Microcracking Study of Carbon Fiber Composite Materials for Starshade Petals
We have conducted materials tests of representative carbon composite materials for starshade petals. The principal goal of these tests is to assess whether the level of microcracking of the materials as the petals are stowed or deployed (or even as they are handled during assembly) is likely to cause a problem with meeting the tight dimensional tolerances required for starlight suppression. Cracking can cause unexpected deformation, and the cracks can act as reservoirs for gases. It may also lead to roughening of the optical edge whose sharpness minimizes undesirable scattering of light. We use mechanical tests, microscopy, acoustic tomography and small‐angle x‐ray scattering to characterize the microcracking. These tests have found no large cracks (>100 microns) at bending strains up to 0.005, the strain at which audible popping occurred. The stress‐strain curves did not display any anomalies until a precipitous drop in stress at a strain of ~0.012 due to the formation of a transverse crack. Analysis of SAXS data is on‐going which will quantify microcrack distributions at the sub‐micron level. We find no large cracks (>100 microns) at the design strain tolerance for non‐critical structural elements (0.005); analysis is on‐going to quantify nanoscale cracking at the strains of critical structural elements (0.0005).