Abstract. The feasibility of the use of embedded strain gauges for the measurement of internal deformation in laminated composite material has been ...
Key Engineering Materials Vol. 144 (1998) pp. 251-260
© 1998 Trans Tech Publications, Switzerland
Strain Measurement in Composite Materials
Using Embedded Strain Gauges
S. Aloisi, U. Galietti and C. Pappalettere Politecnico di Bari, Dipartimento di Progettazione e Produzione Industriale,
Viale Japigia 182. 1-70126 Bari, Italy
Keywords: Embedded Strain Gauges, Laminate Composite, ILSS
Abstract The feasibility of the use of embedded strain gauges for the measurement of internal deformation in laminated composite material has been investigated. The methodology consists in inserting the strain gauges among the layers of the laminate before polymerisation in the oven. High Temperature Carbon Fibre - Epoxy Matrix composite has been used in this study. A Scanning Laser Acoustic Microscope analysis was performed to check whether the embedding technique caused problems of delamination between the gauges and the layers of composite and between the layers. Tensile, . bending and InterLaminar Shear Stress (ILSS) evaluation tests were carried out to assess the feasibility of embedded strain gauges to measure the internal deformation in laminate composites. The results obtained showed good agreement with theoretical values from simple calculations. 1. Introduction Nowadays there is a great interest in techniques able to provide useful information on the internal stress-strain state of a generic material component and laminate composite in particular . The development of a new class of material, the SMART material, gives a strong impulse to implement monitoring techniques. The monitoring of the strain state in the material gives better possibility to control the occurrence of critical events during in-service conditions. The embedded strain gauge technique has been recently used in biomechanic applications [1,2]. in process monitoring [3] and to validate embedded grid moire technique [4]. It is remarkable that in most cases the technique was considered reliable since the gauges were embedded in homogenous and isotropic material, typically epoxy resin or polycarbonate [5]. The aims of this work are to demonstrate the feasibility of a procedure of application of embedded strain gauges in a laminate composite and to characterise the material determining also the InterLarninar Shear Stress.
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2. Experimental set up 2.1 Material and embedding process High Temperature Carbon Fibre - Epoxy Matrix (Pre-Preg) composite has been used in this analysis. It is a 8 layer unidirectional laminate composite with the graphite fibres placed with a zero angle with respect to the loads in the epoxy matrix. Four square plates (298 x 298 mm 2) were produced with embedded strain gauges. In the first two plates 3 EA 06 250 BF 350 (0.025 mm thick) and 3 CEA 06 250 UW 350 (0.075 mm thick, Polyimide-encapsulated) gauges were placed as shown in Fig. I. In the other plates only CEA strain gauges were placed in the same manner; moreover in the second set of plates the soldering contacts were insulated with a 001 mm thick layer of PLY-OOI KAPTON FlLM. All the materials used in this study are by Micro Measurements - USA (MM). The strain gauges have been placed between the layers using a mask, since it was impossible to ensure the exact position of the strain gauges after the polymerisation. The wires, insulated by painting, were brought out by very small holes through the section. The plates were then placed in the oven for polymerisation in the oven in vacuum conditions. AU plates were reinforced at two opposed ends, in order to ensure enough strength where the grips of the loading machine acted. Then 6 rectangular specimens (cross section 25.4 x 1.3 mm 2) with and 3 specimens (to be used as reference) without embedded gauges were obtained from each sheet. 1.3
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Fig. I: Geometry of the plates, specimens and location of the strain gauges. The name of the gauge results from the ply number it has been placed at.
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2.2 Integrity of the specimens A Scanning Laser Acoustic Microscope analysis was performed to check whether the embedding technique caused problems of delamination between the gauges and the layers of composite and between the layers. The SLAM technique operates as follows [6,7]. A plane ultrasonic wave at frequencies of lO, 30 or lOO MHz emitted by a piezoelectric transducer goes through the thickness of a specimen, placed in a liquid, and is partially absorbed by the specimen. A laser based optical system detects the ripple on the mirrored surface and an holographic image of the entire field is obtained. Since the ultrasonic wave depends on the continuity of the specimen along the thickness, delamination, cracks, voids and porosity are detected as results of a low intensity ripple area. Performing this technique on the laminate composite specimens, it was also possible to determine the relative position of the gauges in the specimens.
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