Laminate Failure

A possible adjunct to the laminate design procedure is a specific laminate failure criterion that is based on the maximum strain criterion. In such a criterion, all lamina failure modes are ignored except for fiber failure. That is, matrix cracking is regarded as unimportant. The criterion is exercised by finding the strains in the fiber directions of each layer. When these strains exceed the fiber failure strain in a particular type of layer, then that layer is deemed to have failed. Obviously, more laminae of that fiber orientation are needed to successfully resist the applied load. That is, this criterion allows us to preserve the identity of the failing lamina or laminae so that more laminae of that type (fiber orientation) can be added to the laminate to achieve a positive margin of safety. 

For most constructions and applications it is essential to face the reverse of the composite board with a balancing laminate similar in construction and moisture content to the decorative face laminate (failure to do this results invariably in differential movement of the faces and problems with bowing of the board). 

As at this stage, accurate modeling of failure after hrst damage is very complicated and not very accurate. For general laminates, failure theories used in practice conservatively equate hnal failure with failure of the first ply in the laminate. The procedure requires determination of the stresses or strains in each ply in the ply axes and uses one of the many failure criteria. The steps followed in a typical failure analysis are as follows ... 

In the absence of strength data for the laminae and the laminate, failure of the laminate should be examined using the Tsai-Wu criterion, see... 

P(8) Bolt shear failure (Figure 5.11 (d)) is caused by high shear stress in the fastener. Normally the glass FRP laminate failure occurs before fastener shear failure. 

When using the Hart-Smith approach, the laminate failure envelope is constructed by superimposing the failure envelopes of its constitutive laminae or alternatively by using failure data measured at the laminate level to construct a failure envelope in the same manner as for a single ply. 

Laminate failure analysis predicts the criticality of a loading condition by comparing actual ply stresses and strains with ply stresses and strains corresponding to the failure of the laminate. Failure criterion functions are used to take into account the combined effect of stress or strain components. The criticality of the loading condition is indicated with a reserve factor which defines how much the load can be increased before failure occurs. The importance of the analysis is evident. The need for a computer code is also obvious since the failure analysis is even more complicated than the load response analysis. 

Laminate failure envelope analysis determines the combinations of selected load components that cause the failure of a laminate. Envelopes are constructed from the results of consecutive failure analyses, A computer code for creating failure envelopes is useful since the envelope in graphical form displays clearly the load-carrying capability of the laminate. Such a tool should allow the designer ... 

Interlaminar shear stress n. Shear stress between layers of a laminate, an important cause of laminate failure and delamination. 

The use of a fracture mechanics approach based on the strain energy release rate to assess failure in rubber-cord laminated structures is reviewed. The mechanics of crack propagation are considered for cracking either between the plies or around individual cords, and also for crack initiation and growth near cord ends. The ability of the approach to predict the effects of various design and construction parameters on laminate failure is also discussed. 9 refs. 

Laminate failure analysis was performed using a step-by-step stiffness reduction pro-... 

As discussed in [1], we assume that once an interface fails, total laminate failure results, then the lowest interlaminar failure load should be regarded as the failure load of the laminate. 

Laminate failure stress (ksi) for [ 0/9O]s type laminates (Material AS4/3501-6)... 

If open mode delamination is ifegarded as total loss of load carrying function of the laminate, then the present failure criterion based on interface strength is adequate for laminate failure prediction for fiber-dominated it/4 laminates that are prone to open mode delamination. 

From the experimental results for ic/4 laminates, we note that if delamination does not occur (e.g. [0/90/45/-45]s) then Tsai-Hill criterion can predict laminate strength. If delamination occurs before total laminate failure, then the laminate strength is lower than that predicted by Tsai-Hill criterion. 

Soden, P.D., Hinton, M.J., Kaddour, A.S., 1998b. Biaxial test results for strength and deformation of a range of e-glass and carbon fibre reinforced composite laminates failure exercise benchmark data. Composite Science and Technology 62 (12—13), 1489—1514. http //dx.doi.org/10.1016/S0266-3538(02)00093-3. 

The initial stage of ply failure may cause an entire laminate failure by the fuel leakage effect associated with crack connectivity. That was clearly shown in the case of IM7/5250-4 at -253°C and ISOOKPa. The fuel leakage problem could be resolved by using a liner, but there is still a tradeoff between structural weight increase and protection of fuel leakage. 

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