An aircraft structural fatigue testing campaign can run for months, applying repeated load cycles while strain gauges record how the structure responds. Each data acquisition (DAQ) reading has to be interpreted against the applied load and measurements from other locations. If channels fall out of alignment, the load path and sequence of local events become harder to assess.

Decide which level of synchronisation the test needs: a shared sample clock, aligned sample times or only a common time reference. Verify channel alignment on the installed system with a common event and limits set by the analysis. Control cable and bridge effects, plan channels for the full campaign, and treat health checks, backup and recovery as part of the measurement system.

Why does synchronisation matter in aircraft structural fatigue testing?

Strain readings only make sense against the load that produced them. Channels across the structure must therefore align with each other and with load control, or a peak may be matched to the wrong load state.

Alignment lets teams interpret the load path, compare strain with analysis predictions, and correlate events across distant locations. Each reading may be valid, yet a timing error can distort the relationship the analysis is meant to explain.

Acquisition and load control need defined timing or status exchange. A National Aeronautics and Space Administration (NASA) report on axial torsional fatigue experiments describes synchronised load waveforms and collection of load and deformation data. A second NASA report describes start and stop signals between a measurement system and a shaker controller.

Acceptable timing error depends on signal content and analysis purpose: a slow survey and a short transient may not need the same alignment. The requirement follows from the measurement and analysis objectives.

What does ‘synchronised’ actually mean for thousands of strain channels?

“Synchronised” may mean simultaneous sample instants, aligned timestamps or only a shared time reference. These relationships differ, so the selected requirement should be explicit and verified on the installed system.

Simultaneous sampling captures each channel at the same nominal instant using a sample-and-hold stage or one converter per channel. Multiplexed inputs scan one converter across channels in sequence, creating offsets that may be immaterial for slow signals but matter when relative timing drives analysis. The size depends on design and rate; see this multichannel acquisition overview.

Channel skew is the timing difference between two channels. Jitter is variation in that difference across samples. Across nodes, include both conversion timing and node alignment in a channel-level timing budget.

The Institute of Electrical and Electronics Engineers (IEEE) 1588 standard defines the Precision Time Protocol (PTP) for synchronising networked clocks. PTP clock alignment does not prove that analogue channels were sampled at the same instant.

A reference signal or event distributed to channels on different nodes enables comparison of recorded edges and calculation of the offset. Acceptance limits depend on the intended sample rates and analysis.

How should you plan channel count for a full-scale fatigue test?

Channel planning spans the full campaign, not only the first installation. Permanent channels, periodic strain surveys, and temporary instrumentation all contribute to the count, while supported expansion allows for later changes.

A building block approach can progress from material coupons and structural feature elements to larger components and full scale structures. A NASA study of the Composite Crew Module describes this sequence; a U.S. Federal Aviation Administration report explains testing at representative structural levels. Levels depend on design and substantiation objectives.

A useful plan separates permanent monitoring, survey and temporary channels, and allows for additions after inspection or a change in objectives. The supported expansion path and verified module or node combinations form part of that plan.

A channel dictionary preserves continuity when instrumentation changes. It typically records a stable name, location, gauge type, bridge configuration, excitation, units, calibration, node, sample rate and status, with earlier mappings retained when a gauge is moved or replaced.

VTEST supplies structural test systems that scale from a handful of gauges to thousands of synchronised channels, with 16 strain or voltage channels per box. For full-scale airframe and wing work, this includes the VTI Instruments EX1403A.

Where should acquisition hardware sit relative to the strain gauges?

Acquisition near the gauges shortens analogue runs and can reduce noise exposure and lead wire error, but increases the number of nodes to synchronise, power and maintain. Placement depends on bridge wiring and the test environment rather than a preference for central or local hardware.

Lead resistance in a two-wire quarter bridge adds series resistance, shifting the reading and reducing sensitivity. Matched lead changes can cancel in a three-wire quarter bridge. Remote excitation sensing measures voltage at the bridge to compensate for excitation lead drop; it does not correct all signal lead or bridge arm errors. Shunt calibration applies a known resistor across a selected bridge arm; its interpretation depends on bridge configuration, gauge arrangement, resistor location, and calibration objective.

Two wire staring gauge measurement setups (a) Standard Bridge (b) Bridge with lead wire compensation

Longer analogue routes expose low-level signals to interference, so shielding, grounding, routing and bridge design remain important. Hardware placement depends on cabling, the test environment and maintenance needs.

How do you protect data integrity over a test that runs for months?

Backup, recovery, channel health and data structure form part of the test design because missing or corrupted records may not be repeatable. The campaign recording plan identifies what is captured continuously, what is surveyed at set test states and how data usability is checked.

Continuous recording stores selected channels through a loading sequence; a strain survey takes planned readings at defined load levels. The International Committee on Aeronautical Fatigue and Structural Integrity (ICAF) report describes strain and deflection surveys at selected fatigue spectrum cases, including checks before and after test stops.

Open circuits, disconnected leads, drift, clipping and noise are common channel health concerns. Alarm review and updates to the channel dictionary after repairs support consistent interpretation; trending may reveal a problem before the next test block.

A project data management plan can specify independent copies, retention, export and restore checks in line with the campaign’s recovery needs. Measurement records can retain context through timestamps, calibration, channel definitions and the processing version used for analysis.

Structured, time-aligned measurements and channel metadata also support later analysis, including artificial intelligence and machine learning (AI/ML). For teams that need to analyse this data without moving it off-site, VTEST offers on-premise AI/ML for test data analysis.

What should a structural test data acquisition specification include?

A useful specification connects each test objective to a measurement limit, an installed system check, and a support requirement. Set values from the test article and analysis plan.

For a programme-specific discussion of the measurement requirements, request a quote from VTEST.

How do landing gear tests change the requirements?

Landing gear drop tests call for different measurement priorities, so the channel plan must reflect the event and test site.

A landing gear impact is transient, so higher sample rates than slow fatigue surveys may be needed, with sensor response and interference controls that preserve strain and load. A peer-reviewed drop test study examines dynamic strain and gauge response. VTEST supplies the VTI Instruments EX1200 Series for landing gear impact trials.

FAQ: aircraft structural fatigue testing questions answered

• How many strain gauges does a full scale aircraft fatigue test use?

A study of the Swiss full scale fatigue test at RUAG reports that over 1,100 strain gauges were monitored throughout the test. The reported gauge count describes that programme and is not a general channel planning target.

• What is a strain survey in fatigue testing?

A strain survey records selected channels at defined load levels, using planned snapshots rather than continuous recording of every channel. The sequence depends on the campaign.

• Why does lead wire resistance matter in strain measurement?

Lead resistance in the bridge can shift the reading and reduce sensitivity, especially with long or thin leads. Remote excitation sensing compensates for voltage drop in the excitation leads, while shunt calibration checks the response against a known resistance change. Which method applies depends on the bridge configuration.

• How do you check that thousands of strain channels are aligned in time?

A common event applied to channels on different nodes makes their recorded timing comparable. Analysis needs to establish the acceptance limits, which are recorded and checked again after system changes.

• Should the data acquisition system be synchronised with the load control system?

Strain and applied load usually need a defined timing or status relationship. The captured signals and implementation details determine whether that relationship is preserved; a shared clock label alone does not establish sample alignment.

Sources

• Lukas Schmid and Michel Guillaume, study of the Swiss full scale fatigue test

• NASA, Structural Element Testing in Support of the Design of the NASA Composite Crew Module

• Federal Aviation Administration, Material Qualification and Equivalency for Polymer Matrix Composite Structures

• ICAF, Some Observations to Recent Full Scale Fatigue Tests

• NASA, A Data Acquisition and Control Program for Axial Torsional Fatigue Experiments

• NASA, Test Methodology Development for Experimental Structural Health Monitoring of Composite Materials using Guided Lamb Waves

• Texas Instruments, Multichannel Data Acquisition Systems

• Institute of Electrical and Electronics Engineers, Precision Time Protocol Standard

• Han and colleagues, Landing Gear Dynamic Drop Test Study

• NASA Technical Reports Server, Instrumentation in a Solid Rocket Plume Environment

• NASA Technical Reports Server, Solid Rocket Motor Static Firing Test Stand