3.1 Tensile Performance
The tensile strength is a vital property for evaluating materials. The corresponding tensile stress-strain curves for the plate samples are presented in Fig. 1(a). The stress-strain curve of the samples can generally be segmented into two primary phases. The initial phase corresponds to elastic deformation, which exhibits a linear trend. This phase is further elaborated upon in the inset of Fig. 1(a). The subsequent phase can be attributed to the elastic-plastic deformation, which showcases a nonlinear behavior.
The tensile strength for various GF/PBT composites, namely GF/PBT-0, GF/PBT-85, GF/PBT-100, GF/PBT-115, GF/PBT-130, and GF/PBT-145, are determined to be 67.78, 64.84, 61.94, 58.50, 55.14, and 52.93 MPa, respectively(Fig. 1(b)). A discernible trend emerges from these findings: the tensile strength of the composite samples exhibits a progressive decline as the aging temperature ascends, with the most significant reduction observed beyond 100 ℃. The tensile strain measurements for these composites are depicted in Fig. 1(c). It can be inferred that the tensile strain for the composite materials progressively decreases from 0.058 to 0.054 with the escalation in aging temperature.
Delving into the elastic modulus of these materials, as illustrated in Fig. 1(d), it becomes apparent that the aging temperature does not significantly influence the elastic modulus. Regardless of the aging temperature, all samples manifested an elastic modulus hovering around 2.4 GPa. Thus, the thermal aging process exerts a minimal impact on the elastic modulus of GF/PBT samples.
To corroborate the experimental results, a static tensile stress simulation is conducted on the dog-bone shaped specimen using the FE software ANSYS (2018, USA). The specimen is modeled with hexahedral meshing (Solid 185 element), and a tensile simulation is executed by applying load at a consistent rate of 50 mm/min. The outcomes of the FE simulation are depicted in Fig. 2, and minor plastic deformation appears on the fracture surface.
The simulation reveals congruencies between the specimen deformation, the site of damage, the distribution and magnitude of maximum tensile stress, and the experimental findings. Figure 3 is the tensile stress-strain curve of FE analysis. It is slightly different from the experimental results, as the constitutive model of FE materials differs from that of real materials. Notably, the ultimate failure site is centralized within the midsection of the dog-bone specimen. The current FE model adeptly mimics the specimen's tensile behavior alterations. Such findings substantiate the reliability of the FE model, suggesting its capability to precisely forecast the specimen's behavior up until its peak load. Nonetheless, a holistic assessment of material alterations post-fracture mandates a combination of mechanical testing and fracture condition observations.
Figure 4(a) presents the typical stress-displacement curves from three-point bending tests on composite specimens post various thermal aging treatments. Displacement is gauged at the load application point. For every specimen, these curves exhibit a linear rise during the initial phase, culminating in the ultimate load, which denotes the specimen's breaking point. Evidently, the aged specimens display a markedly reduced stress-displacement response in comparison to the unaged samples, signifying diminished fracture toughness in the former. Figure 4(b) delineates the decline in bending strength with escalating aging temperature, mirroring the tensile strength trend. Concurrently, as depicted in Fig. 4(a), there is a decrease in bending strain correlating with a rise in aging temperature. Both the bending strength and strain show reductions in line with amplified thermal aging temperature. This can be attributed to the polymer matrix deteriorating under intensified temperature aging, leading to more brittle composites. Such alterations might stem from matrix expansion and the onset of microcracks at the interface due to temperature elevations.
Figure 5 shows the results from the ANSYS simulation of the plate specimen undergoing static three-point bending, utilizing a separated displacement coordination model for the simulation. A consistent load application rate of 50 mm/min is employed for bending simulation. The findings indicate congruence between the maximal bending stress and damage locales derived from FE simulation and those observed in the test specimen. The ultimate fracture occurs under the influence of the bottom load. Concurrently, the FE model proficiently illustrates the stress distribution across the GF/PBT composite plate specimens throughout the bending operation.
3.2 Microscopy Investigation of Tensile Specimen
Figure 6 presents SEM cross-sectional images of both the original and thermally aged GF/PBT composite samples. The fracture surfaces display a coarse, linden-like texture. Notably, no evident propagation or expansion of cracks is visible, suggesting that the GF/PBT composite samples maintain a commendable toughness. Additionally, the thermally aged composites, irrespective of the specific aging temperatures, show minimal variations. To delve deeper into the failure mechanisms of GF/PBT composites post thermal aging, a closer examination using high magnification images of tensile fractures is warranted.
Figure 7 displays a high-resolution micrograph of the tensile fracture surface of the GF/PBT composite. It's evident that the glass fibers are entrenched within the PBT matrix. Throughout the tensile fracture, certain glass fibers shatter, while others are extricated from the matrix, revealing a myriad of voids, underscoring the reinforcement role of the glass fibers in the PBT matrix. From Fig. 7(a)-7(b), the majority of glass fibers appear to fracture under tension, with a minority dislodging from the matrix, testifying to the robust adhesion between the PBT and the fibers. As illustrated in Figs. 7(c)-7(f), rising aging temperatures correlate with an escalating number of glass fibers disengaging during tension, resulting in numerous indentations on the matrix surface and fissures at the interface of glass fibers and the PBT matrix. This behavior can be ascribed to the contrasting thermal expansion coefficients of the glass fibers and the PBT matrix. The variance in thermal expansion coefficients during thermal aging introduces residual stress, culminating in interfacial damage and surface fissures between the glass fibers and matrix. When the aging temperature continues to rise, the fracture surface of the glass fiber still exhibits brittle fracture characteristics. At the same time, there is a clear gap between the broken glass fiber and the surrounding PBT, and a large amount of PBT debris is covered on the glass fiber fracture surface. This discovery indicates a significant separation between fibers and matrix at temperatures above 100 ℃, resulting in the formation of gaps. At high temperatures, PBT undergoes pulverization, leading to a large amount of debris after fracture. This change makes the original PBT matrix unable to provide support for glass fibers under tensile force, and at the same time, glass fibers undergo significant embrittlement, resulting in a significant decrease in the strength of GF/PBT materials. In addition, the separation of composite structures under tensile stress leads to a change in the slope of the tensile curve.