Project MMS 7IAG meeting 4 Dec. 2002

Summary

Task 4c Failure criteria

Task 5 A database for adhesives

Prediction of failure in adhesive joints

Calculate stress and strain distributions in the adhesive using stress analysis

Use a criterion for crack initiation based on a critical level of stress or strain to give ultimate or safe load or deformation

Use fracture mechanics to describe crack propagation

Predictive accuracy

Depends onthe validity of the failure criterionthe accuracy of the materials model in the stress analysis

Different materials models give significantly different predictions of stress and strain

Stress analysis of the butt-joint – FE models

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40 45

extension (microns)

load

(kN

)

von Mises

exponent Drucker-Prager

experimental

linear Drucker-Prager

Evaluation of failure criteria

Predict force vs extension for joint specimensCompare with experiment to determine instant of crack initiationIdentify location in adhesive where crack initiatesPredict stress and strain distributions in that location at the instant of failureCompare critical values of stress and strain components in different joint geometries

FE meshes for lap, scarf and T-peel joints

Scarf joints – aluminium adherends

0

1

2

3

4

5

6

7

8

9

10

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

gauge extension (mm)

Forc

e (k

N)

HRB0023

HRB0029

Exponent Drucker-Prager

Cavitation model

Scarfjoint with Aluminium adherendsForce vs gauge extension for gauge lengths of approx 12.5

Crack path in the scarf joint

Possible failure criteria

Critical maximum principal stress

Critical maximum principal strain

Critical effective shear (Mises) stress

Critical hydrostatic stress

Critical volumetric strain

Maximum principal strain - comparison of model predictions

Cavitation model

Exponent D-P

Cavitation model

T-peel - steel adherendsSteel T-peel joint tests

0.5 mm/min

0

0.5

1

1.5

2

2.5

3

3.5

4

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

Extensometer displacement (mm)

Load

(kN

)

HRB202 (0.5 mm/min)

HRB207 (0.05 mm/min)

HRB208 (0.5 mm/min)

Exponent Drucker-Prager

Cavitation model (elastic)

Cavitation model (elastic-plastic)

Crack path in the T-peel joint

Comparison of maximum values of stress and strain components for each joint from both models

FailureExtn

(mm)

Max.princ.stress(MPa)

Hydrostatic stress(MPa)

Max. princ. strain Volumetric strain

Cavitn ExpD-P Cavitn. ExpD-P Cavitn ExpD-P Cavitn. ExpD-P

Lap/steel 0.085 76 81 37 43 0.058 0.053 0.025 0.021

Lap/alum.

Scarf/steel 0.035 64 60 39 50 0.064 0.18 0.047 0.13

Scarf/alum. 0.045 65 60 39 50 0.082 0.28 0.061 0.18

T-peel/steel 0.19 56 57 38 41 0.036 0.042 0.022 0.026

T-peel/alum 0.24 57 57 35 38 0.039 0.044 0.020 0.021

Conclusions – failure criterion

Identifying a realistic failure criterion is still a subject for further researchUse of such a criterion would require an accurate stress analysisThe accuracy of stress and strain calculations depends on the choice of materials model and joint geometryA criterion for safe operation will probably be used

A database for adhesives

Each module in the Toolkit needs access to specific data for different adhesivesThe database will specify these data and identify test methods and conditions for their measurement (by adhesives suppliers)A compromise is needed when specifying the required data- specify too much data and the cost will be too high- specify too little and the capabilities of the toolkit will be

limitedProbably restrict the database initially to structural adhesives

PROPERTY REQUIREMENTS FOR MATERIALS SELECTIONPredominantly single-point data

Tensile modulus GPa ISO 527 Bulk specimens

Shear modulus GPa ISO 11003-2

Yield stress MPaISO 527 For ductile behaviour (failure after yield)

Yield strain

Nominal strain at failure

Stress at failure MPa ISO 527 For brittle behaviour (failure before yield)

Strain at failure

Glass transition temperature

C ISO 11357-2

DMTA curve GPa ISO 6721-4 From –40 to above Tg

Property Units Test method Additional information

Lap shear strength MPa

Aluminium adherends - specify surface treatment

Steel adherends – specify surface treatment

Other – specify material and treatment

T-peel strength

Aluminium adherends - specify surface treatment

Steel adherends – specify surface treatment

Other – specify material and treatment

Toughness kJ/m2 ISO 179Charpy impact – from -40ºC to 23ºC or fracture toughness from joint test

Viscosity Pa.s

Property Units Test method Additional information

Slump

Working life

Density kg/m3 ISO 1183

Water absorption % ISO 62 Saturation value at 23ºC

Volume resistivity ohm.m IEC 60093

Electric strength kV/mm IEC 60243

Property Units Test method

Additional information

PROPERTY REQUIREMENTS FOR STRESS ANALYSIS

Specify post-cure temperature and timeStore specimens at 23ºC, 50% RH for 30 days prior to testing

Property Units Test method Additional information

Tensile modulus MPa ISO 527At -40ºC, 23ºC and 80ºCAt a test speed of 1 mm/min.(or use TAST and a typical Poisson’s ratio value)

Poisson’s ratio ISO 527 (or use typical value)

True tensile stress vstrue plastic strain curve

MPa ISO 527 At -40ºC, 23ºC and 80ºCAt a test speed of 1 mm/min.

Shear stress vs plastic shear strain curve ISO 11003-2

TAST(alternative to tensile data)At -40ºC, 23ºC and 80ºCAt a test speed of 1 mm/min.

Poisson’s ratio curve ISO 527 Over range of strain at 23ºC

Linear expansion coefficient

K-1 ISO 11359-2 Over temperature range between 23ºC and Tg

Property Units Test method Additional information

PROPERTY REQUIREMENTS FOR QUALITY CONTROL AND DURABILITY

Post-cure at recommended cure temperature and time and record these quantities.Specify adhered type and surface treatment. Use steel and aluminium and optional others.

Property Test condition Specimen conditioning prior to test

Lap-shear strength -40ºCDry

23ºC

80ºC

23ºC Expose for 30 days:

Under 50% RH at 23ºC

At 120ºC

Under 95% RH at 70ºC

In water at 23ºC

In alcohol at 23ºC

Other chemicals and temperatures that are optional

Conclusions – adhesives database

These proposals are a basis for discussion and comment- Within MMS 7- By IAG members- Within ISO TC61- By adhesives suppliers