Influence of Thermal Shocks on Residual Static Strength, Impact Strength and Elasticity of Polymer-Composite Materials Used in Firefighting Helmets

Abstract

The article presents results of experimental studies on mechanical properties of the polymer-composite material used in manufacturing firefighting helmets. Conducted studies included static and impact strength tests, as well as a shock absorption test of glass fiber-reinforced polyamide 66 (PA66) samples and firefighting helmets. Samples were subject to the impact of thermal shocks before or during being placed under a mechanical load. A significant influence of thermal shocks on mechanical properties of glass fiber-reinforced PA66 was shown. The decrease in strength and elastic properties after cyclic heat shocks ranged from a few to several dozen percent. The average bending strength and modulus during the 170 degree Celsius shock dropped to several dozen percent from the room temperature strength. Under these thermal conditions, the impact strength was lost, and the lateral deflection of the helmet shells increased by approximately 300%. Moreover, while forcing a thermal shock occurring during the heat load, it was noticed that the character of a composite damage changes from the elasto-brittle type into the elasto-plastic one. It was also proved that changes in mechanical and elastic properties of the material used in a helmet shell can affect the protective abilities of a helmet.

Keywords: degradation; fire helmet; impact test; polymer composite; thermal shock.

Figure 1

Firefighting helmet (a) and helmet shell (b,c) made of Ultramid (PA66-GF25FR) by injection molding.

Figure 1

Firefighting helmet (a) and helmet shell (b,c) made of Ultramid (PA66-GF25FR) by injection molding.

Figure 2

Scheme of testing lateral stiffness of helmet shells.

Figure 3

The course of mechanical load while testing lateral stiffness of firefighting helmet shells.

Figure 4

Instrumentation of an impact hammer DFP 1000 intended to test firefighting helmets: (a) a static head model, (b) the way of placing a helmet on a head model, a laser pointer indicates the point of treating a helmet with the impactor, (c) spherically ended impactor that hits a helmet.

Figure 5

Temperature fields of a tested sample at the beginning (a) and at the end (b) of the thermal processing procedure.

Figure 6

Temperature-time characteristics obtained while stabilizing parameters of a hot air flow.

Figure 7

Firefighting helmet CV102 (a helmet shell and some other elements are made of Ultramid PA66-GF25FR) placed on a static head model, located in a measurement track of a drop hammer DFP 1000, which was treated with the hot airflow (air heater HOTWIND SYSTEM).

Figure 8

Selected characteristics σ−ε obtained in a tensile strength test after the exposure to thermal shocks: (a) 20 °C, (b) 160 °C—1 min, (c) 5 cycles × 100 °C—25 min, (d) 5 cycles × 160 °C—1 min.

Figure 8

Selected characteristics σ−ε obtained in a tensile strength test after the exposure to thermal shocks: (a) 20 °C, (b) 160 °C—1 min, (c) 5 cycles × 100 °C—25 min, (d) 5 cycles × 160 °C—1 min.

Figure 9

Framing chart presenting results of a bending strength test with a thermal shock.

Figure 10

Experimental characteristics of force in the function of an impactor movement depending on thermal shock: (a) standard temperature value (20 °C), (b) extreme temperature value (thermal shock at the temperature of 140 °C for 10 min).

Figure 11

Crack and deformation of the CV102 helmet surface: (a) depending on thermal shock standard temperature value (20 °C), (b) extreme temperature value (thermal shock at the temperature of 140 °C for 10 min).

Figure 12

SEM image of a fracture of Ultramid sample exposed to the heat load of 100 °C.

Figure 13

Results of testing the level of damage of strength and stiffness of the PA66 composite reinforced with glass fiber at fire temperature values: bending test.

Figure 14

Results of testing the speed of damage of bending strength and elastic modulus depending on a temperature value.

Figure 15

Structural elements of a firefighting helmet after the exposure to the impact loading: (a) a helmet shell after an impact test with the energy of 60 J using an impactor with the diameter of 20 mm at the temperature of 140 °C, (b) an amortizing liner at the impact point in a helmet exposed to the heat load of 140 °C.