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On the shear rate, i.e. the materials' flow curve, to get reliable estimates of the volumetric flow rate under given pressure drops or vice versa. Several GNF models will now be covered since they are fairly important in engineering calculations ( 4 , Chapt. 4). However, we will limit our discussion of how other constitutive equations based on continuum mechanics can be constructed (using some of the ideas mentioned above) to merely a verbal sketch of the mathematical process. We will also not...

002

Non-dimensional plot of the load-displacement curves for a DCB specimen with different values of the fracture parameters 4 . is appropriate only when the normalized parameter Er 0 o2h is very small (i.e., a large cohesive strength, a low intrinsic toughness of the adhesive layer, or very thick adherends). In particular, it should be noted that if the peak load is used with Eq. 2.6 to calculate the energy-release rate at fracture, this value will result in an underestimate of the...

0 025 050 075 100 125 150

Distribution of radial stress along fiber fragment length 88 . the fiber axis. Depending on the amount of energy released upon fiber failure, this microcrack may or may not propagate into the matrix and or the interphase, depending on the level of adhesion between fiber and matrix. Then, the resulting stress distribution will depend on the mechanical properties of the fiber, the matrix and the interphase and the extent of the damaged zone around the fiber break. As pointed out by Ko et...

2 H

Where T0 is the (real) amplitude and 5 is the relative phase shift ( loss angle), then Eqs. 192 and 193 yield the components of the complex viscosity as ff 2HT0cos8 V V nR4co0o )' V V jz r4co0n ) If we want to find out how a fluid behaves under extension, we have to somehow 'grip' and stretch it. Experimentally, this is much more difficult than the shear arrangement, especially if the fluid has a low viscosity. Earlier (see Section 5) we saw that it is possible to classify steady extensional...

2 7t

Again, it is the stress intensity factor that distinguishes the crack-tip stress distribution from one loading and crack geometry to another. A large variety of methods exist for determining the stress intensity factor associated with a particular configuration as can be seen from the compilation by Tada et al. 55 , When finite element methods are used for the stress analysis of cracked components, stress intensity factors may be extracted by examining the displacement solution near the crack...

7

Deformed shape of the elastic layer in an adhesively bonded butt joint with rigid adherends when subjected to a positive-shear loading. for uniform adhesive shrinkage. The solution for the uniform edge pressure can be thought of as providing the fundamental singular solution, and the characteristic stress defined in Eqs. 11 and 12 is the stress superimposed with the edge pressure to obtain a stress-free edge. The adhesively bonded butt joint's thin adhesive layer is sheared by...

The durability of adhesive joints

Department of Mechanical Engineering, Imperial College of Science, Technology and Medicine, Exhibition Rd London SW7 2BX, UK One of the most important requirements of an adhesive joint is the ability to retain a significant proportion of its load-bearing capability for long periods under the wide variety of environmental conditions which are likely to be encountered during its service-life. Unfortunately, for the reasons outlined later, one of the most hostile environments for joints involving...

45

Crack growth rate as a function of strain energy 9, experimental results for compression, O theoretical results for compression, experimental results for simple tension. Redrawn from ref. 49 , be used to generate the characteristic fracture energy versus crack speed curves which can then be applied to very complicated real world parts by using numerical methods to map the strain energy in the part and using that data to determine if crack growth is likely. 5. Designing with elastomeric...

P

Cross-sectional view of cone pull-out tests. Starter crack Fig. 25. Cross-sectional view of cone pull-out tests. inversely proportional to the maximum energy release rate for specimens with varying t D ratios. The failure behavior of a series of cone pull-out tests was also explained with the aid of fracturc mechanics and finite element analysis 23J. The specimens tested consisted of a truncated PMMA cone bonded into a matching hole in a PMMA plate using a dear...

F z aw W o

Hence pressure and thrust measurements, via Eqs. 175 and 178, allow one to calculate N and While the cone and plate geometry is the preferred arrangement to obtain the steady viscometric functions, it is limited to low shear rates usually, to those less than 10 s '. At higher shear rates encountered in processing 10 106 s_l), it is customary to resort to capillary rheometry to measure the shear viscosity. Unfortunately, the normal stress differences cannot be obtained from this test. To get Ni...

34

The two representative specimens with different surface preparations revealed that the interface properties significantly affect the crack propagation behavior. Advanced surface preparation techniques enhance the adhesion between the adhesive and the substrates and consequently, the rate dependence of the locus of failure is reduced. Of particular significance, however, is that the debond did not propagate exclusively at the weaker interface. The stress state destabilized the debond, leading to...

4 Summary

This paper has provided an in-depth discussion on the fracture of adhesive joints with a focus on how to deduce the 'intrinsic' fracture parameters of the adhesive layer from experimental results. The approach of using these parameters in cohesive-zone models to predict the fracture of adhesive joints has been outlined. By using this approach, many important issues that are frequently encountered in the fracture testing of adhesive joints, yet are usually ignored by traditional fracture...

Iio [1 W aln91

Various isothermal and non-isothermal flow problems have been solved with the above model fluids. Bird et al. 4 (p. 229) lists some of these solutions for rectangular slits and circular pipes. 5.2. Elastic and viscoelastic models We will now give a brief sketch of some of the other continuum models beyond the simple viscous ones considered above. The purpose here is to show how these models are interconnected and allow the reader to gain a broader scope of the subject without going into the...

2

The authors conclude that a combination of the flexure, torsion, and thermal bending tests can be used to accurately determine the Young's modulus, residual stress, and thermal expansion coefficient, if the Poisson's ratio can be estimated. Combinations of experimental parameters that give accurate results are presented The stiffness of inorganic films may also be determined from the measurement of the propagation velocity of surface acoustic waves, C, which, for a homogeneous material is...

K iK pf HW 2V3 2315

If the crack is at the sub-interface, the local stress intensity factor Kx and Kv can also be expressed as a function of the external load by substituting Eq. 20 into Eq. 17 as K q IKI cos to + (j> + s In ( ) Ku q iT sin a) + 0 + eln(Sf O (21) and G 2P a E B H is the applied strain energy release rate in the specimen. Fig. 29 shows that due to the material mismatch, when the distance between the sub-interface crack and the interface St approaches zero, the corresponding component at the crack...

10

Peel stress at the first ply interface under load. can be of sufficient magnitude to cause interlaminar failure of the adherend prior to adhesive failure. Although such failure was not seen in these joints, probably because of the more brittle nature of the adhesive used compared with the rubber-toughened (CTBN) adhesive used by Adams et al., it was, nevertheless, important to consider the stress distribution within the adherend. As can be seen from Fig. 11,...

O

Blistering of coating due to compressive residual stress inducing buckling. Note that edge normal load, the change in stress due to buckling, Act, is positive. originating in water clusters at microscopic voids. Blistering arises from the coating buckling (see Fig. 12), and an analysis similar to that used to study indentation induced buckling can be used to find the critical compressive stress. A delamination of length 2b is assumed present in the interface, and, in the absence of...

P2

The Auger depth profiling method was then used for those specimens, in which the failure appeared to occur at or near the interface. As shown in Table 5, in the mode I test, the thicknesses of the residual adhesive layer on the failure surfaces were about 250 xm for all the specimens with different surface preparations, which indicated that the failures all occurred in the middle of the adhesive layer in the test regardless of the surface preparation method since the total...

Ffo 7a2rsftr8

Tsf, is the stress free temperature of the adhesive, which was measured using a curvature measurement technique 37 for each adhesive, and was very close to the glass transition temperature of the respective adhesive (the results are also listed in Table 1). T is the test temperature (room temperature in this study). Since the coefficients of thermal expansion of the adhesives increased with rubber concentration and meanwhile the modulus and the glass transition temperature decreased slightly,...

243

The commonly held approximation that the behavior in peel at a given elongation rate is dominated by the viscoelastic properties at the equivalent frequency is shown to be crude at best and misleading at worst. The work of Piau et al. 23 follows a similar pattern. Modeling is done using the measured stress-strain behavior of the adhesive, the measured elongation at detachment from visualization of the peel front and the measured peel force. They show that the...

Adhesive Shear Strain

Development of non-uniform shear stresses in double-lap and double-strap bonded joints. closed-form analysis, in ref. 9 , to have a constant width, independent of the total overlap. Most of the load is transferred there, with very little in the middle of the joint. However, the low-stressed interior is absolutely vital to the durability of adhesively bonded joint. The adhesive will creep, locally, at the ends of the overlap, at quite low sustained loads, because of the severity of the...

Final Failure Of

NOTE THAT DESIGN PROCESS MUST ACCOUNT FOR NONLINEAR ADHESIVE BEHAVIOR, BUT A PRECISE STRESS-SRAIN CURVE IS NOT MANDATORY. AN APPROXIMATION, BASED ON A SIMILAR ADHESIVE, WILL U SUALLY SUFFICE. NOTE THAT DESIGN PROCESS MUST ACCOUNT FOR NONLINEAR ADHESIVE BEHAVIOR, BUT A PRECISE STRESS-SRAIN CURVE IS NOT MANDATORY. AN APPROXIMATION, BASED ON A SIMILAR ADHESIVE, WILL U SUALLY SUFFICE. Fig. 16. Adhesive shear design model based on restricting design limit loads below the adhesive elastic capability....

60

Experimental joint strengths of three steel adherends with AVI 19 adhesive. The predicted lines are from Eqs. 1, 2 and 3. Ciba, which has a measured maximum yield strength in shear of 49 MPa with about 30 shear strain to failure. In Fig. 16, line 1 applies to Eq. 1. It predicts reasonably the strength of the high-strength steel joints, although it is clear that, at high loads, the measured strength is below that predicted. A more brittle adhesive used with these high-strength adherends...

Fully Plastic Adhesive Load Limited By Bond Area

Strength of double-lap bonded joints at minimum service temperature. limiting central adherend thickness and to compute the overlap needed to transfer the adherend ultimate strength (not any nominal design load, as explained above), using Eq. 6 and to add an appropriate manufacturing assembly tolerance. No greater overlap can ever add to the joint strength unless a more complex bonded joint geometry is adopted, as would be necessary for adherends thicker than the upper limit...

The design of adhesively bonded joints

Phantom Works, The Boeing Company, Huntington Beach, CA, USA Designing successful adhesively bonded joints is straightforward, provided that one pays close attention to a small number of critical issues, none of which is difficult to comprehend and none of which is any more difficult to comply with than to ignore. The first is that the joint must never be designed to be weaker than the surrounding structure, unless one is deliberately planning a weak-link fuse with no damage tolerance. The...

10 10

The number, Nf, of cycles to failure for single-lap joints as a function of the maximum load, 7ma. , per unit width applied in a fatigue cycle. The lap joints were cycled at 5 Hz in water at 28 C. The points represent the experimental data filled points indicate where the test was stopped prior to failure whilst the lines are the theoretically predicted lifetimes 74 . and 6, for example for polymeric adhesives is often relatively steep. This implies that, for adhesive joints, the rate...

52

The microindentation technique seems to agree well with the results obtained from the single-fiber fragmentation method. An interfacial shear strength equal to rav 3870 750 psi 26.6 5.2 MPa was obtained for IM6 U fibers using the ITS. This value is slightly higher than the one obtained from the single-fiber fragmentation technique. For IM6-100 , rav 5400 1540 psi 37.2 11.0 MPa and for IM6-600 , rav 6200 1640 psi 42.7 11.3 MPa were obtained also using the ITS. The...

Effect of rheology on PSA performance

Introduction to pressure-sensitive adhesives Pressure-sensitive adhesives PSAs have become familiar materials in our world. Most people have used tapes such as masking tape, electrical tape, or transparent tape, for example Scotch Magic Transparent Tape. All of these have pressure-sensitive adhesives of various kinds coated on a paper or polymeric film backing. The adhesive strength of such materials can vary widely from the easily removable Post-It note to the permanent bonds formed by the...

2 Key advances in the energy argument

Although Newton studied adhesion of glass lenses shortly after Galileo's death, noting ye apparition of a black spot at ye contact of two convex glasses 2 , and measuring the interference fringes which indicated very close contact of the Fig. 3. Apparatus used by Obreimoff to cleave mica and observe its subsequent adhesion. Fig. 3. Apparatus used by Obreimoff to cleave mica and observe its subsequent adhesion. Fig. 4. a Interference fringes seen in gap between mica foil and block, b...

3 Adhesive cracking

In this case, cracks run along the interface between two materials due to interactions between the stress field in the adhesive layer and spatial variations in fracture properties. The cracks are not generally free to evolve as mode I cracks, as was the case for cohesive cracks, and mixed-mode fracture concepts combinations of tension and shear have to be considered. Mode II or shear components are induced, even in what appear to be nominally mode I loadings, due to differences in moduli about...

Measurement methods for fibermatrix adhesion in composite materials

HERRERA-FRANCO b Departments of Chemical Engineering and Materials Science and Mechanics, Composite Materials and Structures Center, Michigan State University, East Lansing, Ml 48824-1326, USA h Centro de Investigaci n Cient fica de Yucat n, A.C., Calle 43 130, Col Chuburn de Hidalgo, C.P. 97200. M rida. Yucat n, M xico 1. Fiber-matrix adhesion measurement methods It is well known that the level of adhesion between fibers and matrix affect the ultimate mechanical...

M M U

1E-4 1E-3 1E-2 1E-1 1E 0 1E 1 1E 2 r mm Fig. 11. Comparison of linear-elastic and elastic-perfectly plastic finite-element solutions with asymptotic solution for stress in front of an unbonded inclusion embedded, within an epoxy disk, with h 18 mm and AT -100 C. 6. Limits on the applicability of a Kac failure analysis Two basic requirements must be met before the Kac criterion can be applied. One obvious requirement is that failure must initiate at the interface corner. The other requirement is...

Shear Lag Analysis Volkersen

Volkersen Shear Lag Equation

X - transverse shear stress y - distance from neutral axis V - applied lateral shear load b - width of beam at section Q - 1st moment of area of partial section I - 2nd moment of area Fig. 9. When subjected to lateral shear loads, transverse shear stresses are required within the beam to allow buildup of axial stresses associated with changing bending moment. such lateral loads, the axial stresses vary not only from top to bottom of the beam, but also along the length due the change in bending...

Peel Adhesive Joint

Which are the two boundary conditions that allow us to determine the final solution for the shear lag model of the lap joint where P is the axial load per unit width of the joint. For the balanced adherend case where the , t, products for the upper and lower adherends are the same, the coefficient for the hyperbolic sine term becomes zero, and the shear stress distribution is symmetric about the center of the joint. Adams and Wake 11 have provided insightful figures illustrating the shear...

D

For peel loading, bending of the flexible adherend results in relative shear deformations within the bondline, again demonstrating coupling often seen between shear and peel stresses in bonded joints. the adherend results in a relative horizontal translation of the adherend surface with respect to substrate. Thus a peeling action produces not only peel stresses, but also shear stresses. Clearly, the stresses present within even the most simple joint configuration can be quite complex...