The delamination is an usual problem in composite materials structures. Basically, it is a separation of the material plies due to several reasons. Normally, these are impact, lay-up issues and fatigue. In addition, the delamination can be due to impact and fatigue or all those causes together. In terms of manufacturing problems, it usually occurs due cutting or drilling processes on the laminate, that leave free edges from where the delamination starts. The most common cause is impact, because shanks are a commonplace in racing field. Hence, it can initiate a crack. If this is not detected, it propagates along the laminate. When the crack reaches the surface, the structure fails due to delamination by fatigue. The main difficulty when dealing with the delamination, is its detactability. Even the ultrasonic transducer has trouble to detect any delamination, because the consecutive bonding surfaces absorbs the ultrasonic waves. This is one of a sort of non-destructive technique (NDT) used to detect the delamination. The problem is that, the delamination is a sort of out-of-plane issue that starts from inside and propagates until the surface. This is the point at which the delamination is detected. Actually, the bonding surface is detected as a big delamination by the transducer. As close the surface the delamination is, the easier is its detectability. The problem is that, usually when it is very near the surface, the failure is about to occur. Therefore, the main problem of composite material components is an out-of-plane deformation, a separation of the plies across the laminate, which is called delamination. This article will describe an overview about this very important issue in racing car composite chassis.
Delamination effects
Most of the composite material structures are based in adhesive joining. When delamination occurs, the main effect is that, the laminate will work as partially bonded one. Hence, the laminate lose strength and stiffness. The first is lost, because the laminate can not transmit the load as in the undamaged one. In the case of the stiffness, the lay-up characteristic is more important. In other words, an unidirectional laminate exhibit a delamination without a significative decrease in stiffness. However, this can not occur in complex laminates. The delamination is capable to reduce the laminate into several sub-laminates. Each of them will have its own characteristics, which result in an unpredictable structure in terms of mechanical behaviour. In other words, a structure with regions of different strength, stiffness and coupling between in-plane and out-of-plane deformations. The reason is that, each sub-laminate composes the overall laminate. If this is a balanced one, there is a small coupling between the in-plane and out-of-plane deformations. As a result, the stress conditions are deteriorated.
Delamination scenarios
This failure mode can occur in two different scenarios, A and B. The scenario A is characterized by the fact that, all plies are still carrying the loads. The scenario B occurs when some plies are not carrying loads since they had failed, thus there are less plies effectively working. The scenario A is also characterized by the stress distribution over the sub-laminates. Then the value of the failure deformation coincides with the one for the most rigid ply. Conversely, the weakest plies will share the lowest amount of stress. In the case of the scenario B, the failed plies do not bear any load. Hence, the remaining ones will carry more load as usual. The failure of the entire laminate depends on the level of the contribution of the failed plies. Therefore, the laminate modulus depends on the plies that had failed.
Examples
These scenarios can be better understood by a laminate example. Considering a [0/90/0]° laminate, if this had exhibited delamination according to the scenario A, the plies at 90° might be the first one to failure. However, if the load is no longer shared equally between the plies, then the 0° ones became the most critical. The reason is that, they will carry the most part of the loads. If the same [0/90/0]° laminate is exhibiting delamination by the scenario B, the result is different. In this case, some plies are already broken, thus the delamination propagates along the interface. The first plies to failure, again, might be the ones at 90°. Then the delamination will propagate through the interface and the 0° plies will not carry any loads.
Delamination instability
Another effects due to the sub-laminates are that, their unbalance also results in some coupling extensions, which is bending, and the compression instability. This last one occurs, because each sub-laminate is a more slender structure, thus the laminate is more prone to suffer instability.
Mitigating delamination
Although the delamination is rather impossible to be completely avoided, it is possible to mitigate its occurrence. This is done by reinforcements and/or modifications on the edge of the laminate. Actually, the great problem of the free edges is that, they are free surfaces, which there is a significant possibility to exhibit defects due to tooling or cutting or shear mismatches. For those possible issues, it is necessary to perform edge reinforcements and/or modifications.
Edge reinforcements
Regarding the edge reinforcements, there are three types, the edge cap, the stitching and the interlayed adhesive layers. The free edges usually exhibit shear misalignments. One technique to avoid that is the stitching. This is usually done for textile fiber. It is based in stitching the plies across the laminate perimeter. When shear misalignments occurs, if the plies are not stitched, they can deform at different directions. The stitching keeps the plies together and concentrates peer and shear forces between the plies at the edge of the laminate. This process is done previously the ply consolidation. The other reinforcement is the interlayer adhesive layers. These are based in layers of adhesive between the fibers, but only at the edges. The adhesives are usually thick and with a very low Young’s modulus with respect to adherents. This is a great reinforcement against stress concentrations mainly for laminates as aluminum and glass fiber composites. These two compounds have completely different thermal expansion coefficients. Then, the stress concentration will arise at the edges. A thick and soft laminate compensates this difference, thus reducing the stress concentrations. If a thick and rigid adhesive was applied, the stress concentrations would increase. The order of magnitude of the adhesive is of fraction of millimetres. Hence, to reduce the delamination at the edges, a soft polymer based adhesive is used to decrease the stress concentration. It is a quite simple and effective method.
Edge modifications
Another approach for mitigating the delamination is by edge modifications. These can be performed by three methods, the ply termination, the notching and the tapering. The ply termination is commonly used in laminates with plies disposed at different orientations. Usually, when the plies are arranged in the cascading configuration, the different orientations enhance the shear disagreements. Then the shear stress at the edges is also enhanced. The objective of the ply termination is the ply drop, which is the cut-out of one of the plies just near the edge. This is done in order to have two plies with the same orientation together just at the edges. For instance, in a cross-ply laminate with [0/90/0/90/0]° ply orientations it is possible to notice that, at the middle there is a shear disagreement given by [0/90]° plies. If the ply at 0° was cut just near the edges, the configuration at this region would be [0/90/90/0]°. Therefore, a possible critical interface would be removed reducing the stress concentrations. Another method of modification is the notching. It is usually applied for long laminates. The problem of these is that, the longer the edge, the higher the stress concentration. This method proposes to machine or drill a notch in order to divide a long edge in two or more smaller ones. The notches work as a sort of interruption of the stress concentrations. The last possible modification is the tapering. As the name suggests, the edge is tapered in order to reduce the stiffness at that region. The lower stiffness helps to reduce the stress concentrations.
References
- This article is based in the notes taken during the Composite Materials course attended by the author in Muner Advanced Automotive Engineering Master Degree.
Cover Figure
- Wikipedia.
Automotive Papers 