Structural validation of off-highway vehicles using service data Colin J Dodds Dodds and Associates 20th May 20151Dodds and Associates Website: Contact: Phone:

Test Engineer’s Handbook Structural testing of complete vehicles, aggregates and components in the laboratory 20th May 2015Dodds and Associates2 Available at Password = walterscott

Summary 20th May 2015Dodds and Associates3 Limited number of prototype vehicles and components available for structural validation. The service loading is uncertain and often abusive The weld quality is variable! Validation program Predefined reliability Limited field measurements on 3-4 prototypes Classical life estimation methodology Introducing the “Gassner” life estimation methodology Results compared with warranty data

Test vehicle & methodologies Vehicle – off-road BHL Specimen Back-hoe 2 methodologies – Classical (Wöhler) BS 7608 – Gassner prediction based on maximum stress 20th May 2015Dodds and Associates4

The test specimen 20th May 20155Dodds and Associates

Duty cycle events 20th May 20156Dodds and Associates

Modelling – boom Trenching event 20th May 2015Dodds and Associates7 Max bucket force Combination of Multi-Body Simulation & FEA Courtesy ARAI, Pune First approach at design validation

Poor correlation CAE vs. measurement 20th May 2015Dodds and Associates8

20th May 2015Dodds and Associates9 Static load cases to check the force-stress relationship 31 different load cases covered! Courtesy ARAI, Pune

Let’s get some service data 20th May 2015Dodds and Associates10 Classical (Wöhler) BS 7608 methodology published in SAE

Life estimation Classical approach 20th May Dodds and Associates n N

Gauge location and terminology are very important 20th May 2015Dodds and Associates12 All gauges should be placed such that the centre of the gauge is 0.5t from weld toe and the gauge length should be <0.2t where t is the plate thickness. If this is not possible then, the leading edge of the gauge should be 0.3t from the weld toe. LOCAL STRESS NOMINAL STRESS 200Mpa CAE 100Mpa Measure

Data acquisition 7-8GB of data collected 74 channels of strain data (88 total) Approx. 12 hours of data on multiple events Trenching Rock chipping Rock unsettling Crane application Tree uprooting User specific events Time to collect data 14 days 20th May 2015Dodds and Associates13 Trenching MD

Data acquisition 20th May 2015Dodds and Associates14 Tree uprootingDuck walking

Events the designer never thought of 20th May 2015Dodds and Associates15 Did we forget something? Truck loading BH hitting ground

Yet another event! 20th May 2015Dodds and Associates16 The Times, 19 th May 2015

The problem with modelling! 20th May 2015Dodds and Associates17 Model trenching Trenching HD If the end conditions do not match the results will not

Weld challenges 20th May Dodds and Associates

Weld classification 20th May Dodds and Associates

Weld classifications in specimen 20th May 2015 Dodds and Associates 20 D-classF2-class

BS 7608 Weld SN curves 20th May Dodds and Associates N 50 SN curves

Typical scatter in weld life data 20th May Dodds and Associates Recommended Hot-Spot Stress Design S-N Curves for Fatigue Assessment of FPSOs, S J Maddox

Measured stress & load spectrum: Trenching 20th May 2015Dodds and Associates23

Measured stress & load spectrum: Craning 20th May 2015Dodds and Associates24

Measured stress & load spectrum: Rock chipping 20th May 2015Dodds and Associates25

Load spectra – all events 20th May 2015Dodds and Associates26

Create Load spectrum for duty cycle 20th May 2015Dodds and Associates27 Duty Load spectrum Load spectrum A, B, C Concept Example: Sum of 3 load spectra

Estimate percentile max stress in worst case event 20th May 2015Dodds and Associates28 Max value in Δt Stress time series Max value PDF Max value CDF Scale load spectrum to maximum value

Reliability concept Equate the weakest component to the highest load 20th May Dodds and Associates

Methodology overview 20th May 2015Dodds and Associates30 Palmgren-Miner

Present results 20th May 2015Dodds and Associates31 Numbers represent strain gauge locations Scatter in SN curve Uncertain service loads Weld quality and residual stress!

A better approach The designer can apply it! 20th May 2015Dodds and Associates32

Life estimation: Gassner approach 20th May 2015Dodds and Associates33 Load spectra & corresponding SN curves

Design life concepts 20th May 2015Dodds and Associates34 Wöhler & Gassner life curves V. Grubisic Gassner established the topic of "Betriebsfestigkeit" (operational fatigue strength) which can be described as follows: dimensioning (sizing) of a component for finite, but sufficient fatigue life under variable loads

Wöhler & Gassner SN curves 20th May Dodds and Associates

Simplicity Gassner life estimation 20th May Dodds and Associates

Gassner life estimation concept 20th May 2015Dodds and Associates37

Concept, continued 20th May 2015Dodds and Associates38 Life is estimated based on an assumed Gaussian load spectrum (which has a low probability of occurrence) with the maximum range equal to the maximum range in the duty cycle. The method has been proven to give a safe working life estimate. The red dot is the design life (20,000 hours) at the required reliability based on this Gaussian load spectrum. Depending on the real load spectrum of the duty cycle, A, B or C, life is estimated at points 1, 2 or 3. and is independent of the shape of the duty cycle. Point 1 is to the left of the 10% failure probability for the weld and hence will have a very low probability of failure if at all. Point 2 is the same as the design life even though the actual N 10 life will be longer as the duty cycle is less severe. Point 3 is to the right where the probability of failure will be > 10%. In this case the duty cycle lasts longer than the design duty cycle (20,000 hours)

Gassner step 1 Create the filter 20th May 2015Dodds and Associates39 Use trenching data from Gauge B14 To create frequency filter for the Gaussian data Convert to Stress

Filters from other gauges 20th May 2015Dodds and Associates40 The frequency spectra of all gauges on the component have the same shape

Gassner step 2 Create a Gaussian signal Gaussian white noise through the filter Compare – Signals – PSDs – Load spectra 20th May 2015Dodds and Associates41

20th May 2015Dodds and Associates42 Compare the Gaussian and service stress signals

20th May 2015Dodds and Associates43 Compare the filter and response PSDs

20th May 2015Dodds and Associates44 Compare the Gaussian & service load spectra Max range 350MPa Number of cycles = 1.25E4 to cut-off Time = 22min Gauge B14

20th May 2015Dodds and Associates45 Compare level crossings Red = Gaussian Blue = service

20th May 2015Dodds and Associates46 Other service load spectra

Load spectra from a road vehicle chassis 20th May 2015Dodds and Associates47 Stress cycles/km km

Gassner step 3 Generate the Gaussian SN curves

Test specimens under random loading Compared with sinusoidal tests the random test will take 50 ~ 100 times longer per specimen 20th May 2015Dodds and Associates49 Not an impossible task – Random loading tests on motorcycle frame welds

Compare the Gassner life curve with estimated lives from field data using Wöhler curve 20th May 2015Dodds and Associates50 Gassner line Life estimates from sampled field data

Easy to understand & use 20th May 2015Dodds and Associates51

Does it work? 20th May 2015Dodds and Associates52 Proc. IMechE J Rail & Rapid Transit 226(5)

Design stress for good welds for a 99.9% Reliability in 20,0000hrs 20th May 2015Dodds and Associates53 Note: 14,000 was life of BH in a 20,000 total life Max allowable STRESS RANGE = 108MPa

Gassner as a design tool 20th May 2015Dodds and Associates54