Measurement and Analysis of Residual Stresses and Warpage in Fiber Reinforced Plastic and Hybrid Components

Abstract

Glass/carbon fiber reinforced plastic (GFRP/CFRP) and hybrid components have attracted increasing attention in lightweight applications. However, residual stresses induced in the manufacturing process of these components can result in warpage and, eventually, negatively affect the mechanical performance of the composite structures. In the present work, GFRP, CFRP, GFRP/steel and CFRP/steel hybrid components were manufactured through the prepreg-press-technology always employing the same process parameters. The residual stresses of these components were measured through the hole drilling method (HDM), based on an adequate formalism to evaluate the residual stresses for orthotropic materials including the calculation of the calibration coefficients via finite element analysis (FEA). In FEA, the real material lay-up and mechanical properties of the samples were considered. The warpage induced by residual stresses was measured after the samples were removed from the tool. The measured residual stresses and warpage of four different types of samples were compared and results were analyzed in depth. The results obtained can be extended to other hybrid materials and even could be used for designing multi-stable laminates for application in adaptive structures. Moreover, the effects of the drilling process parameters of HDM, e.g., the drilling speed, the drilling increment and the zero-depth setting, on the resulting residual stresses of GFRP were investigated. The reliability of residual stress measurements in GFRP using HDM was validated through mechanical bending tests. The conclusions concerning the choice of optimal drilling parameters for GFRP could be directly applied for other types of samples considered in the present work.

Experimental Analysis of Residual Stresses in CFRPs through Hole-Drilling Method: The Role of Stacking Sequence, Thickness, and Defects

“…Furthermore, matrix C (constituted of the coefficients C kl ) represents the relationship between the residual stresses and the relaxed strains and depends on the material properties and thickness of the sample, the depth of the hole, as well as the geometry of the strain gauge. More details on calculating the calibration coefficients C kl by separately imposing boundary conditions can be found in [11].…”

“…Alternatively, the hole-drilling method (HDM) is a well-known approach for measuring the residual stresses in composites, being capable of providing in-depth residual stress profiles [11]. The HMD has been standardized in ASTM E837-13a [12], specifying measurement procedure and range, minimum requirements of instrumentation, algorithms, and coefficients for the computation of uniform and nonuniform stress distributions, as well as error sources.…”

“…However, this assumption does not hold true for orthotropic materials. For accurate analysis of the residual stress state in orthotropic materials, such as CFRPs, the standard model [12] has been adapted, as can be seen in [1,11,13].…”

“…Both results were in good agreement. In other studies [13,21], calibration coefficients for glass/carbon fiber reinforced plastics (GFRP/CFRPs) were calculated based on their real geometry, thickness, and stacking sequence. The relation between the coefficients and the residual stresses was studied.…”

The Use of CFRP for Structural Reinforcement—Literature Review

“…The residual stresses of the steel-CFRP components were measured by the borehole method, based on an appropriate formalism for evaluating residual stresses for orthotropic materials, including the calculation of calibration factors by finite element analysis. The finite element method takes into account the actual material layout and mechanical properties of the specimens [23].…”