Background

Efficient biaxial strain control to study fatigue damage growth in composite materials at the coupon scale level under realistic multi-axial stress states observed in structures is needed.

Objective

Two biaxial cyclic strain control algorithms, referred to here as the active and the passive control method, are presented as a fatigue testing strategy for composite materials.

Method

The strain is measured at a sampling frequency of 116 Hz through a real-time digital image point tracking system. The active control method utilizes a cascade control algorithm to accommodate the low sampling rate of the digital image point tracking system relative to the servo-hydraulic PID controller. The passive control method is a conditional control algorithm where the force command is adjusted when the measured peak-valley strain violates the predefined strain tolerance. The performance of these control techniques are initially evaluated on uniaxial coupon specimens to investigate the effect of testing frequency, strain magnitude, accuracy of the stiffness parameter and the tolerance band. Subsequently, the two control methods are verified on cruciform specimens under different biaxial strain states.

Results

Under biaxial loading, the peak feedback strains had a maximum error of 6.4% and 9.0% for the active and the passive control method respectively, depending on the required biaxial strain state.

Conclusions

The active control method was found to offer a better accuracy relative to the passive control method. However, it suffered from a lower fidelity as the test stopped whenever the real-time digital image point tracking system lost track of the point markers.

中文翻译：

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复合材料双轴应变控制疲劳试验策略

背景

需要有效的双轴应变控制来研究在结构中观察到的真实多轴应力状态下在试样尺度水平上复合材料的疲劳损伤增长。

客观的

两种双轴循环应变控制算法，这里称为主动和被动控制方法，作为复合材料的疲劳测试策略。

方法

通过实时数字图像点跟踪系统以 116 Hz 的采样频率测量应变。主动控制方法利用级联控制算法来适应数字图像点跟踪系统相对于伺服液压 PID 控制器的低采样率。被动控制方法是一种条件控制算法，当测量的峰谷应变违反预定义的应变容差时，调整力命令。这些控制技术的性能最初在单轴试样上进行评估，以研究测试频率、应变大小、刚度参数的准确性和公差带的影响。随后，在不同双轴应变状态下的十字形试样上验证了两种控制方法。

结果

在双轴加载下，根据所需的双轴应变状态，主动和被动控制方法的峰值反馈应变的最大误差分别为 6.4% 和 9.0%。

结论

发现主动控制方法相对于被动控制方法提供更好的精度。然而，它的保真度较低，因为每当实时数字图像点跟踪系统失去对点标记的跟踪时，测试就会停止。