Identification of process-induced residual stress/strain distribution in thick thermoplastic composites based on in situ strain monitoring using optical fiber sensors

Abstract

In thick thermoplastic composite laminates, nonuniform temperature distribution arises in the through-thickness direction during high-rate manufacturing processes. This causes the so-called thermal skin-core effect. The surface region solidified in advance constrains shrinking of the inside region, so nonuniform residual stress/strain distribution arises in the through-thickness direction. This study quantitatively clarified this mechanism and identified the amount of residual stress/strain by utilizing fiber optic–based internal strain measurement and process simulation. First, in-plane transverse strain of thin carbon fiber/polyphenylenesulfide laminates was measured using fiber Bragg grating sensors to determine two key parameters for stress/strain simulation; thermal/crystalline shrinkage strain and composite stiffness. Abaqus-based simulation using these properties was then performed to calculate stress/strain distribution in thick laminates. The simulated strain agreed well with the measured value and it was confirmed that the residual stress developed in a relatively low temperature range. In addition, transverse three-point bending tests were conducted to validate the amount of residual stress calculated by the simulation. The bending strength increased by the thermal skin-core effect and the amount of strength increase coincided with the simulation, confirming the validity of the simulation. Extension of the proposed approach to the evaluation of the morphological skin-core effect is also discussed.

Assessing residual stress redistribution during annealing in thick thermoplastic composites using optical fiber sensors

“…Developments of parabolic residual stress distribution due to thermal skin-core effect; surface (inside) region has residual compressive (tensile) stress. 4 This present study clarifies development and redistribution behavior of residual stress through both molding and annealing using multiple optical fiber sensors deployed in the thickness direction, which was utilized in our previous study. 4 The focus is on the behavior in the composite transverse direction dominated by the matrix resin properties because the transverse property determines residual stress and shape distortion.…”

“…The strain at the beginning of cooling is set to zero as a reference point. Even though the results of the SC and MC conditions have been already included in our previous article, 4 they are presented again for completeness. The FBG sensors started to measure significant compressive strain due to crystallization and solidification below solidification temperature, regardless of cooling conditions.…”

“…The surface region solidified in advance constrained shrinking of the inside region (Figure 1), so compressive strain was smaller in the inside region. 4 Since the maximum temperature difference between the surface and the core generated during cooling was larger under the FC condition, it was expected that the thermal skin-core effect and thus strain difference between the surface and the inside would be more significant under the FC condition. However, the strain difference after cooling was larger under the MC condition.…”

“…2,3 The authors have investigated the thermal skin-core effect generated in unidirectional (UD) carbon fiber/polyphenylenesulfide (CF/PPS) laminates using a new approach combining fiber-optic-based strain monitoring and process simulation. 4 Meanwhile, under a molding condition with a higher cooling rate, the cooling rate is also nonuniform in the thickness direction in addition to the temperature. When the matrix resin of the thermoplastic composite is semicrystalline, the crystallization behavior of the matrix depends on the cooling rate, 5 so nonuniform distribution of degree of crystallinity (DOC) is formed in the thickness direction.…”

Simultaneous Measurement of Temperature and Mechanical Strain Using a Fiber Bragg Grating Sensor

“…Jacobsz and Jahnke [ 10 ] utilized FBG sensor to detect the leak of water pipelines. Tsukada et al [ 11 ] evaluated the residual stress arose in thick thermoplastic composite laminates during high-rate manufacturing processes using FBG sensors. Jang and Kim [ 12 ] utilized FBG sensors to determine the impact location on a composite stiffened panel using a triangulation method.…”