Materials

Different material tests could result in different material mechanic character.

The typical material test for metal is tensile test. With this test strain-stress curve, yield point, necking point and failure point of material could be observed.


Figure 1. Force (F) and Length (l) are Measured
Engineer strain-stress curve could be generated by:(1)
σ e = F S 0 MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeq4Wdm3aaS baaSqaaiaadwgaaeqaaOGaeyypa0ZaaSaaaeaacaWGgbaabaGaam4u amaaBaaaleaacaaIWaaabeaaaaaaaa@3C79@
(2)
ε e = Δ l l 0 MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeqyTdu2aaS baaSqaaiaadwgaaeqaaOGaeyypa0ZaaSaaaeaacaqGuoGaamiBaaqa aiaadYgadaWgaaWcbaGaaGimaaqabaaaaaaa@3DB6@
Where,
S 0 MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4uamaaBa aaleaacaaIWaaabeaaaaa@37B5@
Section area in the initial state
l 0 MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiBamaaBa aaleaacaaIWaaabeaaaaa@37CE@
Initial length
In this Force-elongation curve or engineer stress-strain curve, three points are important.
  1. Yield point: where material begin to yield. Before yield you can assume material is in elastic state (the Young's modulus E could be measured) and after yield, material plastic strain which is non-reversible.
    • Some material in this test will first reach the upper yield point (ReH) and then drop to the lower yield point (ReL). In engineer stress-strain curve, lower yield stress (conservative value) could be taken.
    • Some material can not easily find yield point. Take the stress of 0.1 or 0.2% plastic strain as yield stress.
  2. Necking point: where the material reaches the maximum stress in engineer stress-strain curve. After this point, the material begins to soften.
  3. Failure point: where material failed.


Figure 2.
Rm
Maximum resistance
Fmax
Maximum force
ReH
Upper yield level
ReL
Lower yield level
Ag
Uniform elongation
Agt
Total uniform elongation
At
Total failure strain

True stress-strain curve which is requested in most materials in Radioss, except in LAW2, where both engineer stress-strain and true stress-strain are possible to input material data.

In Figure 3, find engineer stress-strain curve (blue) by using:(3)
σ t r = σ e exp ( ε t r )
(4)
ε t r = ln ( 1 + ε e )
The result is true stress-strain curve (red). Plastic true stress-strain curve is shown in green, which plastic strain begin from 0. This green plastic true stress-strain curve is what you need, as in LAW36, LAW60, LAW63, and so on.


Figure 3.
The true stress-strain curve is valid until the necking point of the material. After the necking point, the material curve has to be defined manually for hardening. Using a different material law, Radioss will extrapolation the true stress-strain curve to 100%.
  • Linear extrapolation: If stress-strain curve is as function input (LAW36), then stress-strain curve is linearly extrapolated with a slope defined by the last two points of the curve. It is recommended that the list of abscissa value be increased to a value greater than the previous abscissa value.
  • Johnson-Cook: After necking point, Johnson-Cook hardening is one of the most commonly used to extrapolate the true stress-strain curve.(5)
    σ y = a + b ε p n MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeq4Wdm3aaS baaSqaaiaadMhaaeqaaOGaeyypa0JaamyyaiabgUcaRiaadkgacqaH 1oqzdaWgaaWcbaGaamiCaaqabaGcdaahaaWcbeqaaiaad6gaaaaaaa@4095@

    However, it may overestimate strain hardening for automotive steel, In this case, combination of swift-voce hardening is more accurate.

  • Swift and Voce: After necking point, use one of the following equations to extrapolate the true stress-strain curve.
    Swift model
    σ y = A ( ε ¯ p + ε 0 ) n MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeq4Wdm3aaS baaSqaaiaadMhaaeqaaOGaeyypa0JaamyqamaabmaabaGafqyTduMb aebadaWgaaWcbaGaamiCaaqabaGccqGHRaWkcqaH1oqzdaWgaaWcba GaaGimaaqabaaakiaawIcacaGLPaaadaahaaWcbeqaaiaad6gaaaaa aa@43C6@
    A MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyqaaaa@36BD@ and n MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyqaaaa@36BD@ are positive.
    Voce model
    σ y = k 0 + Q [ 1 exp ( B ε ¯ p ) ] MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeq4Wdm3aaS baaSqaaiaadMhaaeqaaOGaeyypa0Jaam4AamaaBaaaleaacaaIWaaa beaakiabgUcaRiaadgfadaWadaqaaiaaigdacqGHsislciGGLbGaai iEaiaacchadaqadaqaaiabgkHiTiaadkeacuaH1oqzgaqeamaaBaaa leaacaWGWbaabeaaaOGaayjkaiaawMcaaaGaay5waiaaw2faaaaa@4A28@
    k 0 MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4AamaaBa aaleaacaaIWaaabeaaaaa@37CD@ , Q MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyqaaaa@36BD@ and B MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyqaaaa@36BD@ are positive.
    Combination of Swift and Voce model (LAW84 and LAW87)
    σ y = α [ A ( ε ¯ p + ε 0 ) n ] S w i f t h a r d e n i n g + ( 1 α ) { k 0 + Q [ 1 exp ( B ε ¯ p ) ] } V o c e h a r d e n i n g MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVeea0le9v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk 0xbba9q8WqFfea0=yr0RYxir=Jbba9q8aq0=yq=He9q8qqQ8frFve9 Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeaaeaaakeaacqaHdpWCda WgaaWcbaGaamyEaaqabaGccqGH9aqpcqaHXoqydaagaaqaamaadmaa baGaamyqamaabmaabaGafqyTduMbaebadaWgaaWcbaGaamiCaaqaba GccqGHRaWkcqaH1oqzdaWgaaWcbaGaaGimaaqabaaakiaawIcacaGL PaaadaahaaWcbeqaaiaad6gaaaaakiaawUfacaGLDbaaaSqaaiaado facaWG3bGaamyAaiaadAgacaWG0bqbaeqabeqaaaqaaaaacaWGObGa amyyaiaadkhacaWGKbGaamyzaiaad6gacaWGPbGaamOBaiaadEgaaO Gaayjo+dGaey4kaSIaaiikaiaaigdacqGHsislcqaHXoqycaGGPaWa aGbaaeaadaGadaqaaiaadUgadaWgaaWcbaGaaGimaaqabaGccqGHRa WkcaWGrbWaamWaaeaacaaIXaGaeyOeI0IaciyzaiaacIhacaGGWbWa aeWaaeaacqGHsislcaWGcbGafqyTduMbaebadaWgaaWcbaGaamiCaa qabaaakiaawIcacaGLPaaaaiaawUfacaGLDbaaaiaawUhacaGL9baa aSqaaiaadAfacaWGVbGaam4yaiaadwgafaqabeqabaaabaaaaiaadI gacaWGHbGaamOCaiaadsgacaWGLbGaamOBaiaadMgacaWGUbGaam4z aaGccaGL44paaaa@7D8E@


Figure 4.

Here, α is weight of Swift hardening and Voce hardening. Here one Compose script as example to fit the Swift hardening parameters A MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyqaaaa@36BD@ , ε 0 MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeqyTdu2aaS baaSqaaiaaicdaaeqaaaaa@3884@ , n MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyqaaaa@36BD@ and Voce hardening parameters k 0 MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4AamaaBa aaleaacaaIWaaabeaaaaa@37CD@ , Q MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyqaaaa@36BD@ , B MathType@MTEF@5@5@+= feaagKart1ev2aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyqaaaa@36BD@ with input stress-strain curve.