**2. Materials and Methods**

#### *2.1. Design of the Tested Harness*

For the purpose of the test, a fully articulated harness from serial production has been chosen. However, all non-load-bearing elements that increase pilot comfort were intentionally removed. The aim was to extract only the structural frame to obtain more variability in the positioning of the measuring elements. Because the removed elements do not affect the strength of the harness, similarity to a fully equipped harness intended for real use is guaranteed. The complete scheme of used materials is highlighted in Figure 1.

**Figure 1.** Scheme of used materials.

Straps of the same width were used for the entire harness. The only differences are in their declared strengths. Regarding the buckles, the exact types are also shown. The names stated in Figure 1 are the trade names of the buckles. It is therefore possible to trace their exact parameters, which will be discussed further in the following section.

#### *2.2. Equipment for Measuring Forces in Webbing*

Measuring the forces in a flexible structure, such as parachute webbing, is a very specific issue. The main reason is that straps in the final configuration are not uniformly loaded in most cases. It is necessary to consider, for example, the partial loading of the sides of the strap due to the seating around the dummy's body. Hence, there was a need to develop a custom measuring element. The major request was also to use elements with minimum dimensions to have the possibility of implementing them into each webbing. The minimum dimensions of the feature also ensured the smallest possible influence on the harness' structural characteristics. In addition, the aim was to eliminate any undesirable loads, for example, from bending. When selecting the positions for the load cells, consideration was given to placing them in positions where only tension could be expected, and the bending component was eliminated as much as possible.

Regarding the design of the load cells, modification of the buckle frames was determined to be the most appropriate and least costly option. The aim was to ensure that the stiffness of the structural node was not adversely affected by the incorporation of the individual measuring components. Therefore, the same carabiners were always selected for the design of the strain gauges as those already used in the design chain. For the purpose of this study, only two types of buckle bases were required. The stronger type is a carabiner with the trademark PS 22040-1 and a declared strength of 2500 lbs, while the weaker type

is for use in chest webbing and has a declared strength of 500 lbs. Figure 2 shows the initial geometry of the buckles used before any modifications. Figure 2 highlights the initial geometry of the used buckles before any modification. To obtain the basic frame for strain gauge installation, it was necessary to remove the moving part used to fix the strap in the tightened position.

**Figure 2.** Selected buckles for the frame preparation to create the measuring load cell.

Once a clear rectangular base plate was prepared, four strain gauges were installed on the degreased surface, one on each side of the buckle. The intention was to reach a full bridge connection that will ensure accurate measurement of the force, regardless of whether the buckle is loaded symmetrically or not. This assumption has been confirmed during the calibration, which has been performed for all six load cells. In the procedure, the carabiners were loaded evenly and unevenly, in the sense that one side of the carabiner was loaded more than the other. This process verified that the total measured force did not vary from case to case. Repeating the above approach for each load cell separately guarantees accurate measurement of all elements, regardless of any manufacturing tolerances of the buckle or inaccurate placement of the strain gauge on its surface. The specific regression curves of these load cells are shown in Figure 3. The regression dependence is linear over the entire applied load range, and therefore, a very high accuracy of force measurement can be assumed. The dependence of the material tension on the total loading force generated by the tensile test machine is shown.

**Figure 3.** Exact buckle plate calibration curve: (**a**) curve related to baseplate PS22040-1 and (**b**) curves related to baseplate PS 70101-1.

The final load cell implemented into the harness structure is highlighted in Figure 4. It was necessary to provide mechanical protection around the strain gauge against damage when the harness settles on the metal dummy. During the first moments of loading, significant movements occur. The area around the strain gauges was sealed with hot melted glue. This method of protection proved to be sufficient as no damage occurred during the test.

**Figure 4.** Final load cell manufactured on PS22040-1 baseplate sewn into structure.
