Material Characterization of Self-Sensing 3D Printed Parts
13.2 Materials and Methods
13.2.1 3D printing of the Embedded Sensors
The 3D printing of the parts and the sensors were performed with a LulzBot TAZ 5 printer which was upgraded with a LulzBot TAZ Dual Extruder Tool Head v2 (Fig.13.1a). Painter’s tape was applied to the print bed to enhance adhesion during the build process. Strain gages (C2A-06-125LW-350) were acquired from Vishay Precision Group (Northvale, NJ).
Embedded sensors were printed using PLA and conductive PLA. In-PLA PLAdium Natural filament (2.85 mm diameter) was purchased from Taulman3D (Saint Peters, MO) and black conductive PLA filament (2.85 mm diameter) was purchased from Proto-Pasta (Vancouver, WA). Conductive PLA is composed of Natureworks 4043D PLA, a dispersant, and conductive carbon black [5].
Fig. 13.1 Experimental setup: (a) 3D printer setup, (b) Print orientations used during the testing
Table 13.1 Print parameters
used to print the specimens Parameter Test setting
Filament diameter 2.85 mm
Infill 50% and 75%
Layer height 0.5 mm
Layer thickness 0.5 mm Nozzle diameter 0.5 mm Nozzle temperature 215ıC Perimeter thickness 1.01.5 mm Print bed temperature 60ıC
Print speed 50 mm/s
To ensure repeatability during the fabrication of specimens, the print parameters were fixed throughout the study. The print parameters used are summarized in Table13.1. The specimens were printed at the 0ı, 90ı, and 45ıorientations (Fig.13.1b), where each layer for the 0ıand 90ıorientations were printed the same (longitudinal for 0ıand transverse for 90ı) and the layers for the 45ıorientation switched between 45ıand45ı. The purpose of testing the materials at these orientations was to satisfy the material property components of the composite laminate theory equations for the Hanshin failure criteria [6].
13.2.2 Testing of 3D Printed Sensor
Currently, there is no standard failure criterion accepted by industry designers for composites under general loading conditions [7]. Therefore, the Hanshin failure criteria are required for evaluating initial, subsequent, and final failure while the printed test specimens are under axial and bending loading. Hanshin failure criteria apply for in-plane ply failure modes and are compatible with the shells, or the outer perimeter layer, of the print specimens [6]. Therefore, the shell thickness is assumed to be a continuation of the composite ply. Through experimental testing, mechanical properties can be measured for Hanshin failure criteria evaluation of 3D printed parts. These mechanical properties include the Young’s modulus (E) in the extrusion direction (E0) and 90ofrom the extruded direction (E90), Poisson’s ratio () in the 0o–90odirection (0–90), and the in-plane shear modulus (G0–90) [6]. These properties are necessary to determine the failure of fiber rupture in tension, fiber buckling and kinking in compression, matrix cracking under transverse tension and shearing, and matrix crushing under transverse compression and shearing [6]. While the Hanshin failure criteria requires mechanical properties in the 0oand 90o orientations, this study also investigates the 45oorientation since it is often the default layup in many 3D printing programs.
13.2.2.1 Electrical Properties Temperature Sensitivity
The electrical properties of the conductive PLA were analyzed to determine the characteristic response of the printed sensors.
The sensor developed was a strain gauge (referred to as a U-Gage). Fig.13.2apresents an industry standard strain gauge (left) and the additive manufactured U-Gage (right). The filament manufacturer (Proto-Pasta) characterized the conductive PLA with a volume resistivity of 30 Ohm-cm for printed parts with perpendicular layers and a volume resistivity of 115 Ohm-cm for printed parts along the Z axis [8]. However, it was not clear under which conditions (i.e., fill density, layer thickness, orientation, other print parameters) these specifications were determined. Therefore, these characteristics were not able to be validated. Due to the lack of adequate information about the print parameters from the manufacturer, tests were conducted to determine the electrical characteristics of the conductive PLA specific to the print parameters used throughout the project.
To measure the temperature coefficient, three U-Gages were placed in a laboratory oven and their resistances were measured with respect to the change in temperature using a multimeter, as shown in Fig.13.2b. Additionally, due to the conductive PLA being highly hygroscopic, the humidity of the testing environment was monitored throughout the tests.
The oven temperature and relative humidity were measured using the Omega RH820U Humidity Temperature Meter.
Resistance measurements were taken 10 minutes after the desired environment temperature was reached to ensure thermal equilibrium and the resistances of the U-Gages were stabilized. The resistances were measured during temperature increases and decreases four times to be able to monitor for a hysteresis effect.
Fig. 13.2 Testing to determine the temperature coefficient: (a) Industry standard strain gauge (left) and an additive manufactured U-Gage (right), (b) Experimental setup for the temperature coefficient tests
Fig. 13.3 Tensile testing setup to determine the Young’s modulus under axial loading
13.2.2.2 Mechanical Properties
Determination of Young’s Modulus by Tensile Testing
Tensile tests were performed on conductive PLA coupons for sensor mechanical characterization as well as PLA coupons for the base material mechanical properties required for analyzing the Hanshin failure criteria. These tests obtain the Young’s modulus under axial loading in the longitudinal and transverse directions at 50% and 75% infill. ASTM standard D3039/D3039M for polymer matrix composite materials was adhered to [9]. All tensile tests were performed on a Mark-10 test stand (Fig.13.3), the grips for the Mark-10 test stand were of type G1061-2, and the max allowable load was 1100 N.
Fig. 13.4 Cantilever beam testing: (a) Experimental setup, (b) Load being applied to the beam
Determination of Young’s Modulus by Cantilever Beam Testing
The Young’s modulus under bending load, gauge factor, and Poisson’s ratio of PLA at 50% and 75% infill was obtained from a cantilever beam experiment. At each increment of applied load, the strain, resistance, and deflection from a strain gauge was recorded with a P3 Strain Indicator (Vishay Precision Group), multimeter, and micrometer (Fig.13.4). Two commercial strain gauges were adhered to a PLA beam specimen; these strain gauges were attached on the top and bottom of the beam as well as applied perpendicular to each other to calculate the Poisson’s ratio. The CAD model of the PLA beam includes a 4 mm diameter extruded cut hole from the top of the beam, 7.75 mm away from the cantilever end. This hole serves as a consistent location for an incremental load to be applied. To ensure the Young’s modulus was obtained from the elastic region of this experiment, the maximum load applied was 1 kg (9.81 N). After testing concluded, a final visual check of strain approaching zero was conducted to verify elasticity within the tested specimen.