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Adhesive bonding is increasingly being used for composite structures, especially in aerospace and automotive industries. One common joint configuration used to test adhesive strength is the single-lap shear joint, which has been widely studied and shown to produce significant normal (peeling) stresses. When bonding composite structures, the normal stresses are capable of causing delamination before the adhesive bond fails, providing inconclusive engineering data regarding the bonding strength. An alternative test is the block shear joint, which uses a shorter sample geometry and a compressive-shear loading to reduce normal stresses. Analytical models proposed by Goland and Reissner and Hart-Smith are used to compare the edge-bending moment for the two joint configurations. The stress distributions along the bondline are also compared using finite element analysis. Experimental tests are conducted to evaluate these analyses and the failure modes of each configuration are recorded. Block shear samples demonstrate a joint strength over 100% higher than single-lap shear specimen bonded with the same adhesive material. The lower joint strength measured in single-lap shear is found to be potentially misleading due to delamination of the composite adherend.

In this paper, the adverse effects of sea water environment on the fatigue life of woven carbon fiber/vinyl ester composites are established at room temperature. The fatigue life, defined as number of cycles to failure is determined for dry and sea water saturated composites. It is observed that the presence of sea water decreases the fatigue life of woven carbon fiber/vinyl ester composites, i.e., sea water saturation reduces the numbers of cycles to failure. The cycles to failure are comparable between dry and sea water saturated samples at lower strain ranges, but are drastically different at higher strain ranges. That is, the average measured reduction is between 37% at 0.46% strain range to 90% at 0.62% strain range. This implies that the influence of sea water saturation on the fatigue life is more pronounced when the maximum cyclic displacement approaches maximum quasi-static deflection. In addition, microstructural damage modes are identified at different stages of fatigue loading, where both dry and sea water saturated composites manifest similar damage modes that include, matrix cracking initiated near the top surface of the composites, progressive crack growth manifested as intralaminar matrix cracking, and specimen failure via fiber breakage. Due to the aforementioned reduction in flexural fatigue responses and damage mechanisms observed in woven carbon/vinyl ester composites exposed to sea water environment, special consideration is required while designing critical load bearing components in offshore marine applications for long-term survivability of structures. evaluation [4,5,6]. Often many structures are subjected to fluctuating and vibrating loads, typically categorized as fatigue loading, which are known to cause premature structural failure. This paper studies the influence of one particular load case, flexural cyclic loading (flexural fatigue), on woven carbon/vinyl ester composites under two different environmental conditions of dry and sea water saturation at room temperature.Fatigue phenomenon is characterized by the failure caused by repeated loading, which initiate and propagate cracks as loading cycles increase. These types of loads can be steady, variable, uniaxial, or multiaxial. The cyclic (fatigue) load levels needed to cause failure is often less than the maximum quasi-static load, making this an important parameter to consider during design. Synthesis, analysis and testing are necessary procedures to develop a product with durability [7] for fatigue design as fatigue failures in structures implicate huge costs. Fatigue testing methods and design criteria for FRPCs can be challenging due to the complex damage mechanisms, which can potentially cause large scatter in fatigue life data and correspondingly increase the challenges associated with fatigue-predictive modeling. Temperature and environmental interactions with fatigue loading further generate intricate manifestations of failure modes within the material. Particularly, understanding the fatigue behavior of high-strength carbo...

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Sandwich composites and syntactic foams independently have been used in many engineering applications. However, there has been minimal effort towards taking advantage of the weight saving ability of syntactic foams in the cores of sandwich composites, especially with respect to the impact response of structures. To that end, the goal of this study is to investigate the mechanical response and damage mechanisms associated with syntactic foam core sandwich composites subjected to dynamic impact loading. In particular, this study investigates the influence of varying cenosphere volume fraction in syntactic foam core sandwich composites subjected to varying dynamic impact loading and further elucidates the extent and diversity of corresponding damage mechanisms. The syntactic foam cores are first fabricated using epoxy resin as the matrix and cenospheres as the reinforcement with four cenosphere volume fractions of 0% (pure epoxy), 20%, 40%, and 60%. The sandwich composite panels are then manufactured using the vacuum assisted resin transfer molding process with carbon fiber/vinyl ester facesheets. Dynamic impact tests are performed on the sandwich composite specimens at two energy levels of 80 J and 160 J, upon which the data are post-processed to gain a quantitative understanding of the impact response and damage mechanisms incurred by the specimens. A qualitative understanding is obtained through micro-computed tomography scanning of the impacted specimens. In addition, a finite element model is developed to investigate the causes for different damage mechanisms observed in specimens with different volume fractions.