Failure criteria for Adhesives. Sainath Kadam, 3mE 19 oktober 2014

Transcription

1 Failure criteria for Adhesives Sainath Kadam, 3mE 19 oktober 2014 () Failure criteria for Adhesives 1 / 59

2 Outline 1 Introduction 2 Testing 3 Data analysis 4 FEM 5 Results and Conclusions () Failure criteria for Adhesives 2 / 59

3 Next subsection 1 Introduction Motivation Research Question Workflow Failure Criteria Types of failure 2 Testing 3 Data analysis 4 FEM 5 Results and Conclusions () Failure criteria for Adhesives 3 / 59

4 Motivation Figuur: Advantages of adhesives over other joining techniques. () Failure criteria for Adhesives 4 / 59

5 Next subsection 1 Introduction Motivation Research Question Workflow Failure Criteria Types of failure 2 Testing 3 Data analysis 4 FEM 5 Results and Conclusions () Failure criteria for Adhesives 5 / 59

6 Research Question Research Question To postulate or formulate the best stress based criterion applicable to any adhesive joint to evaluate its strength or allowable stress accurately. () Failure criteria for Adhesives 6 / 59

7 Next subsection 1 Introduction Motivation Research Question Workflow Failure Criteria Types of failure 2 Testing 3 Data analysis 4 FEM 5 Results and Conclusions () Failure criteria for Adhesives 7 / 59

8 Workflow Figuur: Workflow during research. () Failure criteria for Adhesives 8 / 59

9 Next subsection 1 Introduction Motivation Research Question Workflow Failure Criteria Types of failure 2 Testing 3 Data analysis 4 FEM 5 Results and Conclusions () Failure criteria for Adhesives 9 / 59

10 Failure Criteria Stress based failure criteria 1 Maximum shear 2 Von Mises 3 Mohr-Coulomb 4 Drucker-Prager 5 Fracture based 6 In-plane shear 7 Peal 8 Mean stress () Failure criteria for Adhesives 10 / 59

11 Next subsection 1 Introduction Motivation Research Question Workflow Failure Criteria Types of failure 2 Testing 3 Data analysis 4 FEM 5 Results and Conclusions () Failure criteria for Adhesives 11 / 59

12 Types of failure Figuur: Cohesive failure. Figuur: Adhesive failure. () Failure criteria for Adhesives 12 / 59

13 Next subsection 1 Introduction 2 Testing Test setups 3 Data analysis 4 FEM 5 Results and Conclusions () Failure criteria for Adhesives 13 / 59

14 IBS Stein test setups Figuur: Setup for tensile, shear and bending tests. () Failure criteria for Adhesives 14 / 59

15 Torsion test setups Figuur: Model of the torque test. Figuur: Setup showing torque sensor, couplings, torque wrench. () Failure criteria for Adhesives 15 / 59

16 Lap shear test setups Figuur: Instron setup for lap shear test. Figuur: Preparing lap shear sample. () Failure criteria for Adhesives 16 / 59

17 Next subsection 1 Introduction 2 Testing 3 Data analysis Identifying outliers Identification of yield 4 FEM 5 Results and Conclusions () Failure criteria for Adhesives 17 / 59

18 Identifying outliers Figuur: Time series plot - shear tests Figuur: Identifying outliers () Failure criteria for Adhesives 18 / 59

19 Next subsection 1 Introduction 2 Testing 3 Data analysis Identifying outliers Identification of yield 4 FEM 5 Results and Conclusions () Failure criteria for Adhesives 19 / 59

20 Identification of yield Plasticity advance Complete yield () Failure criteria for Adhesives 20 / 59

21 Identification of yield Figuur: Representative stress-strain curve showing stages in plasticity. Figuur: Stress-strain curve from tested lap shear. () Failure criteria for Adhesives 21 / 59

22 Next subsection 1 Introduction 2 Testing 3 Data analysis 4 FEM FEM modeling Stresses at a distance FEM analyses 5 Results and Conclusions () Failure criteria for Adhesives 22 / 59

23 Geometry & Stress locations Figuur: Modeling aspects considered & stress locations. 1 Adhesive fillet radius = 2 adhesive thickness. 2 Adherend fillet radius = 1 / 10 adhesive thickness. 3 Check stresses in FEM at the location of failure. 4 For cohesive failure check stresses in center layer. 5 For adhesive failure check stresses at interface layers. () Failure criteria for Adhesives 23 / 59

24 Meshing 1 Stress gradients in the transverse direction are low. Mesh density can be low. 2 Also mesh is coarse away from the interest zone. Figuur: Overall FEM modeling. () Failure criteria for Adhesives 24 / 59

25 Next subsection 1 Introduction 2 Testing 3 Data analysis 4 FEM FEM modeling Stresses at a distance FEM analyses 5 Results and Conclusions () Failure criteria for Adhesives 25 / 59

26 Stresses at a distance - Center layer Figuur: Distribution of stresses in center layer of adhesive. Figuur: Closeup of the stresses at the edges. () Failure criteria for Adhesives 26 / 59

27 Stresses at a distance - Interface layer Figuur: Distribution of stresses in interface layer of adhesive. Figuur: Closeup of the stresses at the edges. () Failure criteria for Adhesives 27 / 59

28 Next subsection 1 Introduction 2 Testing 3 Data analysis 4 FEM FEM modeling Stresses at a distance FEM analyses 5 Results and Conclusions () Failure criteria for Adhesives 28 / 59

29 Types of tests and FEM analyses Figuur: Torsion test Figuur: Bending test () Failure criteria for Adhesives 29 / 59

30 Types of tests and FEM analyses Figuur: Shear test Figuur: Tensile test () Failure criteria for Adhesives 30 / 59

31 Types of tests and FEM analyses Figuur: Lap shear test () Failure criteria for Adhesives 31 / 59

32 Next subsection 1 Introduction 2 Testing 3 Data analysis 4 FEM 5 Results and Conclusions Results Way of Working Conclusions () Failure criteria for Adhesives 32 / 59

33 Results - Center layer Figuur: Comparison of the Failure criteria for center layer. () Failure criteria for Adhesives 33 / 59

34 Results - Interface layer Figuur: Comparison of the Failure criteria for interface layer. () Failure criteria for Adhesives 34 / 59

35 Next subsection 1 Introduction 2 Testing 3 Data analysis 4 FEM 5 Results and Conclusions Results Way of Working Conclusions () Failure criteria for Adhesives 35 / 59

36 Way of Working Figuur: Way of working. () Failure criteria for Adhesives 36 / 59

37 Next subsection 1 Introduction 2 Testing 3 Data analysis 4 FEM 5 Results and Conclusions Results Way of Working Conclusions () Failure criteria for Adhesives 37 / 59

38 Conclusions Main Conclusions 1 Selection of failure criteria based on the mode of failure. 2 Cohesive failure Drucker-Prager criterion. 3 Adhesive failure Fracture criterion. Other Conclusions 1 Stresses at a distance realistic estimate of stresses near edge. 2 Adhesive and adherend fillet avoid steep stress gradients near edge. 3 Results from cohesive failures could be used for different substrate materials. 4 Adhesive failures is substrate dependent. () Failure criteria for Adhesives 38 / 59

39 Comments Comments 1 Yield as failure Yielding varies with tests. Stability is critical. 2 Strain and Energy criteria Linear analysis, strain as good as stress. 3 Shrinkage Edge effects predominant in comparison. 4 Strain rates Strength decreases with decreased strain-rates. 5 FEM Non-linear model sensitive to different types of loads 6 Testing Strains need to be accurately measured. () Failure criteria for Adhesives 39 / 59

40 THANK YOU Questions () Failure criteria for Adhesives 40 / 59

41 Number of tests - Scotweld 9323 Test type for Scotchweld 9323 Number of tests IBS1 pin lateral shear test 156 IBS1 bending test 156 IBS1 bending test with corner beads 26 IBS1 bending test at 1/10th speed (0.1mm/s) 26 Lap shear test 9 IBS1 tensile test 364 IBS1 bending test for cracked specimen 52 IBS1 bending test at 1/10th speed (0.1mm/s) 26 Torsion test 156 Tabel: Table with list of tests and the number of tests conducted for Scotweld () Failure criteria for Adhesives 41 / 59

42 Number of tests - Araldite 2030 Test type for Araldite 2030 Number of tests IBS1 pin lateral shear test 130 IBS1 bending test 156 IBS1 bending test with corner beads 26 IBS1 bending test at 1/10th speed (0.1mm/s) 26 Lap shear test 9 IBS1 tensile test 156 IBS1 bending test for cracked specimen 52 IBS1 bending test at 1/10th speed (0.1mm/s) 26 Torsion test 156 Tabel: Table with list of tests and the number of tests conducted for Araldite () Failure criteria for Adhesives 42 / 59

43 Summary test results - Scotweld9323 Test type for Scotchweld 9323 Nominal stress (MPa) Torsion test 33 IBS1 tensile test 54 IBS1 pin lateral shear test 27 IBS1 bending test 5 Lap shear test 18 Percent standard deviation in pin tests Percent standard deviation in all tests Tabel: Table with summary of the test results for Scotchweld () Failure criteria for Adhesives 43 / 59

44 Summary test results - Araldite2030 Test type for Araldite 2030 Nominal stress (MPa) Torsion test 28 IBS1 tensile test 15 IBS1 pin lateral shear test 4 IBS1 bending test 2 Lap shear test 11 Percent standard deviation in pin tests 97.6 Percent standard deviation in all tests Tabel: Table with summary of the test results for Araldite () Failure criteria for Adhesives 44 / 59

45 Goodness of fit - Weibull Figuur: Goodness of fit characteristics Figuur: Fit for different probability distributions () Failure criteria for Adhesives 45 / 59

46 Remarks Further study 1 Formulate non-linear material models. 2 Shear and compressive tests on bulk samples. 3 Shrinkage related residual stresses. 4 Swelling and its effects on strength. 5 Visco-elastic behavior effecting creep and impact analysis. 6 Fatigue. 7 Modeling the joint with contact elements with appropriate stiffness. () Failure criteria for Adhesives 46 / 59

47 Results - Center layer Criteria Torsion Tensile Shear Bending Lap STD w/ lap Max. Principal Maximum Shear Von Mises Peel In-plane shear Mean stress Fracture stress Mohr-Couloumb Drucker-Prager Tabel: Summary of results for center layer. These results are the results of center layer for Scotchweld STD represents Percent standard deviation for results for each criterion. Last column includes the lap shear results in the STD calculations. () Failure criteria for Adhesives 47 / 59

48 Results - Interface layer Criteria Torsion Tensile Shear Bending Lap STD w/ lap Max. Principal Maximum Shear Von Mises Peel In-plane shear Mean stress Fracture stress Mohr-Couloumb Drucker-Prager Tabel: Summary of results for interface layer. These results are a combination of the results from top and bottom layer results for Araldite STD represents Percent standard deviation for results for each criterion. Last column includes the lap shear results in the STD calculations. () Failure criteria for Adhesives 48 / 59

49 Sensitivity - Material properties Figuur: Sensitivity of Von Mises peak stress (MPa) at adherend corner to material properties. 1 Stresses increase with decrease in Poisson s ratio. 2 Stresses increase with increase in Young s modulus. 3 Sensitivity is different for different components of stress. 4 Stresses are less sensitive to changes in Young s modulus. () Failure criteria for Adhesives 49 / 59

50 Sensitivity - Mesh Figuur: Von Mises peak stress at adherend corner for different mesh size. 1 Critical mesh size is 1/3rd the adhesive thickness 2 As mesh density increases the stress levels increase. 3 In this case we have 30% increase in stresses for twice as many nodes at the corner. () Failure criteria for Adhesives 50 / 59

51 Stress locations Figuur: Stress locations. () Failure criteria for Adhesives 51 / 59

52 Material models 1 Linear Von Mises material model is used where the tensile and compressive strengths are believed to be equal which is not true for adhesives. 2 Improvement is the Linear or Extended Drucker-Prager material model which is sensitive to hydrostatic stresses hence depicting a truer behavior of the adhesive. 3 Advantage of non-linear material models effect of material singularities is reduced. 4 Disadvantage requires more detailed testing for non-linear material properties. Stresses from FEM analysis are not scalable. () Failure criteria for Adhesives 52 / 59

53 Failure criteria Drucker-Prager criterion S yt ( m 1 2 m )(σ 1 + σ 2 + σ 3 ) + ( m m ) (σ1 σ 2 ) 2 + (σ 2 σ 3 ) 2 + (σ 3 σ 1 ) 2 2 Pressure dependent criteria for determining if a material has yielded or failed. Usually used to study plastic deformation of soils, also polymers. () Failure criteria for Adhesives 53 / 59

54 Failure criteria contd. Maximum shear stress criterion S yt max( σ 1 σ 2, σ 2 σ 3, σ 3 σ 1 ) The material remains elastic if the 3 principal stresses are equivalent. However as one of the principal stresses becomes larger or smaller, then material is subjected to shearing. () Failure criteria for Adhesives 54 / 59

55 Failure criteria contd. Mohr-Coulomb criterion S yt m + 1 (2 m) max( σ 1 σ 2 + K(σ 1 + σ 2 ), σ 1 σ 3 + K(σ 1 + σ 3 ), σ 2 σ 3 + K(σ 2 + σ 3 )) It includes the response of material under compressive and tensile stresses. Generally applied to materials with larger compressive than tensile strength. () Failure criteria for Adhesives 55 / 59

56 Failure criteria contd. Von Mises criterion (σ1 σ 2 ) S yt 2 + (σ 2 σ 3 ) 2 + (σ 3 σ 1 ) 2 2 Fracture stress criterion S yt σ Z 2 + σ XZ 2 + σ YZ 2 Equivalent stress criteria developed from the combination of in-plane shear stress and the normal stress acting on that plane. () Failure criteria for Adhesives 56 / 59

57 Failure criteria contd. In-plane shear stress criterion S yt σ XZ 2 + σ YZ 2 Peel stress criterion S yt σ Z Mean stress criterion S yt σ 1 + σ 2 + σ 3 3 Figuur: Normal stress perpendicular and in-plane shear stresses in the plane of the surface. () Failure criteria for Adhesives 57 / 59

58 Misceleneous Comments Yield as Failure 1 Adhesives are ductile. Some tests show more yielding than others before failure. Good idea to consider yield as failure. 2 Beyond yield the joint may not break off, but will affect stability of joint. Strain and Energy criteria 1 For the linear part of the response to loading it does not matter if we consider strain or stress criteria. 2 Von Mises is based on the distortion energy. Although no direct energy criteria is evaluated. Shrinkage 1 Residual stresses and temperature effects are present. Far more dominant is the edge effect with stresses predominantly from loading. () Failure criteria for Adhesives 58 / 59

59 Misceleneous Comments contd. Strain-rates 1 Not directly a part of analysis. Some tests were conducted to find out the effect of loading rate on strength. 2 Strength decreases with decreased strain-rate. FEM 1 Non-linear analysis is computationally very expensive. 2 More than just a non-linear material model, we need to take into account the difference in response of adhesive under tensile/shear/compressive loads. Testing 1 More accurate measurement of strains needs additional setting up of gages. Considering strain criteria might be interesting. 2 Machine strains can be calculated, but is still approximate. () Sensitivity of stresses Failure criteria to Poisson s for Adhesives ratio is critical. 59 / 59