Design, manufacturing and testing of pull-out and shear-out insert allowable for sandwich applications

This paper aims to present the procedures necessary to determine the insert allowable for a composite sandwich, considering that the inserts were the most commonly used means to install equipment on the composite structure of Clean Sky 2 (CS2)-RACER compound helicopter.

The installation of the equipment inside of the airframe shall comply with the certification regulations, especially in relation to the inertial factors. Establishing of the needed number of inserts to fix the equipment is directly linked to the allowable coming from coupon tests. The materials and test procedures to which they were subjected are part of the process qualification used in the development of the CS2-RACER Main Fuselage. The samples were tested in two different static mechanical loadings, consisting of pull-out insert and shear-out insert tests. The mechanical behaviour and failure mechanism of the materials were evaluated using optical and scanning electron microscopy.

The insert installation on the sandwich structure influences the behaviour and mechanical properties during pull-out and shear-out testing.

The limited data available in standardized documents related to insert testing makes it difficult to compare results with certified baseline values.

To reduce the effort of selecting the optimized insert system, specific parameters are included in analytical pre-sizing, i.e. type of loads, geometry, materials, failure modes, special conditions such as manufacturing and testing.

The results of the study presenting the design, manufacturing and mechanical testing of pull-out and shear-out inserts used in composite materials sandwich-type coupons provide valuable information regarding the insert allowable determination.