Mixed mode tests

Clearly both mode I and mode II tests are important in the evaluation of fiber reinforced composites. In structures, however, delaminations grow under a mixture of mode I and mode II loading, and also there are obvious advantages in test methods where both mode I and mode II data can be obtained simultaneously with a single test. One such mixed mode test is also covered in the ESIS protocol [146] and is briefly outlined below. 

As with mode II, a common feature of mixed-mode testing is that the failure path is usually not in the center of the adhesive layer, but deviates to run close to the interface with the upper substrate. It has been shown that this makes only a very small difference to the applied mixed-mode ratio, but the locus of failure being close to an interface may have other effects on the test (Dillard et al. 2009). It is interesting to note that such studies highlight that profound effects can be observed if the mode mix alters the path taken by the crack. A fracture path between the adhesive and the carrier in a film adhesive, or within a polymeric fiber-composite substrate, can have a significant and deleterious effect on the fracture resistance of a joint. 

Fig. 3.33. CLS specimen for mixed mode interlaminar fracture tests.

A series of SPE sorbents ranging from nonpolar, to mixed-mode, to polymeric, were tested for their performance in the STA of a diverse range of drugs (17 analytes) in whole blood [102], In this experiment a C8-modified silica material was... 

Spherisorb propyl SCX (3 pm) Spherisorb phenyl SCX (3 pm) Symmetry SCX (3 pm) Spherisrob Mixed Mode (3 pm) Evaluation of various SCX phases, compariosn of electroosomotic mobility at various pHs, separation of antidepressants, amitriptyline, nortriptyline, neutral test mixture MeCN-50 mM phosphate 70 30, pH 2.3 MeCN-20 mM borate 70 30, pH 9.0 [97]... 

All samples except chrome failed in a mixed cohesive-adhesive mode. Tests were run 1-2 days after preparation of the test specimens. Since PTHF may crystallize slowly even in the presence of the compounding diluents, there could be an important effect of aging. 

Block polymer B differs substantially in its failure characteristics from BP A polycarbonate. For the block polymer a mixed failure mode predominates in three-point bend tests of notched specimens from —100°-90°C. In the mixed mode craze breakdown and plane strain fracture occur first inside the specimen subsequently shear failure occurs in the surface regions of the specimen. Shear lips (11) are formed as a result. Shear lips are also found on the notched Izod impact fracture surfaces of block polymer B, implying that the same mixed mode of failure occurs under high speed loading conditions. 

Ductile-brittle transitions are more accurately located by variable temperature tests than by altering impact speed in an experiment at a fixed temperature. This is because a linear fall in temperature is equivalent to a logarithmic increase in straining rate. The ductile-brittle transition concept can be clarified by sketches such as that in Fig. 11-25. In the brittle region, the impact resistance of a material is related to its LEFM properties, as described above. In the mixed mode failure... 

In PEI (Fig. 37), the shear fracture process never fully develops (under the tested conditions). A mixed mode develops, with some elements of shear fracture, but without the epsilon plane strain shear bands, a true 45° shear fracture process cannot develop. The lifetime enhancement is not as substantial, as seen in PC and PEC. 

This paper presents results from a study of assemblies composed of glass fibre reinforced epoxy composites. First, tests performed to produce mixed mode fracture envelopes are presented. Then results from tests on lap shear and L-stiffener specimens are given. These enabled failure mechanisms to be examined in more detail using an image analysis technique to quantify local strain fields. Finally the application of a fracture-mechanics-based analysis to predict the failure loads of top-hat stiffeners with and without implanted bond-line defects is described. Correlation between test results and predictions is reasonable, but special attention is needed to account for size effects and micro-structural variations induced by the assembly process. 

The paper is presented in three parts. First, the tests employed to determine the mixed mode fracture envelope of a glass fibre reinforced epoxy composite adhesively bonded with either a brittle or a ductile adhesive are briefly described. These include mode I (DCB), and mixed mode (MMB) with various mixed mode (I/II) ratios. In the second part of the paper different structural joints will be discussed. These include single and double lap shear and L-specimens. In a recent European thematic network lap shear and double lap shear composite joints were tested, and predictions of failure load were made by different academic and industrial partners [9,10]. It was apparent that considerable differences existed between different analytical predictions and FE analyses, and correlation with tests proved complex. In particular, the progressive damage development in assemblies bonded with a ductile adhesive was not treated adequately. A more detailed study of damage mechanisms was therefore undertaken, using image analysis combined with microscopy to examine the crack tip strain fields and measure adherend displacements. This is described below and correlation is made between predicted displacements and failure loads, based on the mixed mode envelope determined previously, and measured values. 

Figure 12. Top hat stiffener pull-off test set-up (a) mode I, (b) mixed mode I/II.

Fracture mechanics characterisation tests have been performed to determine the mixed mode fracture envelope of an epoxy bonded glass/epoxy composite. Analysis of lap shear, and L-stiffener geometries has shown that for this relatively brittle adhesive reasonable first estimations of failure loads can be obtained for both cracked and uncracked specimens. An image analysis technique has been developed which enables failure mechanisms to be... 

Ducept F, Gamby D, Davies P, A mixed mode failure criterion from tests on symmetric and asymmetric specimens Comp. Sci Technology, 59, 1999, p609-619. 

In the present work, a novel experimental method was proposed to evaluate the modes I-fll fracture toughness of the adhesive interface between concrete and carbon fiber sheets. The validity of the evaluation formula was studied on the basis of the results of finite element analysis. The mixed mode fracture toughness and fracture criterion of the adhesive interface were also studied on the basis of the results of fracture toughness test using the proposed method. 

Fig. 12. Experimental apparatus of the mixed mode fracture toughness test.

The mixed mode fracture toughness tests were carried out on a universal testing machine at loading rate of 1 mm/min. The loading angle was varied to study the criterion of mixed mode fracture as = 0°, 15°, 30°, 90° (G /G=0.01, 0.31, 0.55, 0.95). 

Figure 15 shows the diagram of the mixed mode fracture toughness, G. The abscissa shows the mode I component of the mixed mode energy release rate, G[. The ordinate shows the mode II component of the mixed mode energy release rate, G[j. Each plot is the average value of 5-10 test data. 

Fig. 15. Mixed mode fracture toughness of the adhesive interface between concrete and carbon fiber sheets each plot is the average value of 5-10 test data.

In order to apply the characterization and structural design of the concrete bridge structures retrofitted with tensioned carbon fiber sheets, the mixed mode disk specimen was proposed and applied to the mixed mode fracture toughness tests. The following conclusions were derived from the results and discussions ... 

For obvious reasons, fr ture mechanics studies have concentrated on opening mode (Mode 1) fractures, and only a few studies have been made of other fracture modes. In one such study, Bascom etaL performed mixed-mode fracture tests on epoxy resin adhesives, vnth the epoxy layer at 45° to the applied stress . This test combines the opening Mode I widi die in-idaiK shear Mode II. The authors calculated a mixed-mode fracture etffirgy G/ jjc from the results, and obtained a... 

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