Understanding reliable materials testing

Zwick ZHN universal nanomechanical tester

Andy Pye attended a materials testing forum at Zwick HQ in Ulm to find out more about the practical implications of achieving reliable test results.

The 24th International Forum for Materials Testing was recently held at the headquarters of Zwick in the German city of Ulm, where Environmental Engineering’s Andy Pye spoke to the company’s Head of Industry Management, Johannes Bührle to find out what the achievement of reliable test results means in practice.

Reliable test results are a fundamental and highly topical aspect of materials testing. There are four key elements to reliability:

* Accuracy: Machines must operate with precision, meaning that the average of a set of test results is very close to the reference value.
* Repeatability: When tests are repeated under the same conditions, the results obtained are closely grouped.
* Reproducibility: When machines performing the same test are compared, the results are reproducible.
* Traceability: The continuous documentation of calibrations and tests makes all results transparent and traceable.

Accuracy

In general, measurements may be:

* Precise but not true
* Not true and not precise
* True but not precise

But only measurements which are ‘true’ AND ‘precise’ are ‘accurate’.

During a scheduled calibration, the measurement chains of a testing machine are compared with a national measurement standard. Calibration means establishing the difference between the actual value and the set value.

Regular checks ensure that test results are only obtained using correctly functioning testing equipment. Equipment for use in calibration must possess a higher level of accuracy.

Zwick machines are calibrated against the following parameters: force (tensile and compressive); length; hardness; and mechanical work. Documentation is provided as per regulations stipulated by DAKKS, Germany’s National Accreditation Body. As the government-appointed accreditation body, this organisation provides the accreditation of conformity assessment bodies. Accreditation by DAkkS demonstrates the technical competence and independence of laboratories, certification and inspection bodies or verifiers.

Accurate results are also ensured by means of automatic real-time correction of machine deformation. Zwick’s testXpert II measurement and control software provides a correction curve, ensuring that control is directed through the corrected channel, allowing displacement targets to be attained exactly.

Zwick’s testXpert II measurement and control software supports applications that range from yield strength measurement to the determination of specialist properties such as heat deflection temperature and Vicat softening point of plastics.

In the case of brittle specimens, or in creep and fatigue tests, the exact alignment of the load string is critical to obtaining accurate test results. The testXpert II alignment SW module determines the angular error and offset error using a strain-gauged alignment transducer. The severe demands of tensile modulus measurements on plastics mean that high-accuracy extensometers are an essential requirement. Determination of the tensile modulus of plastics is performed using both contact and non-contact extensometers.

Repeatability

‘Repeatable’ refers to a statistical measure for measurements in a laboratory over a short period under the same conditions, usually with a single operator and testing instrument.

Automated testing ensures highly uniform insertion of specimens into the testing machine and exact repetition of test sequences. Automated testing can be expected to generate significant improvements in standard deviation. Typical figures might be:

* tensile strength by 11%
* anisotropy (r20) by 26%
* tensile modulus by 48%

Compression tests on long-fibre-reinforced composites are particularly demanding; specimen alignment must be very accurate to prevent bending and buckling during the test. Using the correct test fixture is essential. An axial offset of just 10 µm will result in an invalid test to ASTM D6641, due to bending.

In addition various easily adaptable fixtures are supplied with the machine, including a flexure test kit for 3 and 4-point flexure tests and a hydraulic compression test kit (HCCF). The HCCF test device that meets the requirements of Airbus Standard AITM 1.0008, which describes both shear loading and combined loading. It enables considerably more valid tests to be performed compared with the standard compression fixture.

A Combined Loading procedure is suitable for the testing of fibre composites under the higher loads that occur with larger specimen cross-sections. Part of the compressive force is loaded via the specimen clamping, the rest on the ends of the specimen. The length of the specimen is matched precisely to the length of the jaws. Very high requirements are placed on the processing of specimen ends, as in the end-loading procedure.

The application of clamping force is described as a hydraulic parallel clamping principle. Studies showed that more valid break images are achieved when clamping peaks at the transition between free clamping length and clamping area are avoided by the structural design of the jaws. In the Airbus standard, this is described as “soft load introduction”.

Reproducibility

‘Reproducible’ refers to a statistical measure for measurements over a lengthy period, usually with multiple operators and testing equipment.

* precision under reproducibility conditions
* same measurement procedure
* similar measuring equipment
* different locations or times for measurement
* different operators performing the measurement
* different test conditions

Measuring instruments with a smaller results spread, means that a larger assured tolerance band is available for the production process.

To ensure a high degree of reproducibility, over 600 Standard test programs, operated by simply selecting the test program in testXpert II, are pre-configured in accordance with test standards and contain the terminology used in the industry concerned.

testXpert II Re-Run with videoXtens enables retrospective determination of true (= correct) strain values in the event of specimen breaks outside the original gauge-marks.

Traceability

Traceability is the continuous documentation of the calibrations of all measurement chains and thereby the establishment of a reference to a national or international measurement standard.

testXpert II provides sophisticated user management with freely configurable user profiles and detailed rights management. This enables authorisation levels to be shown in the software and allows menu items, standard actions (such as printing reports) or test programs to be authorised or blocked individually, reducing the probability of critical input errors. Blocking user accounts in the event of unauthorised log-in attempts allows manipulation to be prevented, while attempted access of this kind can be reported via a notification system.

Medical Devices Directive 93/42/EEC and FDA 21 CFR Part 11 regulations require testing systems used in the medical sector to be validated at regular intervals. A key element of the validation of processes in medical engineering and the pharmaceutical industry is the technical inspection of individual systems and instruments.

The ‘Expanded Traceability’ function provides all the tools needed, in conjunction with organizational procedures, to fulfil the essential conditions of the requirements set out in FDA 21 CFR Part 11.

Electronic records

The ‘electronic records’ function allows complete, non-manipulatable documentation of all actions and changes performed in testXpert II. The level at which actions are to be logged and, if necessary, explained is specified by the user in accordance with his regulatory guidelines (e.g. changes to a test-relevant parameter such as test speed). This data is stored in the audit trail.

Electronic signature

The ‘electronic signature’ function allows documentation of assumption of responsibility plus a change to ‘paper-free documentation’, as the signature on the test report can be replaced by a digital signature for the test program or series by entering user identification and password in testXpert II. It is possible to specify precisely how many people must sign and who is authorised to do so. The test program and test series are safeguarded against unauthorized changes when in signed status.

Video Capturing

The Video Capturing Plus option enables display and documentation of test sequences. A video is recorded parallel to the test on webcam or camcorder. Integrated hardware synchronisation enables frame-accurate synchronisation of the testXpert II test data and the video. When the video is played back the corresponding test data are displayed in the curve graphic via crosshairs; alternatively the user selects any desired measured value line via the crosshairs and automatically obtains the corresponding video image. In this way the test sequence can be visually evaluated, even retrospectively, or a special material behaviour can be documented.

Andy Pye

Andy Pye

Andy Pye is a graduate of Cambridge University and has had a high profile career in the technical press as well as being a pioneer in web publishing.
Andy Pye

Latest posts by Andy Pye (see all)

About Andy Pye

Andy Pye is a graduate of Cambridge University and has had a high profile career in the technical press as well as being a pioneer in web publishing.

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