PSI - Issue 72

P.M.M. da Silva et al. / Procedia Structural Integrity 72 (2025) 69–76

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reinforced composites (Vaidya et al. 2022). SMC is a polyme ric molding compound in the form of a ‘sheet’, also known as a prepreg. It essentially consists of a thermosetting resin matrix with glass fiber reinforcement and mineral fillers (Gortner et al. 2022). This material is suitable for structurally reinforced parts, finished on both sides. SMC is made up of several components, main and secondary. As main components, the unsaturated polyester is a binding agent for the various components that promotes material flow, and the glass fiber the component that gives the final part strength and durability. The secondary components are the thermoplastic additives, catalysts or inhibitors, release agents, thickeners, styrene, and mineral fillers (Kandelbauer et al. 2022). Depending on the required properties, such as the mechanical strength, flammability, density, and heat resistance, the percentages of each component can be adjusted. The value of SMC is the diversity in design, product size, and dimensional stability. SMC is processed by hot compression, using a compression mold with two halves that provide the desired design. For the mechanical characterization, the tensile test is the most widely used because it is easy and quick to perform, it has excellent reproducibility, and it is documented by various standards, such as the DIN EN ISO 527, ena bling to estimate the Young’s modulus ( E ) and tensile strength (  f ). The geometry can be circular, rectangular or square, among others, depending on the standard used. Since bulk specimens are made up exclusively of the material in question means that there is no influence from the adherends when compared to the butt joint method (Zuo and Vassilopoulos 2021). Shear tests are mainly used to estimate the shear modulus ( G ) and the shear strength (  f ). The biggest difficulty is obtaining a specimen with a precise cut in a representative sample volume, so that the methods present a satisfactory G measurement. Some of the methods are detailed in standards such as ISO14129 or ASTM D9051. The Iosipescu method is aimed at solid composite laminates, it requires opening a notch in the specimen, and the force is applied directly to the notch. The Block Shear (BS) test is governed by the ASTM4501 standard. This test has mainly been applied to characterize adhesives, although it can also be applied to solid samples (Adams 2010). Fracture toughness characterization is based on fracture mechanics concepts. This area studies the fracture behavior of components containing defects that can lead to catastrophic failure. Thus, it is vital to know the conditions under which these cracks propagate (Medina 2014). Three fundamental fracture modes exist, namely mode I or tensile mode, mode II or in-plane shear mode, and mode III or out-of-plane shear mode. In most cases, mode I fracture characterization is analyzed using the DCB test, in which the crack is subjected to a tensile stress. It is a relatively simple test to carry out and is documented in standards such as ISO 15024. Its main purpose is to obtain G IC (Xu and Ding 2020). The DCB test consists of applying tensile forces to the crack tip, by loading the specimen through hinges or loading blocks. The shear fracture characterization of materials is increasingly important since delamination is a significant failure mode (Davies et al. 2005). Several methods have been studied to standardizing and validating this characterization (Masters 1987). The main methods are the ENF, End-Loaded Split (ELS), and 4-Point ENF (4ENF). The ENF test is the most widely used because it is simple to implement and it is widely validated, producing accurate results. The ENF test consists of bending the specimen under a three-point arrangement. However, crack propagation may be unstable and monitoring it is complex (Figueiredo et al. 2018). SMC materials have been scientifically addressed under different perspectives. Nony-Davadie et al. (2019) experimentally assessed the anisotropic of advanced carbon fiber sheet molding compound composites (AC-SMC) under static and fatigue solicitations. Randomly oriented and highly oriented composites were evaluated. For the two tested solicitations, randomly oriented AC-SMC showed a two-stage damage evolution without any damage saturation prior to the samples’ failure. X -ray radiography and scanning electron microscopy (SEM) observations revealed microcracks appearing between and inside bundles. The damage evolutions of the two orientations of SMC displayed different kinetics and improved properties for the highly oriented SMC specimens. Trauth et al. (2018) characterized, under static and dynamic loads, a structural glass or carbon fiber SMC. For the glass fiber SMC at high testing rates, the strength and energy significantly increased. On the other hand, carbon fiber SMC revealed no measurable rate sensitivity. Anisotropic properties resulted from material flow during compression molding due to fiber orientation. Assmuth and Meschut (2022) evaluated the bonding properties and failure of adhesively-bonded SMC adherends, considering additives to increase the joint durability. SMC were bonded with both structural and semi-structural adhesives, and peel tests were carried out in pristine conditions, and under thermal and hygrothermal degradation. The test results showed that the joints are dependent on the used adhesive and possible fiber bridging. This work addresses the SMC’s mechanical behavior and fracture toughness with different glass FVF. Bulk and block shear mechanical tests were carried out. By DCB and ENF tests, G IC and G IIC , respectively, were estimated.