The fracture toughness and fatigue crack propagation of the

metal welded joints

Reporter: Shi LeiTutor: Chen Xu2012.11.7

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Contents

Metallographic structure analysis of welded joints

The principles & measurement of fracture toughness

The principles & measurement of fatigue cracks propagation

Metallographic structure analysis of welded joints

A metal welded joint is consisted of : weld metal area, bond area, heat affect zone and base metal area.

Weld metal area: is solidified by liquid metal in a weld pool. It has been considered as a kind of forging organization. The crystalline grains are columnar which are normal to the bottom of the weld pool. Low melting point impurities, such as S and P, are easy to form segregation in the center of the weld line, which decrease the plasticity of the weld and cause heat cracks. Thus the strength of the weld metal is usually not lower than the base metal.

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The organization of HAZ of not easy quenching steels(low carbon steel and low alloy steel):1.Fusion zone2.GCHAZ3.GRHAZ4.ICHAZFusion zone：is the junctional zone between the weld metal and base metal.Heating temperature： 1490～1530℃（固、液相线之间）Structure： (unmelt but grown by overheat） coarse-grained structure and（ partly solidified）forged structure。Characterization： this area is very narrow with not uniform structure, and the strength & plasticity is very poor. So it is the origin of cracks and local brittle fracture.

GCHAZ：is close to the fusion zone.

Heating temperature： 1100℃～1490℃（ 1100℃～固相线）Structure：Overheated structure of coarse grains.Characterization：The width is about 1 to 3 mm, and plasticity & toughness is decreased.

GRHAZ：is close to the GCHAZ.Heating temperature：850℃～ 1100℃（ AC3至1100℃）Structure：Uniform and fine ferrites and pearlites(approximately normalizing organization).Characterization：The width is about 1.2 to 4.0 mm, and its mechanic performance is better than the base metal.

ICHAZ：is close to the GRHAZ.Heating temperature：AC1～ AC3

Structure：Not uniform ferrites and pearlites.Characterization：The phase of partial organization has been changed. The crystalline grains are not uniform, so the mechanic performance is poor.

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The HV values of the weld:essentially represent the performances of different metallographic organizations.So it is easy to distinguish the different weld zones with the distribution of the HV values.

The principles & measurement of fracture toughness

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Linear Elastic Fracture Mechanics(Stress intensity factor),(Crack extension force),(Radius of plastic zone)

Small scale yielding elastoplastic materialModified ,

R=2

Elastic-plastic Fracture Mechanics(Crack tip open displacement),

(Integral J),

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Arrest of the plastic zone:

1)Even fracture: Breadth is large. In plane strain state. The plastic zone is small.2)Oblique fracture: Breadth is small. In plane stress state. The plastic zone is large.3)Mixed fracture: Breadth is within a certain range.

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The measurement of fracture toughness(GBT 21143-2007 ):

Test sample: Support:

Devices: Hydraulic fatigue machine, extensometer, X-Y recorder

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The measurement of

If

Then

Cracks arrangement:

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𝛿=𝛼+𝛽∆𝑎𝛾

The measurement of

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J

The measurement of J

The principles & measurement of fatigue cracks propagation

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Cracks initiation: The cracks initiation of single-phase alloy are usually caused by slip band, while those of heterogeneous alloy are caused by local plastic deformation between heteroatoms and matrix.

The law of fatigue cracks propagation: The components exerted by high cycle fatigue load (Elastic or small scale yielding) are only considered. The relationship of (crack growth rate) and (range of stress intensity factor) can be described as the following equation:

𝑑𝑎𝑑𝑁 =𝐶 ∙𝐾 𝑛

The logarithmic form:

𝑙𝑔 𝑑𝑎𝑑𝑁=𝑙𝑔𝐶+𝑛𝑙𝑔𝐾

The Paris law:

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The three stages of fatigue cracks propagation:

(I)The first stage: cracks initiation When , the cracks can not be propagated or the .

(II)The second stage: steady expansionThe Curve is approximately linear, which is in accord with the Paris Law. The is about to .

(III)The third stage: rapid expansionWhen >, cracks begin to expand unstably and rapidly. The > . When , the unsteady fracture is happened.

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The measurement of fatigue crack growth rate(GB-T6398):

Secant method:

Increasing polynomial:

𝑙𝑔 𝑑𝑎𝑑𝑁=𝑙𝑔𝐶+𝑛𝑙𝑔𝐾

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Thanks for your attention!