Precise Determination of Thicknesses of Multilayer Polyethylene Composite Materials by Terahertz Time-Domain Spectroscopy

Abstract

The multilayer structure of an ultra-high molecular weight polyethylene (UHMWPE) composite material was investigated in the terahertz (THz) spectral range by means of time-domain spectroscopy (TDS) technique. Such structures consist of many alternating layers of fibers, each being perpendicular to the other and each having a thickness of about 50 μm. Refractive indices of two composite samples and of a sample composed of four single layers (plies) having the same fiber orientation were determined for two orthogonal orientations of the electric field in a transmission TDS system. The birefringence of a single layer was measured, and the origin of this phenomenon is discussed. Using the TDS system in reflection, the formation of many pulses shifted in time was observed originating from reflections from interfaces of successive layers caused by the periodic modulation of the refractive index along the propagation of the THz radiation. This phenomenon is theoretically described and simulated by means of a transfer matrix method (TMM). A time-domain fitting procedure was used to determine thicknesses of all layers of the composite material. The reconstructed waveform based on the optimized thicknesses shows very good agreement with the measured waveform, with typical differences between measurements and simulations J Infrared Milli Terahz Waves (2015) between 3 and 7 μm (depending on the sample). As a result, we were able to determine the thicknesses of all layers of two multilayer (~200 plies) structures by means of the reflection TDS technology with high accuracy.

Continuous Wave THz System Based on an Electrically Tunable Monolithic Dual Wavelength Y-Branch DBR Diode Laser

“…The actual thickness of the sample can be calculated from the known refractive index (n) of the material. Using n = 1.54 from [17,18] and using the equation Δl = (n − 1)d s where Δl is the optical thickness and d s is the sample thickness, we obtain d s = 2.6 mm. This is in reasonable agreement with the thickness of 3.1 mm obtained by a Vernier caliper but suggests that the refractive index in our sample at 300 GHz is slightly lower than the value given in [18].…”

Defect Detection of Adhesive Layer of Thermal Insulation Materials Based on Improved Particle Swarm Optimization of ECT

“…A method to find counterfeit drugs quickly and reliably based on transmission spectroscopic terahertz (THz) measurement technique was developed in [6]. Palka and Krimi [7], Li and Ding [8] proposed a method based on terahertz time-domain spectroscopy for thickness detection of composite materials, which can obtain the thicknesses of all of the layers of the composite materials based on a time-domain fitting procedure. A method based on laser ultrasonic detection of drilling-induced delamination was presented for the test of composite laminates in [9], where the laser ultrasonic C scan was used to test composite laminates, and the morphologies, dimensions, and positions of drilling-induced delamination can be obtained.…”

Terahertz imaging of composite materials in reflection and transmission mode with a time-domain spectroscopy system

“…Recently, Terahertz technology has emerged as a promising technology for performing NDT. [1][2][3][4][5][6] Examples of applications of THz technology within NDT include inspection of car paint layers 7 and inspection of coating thickness for pharmaceutical tablets.…”