The mechanical properties of adhesive joints are generally evaluated based on their strength and toughness. The best-known method for evaluating strength is the single-lap joint (SLJ) test. Springs are sometimes used when applying creep loads to SLJ specimens under humid conditions [20,21]. Spring-loaded SLJ specimens have also been immersed in hot water or exposed on ship decks [16,22]. The double cantilever beam (DCB) test is commonly used to evaluate fracture toughness [23,24]. In the static fracture test, the DCB specimen is set to a mechanical tensile testing machine and loaded in mode I at a constant opening speed. Three parameters, the crack length, load, and opening displacement, are required to calculate the fracture toughness of the adhesives in the DCB tests. However, the optical measurement of the crack length causes large errors in the calculation of fracture toughness. Therefore, new approaches for crack measurement methods using digital image correlation and mechanoluminescence have been proposed [25,26]. Moreover, a method to reduce one of the three parameters has been proposed: the compliance-based beam method (CBBM) [27]. To apply a creep load to the DCB specimens, a tensile testing machine can be used in the same way as for static tests [8]. However, in this case, it is difficult to maintain a specimen exposed to humid conditions. Another method of testing creep crack resistance is to insert a wedge into the DCB specimen, which is the Boeing wedge test [28,29,30,31]. In this case, creep-loaded specimens can be exposed to different environmental conditions to investigate their degradation [32,33,34,35]. Conversely, optical crack measurement is required to calculate the fracture toughness, leading to concerns regarding increased calculation errors. Studies have been conducted to accurately measure the crack length in wedge tests [36,37]; however, these techniques are not a panacea. To overcome these weaknesses, a spring-loaded DCB test has been proposed for hot water immersion tests but remains largely untested to date [38]. Similar to the spring-loaded SLJ test, the spring exerts a creep load on the specimen; however, the displacement increases and the load decreases as the crack propagates. By measuring the load and displacement over time and applying the CBBM, a change in the fracture toughness can be obtained. Therefore, it has the potential to evaluate crack resistance under a combination of creep load and immersion without measuring the crack length. Although no studies have focused on the effectiveness and accuracy of the spring-loaded DCB test method so far, this test method deserves more attention.
ist spring design software crack
where kspring is the spring constant. The spring-loaded DCB test includes two stages: load applying stage (stage 1) and crack propagation stage (stage 2). In the first stage, the crack length is considered constant as a=a1, where a1 is the crack length at the beginning of the first stage. Therefore, the compliance in the first stage C1 keeps constant. From Equations (2) and (4), the opening displacement in the first stage can be expressed as
The macroscopic image of the fractured surfaces of the specimens after the spring-loaded DCB tests. White triangles: initial crack positions; light-green triangles: failure mode changing points; red triangles: crack positions after creep immersion tests. Numbered locations are magnified using a microscope in Figure 12.
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