1 School of Civil Engineering, Xi'an University of Architecture and Technology, Xi'an 710055, China 2 School of Architecture, Chang'an University, Xi'an 710061, China
Cite this article:
Binshan YU,Sheliang WANG,Tao YANG,Yujiang FAN. BP Neural Netwok Constitutive Model Based on Optimization with Genetic Algorithm for SMA. Acta Metall Sin, 2017, 53(2): 248-256.
Systematic study was conducted on the variation regularity of stress-strain curve, feature point stress, dissipated energy and equivalent damping ratio of shape memory alloy (SMA) wires changed with wire diameter, strain amplitude, loading rate and loading cyclic number. By nonlinearly modeling experimental results for SMA using the neural network intelligent algorithm (a neural network algorithm with back-propagation training) and optimizing the initial weight and threshold value of neurons using genetic algorithm, a new BP neural network constitutive model for SMA optimized with genetic algorithm is established. This model successfully overcomes the shortcomings of other mathematical models such as the phenomenological Brinson, by which the various influence factors to mechanical properties in an experiment for SMA are hardly simulated exactly. In fact, the average error between experimental and simulated results is only 1.13% by using this model, much better than conventional BP neural network models. The results show that the BP neural networks constitutive model optimized with genetic algorithm can not only predict accurately the superelastic performance of SMA under cyclic loading, but also avoid the no convergence problem caused by concussion of BP network due to the improper initial weight and threshold value set up. Furthermore, this model would be a better model than others because of fully considering the dynamic influence of loading/unloading rate on SMA experiments.
Fund: Supported by Nation Natural Science Foundation of China (No.51678480), Co-ordinator Innovation Projects Foundation of Shaanxi Province (No.2013SZS01-S02), and Industry-Foundation Research Project of Shaanxi Province (No.2014K06-34)
Table 1 Test conditions used for measuring SMA superelastic properties
Fig.1 Simplified four-line constitutive curve and feature points (a, b, c, d—feature points; E, E’— elastic modulus)
Fig.2 Effects of diameter on stress-strain curve of SMA wire
Diameter / mm
σa / MPa
σb / MPa
σc / MPa
σd / MPa
ΔW / (MJm-3)
ξ / %
0.5
483.83
585.69
331.04
203.72
12.43
6.49
0.8
447.62
527.20
358.10
139.26
12.22
6.01
1.0
420.17
502.93
331.94
118.23
10.52
5.34
1.2
349.26
464.20
247.57
70.74
9.63
5.00
Table 2 Mechanical properties of SMA wire with different diameters
Fig.3 Effects of strain amplitude on stress-strain of SMA wire
Strain amplitude / %
σa / MPa
σb / MPa
σc / MPa
σd / MPa
ΔW / (MJm-3)
ξ / %
3
426.90
496.56
260.65
120.96
4.46
4.18
6
420.17
509.30
254.65
101.86
12.70
6.09
8
432.90
515.66
254.65
70.03
20.76
6.60
Table 3 Mechanical properties of SMA wires with different strain amplitudes
Fig.4 Effects of loading rate on stress-strain of SMA wire
Loading rate mmmin-1
σa / MPa
σb / MPa
σc / MPa
σd / MPa
ΔW / (MJm-3)
ξ / %
10
420.17
509.30
254.65
101.86
12.70
6.09
30
426.54
515.36
280.11
107.59
12.31
6.25
60
420.17
502.93
326.04
109.86
11.93
6.15
90
420.17
502.93
331.94
118.23
10.52
5.34
Table 4 Mechanical properties of SMA wires with different loading rates
Fig.5 Effects of loading/unloading cycle number n on mechanical properties (a) stress-strain (b) feature point stress (c) dissipated energy ΔW (d) equivalent damping ratio ξ
n / cyc
σa / MPa
σb / MPa
σc / MPa
σd / MPa
ΔW / (MJm-3)
ξ / %
1
604.79
604.79
273.75
178.25
6.843
6.11
2
560.23
572.96
254.65
171.89
6.190
5.81
3
541.13
560.23
241.92
171.89
5.796
5.44
5
515.66
541.13
241.92
165.52
5.481
5.18
10
483.83
509.30
222.82
159.15
5.035
4.76
15
440.73
496.56
222.82
159.15
4.769
4.48
20
439.27
483.83
216.45
152.79
4.603
4.37
25
432.90
477.46
216.45
152.79
4.461
4.18
30
432.90
477.46
216.45
152.79
4.438
4.16
Table 5 Mechanical property parameters of SMA wires with different loading/unloading cyclic numbers
Fig.6 Flow chart of BP network optimized by genetic algorithm
Fig.7 BP network topology for austenite SMA constitutive relationship
Fig.8 Topology of BP network constitutive model
Fig.9 Training process
Fig.10 Variation curves of objective function of genetic algorithm changed with hereditary algebra
Fig.11 Comparisons between test curves and BP network constitutive curves without optimization
Fig.12 Comparisons between SMA test curves and pre-optimized /post-optimized BP network predicted curves under the loading rates of 10 mm/min (a), 30 mm/min (b), 60 mm/min (c) and 90 mm/min (d)
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