“…That is, for a given ΔG, the value of da/dN is actually observed to decrease as the value of the Rratio increases. This behaviour is also seen in the experimentally-determined da/dN versus ΔG curves presented in [10,34,35], see . In these figures the results shown are from Mode I DCB fatigue tests, apart from the data shown in Figure 7 where fatigue results [10] from asymmetric double-cantilever beam (ADCB) specimens consisting of aluminiumalloy substrates bonded using a single-part, heat-cured, toughened-epoxy adhesive are given.…”
Section: Resultsmentioning
confidence: 52%
“…These observations, coupled with the introduction by the US Federal Aviation Administration (FAA) of a slow crack-growth approach to certifying composite and adhesively bonded structures [4], has led to a renewed interest [1,[5][6][7][8][9][10][11][12][13] in how to predict the growth of delaminations and disbonds under cyclic-fatigue loads. The precise wording used in the FAA Airworthiness Advisory Circular (AAC) 20-107B [4], which covers both composites and adhesively-bonded joints, is:…”
“…That is, for a given ΔG, the value of da/dN is actually observed to decrease as the value of the Rratio increases. This behaviour is also seen in the experimentally-determined da/dN versus ΔG curves presented in [10,34,35], see . In these figures the results shown are from Mode I DCB fatigue tests, apart from the data shown in Figure 7 where fatigue results [10] from asymmetric double-cantilever beam (ADCB) specimens consisting of aluminiumalloy substrates bonded using a single-part, heat-cured, toughened-epoxy adhesive are given.…”
Section: Resultsmentioning
confidence: 52%
“…These observations, coupled with the introduction by the US Federal Aviation Administration (FAA) of a slow crack-growth approach to certifying composite and adhesively bonded structures [4], has led to a renewed interest [1,[5][6][7][8][9][10][11][12][13] in how to predict the growth of delaminations and disbonds under cyclic-fatigue loads. The precise wording used in the FAA Airworthiness Advisory Circular (AAC) 20-107B [4], which covers both composites and adhesively-bonded joints, is:…”
“…Azari et al, 3 Ripling et al, 5 Jethwa and Kinloch 6 and Curley et al, 7 Mall et al, 8 Pirondi and Nicoletto 9 and Martin and Murri. 10 As a result, Martin and Murri 10 and Jones et al 11 concluded:…”
Section: The Value Of the Exponentmentioning
confidence: 99%
“…As commented earlier, in the Paris crack‐growth equation, the rate of crack growth per cycle, da / dN , is assumed to be linearly related to either ( G max ) m or (Δ G ) m where the exponent m is a constant that is determined experimentally. Unfortunately, for structural adhesives and fibre‐composite materials, the value of the exponent, m , in this relationship tends to be relatively large, for example, Azari et al , Ripling et al , Jethwa and Kinloch and Curley et al , Mall et al , Pirondi and Nicoletto and Martin and Murri . As a result, Martin and Murri and Jones et al concluded: For composites, the exponents for relating propagation rate to strain‐energy release rate have been shown to be high especially in Mode I.…”
A B S T R A C T The present paper examines crack growth in a range of aerospace and automotive structural adhesive joints under cyclic-fatigue loadings. It is shown that cyclic-fatigue crack growth in such materials can be represented by a form of the Hartman-Schijve crackgrowth equation, which aims to give a unique and linear 'master' representation for the fatigue data points that have been experimentally obtained, as well as enabling the basic fatigue relationship to be readily computed. This relationship is shown to capture the experimental data representing the effects of test conditions, such as R-ratio and test temperature. It also captures the typical scatter often seen in the fatigue crack-growth tests, especially at low values of the fatigue crack-growth rate. The methodology is also shown to be applicable to both Mode I (opening tensile), Mode II (in-plane shear) and MixedMode I/II fatigue loadings. Indeed, it has been demonstrated that the fatigue behaviour of structural adhesives under both Mode I and Mode II loadings may be described by one unique 'master' linear relationship via the Hartman-Schijve approach.Keywords adhesives; fatigue crack growth; Hartman-Schijve equation; joints; mode mix; safe life.
N O M E N C L A T U R E a ¼ crack lengthA ¼ a constant in the Hartman-Schijve equation da/dN ¼ rate of crack growth per cycle D ¼ a constant in the Hartman-Schijve crack-growth equation G ¼ strain-energy release rate G c ¼ quasi-static value of the fracture energy G max ¼ maximum value of the applied strain-energy release rate in the fatigue cycle G min ¼ minimum value of the applied strain-energy release rate in the fatigue cycle ΔG ¼ range of the applied strain-energy release rate in the fatigue cycle, as defined later
“…Generally, the Paris relations for the fatigue crack growth in metals are usually characterized by the parameter △K. Because G is proportional to K 2 [33] and K is proportional to the applied force P, both P G and △G include the parameter △K in the following manner:…”
Two Paris-type models are proposed to characterize the fatigue delamination growth (FDG) behavior in CFRP laminates. The crack-driving forces denoted by two definitions of the strain energy release rate (SERR) range against the fatigue delamination resistance of composite materials are introduced to control the FDG. The first definition is the square of the difference between the square root of maximum SERR and that of minimum SERR. The second one is the arithmetic difference between the maximum and minimum SERR. A series of fatigue delamination tests of CFRP multidirectional laminates under various stress ratios and mode-mixity ratios were carried out. Among different Paris relations, the FDG rate corrected by the first model exhibits a linear relationship with R-ratio dependency, and the second model further removes the R-ratio dependence, under different mode-mixity ratios, on a log-log scale. The experimental results indicate that the presented models can provide more generic descriptions and a proper interpretation of the FDG behavior in CFRP multidirectional laminates.
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