Comparison of Embedded and Conventional Strain Gauges on Glass Fiber-Reinforced Coupons

Electrical strain gauges from HBM are well established all over the world. Different applications such as static material and durability testing of coupons and components rely on the high quality and the wide portfolio of over 2000 different strain gauge types.

Together with IPC, the industrial technical center for plastic and composite innovation in France, an investigation based on testing in compliance with the ISO 527-4 standard was performed to compare strain gauges embedded in the composite material (LI66-10/350) with conventional strain gauges installed on the surface of the test structure (LY21-6/350). The test was performed on glass fiber-reinforced plastic coupons.

Points of Interest

1. Does the integration of the strain gauge into a fiber material have an influence on the test specimen?

2. Do the strain gauges installed on the surface create a different measurement signal than the ones that are integrated?

Strain Gauges used for Testing

LI66-10/350 (embedded strain gauge from HBM)

10mm grid length
350 Ohm
Pin length: 15mm
Temperature range: -40°C to +180°C (-40°F to +356°F)

LY21-6/350 (conventional HBM strain gauge)

6mm grid length
350 Ohm
Temperature range: -200°C to +200°C (-328°F to +392°F)

Setting Up the Tests

The coupons were measured under the following test conditions:

Coupon no. 1 with LY21 strain gauge on surface + embedded LI66 strain gauge
Coupon no. 2 with LY21 strain gauge only, installed on the surface
Coupon no. 3 without any strain gauges (tested with extensometer)

The coupons were tested with an uniaxial tensile test until failure at a velocity of 2mm/min and a preload of 50N. The testing conditions were 23°C with 50% relative humidity.

Coupon no. 1 with LI66 installed on the surface and embedded LY21.

The Results

The strain gauges give very similar results for the stress/strain measurements. Embedding the LI66 strain gauge into the composite material has no measurable impact on the test coupon. The LY21 strain gauge installed on the composite material does not show any measurable effect or delamination.
The crack position of the coupon is at the same area for all tested coupons.
The graphs present the stress/strain behavior until failure of strain gauges (signal lost). Ultimate failure of composite is around 400MPa for the three coupons. The test coupon with the embedded strain gauge shows a quicker failure since connection between the pins and the wires was lost.

Coupon no. 1, no. 2 and no. 3 after testing.

Embedded LI66 vs. Conventional LY21 Strain Gauge

About IPC

IPC (Centre Technique Industriel de la Plasturgie et des Composites) is the industrial technical center whose expertise is dedicated to plastic and composite innovation in France. Since 2016, the profession has thus new means to support all companies, especially small and medium-sized businesses, whatever the process used, thanks to a contribution instituted to finance R & D, innovation, transfer of technologies and skills.