7 Ensayo de Compresión Teoría

Understanding Material Properties and Compression Testing

Introduction to Material Comparison

• The session begins with a comparison of different metals, highlighting their ductility and resilience. The speaker emphasizes the need to analyze graphs or tables for understanding material performance.

Key Concepts in Material Strength

• Three fundamental concepts are introduced:

• Resistance: The ability of a material to withstand applied forces.

• Ductility: The extent to which a material can deform without breaking.

• Toughness: Defined as the energy absorbed by a material before fracture.

Distinguishing Material Properties

• Clarification is provided on common misconceptions:

• A more resistant material does not always mean it is more ductile or tough.

• Ductility refers specifically to how much a material can be shaped before failure.

Graphical Analysis of Metals

• Visual aids are used to illustrate the properties of various metals tested in the laboratory, including cast iron, aluminum, and structural steel.

• Cast iron is noted for its low resistance and fragility compared to other materials like structural steel.

Comparative Analysis of Metals

• Structural steel shows significant deformation under stress, while cast iron exhibits lower elasticity but higher ultimate strength.

• Among the tested materials, structural steel emerges as both ductile and tough due to its larger area under stress-strain curves.

Transitioning to Concrete Testing

• After discussing metal properties, attention shifts towards concrete testing methods. Previous tension tests are contrasted with upcoming compression tests.

Understanding Compression Forces

• The speaker explains how applying negative loads (compression) affects materials differently than tensile loads.

• Concrete is highlighted for its high compressive strength but low tensile strength, emphasizing the importance of understanding these behaviors in practical applications.

Analogies in Material Behavior

• Various analogies are drawn between compression testing and real-world applications such as forming processes.

Types of Failure Under Compression

• Two primary types of failure modes during compression are discussed:

• Crushing: When materials fail due to excessive load.

• Buckling: Occurs when slender structures bend under compressive forces.

Conclusion on Compression Testing Dynamics

• An exaggerated example illustrates how dimensions affect behavior under compression; longer columns may buckle while shorter ones crush directly under load.

Understanding the Concept of Radius of Gyration

Definition and Importance

• The radius of gyration is defined as the ratio between length and area, indicating how tall or thick a structure is. This relationship helps determine whether a material behaves as a column or under compression.

Calculating Radius of Gyration

• The radius of gyration (r) can be calculated using the formula: r = sqrtI/A , where I is the moment of inertia and A is the cross-sectional area. It indicates if a structure will work in compression or as a column based on its value.

Working Conditions for Structures

• If the radius of gyration is less than 40, it indicates that the structure will behave like a column; values between 40 and 60 suggest an intermediate state, while values above 60 indicate behavior under compression.

Testing Material Properties

Sample Selection and Testing Methodology

• To test materials, samples are selected according to ASTM standards. These samples are placed in a universal testing machine where load is applied to measure deformation.

Measurement Techniques

• During testing, loads are applied incrementally while observing how the sample deforms under pressure. Measurements include initial length and deformation in millimeters.

Analyzing Stress vs. Deformation

Data Collection Process

• After applying loads, stress values are calculated by dividing load by initial area. This results in stress versus deformation data which helps analyze material behavior under different conditions.

Unique Behavior Under Compression

• Unlike tensile tests where materials elongate until failure, compressive tests show that materials may initially appear to "rise" due to increasing area needing more force to compress further.

Failure Modes in Material Testing

Observing Fiber Alignment

• For accurate results during testing, all fibers must remain aligned; misalignment can lead to incorrect readings or unexpected failure modes such as bulging or bending.

Types of Failures Identified

• Two primary types of failures can occur: bulging (where the specimen expands outward) and flexural failure (due to misalignment). Proper alignment during testing is crucial for reliable results.

Friction Effects on Test Results

Impact of Friction on Compression Tests

• Friction between the sample and machine components can affect test outcomes significantly. As compression occurs, friction may cause uneven distribution of forces leading to inaccurate measurements.

Mitigating Friction Issues

• Using lubricants like Teflon or graphite can help reduce friction effects during tests, allowing for more consistent results across different specimens.

Material Deformation Limits

Understanding Deformation Threshold

• Materials typically can deform up to 70% before failing structurally; however, even at 20% deformation significant damage may occur rendering them unusable for practical applications.

Practical Implications for Testing

Material Testing and Properties

Understanding Material Deformation and Strength

• Discussion on the structural integrity of materials at 45 degrees, emphasizing that certain materials cannot deform. The speaker suggests testing up to a fracture point but indicates limitations in their current approach.

• Introduction of concepts like maximum stress and fracture stress, noting that they can only be estimated under specific conditions, particularly for materials produced in Paraguay.

• Explanation of resilience metrics: unit resilience and total resilience can be calculated, but tenacity is limited to 30% due to the inability to observe fractures directly.

Types of Test Specimens

• Overview of three types of test specimens: short specimens with varying diameters and lengths. The speaker describes how these dimensions affect testing outcomes.

• Detailed description of specimen dimensions: one type has a diameter-to-length ratio of 1:3, while another has a ratio of 1:10. This affects the height and overall performance during tests.

Calculating Moment of Inertia

• Calculation methods for circular sections are discussed, including formulas for determining the radius based on moment inertia. The relationship between diameter and radius is emphasized.

• Further elaboration on specimen ratios; specifically, how different ratios impact testing results when subjected to compression forces.

Compression Testing Procedures

• Description of the compression test setup where specimens are compressed to gather data on load versus deformation (delta).

• Emphasis on obtaining initial values from tension tests; graphical representation will help visualize material behavior under stress conditions.

Final Thoughts on Material Behavior

• Discussion about limitations in measuring ultimate strength due to excessive deformation during tests. Acknowledgment that some properties may not be accurately captured through standard methods.

Understanding Tension and Compression in Materials

Graphing Tension and Compression

• The speaker discusses a graphical representation of tension tests, illustrating the four quadrants to analyze material behavior under stress.

• A negative effort is introduced, indicating that certain values will be less than expected; this leads to a discussion on how to graph these properties accurately.

• The importance of understanding the validity of the graph is emphasized, noting that it should only represent areas where all fibers are under compression.