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

Abstract

In thick thermoplastic composite laminates, nonuniform temperature and cooling rate distribution arises in the through-thickness direction during cost-effective high-rate manufacturing processes. Annealing is often carried out after molding to homogenize degree of crystallinity (DOC) and to reduce residual stress. Even though the change in the residual stress/strain distribution occurring inside thick laminates by this heat treatment is practically important, the changing process and the detailed mechanism are not sufficiently clarified. This present study addresses development and redistribution behavior of residual stress through both molding and annealing using multiple optical fiber sensors deployed in the thickness direction. This article begins by explaining about process monitoring of thick laminates to discuss process-induced strain distribution depending on cooling conditions during molding. Next, strain monitoring is performed during annealing, and the strain change caused by cold crystallization is clarified. Finally, the residual stress distribution is evaluated by a transverse three-point bending test, and the validity of the redistribution mechanism deduced from the strain measurement is confirmed.

Comparison of DOFS Attachment Methods for Time-Dependent Strain Sensing

“…However, the integration of OF sensors inside the component, or attaching them on the component surface, is still a challenge for many practical applications [ 1 ]. A limited scope literature review identifies basic types of attachment methods [ 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 ] for fixing the optical fibers, as shown in Table 1 . Structural engineering applications (concrete, timber, and steel) tend to adhere the OF directly on the surface by a rigid glue [ 2 ], pre-embed the OF in a package filled with rigid glue or soft rubber [ 5 , 6 , 7 ], or attach specialized optical cables to the component [ 8 ].…”

“…Similar methods are adopted for polymers and polymer composites [ 9 , 10 ]. In addition, the OF can be embedded directly inside the polymer or composite components during the manufacturing process [ 11 , 12 , 13 , 14 , 15 ]. Polymer matrix surrounding the OF enables the strain transfer and protects the sensor.…”

Review of microwave techniques used in the manufacture and fault detection of aircraft composites

“…There is ongoing research on other non-destructive testing (NDT) techniques for more accurate evaluation and convenient implementation, such as acoustic emission (2)(3)(4)(5)(6)(7)(8) , guided waves (9)(10)(11)(12)(13)(14)(15)(16)(17)(18) (e.g. piezoelectric transducer-based (19,20) and angle beam transducer-based (21,22) ), air-coupled ultrasonics (23)(24)(25)(26)(27)(28)(29)(30) , thermography (31)(32)(33)(34)(35)(36) , optical fibre sensing (37)(38)(39)(40)(41)(42) , digital image correlation (DIC) (43)(44)(45)(46)(47)(48) , electromagnetic testing (49)(50)…”