TD934

Instrumented ImpactTester

Instrumented Impact Testers

The Tinius Olsen / MPM instrumented strikersystem is the most accurate dynamic forcemeasurement system in the world. The mostimportant advantage of instrumenting thestriker with strain gages is that the appliedforce and the energy during impact can bemeasured. In the case of a drop towermachine, this is the only means for energydetermination, but for a pendulum machine,the energy is determined by an opticalencoder and also independently by theinstrumented striker. In addition to energymeasurement, additional data such as thecharacteristic loads, deflections, andenergies can be obtained.

Accurate measurement of the force duringimpact is an especially challenging task dueto the dynamic nature of the load and theshort time period over which the impactoccurs (typically 1 to 5 milliseconds). Thisis compounded by the requirement of atleast 10,000 data points to accuratelyrepresent the applied force.This data sizerequirement and short test duration meansthat the storage of the force-time data mustbeprecisely triggered and this isaccomplished bycontinuously monitoring theload signal and saving data when asignificant rise in load occurs. Rapid loadchange events, such as acceleration of thespecimen up to the striker speed, and brittlecrack propagation,require a systemresponse capability of at least 50kHz forconventional specimens and > 100 kHz forminiature specimens. The Tinius Olsen /MPMsystem satisfies all of theserequirements.

The powerful Impact™ software displays therawvoltage- time data along with the testparameters and allows up to 1,000,000 datapoints to be spread over any time range froma few microseconds to a few seconds. Thisis very important for brittle tests wheremany data points are required over a shorttimeinterval (~0.1 msec). A serious dataacquisition limitation arises because the PCprocessor is too slow to acquire many datapoints over a short time interval,so wesolved this problem by providing a 20 MHzoscilloscope card that stores the datadirectly, once the test is complete, data isdownloaded from the board to the PC.

Knowing the mass of the striker and using aspecial strain gage system, the force - timecurve can be measured and converted to anacceleration-time curve. This curve can thenbe mathematically integrated to yield thevelocity-time curve; thisvelocity -time curvecan, in turn, be mathematically integrated togive the striker displacement -time curve.

These numerical integrations permit a force -displacement curve to be constructed. Sincethe work(or energy) of a system is the areaunder the force - displacement curve, theresultant force - displacement data can itselfbe integrated to give the energy absorbed bythe specimen in fracturing.

The four critical (or characteristic) load pointsare the general yield load (applicable tometals), peak load, brittle fracture load, andbrittle fracture arrest load. The general yieldload corresponds to yielding across theentire uncracked ligament. For tests in thetransition region, the peak load occursshortly after the formation of a sharp crackalong the entire notch surface and is anindicator of crack formation in the testspecimen. In this region, a small amount ofstable crack growth precedes rapid brittlefracture which in turn is seen on the force-displacement curve as a precipitous forcedrop.

Data from instrumented tests have beenused to measure the conventional impacttest parameters (41 J transition temperatureand upper shelf energy), and these datahave also been used to develop usefulengineering correlations. In terms ofstructural integrity assurance, the mostimportant of these are static and dynamicfracture toughness correlations.

Fig 1. This shows an instrumented Charpy striker

complete with strain gages.

Fig 2. Instrumented impact system

computer running Impact 4.3

software, with oscilloscope data

collection card.

Fig 3. Instrumented impact system with

in-situ heating and cooling system.

Instrumented Impact Testers

The Tinius Olsen / MPM instrumentedimpact test system includes the followinghardware and software:

· strain gage amplifier (will beinstalled in test machine computer)

· instrumented striker with cable (soldseparately)

· oscilloscope card installed incomputer

· ImpactTM v4.3 software package· system manual

The Tinius Olsen / MPM oscilloscope cardhas the following features:

· 12- bit capability (reducesuncertainty to less than 2 mV on the10 V full scale which is below thecalibration load cell accuracy of0.1% full scale.)

· allows any number of data pointsper test up to 1,000,000 points(10,000 to 50,000 points are morethan adequate for most tests)

· separate arming of the card. Thestandard card is single channel, buta dual channel scope can also beprovided.

Impact v4.3 software is the data acquisitionand analysis software, operating underMicrosoft Windows 2000™, that is powerbehind the system. The software acquiresthe instrumented signal and, optionally, theoptical encoder energy and the dial energy(for test machines equipped with thishardware), analyses these data and providesa test report which summarizes the absorbedenergy as well as important information fromthe test such as test temperature, specimenidentification, etc.

The post-test analysis portion of the programprovides data, in addition to absorbedenergy, that can be used to characterize theimpact behavior of materials and estimatetheir plane strain fracture toughness. Thefollowing data can be obtained using theanalysis program:

general yield load(for metal Charpy specimens)peak loadbrittle fracture initiation loadbrittle fracture arrest loadcrack initiation energycrack arrest energytotal absorbed energy

These data can be used to determine otheruseful quantities such as:micro cleavage fracture stress (for metals)fractures toughnesscrack arrest toughnessdefinition of the intersection of the lowershelf and transition region (for materialswhich exhibit transitional fracture behavior)definition of the onset of the upper shelf (formaterials which exhibit transitional fracturebehavior)

The software includes several other analysisfeatures as well. The data can be read backinto the program at any time to performadditional analysis, change units, or print outtest reports. In addition, the softwareincludes a statistical process control modelfor tracking changes in key materialvariables.

Hardware

Software

Fig. 4. This screen shows the raw voltage-time data which is displayed with the test parameters.

This machine was equipped with a 20,000 count encoder, allowing the pendulum velocity

to be measured.

Fig 5. This screen shows an expanded

time axis on the previous screen.

This uses a 10,000 point acquisition

spread over 10msec. The MPM/Tinius

Olsen system allows up to 1,000,000

data points to be spread over any time

range from a few microseconds to

several seconds.

© 2005, Tinius Olsen 4/05

1065 Easton Road, Horsham, PA 19044 USA(215) 675-7100 Fax (215) 441-0899

www.TiniusOlsen.com info@TiniusOlsen.com

6 Perrywood Business Park, Honeycrock Lane,Salfords, Redhill, Surrey RH1 5DZ England+44 (0) 1737 765001 Fax +44 (0) 1737 764768

Contact Your Local Representative:

Specifications subject to change without notice

Instrumented Impact Testers

Another useful feature of the software is theTable function. With this function, users candefine the test parameters to be included ina table and the order in which theparameters appear in the table. Users thensimply point to a folder and the software willautomatically open each file (can be forhundreds or thousands of tests), extract thedata, and construct the data table (all of thepost - test analysis options work this way).The table can be printed or exported tovarious spreadsheet programs. The softwarealso allows the data files to be organized bygroups.

In summary, theTinius Olsen / MPMinstrumented striker software incorporatesthe following features:

· data acquisition and analysisprogram provides flexible dataanalysis capability

· test parameters can be easilychanged to accommodate testingrequirements

· virtual digital voltmeter for easy loadcell balance verification and shuntcalibration check

· the optical encoder andinstrumented striker energy,percent shear, and lateralexpansion data can be stored in theinstrumented data file

· ASCII data file storage formatmakes specialized post-testanalysis and plotting easy

· ImpactTM v4.3 supports the MPMoscilloscope card, Nicolet scope,and Tektronix scope (the widestrange of features and functionalityis obtained with the MPM PC card)

· characteristic loads (general yieldload, peak load, brittle fracture load,and crack arrest load), energies,and displacements are reportedalong with total energy absorbed bythe specimen

· users manual and extensive built-inprogram help

Fig 6. Graphs showing the integration plots on the data acquired.

Fig 7. This shows the characteristic load report with all the critical points calculated and shown.