MIL-HDBK-17-3F Volume 3.Chapter 12-Lessons Learned B-6.For sandwich structure used as a walking 3.1 PCF(50 g/m)core density will result surface,a core density of 6.1 PCF (98 in heel damage to the walking surface. g/m)is recommended. B-7.Do not use honeycomb core cell size Prevents dimpling of face sheets. greater than 3/16 inch(4.8 mm)for cocur- ing sandwich assemblies (1/8 inch (3.2 mm)cell size preferred). B-8.When core is required to be filled around Prevents core crushing and possible lami- bolt holes,etc.,this should be done using nate damage when bolt is installed. an approved filler to a minimum of 2D from the bolt center. B-9.Two extra layers of adhesive should be Curing pressures tend to cause the inner applied to the inner moldline at the core skin to "bridge"in this area creating a void run out (edge chamfer).This should be in the adhesive(skin to core bond). applied a minimum of 0.6 in.(15 mm) from the intersection of the inner skin and edge band up the ramp and a minimum of 0.6 in min 0.2 in.(5 mm)from that point into the 0.2 in min edge band. Honeycomb Core .2 Extra Layers of Adhesive B-10.The use of honeycomb sandwich con- Thin skin honeycomb is susceptible to struction must be carefully evaluated in impact damage,water intrusion due to terms of its intended use,environment, freeze/thaw cycles,and is difficult to re- inspectability,repairability,and customer pair. acceptance. 12.3.1.2 Bolted joints C-1.Design the joints first and fill in the basic Optimizing the "basic"structure first com- structure afterwards. promises the joint design and results in low overall structural efficiency. C-2.Joint analysis should include the effects Shimming can reduce joint strength. of shimming to the limits permitted by drawings. C-3.Design joints to accommodate the next To accommodate routine MRB and repair larger fastener size. activities. C-4.Bolted joint strength varies far less with The stress concentration factor,K,is percentage of 0 plies in fiber pattern highly dependent on 0 plies. than does unnotched laminate strength. C-5.Optimum single-row joints have approxi- Optimum single-row joints operate at mately three-fourths of the strength of higher bearing stress than the most criti- optimum four-row joints. cal row in an optimized multi-row joint. 12-15MIL-HDBK-17-3F Volume 3, Chapter 12 - Lessons Learned 12-15 B-6.For sandwich structure used as a walking surface, a core density of 6.1 PCF (98 g/m3 ) is recommended. 3.1 PCF (50 g/m3 ) core density will result in heel damage to the walking surface. B-7.Do not use honeycomb core cell size greater than 3/16 inch (4.8 mm) for cocur￾ing sandwich assemblies (1/8 inch (3.2 mm) cell size preferred). Prevents dimpling of face sheets. B-8.When core is required to be filled around bolt holes, etc., this should be done using an approved filler to a minimum of 2D from the bolt center. Prevents core crushing and possible lami￾nate damage when bolt is installed. B-9.Two extra layers of adhesive should be applied to the inner moldline at the core run out (edge chamfer). This should be applied a minimum of 0.6 in. (15 mm) from the intersection of the inner skin and edge band up the ramp and a minimum of 0.2 in. (5 mm) from that point into the edge band. Curing pressures tend to cause the inner skin to "bridge" in this area creating a void in the adhesive (skin to core bond). B-10.The use of honeycomb sandwich con￾struction must be carefully evaluated in terms of its intended use, environment, inspectability, repairability, and customer acceptance. Thin skin honeycomb is susceptible to impact damage, water intrusion due to freeze/thaw cycles, and is difficult to re￾pair. 12.3.1.2 Bolted joints C-1. Design the joints first and fill in the basic structure afterwards. Optimizing the “basic” structure first com￾promises the joint design and results in low overall structural efficiency. C-2. Joint analysis should include the effects of shimming to the limits permitted by drawings. Shimming can reduce joint strength. C-3. Design joints to accommodate the next larger fastener size. To accommodate routine MRB and repair activities. C-4. Bolted joint strength varies far less with percentage of 0° plies in fiber pattern than does unnotched laminate strength. The stress concentration factor, Kt, is highly dependent on 0° plies. C-5. Optimum single-row joints have approxi￾mately three-fourths of the strength of optimum four-row joints. Optimum single-row joints operate at higher bearing stress than the most criti￾cal row in an optimized multi-row joint