A peridynamic model for sand impact damage has been developed. Model development has focused on simulating impacts of sand particles on ZnS traveling at velocities consistent with aircraft take-off and landing speeds. The model reproduces common features of impact damage including pit and radial cracks, and, under some conditions, lateral cracks. This study focuses on a preliminary application of the model to systems with coatings. Two different coating materials were included in the modeling effort. The required material parameters of critical stretch, which governs the material failure and fracture, and yield strength, were extracted from peridynamic simulations of experimental results of indentation tests. The model was then applied to impact conditions for which experimental measurements of damage characteristics were available. Peridynamic approach enabled investigation of the mechanical properties of interfaces between the coatings and the substrate. This study demonstrates current computational approach and its quantitative ability to simulate sand impact events.
In this study, effects of coating material and thickness on damage characteristics of an electro-optical material impacted by small particles have been investigated. Six distinct sample types were prepared: (1) multispectral zinc sulfide (MS-ZnS), (2) MS-ZnS with rain erosion protection (REP), (3) MS-ZnS with thin durable antireflection coating (DAR), (4) MS-ZnS with thick DAR, (5) MS-ZnS with REP and thin DAR, and (6) MS-ZnS with REP and thick DAR. The samples were subjected to impact by glass spheres of diameter ranging from 150 micrometers to 700 micrometers at velocities ranging from 40 m/s to 200 m/s. The impact events were captured using two high-speed cameras at a rate of 125,000 frames per second. Taking advantage of the transparency of the samples, one camera captured the impact events from a vantage point normal to the impacted plane, on the other side of the impacted face. The other camera captured the events from a side view. Using the synchronized footage from both cameras, impact sites were paired with the impacting spheres and their velocities were measured. Nearly two hundred impacts were analyzed. The analysis included optical and scanning electron microscopy. Through the characterization effort, impact pit diameter, lateral crack diameter, and radial cracks lengths were measured. When coatings were present, delamination and peeling of the coatings were quantified. The trends in damage morphologies as functions of impacting sphere diameter and inbound velocities were explored. Thresholds for lateral and radial cracking phenomena were identified. The measurements were compared against theoretical values from indentation theory.
Erosion of external optics in sensing systems leads to degradation in performance. Sand impact is the most common form of window and dome erosion. Erosion from sand impact can be separated into two categories: erosion generated while on the wing of a platform in flight (known in this paper as “Platform Induced erosion”) and erosion generated on the ground during high wind events like sand storms (known in this paper as “natural environment erosion”). When qualifying materials for external optics, both categories of sand impact erosion must be evaluated. Platform Induced erosion testing is primarily performed at facilities such as the US Air Force's Particle Erosion Test Facility using a testing setup commonly known as the “sand rig”. Natural environment erosion testing, commonly referred to as “blowing sand”, is performed at various facilities around the United States with varying test setups. When interpreting data from a test setup, it is desirable to understand the variability of data collected over multiple runs and from multiple days. Understanding the sources of variability is also important. This paper presents information collected by Raytheon on fused silica test samples at the Particle Erosion Test Facility’s “sand rig” to assess the variability of data obtained from this facility. Possible sources of variability will be discussed and recommendations for performing sand erosion testing will be presented.
Significant anisotropy in as-deposited CVD ZnS at several length scales has been demonstrated through investigation of
structural and optical properties. Compressive strength of cylinders of CVD ZnS oriented in the growth direction is
~50% higher than cylinders taken perpendicular to the growth direction. Lattice parameter measurements of mandrel
side (first-to-grow) material is ~0.4% smaller than growth side (last-to-grow) material in a cored sample representing
~500 hours of CVD growth, indicating significant strain along the growth direction. X-ray diffraction also shows
evidence of preferred orientations for hexagonality which differ depending on position in the growth history. In crosssection,
the cored sample shows several large bands which are correlated with different degrees of infrared absorption
and BTDF scattering. However, no universal trend is found that applies to the whole length from the mandrel to the
growth side regarding optical properties. The extinction in the visible and infrared is lower for measurements
perpendicular to the growth axis than parallel to it, possibly due to scattering from the growth bands.
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