Summary of Metal Fracture Modes and Fracture Surface Analysis

Ⅰ Introduction

Metal materials often face failure scenarios such as fracture, corrosion, wear, and deformation in various applications. Among these, fracture is considered one of the most dangerous failure types and has always received widespread attention. When a metal splits into two or more pieces under stress (sometimes also affected by heat or corrosive media), it is referred to as complete fracture. When there are cracks within the metal, it is called incomplete fracture.

Ⅱ Types of Metal Fracture

1. Classification by Degree of Plastic Deformation Before Fracture

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Ductile Fracture

Also known as ductile fracture or plastic fracture.

Fracture Surface Characteristics: Macroscopically, the fracture surface of a ductile fracture shows significant plastic deformation, changes in fracture size, the presence of fibrous and shear lip regions, and a dull color. Microscopically, it exhibits dimple patterns, fibrous structures, radiating and herringbone ridge patterns.

Determination: Generally, it is defined that if the reduction of area of a smooth tensile specimen is greater than 5%, it is considered a ductile fracture.

▲ Macroscopic Surface of Ductile Fracture

▲ Microscopic Surface of Ductile Fracture

Brittle Fracture

Fracture Characteristics: The characteristics of brittle fracture at both the macroscopic and microscopic levels are a lack of significant plastic deformation and relatively smooth, shiny surfaces. These features arise because the material doesn't undergo sufficient plastic deformation during fracture, causing cracks to propagate rapidly and form neat, smooth fracture surfaces. At the microscopic level, cleavage steps, river patterns, and tongue patterns further reveal the fracture mechanisms and crack propagation paths of brittle fractures.

Hazard: Generally, there are no prior signs, and it occurs suddenly, often leading to severe consequences.

Determination: A reduction in cross-sectional area of less than 5% in smooth tensile specimens may indicate minimal uniform plastic deformation, which is considered brittle fracture.

▲ Brittle Fracture Macroscopic Surface

▲ Brittle Fracture Macroscopic Surface

2. Classification by Crack Propagation Path

Transgranular Fracture

Characteristics: The crack propagates through the interior of the grains.

Nature: It can be either ductile or brittle.

▲ Transgranular Fracture Schematic

▲ Transgranular Fracture Micrograph

Intergranular Fracture

Characteristics: The crack propagates along the grain boundaries.

Nature: Mostly brittle fractures.

▲ Intergranular Fracture Schematic

▲ Intergranular Fracture Micrograph

3. Classification by Stress Type and Relative Position of Fracture Surface Orientation to Stress

Tensile Fracture

Fracture surface orientation is perpendicular to the maximum normal stress.

Nature: Mostly brittle fracture, but can also show significant plastic deformation.

Shear Fracture

Fracture surface orientation aligns with the direction of maximum shear stress, forming a 45° angle with the maximum normal stress.

Nature: Ductile fracture.

 

4. Classification by Load Nature and Stress Cause

Fatigue Fracture

Definition: Fracture occurring in materials under alternating loads.

Mechanism: Result of the combined effect of alternating stress and time.

▲ Fatigue Fracture Caused by Combined Tension and Compression

Environmental Fracture

Definition: Low-stress fracture of materials caused by environmental factors.

Classification: Mainly includes stress corrosion cracking and hydrogen embrittlement.

▲ Corrosion Fracture

II. Methods of Metal Fracture Surface Analysis

The three-step method for analyzing metal fracture surfaces is a systematic process aimed at understanding the causes, mechanisms, and properties of material fractures by observing and evaluating the fracture surfaces.

Step 1: Macroscopic Observation

Purpose: To perform an initial observation with the naked eye, magnifying glass, or low-power optical microscope to gather macroscopic information about the fracture surface.

Content

Overview Observation: Initially observe the general appearance and interrelationships of various regions of the fracture surface using the naked eye and low-magnification lenses.

Analysis of Fracture Nature: Make a preliminary determination of the nature of the fracture, such as ductile fracture, brittle fracture, etc.

Judgment of Crack Origin: Analyze the location of the crack origin and the direction of crack propagation.

Evaluation of Metallurgical and Heat Treatment Quality: Use macroscopic observation to preliminarily assess the metallurgical quality and heat treatment quality.

Recording: Photograph and record the results of macroscopic observations for subsequent analysis.

Step 2: Microscopic Observation

Purpose: To conduct a more in-depth observation using high-power microscopes (such as metallographic microscopes and scanning electron microscopes) to obtain microscopic information about the fracture surface.

Content

Direct Observation of Fracture Surface: Observe the microscopic morphological characteristics of the fracture surface using metallographic microscopes or scanning electron microscopes.

Observation of Fracture Profile: Further understand the microstructure, crack propagation path, and fracture mechanism.

Verification of Macroscopic Information: Use microscopic observation to further verify the information collected during macroscopic observation.

Determination of Fracture Nature: More accurately determine the nature, propagation speed, and origin location of the fracture based on microscopic observation results.

Identification of Fracture Cause: Through comprehensive analysis, identify the cause and mechanism of the fracture.

Step 3: Quantitative Analysis

Purpose: To conduct more in-depth research on the fracture surface through quantitative testing and analysis to obtain more accurate performance data.

Content

Chemical Composition Analysis: Analyze the chemical composition of the fracture material, including the content of major elements, impurities, and non-metallic inclusions.

Mechanical Property Testing: Perform tensile, impact, hardness, and other mechanical property tests on the fracture material to evaluate its strength, toughness, and hardness.

Fracture Morphology Analysis: Use instruments such as scanning electron microscopes (SEM) for quantitative analysis of fracture morphology, such as crack length, width, depth, etc.

Fracture Mechanism Evaluation: Evaluate the fracture mechanism, such as ductile fracture, brittle fracture, etc., based on quantitative test results.

Comprehensive Evaluation: Combine results from macroscopic observation, microscopic observation, and quantitative analysis to comprehensively assess the material's performance.

Through the three-step method of metal fracture surface analysis, one can gain a comprehensive and in-depth understanding of the causes, mechanisms, and properties of metal material fractures, providing important references for the design, manufacturing, and use of materials.