A Preliminary Assessment of Thermoplastic Composite Welding for In-Space Applications by the Thermoplastics Development for Exploration Applications (TDEA) Project

Thermoplastic composite (TPC) welding shows promise for future in-space structural joining applications. NASA’s Thermoplastics Development for Exploration Applications (TDEA) project contributed to this long-term objective through hundreds of welding trials and evaluations on various joint configurations. This report summarizes the key findings from these welding trials and assesses the potential for in-space applications of this technology. The welding trials include coupon-scale assessments of resistance, induction, and ultrasonic welding methods in a lab environment. The weld methods are applied to five TPC materials including amorphous and semicrystalline resins. A single lap-shear configuration is used with welds conducted on individual specimens (i.e., spot welds) relevant for assembling truss structures. Defects, such as unwelded regions, delamination, porosity, or distortion (thinning, fiber pushout) occurred for all weld methods and material combinations. Defects were detected by nondestructive evaluation (NDE) and verified by optical microscopy. Despite the defects, high lap-shear strength (LSS) and low coefficient of variation (COV) were achieved for some combinations of weld method and material system. In general, the extent of defects was lower and the LSS was higher when the welding organization had substantial previous experience with the material system. This highlights the sensitivity of this technology to weld processing parameters and the need for extensive process development. Based on the coupon-scale welding experience, the welding method advantages, limitations, and technology gaps are summarized. For the three weld methods, accurately controlling the pressure and temperature during welding is essential to produce a strong, well-consolidated weld. However, desirable pressure and temperature histories can be difficult to obtain since they are not controlled directly during the welding processes examined herein. Welded joints between adherends that have different resin systems or made with different manufacturing processes using similar resin systems are also assessed and show potential for a variety of applications.

A specific application was chosen by the TDEA project as a test case to evaluate manufacturing scale-up. The application is for a 168-ft-tall truss tower to be located at the lunar south pole and constructed with welded joints on the lunar surface. A subelement-scale structural joint representing the most highly loaded joint was manufactured and tested. Ultrasonic welding was selected for this application. In preparation for the subelement fabrication, scale-up welding trials were also conducted. Two ultrasonic joining processes were successfully developed and tested to loads exceeding the design requirement. All manufacturing was completed in a lab environment. To help address the applicability of this technology for in-space applications, material properties of specific interest for space environments such as outgassing and coefficient of thermal expansion (CTE) are reported. Ultrasonic welding trials with lunar regolith contamination were conducted and show low sensitivity to contamination. This low sensitivity is important for welding on dusty planetary bodies. Finally, a summary is provided of a preliminary study showing the feasibility of weld reassembly at the coupon scale to allow for repair or reconfiguration to meet future mission needs. Overall, the results show that strong welds that meet design requirements are possible, and is demonstrated at the coupon and subelement scale. Substantial process development effort is required to obtain this outcome for a particular weld method, material, and welding configuration and to control joint strength variability. Future work is needed to develop a better understanding of the weld processes, adapt them to space environments through trials in thermal vacuum chambers, and characterize the structural behavior of the welded joints under relevant environments.