Structural Testing of Stress and Fatigue
Fatigue and Durability Tests
A fatigue test or durability test is required to validate the lightweight construction of vehicle and machinery components. HBM offers complete measurement chains optimized for these tests including strain gauges, force and displacement transducers, data acquisition systems and software.
Fatigue Test of Materials and Structures
Driven by the need to increase the energy efficiency of vehicles and machines, lightweight construction of mechanical parts and components need to go close to the fatigue life limits of established or new materials. Nevertheless, for safety relevant structural components such as vehicle axes and wheels, a fatigue failure during operation must be excluded by all means. Likewise, a fatigue failure of critical functional components such as crank shafts or connecting rods leading to a defect of the entire vehicle or machine must also be avoided.
For a lightweight construction, a simulation model is used in combination with durability testing: A fatigue test of each material is used to determine its fatigue life, i.e. their fatigue curve (Woehler curve) or variable amplitude fatigue curve (Gassner curve). These material characteristics are an important input for durability simulation models. FEM based simulation models are typically used in early design phases to assess design alternatives or to identify critical spots of structural components.
The durability test in the lab begins with the first physical prototypes of parts or components. In early stages, dynamic load test may use a simplified block program to compare design variants. In later stages, real-world load data recorded in the field are replayed on the durability test stand to determine and verify the absolute fatigue life of the chosen construction.
Besides the long-term dynamic load test to determine the fatigue life of the test specimen, the durability test may also include a static load test to determine the maximum static force or stress that the test specimen can tolerate, such as a tensile test, compression test, flexure test or torsion test. In a characterization test, the stress distribution across a structure is determined to validate an FEM model.
For fatigue testing and durability testing from HBM:
- Complete measurement chain of strain gauges, data acquisition systems and software, with all components working together perfectly
- Complete software solution for the application, ready-to-start without programming
- High immunity against electromagnetic interference and thermal drift due to carrier frequency technology, distributable amplifier modules and auto-adjustment on all channels
- Compensation of cable effects with 4-, 5-, and 6-wire circuit-based patented amplifier technology
- Broad portfolio of electrical strain gauges with short delivery times, including pre-wired gauges for immediate use
- Optical strain gauges and interrogators for high strain fatigue tests of compound materials
Screenshots of HBM's catman data acquisition software
Span pair cycle counting
Online and post-process cycle counting of the applied load collective using the span-pair cycle counting method, e.g. visual comparison of load collectives
Rainflow cycle counting
Online and post-process Rainflow cycle counting of the applied load collective, either as Range-Mean matrix or as From-To matrix
Visualization of peak values of response over stimulus (e.g. displacement over force) for all cycles to detect a possible onset of damage to the test specimen, including alarming upon exceedance of tolerance limits
Alarms upon unexpected events
E-Mail, visual or sound notification of the user and log messages when measurement values move outside of expected ranges
Pre-defined online and post-process calculation of principal strains and stresses from rosette strain measurements
Easy channel configuration
Quick and comfortable channel configuration by simply dragging and dropping the suitable strain gauge bridge type from the sensor database onto the channels to be configured
Health check of strain gauge channels:
Check of strain gauge channels for correct wiring by comparison of measured vs. expected bridge unbalance when applying a shunt resistor
Recording of selected cycles
Sequence plans to record selected cycles only during long-term tests (e.g.: record 1 full cycle out of every 10 cycles), including automatic detection of the cycles
Integration into Zwick material testing machines
Integration of complementary strain measurements with strain gauges on the probe into Zwick material testing machines, e.g. for the verification of the alignment of the probe holders or for Compression After Impact (CAI) tests of compound materials
Optical strain measurements
Integration of interrogators for optical Fiber Bragg strain gauges and automatic conversion of the measured peak wavelengths (in nm) into strains (in µm/m)
Recording of peaks and valleys
Sequence plans to record peaks and valleys only during long-term tests (e.g.: record peaks and valleys of all cycles, or just for 1 cycle out of every 10 cycles), including automatic detection of the cycles