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Highlights • Experimental mode II test campaign of adhesive joints. • Comparison among ENF, ELS, 4ENF and MMB test results. • Comparison between LEFM and NLFM (J-integral) data reduction methods. Abstract The objective of this work is to define reliable test methodologies and data reduction methods to obtain comparable results among the most common mode II test methodologies: the End-Notched Flexure (ENF), the End-Load Split (ELS), the 4-point End-Notched Flexure (4ENF), and the Mixed Mode Bending (MMB) test at 100% of mode II. For this reason, an experimental test campaign consisting of the abovementioned four mode II test methods was carried out. The J -integral was implemented in the ELS, the ENF and the MMB tests and a J -integral closed form was developed and implemented for the 4ENF test. Then the J -integral was compared to the data reduction methods based on Linear Elastic Fracture Mechanics (LEFM). The results obtained from the J -integral based methods were independent from the test methodology and obtained very good agreement among the tests. In addition, from the experimental results the advantages and drawbacks of the different test procedures are analysed and discussed, concluding that the ELS test is the most suitable to measure mode II fracture toughness of adhesives. [ABSTRACT FROM AUTHOR]

Highlights • Definition of a methodology to obtain propagation values in an ELS test. • Stability and full Fracture Process Zone formation during the test are guaranteed. • Large deflections and adherend failure are avoided. • The method provides working regions for a wide range of adhesives and resins. Abstract The definition of the specimen dimensions in mode II fracture tests of bonded joints, where large Fracture Process Zones (FPZ) occur, is still an issue to be solved. Several conditions shall be met in order to obtain propagation values in bonded joints testing; stability of the test, full FPZ formation, and avoidance of large deflections and adherend failure. This work presents a methodology that indicates the specimen dimensions needed to obtain propagation values in End Loaded Split (ELS) tests. In addition, suitable specimen dimensions for a wide range of thin bonded joints between CFRP adherends is presented, solving the issue of defining suitable specimen dimensions to test adhesives to ensure steady state propagation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Constant energy release rate (G) applied to DCB-like specimen by loading the roller wedge with a constant load. • Creep crack growth rate curves obtained with the RWD test setup by testing multiple specimens at different G levels. • For Araldite 2021-1 it is very likely that the mechanical properties are overestimated if it is only quasi-static tested. • If the creep crack growth rate gets lower the fracture surface transitions from cohesive to adhesive failure. Understanding the performance of a bonded joint over time is essential for the design of durable bonded joints and maintenance protocols. Viscoelastic creep crack growth and how it affects the mechanical behaviours of an adhesive is relevant information for a durable design. For this purpose, a method to obtain the average crack growth rate (da / d t) as a function of the energy release rate (G) was developed. The proposed roller wedge driven (RWD) creep crack growth methodology can provide creep crack growth rate curves for a constant applied energy release rate. The RWD test setup was designed by the authors to test mode I DCB-like specimens by using a roller wedge. An advantage of using a moving wedge is that, on average, the crack growth rate equals the displacement rate of the wedge. By changing G for different specimens, a G vs da / d t curve can be obtained for the methacrylate adhesive Araldite 2021–1. The power law regression line of the G vs da / d t curve provides a Pariś law-like equation. Data have shown that applying an energy release rate that is relatively low compared to the fracture toughness of Araldite 2021–1, found by quasi-static testing, will result in creep crack growth. Furthermore, a transition from cohesive to adhesive failure has been observed when the applied energy release rate is lowered. For durability design of bonded joints it must be considered that only using data from quasi-static testing will very likely overestimate the durability of the bonded joint. [ABSTRACT FROM AUTHOR]

• A relatively small and simple mode I fracture toughness test setup potentially used without a test machine. • A method based on a roller wedge that could potentially be an alternative to the DCB test method. • Assuming zero friction for a roller wedge overestimates the calculated fracture toughness of the bonded joint. • Force measurement to determine mode I fracture toughness is a more robust method compared to adherend rotation measurement. In structural bonded joint design, mode I fracture toughness is a key mechanical property. Using a sliding wedge driven test to measure the fracture toughness of an adhesive is a good alternative to the standardised DCB test. However, with a sliding wedge driven test, the friction between the wedge and the specimen is difficult to determine and has an influence on the fracture toughness data reduction. In this work, we present a relatively small and simple mode I fracture toughness test setup with a roller wedge, which can potentially be used without a test machine to make a quick and affordable approximation of the mode I fracture toughness. DCB test results of the specimens are used as a reference to compare the roller wedge driven test method against. Results show that the friction of the roller wedge is significantly lower than a sliding wedge, and thanks to the low friction of the rollers the required driving force of the wedge is likewise low. Therefore, a human hand can apply a high enough force to the wedge by rotating a threaded bar to push down the wedge. Controlling the displacement rate by rotating a threaded bar by hand is difficult, therefore this method appears to be only suitable for non-rate sensitive adhesives. By comparing the roller wedge force and J-integral data reduction method, it has been shown the roller wedge force data reduction method is less sensitive to measurement errors. The proposed Roller Wedge Driven test method could potentially be an alternative mode I fracture toughness test method for bonded joints. [ABSTRACT FROM AUTHOR]

Expressions for the energy release rate and compliance are defined for some asymmetric beam like specimens. The expressions are derived taking into account the Timoshenko beam theory with elastic foundation. The energy release rate is expressed by the applied loads or by a combination of loads and rotation angles, the latter allows to characterize bonded interfaces without taking into account the elastic foundation properties. The condition of stability under displacement controlled test is analysed for typical specimens. The results presented are useful for the characterization of fracture toughness of bimaterial interfaces. • Expressions for the ERR and compliance are defined for asymmetric beam specimens. • ERR is expressed by applied loads or a combination of loads and rotation angles. • The condition of stability is analyzed for typical test. • The results are useful for the characterization of bimaterial interfaces. [ABSTRACT FROM AUTHOR]

• Effect of environmental conditions on mode II fracture behaviour. • An inverse procedure has been used to obtain the J-integral curve only from the load–displacement curve. • The participation of temperature and ageing on the mode II fracture behaviour is analysed and discussed. • A comparative study has been done among an inverse method and the classical data reduction methods. The effect of environmental conditions on the fracture behaviour of two types of adhesively bonded joints (wet-aged and non-aged) is experimentally studied under mode II loading. End Loaded Split tests were performed at various temperatures (−55 °C, room temperature (RT) and 80 °C) on non-aged and aged specimens. The non-aged specimens were stored in a laboratory under controlled conditions at RT (23 °C/55 RH) while the wet-aged specimens were exposed to accelerated ageing in an environmental chamber at 70 °C/85% RH for four years. The data reduction was applied using an inverse method and two other direct methods. A good agreement between these methods is observed. The analysis of the obtained results shows that extremely long aging times considerably affect the fracture response of bonded joints under mode II (the fracture toughness of the wet-aged specimens degraded by more than 30%). Furthermore, testing at high or cryogenic temperatures has a significant effect on the fracture response. Indeed, testing at high temperatures was observed to increase the fracture toughness (this increase ranged from 8% to 80%) while cryogenic temperatures increased the brittleness of the adhesive which caused a substantial reduction (more than 25%) in the fracture toughness of the bonded joint. [ABSTRACT FROM AUTHOR]

• Effect of environmental changes on mode I fracture behaviour. • Inverse procedure used to obtain the cohesive law only from the load-displacement curve. • The contributions of temperature and ageing on the mode I fracture behaviour are analysed and discussed. • Comparative study has been done among an inverse method and the classical data reduction methods. The influence of different harsh environmental conditions on the fracture behaviour of secondary bonded adhesive joints is experimentally investigated using mode I DCB tests. Two types of adhesively bonded joints under wet-aged and non-aged conditions were tested at three temperatures: −55 °C, room temperature and 80 °C. The wet-aged specimens were exposed to accelerated ageing in an environmental chamber at 70 °C/85% RH for four years; meanwhile, the non-aged specimens were stored in the laboratory under controlled conditions at room temperature. It was observed that extremely long ageing time has a significant effect on the fracture response of the bonded joint, while the temperature increase is holding a slight impact. Freezing temperature leads to more brittle behaviour and a reduction in the fracture toughness. [ABSTRACT FROM AUTHOR]

• A new simplified data reduction method based of the Wedge-Driven Test is proposed. • G IC values obtained from the DCB test and the WDT+ are in very close agreement. • The WDT+ is fully-objective and avoids crack length measurement or COF estimation. • The range of test velocities at which quasi-static conditions are met is increased. • The WDT+ is suitable for any type of adhesive and bondline thickness configuration. The resistance to fracture of a bonded joint is usually evaluated through the experimental characterisation of its fracture toughness. Different test procedures and data reduction methods are available in the literature, such as the ISO-25217 test standard. The need for crack length measurement has led to inaccurate results when large adhesive thicknesses or flexible adhesives were studied. Advanced test protocols based on the insertion of a wedge, such as the Wedge Test (WT) in ASTM D3762 or the Wedge-Driven Test (WDT), simplify the test procedure, but most such procedures do not allow for the measurement of fracture toughness. Further, experimental results are scarce in the literature, especially when dealing with bonded joints and considerable thicknesses. In the present work, an advanced data reduction method based on the WDT is presented: The Wedge-Driven Test Plus (WDT+). The fracture toughness of specimens made by bonding two aluminium adherends with a rigid and a flexible adhesive was measured, and the effect of wedge thickness and test speed at low displacement rates on the test results was analysed. The results were compared with static tests performed in a double cantilever beam (DCB) following ISO-25217. It has been demonstrated that the proposed data reduction method is less sensitive to test speed and enables different adhesive types, and bondline thickness and wedge thickness configurations, thereby improving the WDT available in the literature. [ABSTRACT FROM AUTHOR]

Abstract: The mechanical performance of composite components repaired with bonded patches depends on the moisture content of the adherents. There is a need to define drying procedures which guarantee the quality of the bonded joint. An experimental investigation has been carried out on the effect of pre-bond moisture on the fracture toughness (GIC) and failure behavior of repair joints bonded with adhesive films. Substrates were conditioned by immersion in distilled water at 70°C for 336h and subsequently dried for 1h or 24h to achieve different moisture contents before bonding. Specimens of bonded joints were tested under mode I loading at room temperature. The fracture toughness (GIC) decreased with increasing pre-bond moisture level in the substrates. Fractographic inspection revealed an enhancement of the cohesive mode of failure as the pre-bond moisture decreased. Extending the duration of the drying operation of the substrate with pre-bond moisture, caused an improvement in the fracture toughness of the joint, although not a full recovery of the reference values obtained in absence of pre-bond moisture. [Copyright &y& Elsevier]

Adhesives in bonded structures are exposed both to external loads and environmental conditions; durability studies are currently needed to assess their service lifetime. Conditioning strategies differ in considering external load conditions (such as stressed or not stressed) for the durability analysis of double cantilever beam (DCB) bonded joints. Different test procedures such as ASTM D3762 (wedge testing) or ISO-25217 (DCB testing) exist to characterise the evolution of the fracture strength and toughness found in bonded joints. These methods depend on crack-length measurements, however, and achieving an accurate visual determination may be difficult due to the large fracture process zones (FPZs) that develop in the adhesive layer, especially in flexible or degraded bonded joints. To compensate, crack-length-independent data-reduction methods such as the compliance-based beam method (CBBM) or the J-integral method can be used, but experimental research is lacking on the suitability of these methods in ageing tests. A lack of consensus also exists in testing methodologies to evaluate the durability of bonded joints, especially when examining flexible bonded joints. The present work evaluates the influence of damage on fracture toughness within flexible bonded joints exposed to service conditions. Wedge tests and DCB tests are conducted using DCB specimens bonded with a flexible structural adhesive, proving that the degradation of flexible bonded joints exposed to environmental conditions is significantly accelerated when external loads act on them. The findings show that crack length estimation is affected due to environmental effects and thus, that crack-length-dependent test methods are not applicable in ageing tests. • The effect of a sustained load accelerates degradation in flexible bonded joints. • Wedge Test is not applicable for flexible bonded joints due to adhesive relaxation. • Large deviations in G I C values when crack-length-dependent methods are used. • Adhesive weakens when exposure combines load and environmental conditions. • The area affected by the applied load grows linearly with increased exposure time. [ABSTRACT FROM AUTHOR]

This paper aims to discuss the effect mode II type tests have on the cohesive law of bonded joints. For this reason, an objective inverse method has been developed to extract the cohesive law for experimental load-displacement data using an analytical procedure to represent the fracture process zone of mode II tests. The method is implemented on the more popular and standardized mode II tests: End-Notched Flexure and End-Load Split. The analysis of the experimental data shows that the cohesive law is independent of the test type. In addition, the obtained fracture toughness calculated by integrating the cohesive law shows good agreement with the J-integral method. [ABSTRACT FROM AUTHOR]

Abstract This study presents a novel method to determine the mode II cohesive law of unidirectional carbon fiber reinforced polymer laminates, employing the End Notched Flexure (ENF) test. The fracture toughness in mode II (G) and crack shear displacement (Δ t) are determined based on a compliance variation data reduction method (BTBR) by processing the global load displacement curve, without monitoring the crack length and the crack shear displacement during the test. The procedure is validated by finite element analysis including cohesive zone modeling. The results of the method are compared to those obtained with the Direct Method, where the fracture toughness is determined based on the rotations of the load introduction points and the crack shear displacement is directly measured. It is concluded that the proposed data reduction scheme is suitable to obtain the mode II cohesive law using only the load and displacement data obtained from the testing machine, without any external displacement measurement technique and without any assumption of the form of the cohesive law. [ABSTRACT FROM AUTHOR]

• Mode I fracture toughness is influenced by the stress state, governed by t a and B. • Elasticity has a strong effect on the fracture behaviour of flexible bonded joints. • The lateral contraction due to elasticity is independent to the bondline thickness. • Method to predict fracture energy of bonded joints based on stress state analysis. The stiffness and strength of a bonded joint mainly depend on the geometry of the assembly and the constitutive behaviour of its constituent parts (adherends, adhesives and interfaces). The adhesive itself may sustain a significant hydrostatic contribution to fluctuation along the bonded area, strongly influencing the local joint stiffness and strength. A considerable amount of work has been undertaken to independently evaluate the effect of the width and thickness of bonded specimens on strength or toughness. However, experimental results are lacking, especially when dealing with thick low modulus/hyperelastic adhesives. In the present work, the influence of the width-to-thickness ratio on the equivalent mode I fracture toughness of a flexible bonded joint is evaluated. Thus, an analytical model capable of predicting the fracture toughness of double cantilever beam (DCB) flexible bonded joints for a given thickness and any width has been developed. The analysis is supported through an experimental testing regime with DCB specimens made by bonding two aluminium adherends with a thick silicon-based adhesive layer, proving that the fracture behaviour of flexible bonded joints is dominated by the elastic behaviour of the adhesive layer. [ABSTRACT FROM AUTHOR]