Krishnamurthy Sriram, Badcock Rodney A, Machavaram Venkata R, Fernando Gerard F
Engineering System Department, Royal Military College of Science, Shrivenham SN6 8LA, UK.
School of Metallurgy and Materials, University of Birmingham, Birmingham B15 2TT, UK.
Sensors (Basel). 2016 May 28;16(6):777. doi: 10.3390/s16060777.
Residual stresses in fibre reinforced composites can give rise to a number of undesired effects such as loss of dimensional stability and premature fracture. Hence, there is significant merit in developing processing techniques to mitigate the development of residual stresses. However, tracking and quantifying the development of these fabrication-induced stresses in real-time using conventional non-destructive techniques is not straightforward. This article reports on the design and evaluation of a technique for manufacturing pre-stressed composite panels from unidirectional E-glass/epoxy prepregs. Here, the magnitude of the applied pre-stress was monitored using an integrated load-cell. The pre-stressing rig was based on a flat-bed design which enabled autoclave-based processing. A method was developed to end-tab the laminated prepregs prior to pre-stressing. The development of process-induced residual strain was monitored in-situ using embedded optical fibre sensors. Surface-mounted electrical resistance strain gauges were used to measure the strain when the composite was unloaded from the pre-stressing rig at room temperature. Four pre-stress levels were applied prior to processing the laminated preforms in an autoclave. The results showed that the application of a pre-stress of 108 MPa to a unidirectional [0]16 E-glass/913 epoxy preform, reduced the residual strain in the composite from -600 µε (conventional processing without pre-stress) to approximately zero. A good correlation was observed between the data obtained from the surface-mounted electrical resistance strain gauge and the embedded optical fibre sensors. In addition to "neutralising" the residual stresses, superior axial orientation of the reinforcement can be obtained from pre-stressed composites. A subsequent publication will highlight the consequences of pres-stressing on fibre alignment, the tensile, flexural, compressive and fatigue performance of unidirectional E-glass composites.
纤维增强复合材料中的残余应力会引发许多不良影响，如尺寸稳定性丧失和过早断裂。因此，开发加工技术以减轻残余应力的产生具有重要意义。然而，使用传统无损技术实时跟踪和量化这些制造引起的应力的发展并非易事。本文报道了一种由单向E玻璃/环氧树脂预浸料制造预应力复合板的技术的设计与评估。在此，使用集成式称重传感器监测施加的预应力大小。预应力装置基于平板设计，可实现基于热压罐的加工。开发了一种在预应力之前对层压预浸料进行端部接片的方法。使用嵌入式光纤传感器原位监测加工引起的残余应变的发展。当复合材料在室温下从预应力装置卸载时，使用表面安装的电阻应变片测量应变。在热压罐中加工层压预成型件之前施加了四个预应力水平。结果表明，对单向[0]16 E玻璃/913环氧树脂预成型件施加108 MPa的预应力，可将复合材料中的残余应变从-600 με（无预应力的传统加工）降低至接近零。从表面安装的电阻应变片和嵌入式光纤传感器获得的数据之间观察到良好的相关性。除了“抵消”残余应力外，预应力复合材料还可获得增强材料更好的轴向取向。后续出版物将重点介绍预应力对单向E玻璃复合材料的纤维排列、拉伸、弯曲、压缩和疲劳性能的影响。
