Evaluation of residual stress development in FRP-metal hybrids using fiber Bragg grating sensors

Abstract

This paper presents experimental measurement methods for the determination and evaluation of process related thermal residual stresses in fiber metal laminates. A cure monitoring system with fiber Bragg grating (FBG) sensors is used to measure the in-plane strains during processing of carbon fiber reinforced plastic (CFRP)-steel laminates. The simultaneous measurement captures the thermal expansion during the heating stages, the cure shrinkage, and the cooling thermal shrinkage. The results enable the characterization of the co-cure bonding process and the stress transfer between the metal and FRP-layers during the creation process. The residual strains, which are used for calculation of the residual stresses, are recorded at room temperature after manufacturing. In addition, an advanced method using FBG-sensors and the deflection of asymmetric hybrid specimens was developed to validate the gained residual stress data. Asymmetrical specimens are created by removing selected layers after cure. Quantitative evaluation is achieved by determination of their curvature and measuring the strain changes with the embedded FBG-sensors. For validation, the methods were successfully demonstrated on two different curing cycles with different resulting residual stress levels. The simultaneous strain measurement enables the investigation of stress development and delivers more in-depth process knowledge for further optimization of the manufacturing process.

Adhesion of Multifunctional Substrates for Integrated Cure Monitoring Film Sensors to Carbon Fiber Reinforced Polymers

“…The curing reaction can be monitored via various physical effects, since a wide variety of properties change during polymerization [1,2]. Structure-borne sound methods are based on the change of mechanical properties [3], refractive methods use the change of the refractive index [4], strain-based methods such as Fiber Bragg Gratings measure the chemical shrinkage during the curing reaction [5], thermodynamic methods such as Differential Scanning Calorimetry measure the heat released during the exothermic reaction and dielectric methods are based on the change of dielectric properties of the epoxy resin [6]. Dielectric monitoring of the curing reaction using film sensors has several advantages over the other methods mentioned.…”

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

“…Section 3). The position of the grids 1 to 8 is shown in Figure 3, and eight different combinations of strain gauge grids, (1,2,3), (2,3,4), (3,4,5), (4,5,6), (5,6,7), (6,7,8), (7,8,1), (8,1,2), are evaluated for determining residual stresses. An anomaly in the results due to local heterogeneity and defects can be immediately detected by using this special strain gauge.…”

“…These residual stresses often cause warpage of the composite after it is removed from the tool. In addition, the residual stresses also can induce defects such as delamination in the absence of any external loads [8]. The magnitudes of these stresses depend on material properties, stacking sequences, overall dimensions as well as the curing cycle and cooling conditions [4,8].…”

Automated, Quality Assured and High Volume Oriented Production of Fiber Metal Laminates (FML) for the Next Generation of Passenger Aircraft Fuselage Shells

“…Fiber-Bragg-gratings (FBG) and strain gages (SG) were used simultaneously to measure strains in the curing composite layers and the thin metal foils, respectively, at the same time (see Figure 8a). A study has been successfully conducted in the project, to validate the SG and FBG measurement techniques and to validate the specific temperature compensation factors, which were determined for the optical sensors in previous investigations [12].…”