Depth of penetration criteria on metallic surfaces for use in MMOD risk assessment

System level assessment of hypervelocity impacts by micrometeoroids and orbital debris (MMOD) relies on the definition of the spacecraft geometry and trajectory, the natural environment of the micrometeoroids and induced environment of the orbital space debris, ballistic limit equations and the failure criteria. The definition of the MMOD environments provides the particles flux and when is combined with the ballistic limit equations will determine the number of the critical penetrating particles that could result in the failure of the underlying component is calculated and is used to calculate the risk based on some failure criterion. Spacecraft geometry provides the shielding configuration over the spacecraft critical body which defines the selection of the ballistic limit equations to be used in the risk assessment. The definition of the failure criterion for metallic pressure systems involves the definition of the allowable depth of penetration that could result in leakage or burst of the component. This paper addresses the definition of the allowable depth of penetration of generic metallic tanks from MMOD impacts. The allowable penetration depth of metal tanks is based on a fracture mechanics approach calibrated using biaxially stressed coupons tests subjected to Hypervelocity Impacts (HVI). The planar crack-crack spacing was based on the craters spacing distribution of the HVI coupon tests. The Stress Intensity Factor (SIF) as a function of crater depths and crater spacing and applied remote stress is calculated using NASGRO®, a linear fracture mechanics software. The calculated SIF is compared with the material fracture toughness to determine if the craters result in a failure of the coupons under biaxial stress. This work resulted in a recommended allowable depth of penetration of 20% on the surfaces of metallic pressure vessels on spacecraft.