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Abstract

The interest in fiber-reinforced thermoplastic composites for automotive, aerospace, and other structural applications has significantly grown because of the need for high-performance, lightweight, and recyclable materials. Over-molded thermoplastic composites combine the benefits of discontinuous and continuous fiber reinforced thermoplastic composites, such as great design flexibility and improved mechanical properties. This research focuses on the characterization and testing of hybrid thermoplastic composites made of long fiber reinforced thermoplastic (LFT) composite and unidirectional (UD) glass fiber thermoplastic composite by compression over-molding. The aim is to assess the mechanical properties of these hybrid composites with both quasi-static testing such as three-point bend and dynamic testing such as the impulse excitation technique (IET). Moreover, micro-CT complemented by burn-off tests was utilized to analyze the extent of variability concerning fiber orientation, fiber length, and fiber content across different regions of the molded panels. Results suggest important relationships between mechanical performance and fiber orientation. The IET results coincided with those from the three-point bend testing, indicating its potential as a suitable nondestructive technique for testing hybrid composites comprised of discontinuous fiber and continuous fiber thermoplastic composites. This study offers detailed information about the interrelationships among the processing, microstructure, and mechanical properties of hybrid glass fiber polypropylene composites while demonstrating the potential of using the impulse excitation technique for testing the hybrid composite.

Keywords

Composite materials over-molding compression molding non-destructive analysis flexural property

Introduction

Demand for high-strength and lightweight materials for automotive and aerospace applications has made fiber-reinforced thermoplastic composites (FRTCs) of great interest. They offer an appealing combination of strength, toughness, short cycle times, and great recyclability,^1–5 making them great alternatives to conventional metallic materials and thermoset composites.^2,6–8 In addition to their great mechanical properties, FRTCs offer excellent corrosion resistance, good damping capacity,^9,10 and near-unlimited shelf life. The processability of FRTCs also allows them to be combined with dissimilar materials for hybridization, such as with other thermoplastic composites or metals, to create robust and light structural elements. These traits result in reduced fuel consumption and increased energy efficiency in transport systems, which are reconcilable with sustainability goals in the automotive,^11–13 offshore, and transportation industries.^14,15 The two types of thermoplastic composites that are widely used are discontinuous fiber composites, such as long fiber-reinforced thermoplastic (LFT) composites, and continuous fiber composites, such as unidirectional (UD) fiber thermoplastic composites. LFT composites are reinforced by discontinuous fibers with lengths that are greater than the critical fiber length.^1,16 Despite the advantages in ease of process and design, the mechanical properties of LFT composites are limited due to discontinuities and orientation of the fiber reinforcement, which normally results in compromised mechanical performance.^17–19 Compared with LFT composites, UD fiber composites are reinforced by continuous fibers and can be tailored to applications with multi-directional load requirements. In spite of the challenge in molding, UD fiber composite exhibits excellent mechanical properties in its fiber direction.

To overcome the shortcomings of LFT composites and UD fiber composites, hybrid composite materials have been developed to combine continuous and discontinuous fibers in FRTCs. The hybridized material provides balanced performance with directional strength and great processability, and a uniform stiffness profile.^20,21 Continuous fibers provide load-bearing capacity in principal load directions, while discontinuous fibers are used to fill the intricate geometries and increase the off-axis properties.^22,23 The hybridized materials can be manufactured with an over-molding or co-molding process, a process that combines two different materials to create a single part. This method is commonly used in manufacturing composite materials, particularly LFT and UD fiber composites. The compression-based over-molding process can be done by placing the LFT composite and UD fiber composite material together and then molding the two materials together.²⁴ Various forms of continuous fibers have been combined with LFTs to create hybridized materials with enhanced strength and rigidity.^20,21 These hybridized or over-molded LFTs with other forms of the composite can provide weight savings as a structural material, particularly in the automotive industry.^1,16 Previous studies have shown that LFT and different types of continuous fiber-reinforced materials can significantly improve the mechanical properties of the resulting composite.^1,16 The durability and reliability of the final product are crucial factors that need to be considered in the over-molding process. It is essential to ensure that the bonds between the different components of the composite material are strong enough to withstand the service conditions throughout its lifecycle.^22,23 Most of the over-molded composites research has been focused on injection-based processes, and their performance has been evaluated by destructive mechanical testing.^25–33

Non-destructive testing (NDT) techniques offer a complementary approach to evaluating material performance. These methods allow for quality assessment without causing damage, ensuring that the mechanical integrity of the final product meets the required standards. The non-invasive ability of NDT methods aids in the detection and monitoring of the material properties and structural changes over time. Only a few studies on over-molding using the compression molding process have been found, and none of them involves non-destructive analysis.^34,35 Impulse excitation testing (IET) is a non-destructive method to characterize the dynamic elastic modulus from the natural resonance frequency of a material. It is commonly used for ceramics, metals, and metal matrix composites,^36,37 and offers advantages such as specimen preservation, reproducibility, and exclusion of distortion by damage. Active research was done to expand on the capacities of the IET methodology, such as using a square specimen with numerical modal analysis verification,³⁸ modeling of the triphasic concrete composite material,³⁹ and integration of machine learning algorithms.^40,41 Song et al.⁴² compare the dynamic responses from numerical modal analysis and IET data, and a good correlation was found between these two. Tereza et al.⁴³ investigated the use of IET in measuring auxetic materials, especially because of their negative Poisson’s ratio. Comparison between flexural responses characterized by IET and quasi-static bending was found for lime-cement mortar,⁴⁴ glass fiber epoxy (GF/Epoxy),⁴⁵ metal-ceramic,⁴⁶ and carbon fiber epoxy (CF/Epoxy) composites.⁴⁷ The resulting trends vary by material type due to differences in their inherent characteristic responses in damping and relaxation within a multi-constituent composite material. Micro-CT is another non-destructive testing method that is increasingly used to characterize thermoplastic composites.^1,48–50 Short glass fiber-reinforced nylon composites were evaluated using micro-CT, and accurate fiber length and orientation information were obtained.⁴⁹ Woven glass fiber-reinforced polypropylene composite was studied via micro-CT to evaluate internal deformation, out-of-plane strain, and the onset of cracking and delamination.⁵¹ Micro-CT has also been used in 3D-printed thermoplastic composites to analyze the effects of printing parameters such as printing temperature, filament feed rate, printing speed, and layer thickness on the microstructure of the printed composite.⁴⁸ Both IET and micro-CT are used in this work to characterize hybrid composites.

This work focuses on characterization and testing of compression over-molded composites comprised of LFT and UD fiber composites. A hybrid composite was fabricated by over-molding long glass fiber-reinforced polypropylene (PP) with a unidirectional glass fiber PP composite tape. Fiber characteristics of the hybrid composite were quantified using a burn-off test and micro-CT scan. The mechanical performance of the hybrid composites was evaluated using three-point bend testing and impulse excitation testing and compared to that of LFT composites.

Materials and methods

Materials and equipment

Over-molded glass fiber polypropylene (GF/PP) composite panels were manufactured by an extrusion-compression process using a single-screw extruder and a 300-ton hydraulic press (LMG Machinery, USA). The long glass fiber reinforced polypropylene matrix composite pellets (CELSTRAN® PP-GF50-0453 P10/10) were used. This material consists of cylindrical long-fiber pellets approximately 11 mm in length, containing 50 wt% glass fiber with fiber lengths equivalent to the pellet length. For the continuous glass fiber reinforced polypropylene composite, unidirectional fiber tapes (CELSTRAN® CFR-TP PP GF70-13) were used. These tapes have a width of 305 mm and a thickness of 0.25 mm, with a glass-fiber content of 70 wt%. The material supplied by Celanese was used as raw materials. Fiber weight percentages, tensile modulus, and densities provided by the company are listed in Table 1.

Table 1.

Mechanical and physical properties of LFT composite and UD fiber composite tape from Celanese.

Property	UD composite tape	LFT pellets
Fiber weight percentage	70 wt%	50 wt%
Density	1.66 g/cm³	1.33 g/cm³
Tensile modulus	33.92 GPa	11.0 GPa
Thickness	0.25 mm	—
Flexural modulus	33.23 GPa	9.58 GPa
Pellet size	—	∼11 mm

The schematics of the machines and materials are shown in Figure 1(a). A layer of UD GF/PP composite tape was cut to a dimension of 152 x 152 mm and laid inside a mold. The mold was equipped with heaters and a temperature of around 110°C was used. The LFT charge was produced using a single-screw extruder, described in previously published papers,^16,52 to produce a molten charge at a temperature around 210°C. The extruder is equipped with a three-zone heater unit to supply a molten charge with its 60 mm diameter screw. The single-screw extruder operated separately from the compression molding machine. After extrusion, the molten LFT charge was manually transferred from the die outlet and placed onto the center of the UD GF/PP tape inside the heated mold cavity prior to compression (Figure 1(b)). The charge was deposited consistently over the centroid location to allow the LFT to flow uniformly in all directions. A molding pressure of 36 MPa and a two-minute hold time were set before the mold was opened. The over-molded panel before demolding and after demolding are shown in Figure 1(c) and (d), respectively. Baseline panels made with 100% LFT (without UD tape) were also produced using the same molding pressure for comparison purpose.

Figure 1.

(a) Schematics showing over-molding of LFT GF/PP onto UD GF/PP tape; (b) LFT GF/PP charge placed on top of UD GF/PP tape for over-molding; an over-molded GF/PP composite panel (c) before demolding and (d) after demolding.

Fiber characteristic measurement

The mechanical properties of LFT composites are positively correlated with the fiber alignment to their expected load-bearing orientation and fiber length.^1,16,53 Initial assessment of the over-molded panels suggests that the material properties vary spatially due to the inherent anisotropy of the reinforcing fibers and the fiber distribution due to mold flow from the over-molding process. The over-molded composite panel was zoned into 11 sections (11 samples) to establish a profile of mapped-out morphologies and mechanical responses, as shown in Figure 2. The samples have unidirectional fibers along the sample length direction. The LFT composite is on the top, and the UD tape is on the back side of the panel. The samples were labeled from 1 to 11 (from left to right). Fiber content, fiber length, and fiber orientation within the LFT composites were characterized using various methods such as burn-off test and micro-CT. These results will be used to correlate with the mechanical properties characterized by quasi-static mechanical test (three-point bend testing) and dynamic testing (impulse excitation test).

Figure 2.

Samples prepared from the over-molded composite panel for impulse excitation and three-point bend testing. Note. unidirectional fibers are in the sample length direction.

Burn-off Test

A burn-off test was conducted to evaluate the consistency of the fiber content distribution in each sectioned sample. Each sample was heated in a furnace (Ney 2-160 Series II Muffle Furnace, US) at 500°C for 2 hours to completely remove the polypropylene matrix. After the burn-off, the residue was weighed and compared to its pre-burning mass to get the fiber weight percentage. The fiber length measurement was also carried out for the residual to provide additional evidence to correlate the spatial variability in the morphologies and their resulting mechanical properties using ImageJ.⁵⁴ Seventy-five fibers were randomly selected and measured for each sample residue to obtain average fiber length using digital microscope VHX-6000 (Keyence corporation, Japan).

Micro-CT

Micro-CT tests were conducted to spatially observe fiber orientation and distribution within the over-molded composites. Samples from representative locations of the middle (Sample 6) and edge (Sample 1) were selected (Figure 2). Figure 3 shows the location that has been cut and scanned for analysis of fiber orientation. Samples were cut into 5 × 5 × 3.5 mm (length x width x thickness) rectangular pieces to be scanned using a U-CT^UHR micro-CT scanner (MILabs B.V., Netherlands) with settings of 100 kVp, 100 µA and voxel resolution of 10 microns. Scans were reconstructed using ImageJ.

Figure 3.

Sample locations from the over-molded composites for micro-CT scanning.

Impulse excitation testing

Mechanical properties of the over-molded composite panel were measured using IET. The IET method has gained recognition for analyzing the dynamic elastic properties of isotropic materials as it provides an accessible, fast, and consistent evaluation of their mechanical responses. IET tests were conducted using a Grindosonic MK5 (Grindosonic BV, Belgium) following procedures and equations from ASTM E1876 - Standard Test Method for Dynamic Young’s Modulus, Shear Modulus, and Poisson’s Ratio by Impulse Excitation of Vibration⁵⁵ to determine the dynamic properties of specimens. The specimen dimension was set up to be 152 x 13 x 3.5 mm, and the specimens were tested with a support span set to 80 mm. By measuring the fundamental resonant frequency (Hz) of a bar specimen in flexure, the elastic dynamic modulus of the specimen can be calculated using equation (1).⁵⁵ It is noted that the method was initially developed for testing isotropic materials as described in ASTM E1876. However, this method is used in this work to measure the dynamic Young’s modulus for the over-molded hybrid thermoplastic composite comprised of a UD tape and LFT composite. One of the objectives for this work is to validate the effectiveness of this method to measure its dynamic modulus.

E = 0.9465 (\frac{m f_{t}^{2}}{b}) (\frac{L^{3}}{t^{3}}) T_{1}

(1)

for which E is the dynamic Young’s modulus, m is the mass of the bar, b is the width of the bar, t is the thickness of the bar, and L is the length of the bar. f is the fundamental resonant frequency of the bar in flexure, Hz. T₁ is the correction factor for the fundamental flexural mode, which can be defined by equation (2)⁵⁵ when L/t ≥ 20:

T_{1} = 1 + 6.585 {(t / L)}^{2}

(2)

The impulse excitation test apparatus is shown in Figure 4. An exciter with a steel ball attached was used to introduce excitation on the simply supported beam. A piezoelectric transducer was used to receive resonant responses, which would be processed using a Fast Fourier Transform (FTT) algorithm to retrieve the characteristic natural frequency of interest. 50 data points were collected to achieve the desired statistical representation of the measurements.

Figure 4.

The impulse excitation testing setup for measuring the natural frequency of samples including over-molded composites.

Natural frequency data was collected by striking the front side (LFT side) and the back side (UD side) to measure the dynamic Young’s modulus of each specimen. Each specimen was tested at least twice to evaluate the consistency of the natural frequency. The second test was performed right after three-point bend testing to verify no damage occurrence to the specimen. Data from the front and back side of the pure LFT samples were collected for comparison purposes.

Bending test

Three-point bend (3PB) tests were conducted on the over-molded samples following the procedure outlined in ASTM D790 - Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials.⁵⁶ It is noted that those samples do not follow the standard sample configuration because of the non-symmetric characteristic through their thickness. This was done using an Instron 1331 universal testing machine from Instron, Norwood, MA, USA. The sample dimension was set up to be 152 x 13 x 3.5 mm with a gauge length of 56 mm based on the recommended length-to-depth ratio of 16:1 from the standard. The tests were performed in displacement control, and the crosshead speed was set to 2 mm/min. Each specimen was loaded to a maximum displacement of 3 mm, whereupon the associated load–deflection data was recorded for modulus and strength calculations. Figure 5 shows the 3PB testing used for the quasi-static mechanical test. Each sample was tested within their elastic limit without inducing permanent damage to the specimen, which was confirmed by identical flexural stress-strain curves after superimposing for the same specimen as well as the same natural frequency values from IET testing. Each sample was tested on the LFT (front) and GF/PP tape (back) sides to evaluate possible differences in material behaviors and compare results with those gathered from the dynamic test. Each specimen was tested six times in total to verify that no damage occurred during flexural testing, which was confirmed by consistency in the flexural modulus measured from those repeated tests performed on the same specimen.

Figure 5.

Three-point bend test setup for testing hybrid composite samples to elastic limit without inducing permanent damage to the beam.

Results and discussions

Fiber morphologies

Fiber morphologies were expected to be aligned with the transport phenomena of the molten charge governed by the radial squeeze flow from the compression molding process.⁵⁷ Fiber characteristics were measured quantitatively using a burn-off test and qualitatively using a micro-CT scan to establish location-varied baseline characteristics. Fiber weight fraction, fiber length, and fiber orientation were characterized.

Fiber content and length measurement

The fiber fraction of the over-molded composites was characterized using a burn-off test. 11 samples were tested from a panel to map out the fiber distribution. Residues of each sample were measured using a balance with an accuracy of four decimal digits. The measured fiber weight fraction and fiber length are summarized in Table 2. It was found that the fiber weight fraction of the LFT composite remains consistent at an average of 53.3% across the examined samples with a low overall standard deviation (0.23%), regardless of its relevant location over the molded panel. The trend highlights the consistency in fiber content. To examine the variation of results statistically, fiber length statistics were binned into two groups by their relative location over the mold for a t-test to be conducted. Samples 1-3 and 9-11 (Figure 2) were grouped into the edge sample group, and samples 4-8 were grouped into the middle group. The inferential statistics suggest that the middle group is significantly smaller than the edge group (P < .05) using a one-tailed t-test. This serves as evidence to support the expectation of a varying mechanical response between these two groups.

Table 2.

Fiber weight fraction and fiber length measurement of the LFT composite in hybrid composite samples.

Sample location	Fiber weight fraction (wt%)	Average fiber length (mm)
1 (edge sample)	53.0	4.61
2	53.8	4.63
3	53.4	4.37
4	53.0	4.22
5	53.3	3.99
6 (middle sample)	53.2	3.88
7	53.1	3.98
8	53.2	3.91
9	53.4	4.13
10	53.5	4.29
11 (edge sample)	53.3	4.58

Apart from fiber content, fiber length is another essential factor that positively correlates with the mechanical performance of the composites.⁵⁸ An analysis was conducted to investigate the spatial distribution of fiber length over the burn-off residue defined previously over the LFT portion. The average fiber lengths measured for the dataset was 4.19 mm, with a standard deviation of 0.50 mm. Trends of higher fiber length were observed for samples neighboring the mold edges. The fiber length was significantly reduced from the LFT’s 11 mm as stated on the datasheet across the composites due to the single-screw extrusion and compression processes. The average fiber length near the center was found to be slightly shorter probably due to more fiber-fiber interaction with longer interaction time and more constrained condition compared to the fibers flowing away from the center location. The center area has a fiber length of 4.00 ± 0.13 mm, averaged from 5 center samples. The edge area has a fiber length of 4.44 ± 0.20 mm, averaged from 6 center samples. There is approximately 11% increase of fiber length from center to edge. The measured fiber weight fraction of the LFT layer remained consistent across all samples, averaging 53.3 wt%, which represents a slight increase of approximately 3.3% compared to the nominal fiber content of the original LFT GF/PP pellets. Micro-CT imaging further confirmed distinct differences in fiber orientation from the center to the edge of the LFT composite layer, with fibers at the edges aligning more parallel to the sample length and fibers in the center exhibiting increasingly randomized or circumferential orientation. These factors can all contribute to the observed variations in fiber length across the sample. Generally, composites reinforced with longer fibers longitudinally aligned with the load-bearing direction will result in a higher modulus compared to shorter fibers.^2,59–61

Micro-CT analysis

Fiber orientation for the over-molded panel was characterized by micro-CT scanning. Two samples were selected for micro-CT testing: one from the edge (Sample 1) and one from the middle (Sample 6) of the panel to confirm the trend of fiber orientation regarding the mold flow scenarios. Fiber orientations were assessed visually from the reconstructed micro-CT slice images, as automated orientation quantification was outside the scope of the present study. Scanning results were selected and shown in locations A, B, and C (Figure 6). As fibers approached the edges, they followed a circular pattern or were oriented parallel to the charge center. Thus, specimens prepared from the edges had fibers mainly oriented in the sample length direction or oriented with small angles close to this direction. The micro-CT images of Sample A reveal the intricate details of the fiber orientation and length distribution in different areas, highlighting the challenges of achieving uniform fiber distribution in composite materials. These findings have significant implications for the mechanical properties of the composite material. Therefore, it is crucial to consider these factors during the manufacturing process and understand their effect on mechanical properties. The results showed that the fiber orientation in the middle section was random in Sample 6, while the end of the sample had a predominant fiber orientation perpendicular to the sample length direction. Furthermore, similar to Sample 1, it was observed that a circular or parallel orientation of fibers with respect to the charge center existed at the ends of Sample 6.

Figure 6.

Schematic representation of the analyzed regions of the micro-CT scans of Sample 1 (left: a, b, and c) and Sample 6 (right: d, e, and f). “Center” indicates the location where the LFT charge was placed during overmolding, and “center direction” denotes the direction pointing toward this placement location. “Orientation direction” represents the visually assessed fiber alignment within each region. “Sample length direction” refers to the longitudinal direction of the specimen.

Micro-CT imaging provides a method to analyze fiber orientation in composite materials. Findings highlight the importance of understanding fiber orientation in optimizing the production process and improving the mechanical properties of composite materials. The combination of impulse excitation testing and micro-CT analysis can provide valuable insights into the properties and characteristics of composite materials, enabling manufacturers to optimize their processes for improved product quality and performance.

Dynamic and quasi-static mechanical properties

The mechanical properties of the over-molded composites were characterized and compared using IET and 3PB. A general trend of a higher dynamic modulus compared to quasi-static modulus is expected due to a shorter relaxation time allowed for the dynamic modulus, and possible morphological damage induced by excessive deformation during the quasi-static tests.⁶²

Repeatability of IET dynamic modulus measurements for the overmolded composite panels

The consistency of the impulse excitation test method for non-destructive testing of overmolded composite materials was evaluated by analyzing 22 samples. Each sample underwent 3 separate trials and was excited five times on its front and back sides each time. Statistical analysis was performed to assess the consistency of the test method. On each IET sample, measurements of the natural frequency that represent flexural vibration were done five times on both the front and back of the samples to establish a statistically sound dataset. The resonance frequencies were then converted to dynamic modulus to derive an average modulus reading and standard deviation. Figures 7 and 8 show the dynamic modulus of 2 overmolded panels consisting of 11 specimens each. Panel 2 (shown in Figure 8) was later found to contain local delamination in samples 5–9, which explains the lack of overlap among the three repeated IET runs for these locations. Because these defects interfered with the dynamic response, Panel 2 was discarded and not used for subsequent mechanical characterization. This provides a practical example of how local defects can influence IET reproducibility and highlights the method’s sensitivity to internal damage. It is observed that the overall flexural modulus reading does not differ much when comparing data retrieved from the front (LFT side) and back (UD side) of the samples. t-tests were conducted between each back and front datasets to determine if there is any statistical significance between the front and back results. Statistical significance was found in 62% of the datasets tested for the t value higher than the critical value (p > .05). The result indicates that the variance between the front and back groups can differ due to differences in flexural responses from the morphological differences of the fibers. Overall, the dynamic modulus measured by IET is insensitive to the sides where it was measured due to the unifying nature of the beam theorem that embodies the resonance responses in a through-thickness manner. This attribute can be valuable in providing a mapped-out relative properties profile in a nondestructive way.

Figure 7.

Dynamic modulus measured between the front (LFT) and back (UD) surfaces of panel 1 for trial runs A–C. Trials A–C represent repeated IET measurements performed on the same panel to assess test repeatability.

Figure 8.

Dynamic modulus measured between the front (LFT) and back (UD) surfaces of panel 2 for trial runs A–C. Trials D-F represent repeated IET measurements performed on the same panel to assess test repeatability.

The results indicated that the impulse excitation test method is consistent in its measurements across the three trials, with a relatively small standard deviation compared to the range of mean values. Thus, the impulse excitation test method is a valuable tool for evaluating the performance and durability of hybrid composite materials. It shows that there is little deviation between the natural frequencies that are obtained in striking the front (LFT) or back (UD) surface of the samples. It is important to note, however, that the laminate contains a unidirectional (UD) tape layer and hence exhibits orthotropic elastic behavior. ASTM E1876 provides equations for the calculation of a single effective Young’s modulus based on flexural vibration response, but this value represents an apparent or homogenized modulus, not the full orthotropic stiffness matrix. Consequently, the impulse excitation technique in its standard form may not be able to fully capture direction-dependent stiffness effects in laminates with stronger anisotropy or unbalanced layups. This limitation should be considered in applying IET to other laminate architectures because the increased orthotropy may affect the representativeness of the calculated modulus, limiting broader generalization of results.

Comparing flexural response from dynamic and quasi-static mechanical tests

Figure 9 compares the dynamic and quasi-static flexural responses of over-molded LFT-UD composites. Figure 10 compares the dynamic and quasi-static flexural responses for LFT composites. Dynamic flexural responses remain consistent throughout the designated sections, no matter whether it was measured from the front (LFT) or back (UD) of the samples. However, quasi-static responses do vary when comparing the results from the front and the back. The front side, where the LFT was deposited, generally presented a higher flexural modulus as the UD layer underneath provided additional flexural rigidity. It is also observed that the retrieved modulus is consistently higher for both dynamic and quasi-static results for the composite samples near the edges, likely due to the longer and more aligned fiber profile.

Figure 9.

Comparison of dynamic and quasi-static flexural responses for an over-molded (OM) composite panel. The dynamic response corresponds to the averaged results from the Panel 1 trials (A–C).

Figure 10.

Comparison of dynamic and quasi-static flexural responses for an LFT composite panel.

In the analysis of LFT composites, it has been observed that the orientation of fibers is a crucial factor that determines the mechanical properties of these materials. Fiber orientation refers to the alignment of fibers in a particular direction, which directly affects the load-bearing capacity of the composite in that direction. The flow of molten charge from the center (where the charge is placed at) towards the four sides and corners of the mold cavity results in fiber alignment in the edge samples shown in Figure 5. Due to lack of flow at the center location, the fibers are more randomly oriented. This study also shows that fiber alignment can be correlated to the mechanical properties of the composites. When fibers are aligned in a particular direction, the load-bearing capacity of the composite in that direction increases significantly, resulting in improved modulus. In contrast, when the fibers are randomly oriented, the mechanical properties such as modulus of the composite are often less.

The findings also revealed a similar correlation between the modulus values derived from IET and those obtained from flexural testing, indicating that IET can capture the mechanical performance trends across different regions of the over-molded panel with LFT and UD tape. These variations were confirmed through microstructural analyses using burn-off tests and micro-computed tomography. Samples located near the edge of the molded panels showed higher modulus values, which were attributed to fibers aligning more parallel to the direction of applied stress and a longer fiber length. In contrast, samples taken from the center exhibited a more randomized or circumferential (tangential to the center) fiber orientation pattern and shorter fiber lengths, leading to reduced modulus values. This spatial variability in fiber orientation and length, influenced by the flow dynamics of the molten LFT during over-molding, significantly affected the mechanical anisotropy of the panels.

Overall, the representative groups for the edge (Samples 1-3 and Samples 9-11) and middle (Samples 4-8) were compared on their modulus, as shown in Figure 11(a). An average modulus was calculated from data retrieved on the LFT front side of the samples. An average modulus of 10.21 ± 0.4 GPa and 7.09 ± 1.26 GPa were reported for 3PB quasi-static for edge and middle groups for LFT composites. In comparison, IET dynamic properties for LFT were reported for edges and middle groups as 12.26 ± 0.75 GPa and 9.48 ± 1.03 GPa. An average modulus of 11.99 ± 1.70 GPa and 9.93 ± 1.16 GPa were reported for 3PB quasi-static for edge and middle groups for over-molded composites. In comparison, IET dynamic properties for over-molded composites were reported for edges and middle groups as 14.55 ± 2.05 GPa and 12.42 ± 1.02 GPa (Figure 11(b)).

Figure 11.

(a) Grouped data that represent properties of edge and middle samples, and (b) comparison of average modulus for LFT (Panel 3) and over-molded composite (Panel 1) samples measured from impulse excitation and flexural testing.

Conclusions

This work characterized hybrid GF/PP composites manufactured by compression over-molding LFT GF/PP onto UD GF/PP tape and evaluated the influence of local microstructure on mechanical performance. Burn-off testing showed that the LFT layer maintained a consistent fiber weight fraction of 53.3 ± 0.23 wt%, while fiber length decreased from the 11 mm pellet length to an average of 4.19 ± 0.50 mm. A clear spatial trend was observed: fiber length in edge regions averaged 4.44 mm, approximately 11% longer than the 3.98–4.00 mm measured in the center. Micro-CT imaging confirmed that fibers in edge regions aligned predominantly with the sample length direction, whereas central regions exhibited more randomized or circumferential orientation.

Mechanical characterization revealed corresponding spatial differences in modulus. For LFT-only panels, edge samples exhibited an average quasi-static flexural modulus of 10.21 GPa compared to 7.09 GPa in the center, while dynamic modulus from IET measured 12.26 GPa at the edges versus 9.48 GPa in the center. For the over-molded hybrid panels, the quasi-static modulus increased to 11.99 GPa at the edges and 9.93 GPa in the center, with IET values of 14.55 GPa and 12.42 GPa, respectively.

These results demonstrate that IET reliably captures modulus trends driven by fiber length and orientation and provides a practical nondestructive method for evaluating hybrid thermoplastic composites. The strong agreement between IET and flexural testing confirms its suitability for rapid quality assessment in over-molded composite structures.

Footnotes

Acknowledgements

This work is supported by the Materials Processing and Applications Development Center at The University of Alabama at Birmingham, Birmingham, AL, USA.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Yu-Chao Shih

Yongzhe Yan

Haibin Ning

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Characterization and testing of over-molded hybrid composites comprised of discontinuous and continuous glass fiber polypropylene composites

Abstract

Keywords

Introduction

Materials and methods

Materials and equipment

Fiber characteristic measurement

Burn-off Test

Micro-CT

Impulse excitation testing

Bending test

Results and discussions

Fiber morphologies

Fiber content and length measurement

Micro-CT analysis

Dynamic and quasi-static mechanical properties

Repeatability of IET dynamic modulus measurements for the overmolded composite panels

Comparing flexural response from dynamic and quasi-static mechanical tests

Conclusions

Footnotes

Acknowledgements

Declaration of conflicting interests

Funding

ORCID iDs

References