GB/T 21143-2014 Metallic materials - Unified method of test for determination of quasistatic fracture toughness
1 Scope
This standard specifies test methods for fracture toughness, crack-tip opening displacement, J-integral and R-curves for homogeneous metallic materials subjected to quasistatic loading. Specimens are notched, precracked by fatigue and tested under slowly increasing displacement.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
GB/T 8170 Rules of rounding off for numerical values & expression and judgment of limiting values
GB/T 12160-2002 Calibration of extensometers used in uniaxial testing (GB/T 12160-2002, ISO 9513:1999, IDT)
GB/T 16825.1 Verification of static uniaxial testing machines - Part 1: Tension/compression testing machines - Verification and calibration of the force measuring system (GB/T 16825.1-2008, ISO 7500-1:2004, IDT)
GB/T 20832 Metallic materials - Designation of test specimen axes in relation to product texture (GB/T 20832-2007, ISO 3785: 2006, IDT)
ASTM E1820-11 Standard Test Method for Measurement of Fracture Toughness
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
stress intensity factor
K
magnitude of the elastic stress-field singularity for a homogeneous, linear-elastic body
Note: The stress intensity factor is a function of applied force, specimen size, specimen geometry and crack length.
3.2
crack-tip opening displacement
δ
relative displacement of the crack surfaces normal to the original undeformed crack plane at the tip of the fatigue precrack
3.3
J-integral
line or surface integral that encloses the crack front from one crack surface to the other and characterizes the local stress-strain field at the crack tip
3.4
stable crack extension
crack extension which stops or would stop when the applied displacement is held constant as a test progresses under displacement control
3.5
unstable crack extension
abrupt crack extension occurring with or without prior stable crack extension
3.6
pop-in
abrupt discontinuity in the force-displacement curves, featured as a sudden increase in displacement and, generally, a decrease in force
Note 1: Displacement and force subsequently increase beyond their values at pop-in.
Note 2: When conducting tests by this method, pop-ins may result from unstable crack extension in the plane of the precrack and are to be distinguished from: 1) delaminations or splits normal to the precrack plane; 2) roller or pin slippage in three-point bend or compact specimen load trains, respectively; 3) improper seating of displacement gauges; 4) ice cracking in low-temperature testing; 5) electrical interference in force and displacement measuring and recording devices.
3.7
crack extension resistance curves (R-curves)
variation in δ or J with stable crack extension
4 Symbols and designations
The following symbols and designations (specified in Table 1) are used in this standard.
Table 1 Symbols and designations
Symbol Unit Description
a mm Nominal crack length
af mm Final crack length (af=a0+Δa)
ai mm Instantaneous crack length
am mm Length of machined notch
a0 mm Initial crack length
Δa mm Stable crack extension including blunting
Δamax mm Crack extension limit for δ or J controlled crack extension
B mm Specimen thickness
BN mm Specimen net thickness between side grooves
C m/N Specimen elastic compliance
E GPa Modulus of elasticity at the test temperature
F kN Applied force
Fc kN Applied force at the onset of unstable crack extension or pop-in when ∆a is less than 0.2mm offset from the construction line
Ff kN Maximum fatigue precracking force
J kJ/m2 Experimental equivalent to the J-integral, equivalent to the J-integral, with specific values (such as Jc, Ji and Ju) determined by the method in this standard, which are used to characterize the fracture toughness under the condition of non-negligible plastic deformation at the crack tip.
Jc(B) kJ/m2 Size sensitive fracture resistance J at onset of unstable crack extension or pop-in when Δa is less than 0.2mm offset from the construction line (B=specimen thickness)
Jg kJ/m2 J at upper limit of J-controlled crack extension
Ji kJ/m2 Fracture J at initiation of stable crack extension
Jm(B) kJ/m2 Size sensitive fracture resistance J at the first attainment of a maximum force plateau for fully plastic behaviour (B=specimen thickness)
Jmax kJ/m2 Limit of J-R material behaviour defined by the method in this standard
Ju(B) kJ/m2 Size sensitive fracture resistance J at onset of unstable crack extension or pop-in when Δa is greater than or equal to 0.2mm offset from the construction line (B=specimen thickness)
Juc(B) kJ/m2 Size sensitive fracture resistance J at onset of unstable crack extension or pop-in when stable crack extension cannot be measured (B=specimen thickness)
J0 kJ/m2 J uncorrected for stable crack extension
J0.2BL kJ/m2 Size insensitive fracture resistance J at 0.2mm stable crack extension offset from the construction line
JQ0.2BL kJ/m2 J at the intersection of fitting curve and 0.2mm offset from the construction line
J0.2BL(B) kJ/m2 Size sensitive fracture resistance J at 0.2mm stable crack extension offset from the construction line (B=specimen thickness)
K MPa·m1/2 Stress intensity factor
Kf MPa·m1/2 Maximum value of stress intensity factor K during the final stages of fatigue precracking
KIC MPa·m1/2 Plane strain fracture toughness
KQ MPa·m1/2 A provisional value of KIC
q mm Load-point displacement
R mm Rotation radius
1) Distance from the load-line to the rotation center before deformation (CT specimen)
2) Distance from the end face of crack nozzle of the specimen to the rotation center before deformation (SEB specimen)
Rm MPa Tensile strength perpendicular to crack plane at the test temperature
Rp0.2 MPa 0.2% offset yield strength perpendicular to crack plane at the test temperature
S mm Span
T ℃ Test temperature
U J Area under plot of force versus load-point displacement curve
Ue J Elastic component of U
Up J Plastic component of U
Ap J Plastic component of the area under plot of force versus notch-opening displacement curve
V mm Notch-opening displacement
Vg mm Total notch-opening displacement
Ve mm Elastic component of V
Vp mm Plastic component of V
W mm Width of test specimen
Z mm Distance of the notch opening gauge measurement position from the surface of the specimen
δ mm Crack-tip opening displacement (CTOD)
δc(B) mm Size sensitive fracture resistance δ at onset of unstable crack extension or pop-in when Δa is less than 0.2mm crack offset from the construction line (B=specimen thickness)
δg mm δ at the limit of δ-controlled crack extension
δi mm Fracture resistance δ at initiation of stable crack extension
δm(B) mm Size sensitive fracture resistance δ at the first attainment of a maximum force plateau for fully plastic behaviour (B=specimen thickness)
δmax mm Limit of δ-R material behaviour defined by the method in this standard
δu(B) mm Size sensitive fracture resistance δ at onset of unstable crack extension or pop-in when Δa is greater than 0.2mm crack offset from the construction line Value(B=specimen thickness)
δuc(B) mm Size sensitive fracture resistance δ at onset of unstable crack extension or pop-in when stable crack extension cannot be measured (B=specimen thickness)
δ0 mm δ uncorrected for stable crack extension
δ0.2BL mm Size insensitive fracture resistance δ at 0.2mm stable crack extension offset from the construction line
δ0.2BL(B) mm Size sensitive fracture resistance δ at 0.2mm stable crack extension offset from the construction line (B=specimen thickness)
δQ0.2BL mm δ at the intersection of fitting curve and 0.2mm offset from the construction line
v — Poisson's ratio
Note 1: Only the main parameters are given here, other parameters are referred to in the text.
Note 2: The values of all parameters listed are assumed to be those measured or calculated for the test temperature, unless otherwise specified.
5 General requirements
5.1 General
The fracture toughness of metallic materials may be characterized in terms of either specific (single point) values (see Clause 6), or a continuous curve over a limited range of crack extension (see Clause 7). The procedures and parameters used to measure fracture toughness vary depending upon the level of plasticity realized in the test specimen during the test. During the test, the test procedures and parameters to determine the fracture toughness of the material are selected through the gradual understanding of the plasticity level of the test specimen. In all cases, tests are performed by applying slowly increasing displacements to the test specimen and measuring the forces and displacements realized during the test. The relationship between forces and displacements is then used to determine the fracture toughness through the material resistance under characterized crack extension. A standard flow-chart that can be used is presented in Figure 1. Characteristic types of force versus displacement curves obtained in fracture toughness tests are shown in Figure 2.
Figure 1 General flowchart showing how to use this standard
Key:
Note 1: FQ is the maximum force used in the determination of a provisional KIC (see Figure 16).
Note 2: Fc, Fu and Fm correspond to either δc, δu and δm respectively, or Jc, Ju and Jm respectively.
Note 3: Pop-in behaviour is a function of the testing machine/specimen compliance and the recorder response rate.
a Point of fracture.
b Pop-in.
Figure 2 Characteristics types of force versus displacement records in fracture tests
5.2 Fracture parameters
Specific values of fracture toughness are determined from individual specimens to define the unstable crack extension or describe the onset of stable crack extension.
KIC characterizes the resistance to extension of a sharp crack so that:
Foreword i 1 Scope 2 Normative references 3 Terms and definitions 4 Symbols and designations 5 General requirements 6 Determination of fracture toughness for stable and unstable crack extension 7 Determination of resistance curves δ-∆a and J-∆a and initiation toughness δ0.2BL, J0.2BL, δi and Ji for stable crack extension 8 Test report 9 Rounding of determination results Annex A (Normative) Crack plane orientation Annex B (Normative) Stress intensity factor and compliance relationships Annex C (Normative) Power-law fits to crack extension data[15] Annex D (Informative) Determination of δi and Ji Annex E (Informative) Example of test reports Annex F (Informative) Measurement of load-point displacement q in the three-point bend test Annex G (Informative) Derivation of pop-in equations Annex H (Informative) Analytical methods for the determination of Vp and Ap or Up Annex I (Informative) Guidelines for unloading compliance method and electrical potential method Annex J (Informative) Guidelines for load separation method Annex K (Informative) Determination of CTOD value with section method Annex L (Informative) Comparison between this standard and ISO 12135: 2002(E) in terms of clause numbers Annex M (Informative) Technical differences between this standard and ISO 12135: 2002(E) and their justifications Bibliography
