EML 3301C Mechanics of Materials Laboratory Lecture Lab 1 intro Announcements • Upcoming Lectures – Report Writing – Uncertainty – Lab 1 calculations and uncertainties – Start Uncertainty Lecture • Lab 1 Data Acquisition In lab the week of August 29,30,31 – Dial Caliper use, Ta’s will instruct – Data stored on Canvas – Two impact specimens per section, one brittle and one ductile – Text instructions are unmodified at this point except for: • No elastic / plastic work fraction is expected. Lecture • Lab 1 In lab – Students will collect any other needed measurements. • Specimen geometry, …. – Data will be collected • Instructor managed data acquisition • A student(s) will help in testing – Raw Data will be posted. • From LabVIEW – A future lecture will cover developing the discussion items, this is easiest with a LabVIEW VI (or matlab, excel). Impact testing: Background • Why was Charpy testing developed? – Steel exhibits brittle failure at temperatures slightly below our ambient, ductile failure above • A note about qualitative vs quantitative… – This was observed in swords early • Causes were not directly known, so procedures that reduced this phenomenon were iteratively found and sometimes lost – Some ships in the early 20th century exhibited this failure mode in the North Atlantic • Titanic • Liberty Ships Impact testing : Background • Liberty Ships (WW2, ~1940) • Welded from Carbon Steel – Faster than rivets From: https://www.tf.uni-kiel.de/matwis/amat/iss/kap_3/illustr/i3_2_1.html Lab 1: Impact Testing • Impact testing, generally at speeds greater than ~2 m/s • Drop hammer • Pendulum • Rotary Lab 1: Impact Testing • Charpy → simply supported beam loaded by an impact from a swinging pendulum Lab 1: Impact Testing • Izod → cantilever beam loaded by an impact from a swinging pendulum Lab 1: Impact Testing • Specimen geometry is controlled. • A notch is included in the specimen to control the failure Lab 1: Impact Testing • Good at comparing specimens prepared exactly the same, tested exactly the same, on the same machine. • Difficult to generate quantitative results that can be used to predict performance – Yield points not easily found Lab 1: Impact Testing • Most modern impact testing machines are pendulum in type. Impactor Sample Location Lab 1: Impact Testing • The energy dissipated during a test can be estimated by measuring the starting height and ending height of the pendulum – Zero velocity at these two points, no kinetic energy – The potential energy change can be had by measuring the geometric configuration, given the inertial properties of the pendulum Lab 1: Impact Testing • Some of the change in potential energy represents work done on the specimen • Some is lost to other non-conservative issues – Friction in bearings – Air resistance – Elastic storage in structure (elastic defection, then dissipation through vibrations) • The change in energy due to some of the nonconservative issues can be estimated by performing a swing down test. Lab 1: day 2 LabVIEW 2023 Q3 can now be licensed with our license number, install instructions are updated. Thursday lecture is TBA, if UF is open, same format as today Lab 1 data collection is now scheduled for next week. Lab 1: Impact Testing • Swing down test: – Displace the pendulum to an initial testing height – Allow to swing for several periods of oscillation. – Estimate the potential energy lost in a typical swing. • Note that the impactor does not engage a specimen. Lab 1: Impact Testing • Work on Specimen: – The force acting on the specimen during impact can be estimated by considering the strain in the pendulum – This force can be estimated over the duration for the impact, and a work term can be calculated that estimates the work done to the specimen by the pendulum. Lab 1: Impact Testing • The impactor is instrumented: – The relative position of the pendulum with respect to the frame is measured with a rotary position sensor (potentiometer) → two voltages, an excitation voltage and a wiper voltage, are measured to estimate position. – The strain in the pendulum is measured with a ½ bridge strain gage configuration • Two parameters, Vs, Vamp are measured → two Voltage measurements are required to estimate strain. Lab 1: Impact Testing: Instrumentation: Potentiometer • Excitation Voltage – Measure between + and -, constant. Wiper + • Wiper Voltage, – measure between – and Wiper, varies proportionally with position between – and + voltages • Wiper voltage is ratiometric with excitation voltage → wiper voltage is proportional to excitation voltage Resistive Element Lab 1: Impact Testing: Instrumentation: 1/2 Bridge with strain gauges • Excitation Voltage, Vs – Measure between + and -, constant. • Bridge Voltage, Vg – measure between two center nodes, varies ~ proportionally with strain – Vg is typically amplified to aid in data collection • Amplified Bridge Voltage, Vamp – Vg multiplied by a gain and offset • Bridge voltage is ratiometric with bridge voltage → bridge voltage is proportional to excitation voltage Rg1 R4 Vs - DC R3 VG + Rg2 Lab 1: Impact Testing In Lab (test instructions) • 1. Take data for position during a large (~90 degree displacement) perturbation swing down test. (we will do this second) Lab 1: Impact Testing In Lab (test instructions) • 2. Take ~2 seconds of data with the impactor hanging at lowest potential energy position (~zero degrees). Use this data to establish the zero voltages for strain and position for the pendulum. (we will do this first) Lab 1: Impact Testing In Lab (test instructions) • 3. Prepare a ductile sample of bass for impact, and impact test. Capture pendulum position and strain gage readings with the attached SADI DAQ. To perform the test (requires 2 people, TA/Instructor + volunteer): – Measure cross-section of specimen (do before any work with Charpy machine) – Load specimen – Prepare VI to run – Lift impactor to desired height – Start VI – Drop impactor – Save data Lab 1: Impact Testing In Lab (test instructions) • 3. Prepare a ductile sample of bass for impact, and impact test. – Measure cross-section of specimen (do before any work with Charpy machine) Measure area at notch Lab 1: Impact Testing In Lab (test instructions) • 3. Prepare a ductile sample of bass for impact, and impact test. – Load specimen Lab 1: Impact Testing In Lab (test instructions) • 3. Prepare a ductile sample of bass for impact, and impact test. – Prepare VI to run – Lift impactor to desired height – Start VI – Drop impactor – Save data Lab 1: Impact Testing In Lab (test instructions) • 4. Prepare a brittle sample of marble for impact, and impact test. Capture pendulum position and strain gage readings with the attached SADI DAQ. • Note that data will be collected on a lab computer, two specimens per section will be tested, all data will be posted to canvas under the assignments. Lab 1: Data • The data gathered in lab 1 will be posited on Canvas • You will be responsible for retrieving it and manipulating it to derive basic material impact properties. • As noted in the text, corrected change in PE and work is expected for both specimens. Lab 1: Impact Testing : after Lab • Develop a calibration constant that relates the potentiometer signal to angular position of the pendulum. – Use data provided taken with the pendulum in two positions • Forward horizontal • Back horizontal • : 180 degrees apart. Lab 1: Impact Testing : after Lab • Compute the windage and friction loses from the large perturbation swing down test and develop corrections for the impact tests. • Calculate and report a corrected energy absorption number for the brittle and ductile samples (change in PE associated with impact on specimen). Lab 1: Impact Testing : after Lab • Using the strain gage data and pendulum position data, calculate the work done during the impact of each specimen. This will require some data manipulation to extract the relevant data range from a much larger range provided. • Compare and discuss the results from the corrected energy approach and the work approach. Lab 1: Impact Testing : after Lab • Present an uncertainty analysis for the corrected energy number (change in PE) processes. • An uncertainty analysis is no expected for the work approach.
