Advanced crack propagation simulation for eSHM system design

In high tech production industry there is a trend towards combining sophisticated design simulation tools with advanced manufacturing processes to facilitate lightweight designs. More complex shapes are constructed for maximal weight reduction. Correspondingly designing closer to the limit requires advanced structural health monitoring (SHM) systems. In addition to guaranteeing the structural integrity of the product, the SHM system allows for field validation of design assumptions by including the information from product performance within design validation. All products in the field become validation cases for the design assumtions made.
This paper proposes an innovative SHM technique by means of capillary channels embedded in the structure (eSHM) in order to detect cracks before they reach their critical length. In this approach it is aimed to place capilary channels inside the structure. The location of the capilary channels is such that they are in zones where cracks pass during propagation before they reach their critical length. The crack monitoring as such is achieved by measuring air pressure in the capilary channel. Cracks reaching the capillary will cause a decrease in air pressure and an alarm. The well functioning of the system is fully determined by the positioning of the capillary channels.
Advanced simulation techniques are used to determine the location of the capillaries to ensure crack detection. Accuracy of these simulations significantly contributes to the success of the SHM system. This paper shows the use of an advanced finite element (FE) based simulation approach to design the SHM system. It consists of two steps. FE based critical stress zone determination calculations are used to calculate the critical hot spots for crack initiation. SHM capilaries are placed such to detect cracks initiated in these critical zones. Validation of the capillary locations is done by means of Nasgrow based crack propagation simulations. Cracks will be initiated at different locations in the hot zones and propagation simulated. It is shown that each of the cracks propagates through the capillaries. A generic example is envisaged: a gearbox support connection between a gearbox and the system it is build into. However in order to make the study quantitave it is opted to design the eSHM for the torque arm of a wind turbine gearbox. The publically available National Renewable Energy Laboratory (NREL) Gearbox Reliability Collaborative (GRC) gearbox will be used in combination with fatigue loading conditions determined in the framework of this GRC project. To facilitate capillaries of complex shape the section of the torque arm containing the capillaries will be produced by means of additive manufacturing techniques. These techniques are becoming increasingly available at a decreasing cost for plastics and metals and could in future be used for manufacturing parts of structural components of gearbox systems.