Measurement of Interlaminar Tensile Strength and Elastic Properties of Composites Using Open-Hole Compression Testing and Digital Image Correlation

Abstract

Advanced polymeric composites are increasingly used in high-performance aircraft structures to reduce weight and improve efficiency. However, a major challenge delaying the implementation of the advanced composites is the lack of accurate methods for material characterization. Accurate measurement of three-dimensional mechanical properties of composites, stress–strain response, strength, fatigue, and toughness properties, is essential in the development of validated analysis techniques accelerating design and certification of composite structures. In particular, accurate measurement of the through-thickness constitutive properties and interlaminar tensile (ILT) strength is needed to capture delamination failure, which is one of the primary failure modes in composite aircraft structures. A major technical challenge to accurate measurement of ILT properties is their strong sensitivity to manufacturing defects that often leads to unacceptable scatter in standard test results. Unacceptable failure mode in standard test methods is another common obstacle to accurate ILT strength measurement. Characterization methods based on non-contact full-field measurement of deformation have emerged as attractive alternative techniques allowing more flexibility in test configuration to address some of the limitations inherent to strain gauge-based standard testing. In this work, a method based on full-field digital image correlation (DIC) measurement of surface deformation in unidirectional open-hole compression (OHC) specimens is proposed and investigated as a viable alternative to assessing ILT stress–strain, strength, and fatigue properties. Inverse identification using a finite element model updating (FEMU) method is used for simultaneous measurement of through-thickness elastic constants with recovery of the maximum ILT stress at failure for characterization of strength and fatigue S–N curves.

Experimental reexamination of transverse tensile strength for IM7/8552 tape-laminate composites

“…29 and Seon et al. 30 show ILT values that were higher than those obtained from D6415 and overall seem to compare well with Y T results from flexural tests available in the literature for the same material system. 19…”

“…28 Recently, two alternative methods have been proposed based on short-beam and the open-hole compression (OHC) tests. 29,30 Both studies report that D6415 provides overly conservative values of ILT due to systematic presence of porosity in the specimen's curved region. The two methods proposed in Makeev et al.…”

Special Issue on Advances in Digital Image Correlation (DIC)

“…The possibility to bridge more intimately measurements and models is highlighted in [8] where experimental measurements are combined to a model to extract mechanical fields with a certain mechanical admissibility close to a shear crack at bi-material interface. A little further on in the coupling between models and measurements, [9] proposed an interesting methodology to quantitatively characterize mechanical (interlaminar) properties reputed to be difficult to identify using finite element model updating techniques. Among current topics, the coupling of DIC with other types of instrumentation techniques or more generally data fusion is discussed in Article [10].…”

Approximation of Non-Linear Stress–Strain Curve for GFRP Tensile Specimens by Inverse Method

“…To this end, many studies have been conducted to improve the strength of plastics by adding glass or carbon fibers [11,12]. The plastics with such fibers vary the pattern of the stress-strain (S-S) curve, a macroscopic property, and also make it difficult to predict microscopic mechanical properties [13,14]. Unal [15] proposed a method for estimating the S-S curve of a specific or composite material with a large fluctuation in material properties during processing, through fuzzy theory.…”

“…Recently, studies on models for estimating the S-S curve using the response surface method of design of experiment (DOE), which are referred to as inverse methods, have been conducted [16][17][18]. For inverse methods, basic theories are provided by the American Society for Testing and Materials (ASTM) standards; further, studies to improve the accuracy of property data through mathematical modeling that uses power and exponential functions have been proposed [12][13][14][15][16][17][18][19][20][21][22]. However, as mathematical models may vary depending on the materials, inverse methods have been mostly applied to metals and alloy models.…”