Strength transverse

Real situations demand more uniformity in composite properties than can be provided by unidirectional composites. Therefore lamina stacking sequences are made where the fiber orientation is altered to provide good properties in all directions. Lamina composed of fiber and matrix in which the fibers are all parallel to each other are stacked on top of each other with a systematic variation in fiber direction. These lamina are then bonded together and the resulting material has more uniformity in properties. Likewise in short fiber or discontinuous fiber composites fiber orientation is random. Therefore properties in directions other than parallel to the fiber (i.e. off-axis) are important70 . 

Consider the same unidirectional lamina with the stresses now applied perpendicular to the fiber axis as shown in Fig. 12. The local stress at the fiber matrix interface can be calculated and compared to the nominally applied stress on the whole lamina to give K, the stress concentration factor. The plot of the results of this analysis shows that the interfacial stresses at the point of maximum principal stress can range up to 2.6 times the applied stress depending on the moduli of the constituents and the volume fraction of the reinforcement. For a typical graphite-epoxy composite, with a modulus ratio of 70 and a volume fraction of 70 % the stress concentration factor at the interface is about 2.4. That is, the local stresses at the interface are a factor of 2.4 times greater than the applied stress. 

The assumptions used in the model to predict the stress concentration factor assumed homogeneity of the fiber and the matrix up to the fiber-matrix interface. This assumption is not correct in all cases. While glass may be homogeneous in all directions, graphite and aramid are not. The tranverse modulus of graphite varies with the tensile 

On the epoxy side of the interface, high fracture toughness and low residual stresses 72,73) are a requirement for optimum transverse strength in graphite and glass-epoxy 1A) composites. Since the adsorption of epoxy components has been shown to be probable, the local structure of the epoxy at the interphase will most likely not be the same as in the bulk. This local anisotropy caused by the interphase is a limitation in the predictive capability of micromechanical models which do not include the interphase as a component. 

---

Anisotropy increases as the density decreases. The transverse strength is usually between 1 0% and 20% of the longitudinal. 

X (= axial or longitudinal strength in tension Xg = axial or longitudinal strength in compression Y, = transverse strength in tension Yg = transverse strength in compression S = shear strength... 

Brinell hardness Transverse strength Tensile strength... 

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

Cure temperature (°C) a Degree of cure Et (GPa) transverse modulus [standard deviation] XT (MPa) transverse strength [standard deviation]... 

Mechanical testing of the three-step cure specimens indicated that no sacrifice in properties resulted from the modification of the process cycle. The retainment of mechanical properties (transverse strength and modulus) coupled with the reduction in dimensionless curvature for the three-step cure cycles investigated provides another suitable cure cycle modification for reduction of residual stresses in composite materials. Overall processing time has not been increased beyond that specified in the MRC cycle. Thus, with no increase in process time and comparable mechanical properties, the residual stresses have been reduced by more than 20 percent in comparison to the MRC cycle baseline data. 

Interfacial shear strength is a critical property for composites. This previous analysis shows that the interfacial stresses under shear loading can be very large. Therefore as in the case for transverse strength, the interphase itself can be the controlling factor in the level of interfacial shear strength attainable for a given epoxy composite. 

All steels, particularly forgings Assure quality control, particularly look for banding (i.e.) distinct parallel layers of ferrite and pearlite which might cause transverse strength problems and segregation of MnS inclusions in ferrite bands). 

D. B. Zahl, S. Schmauder, and R. M. McMeeking, Transverse Strength of Metal Matrix Composites Reinforced with Strongly Bonded Continuous Fibers in Regular Arrangements, Acta Metallurgica et Materialia, 42, 2983-2997 (1994). 

The results of tension tests upon refractories in the hot state are not available in the literature nor are data relating to the transverse strength of bricks and tiles. This is to be regretted since in many furnace constructions transverse loads must be considered. Again, it seems very probable that the compression test of firebricks will ultimately be replaced by one involving transverse stress. 

TABLE 8—Three-variable correlations nail pull versus core hardness and transverse strength ... 

The efficiency of reinforcement is related to the fiber direction in the composite and to the direction of the applied stress. The maximum strength and modulus are realized in a composite along the direction of the fiber. However, if the load is applied at 90° to the filament direction, tensile failure occurs at very low stresses, and this transverse strength is not much different than the matrix strength. To counteract this situation, one uses cross-pKed laminates having alternate layers of unidirectional libers rotated at 90°, as shown in Figure 3.47c. (A more isotropic composite results if 45° plies are also inserted.) The stress-strain behavior for several types of fiber reinforcement is compared in Figure 3.48. 

Figure 2.9 Tensile test for one-ply composite Exfieriment experiments (3 representative samples) and FEA the simulalion results. The model has the following feamres o-FVF = 44%, iy-FVF = 74.5%, inter-yam matrix layer is 20 pm. The transverse strength is assumed to be 40 MPa, the longitudinal strength is 1280 MPa at 60% FVF. The local stress is used to predict...

---