle Fatigue material data scatter using statistical methods and predict data for the required survival probability values.

Adjustment of Single SN Curves

This section describes how a slope-based SN curve is modified in HyperLife.
Certainty of Survival
If the certainty of survival is not 0.5 and standard error (SE) is not 0.0, an SN curve is modified by shifting SRI1 and FL.
Figure 5.


S R I 1   =   S R I 1   x   C r MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qacaWGtbGaamOuaiaadMeacaaIXaGaaiygGiaabccacqGH9aqpcaqG GaGaam4uaiaadkfacaWGjbGaaGymaiaabccacaWG4bGaaeiiaiaado eacaWGYbaaaa@438E@
FL = FL x Cr MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qacaWGgbGaamitaiaacMbicaqGGaGaeyypa0JaaeiiaiaadAeacaWG mbGaaeiiaiaadIhacaqGGaGaam4qaiaadkhaaaa@4056@
Cr =  10 z x b1 x SE MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qacaWGdbGaamOCaiaabccacqGH9aqpcaqGGaGaaGymaiaaicdapaWa aWbaaSqabeaapeGaamOEaiaabccacaWG4bGaaeiiaiaadkgacaaIXa GaaeiiaiaadIhacaqGGaGaam4uaiaadweaaaaaaa@44A8@
Where z is a z-value in standard normal distribution that corresponds to the certainty of survival.
Surface Condition and Fatigue Strength Reduction Factor
A factor for surface condition (Cs) and fatigue strength reduction factor (Kf) are applied to fatigue limit to modify slope of the SN curve after Nc_stat cycles in the following manner.
F L =   F L   *   C s /   K f MathType@MTEF@5@5@+= feaahqart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qacaWGgbGaamitaiaacMbicqGH9aqpcaqGGaGaamOraiaadYeacaqG GaGaaiOkaiaabccacaWGdbGaam4Caiaac+cacaqGGaGaam4saiaadA gaaaa@4276@
Where Nc_stat is the number of cycles at static failure transition.
Figure 6.
Static Failure
When a specimen fails at less than or equal to a certain low number of cycles, the failure is not considered as a fatigue failure but considered as a static failure. Once the failure is considered as a static failure, the low number of cycles is defined as the number of static failure transition cycles (Nc_stat). In fatigue analysis, stress amplitudes are supposed to be less than the stress amplitude (Sc_stat) corresponding to Nc_stat in an SN curve in order to apply fatigue failure theories. If static failure check is enabled, HyperLife checks whether stress or stress amplitudes in stress history is greater than Sc_stat, and reports a warning when a stress or stress amplitude exceeds Sc_stat.
  • Nc_stat can be specified in Static Failure options of My Material.
  • Sc_stat is defined via alpha in Static Failure options of My Material, which is a scaling factor to the UTS to determine the static failure stress threshold.
Database materials are to be saved to My Material to edit the static failure options.
SN Curve Modification and Static Failure Transition
HyperLife modifies the user-defined SN curve when certainty of survival is not 0.5, surface treatment is applied, surface finish is applied, or fatigue strength factor is applied. In the modification due to surface treatment (Cr), surface finish (Cs), or fatigue strength factor (Kf), SN curve is only modified after Nc_stat because surface treatment, surface finish, or fatigue strength factor should not affect static failure behavior which does not follow fatigue theories.
Once static failure check is enabled, HyperLife modifies slope (b0) near UTS so that number of cycles at UTS can be 1 as depicted in Figure 7. By default, in HyperLife static failure check is set to No Check (Advance options > Result > Static Failure Check > No Check), with static failure check is disabled, slope near UTS in SN curve is not modified. HyperLife checks static failure and continues to evaluate damages of remaining stress history with Continue (Advance options > Result > Static Failure Check > Continue). You can choose an option to stop damage calculation as soon as OptiStruct detects static failure (Advance options > Result > Static Failure Check > Stop). Depending on the UTS value, there can be a special case where the calculated slope b0 becomes 0.0. In this case damage is 1.0 for all the stress amplitudes greater than or equal to Sc_stat.
Figure 7.

Instead of HyperLife calculating b0 using UTS and Sc_stat, you can define b0 directly in Static Failure options in MyMaterial. If b0 is defined, b0 is honored as it is. If b0 is set to 0.0, damage at Sc_stat is 1.0, and damage at stress amplitude greater than Sc_stat is more than 1.0.

You can choose how Nc_stat is defined in Static Failure options. You can directly define Nc_stat. This is the default way to define Nc_stat. The default value of Nc_stat is 1000. Another way to define Nc_stat is to specify Sc_stat. Sc_stat is specified by a fraction of UTS (using alpha field in Static Failure options). Default Sc_stat value is 0.9*UTS. If Sc_stat is specified, HyperLife calculates Nc_stat using the slope of the SN curve after SN curve shift due to certainty of survival.
If static failure check is activated, static failure is reported when the maximum stress is higher than Sc_stat or corrected stress range is more than Sc_stat.
If static failure check (Continue in Static Failure options) is set, the SN curve is modified so that HyperLife can report a damage value of 1.0 when stress range is 2*UTS and 2*UTS is smaller than SRI1. Thus, stress range higher than Sc_stat reports a damage value different from the user-defined SN curve due to the modified b0 slope in the picture. If static failure check is set to STOP, damage is greater than 1 when stress range is more than Sc_stat. HyperLife stops the run if 2*UTS is less than Sc_stat.
Note: Advance options > Result > Static Failure Transition Cycle > Life/Stress defines which option should drive the static failure check.
Static Failure Transition Cycle = Stress
The option defines stress amplitude threshold with alpha*UTS in static failure options in MyMaterial.
A cycle corresponding to the threshold amplitude is the static failure transition cycle Nc_stat. If static failure check is not “no check”, SN slope at static failure region (b0) is determined by UTS and alpha*UTS unless user defines b0. If internally calculated b0 is 0.0, damage of stress amplitude higher than or equal to alpha*UTS is 1.0.
Static Failure Transition Cycle = Life (default)
The option directly defines transition cycle with Nc_stat in static failure options in MyMaterial. If static failure check is not “no check”, the following statements apply to SN curve: SN slope at static failure region is determined by UTS and Stress amplitude corresponding to Nc_stat by default. if b0 is defined by user in MATFAT, the b0 will be used for the SN slope at static failure region . If b0 =0.0, damage value is 1 at life = Nc_stat, and damage for life shorter than Nc_stat is greater than 1.
Overall SN Curve Modification
Figure 8.


Combining factors from certainty of survival, surface condition, fatigue strength reduction factor, and static failure, the final SN curve that is used in damage calculation is depicted in Figure 8.

Adjustment of Multiple SN Curves

The following adjustment is applied to multi-mean stress SN curves, multi-stress ratio SN curves and Haigh diagram.
Certainty of Survival
Uncertainty of fatigue strength of material can be taken into consideration by means of the standard error of log(stress) and certainty of survival.
For example, if the standard error of log(stress) is 0.2, and certainty of survival has to be 99.7%, HyperLife adjusts the multiple SN curves as follows:
  • log(fatigue strength) = log(user defined fatigue strength) – 3 x 0.2
  • Fatigue strength = (user defined fatigue strength ) x 10(-3 x 0.2) .
In the example, user defined fatigue strength is reduced by 3 standard error which corresponds to 99.7% in normalized Gaussian distribution.
Surface Condition and Fatigue Strength Reduction Factor
A factor for surface condition (Cs) and fatigue strength reduction factor (Kf) are applied to fatigue strength in the following manner:
Fatigue strength = (user defined fatigue strength ) * K’
Where,

K’ = 1.0 for N <= 1000

K’ = Cs/Kf for N > Nc1

log(K’) = log(Cs/Kf) x (3-logN) / (3-logNc1) for 1000 < N < Nc1

Nc1 : transition point

Standard Error of Cyclic Stress-Strain in Strain Life

The Standard Error of Cyclic Stress-Strain curve is defined via the SEc field for EN fatigue. The value of SEc is used to modify the cyclic strength coefficient as:

K'=K'× 10 zn'S E c MathType@MTEF@5@5@+= feaahGart1ev3aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4saiaacE cacqGH9aqpcaWGlbGaai4jaiabgEna0kaaigdacaaIWaWaaWbaaSqa beaadaqadaqaaiabgkHiTiaadQhacqGHxiIkcaWGUbGaai4jaiabgE HiQiaadofacaWGfbWaaSbaaWqaaiaadogaaeqaaaWccaGLOaGaayzk aaaaaaaa@475F@

Where,

K' = Cyclic strength coefficient.

n' = Strain Cyclic hardening exponent.

z = value of normal distribution calculated using the certainty of survival.

SEc is input via the Strain Life material properties in My Material Context.

References

1 Yung-Li Lee, Jwo. Pan, Richard B. Hathaway and Mark E. Barekey. Fatigue testing and analysis: Theory and practice, Elsevier, 2005