Interface strength

Combination of Eq. 7 or Eq. 8 with the Young-Dupre equation, Eq. 3, suggests that the mechanical work of separation (and perhaps also the mechanical adhesive interface strength) should be proportional to (I -fcos6l) in any series of tests where other factors are kept constant, and in which the contact angle is finite. This has indeed often been found to be the case, as documented in an extensive review by Mittal [31], from which a few results are shown in Fig. 5. Other important studies have also shown a direct relationship between practical and thermodynamic adhesion, but a discussion of these will be deferred until later. It would appear that a useful criterion for maximizing practical adhesion would be the maximization of the thermodynamic work of adhesion, but this turns out to be a serious over-simplification. There are numerous instances in which practical adhesion is found not to correlate with the work of adhesion at ail, and sometimes to correlate inversely with it. There are various explanations for such discrepancies, as discussed below. 

Basically, the term [fi(a /2E)Ih is the visco-plastic dissipation term and the term [1/2ctoLc/] is the intrinsic interface strength term. This equation can be further rearranged to... 

Let us examine this relation for typical values of the A/B interface = 1 (max energy dissipation in the A layer) I = 1 (max strength of interface with influxes) E = 12,000psi (Tq = 4000 psi /t = 30 mils (10 in) Lc = 4 x lO" in. We obtain for both terms G = 20 pli (energy dissipated) + 0.08 pli (true interface strength with max influxes), or G 20 pli, which says that the measured peel strength is dominated by visco-plastic deformation processes. 

The selection of a suitable matrix for a composite material involves many factors, and is especially important because the matrix is usually the weak and flexible link in all properties of a two-phase composite material. The matrix selection factors include ability of the matrix to wet the fiber (which affects the fiber-matrix interface strength), ease of processing, resulting laminate quality, and the temperature limit to which the matrix can be subjected. Other performance-related factors include strain-to-failure, environmental resistance, density, and cost. 

Thomason, J. L. and Schoolenberg, G. E., An investigation of glass fibre/polypropylene interface strength and its effect on composite properties, Composites, 25, 197 (1994). 

Andersons, J. and Tamuzs, V. (1993). Fiber and interface strength distribution studies with the single fiber composite test. Composites Sei. Technol. 48, 57-63. 

Shih, G.C. and Ebert, L.J. (1986). Interface strength effects on the compressive-flexure/shear failure mode transition of composites subjected to four-point bending. J. Mater. Sci. 21, 3957-3%5. 

Bader, M.G., Charalambides, B, Ling, J. (1991). The influence of fiber-matrix interface strength on the tensile strength and failure mode in uniaxial CFRP. In Proc. ICCM-VIII, Composites Design. Manufacture and Application (S.W. Tsai and G.S. Springer, eds.), SAMPE Pub. Paper 1II. 

Maximum allowable interface strength for interface delamination ... 

Composite system (fiber/ matrix or coating) a Required interface strength, ol (MPa) Calculated transverse strength, oj (MPa) ... 

Bader M.G., Bailey J.E. and Bell 1. (1973). The effect of fiber-matrix interface strength on the impact and fracture properties of carbon fiber-reinforced epoxy resin composites. J. Phys. D Appi. Phvs. 6, 572-586. 

Figure 3.14. CNT/polymer nanocomposites observed in SEM (a) and (b) P(S-ABu)/MW CNT films surface respectively prepared by evaporation and film formation or freeze-drying and hot-pressing but showing similar fillers distribution (c) and (d) PS matrix containing ungrafted or PS-grafted N-doped CNT a fracture performed at ambient temperature highlights the difference in fillers/matrix interface strength. Scale bars 1 pm.

Generally, realizing effective reinforcement of CNTs in polymer needs two factors (21) (1) good dispersion in matrix, and (2) strong interface strength between CNTs and polymer. Dispersion is probably a more fundamental issue. Nanotubes must be uniformly dispersed as isolated nanotubes and individually coated with polymer. This results in a more uniform stress distribution and minimizes the presence of stress-concentrated centers. Furthermore, strong interface between CNTs and polymer leads to efficient stress transfer... 

When O ions were in the 0 charge state, the anions were found to be very reactive, with maximum interface adhesion for V and decreasing monotonically towards each side in the d series, but with Cu anomalously stabilized. The interface strength of 0 -terminated a-Al203(0001) is estimated to twice that of the Al + terminated. This picture is in some... 

It was mentioned above that the simulation method of Termonia [67-72] can be used to calculate the stress-strain curves of many fiber-reinforced or particulate-filled composites up to fracture, including the effects of fiber-matrix adhesion. Such systems are morphologically far more complex than adhesive joints. Many matrix-filler interfaces are dispersed throughout a composite specimen, while an adhesive joint has only the two interfaces (between each of the bottom and top metal plates and the glue layer). If one considers also the fact that there will often he a distribution of filler-matrix interface strengths in a composite, it can be seen that the failure mechanism can become quite complex. It may even involve a complex superposition of adhesive failure at some filler-matrix interfaces and cohesive failure in the bulk of the matrix. 

Gan, L., Wang, J., and Pilliar, R.M. (2005) Evaluating interface strength of calcium phosphate sol-gel-derived thin films to H6A14V substrate. Biomaterials, 26 (2), 189-196. 

Thomason, J.L. Schoolenberg, G.E. (1994). An Investigation of Glass Fibre/Polypropylene Interface Strength and its Effect on Composite Properties. Composites, Vol.25, No.3, pp. 197-203 ISSN 0010-4361... 

Wei, Y. and Hutchinson, J.W. (1998) Interface strength, work of adhesion and plasticity in the peel test. International Journal of Fracture, 93, 315-333. 

Table 4 reports the interface shear stresses that have been extracted from the hysteresis loops, as well as pull out lengths and matrix crack spacing distances. These latter data provide also a measure of interface strength. Both sets of data indicated the same trends, and they support the above statements. The interface shear stresses (t) obtained for the minicomposites reinforced with Hi-Nicalon S fibers are quite low (Table 4). Weak interfaces could be logically expected for this Hber/interphase system, fhe current values of x fall within the range of data detennined on SiC/SiC minicomposites reinforced with Nicalon or Hi-Nicalon fibers [7, 12, 14]. Furthermore, it...

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