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Why drop weight testing?

Kayaks experience two types of loading that develop stresses in the hull and deck.  Global loading arises from the interaction of the boat with the water and is dependent on the weight of the boat, the paddler, the cargo, and the water conditions.  Global loads produce global stresses that are well distributed throughout the structure.  Local loading arises from the interaction of the boat with hard objects and results in highly concentrated stresses.  Most damage sustained by kayaks is the result of local loading. 

A few years ago, we tested some cedar strip samples that Nick Shade had put together.  At the time, we had access to a servohydraulic test frame at UC Davis, with fixtures for three-point bending.  Nick did a nice job of presenting the results on his Guillemot Kayaks web site under
Materials Testing on the Kayak Design page.  Perhaps the most striking behavior that the three-point bend testing demonstrated was a strong effect of grain orientation on sample strength.  The most common description of the structural response of cedar strip lay-ups is that of a sandwich, in which the wood serves as a lightweight core for the glass/epoxy face sheets.  Ardent proponents of the sandwich description continue to argue that the glass/epoxy carries the loads, while the wood simply increases the (area) moment of inertia of the sandwich.  While this view is a useful introduction to the structural response, the three-point bend results clearly demonstrated that the wood carries a significant load in the direction parallel to the grain.

While useful, two aspects of three-point bend testing should be considered for the purpose of evaluating kayak lay-ups.  One is that the test best evaluates the response to global loading in one orientation at a time.  Another observation is that the test requires that local contact forces, from the three rollers, do not interfere with the global response.  In reality, kayaks experience both in-plane biaxial loading and out-of-plane contact forces that superpose. We can address these two observations by testing plate samples supported on all sides and subjected to localized loading in the center of the sample.  Drop-weight testing is a relatively simple and inexpensive method for measuring the relative resistance to local loading of the lay-ups used to build kayaks.

The drop tower built for this work, shown in Figure 1.1, consists of three linear motion rods, two fixed crossheads, and a sliding crosshead that carries the drop weight, load cell and impact head.  The sample is placed on a simple 6 in diameter circular support beneath the lower crosshead.  For most of the work completed so far, the weight of the crosshead has been selected to cause complete perforation of the sample.  The nominal drop height has been 20.25 in resulting in an impact velocity of around 125 in/s (7 mi/hr).  A displacement sensor is mounted to one of the linear motion rods and contacts the mass support during impact to provide a displacement record of the event.  Signals from the load cell and displacement sensor are input to a digital oscilloscope and recorded at 90 kHz.
Figure 1.1.  The drop tower setup.

1. Fixed upper crosshead
2. Release clip
3. Sliding crosshead
4. Guide rods
5. Lead weights
6. Impact head
7. DVRT bracket
8. Sample support ring
9. Signal conditioners, data logger (replaced by digital oscilloscope), and computer
A representative load-displacement plot is shown in Figure 1.2.   Four key features are indicated on the plot.  Feature 1 is characteristic of the first damage event during loading.  Feature 2 is the peak load for the test and indicates the onset of catastrophic failure.  Feature 3 is typical of the combined effects of rapid failure (load decrease) and ringing in the load train.  Feature 4 shows interaction of the failed sample with the sides of the support and the rubber stop block that cushions the drop-weight at the end of the test.
Figure 1.2.  Typical load-displacement curve for a drop weight test.  The curve shows (1) signal characteristic of the first damage event, (2) peak load and onset of catastrophic failure, (3) rapid load drop and load train ringing, (4) interaction of the sample with the support.
Any feature of the load-displacement record can be used to compare different lay-ups, but because the maximum load signals the onset of catastrophic failure, the energy to peak load is a good general measure for ranking the resistance to local loading.  The energy to peak load is simply the area under the load-displacement curve from zero load to the peak.  By ranking lay-ups in terms of energy to failure, we account for both load and displacement.  Propagation energy, the area under the load curve from the peak load to complete failure, is an indicator of how much damage a lay-up can sustain.  However, the propagation energy can contain contributions from interaction between the sample and the support, so it carries more uncertainty than does the peak energy value.
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