BWBN

Bouc-Wen-Baber-Noori Model

The BWBN model is an extension of the BoucWen model with stiffness degradation, strength degradation and pinching effect.

Syntax

material BWBN (1) [2...18]
# (1) int, unique material tag
# [2] double, elastic modulus, default: 2E5
# [3] double, yield stress, default: 4E2
# [4] double, hardening ratio, default: 1E-2
# [5] double, \beta (>0), default: 0.5
# [6] double, exponent n (>0, normally >=1), default: 1.0
# [7] double, initial \nu (>0), default: 1.0
# [8] double, slope of \nu (>0), default: 0.0
# [9] double, initial \eta (>0), default: 1.0
# [10] double, slope of \eta (>0), default: 0.0
# [11] double, initial \phi (>0), default: 1.0
# [12] double, slope of \phi (>0), default: 0.0
# [13] double, \zeta (1>\zeta>0), default: 0.0
# [14] double, slope of A (>0), default: 0.0
# [15] double, p (>0), default: 0.0
# [16] double, q (>0), default: 0.0
# [17] double, \lambda (>0), default: 1.0
# [18] double, density, default: 0.0

History Variable Layout

location
value

initial_history(0)

z

Theory

The Wikipedia page contains sufficient information about the formulation of BWBN model. Some normalizations are carried out compared to the original model.

The evolution of internal displacement z(t)z(t) is governed by the differential equation,

ηΔz=hΔuuy(Aν(γ+sign(zΔu)β)zn).\eta\Delta{}z=h\dfrac{\Delta{}u}{u_y}\left(A-\nu\left(\gamma+\text{sign}\left(z\cdot\Delta{}u\right)\beta\right) \Big|z\Big|^n\right).

Then,

F=aFyuuy+(1a)Fyz.F=aF_y\dfrac{u}{u_y}+\left(1-a\right)F_yz.

For state determination, zz is solved iteratively by using the Newton method. The evolutions of internal functions rely on the dissipated energy ee, which is defined to be a normalized quantity.

e=(1a)z du.e=\left(1-a\right)\int{}z~\mathrm{d}u.

The trapezoidal rule is used so that

Δe=(1a)2z+Δz2Δu.\Delta{}e=\left(1-a\right)\dfrac{2z+\Delta{}z}{2}\Delta{}u.

The evolutions are

ν=ν0+δνe,η=η0+δηe,A=1δAe,h=1ζ1exp((zsign(Δu)qzuζ2)2),ζ1=ζ(1exp(pe)),ζ2=(ϕ0+δϕe)(λ+ζ1),zu=1νn=ν1/n.\begin{align*} \nu&=\nu_0+\delta_\nu{}e,\\[3mm] \eta&=\eta_0+\delta_\eta{}e,\\[3mm] A&=1-\delta_Ae,\\[3mm] h&=1-\zeta_1\exp\left(-\left(\dfrac{z\cdot\text{sign}\left(\Delta{}u\right)-qz_u}{\zeta_2}\right)^2\right),\\[3mm] \zeta_1&=\zeta\left(1-\exp\left(-pe\right)\right),\\[3mm] \zeta_2&=\left(\phi_0+\delta_\phi{}e\right)\left(\lambda+\zeta_1\right),\\[3mm] z_u&=\sqrt[n]{\dfrac{1}{\nu}}=\nu^{-1/n}. \end{align*}

Parameters

Strength degradation is controlled by uu. To disable it, set δν=0\delta_\nu=0.

Stiffness degradation is controlled by η\eta. To disable it, set δη=0\delta_\eta=0.

Pinching is controlled by hh. To disable it, set ζ=0\zeta=0 or p=0p=0.

Examples

Vanilla Model

The default behavior is similar to a bilinear hardening material.

material BWBN 1
materialtest1d 1 4E-4 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 80

Strength degradation

A positive δν\delta_\nu enables strength degradation.

material BWBN 1 2E5 4E2 0 .5 1. 1. 1E0 1. 0. 1. 0. 0. 0. 0. 0. 1. 0.
materialtest1d 1 4E-4 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 80

A positive δA\delta_A has the similar effect.

material BWBN 1 2E5 4E2 0 .5 1. 1. 0. 1. 0. 1. 0. 0. 1E0 0. 0. 1. 0.
materialtest1d 1 4E-4 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 80

Stiffness degradation

A positive δη\delta_\eta enables stiffness degradation.

material BWBN 1 2E5 4E2 0 .5 1 1 0 1 1E1 1 0 1 0 1 1 0 0
materialtest1d 1 4E-4 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 80

Pinching Effect

The pinching effect is governed by ϕ0\phi_0, δϕ\delta_\phi, ζ\zeta, pp, qq and λ\lambda.

material BWBN 1 2E5 4E2 0 .5 1. 1. 0. 1. 0. 1. 1E1 1. 0. 1E1 1E0 1. 0.
materialtest1d 1 1E-3 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 80

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