MEng Project in Mechanical Design
General Information
Contact: Paul Dawson
Office: 196 Rhodes Hall
Email: prd5@cornell.edu
Background
As trees age, they inevitably are subjected to wounds that break the protective outer bark and allow
for invasion by various microbial pathogens, including fungi that decay the wood and threaten their
structural integrity. Trees respond to such damage by “compartmentalizing” the pathogens – a process
that allows new, decay-free wood to form after the wounding event; the new wood isolated from that
present at the time of wounding by biochemical barriers created by the tree in response to the wound.
Internal deterioration of stem and branches may continue unabated and unnoticed because just a few
healthy annual rings of new wood are sufficient to sustain a completely healthy, symptomless crown.
Thus, one of many challenges to tree health care professionals is to identify potential “hazard trees”
before they collapse in the face of wind, snow, or other natural forces, causing catastrophic damage
to life and property as they fall.
Microbial degradation of wood in trees is the last step in a succession of events that begins with
exposure of the wood to external microbes when bark is removed and ends when the cell wall contents
(cellulose, hemicellulose, lignin, and pectin) lose all structural integrity via enzymatic degradation.
Although the wood in different species of trees varies with respect to susceptibility to and rate of
decay, tree care professionals responsible for determining whether a tree is “hazardous” and should
be removed usually make their assessment based on the ratio of sound wood to decayed wood. If the
diameter of the decay column in the center of the tree is greater than 30% of the combined width of
the sound wood on either side, then the tree is at risk of breakage and is marked for removal. Given
the value of life and property at risk, there is usually no further assessment of the rate or progression
of decay; it is an “either/or” decision.
In virtually every instructional guide to hazard tree assessment, one of the first “tests” for lost
structural integrity in an otherwise healthy tree is to pound on the tree with a rubber mallet and listen
for changes in the sound that emanates from the tree thereafter. A person with a well-trained ear is
said to be able to use this technique, alone, to detect decay with as much precision as might otherwise
be had with far more sophisticated equipment like a Shigometer (measuring changes in resistance to
a pulsed electrical current), a Resistograph (measuring changes in resistance of a drill bit to passage
through the wood) or a sonic tomograph(measuring changes in passage of mechanical vibration).
Design Goal and Specifications
The purpose of this project is to incorporate new technology into the first option above – “sounding”
with a mallet – so as to replace the subjective interpretation of the sound in response to a tap
with the mallet with an electronic measure of the response, recorded in the mallet head. Specifically,
we want to design, build and test a device that is capable of determining if a tree has lost structural
integrity due to excessive amounts of decay, that meets the following requirements:
The devise must:
- (a) be portable (easily carried and handled by a single person), (b) operate on standing trees, and (c) provide readings without substantially damaging the trees tested.
* The devise must provide a determination within 10 minutes, including the time to set up a test and interpret the measurements.
* The devise should estimate the degradation of strength to within 10% of the competent wood strength with a 90% confidence.
* Supporting algorithms (formulae) and/or software must be provided and documented.
* The device should cost no more than $500.
Project Plan
The project will be divided into two phases. The first phase involves determining how the
acoustic signal (sound) differs with the degree of decay and what instrumentation can be used
to accurately measure the differences. The first phase will end with conceptual designs for
the device and associated methodologies for its use. The second phase involve completing a
detailed design, building the device, and evaluating its performance.
Organization
The design project will be carried out by a design team of 2-4 students and will be conducted
during fall and spring semesters. The team will meet weekly to discuss progress. Meetings
with external collaborators will be scheduled as needed. The team will research aspects of
the mechanical properties of wood and capabilities of acoustic measurements leading to design
concepts during the fall semester (Phase 1). Detailed design, construction and testing will be
carried during spring semester (Phase 2).
Collaboration
The project will supplement ongoing research and extension by Professor George Hudler, Department
of Plant Pathology.